HydraRaptor

Bearings, Bushings and Bars

2012-01-07T17:57:00.000Z

My last post started a discussion about why I got only a few hundred hours of use from PLA bushings and in particular commercial IGUS bushings. I think I mounted the IGUS bushings well enough. I printed PLA holders and reamed them to a 10mm bore, which gave a nice press fit.

I had intended to use a small self tapping screw to retain the flange but found I didn't need them. That is what the two holes are for. They are triangular because they are polyholes.

The holders have slotted screw holes and were screwed to the underside of my Prusa's Dibond Y carriage. I started with them loose and then tightened the screws as I ran the axis up and down to ensure they were aligned well. I then applied lithium grease.

When first fitted they had no slop and very low friction. After a few days of continuous use the holes in the bushings had elongated and there was noticeable slop. At that point I replaced them with LM8UU bearings in prototype bearing holders I designed for the Mendel90.

These have run for thousands of hours with no noticeable wear. They do have more friction than bushings though. It seems higher to start with but they seem to "wear in" quite quickly and it drops.

My suspicion was that the surface quality of the stainless steel rods that I used was to blame, so I have just had a look with a microscope. I used a cheap USB "Traveller" microscope from Aldi and a times 4 objective lens. The magnification is much greater than that though when photographed and blown up to screen size.

Here are a couple of pictures of an off-cut from the stainless steel rods I used on my Mendel: -

Obviously you can only have a small strip in focus due to the curvature of the rod but you can see it looks far from smooth. The difference between the pictures is mainly the lighting angle.

Here is a mild steel rod bought on eBay, sold for Reprap use, so probably typical of what most people use: -

Quite a lot smoother, so hopefully most people get better life from PLA bushings than I did.

Here is a bright steel rod from a 2D printer, or maybe a flat bed scanner, I can't remember which, but it will have used bushings: -

It seems to have a finer grain structure but doesn't look particularly smooth.

And here is a "precision round rail (Induction Hardened)" sold for use with linear bearings that I got from Zapp Automation.

It looks the best out the four, so I guess you get what you pay for.

I think for soft bushings to last you need high quality rods. LMUU bearings seem to be more tolerant.

Mendel90 axes

2011-12-30T18:58:00.000Z

Bearings

With the stainless steel bars that I use I found that PLA bushings only last a few hundred hours before they wear out. I tried Igus plastic bushings and they only lasted about the same length of time. I think you need ground rods rather than rolled to get a smooth enough surface for bushings. Possibly the lithium grease that I used was not suitable for plastic as I am sure other people must have got better life out of bushings.

The ball bearings on my Mendel have proved very durable but they do wear flats on the rods after about a year of continuous use. This wouldn't be a problem except that the rods wear more in the middle, which leads to inconsistent Z height eventually. You can turn the rods to put the flats underneath and get many more years life.

I have run some LM10UU bearings for over a year non-stop and they have not worn the rods noticeably. I did have an LM8UU bearing suddenly decide it only wanted to go one way on my Prusa's X-axis. It just needed some oil to make it work again. I think the X-axis tends to dry out because it runs over the heated bed.

I made the Mendel90 prototype with 10mm rods because I had noticed the 8mm rods sag a little on my Mendel, that has a heavier bed and extruder though. 10mm rods cost quite a lot more than 8mm and the plastic parts get bigger so I intend to make an 8mm version and see how it compares.

X-Axis
The X axis is similar to the Prusa but I have changed a few things: -

Note the axis is shortened in this picture, the belt has a twist not shown and a loop round the tensioning screw.

I lowered the idler and the motor to be in line with the bars because I noticed on my Prusa that the belt tension tended to bow the bars upwards slightly at the ends. It does mean the belt is a bit closer to the heated bed but I haven't noticed any ill effects.

I swapped the positions of the Z bars and Z leadscrews so that the bearing holders face inwards. That means the belt tension tends to push the bearings into their holders rather than pulling them out. That allowed me to get rid of the cable ties.

There are clamps for the X-bars so they don't have to be exactly the right length. They can be adjusted a few mm lengthwise and then locked in place. The holes are open ended at the idler end to allow the bars to be removed without removing the Z-bars first.

The motor housing is a box shape to keep it rigid while still having only relatively thin walls. The hole in the top is for the wires and lets any heat out.

I didn't use a 608 skate bearing for the idler. They might be cheap and available world wide but I found they didn't work on my Prusa, whereas the 624 bearings used on the Sell's Mendel do work. Ball bearings have a chamfered edge, the bigger the bearing the bigger the chamfer and M8 washers are thicker than M4 washers. With 8mm bearings that leaves a gap big enough for the belt to ride down and bind, whereas with 4mm bearings the gap is much smaller so the belt simply brushes against the penny washer, rather than jamming.

I prefer a bearing to a printed pulley with flanges or a crown pulley because if I am using a metal drive pulley for accuracy it does not make sense to have a printed idler.

I haven't added it to the model yet, but there is a half twist in the long return path of the belt so that the smooth side goes over the idler, not the teeth, to avoid any cogging. The twist in the belt doesn't seem to cause any problems, if it did I could revert to the technique here: hydraraptor.blogspot.com/2011/06/half-belt-hack

The belt tensioning is as Greg Frost's design: The ends of the belt are locked in place by clamps with mating teeth. A screw tightens a Nyloc nut against a loop of the belt.

The carriage is the full size of the extruder with the bearings optimally placed in a triangle and the belt attached at the ends. It does mean the carriage is a bit bigger than most but it makes best use of the space to achieve stability. I.e. the travel is limited by the extruder, so there is no point making the carriage smaller, other than reducing print time.

The carriage follows the rod on the two bearing side and only needs to be prevented from rotating around it by the third bearing. In order not to be over constrained the third bearing is suspended by thin but tall struts. That allows it to float horizontally but it is constrained vertically. This prevents binding in the event of the rods being slightly miss-aligned.

The underside of the carriage is shelled and ribbed to save print time but keep it rigid. That has been my philosophy on the design, strength through complexity of shape rather than chunkiness. Whereas other people have tried to reduce the printed parts to a minimum I have tried to put functionally first.

I found that I could not make bearing clamps in the horizontal direction with enough grip so I use cable ties as well on these. The bearings rest at each, end so a single tie in the middle is sufficient to keep them stable.

Y-Axis
The Y axis sits on a flat sheet ensuring the bars lie in the same plane. Only three bearings are needed so the rod on one side can be shorter as it no longer needs to attach at the very front and back. The X-axis also uses three bearings and Z four, making the total ten, which is convenient as they tend to be sold in packs of ten. The belt is also shorter because the motor and idler can be brought inside the axis travel.

The Y motor bracket is a lot more rigid than the Prusa version due to its boxy shape and being screwed to the base instead of hung from bars. The bar clamps are also hollow boxes.

The bearing holders are the same as the ones on the carriage using tie wraps .

Alignment is easy, all the bar clamps and bearing clamps have slotted screw holes allowing a little side to side movement. Initially all the screws are left loose. The long bar is set at right angles to the gantry using a set square and then the bar clamp screws are tightened. The bearing clamps on that side are then tightened. The y-carriage can then be moved backwards and forwards to pull the second bar into alignment before those are tightened. On my todo list is to float the third bearing like I have done on the carriage.

Again the belt has a half twist in the lower return path, not shown on the model. Belt tensioning is easy because the idler has a slot to allow it to be adjusted. The single mounting hole also allows the angle to be adjusted to centre the belt. I plan to move it to the front and put the motor at the back as it makes the wiring shorter and the idler adjustment more accessible. I used two 624 bearings side by side to allow the belt to wander a bit without binding. I seemed to need that on Y but not X. I may move to two on the X-axis as well to give a completely frictionless arrangement.

If you are wondering what the two large holes in the base are, they are there so that dual shaft motors can be used.

Z-Axis
I moved the motors to the bottom to eliminate the possibility of the couplers slipping off. I made the couplers as skinny as possible to get the bar close to the lead screw. That makes the X ends smaller and allows the Z bar to rest on top of the motor giving a metal connection from the base to the top limit switch minimising the effect of the wood shrinking and expanding. For normal Reprap software it probably needs an adjustable bottom limit switch instead.

Note the axis is shortened in this picture.

The Z bars are automatically parallel to the gantry because the distance at the top and the bottom is set by printed parts. The bar clamps at each end of the rods are identical allowing the axis to be made vertical with a set square. This is done at the left hand side and the other side is made parallel by moving the axis up and down before tightening the screws.

I kept the facility for anti-backlash nuts and springs but the only machine I needed to fit them on was my Prusa. I am not sure why, but even the weight of an extra motor was not enough to overcome the backlash with gravity. I think it must have either been due to binding or perhaps the grease I used was thick enough to need some force to squeeze it out of the way. I needed stiff springs and I had to turn up the z-motor current after fitting them. The advantage of not fitting them is it gives some protection against a head crash as the maximum force you can apply downwards is the weight of the X-axis and extruder.

I considered using a single motor and linking the screws with bevel gears and a drive shaft. That would be cheaper than a second motor or a belt but I stuck with two motors for simplicity at the moment.

Bed
I have previously used 6mm aluminium tooling plate with aluminium clad power resistors for my heated beds. These work well but they are heavy. The Prusa PCB heater with a 2mm glass sheet on the top makes a much lighter solution. The picture above shows it clamped down with penny washers but bulldog paper clips work better.

I use 3mm Dibond for the Y-carriage because it is light, stiff and stable. I tried 6mm MDF on my Prusa but it warped due to the heat and the bed never stayed level for long. I don't know how other people manage to use it.

The best bed mounting solution I have tried so far is 20mm brass hex pillars. I tap the carriage holes M3 and screw the pillars into it. I can then level the bed by adjusting them and use the screw in the top to lock the position. I don't like to use springs because they let the bed wobble.

To level the bed I put M3 washers under the back two pillars and screw them tight and lock them. I then twist the Z motors by hand to make both sides level at the back relative to the nozzle. I then adjust the front two pillars to get the bed level front to back.

The process is easy but tedious because all the adjustments interact to some extent, so you have to keep going round them. It would be better if the bed had a single mounting hole at the front in the middle, as you only need one adjustment to get the bed level from front to back. I need to make a smaller version of my Z-probe so I can auto level the bed.

I like to use an air gap under the bed for insulation so that I can cool it rapidly with a fan at the end of the build to make the parts release easier. The air gap provides enough insulation but the Dibond below still gets to around 50°C. I added a heat shield made from corrugated cardboard covered in aluminium foil tape and the Dibond no longer gets warm at all.

The slots are to clear the screw heads. I stuck it down with double sided tape but that did not hold so I added bulldog clips. If I was making another I would bolt it down.

I haven't made any measurements yet but I think the difference in temperature between the middle and the edges is bigger than my aluminium beds. I Intend to try adding printed baffles at the front and the back to stop the movement of the bed pushing cold air under it.

I think I can improve the temperature distribution by changing the PCB pattern. The problem at the moment is that if the middle runs a bit hotter then the tracks local to it will have a higher resistance than those at the edges, which are connected in series with it. That means the middle will get more voltage and become even hotter relative to the edges, positive feedback. A better arrangement would be to have concentric rings of tracks running through areas that are likely to be the same temperature, wired in parallel. That way if the middle got hotter it would only have tracks near the middle in its circuit, so the increase in resistance would lower the current and give some negative feedback.

Another thing I would change would be to remove the silk screen from the top layer as it has some thickness that will reduce the thermal contact with the glass.

Cables
Larger CNC machines use cable chains to enforce a minimum bend radius on moving cables to stop them breaking. There have been several printable versions on Thingiverse but I feel they would give more friction than desirable for a small machine like this. Ribbon cables are very flexible in one direction and are surprisingly rated for 300V, 1.4A and 105°C.

For the heated bed I use ten wires in each direction plus 2 for the thermistor. I clamp it at both ends with a thin strip of polypropylene about 0.5mm thick. That forms the equivalent of a miniature cable chain but with very low friction. Here is the one under the bed: -

This one feeds the X motor and the extruder: -

The rest of the wiring is done on the back of the gantry with printed cable clips : -

The fan on the left is a powerful 80 CFM fan that I use to cool the bed from 110°C to 30°C in about 6 minutes.

The only down side of ribbon cable is that you get some inductive cross talk from the motor signals to the endstops. That doesn't affect my firmware as I only read the endstops during homing and a simple retry loop sorts that out. For firmwares that constantly monitor the endstops a simple RC filter on the inputs should fix it.

This version of the machine I call the Sturdy model. It uses 10mm rods, M4 fasteners and has a build area slightly bigger than a Mendel: 214 x 214 x 150mm. The next version I try will use 8mm rods, M3 fasteners and have an acrylic frame. I will reduce the build area to 200 x 200 x 140mm, same as Mendel so it will be more of an equivalent. I will also make a Huxley equivalent with NEMA14 motors and 6mm rods. The Mendel sized variant will cost a bit less but I doubt the Huxley will be any cheaper.

Mendel90 extruder

2011-12-29T22:26:00.000Z

The Mendel90 parametric design starts from the extruder dimensions and works outwards. I used a Wade's extruder for the Mendel sized version of the machine (I will need to sort out a smaller extruder for the Huxley sized version). My starting point was the Prusa version of Wade's. I tidied it up a bit aesthetically and made a few tweaks to the design and that had the side effect of making it easier for Skeinforge to slice correctly. The old version caused it to think layers were bridges erroneously. It now looks like this: -

The functional things I tweaked were: -

I added nut traps for captive hex head bolts. That allows me to fasten it under the carriage with a couple of wing nuts, so I can swap extruders very easily.
I brought the front of the bearing holder forwards 2mm. That stops the idler closing fully, which makes it easier to feed in new filament and allows the hobbed bolt to be removed without having to remove the idler. The downside is it would be less tolerant of smaller diameter hobbed bolts.
I made the idler bolt holes slightly further apart so that I could make them larger without intruding into the bearing holders.
I added a slot around the top of the hole for the insulator. When it was simply a blind hole it had radiused corners at the end due to the fact that the filament has a minimum bend radius. That meant that, unless the insulator was chamfered, it did not go all the way to the end of the hole.

I use hobbed bolts and 10mm hot ends from reprap-fab.org. Wolfgang makes the bolts so that the big gear can be spaced off from the bearing with 5 washers. That allows the small gear to be placed the right way round, allowing the big gear to be removed easily. M8 washers can vary in thickness so I made a printed spacer 7.5mm long to replace them.

I don't use Greg's accessible version of the extruder because I never remove the idler. Once I have got the spring tension correct I don't like to change it. If I need to clean the hobbed bolt I simply reverse out the filament, remove the nut and then remove the big gear and hobbed bolt. It only needs cleaning if there has been a malfunction due to a filament tangle or a nozzle blockage.

To make the nut easy to remove, rather than use lock nuts or a Nyloc, I use a single nut and a weak spring. The spring stops the nut vibrating loose and gives enough pressure to keep the bolt in the correct position but it can be removed without using a spanner.

The extruder is the only part of the machine that wears out, so I have made it easy to swap out by adding a 9 way D type connector. D connectors screw together and have good strain relief for the cable, so they are reliable when subjected to constant movement. They are also rated for 5A per pin and 125°C, which is a good margin for this application.

I attach the connector with a bracket that is screwed to the motor by removing two of the motor's screws and replacing them with screws that are 5mm longer.

I have several extruders with difference nozzle sizes that I can change very quickly.

Mendel90

2011-12-25T23:27:00.000Z

I never understood why Mendel has a triangular prism frame. The way I see it, the frame only has two functions: - To hold the Y bars in a flat plane and to support the tops of the Z bars. It isn't good at doing either:

The main forces on the Z bars are in the direction of the X-axis and the frame has no strength in that direction. It wobbles when the X-carriage changes direction.
It also doesn't ensure the Y bars are in a flat plane because there is nothing to ensure one end triangle is not rotated slightly relative to the other.

After a trip down a cobbled street in Sheffield my Mendel behaves as if one corner of the bed is lower than the other three. This is impossible because it has a flat sheet of glass on it, but it isn't obvious what needs to be adjusted to fix it but it must be the ends of the Y -bars. The bed needs to be level to within about 0.05mm for good results printing 0.3mm layers without a raft. That is difficult to achieve when the Y axis is strung from bars at opposite sides of the machine.

Other problems are: -

It gets smaller at the top, so the maximum Z travel is limited by the extruder colliding with the bars.
The sizes of the Z axis and the Y axis are tied together, so you can't change one without the other.
It is difficult to adjust the axes so that they are orthogonal to each other and keep them that way if the machine is moved.

This machine is my attempt answer to these problems. I am calling it Mendel90 as I can't think of a better name at the moment. The 90 is to emphasise that the frame is based on right angles rather than 60 degree triangles.

Two flat sheets are mounted at right angles to form the XY and XZ planes. Two buttresses maintain them at right angles to each other. This relies on the sheets being cut at perfect right angles but in the UK you can buy sheet materials such as MDF or acrylic cut to size and they have good right angles. The only cutting I had to do was to cut the arch out with a jig saw. It doesn't need to be accurate and it could be done with a hand saw. The piece removed could be used to make the Y carriage, depending on the material.

The buttresses are bigger than they need to be. I took them all the way back to give me plenty of room to mount my non-standard electronics, but it also has the advantage that the machine will sit on five of the six faces, making it easy to work on.

If the anti-backlash springs are fitted to the Z-axis it should print in all those orientations as well, which would be interesting to try. When printing directly on glass, parts come loose when the bed cools. If the machine was on its back they would fall out the bottom. Who needs an ABP? It might also solve the PLA ooze during warm up problem.

The gantry could be unscrewed and laid on its back over the top of the Y axis to make the machine more compact for travelling. In this case the buttresses could be slimmer to allow it to become even more compact.

I used B&Q style fixing blocks to fasten the sheets together.

I bought some of these and I printed some. They are a lot faster to print than Mendel frame vertexes! The economics are interesting: they are cheaper to print than buy, but while my machines are fully occupied making parts to sell, it is more economical for me to buy them. The printed ones are actually more accurate than the injection moulded ones! The holes are all over the place. I think they must be formed by removable cores and the tool must be worn allowing them to move.

I drilled pilot holes using a paper template. I did this by exporting DXF files of the sheets from OpenScad. I then hacked together a Python DXF reader and an SVG writer to make a program that generated drill centres. I printed them on a large plotter but it could be done with A4 sheets tiled together like the Darwin bed template.

The design is modelled in Openscad, down to the nut and bolt level, and is fully parametric so you can make any size machine and scale the rod diameters and motor sizes if necessary. The only limits are that eventually belts would need to be replaced by rack and pinion above a certain length. It also automatically generates a complete bill of materials for anything in the model.

See also: mendel90-extruder and mendel90-axes

Merry Christmas!

Half belt hack

2011-06-26T17:52:00.001+01:00

I found that I didn't have enough belt to complete the x-axis of my Prusa, but I did have a couple of offcuts about half the required length. Since less than half the belt actually passes over the motor pulley I simply joined them in the middle. My first idea was to print a two part clamp. Another idea was to use heat shrink sleeving, but in the end I simply tied them with some wire.

I joined them back to back so that the teeth mesh, keying them together. This has the beneficial side effect that the smooth part of the belt goes round the smooth idler pulley.

It might actually be worth doing this to get smoother running, even if you do have a belt long enough. Also if you are on a tight budget the second half does not need to be toothed belt at all. It could be packaging strapping or steel wire, etc.

Yet another Prusa Z-coupling

2011-06-25T21:44:00.001+01:00

I finally got around to building the Holiday Prusa Mendel I printed over Christmas. I had a few problems with some of the comedy parts and had to revert to using some of the more up to date ones that I sell.

I didn't find the Z couplings worked very well. The requirements are to couple the M8 threaded rod to the 5mm motor shaft exactly coaxially and with no vertical play, but with some angular flexibility to cater for slightly bent threaded rods or any slight angular misalignment.

The rods are not held very coaxially because the clamp is not symmetrical. The alignment depends on how much the two independent clamps are squeezed, which depends on the exact diameter of the shafts relative to the printed diameter of the part.

They are not very flexible either because they have to be strong enough to support half the weight of the X-axis and the extruder. The direction of pull is in the weak direction of the part that tends to de-laminate it, consequently I print them 100% fill to make them strong enough. I would imagine that if there is any wobble in them the constant flexing would eventually fatigue the part and cause it to break.

I looked around at the various attempts to improve these, but I wasn't happy that any satisfied all the requirements above. I did find two sources of inspiration though:

This one by keegi uses a piece of tubing to provide the angular flexibility and it also helps to grip the smooth motor shaft.

This one by Griffin_Nicoll has the strong direction of the part in the right direction, but suffers the same problem as the original because it has two independent clamps. That is easily solved by removing the split in the top section, but then it would be difficult to grip the smooth motor shaft without the clamp halves being exactly parallel, which would depend on the exact shaft and part sizes. It also has no obvious flexibility. Putting the tubing on the motor shaft solves both these problems.

I hacked Griffin's script to make this version: -

I removed the split, changed the holes and the nut traps to fit M3 and changed the motor shaft diameter to 7mm, which is for a 5mm shaft with tubing on it.

Here it is mounted: -

Both halves are identical inside so not matter what the shaft size is they will always centre and align the shafts automatically. The sleeving allows the shaft to flex angularly and also makes a very firm grip on the motor shaft. The part bears weight along its strong direction and is not required to flex at all, so should last forever. Another possible benefit is if the part is made from PLA it is somewhat insulated from the motor shaft by the tubing, so there is less chance it will melt.

I haven't run the axis yet, but it turns very easily manually and there is no wobble at all. I will include these in my kits from now on and I will include the short piece of tubing as it would be annoying to have to buy just 30mm. Note it does require four extra M3x20 bolts, nuts and associated washers.

The files are here on Thingiverse.

FR4 fail

2011-06-14T23:02:00.002+01:00

Well it seems that FR4 only lasts for about a week. The grip slowly fades making the parts very easy to remove. In fact they all pop off as the bed cools below 60°C and slide about due to the fan and the bed's final movement to the front. The odd small part falls down inside the machine.

If I mounted my machine so it was inclined at 45° they would all fall out the front and could be directed by a chute into a hopper and the machine could then build continuously unattended. Who needs a conveyor belt! The only problem is the grip is now not enough to hold the bigger parts during the build.

I have tried cleaning with acetone but it doesn't seem to help. I suspect the high temperature is making the epoxy more brittle and less sticky. I will be able to prove that when the FR4 without copper on HydraRaptor fails. If I then turn it upside down and it still works on the under side then it is not a temperature ageing effect. If the other side is still working then it must be a reaction to the ABS or the acetone that is the problem.

It is shame because I much prefer a solid substrate to tape. Something like polyimide and fibreglass laminate would probably be ideal but it is hundreds of dollars for a piece big enough.

Wolfgang has posted a mystery material to me that sounds promising, so back to PET tape until it arrives. My friend Tony found that Farnell sells it in wider rolls. It seems to be a bit thicker as well, so is easier to apply, but a lot more expensive than the stuff from BestOfferBuy.com.

ABS on FR4

2011-06-10T22:06:00.160+01:00

I have been printing both ABS and PLA on PET tape for more than a year now. It works well and lasts for many months, but eventually the silicone adhesive fails and it blisters. Applying it is fiddly to avoid any overlap but also not leave gaps between the adjacent runs of tape. I have been on the lookout for a solid material to avoid these pitfalls.

Stoffel15 (Wolfgang) told me that FR4 fibreglass PCB material works well. FR4 is the most common PCB material and is a glass fibre and epoxy resin laminate. It will handle solder re-flow temperatures (~ 240°C) for short durations and can be used continuously at 140°C. As I haven't worked on single sided PCBs for many years, I had forgotten what the surface of the raw material looks like. It is actually smooth and glassy, so ideal as a bed material.

I ordered some single sided PCB material from Farnell. It works fantastically well. It seems to have a bit more grip than PET and has the advantage that there are no lines on the part from the joins in the tape. It also has no give in it, so I don't get any blistering at sharp corners like I did with tape, sometimes leaving shallow dimples.

Another advantage is that when the object cools it tends to break free because it contracts more than the bed does. With tape there is some compliance, so it usually stays stuck when the object cools and it is often hard to remove parts. With FR4, if you get the layer height spot on, the parts break free of their own accord, and if not, are very easy to snap off. This vertex bracket was loose after the bed cooled to 50°C.

Yet another advantage is that I stick the tape to a steel plate 0.9mm thick that weighs 280g. The FR4 is 1.6mm thick but it only weighs 134g, so less than half the mass.

I also tried some plain FR4 without copper and that seems to work just as well. It is 0.9mm thick and weights only 75g. The disadvantage is it is bright yellow, which makes it hard to see the white plastic on it.

I have printed a full set of Mendel parts so far on FR4 and every part has come out perfectly flat, and was easy to remove.

I don't know if it will degrade over time, but there is no sign of surface damage so far. The dark features on the picture above are marks on the aluminium plate underneath.

The nice thing about the z - probe I have on HydraRaptor is that I can change the bed without any calibration.

This is what the underside of an object looks like.

I used the same temperature I used for PET tape, which is 140°C for the first layer and 110°C after that.

I haven't tried PLA yet, but my guess is it will stick because it seems to stick to a superset of things ABS sticks to.

Great tip Wolfgang!

In the past I tried FR2 (SRBP, Paxolin) but that did not work, probably because it had a matt surface. I also tried some CAT7FR, which is another type fibreglass PCB material, but again it had a matt surface and did not work very well. I was able to build a flat object on it, but the first layer outline did not stick properly, so some holes were a bit scrappy.

The copper on the bottom of the single sided material could be used as a heater like the Prusajr heated bed design.

Reliable connections

2011-06-09T21:44:00.000+01:00

After eliminating lots of other sources of unreliability in my machines, electrical connection failures are now the most common failure mode.

The latest failure on my Mendel was that it started leaving a 10mm gap in the outline rectangle that it draws around the objects. Since a bed full of objects still seemed to build OK I decided perhaps it was due to an air bubble in the extruder while it was warming up. However, one time I saw the extruder motor stall and realised it was actually a bad connection.

I have come to realise that simple friction fit connectors do not work in the environment of these machines. I tried re-seating the motor plug but that did not fix it, so I figured the cable must be faulty. I wired both coils in series to my multimeter and waggled the cable until it went open circuit. That allowed me to locate the break and it was, as could be expected, at the point where the cable bends the most, i.e. just below the cable clamp on the top right of this picture: -

On reflection this was not a good arrangement as the cable is only just long enough for the extremes of travel, so it is forced to bend sharply both ways at the clamp. After millions of movements the strands break one by one but the insulation holds the ends together making it only lose contact when it is stretched. When I pulled the ends of the wires three of them snapped very easily, indicating most if not all the strands were broken.

I had a similar problem with the mains wire to a heated bed a while ago. In that case the arcing melted the insulation and allowed the live and neutral to short out, blowing a hole in outer sheath of the cable. Not good! Normally you expect a fuse to protect against a cable fire, but if all the strands start breaking, reducing the current capability, or it breaks and arcs, the fuse offers no protection against fire. Even a low voltage heated bed could fail in this way because of the high current.

The XY table of HydraRaptor uses 9 way D-type connectors. These have been totally reliable moving connections because they are screwed together and have gold plated pins and proper strain relief. The professional stepper motor drives on HydraRaptor have screw terminal blocks for their connections, and again they have proved totally reliable. In contrast all the friction fit connectors fail if there is any movement or vibration of the wire. Some even burn out despite being run at well below their current rating. The contact resistance rises and they then start to heat up.

I rewired my Mendel extruder using a 9 way D-type at the extruder and a longer loop of cable. That necessitated resiting the extruder controller and I also replaced all of its 0.1" MTA connectors with screw terminal blocks. The wires could go straight into these but I added ferrules to allow them to be more easily removed and replaced. I just push the wire into the ferrule and then squeeze it with pliers.

I reprapped a bracket to attach the DB9 connector to the back of Wade's extruder bracket.

The pins are four motor connections, two heater, two thermistor and one heatsink fan that shares a 12V feed with the heater.

Here is the new arrangement :-

The cable loop is much longer, so it bends through a much smaller angle. The top end goes through two cable clamps before it goes to the extruder controller. I found that if you put a bunch of wires though a single clamp you can get some movement at the other side of the clamp. Using two eliminates any movement of the wire relative to the board, less critical now I that have screw terminals, but still a good idea.

It should last a lot longer than the previous cable (which lasted for 15 months of continuous use) and can be easily replaced. I have seen people use corrugated tubing to protect the cable, but I didn't fancy adding any more drag on the extruder as it would increase backlash.

Interestingly, although my extruder stepper motor connections have failed several times, I have never damaged the Allegro driver chips.

Auto bed leveling

2011-04-25T23:46:00.002+01:00

One thing I find tedious is leveling the bed of my machines, so I decided to make use of the Z-probe on HydraRaptor to measure the incline of the bed and compensate for it in software.

First I had to increase the resolution of my Z axis. When I first built the machine I did not realise that I would need fine resolution on Z, so I used an old 24V unipolar motor that I had in my junk collection. With half stepping it gave me 0.05mm resolution. I thought that it was a 200 step motor and that the lead screw had a pitch of 20mm. It turns out it must have been a 250 step motor because the pitch is actually 25mm. I replaced it with a Keling KL23H251-24-8B NEMA23 left over from my Darwin and I now drive it with a x10 microstepping controller the same as I use on X and Y. That gives me a resolution of 0.0125mm and also makes it the fastest axis on my machine. It can easily do 150 mm/s but seeing the nozzle approaching the bed at that speed and then stopping within 0.3mm is very unnerving, so I limit the speed to 50mm/s!

I no longer need the heat sink and fan because the new motor is more efficient and is directly mounted on the axis, which takes the heat away.

I use 6mm aluminium tooling plate so I make the assumption that the bed is a flat plane (rotated slightly around the X and Y axes and offset a little in Z). That means I only need to measure the height at three arbitrary points in order to characterise that plane. I then use the method here to calculate the equation of the plane in the form ax + by +cz +d = 0. The method puts two vectors through the three points and takes the cross product to get a vector at right angles to both of them. That is the normal to the plane and its components are the coefficients a, b and c. Substituting the first point into the equation gives d.

It is important that the three points are ordered anti-clockwise, otherwise the normal vector would point downwards and the machine would try to build the object upside down under the surface of the bed!

Given the bed's plane I then have to make the model coordinates relative to that inclined plane and transform them to the coordinate system of the machine. To do that I calculate two orthonormal basis vectors on the plane using it's equation and use the normalised normal vector for the third. I then multiply the model coordinates by those vectors and add the origin to find where they are in the machine's coordinates. Here is the Python code I used: -

class Plane:
    "A plane in 3D."

    def __init__(self, p0, p1, p2):
        "Construct from three anti-clockwise points"
        #
        # Calcluate the normal vector
        #
        v1 = p1.minus(p0)
        v2 = p2.minus(p0)
        normal = v1.cross(v2)
        if normal.z < 0:
            raise Exception, "Probe points must be anti-clockwise"
        #
        # Coefficients of the plane equation ax + by + cz + d = 0
        #
        a = normal.x
        b = normal.y
        c = normal.z
        d = -a * p0.x -b * p0.y -c * p0.z
        #
        # Generate three basis vectors aligned with the plane
        #
        self.origin = vector( 0, 0, -d / c)

        self.k = normal                      # k axis is simply the normalised normal
        self.k.normalize()

        px = vector( 1.0, 0.0, -(a + d) / c) # an arbitrary point on the x axis: x = 1, y = 0
        self.i = px.minus(self.origin)       # find direction to it from origin
        self.i.normalize()                   # make a unit vector

        self.j = self.k.cross(self.i)        # make a third vector mutually at right angles to the other two
        self.j.normalize()                   # make a unit vector, probably is already

    def transform(self, p):
        "Transform a point to be relative to the plane"
        return self.origin.plus(self.i.times(p.x)).plus(self.j.times(p.y)).plus(self.k.times(p.z))

To test the principal I put a 1mm thick washer under one corner support of the bed to give it an extreme slant compared to normal misalignment. I then built a 100 x 100 x 5mm cube with 0.35mm layers. This would normally be impossible without the bed being level to a small proportion of the layer height. The result was that it came out fine.

As the nozzle traverses the object in XY the Z axis moves a few microsteps. It is barely visible but I can hear it and feel it if I hold the stepper motor shaft. The object is built perpendicular to the plane of the bed, so the sides are very slightly slanted with respect to the machine axis and the nozzle. I am not sure how well it would work on Mendel as the z-axis is geared down so much. It would probably still work as the movement required is so small when the bed is reasonably level. I can't test it as there isn't room for a z-probe on my carriage due to the large heat sink.

Auto z-probe

2011-04-04T13:38:00.004+01:00

A niggling problem I have with Hydraraptor is that the z-axis calibration varies with the weather and how much it is used. This is because the frame is made from wood, which absorbs atmospheric moisture and expands. When the machine is running constantly the heat from the bed dries it out and it plateaus at a low z-value. If I don't use it for a while the z-axis gets higher by as much as 0.5mm in wet weather and the first few builds need large adjustments. When printing raft-less the initial layer height needs to be accurate to about 0.05mm for 0.3mm layers.

When it was configured as a milling machine I made a tool height sensor to solve the problem. It doesn't work for FFF though because the nozzle usually has hot plastic dribbling from it and it also wastes some of the build area.

To solve the problem I designed a z-probe that hangs below the nozzle at the start of the build but then retracts itself after the measurement. It consists of a weighted metal rod that slides through a couple of plastic guides. It has a plastic flange on the top that depresses the plunger of a light action micro switch. In measurement mode the rod protrudes about 10mm below the nozzle. When the measurement is completed the axis descends to place the nozzle close to the bed. The rod lifts until the attractive force of two Neodymium magnets causes it to be pulled about 5mm above the nozzle and held there until the start of the next build.

Here it is installed on the axis.

I used a Meccano worm gear as an improvised weight to ensure the micro switch is activated, much cheaper options exist! The actual weight is surprisingly not very critical. It must be enough to activate the switch reliably but not too heavy for the magnets to lift.

The operating procedure is as follows: The machine warms up the bed and the extruder and waits for a couple of minutes for the nylon pillars that support the bed to expand fully. It then extrudes a length of filament with the z-axis at the top and gives an audio prompt on my computer. I grab the filament and snap it off and then lower the z-probe, which closes the switch and instructs the machine to start.

The axis descends rapidly to place the rod 1mm above the centre of the bed. It then descends in 0.1mm steps until the switch opens. Then it ascends in 0.01mm steps until the switch closes again and that gives the Z calibration point, which is a known distance (about 10mm) above the bed. The nozzle then descends to 1mm above the bed to retract the probe before it moves to the start point.

Here is a video of the sequence.

It could also lower the probe automatically simply by having a bracket near the top of the z-axis to catch the flange as the axis rises past it. The reason I don't do that at the moment is because I use the act of manually lowering the probe as a cue to the machine that I have removed the start extrusion.

The design is on Thingiverse.

Spot on flow rate

2011-03-13T14:53:00.001Z

I have been doing some fine tuning of flow rate recently. I had previously noticed that PLA appears to need a slightly lower flow flow rate than ABS. I didn't notice this with HydraRaptor but I did when I changed from PLA to ABS on my Mendel, which has a Wade's extruder. My theory was that PLA feeds faster than ABS for the same rotational speed of the pinch wheel because, being much harder, it sits on the crests of the teeth and hence is driven by a larger effective pinch wheel diameter than ABS, which sinks in further. This effect is more extreme with a smaller pinch wheel. HydraRaptor has a 13mm pinch wheel compared to just 5mm for the hobbed bolt in my Wade's.

Other people have claimed that ABS changes density when it is extruded. I didn't believe that so I did an experiment to investigate.

I programmed HydraRaptor to extrude 100mm of ABS. I put a mark on the feedstock about 120mm away from the top of the extruder and measured how far the mark moved. I also measured the length and diameter of the extruded filament and I also weighed it and a 100mm sample of the feedstock. These are the results: -

Filament input to the extruder: 105mm of 2.98mm ABS equals 732mm³, weighs 0.777g, density 1.06 g/cm³.
Filament extruded: 2.89m at 0.56mm diameter equals 712mm³, weighed 0.764g, density 1.07 g/cm³.

So on the face of it the volume has gone down by 3% and the weight by 2% giving a slight increase in density. This could be explained by some volatile compounds boiling off, which they do, but I think it is mainly measurement error. In particular the diameter measurements have a big effect because of the square law for area. I took four measurements and averaged them but that is not many along 3m of extruded filament. Also the electronic scale I used to weigh the filament does not have a very stable display as it is only a cheap instrument. It is certainly a lot less than the 15% I have seen reported though.

I also extruded "100mm" of PLA and that actually fed 110mm, showing that with a 13mm pinch wheel it feeds about 5% faster. With a 5mm hobbed bolt I would expect that to be about 12%, which starts to become very noticeable.

So I corrected the pinch wheel diameter in my software for the correct value for ABS and added a bodge factor for PLA. That left the flow rate a bit too low as it has previously been producing good looking objects with the overfeed, so I reviewed the maths I was using.

I have always extruded filament with a 1.5:1 width over height ratio and use a flow rate that would fill a circle 1.25 times the layer height. That was because I originally observed that you need to squash the filament to 0.8 times its diameter to get a good bond and that makes the width about 1.5 times the height. However, that only gives a packing density of 82%, which is a bit low. If you increase the flow rate so the infill is 100% then the outlines will be too wide. This is because the infill can occupy the full rectangular cross section of the filament road, but the outline, being unconstrained, will not have straight sides, so will be wider.

I reasoned that the outline will be extruded with a flat top and bottom where it is constrained between the nozzle and the bed but the sides will most likely be semicircular due to surface tension effects. This led me to a formula that gives the width from the notional extrudate diameter and the layer height.

Equating the two areas gives pd²⁄ 4 = ph²⁄ 4 + h (w - h). So w = h + p(d²⁄ h - h) ⁄ 4 allowing the width to be predicted from the layer height and the flow rate.

Calling the aspect ratio a = w ⁄ h and re-arranging to get the flow rate to make the desired width gives: d = h√(1+ 4(a - 1) ⁄ p). For an aspect ratio of 1.5 d = 1.28h. I had previously been using 1.25h which is about 5% too low but was compensated for by the pinch wheel overfeed. I made a single walled box with the corrected pinch wheel diameter and the new formula and verified that the walls were 1.5 times the layer height.

I also used the same flow rate for the infill, but that can be increased up to the full area of the rectangle w×h. Because the outline and infill use different flow rates there is a small deficit of plastic where they meet, as this model shows: -

This can be fixed by using the infill perimeter overlap ratio setting in Skienforge, but how much? The deficit in area is a rectangle h ⁄ 2 × h minus a semicircle of diameter h, i.e. h²⁄ 2- ph²⁄ 8. If the infill overlaps by a distance x then it contributes an area x × h. Equating these gives x = h (0.5 -π/8).

Converting to a ratio of w gives x/w = (0.5 -π / 8) / a. For a = 1.5 that gives an overlap of 0.07 leading to a "fully stuffed" model where the solid layers are 100% plastic.

In practice that leaves no room for error and requires the nozzle to force the plastic into the corners of the rectangular channels like an injection molding machine. I found I get a better looking object with the volume reduced to 90% of that value. So for the infill I use the formula d = h√(0.9 × 4a ⁄ p) giving d = 1.31h for a = 1.5, making the optimum flow rate for the infill about 5% more than the outline. I also use an overlap value of 0.05 giving the theoretical packing arrangement below.

Running the new equations on my Mendel certainly produces nice looking objects:

At least four people I have sold parts to have commented they look as good or better than parts they have seen from a commercial machine. I use filament about twice the diameter that commercial machines use, which results in more visible layers and rounded corners, etc, but apart from that I must be close now.

Polyholes

2011-02-05T15:25:00.001Z

When Reprap machines print holes they tend to come out undersized, even if the linear dimensions of an object are spot on. There are several effects that all make holes smaller than they should be: -

Faceting error
When CAD systems convert cylinders to triangles they produce a polygonal prism, so holes represented in an STL file are polygons with their vertices on the circumference of the original circle. That means the sides of the polygon are inside the circle, shrinking it by cos(π / n).

You need 10 vertices to reduce the error to 5% and 22 for 1%. So this error quickly becomes small as n increases but that creates another error:-

Segment pausing
When a circle is broken into a lot of little segments the start up time for a segment becomes significant. Reprap in the past has suffered from this really badly and I am unsure what the current status is. Slow serial comms and complex floating point firmware add pauses where extra filament can ooze from the nozzle.

I have never suffered from pausing because I use a 100Mbit Ethernet connection, which has a very low latency, and the data is transmitted in binary and in the units my firmware works in. This means that no further processing is required other than calculating which of the three axes has to go the furthest. However, I use trapezoidal acceleration on each segment, so for very short segments the average speed will be a little lower.

Arc shrinkage
When a flat strip of filament is bent into an arc there is too much plastic on the inside of the curve and too little on the outside. That makes both the inside and outside edges a smaller diameter than they should be. Adrian calculated a formula for it here: http://reprap.org/wiki/ArcCompensation. The formula comes out with a figure that is too small though. I think there is a secondary effect:

Corner cutting
When filament is dragged round a corner it likes to take a short-cut. This depends on how elastic the filament is and how much it is being stretched. I think when the nozzle moves in a circle the filament is continually trying to cut the corner and ends up forming a smaller diameter circle. I think this is the dominant effect on my machines.

Obviously, if you lie to Skeinforge about how wide your filament is that will make holes even smaller, but that is just a calibration problem.

Ideally all these effects should be compensated for in the slicing software but what has happened instead recently is that people are using parametric values in OpenScad to tweak the holes to come out right on their machines. That is the wrong approach because when the holes comes out smaller than they should be, without the slicing software compensating for it, then the infill doesn't meet it as tightly as it should do.

When I started printing Prusa Mendel parts I found the values in the configuration file far too big. I have also noticed this when downloading some designs from Thingiverse. That implies that my holes shrink less than a lot of other peoples, which is odd because all the effects above don't depend on the machine, apart from segment pausing.

Some of the holes in Josef's parts are octagonal. That made me realise that polygons with low vertex counts don't shrink. The inside of the hole is defined by straight lines and they get extruded in the correct place. What does happen though is that the corners of the polygon are rounded. As long as the polygon has a small number of vertices, the corners are far enough from the circle that they can be rounded without impinging on it. The ideal number of vertices is when the corner cutting just meets the circle.

I decided to investigate this using OpenScad. I made a script that generates holes from 1 to 10mm with vertex counts from 3 to 8, 10, 16 and 32. The diameter of the holes is increased to make the polygon edges tangential to the circular hole. I.e. removing the faceting error by dividing by cos(π / n).

difference() {
    cube(size = [95,125,3]);
    for(i = [1:9]) {
        assign(v=[3,4,5,6,7,8,10,16,32][i - 1]) {
     assign(shrink =  cos (180 / v)) {
                echo(v,shrink);
                for(d = [1:9]) {
                    translate([d * d + 5 - ((v == 3) ? 3 : 0), 13 * i, 0]) 
                         cylinder(h= 20, r = (d/2)/shrink, $fn= v);
                }
            }
        }
    }
}

I printed the resulting shape on HydraRaptor and used drill shanks to gauge the hole sizes. Not terribly accurate as the shanks tend to be a little smaller than the tip. I inserted the drills in the highest vertex count hole that it would fit in.

A pattern emerged that the seemed to indicate the maximum number of vertices you can have before the hole shrinks is twice the hole size in mm. The only drill I couldn't fit was the 1mm drill because you can't have a polygon with only two sides. The "1mm" triangular hole did at least leave a hole though, whereas higher polygon counts fill in completely.

To test this simple rule I made a new shape with holes from 1mm to 10.5mm in 0.5mm steps with the number of vertices set to twice the diameter and the diameter increased by cos(π / n).

module polyhole(h, d) {
    n = max(round(2 * d),3);
    rotate([0,0,180])
        cylinder(h = h, r = (d / 2) / cos (180 / n), $fn = n);
}


difference() {
 cube(size = [100,27,3]);
    union() {
     for(i = [1:10]) {
            translate([(i * i + i)/2 + 3 * i , 8,-1])
                polyhole(h = 5, d = i);
                
            assign(d = i + 0.5)
                translate([(d * d + d)/2 + 3 * d, 19,-1])
                    polyhole(h = 5, d = d);
     }
    }
}

I found that all my drills bigger than 1mm fit. The large ones are a snug fit and the smaller ones a little loose, probably because with only a few tangential points touching there is little friction.

These two tests where done on HydraRaptor extruding 0.375mm filament from a 0.4mm nozzle. I printed this the test again on my Mendel with 0.6mm filament through a 0.5mm nozzle and the drills still fit, so it seems universal, at least amongst my machines. It would be interesting to see if others get the same result, so I have put the files on Thingiverse.

My goal is to work out how to print circular holes the correct size, but this seems like a good hack for OpenScad designs to allow holes to come out the right size, regardless of the printer or whether it compensates hole diameters. For example, one would expect circular holes to come out right on a professional printer, so if you have oversized circular holes in your model they will come out too big. However, if you use these low vertex count polygonal holes they should still come out the right size as one would also expect a professional printer to print polygons at least as accurately.

ABS on PETG

2011-01-10T23:54:00.008Z

I have been using PET tape on my heated bed for a long time now. It works very well as long as I clean it with acetone about every 100 hours. It does need a high temperature (145°C) for the first layer with some types of ABS though .

It seems to last forever, the only failure mode is that large thick objects with sharp corners can defeat the adhesive and raise blisters at the corners near the edge of the bed. I solve that by building little heat shields to keep the corners warm. I am always on the lookout for something better though. It would be nice to get rid of the lines where the tape butts against itself.

A friend gave me a sheet of 1mm thick PETG to try. I clipped it onto my heated bed, and thinking it would behave like PET tape, I ran a build using the same temperatures.

Big mistake, PET has a glass transition at 75°C so it went soft and floppy. The object stuck to it very well and was hard to remove, but after getting a knife under one corner, it peeled cleanly. However it left an impression in the PETG.

The base of the object is flat but the filaments are more ridged because they sank into the sheet rather than being squashed.

When the sheet cooled down it warped badly, so that was the end of that experiment. I did have a small offcut though so I tried again at 70°C.

This time the object warped badly. It stayed stuck to the PETG but it warped the sheet. The adhesion was less and the object was easily peel-able. The PETG warped where the object was but the rest of it stayed flat. The heat of the object must have been enough to tip it over its glass transition locally. It left an impression, but not as deep as the first time.

The filaments on the bottom were squashed tighter, not as smooth as when using tape.

So a failed experiment. It is a shame because at high temperatures it bonds very well but, unlike PC, it still peels, but it is no good if it doesn't remain rigid. Wikipedia does say that PETG has a lower melting point than PET. It doesn't mention how it affects Tg, but it gives the Tg of PET as 75°C. Odd then that PET tape doesn't go soft at 75°C. My next trial will be Mylar, which is another form of PET (BoPET).

Frequency limit

2010-12-31T00:11:00.003Z

I currently do my infill on Mendel at 36mm/s. The machine can go faster but the extruder flow rate maxes out at about 40mm/s when extruding ABS at 0.6mm, so 36 is a good safety margin for reliability and quality.

Although the speed is limited there is no real limit on how fast it can change direction. Suppose you make something 2.4mm wide with 0.5mm filament. E.g. a Mendel spring: -

Each wall will be 0.6mm wide leaving a 1.2mm gap in the middle. That gets filled with a zigzag infill where the head moves to within 0.3mm of each wall, so the head moves about 0.6mm on each stroke. At 36mm/s that makes 30 complete oscillations every second. 30Hz is a pretty high frequency for a mechanical system!

What actually happens is my y-axis starts to resonate. Over a few cycles the amplitude of the oscillation builds up and the infill overshoots the outline leaving a serrated edge.

The torque of a stepper motor is zero at rest and increases as it is displaced, so in that respect it behaves like a spring. That springiness together with the inertia of the rotor gives a resonance at hundreds of Hertz, known as mid band resonance. When the load is rigidly coupled, as in this case, the mass of the load brings the resonant frequency down.

As I don't get any missed steps I think the springiness might actually be in the belt rather than the motor. Timing belts have metal cables in them so that they don't stretch, but that makes them stiff, so they don't like to bend round a tight radius. That means the belt has some springiness being pulled round the pulley. A bigger pulley would be better but that would reduce the effective stiffness of the motor, so might actually make things worse. A lighter bed would be good but I haven't found a way to ensure it is flat without going to 6mm tooling plate.

I fixed the problem in software by slowing down the infill that has a high frequency content. I examine each infill path, one axis at a time, and convert it into a list of lengths between changes in direction. I then find the shortest wavelength over three cycles (less than three cycles is not long enough for the resonance to build up). I do this for X and Y directions and save the shortest of the two wavelengths. When I extrude the path I work out the frequency from the pre-calculated wavelength and the desired speed. I then compare that with a limit for each machine and reduce the speed if the frequency limit would be exceeded. I could have a separate frequency limit for each axis but I don't like the idea that the orientation of an object affects how it builds, so I pick the worst axis when deciding the limit.

I set the frequency limit to 20 Hz on my Mendel and 16 Hz on HydraRaptor. HydraRaptor does not show the overshoot problem, but it makes horrible growling noises and shakes the house. The machines make more interesting noises now because each infill run that hits the limit is extruded at an arbitrary lower speed. The overshoot is completely cured.

The builds are a bit slower and in some cases a long infill path will be slowed down by a short section that is high frequency, often a section between a hole and the outline. A more complicated solution would be to isolate the high frequency section and extrude the rest of the path at full speed.

Tip top top layer tip

2010-12-29T14:46:00.002Z

When I first started printing on my Mendel I found it difficult to get the top layer infill solid and meeting the edges. It behaved differently to HydraRaptor, but since it was a different bot and extruder and I had also changed to a different type of ABS and updated Skeinforge it was hard to work out what the problem was.

The first problem I identified was backlash caused by the filament dragging on the carriage. I fixed that by switching from basket feed to spool feed, see hydraraptor.blogspot.com/2010/07/bit-of-drag.html. That made a big improvement but I also set the "Infill Perimeter Overlap" ratio to its default value of 0.15, where previously I had used 0, and also increased the amount of plastic above the theoretical 100% value.

That is the way it stayed until very recently when I made a discovery about Skeinforge. A new parameter had appeared when I updated: "Infill Interior Density over Exterior Density" ratio, which defaults to 0.9. This seems like a good idea to make inner solid layers a bit less dense. It helps if the bottom layer is a bit too low by giving somewhere for the excess plastic to go. As I was using a little excess plastic anyway it seemed a good idea.

I had noticed that some outer surfaces are never well filled even when other surfaces on the same object are. Here is an example in the bottom of the well in this bracket.

I only realised recently that this was because the 0.9 is applied to some exposed surfaces, not just to internal ones. I set the value to 1.00 and things got a lot better. Not only does it fix the problem above, but it helps to make the other top surfaces solid. I normally use three solid layers to get a good surface on top of sparse infill. But with the first two at only 90% the top layer is still lacking in plastic. That is why I had to use a higher flow rate than theory predicted. Once I got rid of this parameter I could reduce the flow rate and still get a solid top surface. In fact, I can get a reasonable top surface with only two solid layers now.

Another side effect of having the flow rate too high to compensate for the layers below being only 90% was that the top layer was being forced in. When the infill goes from two different directions and meets in the middle I was getting a ridge because the plastic would be being forced into a channel that was a bit too small for it.

Yet another issue I had noticed was that some side walls were inexplicably lumpy. I.e. not in positions where the filament starts or stops. Examining the slices I realised that it was caused by the infill displacing the outline. This was because I had a 15% overlap. Since I made the inner solid layers solid I found I don't need this any more and those bumps have gone away.

So in summary I was using excess flow rate and infill overlap to compensate for inner solid layers (and some outer ones) not being 100% solid. The side effects were lumpy walls and ridges on the top surface.

Round robin

2010-12-28T13:38:00.001Z

I have been making a few small tweaks to my host software to improve quality recently. One such tweak is the order in which islands of an object (or objects) are visited. By "island" I mean a closed outline and the holes and infill that it encloses. Skeinforge seems to always go for the nearest island, so when it finishes a layer it starts the next layer on the island it has just done and revisits the others in the reverse order.

This means that the plastic is added to the hottest island first and the coldest last. When an island is small it can mean that the layer below is still molten when the next layer is added. I simply reverse the order of every second layer so that the islands are visited in a round robin order. That means they all get the same time to cool down before the next layer is added.

The only downside is one extra long head move each layer from the last to the first island. If your machine leaves strings that is not ideal but mine hasn't since I started reversing the extruder. That also makes the Comb and Tower modules of Skeinforge redundant.

Crackers

2010-12-26T01:08:00.004Z

My wife has assembled her own Christmas crackers from kits in recent years. She puts in much better gifts than even the more expensive commercial ones contain. It did backfire one year when she put a handkerchief in one and it ended up with a powder burn from the explosive!

This year she asked me to make some reprapped boxes instead to contain the usual cracker contents and look decorative on the table. The explosive element to be provided by a party popper. This is what I came up with: -

Having zero artistic ability myself: the star is Christmas star by andrewar from Thingiverse and the tree was grafted from the frame vertex of the Holiday Prusa Mendel by kliment.

My contribution to the design is the box. The base dimensions were determined by the hats my wife wanted to use and the height by the party popper diameter. This one also contains a magnetic bookmark, two chocolates, two PLA snowflakes and a charade instead of the usual bad joke or motto.

The lids had to be printed hollow side down because of the raised design on top. The gap is too big to be spanned without a lot of droop, so I used the support facility in Skienforge. I set the "support gap over extrusion perimeter ratio" to 10 to make it easier to remove and waste a little less plastic. I have no idea why the ends of the support are all in slightly different places.

It was still quite tedious to remove, so I tried Adrian Bowyer's technique of using oil to reduce the bonding. I knew the roof of the lid started at 8mm, and my host software prints the height of the current layer, so I just waited until it had finished the support and painted it with machine oil using small paint brush, while dodging the head. It worked very well and made the support easy to remove.

Here you can see the scars left behind, probably where I missed with the oil: -

I removed the scars by waving a hot air gun over the plastic.

The unsupported area sags a little and that makes a visible pattern on the top as there are only three solid layers. I think that actually makes it look more decorative by adding a textured border: -

The removed supports could be glued together and used as streamers.

These cracker replacements went down very well with both our families. They make a lot less mess on the dinner table and could also be reusable, but they all asked to keep the boxes, which was of course our original intention.

The files are available here.

Merry Christmas!

101 Mendels

2010-12-18T12:33:00.003Z

From March up until a week ago I have run my Mendel as close to 24/7 as I can and it has printed 101 Mendels, with a bit of help from HydraRaptor. During all that time I have been able to sell them as fast as I could print them but there has been a dip in demand running up to Christmas, so I stopped printing on Monday, having built up a small stock.

I have shipped parts to England, Scotland, Isle of Man, Ireland, Sweden, Denmark, Belgium, Netherlands, Germany, Austria, Poland, France, Spain, Portugal, Italy, Tenerife, USA, Canada, Australia and Singapore.

It seems weird now to have a quiet house and not have to stay up until midnight every night to start the overnight build. It does mean that I have time to blog again though, and print things that are not Mendel parts.

I have been printing parts of a Milestag laser tag gun for a friend of mine. I recommended CoCreate to him and he has taken it and run with it. His first design is way more sophisticated that anything I have managed so far. It is a large device broken up into parts that just fit on my 200mm bed. Here is one of them: -

You can see the rest in Tony's blog http://funwithelectrons.blogspot.com/2010/12/milestag.html.

I think a machine printing 101 copies of itself must be a bit of a milestone in the RepRap project. That is about 100kg of plastic and not far off 4800 hours of printing in about 6000 available. It is testimony to the reliability of the mechanical design and if anything, the quality of the parts is getting better as I tweak the settings.

Crème brûlée

2010-12-16T23:49:00.002Z

This is what happened when the thermocouple fell off my heated bed: -

It happened while both myself and my wife were at work so the machine finished the build. When I came home the room stank of fumes.

The bed temperature will have been limited to about 170°C by the thermal cut out I have in series with the heater for safety. Since it was making a bed of six and it went wrong about 1/3 of the way through the build, they will have been cooking for about 4 hours.

Unsurprisingly the bottom of the object shrank and went brown. What was surprising was that the bottom layer became transparent and glass like. So glass like that I cut my finger on it. The meniscus edge was razor sharp. It seems to have softened over time though, this happened a few weeks ago.

Perhaps it might be a useful process if you want a transparent window on the base of an object. You could lay down a single layer and then cook it for a few hours at 170°C and then deposit the rest of the object on top of it.

Monthly maintenance

2010-11-13T23:35:00.000Z

So, after just over a month more of continuous use of my Mendel, I noticed the filament not spanning gaps well. It had also gone curly again. I measured it at about 0.6mm extruded into fresh air, so decided it was time to bore out the nozzle again. I do this with a 0.5mm bit held between my fingers with the nozzle hot. This restored the diameter to over 0.7mm again, so it is able to extrude 0.6mm filament with enough stretch to span gaps. Looks like this needs to be once a month maintenance.

Another failure I had was two of the bed support lugs sheared off the 360 y-bearings: -

These are under more load on my machine because I have a heavy metal bed. They also get some strain when parts are being removed from it. Rather than strip the machine down and replace the y-bearings, I made a new part that sits between the two y-bearings and supports the bed on three rather than four points.

If the bed is slightly flexible, for example when made from Dibond, then all four corners can be levelled independently. When it is stiffer, for example 6mm aluminium, then you can only adjust three points independently. In fact, one of those can be fixed and then there are only two points that need adjusting.

I made this using the support material option of Skienforge for the first time. To use it I have to enable the raft module but then disable the raft by setting the base layers and interface layers to zero. Without the cross hatch option the support material is easier to remove, but it tends to come away from the bed. For raft-less support the first layer of the support could do with being solid.

Other persistent problems I have are connectors losing contact, so reseating them once a month is good idea. The constant vibration and heat cycling seems to make connectors unreliable. Screw terminals with ferrules over the wire end seems to be the way to go.

The M8 nuts on the frame shake loose, I wish I had used lock washers! Also the grub screw in the pulleys eventually work loose after months and the one in the extruder drive gear needs tightening after a few weeks. It seems to be impossible to keep anything tight in plastic, especially when it is oscillating backwards and forwards. The plastic gives a little and that movement causes screws to work loose. Perhaps some thread-lock in the set screws would do the trick, but I am not certain that the screw might need to be tightened to take up slack caused by the plastic creeping.

Of course running a machine 24/7 is not what most users will do, so it will take many months of normal use before these types of fault manifest.

Rebore

2010-09-28T20:18:00.004+01:00

My Mendel has been very reliable and consistent running virtually 24/7, but about a week ago, after putting on a new reel of plastic things started to go wrong. The initial symptoms were that small parts built fine, in fact I printed a mini Mendel or Huxley that came out well: -

It took just two full Mendel beds, plus a few parts on HydraRaptor. I did the gears on Hydra for accuracy and the Bowden clamps at 100% fill because they look weak to me for the job they are intended to do. The plastic weighs 335g (including a Wade's extruder), slightly more than 1/3 of a Mendel by weight but the print time is about 1/2, because small parts need finer filament. I printed most of these at 0.5mm whereas I do a lot of Mendel at 0.6mm.

But getting back to the problem, the quality of large parts had started to fall off a bit. They were coming out with blobs on the outside formed by the nozzle oozing as it moves from one object to another. These were not well bonded, so they could be simply scraped off with a fingernail, but something I had tuned out ages ago. Another change was that it was not doing 45 degree overhangs well, so it left filament hanging down in the tops of tear shaped holes. Again, not a big problem as they just get drilled out anyway.

I started to suspect the temperature was too high so I pushed the thermistor well into the heater block. Then the filament started jamming after the first layer (which I do very slowly). After a few attempts the extruder drive gear broke where the captive nut for the grub screw is. This seemed more like the temperature was too low, so I suspected the thermistor was no longer reliable. I decided to rebuild the heater assembly as my last one was put together in a hurry from parts left over from an experiment. It had been in the wars as well, being entombed in ABS and hacked out again, not to mention running almost continuously for about 2500 hours. Originally the thermistor was glued in with RTV silicone, but that was long gone and it relied on the wires holding it in place.

Since my original heater hack using a vitreous enamel resistor I had moved on to a smaller resistor on Hydra and found that worked better. The surface area of the block is a lot less and that is where most of the heat is lost from, so the amount of power required goes down. It also warms up faster of course, both due to less heat being lost and also less thermal mass. The resistor I have settled on is a Vishay / Sfernice RWM04106R80JR15E1

The thermistor is drilled as close as I dare to the thread for the nozzle and then counter-bored so that the entrance is wide enough for the PTFE sleeving. The wires have PTFE insulation to withstand the temperature and the resistor is soldered with 300°C HMP solder. I think I could also get away with ordinary unleaded solder as well because of the length of the resistor leads, but I didn't want to chance it.

After a tip from Giles I used Rothenberger high temperature glass rope adhesive to glue the resistor and the thermistor. It sets in only half an hour, which is a big advantage over other things I have tried. I also used it to stick ceramic tape on the outside of the block to insulate it.

When I first heated it up the adhesive bubbled causing a downward slope in the temperature graph. I thought at first the thermistor had been dislodged by the blistering, but I think it was just temporarily cooled by the out-gassing. I should have heated it much more slowly the first time I think.

The new heater works much better than the old one. The warm up time to 255°C is about 280 seconds, whereas the old one took about 400 seconds (the bed takes about 350 seconds to get to 140°C). It also runs at about 70% to maintain 240°C while extruding, whereas the old one needed about 90%. The bang-bang control cycles much faster and only deviates by one degree. That is because of the close proximity of the thermistor to the heater. Because it is mounted between the heater and the barrel I can be sure the swing at the barrel is even less. I calibrate against a thermocouple inside the barrel, so any temperature difference across the block is calibrated out. It should be negligible though because the thermistor is also very close to the barrel and aluminium is a very good conductor. The extra power needed to heat the ABS when extruding 0.6mm filament at 32mm/s is about 10%, i.e. ~2W.

The new improved heater didn't solve any of my problems though. While reassembling the extruder I tried pushing filament through by hand. It was much harder than I remembered it was when I first built the extruder. At this point I was beginning to suspect the plastic was different in some way although it looked identical and was part of the same purchase.

I noted that the filament was coming out very curly. That was something I had noticed happening on both my machines when I do a test extrusion, but I had ignored it. I measured the diameter though and found whereas it normally swells to 0.7mm this was coming out oval and about 0.5mm by 0.6mm. It all fell into place then. I have read that the difference between straight hair and curly hair is whether it is round or oval. The only way the filament could be oval is if the nozzle aperture is no longer round. I put a 0.5mm drill bit through it and it started to extrude round, straight, 0.7mm filament again. The hole must have been partially occluded by the burnt plastic that tends to glaze the end of the nozzle. That caused the plastic to come out thinner and faster. It was fine when making objects with 0.5mm filament because it was still being stretched but when building with 0.6mm filament it was being compressed, so would hang loose if given the chance. The smaller hole increased the barrel pressure, which is why it oozed. The plastic would be compressed more, so require more backing up to release the pressure and stop the flow. Also the extra pressure was too much for the pinch wheel when extruding at the top flow rate I use, which is 0.6mm at 32mm/s. I think the M8 hobbed bolt is below the ideal diameter for softer plastic like ABS.

I also re-bored HydraRaptor (with a 0.4mm drill) and that stopped the filament being curly as well. It seems nozzles need occasionally re-boring. I had assumed that the hot flow of high pressure plastic would have kept the hole clean, but not so.

So a simple fault had my machine out of action for days because I didn't recognise what the symptoms meant collectively.

Some corners like it hot

2010-09-12T00:20:00.001+01:00

Large objects with sharp corners, such as the Mendel z-leadscrew-base, produce enough stress to form a blister in the PET tape on my heated bed. These can only be flattened again by pricking the tape. I can't understand how air gets in and cannot get out again, but that is what seems to happen.

The blisters leave a small indentation in the object's base. It is only an aesthetic problem because the base remains flat, i.e. it doesn't rock on a flat surface.

Sometimes the blister allows the corner to peel from the bed towards the end of a build, allowing the corner to curl upwards a little. Generally I can avoid that by cleaning the bed with acetone before problem builds. I also use hexagonal infill on those parts and only two solid layers rather than three in an attempt to reduce the stress. When I design my own parts I round the corners, where possible, to prevent such problems.

A solution may be to use a sheet of PET rather than PET tape, but then you need to find a way of holding it down. One thing I have noticed though is that when I build a bed with four of the z-brackets closely packed the corners on the inside don't blister or lift. That must be because the air around them is hotter. As an experiment I added some little plastic walls to the build to act as baffles to keep the heat in as the bed moves through cooler air.

These have a 5mm thick base to help keep the tape flat and are 1mm away from the edge of the object. They work well and stop the blisters forming at the corners. They are very similar to Forrest's apron technique but their primary function is thermal rather than mechanical. A more general technique would be to build a thin wall all the way around the perimeter of the objects to cocoon them. I expect that would only need to be one filament thick and perhaps might give a similar effect to having a heated build chamber.

Freezing your bits off

2010-08-24T13:19:00.000+01:00

Since I started cleaning my PET tape with acetone it can be hard to remove the parts from it sometimes. Somebody suggested trying freezer spray a while back, so I gave it a go.

I got this Arctic Spray, which is intended for freezing water pipes so that you can work on them without draining the water. I must admit I wouldn't fancy having a strict time limit if I was plumbing, you would have to be sure you had all the right tools and materials to start with. My occasional forays into plumbing rarely go to plan and usually involve a trip to B & Q in the middle.

I tried it first on an ABS part before the bed had cooled for any length of time, so it would be at about 100°C and the parts still soft. The part curled up at the edges and so came off easily. I thought I had ruined it by making it warp, but to my surprise it became flat again when it cooled. Still that seems a bit risky, and the spray isn't cheap, so now I cool the bed to 50°C with a fan and then spray any stubborn parts that I can't pull off. It works a treat but I don't know how long a can will last. It would have to be a lot of uses to make it worth the cost: £4.49 plus £2.20 on eBay. Hitting them with a block of wood and a hammer is a lot cheaper!

Friday the 13th

2010-08-23T00:21:00.002+01:00

When I got up last Friday morning my PC had this on the screen: -

This is despite the fact that I had automatic updates set to ask me before installing. This was the result :-

The PC had stopped talking to my Mendel about half way through a seven hour build, so the axes had stopped moving but the extruder was left running for a few hours. The result was that the extruder was encased in a solid ball of ABS about the size of a tangerine. Thanks Microsoft! What I actually said at the time was less polite!

Of course I was planning to put some safeguards in the firmware and also run the machine from an SD card, but have never quite got round to it as I have been printing virtually non-stop for months.

I couldn't remove the extruder from the carriage because the blob was too big to go through the gap, so I had to dismantle the x-axis to release the carriage.

I have seen this happen to other people and it wrote off the extruder, but I thought this one should survive because it is mostly metal underneath the blob.

I tried using a loop of hot nichrome to slice bits of it off. The nichrome cut through OK, but the ABS closed up behind it, so it achieved nothing.

Next I tried a small circular saw attached to a Dremel. That worked OK, but threw off sawdust and bits of ABS hot enough to burn, even through light clothing. I got the bulk of it off that way but when I nicked one of the heater wires I decided to stop.

I got some more off by heating it with a hot air gun and pulling lumps off with a pair of pliers. That was OK but the whole extruder got too hot to hold and it was starting to soften the carriage.

I got the remainder off by running the heater up to 200°C and using a knife, wire cutters and pliers. It took me about 3 evenings in total to remove the blob and the machine was out of action for a week while I reassembled it and calibrated it again.

It now takes a bit longer to warm up, and extrudes more filament during the warm up process than it used to. I suspect therefore that the thermistor is reading low and so it is running hotter. It doesn't seem to cause a problem with the ABS that I am using. I had to increase the time I run the extruder to prime it after warm up though.

I also seem to have managed to bend one of my z lead-screws while sliding the x-axis bars in and out. It doesn't matter as the axis is constrained by the z-bars, but annoying as it rattles a bit.

All in all a bit of a disaster. It's running again now though, but I still haven't put a safeguard in my firmware. I will have to develop it on HydraRaptor and load it into Mendel between builds. I have disabled automatic updates!

Rejuvenated Bed

2010-08-09T01:30:00.000+01:00

I put new PET tape on my heated bed at the beginning of July. Since then I have printed 15 Mendels on it, but on the last few I was getting problems with the parts not sticking. That is after about 700 hours of printing and ~15kg of plastic. I occasionally swab it down with Isopropanol to remove grease from finger prints, but Isopropanol is not a solvent for ABS. This evening I tried cleaning it with acetone instead. It dramatically increased the grip level, restoring it to new and making the parts hard to remove again! ABS must leave some traces behind on the surface of the bed and the acetone removes it.

So it looks like PET tape is almost fully reusable. It tends to get the odd blister where the corners of big objects overcome its adhesive and picks up a few scars from the odd accident with a knife. Apart from that it just needs cleaning with acetone about once a month.

A bit of a drag

2010-07-19T21:05:00.008+01:00

One problem I have had, on and off, with Mendel is a tendency for the infill not to meet the outline. This was particularly bad with PLA. I have combated this by having some infill overlap and also extruding the plastic slightly faster than it should be, so that the solid layers are well stuffed. I don't have to do either of these things with HydraRaptor. I couldn't figure out what the difference was until I changed a reel of plastic recently. The first print on the new reel came out like this: -

The gap is always at one side like this, it is as if the infill is not centred within the outline. The reason, I have come to realise, is that the belt has some play in it because it is not infinitely taught. When the extruder pulls filament off a reel it exerts a force on the carriage, which displaces it slightly from where it would come to rest without any external force. Because the carriage moves, the filament only gets pulled from the reel at the local extremes of movement, the rest of the time it is slack. This causes small offsets in the filament paths. In particular, when it is doing zigzag infill it is using filament relatively quickly, so at the end of the zigzag furthest from the centre of the bed it is likely to give a little tug of the reel each time, causing the zigzag to stop short of the outline.

The conclusion is that the filament feed for a belt-driven moving-head machine needs to be very low drag. The hanging basket technique that I used on HydraRaptor is no good because it has to pull plastic out from under its own weight. Making the feed point high above the machine reduces the lateral drag on the carriage, but you can easily get enough vertical drag to deflect the x-bars upwards, or even lift the z-axis slightly because the backlash in the thread is only taken up by the weight of the x-axis.

The reason it was bad with PLA was because I was pulling it from a hanging basket and being very stiff, even a small coil needs a lot of tug.

The system I now use is a vertically mounted spool big enough to take 5kg coils, which last me a couple of weeks.

The bearing is just a stainless steel axle running in PLA bushes, lubricated with some lithium grease. It is low friction, but not as frictionless as a ball bearing. It needs a little friction to stop any in-balance in the coil causing the spool to spin to its low point. Also the faces of the spool need to be quite big to stop a loose loop of filament coming over the side. It pulls tight and jams if that happens.

I can take the spool apart to insert a new coil of plastic.

In general though I have to wind it all off and on again to get it tight and balanced enough to wind off smoothly. Since 5kg is about 800m it takes a long time to wind it onto a garden hose reel and then back on again. Someday I will get round to making a machine to do it for me. In the meantime I will make a second identical spool so that I can just mount the coil on one spool and wind it onto a second one to use.

Meltdown

2010-07-08T21:00:00.008+01:00

While making its 18th child, my Mendel made a real pig's ear of laying down the first layer holes at the start of a build. So bad that the infill did not join to them and started curling upwards. I had to watch it a while before I realised what the problem was. The extruder had come loose and was bouncing up and down when the filament feed stopped and started.

I thought it was just that the bolts had worked loose but after I tightened them it was still moving and there were some worrying crunching sounds, so it was time to strip it down.

The bottom of the heatsink is covered with a sticky deposit. It is some volatile component that boils off the ABS and condenses on cold surfaces,

The main extruder bracket had broken and the carriage didn't look too good either: -

I stripped the carriage down as well and found that it was cracked and severely distorted.

The main problem I realised is my modified hot end. Normally the insulator is locked into the chunky part of the bracket by a couple of M3 bolts through it. I can't get those in because my heatsink is in the way, so I rely on the mounting bolts and the upper carriage to take the extrusion force. On reflection, not a good idea!

The lower carriage is less deformed because the extrusion force does not pass though it.

The heat rising from the bed and the extruder must be enough to soften the carriage and let it deform, but also it seems to have made the ABS weak and crumbly. Even the belt clamps have deformed.

This is after about 3 months of printing though so it isn't a big problem to replace them as long as you have spares. I had just printed a carriage before it failed so it was easy to replace but I had to print another extruder on HydraRaptor. You really need to have a full set of spares on hand, or have two machines.

I did various changes to make it more durable. The main thing is I fitted nuts under the heatsink so that no force goes through the carriage. I also put large penny washers on the top of the extruder bracket to reinforce the lugs. Ideally they should be a bit thicker but that would reduce the Z travel even further. The extruder motor clashes with the frame which reduces the height. Then my heatsink loses another 10mm or so and my heated bed loses 26mm. I am left with about 35mm which is only just enough to build the tallest Mendel part (the lower carriage).

I also used nyloc nuts in the captive positions in the carriage. The wiki advises against this as it may crack the plastic but it doesn't seem to be case with my ABS parts. Ordinary nuts don't stay tight because the plastic creeps.

I intend to fit some sort of heat shield to stop the heat rising from the bed reaching the carriage. In the mean time I have started fitting the front on my cabinet after the first layer is finished, when the bed temperature drops from 140°C to 110 °C. That can be up to 90 minutes into the build, so not convenient.

ABS on PC

2010-07-03T01:00:00.003+01:00

My last heated bed ran for a long time but it finally went pop on Mendel print number 15. The TO220 resistors developed a short to earth about half way though an 8 hour overnight build. It took out a 5 Amp mains fuse and destroyed the 4 Amp solid state relay that was controlling it.

Clearly the cheap TO220 resistors are just not suitable for abusing as heating elements, so I went back to using aluminium clad resistors. The disadvantage is that they are higher profile and need two accurately drilled mounting holes, but they are a lot more robust and cheaper. The more expensive TO220 resistors I used on HydraRaptor are still going strong, but there is nothing to suggest that they are any better in their spec. It is the tab insulation that breaks down though, so it could be just the fact that the voltage is much lower on HydraRaptor.

I have used the Tyco THS10 series at temperatures up to 240°C and not had any fail yet. They are not rated for mains voltage though, so I moved up to THS15 series which are. They are slightly taller, which doesn't actually matter because I use 20mm stand-offs, so there is still sufficient gap. The mounting holes will take an M2.5 screw, but I didn't have any to hand, so I drilled them out for M3. There is just enough room for a screw head with an integral washer, a standard washer would not fit.

I have run the THS10 at about twice their rating so I did the same with these: 9 × 22Ω in series gives a total power of 290W at 240V. That gives a warm-up time of about 4 minutes to 140°C. My extruder takes longer to get to 255°C, so I set them both off together so that the bed has enough time at its steady state temperature for the nylon pillars to expand fully.

The white PTFE clamp is where I attach the thermocouple. The device wrapped in Kapton tape is a 190°C thermal cut-out to prevent melt down if the firmware crashes or the solid state relay goes short circuit. The mains wire has PTFE insulation to handle the temperature. Since the wiring is exposed it should really have an extra layer of insulation to be considered safe, but I am not about to stick my fingers under a hot bed so I didn't bother. If you have children or animals, or are completely risk averse, then you probably should.

I haven't put any magnets on this one yet as I haven't been making use of the ones on the last bed since I started using white ABS on PET tape. The objects mainly come loose when they cool down and are easily removed without having to remove the steel plate and bend it.

ABS on PET tape works well. The grip level seems to degrade much more slowly than Kapton does. After lots of use it becomes easier to remove objects, but then the amount of grip is not quite enough for some parts. I can make most of the Mendel parts time after time, but I have problems with a few. The outer corners lift slightly towards the end of the build of the large Z brackets when the PET is old and I am building more than one at a time.

Not easy to see, but the bottom right corner has lifted by about 0.5mm. It makes no difference to the function of the part but I like to get them completely flat.

At the opposite end of the scale I have problems with the bed springs and the X 360 Z bearing plates. These are very tall compared to their footprint, so as the nozzle bushes past the top of the objects they often ping off the bed due to the small contact area and the high leverage. When the PET is old I have about a 30% reject rate with these unless I do them one at a time.

I had a 5mm sheet of polycarbonate that I have been meaning to try as a bed material for some time. I think that is what is used on commercial machines. It has a high melting point (267°C), so will not melt when the hot filament lands on it. It also has a high glass transitions (150°C) so shouldn't soften on a heated bed.

I clamped it to the aluminium bed with some bulldog clips.

I tried it cold to start with but the ABS did not stick so I tried it at 140°C next. I made a test shape that I am using to research hole shrinkage. It stuck so well I broke it trying to get it off.

I had to use a chisel to get the rest off. Strangely, although the ABS is extruded below the melt point of the PC, so it can't form a diffusion weld, it forms a stronger bond with the PC than to itself.

I dropped the initial bed temperature to 50°C which seemed to be the lowest I could get the first layer outline to stick properly. After the first layer I set the bed temperature to 90°C to reduce the warping stress in the ABS. These are temperatures on the underside of the aluminium, so the top surface of the PC will be something like 15-20°C lower.

I made these tall objects that tend to come unstuck from PET. These held well, in fact, when I removed them, most of the springs and one of the bearing plates left their bottom layer behind. Not really a big problem, the bottom layer becomes a minimalist raft!

For general production I went back to PET tape. I covered a sheet of 1.5mm thick stainless steel and clamped it down with more bulldog clips. I can swap it with a sheet of glass if I need to do PLA. The steel seems to be strong enough to stay flat in the middle when clamped at the edge.

Broken bracket breakdown

2010-07-01T22:06:00.005+01:00

Whilst printing a 16th set of Mendel parts, my Mendel printed a bed of brackets with bits missing: -

On investigation I found the idler bracket on the extruder had broken, so there wasn't any pressure on the pinch wheel.

It lasted a long time before it broke but clearly it wasn't strong enough. Wade made his in PLA, which is harder and I only use two of the four bolt holes, so mine is under more strain.

I made a stronger replacement. It is thicker and a little bit bigger in the other two dimensions. I also made the holes 4.5mm rather than 4mm so it slides on the bolts easier and I capped the ends of the axle holder as mine tended to slide sideways.

The files are on Thingiverse.

Wooden overcoat

2010-06-12T23:30:00.001+01:00

I can make raft-less ABS objects on a heated bed pretty reliably, but when I try to do a whole bed full I get corners lifting on the objects near the edge of the bed. I think the reason is that the air around them is not as hot. If you think about it, with a moving bed machine like HydraRaptor, if you have a single object in the centre of the bed then you have a buffer of hot air around it. The bed only has to move by the dimensions of the object, so if the object is less than half the size of the bed then it remains inside that buffer. When you make objects near the edge of the bed the buffer is smaller and the bed moves further, so you get a double effect. To mitigate this effect I have halved the size of my biggest build trays. This is less convenient as a full bed is about 8 hours giving three shifts a day.

For example, with natural ABS on Kapton I was able to get away with a full bed like this: -

... but with white ABS on PET tape I would always get the odd part around the edge lifting, so I have to do it as two builds now. I bought a new reel of natural ABS from MakerBot but I can't make it work with HydraRaptor. I made a couple of objects but then it started to always jam after a few layers. The reason seams to be that because it is undersized at about 2.8mm, and my barrel has an internal diameter of 3.6mm, molten plastic back-flows up the barrel as far as the cold zone, where it freezes and makes the filament hard to push. This causes the filament to buckle and jam. I don't understand what has changed. The last reel I got from MakerBot a long time ago was the same diameter and I only finished using it recently and had no problems in the same extruder with the same settings. I switched back to white ABS and it works reliably again, so it must be something to do with the plastic.

Another problem with ABS is the fumes. My Mendel extruder seems to give off more fumes than HydraRaptor's does, perhaps because the melt zone is much bigger, and the white ABS seems to smell more acrid than natural ABS. I did a build with a window open to get rid of the fumes but most of the parts then warped, presumably because there was no longer a buffer of warm air around them, but a cool breeze.

In an attempt to tackle both of these problems I built an MDF box around my Mendel.

The front of the box is held on by magnetic door catches. It is sealed by door draft extruder strips and has a window made from plastic from a picture frame. This is glued on with silicone sealant.

The box is tall enough to allow the filament to enter through a single hole in the middle of the roof with a felt gasket that catches the dust.

The fumes are extracted by a tiny fan mounted in a chimney in the roof and piped out through a window vent.

I made a little pipe with a flange that fits into a slot in the vent and taped up the other slots with PET tape. I have another vent in the other window for fresh air in.

This fan is controlled by a spare output on my extruder controller and I have a thermistor to sense the air temperature in the middle of the chamber at the height of the top of the Mendel frame. Together with a small fan to cool the extruder heatsink and a large fan to cool the bed that uses up all the free outputs of my extruder controller, but not for the uses I originally envisaged.

I set the target chamber temperature to 40°C because that is as high as I dare to run the electronics and power supply. With the front closed the small fan cannot hold the temperature down and I have seen it go as high as 50°C without any ill effects. The extruder stepper was then too hot to touch though. Note there is no chamber heater. All the heat comes from the uninsulated bed, extruder and the motors and electronics, so I have actually reduced the total power consumption slightly and gained a heated chamber. To maintain 40°C I have to leave the front open at the bottom. I will add some vents at the bottom of the sides to allow cool air in and perhaps use a bigger fan.

Even with a gap at the bottom of the door I cannot smell any fumes. Since using the chamber nothing has warped provided the first layer outline went down properly as discussed in my last post. It also makes the machine very quiet although it was already much quieter than HydraRaptor.

Black and White

2010-05-24T09:00:00.000+01:00

I bought some new ABS filament from reprapsource.com as it is a reasonable price, the postage from Germany is not too bad and being in the EU there are no customs charges, so it does not get held to ransom by Parcel Force for their ridiculous handling charge.

The advert does not state a colour so I assumed it would be natural, however when it came it wasn't like any ABS I had encountered before. Natural ABS is cream coloured and opaque. This was white and a bit translucent. At first I though it was HDPE, but when bent it bruised, which is a characteristic of ABS.

I ran it first in HydraRaptor. The only issue I had was that it didn't want to stick to the PET tape I was using until I raised the bed temperature to 140°C for the first layer and extruded at 250°C. For subsequent layers I revert to the bed at 110°C and filament at 240°C.

The objects produced look nice in white and seem to be harder than those made in natural. I don't think it is simply pigmented ABS, I think it is a different formulation.

My impressions of using PET tape instead of Kapton tape is that it doesn't seem to give as much grip as new Kapton, but it doesn't degrade. I can make most things on it with HydraRaptor without any warping at all, but Mendel bed springs tend to come unstuck. This is because they are relatively tall and have very little contact area with the bed. If the extruder hits a slight blob on a high layer it will snap the part off. Sometimes the loose part hits another part and starts a chain reaction where they all fall off.

When doing raft-less builds on PET or Kapton it is essential that the first layer outline sticks perfectly and has no gaps in it, especially at the corners. If the first layer is too high it obviously doesn't stick and takes short cuts across the corners. If it is too low it also lifts at the corners though. What happens is that the filament becomes squashed into a flat ribbon. When that tries to bend around a sharp corner the outside has to stretch but instead it lifts and folds over inwards. A difference in z-value of 0.05mm can make all the difference. Increasing the temperature also helps to make the plastic bend around corners. If a corner does not stick perfectly then after two or three layers it will curl up at an angle of about 45°. This effect is not like the corner warping you get on a cold bed. It is much more localised and extreme. Small objects tend to come off during the build if a corner lifts.

With the natural ABS I was using before on Kapton it was far less critical. Objects stuck so well I had to remove them with a hammer or use a flexible bed. With white ABS on PET tape the objects can be removed more easily. Sometimes they just come free when they are cooled.

When I tried the new ABS in my Mendel it took a lot more tweaking to get it to work. The first issue was that I had to increase the feed rate by about 18% relative to what I was using for PLA. My theory is that being softer it presses further into the threaded pulley and so sees a smaller pulley diameter. The hobbed M8 bolt has an internal radius of only about 5mm. The drive pulley on HydraRaptor is about twice that diameter and seems give more grip on softer plastics and doesn't need the 18% bodge factor when switching from PLA to ABS. I just tell it the filament diameter and it just works.

The next problem I had was that holes tended to shrink inwards and not meet the infill as you can see on this piece.

I also find PLA has a tendency to do this on my Mendel but not on HydraRaptor. For a sanity check I built the same object from the same g-code with black ABS.

Notice how much bigger the holes are.

When I was flushing the black out again with the white I noticed that the white had far more die swell and was coming out at about 0.7mm. The black was only about 0.55mm. This means that to extrude at 0.5mm the white is being stretched a lot more, which accounts for why the holes shrink inwards. To test this hypothesis I ran the same g-code again scaling up all the coordinates by 0.6/0.5. This produced a bigger object but the holes are much better.

I then re-sliced the object for 0.6mm filament and that also printed correctly.

So it seems that the white ABS has more die swell than natural or black. In that respect it also reminds me of HDPE. For some reason HydraRaptor is not affected and seems to have less die swell despite having a smaller nozzle, which normally gives more die swell in relative terms because the pressure is higher.

The other thing I discovered is that black ABS does not stick well to PET. It seems a bit greasy.

So with a 0.5mm nozzle if have to build objects at 0.6mm when using white ABS in my Mendel, but with a 0.4mm nozzle on HydraRaptor I can build at 0.375mm or 0.4375mm no problem and holes do not shrink excessively. I am not sure what the difference is, perhaps the length of the nozzle aperture.

PLA on glass

2010-05-14T19:39:00.008+01:00

A while ago Jordan Miller emailed me to say that PLA can be printed on hot glass. He had tried ABS but it did not stick at 90°C, which was the highest temperature his bed would go so I said I would try it at 140°C.

I found a piece of glass the same size as HydraRaptor's bed that was 5mm thick. It used to be the platform of a kitchen weighing scale. It has nice rounded corners, the only problem was that it had an aluminium boss glued to it. I tried to remove it first with a hammer, then I tried acetone and finally I tried a hot air gun. None of these methods worked so I put it in the oven at gas mark 6 for 10 minutes. It then just lifted off with a pair of tongs.

For a quick test I just taped it down with some Kapton tape. It holds firm as long as you do all four sides.

As you can see ABS does not stick to glass at 140°C.

Next I moved the glass onto my Mendel as it was set up for PLA at the time and I couldn't get PLA to stick to PET tape.

I printed a frame vertex on glass with the bed starting at 120°C for the first layer, dropping down to 45°C for the rest of the build.

That stuck well but came off easily when the bed was cooled. Next I tried a new piece of 4mm glass cut to the size of the bed.

That stuck so well that it took several blows with a hammer to to remove each object. One piece chipped when it hit the wall behind! For some reason the new glass seems to stick much better than the old.

The objects come off perfectly flat and glassy.

I dropped the bed temperature to 100°C, which makes them a little easier to remove, just a sharp tap with a hammer rather than a heavy blow! Any lower than that and I have trouble getting the outlines to stick. Jordan uses only 65°C and reports the objects are easy to remove, so I am not sure what I am doing wrong, different PLA perhaps. If I start with the head lower then the plastic rucks up during the first layer infill.

So glass looks like a good bed material for PLA as it comes completely flat and hopefully should not degrade. Jordan reports that finger prints prevent objects sticking but they can be removed with alcohol. Copper clad PCB material has the advantage that you can flex it to remove objects but doesn't give as good a finish.

Plumbstruder

2010-05-03T17:30:00.000+01:00

Brian Reifsnyder asked for volunteers to test his hybrid PEEK and PTFE insulator design, so I used it for the hot part of my Mendel extruder to start with. The drive mechanism is Wade's design.

It worked well at first, requiring little force to extrude PLA, but got harder and harder until eventually it completely jammed. This video below shows that even with the nozzle removed and starting with a completely empty barrel I couldn't push more than about 15mm of filament through it.

The reason was that the PTFE liner had slipped a little leaving a small gap between it and the end of the brass heater barrel.

This makes the extruder jam completely solid. The reason is that PLA goes rubbery above 50°C, so any pressure on it makes it expand width wise and grip the side of the tube. If there is a gap that it can expand into it locks the filament.

I stripped it down, cleaned it out and reassembled it with some washers to hold the PTFE down.

Brian has added a circlip to the design to solve the problem.

I haven't tested this version yet because I ran into another problem before it arrived. When I started using a heated bed for PLA the extruder jammed again. This time it was because the top end of the insulator got hotter than the glass transition of the PLA, so it swelled as it went into the insulator and jammed in the tapered entrance. There was also some leakage around the threads.

The reason it got too hot is a combination of the heated bed, the fact that I used an uninsulated heater with a large surface area, and the fact that the Mendel carriage traps the rising heat.

I decided to try out an idea I had a while ago, which is similar in intent to Brian's scheme. Instead of putting PTFE inside PEEK to stop it expanding I put it inside a 15mm copper pipe. This not only totally constrains it so it cannot swell, it also removes heat from it, shortening the transition zone. I am calling this one Plumbstruder. Here is a sketch of the layout: -

The end of the copper pipe is closed off by soldering an end cap on and then drilling it out to leave a lip to support a PEEK disk which the barrel screws into as well as into the PTFE. That means the PEEK supports the extrusion force, as in Brian's design, but I also use the thread in the PTFE as a seal rather than just having a compression joint.

The copper pipe gets hot so I coupled it to a big heatsink with a copper flange.

I turned this from a solid block of copper a friend gave me (thanks Paul). I soldered it onto the pipe and screwed it onto the heatsink.

I turned the one piece nozzle / barrel from hex stock so it has a nut shaped flange in the middle to make it easy to screw in and also gives the aluminium heater block something to tighten against.

I had to turn down the PTFE to be a tight fit inside the pipe. I was hoping to find a size where the ID of the pipe matched the OD of the PTFE. 22mm copper pipe has an ID of 20mm, so theoretically 20mm PTFE rod would fit. In practice I have found that PTFE rod is about +/- 0.5mm so, unless you were lucky, the fit would not be good enough.

Even with a big heatsink it was getting uncomfortably warm so I added a tiny fan.

I have been using this extruder on my Mendel for a few weeks and it is totally reliable, with no sign of leaking. I think that of all the extruders I have made, this one needs the least force to extrude. I can push plastic through by hand at high speed with ease. For an extruder to work I think the transition zone needs at least two of the following three attributes: short, slippery or tapered. Unfortunately a short transition zone seems to mean using a heatsink, which is not ideal for a moving head machine.

I also think a short melt zone improves the accuracy by reducing the start-stop time. In that respect this design is not ideal, although it is no worse than the standard design.

Flash bang bed

2010-04-30T23:48:00.008+01:00

As my MK3 heated bed on HydraRaptor has been working well I decided to scale it up for Mendel.

Buying aluminium that is flat seemed to be a hit and miss affair until a friend told me that what I need is tooling plate and put me in touch with a company that sells it. They recommended C250 cast machined tooling plate. It wasn't cheap (I got 5 pieces 200 × 200mm for ~ £140) but they are all flat.

I can't find a geometric definition of flatness. It is given as +/- 0.4mm for a 6mm sheet of C250 (I would have preferred 5mm to reduce the mass a bit but that is +/- 0.8mm). I take it to mean that all the points on the surface of a metre square plate will lie in a volume 0.8mm high. For a 200mm piece I expect the deviation to be about 1/5 of that, i.e. 0.16mm assuming it is a single curve rather than wavy. Since the bed can be levelled at the corners the deviation in the middle should be about half that again, 0.08mm, just about acceptable for raft-less printing.

When I tried levelling the bed I ran into a problem though. With my Dibond bed I could level each corner because it can flex a bit. With the rigid aluminium bed I can only level three out of the four corners at a time. When I move the nozzle to each corner in turn it behaves as if two diagonally opposite corners are lower than the other two. That would imply the plate is not flat, but I know it is when I put a straight edge across it. I think this means that the two y-axis bars are not quite level with each other at both ends, causing the bed to twist about the y-axis as it traverses it. I expect it could be corrected by adjusting the frame but I haven't got my head around what to adjust and in what direction yet.

Given that I am using 188W on a 150mm bed on HydraRaptor, to get a similar warm up time I would need 335W. That seems a lot to get from a PSU, so I decided to make it mains driven. I found that I could get 47Ω TO220 resistors cheaper than other values. Five in series across the mains gives about 250W, so I used two strings of five to give 500W. That gives a warm up time of about three minutes.

Equally spacing four or nine resistors on a square is easy but placing ten is an interesting problem. I used the solution to packing ten circles in a square that I found here. This is my layout with 16 magnets as well.

And here it is wired up: -

I used wire with PTFE insulation rated to 300°C. I have an earth connection of course. It would be a good idea to have a second earth in case the first one breaks due to the constant bed movement. I also fitted a 150°C thermal cut out that came out of a microwave oven. With 500W it would get very hot indeed if the control circuit failed.

I intended to mount the magnets the way I did before, by drilling holes not quite through, leaving a rim to retain them. I didn't tighten my drill stop enough and went all the way through so I decided to glue them in with JB-Weld.

I placed the bed onto a sheet of glass with some cling film on it. I then dropped in the magnets and glued them. When I turned it over the next day I found the magnets were sticking up from the surface. The glue must expand as it sets pushing the magnets down and lifting the plate!

I tapped them down with a punch but, unsurprisingly, they fell out the first time the bed was heated. In the end I jammed them in with PET tape. Drilling part way through is a much better solution.

I mounted the bed on top of the Dibond bed with nylon stand-offs.

Not an ideal solution as a lot of z-travel is lost, but the thermal cut-out is quite deep.

I used chocolate block connectors to wire up the mains. To make them safe and provide strain relief for the cables I RepRapped some plastic covers.

The lids just clip on with some tabs that fit into small slots. They didn't fit very tightly, I need to make the tabs bigger and a tighter fit. A boss and a screw hole would have been better I think.

For safety all the wires should be inside the cover as everything accessible should be double insulated. I will make it wider at my next attempt.

The bed worked well for the first few objects I made. Simple bang-bang control gave about 10°C overshoot initially but settles down before the object build starts so does not really matter. One thing I have realised is that the nylon pillars expand about 0.1mm when they warm up so I give them some time to do that otherwise the first layer has varying height.

I got some new ABS from reprapsource.com that turned out to be white, I was expecting natural as that is easier to work with. It seems to need higher temperatures to get it to stick to itself and the bed. I am extruding at 240°C with the bed at 140°C for the first layer and 110°C after that. I built one object like that and then disaster struck. The bed heated to 140°C and levelled off. While the extruder was heating I heard a few pops and crackles. When I looked at the temperature graph I saw the bed temperature soaring. Before I had time to think what was happening there was a loud bang and flash from underneath the bed and the 5A fuse in the plug blew.

What happened was one of the resistors developed a short between its tab and one of the connections. That caused a path to earth which increased the power on the remaining four in the chain. Several of those went short circuit as well in a chain reaction which ended up shorting the mains.What I couldn't explain at first was why the firmware did not turn it off and why the thermal cut-out did not cut the power. It turns out that I had swapped the live and neutral connections in the IEC connector, which meant that the solid state relay and the cut-out were in the neutral connection. As soon as the first resistor shorted it had bypassed all the control, not good!

I had originally chosen the resistors when I was making a bed for PLA at 60°C. Looking at the datasheet they have a maximum operating temperature of 155°C but they are de-rated to zero wattage at that temperature, so by putting 50W into them at 140°C I am grossly over loading them. I have abused AL clad and vitreous enamel resistors in this way and not had any problems but the TO220 seem far less robust. I don't know what they use for the tab insulation but I wouldn't be surprised if it was epoxy. The high voltage may also have been a factor as the ones on HydraRaptor have survived a similar overload so far. They have the same de-rating curve, but are made by a different company.

I rebuilt the bed and changed my firmware to stay inside the power curve by reducing the PWM ratio as the temperature increases. Unfortunately , I found I could only get to 130°C so I had to change the zero power point to 200°C to get to 140°C in a reasonable time. Even then it takes 400 seconds instead of 175.

So far it is holding up, but it is nowhere near as fast as I wanted. A shame because I had bought 50 of the 47Ω resistors, but I think I will have to scrap them and go back to AL clad. The smallest ones that I have used before are not rated for mains voltage so I will need some bigger ones. PCB or stick on silicone heaters are starting to look more attractive!

ABS on PET tape

2010-04-24T15:30:00.003+01:00

I find ABS sticks to Kapton very well to start with, but as it ages, it seems to stick less well. Corners start to lift and eventually builds are ruined. I have tried cleaning it with isopropyl alcohol and with acetone but it makes no difference. Charles Pax has reported that sanding with 220 grit paper makes it stick better. I cannot reproduce this. In fact, I find the opposite effect. It always sticks well when new, and if anything, sanding it makes it worse.

Somebody pointed out a while ago that you can get PET tape that is rated to 250°C. That is not as high as Kapton, but just about adequate for a heated bed when extruding ABS at 240°C.

I bought some and when my Kapton stopped working I decided to give it a try. It seems to work well. The first layer goes down perfectly :-

and the objects stay flat: -

I do the first layer at 240°C with the bed at 120°C and subsequent layers at 220°C with the bed at 110°C. I have made all the parts for an extruder on it so far and it has performed perfectly. The extruder will be on eBay this evening.

It is too early to say if it better than Kapton, but it looks promising.

Dibond bed

2010-04-06T19:49:00.005+01:00

I had been making Mendel parts with my Mendel, using PLA on blue masking tape, as it didn't have a heated bed . When I made a frame vertex on its own it came out completely flat. Larger parts like the z-base brackets warped a little at the corners, but were still acceptable. However, when I made a bed full of parts the warping was much worse. Frame vertexes warped a little and z-base brackets curled up several millimetres and jammed the y-axis, ruining a bed full of parts. I think the reason they warp more is that it takes so long for each layer that the parts are completely cold when the next layer is deposited. The odd thing is that Adrian Bowyer manages to print trays full of parts on blue masking tape without a heated bed. I have added it to the growing list of things that work better in Bath than they do here: AOI and PTFE being another two.

I had some aluminium plate on order but I wanted to knock something up quickly. I figured PLA on blue tape would only need 40-50°C to stop it warping. My bed is made from Dibond, which is 3mm thick and has the following characteristics:

Thickness of aluminium layers 0.3mm.
Core polyethylene, type LDPE.
Surface: lacquering - modified polyester lacquer system.
Temperature resistance from -50 ° C to +80 ° C.
Aluminium grade premium A1Mg aluminium alloy.

The great thing about it is that it appears to come pretty flat and is strong, light and easy to machine. I wondered if the aluminium layer was thick enough to spread the heat. I didn't think heat would flow though the LDPE very well so I mounted 10 47Ω 50W resistors around the top edge. I have found that for some reason 47Ω are cheaper than the 12Ω ones I used on HydraRaptor's bed. I wired them in pairs in series and then all the pairs in parallel giving 18.8Ω. I connected them to my 48V AC transformer with a small solid state relay. The total power is about 120W. Not as much as I use on my aluminium beds, but plenty of power to get to 50°C quickly. In fact, it warms up faster that my extruder does.

An initial test showed that the middle was about 10°C cooler than the edge. Not a big surprise considering how thin the aluminium is and how far the heat has to travel. When I measured the other side the difference was only about 5°C, so I decided to mount it upside down with the resistors on the bottom and the thermocouple on the top.

It works very well, and the objects stay flat. The first multi-part build I did though failed after the first few layers.

The extruder jammed because the top of the thermal insulator got hot enough to allow the PLA filament to go soft before the entrance. The extruder was finding PLA very hard to push anyway and the maximum speed I could get was about 24mm/s of 0.5mm filament. This is because the thermal transition zone is too long. The extra heat rising from the bed must have pushed it over the edge, literally!

The insulator is a combination of PTFE for slipperiness and PEEK for strength, but I think PEEK conducts too much heat. It doesn't help that my heater is not insulated yet and the Mendel carriage traps any rising heat.

I am quite happy with with Wade's drive mechanism but decided it was time to try another hot end design, coming soon ...

I think that for PLA, Dibond and blue tape / Kapton is a good solution. It won't handle the temperatures for ABS on Kapton though, but it might be good for ABS on PMMA or PC.

CU + PLA

2010-04-02T23:19:00.010+01:00

Vik Olliver asked for a volunteer with a heated bed to see if we can extrude onto copper clad board. I didn't think it would stick, but gave it a go anyway.

I first tried ABS onto double sided copper clad FR4 taped to a bed at 120°C. The ABS stuck well enough to extrude the first layer of a 20mm square, but when it cooled down it had no adhesion at all.

PLA at 55°C did exactly the same, but PLA at 130°C stuck very well, so well in fact that I can't get it off with my fingers (the blob was where I aborted the print after the first layer).

Maybe ABS would stick in the same way at an even higher temperature, but maybe not as it is less like glue than PLA. The 120°C / 55°C temperatures are what I use for Kapton, which is why I used them as the starting point.

An interesting aside: I had to measure the PCB to work out the z-height. It is only 1.4mm thick, whereas a standard PCB is 1.6mm. You can also see the grains in the FR4 showing through the copper. This means the board I bought in Maplin for home PCB use is actually the same stock material that they use for the first part of a commercial production process, but when they plate thorough the vias they increase the thickness of the copper all over to get the standard 1oz/inch². I don't know if this is always the case, i.e, that all home made PCBs have less copper than a production one, or whether you can get bare board with 1oz on it already.

Anyway a good result, assuming PLA will resist PCB etchant. Also, it seemed like a potential bed technique. I.e. do the first layer onto hot copper and then cool it to about 50°C for the rest of the object. I tried it with this butterfly: -

It worked perfectly. After the first layer I blew it with a fan to cool it down to 50°C. It took about four layers to get down to that temperature. Since I added the insulation under the bed it takes longer to cool it than it does to heat it.

After it had finished and cooled down to 40°C it was still firmly attached, so I removed it by flexing the PCB.

The base of the object is perfectly flat.

I think for PLA this might be a better technique than Kapton. I can't imagine the PCB wearing out. It could also be self heating with a serpentine track on the other side. I don't know that just taping it down would be strong enough for making large objects. I could solder fastenings on the back if not.

I don't know if there is anything special about copper and PLA, or whether other hot metals and plastic would work . I tried similar things with ABS on AL, but may not have had it hot enough.

Heated bed MK3

2010-03-25T19:25:00.011Z

My first heated bed worked OK but it was slow to warm up and hard to remove objects.

The second one was only ever intended for experimenting with vacuums and magnets but I ended up printing most of my Mendel on it. It worked well but limits the build area.

I have now replaced it with a full size version, using the lessons learned from the first two.

The first bed was the same size as HydraRaptor's table (200 × 200mm) but the build area is only about 150 × 150mm. The warm up time and power are both proportional to area, so I made this one just big enough, i.e. 150×150mm. Removing the 25mm border nearly halves the area!

The other innovations were to make it easier to build. I replaced nine AL clad resistors with four TO220 resistors. These are rated at 50W and 155°C, so are actually totally within spec when I run the table from 48V giving 188W. Instead of having to tap two M2 holes for each resistor these only need a single M3 hole. That is much easier to tap as the tap is a lot sturdier.

They are also lower profile of course. I just noticed that 47Ω ones are less than half the price, so I should have used four of those in parallel instead.

The thermocouple is mounted with a clamp made from PTFE.

Since this bed has a steel plate on top none of the holes need to be blind. That makes drilling and tapping easier as well.

On my previous magnetic bed I placed the magnets in blind holes that were almost all the way through. That required a milling machine to get flat bottomed holes. On this version I just drilled almost all the way though, leaving a lip to retain the magnet.

This is the top side. The magnet in the middle was done with an alternative technique. I drilled a through hole and then jammed the magnet in with a few strands of copper wire. That gets it flush with the surface, giving maximum magnetic force, but it pulled through on first use. I will have to glue it with high temperature epoxy I think.

After a suggestion by Enrique that wool was a good high temperature insulator my friend Steve gave me some carpet underlay made from wool. I used it to insulate the underside, thanks guys.

For the steel plate on the top I used the cover of an old CD ROM. It is only 145mm wide unfortunately. I think it is mild steel with nickel plating. Not as good as the stainless steel springy piece I got from inside a toaster.

So here is the finished article with the biggest bit of Mendel built on it. It was quite hard to remove. I had to remove the steel plate and bend it a little as intended. I had found that things I built recently on the small table could be removed without lifting the plate. I think the Kapton gets less grip as it ages. I tried cleaning it with alcohol and sanding it with very fine emery paper, but that seemed to make it worse if anything. It seems that shiny Kapton gives more grip than matt.

Making Mendel

2010-03-21T15:43:00.037Z

I aimed to build my Mendel in time to show it at the Makerfaire in Newcastle but completely failed. I had two weeks to build it, which I thought was plenty. In actual fact it took closer to three weeks before I got it printing successfully. I had no major problems, just a few snags here and there and a severe underestimation of how long it would take on my part.

Printed Parts

Unlike when I printed two sets of Darwin parts, printing the parts was the easy bit. This was due to three breakthroughs I had at the beginning of the year: -

The heated Kapton bed removed the need for rafts, which not only take a significant time to print, but also can take a lot of manual work to remove.
The extruder fast reverse got rid of all the strings, which also took a long time to clean up, especially from inside the Darwin corner blocks.
The "no compromise" extruder is so reliable that I have the confidence to do multi-part, layer by layer builds, which gets a lot more on the table, allowing longer unattended operation.

I printed the parts with 0.4mm or 0.375mm filament and with 25% infill. For the larger parts I used two outlines for strength. Since the large parts don't need fine detail, I think printing them with 0.5mm filament and one outline would be quicker, but that would need a bigger nozzle.

The weight of the parts, not including the extruder, was only 730g. I printed the outlines at 16mm/s and the infill at 32mm/s, so it's hard to say the total time. Assuming an average speed of 24mm/s at 0.4mm diameter gives about 3 mm³/s. That would put the total time at about 65 hours. I did it as a background task over a few weeks. A lot of the parts were printed as experiments with heated beds.

Rods

I took me an evening to cut all the rods. The method I used was to nail a stop to my workbench to line up the rod against a metre rule.

I then lined a piece of masking tape up with the correct measurement and wrapped it round the rod to mark the place to cut. I also wrote the name of the rod on the tape to make it easy to identify later.

A Black & Decker workmate makes an ideal vice to hold the rods while sawing. I rotate the studding until the thread lines up with the edge of the masking tape. That guides the saw to start in exactly the right place.

I used BZP for all the studding except the z-leadscrews, for which I used A2 stainless steel because it is smoother and generally straighter. I bought the rods from Farnell and even the BZP studding was very straight, a lot better than the stuff you get in B&Q. I also used A2 for all the bars.

It was very hard work sawing the A2 until I switched to a new blade and used Trefolex cutting compound. I am not sure which made the most difference, but I could then cut the A2 much easier than I had been previously cutting the BZP. I wish I had done that earlier, it would have saved a few hours.

Thick Sheets

The thick sheet parts are not really suitable for making by hand, particularly the squashed frog. They have lots of slots, which are hard to make without a milling machine or a laser cutter, etc.

I am not sure exactly what the hole in the bed and the purge plate are for, so I made the bed a simple rectangle with four holes. I am using my own electronics, so I made the two circuit board plates to suite. I simply cut rectangles and I marked the holes and drilled them in the right place, so no need for slots. That just left the squashed frog.

I made a much simpler design with drill centres on it. There is no need for the bulging legs and sloping shoulders. I think they must be just to make it look more like a frog. Fine if you you are CNCing it, but a PITA if you have to make it by hand. Also the holes for the opto tab and the purge plate are mirrored for no apparent reason, so I made it chiral.

This just starts as a rectangle with some holes in it. Then the large slots are made with a saw thin enough to turn in the holes. The outer holes that mount the bearings can be round because they are in a a fixed place, dictated by the holes in the bed. The inner holes need to be slots because the bearings are adjustable. I just left them off the template and marked them with the bearings adjusted and in place.

I made the sheets from 3mm Dibond, which is below the recommended thickness, but seems stiff enough. It is also light weight and very easy to machine.

Thin Sheet

I didn't have any optos, so I used micro switches for my end stops, hence didn't need any thin sheet parts. I simply attached them to the bars of each axis with P-clips. A little RepRapped bracket would be better but I was building this in a hurry, so had gone into bodging mode at this point!

They seem to have sufficient repeatability and certainly will when I replace the electronics with my new design, which will know the motor phase, reducing the uncertainty by a factor of 32. It is the same switch that I have used on the z-axis of HydraRaptor, which has proven totally reliable. They seem to be this one from RS, not cheap.

Belts

These were easy enough to split but, because the reinforcing wires run in a spiral, the blade tends to follow one for a while before managing to cut through it. That leaves a ragged edge with a bit of wire sticking out.

I didn't understand the rationale for slackening the belts until you just don't see backlash when moving one motor detent. I am microstepping anyway, so a motor detent is not significant. I made my belts good and tight.

Snags

I had a few snags with the mechanical assembly: -

The x-axis spacers are too short. The STL files are 5mm shorter than the parts in the STEP assembly. That caused the motor to clash with the nuts on the 360 bearing.

The 180 bearing at the other end was about 10mm from where it should be.

A simple fix was to slide the axis along leaving a 10mm gap to the spacer, the only problem remaining is that the spacers rattle at certain step rates.

The STEP model shows this gap should be only 5mm, but I have been unable to find the discrepancy. My rods and inspection distances are correct and the ends of the rods are flush with the clamps, as they are in the model.

The bed springs seemed to be too long to compress to the length of the bed-height-spacer-31mm_1off, which is not actually 31mm, but 29mm, so I don't know what gives there, I just spaced them a bit higher.

The bolts in the z-bar clamps are too long to allow the bearing to be inserted. I replaced them with shorter ones.

Similarly the bolts in the x-carriage get in the way of the extruder I fitted.

The J3 jigged distance did not seem correct. The distance between the y-bars is set by the J2 distance and the 3 nut spacers.

Extruder

I used Wade's extruder design as I didn't have time to adapt any of my own.

The gears work well, with very little backlash, but the small one has some movement on the motor shaft. It is just a press fit with a flat on the shaft. I need to redesign it with a captive nut and grub screw.

I didn't have a suitable M8 shoulder bolt so I made one from brass by attaching a nut with a pin through it.

I hobbed it with an M3.5 tap. I haven't measured the grip, but I get the impression it is not as high as Wade gets, I am not sure why.

For the bottom half of the extruder I used some parts that Brian was looking for volunteers to test for him.

The insulator is made from PEEK with a PTFE liner. The idea being to get the strength of the PEEK and the slipperiness of the PTFE. It seems to work well with PLA, which is all I have run through it so far.

The barrel is long because it is designed to take nichrome, but I just screwed it into a block of aluminium with a vitreous enamel resistor in it.

This was left over from a previous experiment. I have now moved onto a smaller resistor size, so this block could be smaller. The barrel could be a lot shorter with this arrangement and that would give less ooze and less viscous resistance.

The extruder works well with PLA. The main problem with it is that it mounts at right angles to the x-axis, so the motor severely restricts the maximum height of the z-axis. Another issue is that to remove it you have to remove the motor to get at the bolts. To remove the motor you have to remove the big pulley to get at the motor's bolts, to do that you have to remove the pinch wheel assembly. I.e. to remove the extruder you have to completely disassemble it!

Electronics

To get up and running quickly I used the same electronics that I use on HydraRaptor. The only difference being that I used MakerBot V3 stepper drivers. These use the A3977 chip and give x8 microstepping. That gives an axis resolution of 0.025mm, but more importantly gives nice smooth running.

When the weather was exceptionally dry I found they are very sensitive to static. A discharge to any part of the machine would cause the A3977 to shut down its outputs and draw enough current from the 5V rail to cause the 100mA regulator to current limit. The red LED on the power rail goes dim. Powering off and on again fixes it and there doesn't seem to be lasting damage. I suspect that might not be the case if the 5V rail was not current limited. Apparently the only way to fix it is to add external Schottky diodes. That is very disappointing as one of the nice features of the chip is that it is supposed not to need them. I will investigate further to see if all eight diodes are needed before making my own board.

Firmware

I used the same firmware as HydraRaptor. I just added some compile time conditionals to cope with two pin outs and a different IP and MAC address for each machine. I also had to change from 16bit to 32 bit positional commands because the axes are bigger.

Software

I used the same Python software as HydraRaptor but I had to re-factor it quite a lot to support both machines. I added a class to represent the Cartesian bot which holds the axis resolution, direction, maximum speed and acceleration plus the IP address. I also added a class to represent the extruder controller as I have calibration values unique to each board. I already had classes to represent thermistors and extruders.

I can run both machines at the same time from one PC and, because I only use the Skeinforge output for the toolpath, I can use the same sliced files for either machine. This is despite the fact that they run at different speeds and are loaded with different plastic.

Results

So here is the finished machine: -

And here is a video showing it being tested: -

I am running the X & Y motors at about 0.75A and Z at about 1A. I have set the maximum XY speed to 100mm/s, but I think it could go a lot faster. Z only goes at about 5mm/s because not only is it a threaded rod drive, but it is geared down by the belt and pulleys!

I haven't printed a lot yet, but so far the results look as good as they do from HydraRaptor. The next thing to do is add a heated bed and try ABS.

PMMA on PI

2010-02-17T12:51:00.008Z

Previously I described how I made some lamp shade clips for some friends from PMMA (Acrylic). Unfortunately the clips got lost in the post so I had to run off some more. I used a sheet of acrylic as the bed the first time, but since then I have developed my magnetic bed with PI (Kapton), so it was an interesting experiment to see if PMMA would stick to PI.

The originals were made with the bed at 100°C, but the insulating effect of 4mm of PMMA gave a surface temperature of only~85°C. That was sufficient to form a partial weld strong enough to resist warping. My first test was with the bed at 100°C, but that came loose after a few layers. With the other plastics I have found I need a higher temperature on PI because it does not form a weld. It sticks by some magic that I don't understand, possibly Van de Waals forces.

I tried again with the bed at 130°C and this time it stuck well but was easily peel-able by bending the bed. So another plastic that works on hot polyimide.

I have also been doing multi-part builds one layer at a time, something I hadn't dared to do until recently because my previous extruders were not reliable enough to risk a bed full of objects.

If you get the height just right on the polyimide bed the first layer comes out almost perfect. You can actually make one layer thick objects, i.e. 0.3mm in this case.

These came out very well.

I arrange multiple items by reading the STLs into AOI and orienting and positioning them. I then union them together in pairs and then union all the unions until I have a single object. I then convert that to a triangle mesh and export it as a GTS. If I don't convert first I only get one object in the GTS rather than the composite. I then slice as one object.

These springs show a very rare bug in Skeinforge or my code where it does a move with the extruder on. That is the only time I get any string.

Here is an assortment of smaller parts :-

It is best to group the taller parts together so when it gets to the higher layers the head does not have far to travel.

There were so many in this build that it overlapped the hot part of the table, so a few corners lifted a bit. I think ideally the table should be a bit bigger than the movement to keep some warm air around the objects. Perhaps a wall around the edge would keep some heat in.

I did have the crazy idea of building a wall around the objects as they are made so that there is a small gap of trapped hot air around them. I.e. the machine builds a disposable cocoon around the object to act as a heated build chamber. It would be a bit wasteful of plastic of course but it could be made from a cheap paste if we had a second head.

PLA on a vacuum bed

2010-02-11T23:52:00.006Z

Having successfully made one Mendel z-leadscrew-base_2off as an experiment to try ABS on a vacuum bed, I decided to make the second from PLA to see how well that works on a vacuum.

Three of the corners lifted very slightly during the build (about 0.2mm) but not enough to matter except to a perfectionist.

When the object cooled it did not break the vacuum, unlike ABS. The part was still easy to remove though.

The base is flat apart from the corners and a few shallow dimples.

The quality was very good with no clean-up at all. I recently discovered a simple bug in all my builds after I moved away from 0.5mm filament. With 0.3mm and 0.4mm filament my layer height was 0.24mm and 0.32mm respectively. The problem is my z-axis only has 0.05mm resolution, so layers alternated in height, none of them being spot on. That caused the sides of my objects to not be as flat as they should be as the filament width varied from layer to layer. I now use 0.375mm filament giving a 0.3mm layer.

Suck it and see

2010-02-11T00:57:00.002Z

The magnetic steel and polyimide tape bed works very well for manual operation but I am pursuing the vacuum bed idea for fully automated production. I went through a few designs in my head before actually making anything.

The first idea was to drill an array of holes through an aluminium plate and connect them on the back by milling a network of channels. I would close the top of the channels with some Kapton tape. The problem with that idea was there was then nowhere to mount the heating resistors unless it was on top of the Kapton sealing tape, which didn't seem ideal. My solution to that was to mill a channel into the edge of the plate and wind a coil of nichrome all the way round it. That was my plan until I realised there would then be nowhere to attach the vacuum hose.

A solution might be to use a Kapton or silicone stick-on heater and use it to seal the channels in the underside.

What I actually did was to mill a grid of very fine channels into the top surface allowing me to attach the vacuum hose to the side edge and drill a small hole down to meet it, leaving the bottom free for the resistors and thermocouple.

The channels are about 0.5mm wide and 0.5mm deep on a 5mm grid. I milled them with a 0.3mm conical bit that I bought for milling PCBs.

I used a feed rate of 2mm/s and 0.1mm cut depth per pass. My MiniCraft drill runs at about 20,000 rpm. The results were not very good. I have only ever milled plastic before with HydraRaptor. It struggled cutting aluminium such that the shaft of the drill was being displaced in the direction of the bed travel. It raised a burr about as high as the channel is deep. My friendly local milling consultant told me afterwards that aluminium does not like lots of flutes. He recommended a D-shaped cutter with a single cutting edge and a higher spindle speed.

I sanded the surface flat with 240, 600, 800, 1200 and 1800 grade wet-and-dry sandpaper and then polished it with metal polish. I did this to get as good a seal as possible with whatever was placed on top.

I attached some polythene pipe using an M5 copper welding nozzle screwed into a tapped hole in the side of the plate. I use a tapered tap so that the thread would bind to form a seal. I used Fernox LS-X jointing compound to make sure it was airtight. I think it is silicone, so should handle the temperature.

I was hoping to get a perfectly air tight seal and be able to use a static vacuum generated by a syringe. It doesn't seal fully though. I believe normal vacuum tables use rubber o-rings set into a groove to form a seal. I reasoned that would not work in this case because, whereas sheets of stock for milling are stiff enough to remain flat and squash the rubber, thin films would just bend upwards. My idea was that the thin film would be sucked into the channels and be compliant enough to seal it. I think it fails because the edges of the channels are too rough due to my poor milling.

I first attached a small vacuum pump that I made for my jukebox. It is just an aquarium pump with a pipe attached to the air inlet and the case is sealed with rubber glue.

It is not a very strong vacuum, but it is enough to pick up a CD with a suction cup made from the end of a child's rubber dart. I plan to use it for SMT pick and place soon. I measured it at 960 millibars, which is also the extreme low reading on our barometer. I knew that the vacuum it created was less than the atmospheric variation because I started off with an absolute pressure transducer on my jukebox to detect if a disk had been picked up. I had to change the trip point about twice a year because one setting would not work for both extreme high and low weather conditions. In the end I added a second sensor to make it differential.

I placed a piece of 0.075mm polyimide film on top. This is about twice as thick as the tape I use.

The video below shows the effect of the vacuum. It pulls flat and has some resistance to sliding but is not a very strong grip.

I built a Mendel part at 100°C for my first test. The film stayed flat during the build but a few corners lifted. When the part cooled it broke the vacuum and wrinkled the sheet. It was past the point where it had hardened so the base was perfectly flat apart from where the top corners had lifted slightly during the build.

I measured the temperature of the surface and found that it was 10°C lower than that measured underneath by the thermocouple. I raised the set point to 110°C and made another part. This time only one corner lifted (left side of the boss in the middle) .

Here is a speeded up video of the film releasing as the bed is cooled down to 40°C by a fan.

And here is me simulating removing the object by sliding the film. Ignore the ×16 annotation, I am not that slow!

The next thing I tried was a really big part of Mendel. I didn't trust my weedy aquarium pump to hold it down so I used a 1/4 HP 180W 3 cubic feet per minute pump rated to go down to 0.1 millibars. I bought it for £150 over two years ago to make a vacuum bed for milling but never got round to it, so it has been sitting on a shelf, like a lot of other parts and materials I have bought for experiments but not had time to use.

When connected directly to the vacuum gauge with a length of plastic hose it goes down to about 30 mb. I think I would need better quality fittings and pipe to get down to 0.1 mb. When connected to the vac table it gives 40 mb, so although it does leak, it still gets most of the available downforce from atmospheric pressure, i.e. ~15 lbs / square inch.

The part I made ended in disaster because the vacuum broke during the build. I think it was mainly because the object was not quite centred on the table so its outside perimeter was on top of the last vacuum channels. Before it failed some corners had lifted a little, early in the build, so it looked like the ABS does not stick to film as well as it does to tape.

I centred the table and made a slightly smaller piece. Actually this was the same depth as the last piece, so the perimeter falls about half way between the last two channels. Ideally I think you don't want to be that close to the edge.

I raised the temperature to 120°C, so the top of the film was probably ~110°C.

The film stayed vacuumed down during the build but still broke the vacuum and wrinkled during the cool down period. That means that even with close to the maximum vacuum it cannot hold the contraction force. Not a big surprise as I realised a long time ago the warping can generate a lot more than 15lbs / square inch of pull. The only reason I thought this might work is because the plastic does not warp while it is kept hot and indeed the vacuum holds during the build. The problem is that the object does not stick to the film well enough. One corner peeled early on and lifted further as the build progressed. The rest of the base is flat though, so the part is easily good enough to use. The higher temperature and vacuum meant that the grid lines are just visible on the object base if you get the light right.

So just to make sure I can make an object this size on tape without warping I made the larger part again on my magnetic bed. That failed because the bed slipped part way through. I knew that was a likelihood and that I need to add a couple of dowel pins in the corners, but I didn't want to do that while I was experimenting with vacuums. I gambled on it not slipping and lost.

It did build enough to show that it sticks much better though. The corners stayed down and the build lasted long enough to go way past the point where the corners lifted on the vacuum bed. The base was perfect.

I could only think of three possible reasons why the corners would lift on the vacuum bed and not on the magnetic bed.

The surface of film might be different to tape. After all, tapes have the magic property that the glue only sticks to one side and peels from the other without leaving any residue.
The film is thicker, so has more thermal resistance, which might have some influence.
Perhaps the film can lift a little in between the vacuum channels allowing the plastic to peel away and then be sucked flat again.

My best guess was that the third explanation was the most likely. Perhaps closer spaced channels or thicker film would solve it. I had a sample of 0.15mm film so that was the easiest thing to try next. It stuck much better but towards the end of the build I heard a snapping sound and saw that one corner had lifted.

More importantly though I could see that the other corners were deforming the film upwards as I had suspected. It is hard to see here, but the slightly raised blister of film was over a channel, so subject to the full vacuum force.

This shows that even with a heated bed, there is sufficient warping force at the corners to beat atmospheric pressure. The part ended up with one chamfered corner and dimples in the others.

So in general the experiment is a failure as it does not work as well as the magnetic bed. It does allow easy automated removal though. All you need to do is tape down one edge of the film. When the object cools it wrinkles the film and breaks the vacuum. A fence could then push the object off the bed with not too much force, as the film peels easily. When the object is gone the film springs back to being flat and the vacuum can pull it down again ready for the next object.

Although corners lift, the objects are usable for making a Mendel, it is only an aesthetic issue in this case. I think rounding the corners of the parts would fix it. I guess PLA might stay stuck as it warps less, and ABS only fails in extreme cases.

Thanks to Paul for providing the polyimide film samples and lending me the vacuum gauge.

Quick release bed

2010-01-31T01:10:00.000Z

I am in the process of making a heated vacuum table to hopefully allow automatic ejection of finished objects. In a conversation with Laszlo he mentioned he was planning to use a heated steel bed and use magnets placed around the object to hold down a sheet of Kapton. I turned the idea upside down. Why not stick Kapton tape to a sheet of steel and clamp it to a heated aluminium bed using magnets underneath?

I found a thin sheet of bright springy steel that was part of an electric toaster. My best guess is that it is one of the grades of stainless steel that is magnetic. It is only 0.3mm thick so it is relatively flexible, but it always springs flat. It came from a Kenwood toaster that gave good service until our cleaner suggested to my wife that she should turn it upside down to get rid of some persistent crumbs. The next time it was used it burst into flames because a crumb got wedged between the element and the steel plate and burnt through the nichrome.

I made a tiny heated table from an off-cut of 6mm aluminium. It is only 105mm x 73mm, which is smaller than a MakerBot CupCake bed but I think it is just big enough to make all the Mendel parts.

I have run out of AL clad resistors so I made my own from vitreous enamel ones embedded in aluminium blocks with tin foil. I used two 6.8Ω resistors in series driven from ~ 26V AC. That gives about 50W and a similar warm up time to my larger bed driven with 200W.

I milled flat bottomed holes to within about 1mm of the surface and embedded five neodymium magnets which are held in with Kapton tape.

I used M3 threaded nylon stand-offs as insulated table legs and mounted it onto my XY-table using a sheet of 4mm aluminium / plastic laminate called Dibond. It is very nice material to work with.

The steel plate covered in Kapton tape then sticks to the top of the table. I heated it to 100°C and tried making some ABS objects.

This worked well and the objects were easy to remove by bending the plate and peeling them.

The magnets are strong enough to hold down even big objects. The only problem I had was that the nozzle snagged on the first layer of this object and managed to slide the steel plate, causing the first layer to be offset.

Contrary to popular belief, FFF does require significant force and benefits from a stiff extruder mounting.

A couple of pins in the corners to act as dowels would solve the sliding problem.

Here is a video showing how easy it is to remove the objects: -

It is still a manual process though, so I will pursue the vacuum table idea to attempt to make a bed that can eject the object itself.

Will it stick?

2010-01-23T12:01:00.004Z

ABS sticks very well to hot Kapton, so I wondered what else would stick to it. The first thing to try was PLA. This sticks pretty well to cold masking tape and doesn't warp much, but large objects do have some warping. I figured heating the bed to around 50°C would fix that. Rather than changing from Kapton to masking tape I decided to see if I could stick PLA to Kapton and get a shiny surface as well.

The first bracket was made on cold masking tape so the base has a matt finish.

The second one is on Kapton at 50°C for the first layer, dropping to 40°C after that. My logic was to have the bed just above the glass transition to make it stick and just below afterwards to stop it warping. As you can see one of the hole outlines did not stick properly. The PLA was extruded at 200°C for the first layer and 180°C for the rest.

For the third one the bed was at 55°C falling to 45°C. The outline stuck properly and the base is nice and shiny. The surface imperfections you can see are from gouges in the aluminium bed caused by a slight accident with a decimal point. It caused the nozzle to be rammed into the bed and then the X-Y movement ploughed furrows. These show up through the Kapton tape.

The last one is my first ABS test for comparison.

It was looking good, so I tried something bigger, a Mendel belt splitter jig: -

The left hand corner lifted and the object ended up more warped than it would have been made on cold masking tape.

I tried again with the bed at 55°C all the way through the build. My extruder started jamming so I increased the PLA temperature to 210°C for the first layer and 190°C for the rest, the values I had been previously using on cold tape.

This time it was successful and stayed stuck down: -

The base came out perfectly flat and more transparent: -

The extrusion lines of the three solid base layers are less visible and you can see through to the sparse infill. This is only 25% but the object feels incredibly strong. I get the feeling the hot bed makes things stronger.

There is a bit of a meniscus around the edge. This is mainly because I had a bodge of a -0.1mm offset in the first layer outline to get PLA outlines to stick to tape reliably. I removed the bodge and made this object: -

The base layers are very transparent here, even more so to the naked eye than the camera shows. There is something a little odd with some of the extrusion lanes above the bottom left hole. I think those discontinuities must be the plastic squirming a bit while extruded, which is usually a sign of not being stretched enough.

The top of the object has a small defect: -

There is a small hole above and right a bit of the centre. I think this is because the plastic doesn't span gaps as well without a fan, so it fails to bridge the sparse infill properly. I wasn't watching so I didn't see exactly what went wrong.

The next plastic I tried was HDPE. Not surprisingly it doesn't stick very well to hot Kapton. With the bed at 130°C it stays molten but is quite rubber like. With the bed at 110°C it sets and turns white (because it crystallises I believe). I tried various combinations of these two temperatures but could not get it to stick reliably. I could lay down the first layer of a raft but then subsequent layers would rip it up as the adhesion is very low.

I think the way to do HDPE without a raft is to extrude it onto a thin sheet of HDPE, or maybe polythene, held down by a vacuum and heated to prevent warping. That will have to wait until I build a little vacuum table, hopefully this weekend.

Last on the list was PCL. That sticks very well to Kapton heated to 40°C but it never sets and makes a soggy object.

Before the heated bed I used to build with a fan, and at only 40°C the bed has no trouble holding temperature, so I tried with the fan next.

That worked OK and built a complete object: -

The infill did not stick very well to the outlines of the holes, especially on the downwind side. It probably needs a denser infill, and perhaps some overlap. 25% fill is not really appropriate for PCL as it very soft and flexible.

The bottom is smooth and shiny as expected and it took some effort to peel it off, so I expect large objects could be made. I couldn't experiment further though because the filament started buckling in my extruder.

I can't explain why it worked for a while and then stopped but I tried higher temperature and slower extrusion but could not get it reliable again. The pipe could probably be a few mm closer to the pulley but not much more because it would hit the pinch wheel.

I don't have a lot of use for PCL, other than using it up. Dropping it from the requirements for the extruder would allow me to use a smaller pulley. If you look at the table at the end of this article, you can see that it is only PCL that struggles for grip with a worm pulley. I think I could drop to half the diameter, which would just about bring the gear ratio into the range of a single pair of spur gears. I have a 4" Meccano gear that gives 7:1, so I might try that in my next extruder.

So hot Kapton works well for everything I have tried so far apart from HDPE.

Stepping up to the mark

2010-01-11T00:28:00.015Z

My wife is a very measured person. As well as watching how much power we using she likes to count her footsteps with a pedometer to check if she is getting here daily quota of 10,000. I have bought her several pedometers but they generally come to an early demise due ti inadequate belt clips. The last one fell into the toilet! The first one I bought was the best, but the belt clip broke off.

I promised to RepRap a new clip a long time ago, but only got round to it today. Neither of us could remember what the old clip looked like so I designed a simple one from scratch. To my surprise it printed perfectly on the hot bed, I thought it might need some cooling. HydraRaptor will have automatically dropped to half speed because the layers are so small.

I cut off the remains of the old clip and filed it smooth. I then removed all traces of grease with some isopropanol and welded it on with some MEK pipe cement. A friend gave me it anticipating that I might want to weld ABS someday. It will dissolve and weld ABS and PVC. I think the case is ABS, so it is ideal for the job. It needs 4 hours to cure, so I left it overnight.

It seems to have done the job. I offered to make it in black but my wife wasn't bothered.

The next problem was that the batteries had gone flat in the years she has been waiting for me to fix it. Buying specific batteries is expensive but you can get a mixed selection of 40 for £1. The problem is though that we mainly use the biggest ones so have too many of the smaller ones. I had some that were the right diameter but too thin so I Reprapped some spacers.

I can now make objects side by side one layer at a time with no strings between them. These are probably the smallest things I have made. They are about the size of tiddlywinks.

A bit tricky to keep in place while the battery cover is replaced but they did the job.

So I am back in the good books for a while. I managed to run off two of these as well. They take about 90 minutes each and are perfectly flat and string-less again.

Just the odd lump on the surface at the start or end of an outline. I think that can be easily solved by always starting on an inner shell before doing the outer shell and then finishing the outer shell with a wipe towards the inner one.

Golden wonder

2010-01-10T09:24:00.017Z

My first attempt at extruding ABS onto hot Kapton had "all the stops pulled out" to make it stick, i.e. 120°C bed, nozzle height 0.1mm too low, very slow outline and infill on the first layer (4mm/s). The adhesion was very good so I decided to back off a bit. It is not a good idea to change more than one thing at a time but I did anyway. I got rid of the -0.1mm Z offset and sped up the first layer infill to 32mm/s, leaving the outline at 4mm/s. I also dropped the bed temperature to 80°C. That was too low, the corners lifted about 1mm during the build, but I think the part will still be usable.

The base is still glossy but you can see and feel some valleys between the extrusion "lanes". The next test was a binary chop with the bed at 100°C.

This is perfectly flat, even when off the bed for a day, but the extrusion lanes are still noticeable. The next test was at 110°C.

The extrusion lanes are gone in most places but a few are just visible. The first one that I did at 120°C has no extrusion lanes on it all, just some very slight graining from the Kapton tape that you can also see on the picture above. The tape lines and grain go from bottom right to top left. The extrusion infill slopes bottom left to top right and is only visible on the right hand side of the object. I think perhaps Z has to be a bit lower to get rid of them completely, but it is only important if you want to make something aesthetic, like an instrument panel, for example.

Of course there are the tape join marks. I used unbranded polyimide as it seems to be about half the price of branded Kapton. I got it from here, which is very cheap and free shipping if you don't mind waiting a while. You can get polyimide tape up to 250mm wide, but it is always on a 33m roll, so it gets very expensive. I have ordered a 150mm roll to cover the working area of HydraRaptor's build table. It was £53.71 from here, so very expensive, but a small price to pay for perfection! I don't know when it will arrive as post is a nightmare at the moment. I am still waiting for things from the 17th of December. Parcels are not being delivered because of the snow, so you have to go and collect them, but several letters and packets seem to have disappeared.

Here are all the tests side by side, notice the colour change with temperature, it is a bit exaggerated on the photo : -

I now have a full set of Mendel vertexes including two that I made in PLA that warped slightly (on a cold bed). I moved onto something more ambitious on the warping front: the Mendel x-carriage-lower_1off part. I don't think this is printable in ABS without a heated platform, or air stream, unless you use the apron method developed by Forrest Higgs. For this test I started the bed at 120°C and dropped it to 100°C after the first layer. The logic being that 100°C seems to be enough to prevent warping, but 120°C is needed to get a perfectly smooth finish. It takes a few layers before the temperature has dropped to 100°C as I don't wait for the plate to cool down.

Unfortunately the ancient version of Skeinforge that I use gets one layer wrong on this part. The layer has the central hole missing. The filament didn't span the void very well as it is a very big void, I have no fan running and there is a lot of heat rising from the bed. That caused some filament to stick up and collide with the head. It spun round 90° unscrewing it 1/4 of a turn. Amazingly it did not leak but the nozzle hole must be slightly off centre with respect to the barrel thread, so I got an offset in X and Y above the layer that went wrong. Still, the objective was to test warping and it came out totally flat.

The corners have a dimple that looks like an air bubble, but must be something to do with them trying to lift I think. Apart from these the base is as flat as glass and had it not been for the Skeinforge bug it would have been usable straight off the bed. I cut the membrane out with a knife and drilled through the blinded holes before taking these pictures.

I tried the x-carriage-upper_1off starting the bed at 120°C for the first layer and dropping to 90°C. Again Skeinforge got it wrong, not surprising as the topology is very similar. This time I also dropped the filament temperature to 220°C, so it spanned better and the head did not get spun. A longer snout on the nozzle might be a good idea to avoid collisions with build defects.

Again here it is with the membrane removed.

The corners lifted very slightly but the rest of the base is completely flat. It doesn't rock on a flat surface like an object made on a cold bed would. In fact, the raised corners made it easier to remove from the bed.

So it looks like 100°C bed temperature is the minimum to prevent warping when using Kapton. 120°C for the first layer gives a better aesthetic finish, perhaps with a small negative z-offset. Having the object kept warm seems to allow a lower filament temperature without losing strength. I used to build at 240°C and use 0.5mm for stronger objects. I can now use 220°C and 0.4mm with no sign of de-lamination so far. The lower temperature is good because the ABS out-gasses less and so smells less.

I can't recommend Kapton on heated aluminium highly enough. It has transformed my experience building with ABS completely. I no longer need a raft, which saves a lot of plastic, time and labour to remove it. My objects can be completely flat, smooth and glossy. Together with using a geared stepper extruder drive to completely eliminate ooze it means I just print an object, remove it from the bed and it is ready to use. There is a slight meniscus of plastic around the base, which you might want to remove with a file or a knife.

It has several advantages over acrylic: -

Acrylic is a good insulator, so even 3mm reduces the surface temperature by about 15°C, making it take longer to warm up and harder to control.
It tends to warp as it has a similar glass transition temperature to ABS.
It can be hard to remove the object as it can be permanently welded if you deposit the ABS hot enough.

The way ABS sticks to hot Kapton is different. The Kapton does not melt at all so you don't get a weld no matter how hot the ABS is. I don't know what sort of bond it makes, but it is always peel-able.

While I have been writing this article a friend came up with a brilliant suggestion. Why not use non-adhesive Kapton film, clamp it on the table, possibly with a vacuum? When the build is finished just release it so it can be peeled off the object with ease. I realised that would enable a conveyor belt table to be made. People have suggested this would allow a machine to churn out parts unattended. E.g., stretch a band of Kapton over a heated plate and rotate it when the object is finished and has cooled. The object will then drop off the end.

I still have a couple of problems to solve with the heated bed. The heat spreads downwards and warms my X-Y table. It is not much, I haven't measured it but I would guess to mid 40's C. That is enough to expand the aluminium that the table is made from and open up a gap in the ways so that it has some play and starts rattling. I removed the foam-board to leave an air gap (the logic being that the movement of the table would generate some cooling airflow) and covered the top of the bed with aluminium foil to reflect the heat back. That helped, but not enough. I think I will need to blow cold air over the top of the table with a sheet of something like PTFE to cover the bottom of the heated bed.

The other issue is that having heat around the object rather than cold air blowing on it means that void spanning and overhangs don't work as well as they did. I think I need a jet of warmed air directed at the end of the nozzle to cool filament to freeze it quickly.

Hot metal and serendipity

2010-01-05T16:48:00.014Z

I couldn't get to work today because we had seven inches of snow during the night and a couple more today, so I had an extra day of RepRapping.

So my extruder is back working after re-fixing the thermistor with some RTV silicone. I get a degree or two more temperature swing with silicone compared to Cerastil, so not ideal, but it is workable. I think the plastic has such a high specific heat capacity and thermal resistance that it probably averages out the temperature swings anyway.

I switched to ABS to make a change from PLA as I am now able to use my 5kg spool of oval ABS that has always been two wide for my previous extruders. The bore of this one is 3.6mm, which is actually a bit on the big side for 3mm filament. I think about 3.3mm would be the best compromise.

My first experiment was to see if I could extrude directly onto my heated aluminium bed. My initial attempts failed to stick, even at 110°C, but I found that I could lay down a raft. I always cool the raft before applying the first layer of the object (I also drop the temperature of the first layer to 190°C), otherwise it welds too strongly to remove. When I cooled the raft it detached from the bed, presumably because it shrinks.

I reasoned if I could get the raft to stick then I should be able to get the object to stick. The difference is I do the first layer of the raft at 4mm/s and have the head lower than I would normally, so that the filament is squashed more. I tried making the first layer of the object at 4mm/s and a little lower than it should be. It almost worked so I upped the temperature to 120°C and tried again. This time I was able to make one of Zaggo's whistles.

When it came to making the pea it got too hot and started moving around.

Normally I would use a fan on ABS to get small items to hold their shape, but obviously blowing cold air onto a hot base is going to waste a lot of power. The fix I have in mind is to blow a very small jet of air at the same temperature as the base and aim it just below nozzle. Hopefully by keeping the jet small I can avoid the sort of power that hair dryers use. Adding the heated bed has increased the power consumption of my machine by about 50W, which has more than doubled it.

When I cooled the finished object and the bed to 40°C, by running the fan, the object simply lifted off. At 120°C the ABS is like a soft rubber or gel. It clings to the aluminium, but will peel off with very little force. When it cools it becomes completely detached.

The bottom of the object is smooth and shiny and perfectly flat. I can actually see part of one of the swirls that are on my bed if I catch it right in the light. That means the plastic takes the texture of the base, so you could pattern and texture it in the same way as injection moulds.

The next thing I tried was a Mendel vertex bracket as these are big enough to warp. It managed the outline, but when it started doing the outlines of the holes the filament failed to stick so I aborted that build.

The obvious way to get more grip is to use a sheet of acrylic as many people report that works well. I have a couple of problem with that though. Acrylic is a good insulator so the temperature control becomes more difficult. It tends to warp unless it is held down at the edges. I don't have any bolts long enough to mount it on my bed with the frame on top. I ordered some 2BA studding last year, but all the post from just before Christmas has gone missing.

I looked around for a piece of metal with some texture and found some aluminium with a satin finish painted with metal primer, from a very old experiment. It looked promising to start with: -

But it soon snagged and started ripping it up again: -

However, as you can see, I held the plate down with Kapton tape and by accident part of the object was extruded onto the tape. It stuck well to the Kapton but was peel-able. This looked extremely promising. Kapton on top of aluminium could be the perfect bed material for ABS. It looks like it will be reusable many times, as masking tape is for PLA.

The bracket stayed perfectly flat during the build. I cooled it with the fan to 40°C. It was quite difficult to remove. In the end I put a penknife under one edge and tapped it with a hammer. It came off cleanly and with a perfectly flat base with a glassy appearance.

The only blemishes are the gaps in the tape, what looks like an air bubble in the tape, and the dent from my penknife.

The base is a slightly golden colour and that extends up for the first few layers so I think the bed was a bit too hot. I had it at 120°C and the first layer at 4mm/s, so I will have to back track a bit and see if I can get away with a lower temperature and faster first layer, but this is looking very good. No warping, no raft, a cheap reusable bed material and a mirror finish.

Extruder broke already

2010-01-03T21:35:00.001Z

Well my best attempt at making a reliable extruder again resulted in one that only lasted a few weeks! The brass worm pulley that was pushed onto a splined shaft worked loose while extruding PMMA.

PMMA is quite hard work to extrude, but probably no worse than HDPE. On reflection splines into brass are not going to hold the massive force that occurs at 2mm radius. A better idea would be to have a boss on the side of the pulley and use a set screw onto a flat on the shaft. I would also add smaller diameter bosses at each side to meet the centre rim of the bearings. That would automatically position the pulley dead centre.

But to do that I would have to make a new pulley cutting jig and redesign the motor bracket to be a bit wider. I would need a working extruder to make the new bracket of course, so I decided to bodge the existing design.

I drilled out the centre of the pulley to 6mm and then reamed it to 6.4mm. I then turned a steel hub from a piece of hex pillar. I made it about a tenth of a millimetre oversized, added a chamfer to the hole in the pulley and forced it in with a vice, creating a very tight fit.

I didn't trust that to hold on its own so I left a hex flange on the other side and soldered it to the brass: -

Certainly not my best soldering, but bodging is bodging. The hub is twice as wide as the wheel and steel is harder than brass, so it should have a much better grip on the splines. I don't know if it will last or not. The constant back and forward motion of the anti-ooze fix means that if anything is weak it gets worked loose.

With the repaired extruder I made a third lamp shade clip leaving 1mm of the acrylic rod left above the pulley, how lucky is that?

Then I pushed my luck too far. When I bought the 3mm PMMA rod I also got a 2mm rod to compare results. Stiffness of a rod is a fourth power on diameter I think, so 2mm filament is five times more flexible than 3mm.

This would certainly be feasible to use in coils as it has a similar minimum bend radius to 3mm PLA, we just need to find somebody to supply it in that form at a reasonable price. 2mm rods are even more expensive than 3mm rods, £1.24 on eBay as opposed to £1.49, but are only 44% of the volume!

I decided to give it a try in my newly repaired extruder by printing a whistle. I had to scale it down because with 0.4mm filament it would use more than 1m of 2mm filament, so I printed the same g-code using 0.3mm filament and scaled the dimensions accordingly.

It managed to print a couple of layers and then the extruder jammed. I think the problem is that with a 3.6mm bore and 2mm filament there is too much of a gap, so molten plastic can flow upwards and freeze in the cold part of the tube above the taper. I think it would work fine with an extruder designed for 2mm filament. The drive mechanism just about works because although it does not have as much grip, it only needs 44% of the force that 3mm filament needs. The barrel and heater block would need a smaller bore though and could be made smaller. Similarly the smaller motor I used before would have plenty of torque, in fact a high torque NEMA14 should work.

So there are a lot of advantages to using 2mm feedstock like commercial machines do, BUT stiffness falls as a forth power, but force required only falls as a square law, so I expect soft plastics like HPDE, PP and PCL may buckle when being fed. Certainly the gap between the pinch wheel and the barrel entrance would need to be very small.

I fixed the jam by putting a drill down the hot barrel and hitting it with a hammer. That fixed it and I hand fed some ABS before reassembling the extruder. After assembly it would not work at all. The thermistor had shorted out to the metal work!

Nothing much to see from the outside, just a weird furry slimy deposit on the back of the AL tube and a green stain on the thermistor lead that was shorted.

I cannot get to the thermistor or heater without removing the PTFE cover, but that can't be removed without unscrewing the barrel, another slight design flaw. If I had tapped the stainless steel pipe all the way up I could just unscrew it from the AL tube that surrounds it, but it is really hard work tapping stainless steel.

I unscrewed the barrel while the extruder was hot to reveal this mess: -

The plastic that leaked when I first built the extruder has been stewing for weeks and has boiled down to something resembling bitumen. I expect the more volatile products condensed on the cold AL tube above it forming the Vaseline like deposit.

I couldn't tell why the thermistor was shorted because it came away with the PTFE cover. The Cerastil that I glued it in with seems to have decomposed in the chemical soup around it. My last few attempts at sticking thermistors with Cerastil have not been very successful. I am not sure if I mixed it to the wrong consistency, or if it is now too old to cure properly. It doesn't look any different, but instead of rock hard cement I seem to get something crumbly.

I cleaned it all up and stuck the thermistor back in with RTV silicone. I am sure it is not as conductive as Cerastil, but over such a short distance (between the thermistor and the wall of the hole it is in) I am hoping it will not have much effect.

I made the hole for it a bit deeper and opened out the top so it was big enough to accommodate the PTFE sleeving as well. That should keep it from touching the metal. It is surprisingly difficult to glue something into a small hole with a viscous glue. It is hard to get the glue to go down the hole without leaving an air pocket. A better idea might be to drill out a small screw, all the way through, fill it with glue from both sides. Then when it has set simply screw it into a tapped hole in the heater block.

I am waiting 24 hours for the silicone to cure now, so back to work tomorrow and less blog posts.

New Year New Plastic

2010-01-01T20:49:00.004Z

Just over a year ago a friend asked me to make replacement for a broken clip that was part of a light fitting. It was not too difficult to model and I made a copy in ABS with 0.3mm filament, 0.24mm layers.

It did the job mechanically, but with one obvious aesthetic problem: -

The original clips were made from transparent polycarbonate and all I had at the time was green ABS. I didn't use PLA because I worried the lamp could easily get hot enough for the clip to go soft and drop the shade. The only transparent thermoplastic that I could get hold of in filament form was PMMA (AKA acrylic / Perspex, etc), which is available in 1m rods. It is too stiff and brittle to use in my previous extruder, so I promised to have a go when I moved to a pinch wheel design.

The first attempt was a complete failure. It melts at 130 - 140 or 165°C depending where you read. It has a relatively high glass transition, 100-114°C, again depending where you read. I found I could extrude it with a fair amount of force at 180°C. It is very viscous with plenty of die swell. I couldn't get it to stick to anything, including itself, at that temperature though. It isn't sticky like PLA, so it wouldn't stick to masking tape. The obvious second choice was a sheet of acrylic as all thermoplastics will stick to themselves.

The general rule of thumb to make plastic weld to itself is that the average of the temperature of the hot part and the cold part has to be higher than the melting point. So to get it to stick to the base, which is at room temperature it would have to be extruded at twice the melting point minus the ambient temperature. The only plastic that seems to break this rule is PLA which melts at 160°C but will bond to itself at 180°C. I think it is something to do with it having a low glass transition and / or that it is sticky like a glue when it is molten.

I upped the temperature to 240°C but it started to hiss and smoke and still did not bond to the base. Lots of places quote the boiling point of PMMA to be 200°C! I dropped the temperature back to 220°C and it is much happier, but still does not stick .

So the only way to make it bond with itself is to raise the ambient temperature. Cue the heated bed. I set the temperature of my aluminium plate to 100°C, the hottest it can safely be below the glass transition. I taped a small scrap of 3mm acrylic sheet to the middle of the bed with Kapton tape. From my experiments before I estimate the surface temperature would be about 85°C. That gives an interface temperature of about 150°C and that seems to be enough to get it to bond to itself.

Here is a short video of 0.3mm PMMA filament being extruded at 16mm/s: -

Here is the finished object: -

It was not too difficult to release from the bed with a penknife once the bed has cooled that is, I keep forgetting that it is hot! The bed takes ages to cool unless I blow it with a fan.

I am very pleased with the final result. I only had 1 meter of 3mm filament to get this right and I managed to find a suitable bed material, temprature settings and make three clips. The build quality is excellent even if I say it myself.

So another useful material in the RepRap arsenal. Apart from HDPE I think it has the highest working temperature. It is very stiff and brittle though. I had a couple of jams due to it snapping where it enters the extruder barrel. The alignment is not quite right because being so hard it does not press into the worm pulley as far as other plastics. The extruder could do with an adjustment there perhaps, or a bigger entrance to the pipe.

It is a bit more transparent than PLA. It smells a bit more when it is extruded, but it is not an unpleasant smell, I would describe it as sweet and aromatic. The major downside is that it is only available in rod form, so the biggest object you can make in one go is 7 cm³ and at £1.49 per meter on eBay, it is comparatively very expensive.

Hot bed

2010-01-01T15:58:00.006Z

Making a heated bed to combat warping has been on my "to do" list for a long time. In fact I ordered the materials more than a year ago. My plan was to use an aluminium plate with many small power resistors screwed on the back.

The plate is 8" square to match my table and 6mm thick. A friend with a CNC machine shop kindly machined it for me. It saved me a lot of hard work with a hacksaw and file and looks a lot better as well.

I estimated that it would need about 50W to raise the temperature to 100°C, so I aimed for 100W to give a reasonable margin for control. I used 9 10W 12Ω resistors wired in parallel. Driven from 12V this would take 9A giving a power of 108W.

The holes in the resistors are only big enough for M2 screws. I drilled blind holes and tapped them with a plug tap, actually a broken tapered tap that I ground to a flat end.

Tapping small holes in aluminium is tricky, that was how the tap came to be broken in the first place. The correct size hole for M2 is 1.6mm but I drilled it 1.7mm to make it easier to tap. In fact aluminium is so ductile that the peaks of the thread are still the correct diameter. I.e. the 1.7mm drill would not fit the hole after tapping and the thread was a good tight fit on the bolts. I used paraffin for lubrication.

Soldering the resistors was fun.

I used stout wire to handle 9A and high temperature solder because I fancied using it as a hot plate for soldering. My 50W iron did not have enough power to melt the solder when the resistors were mounted with two thick copper wires leading from them. To get round that I placed it on a silicone matt and powered it up to raise the temperature to 100°C and then soldered it while hot and live, not something I would recommend. As the iron bit is grounded I had to solder all the 0V connections first and then swap the polarity.

The original plan was to power it from a 12V PC power supply and switch it with a big MOSFET. Initial tests with a bench power supply showed it took about 15 minutes to warm up to 80°C. When calculating the power I had forgotten take into account the specific heat capacity of the thick sheet of aluminium. I didn't want to add 15 minutes to the build time, so I decided to double the power. I have abused these resistors before and got away with it. I changed the wiring slightly to make a series parallel combination with a total resistance of 12Ω and fed it from 48V AC giving 192W.

I used a big 350W transformer and controlled the mains to it with a solid state relay. Since the temperature is controlled there is no real point in using a regulated DC supply. It is much more efficient to use AC and avoid the losses associated with rectification and smoothing. It also allows me to use the same control hardware and firmware that I used for the SMT oven.

I made some PEEK insulating stand-offs to mount it on my XY table with a gap of about 6mm below the resistors: -

I wrapped the feed points around two of these to make the transition to a lower temperature with PTFE sleeving before using normal flex to handle the movement of the table.

I also added some foam board to insulate the top of my X-Y table.

This just fills some of the air gap under the plate to prevent air circulating and convecting heat downwards.

I made some PTFE washers to go under the nuts that hold it down by slicing up a failed extruder insulator: -

These deformed considerably when I heated the table to 230°C, highlighting why PTFE insulators fail when used in an extruder.

Here is the final result mounted on the machine: -

I added Kapton tape around the edge as I thought it would stop hot air escaping from underneath, but it didn't seem to make a lot of difference.

Here is the open loop response at full power: -

Although it can reach the required temperature, it is much too slow for SMT soldering. It needs to be able to rise at about 1°C / second for that. So I will stick to using the oven for soldering for now. I was hoping to be able to paste boards, place components and then solder with the board still on the table, but it obviously needs a lot more power.

Here is the response using bang-bang control from the host at one second intervals.

Some analysis: the initial rise rate is about 20°C in 75 seconds. The specific heat capacity of aluminium is 0.9 J /gK and the total weight of the bed plus resistors is 700g. So with 192W the time taken to rise 20°C should be 0.9 × 700 × 20 / 192 = 66 seconds, reasonable agreement as we ignored any heat loss.

The initial fall rate is 5°C in 85 seconds while at a temperature of ~80°C above ambient. So the rate of heat loss is 0.9 * 700 * 5 / 85 = 37W. Looking at the steady state the power is on for about 1 in 6, which would be 32W, so again reasonable agreement.

The plate is ~ 200mm square so its area is 0.04m² so it looks like we need about 1kw / m² to reach the sort of temperatures needed for HDPE and probably twice that to have reasonable warm-up time and control. Mendel's build area is also 200mm square, so would require a similar power.

You might have noticed the thermocouple is covered with a piece of ceramic cloth in the photo above. This is what happens if it is just stuck down with Kapton tape:-

You can see that as the temperature rises you get increasing thermal noise. Even with the ceramic cover in place you can see similar noise on the open loop test when the temperature was much higher. I think the reason for this is the convective air currents causing chaotic air turbulence. If you think about it you have hot air rising but, away from the edges, the only way cold air can replace it is by falling through the rising air.

A better place to put the thermocouple would be under the bed to avoid the convection currents, but I wanted to try controlling the surface temperature when it was covered by a bed material. Here is what happens with the thermocouple on top of a 3mm thick sheet of smoked acrylic: -

The set point is 95°C in this case. Clearly a case where bang-bang does not work too well, with 5°C overshoot and 3°C undershoot.

The acrylic loses about 15°C between the bottom and the top surface. That makes it curl upwards, so it would need a frame around the edge to hold it down. Fortunately I have one made from HDPE laminated with aluminium so it should stand the heat. It also adds a significant time lag.

Another problem is that acrylic has a glass transition at about 114°C. When the control was of the top surface temperature, the bottom surface exceeded that during the overshoots and went soft.

So I will need to implement PID for top surface control, but I had a suspicion that a transformer was not going to like PWM into its primary much. Anyhow I put the thermistor back onto the plate and moved the bang-bang control from the PC to the firmware in preparation for building something. Bang-bang was an apt name for what happened next. When the temperature crossed the set point it started dithering the mains on and off. The transformer sounded like it wanted to jump off the desk and then blew its 3A anti-surge mains fuse.

The solid state relay turns the power on at the zero point crossing of the mains, and off when the current is zero. Current builds up slowly through an inductor so what could possibly be wrong? I had noticed big transformers thump when you connect them to the mains, but I had always assumed it was because the secondary usually has a big smoothing capacitor to charge up. However, this was a purely resistive load, and even with no load attached the transformer thumps on start-up, so some reading up on transformer theory was required!

It turns out that transformers take a big surge current and turning on at the zero crossing point is actually the worst point to turn them on. The reason is that when a transformer is running, being an inductor, the current lags behind the voltage by 90°. So normally when the voltage is crossing zero, the current is at its maximum reverse polarity and over the next half cycle of voltage it goes though zero and then to its maximum positive value: -

If the current starts at zero then over the first half cycle it will rise to twice its normal value: -

That would not be too bad except for the fact that transformers usually run with their core close to magnetic saturation for efficiency reasons. That means the core saturates during start-up. The inductance disappears and then the only thing limiting the current is the DC resistance of the primary, about 3.3Ω in my case, so the current can be enormous. Counter intuitively, the best time to turn a transformer on is when the mains is at its peak voltage.

So I learned something I didn't know about transformers. The fix was simple, I added a solid state relay to the secondary circuit and plugged the transformer into the mains. Bang-bang control then was able to pulse it very quickly due to the dithering caused by noise, which ends up with some proportional control, that reduces the temperature swings to a fraction of a degree.

So I can add PID control firmware and it can be shared with my oven control but I have an extra solid state relay and a big transformer. A better solution would be to pick the resistor values to give the correct wattage when wired in series across the mains. Of course mains on a moving table is not the safest design. I would use a heavy three core flex with an earth lead to the plate and a second independent earth strap for safety.

It turns out the first use for my heated bed was not to combat warping, but actually something more essential, details coming soon ...

Cooking with HydraRaptor

2009-12-29T12:30:00.001Z

I needed to assemble some PCBs recently, so I set about making a temperature controller for my SMT oven.

First I had to replace the solid state relay on HydraRaptor.

Solid state relays are triacs with an optically coupled input, zero crossing switching and built in snubbers. I used it for controlling a vacuum cleaner when milling. It was massively overrated but for some reason it failed some time ago. I replaced it with a cheaper one and added a varistor across the mains input to kill any transients, as that is the only explanation I can think of for the old one's demise.

The next task was to write a simple graphing program in Python. I tested it by plotting the response of my extruder heater.

With bang-bang control it swings +/- 2°C with a cycle time of about ten seconds.

Here is the code for the graph: -

from Tkinter import *

class Axis:
  def __init__(self, min, max, minor, major, scale):
      self.scale = scale
      self.min = min
      self.max = max
      self.minor = minor
      self.major = major

class Graph:
  def __init__(self, xAxis, yAxis):
      self.xAxis = xAxis
      self.yAxis = yAxis
      self.root = Tk()
      self.root.title("HydraRaptor")
      self.last_x = None
      frame = Frame(self.root)
      frame.pack()
      xmin = xAxis.min * xAxis.scale
      xmax = xAxis.max * xAxis.scale
      ymin = yAxis.min * yAxis.scale
      ymax = yAxis.max * yAxis.scale
      width  = (xmax - xmin) + 30
      height = (ymax - ymin) + 20
      #
      # X axis
      #
      self.canvas = Canvas(frame, width = width, height = height, background = "white")
      for x in range(xmin, xmax + 1, xAxis.minor * xAxis.scale):
          self.canvas.create_line(x, ymin, x, ymax, fill = "grey")
      for x in range(xmin, xmax + 1, xAxis.major * xAxis.scale):
          self.canvas.create_line(x, ymin, x, ymax, fill = "black")
          if x == xmin:
              anchor = "nw"
          else:
              anchor = "n"
          self.canvas.create_text((x, ymax), text = x / xAxis.scale, anchor = anchor)
      #
      # Y axis
      #
      for y in range(ymin, ymax + 1, yAxis.minor * yAxis.scale):
          self.canvas.create_line(xmin, y, xmax, y, fill = "grey")
      for y in range(ymin, ymax + 1, yAxis.major * yAxis.scale):
          self.canvas.create_line(xmin, y, xmax, y, fill = "black")
          if y == ymin:
              anchor = "se"
          else:
              anchor = "e"
          self.canvas.create_text((xmin, ymax + ymin - y), text = y / yAxis.scale, anchor = anchor)
      self.canvas.pack()
      self.canvas.config(scrollregion=self.canvas.bbox(ALL))
      self.root.update()

  def scaleX(self,x):
      return x * self.xAxis.scale

  def scaleY(self,y):
      axis = self.yAxis;
      return (axis.max + axis.min - y) * axis.scale

  def plot(self, line, colour = "blue"):
      for i in range(len(line) - 1):
          self.canvas.create_line(self.scaleX(line[i][0]),
                                  self.scaleY(line[i][1]),
                                  self.scaleX(line[i+1][0]),
                                  self.scaleY(line[i+1][1]), fill = colour)
      self.root.update()


  def addPoint(self,p, colour="red"):
      x = self.scaleX(p[0])
      y = self.scaleY(p[1])
      if self.last_x != None:
          self.canvas.create_line(self.last_x, self.last_y, x, y, fill = colour)
      self.last_x = x
      self.last_y = y
      self.root.update()

  def __del__(self):
      self.root.mainloop()

The third task was to interface a thermocouple to HydraRaptor. I had a spare analogue input, so I attached one of Zach's thermocouple sensor boards to it. I tested it by attaching the thermocouple to a light bulb with Kapton tape. I then ran a program that turned the bulb on and then off and graphed the temperature response.

As you can see there is a ridiculous amount of noise on the readings. I tracked this down to switching noise on HydraRaptor's 5V rail, which is generated by a simple buck converter from a 24V rail. The AD595 datasheet claims that it has a power supply sensitivity of only 10mV/V so the error should have been a small fraction of a °C. All I can assume is that its rejection of high frequency noise is far less than its DC supply rejection. In fact, pretty much all the supply noise appears on the output.

I fixed it by filtering the supply with a simple RC filter consisting of a 1K series resistor and a 22uF capacitor. I fitted these to the thermocouple board in the unused holes intended for an alarm LED and its series resistor. The power is fed in via the anode connection for the LED. It feeds to the supply rail via the 1K fitted in the R1 position. The positive lead of the capacitor goes into the original +5v connection to the board. The negative lead goes to the GND connection together with the ground lead. This mod will be required whenever the 5V rail comes from a switch mode supply rather than a linear regulator.

Here is the much improved graph with the filter fitted: -

The next thing I tried was bang-bang control of the oven to a fixed temperature with the thermocouple attached to a scrap PCB. No great surprise that there is massive overshoot due to the thermal lag caused by the loose coupling of the PCB to the heating elements via air.

It is obvious some form of proportional control is required, so I implemented PWM control of the mains supply to the oven. As triacs don't turn off until the end of the mains cycle there is no point in varying the pulse width in less than 10ms increments (in the UK). So I implemented a simple firmware scheme where I can specify how many 10ms units to be on for out of a total period, also specified in 10ms units. Setting the period to 1 second allows the heating power to be expressed in 1% units.

My original plan was to implement a PID controller, but after examining the required soldering profile I decided a much simpler scheme would probably perform better.

The is a profile for tin-lead solder that I got from an Altera application note. I mainly use leaded solder at home because the lower melt point gives a much bigger margin for error, it wets and flows a lot better, the fumes are less toxic and it doesn't grow tin whiskers.

Looking at the profile you can see the times are not too critical, but the temperatures are. I reasoned I could simply apply fixed powers to get the right temperature gradient until each target temperature was reached. To get round the overshoot problem I simply measured the overshoot and subtracted it from the target temps.

After a little experimenting I got this profile, which looks pretty good to me: -

The blue line is the target profile, red is actual and the green lines show the time at which each target was reached.

The preheat slope and re-flow slope are simply full power until the temperature is equal to the target minus the overshoot. During the first half of the soak period I had to ramp the power from 0 to 50% to get it to turn the first corner without overshoot. When the reflow peak minus the overshoot is reached I simply turn the oven off. When it gets to the cool section I open the oven door.

Here is the code: -

from Hydra import *
from Graph import *

profile = [(10,20), (120,150), (210,180), (250,210), (330, 180), (420, 20)]

slope = 140.0 / 100
overshoot = 15.0
pre_overshoot = 25

preheat_temp = 150.0
soak_temp    = 180.0
soak_time    = 90.0
reflow_temp  = 210.0
melt_temp    = 183.0

preheat_slope = (soak_temp - preheat_temp) / soak_time

s_preheat = 1
s_soak = 2
s_reflow = 3
s_cool = 4

def interp(profile, x):
  i = 0
  while i < len(profile) - 1 and profile[i + 1][0] < x:
      i += 1
  if i == len(profile) - 1:
      return 0
  p0 = profile[i]
  p1 = profile[i+1]
  return p0[1] + (p1[1]-p0[1]) * (x - p0[0]) / (p1[0] - p0[0])


def oven_cook(profile):
  hydra = Hydra(True)
  try:
      xAxis = Axis(min = 0,  max = 500, minor = 5,  major = 25, scale = 2)
      yAxis = Axis(min = 10, max = 250, minor = 5,  major = 20, scale = 2)
      graph = Graph(xAxis, yAxis)
      graph.plot(profile)

      t = 0
      state = s_preheat
      m_state = s_preheat
      hydra.set_mains(100,100)
      while t < xAxis.max:
          sleep(1)
          temp = hydra.get_temperature()
          print temp
          graph.addPoint((t, temp))
          #
          # Control the power
          #
          if state == s_preheat:
              if temp >= preheat_temp - pre_overshoot:
                  hydra.set_mains( 0, 100)
                  t_soak = t
                  state = s_soak
          elif state == s_soak:
              power = (t - t_soak) * 100.0 / soak_time
              if power > 50:
                  power = 50
              hydra.set_mains(int(power), 100)
              if temp >= soak_temp - overshoot * preheat_slope / slope:
                  hydra.set_mains(100,100)
                  state = s_reflow
          elif state == s_reflow:
              if temp >= reflow_temp - overshoot:
                  hydra.set_mains(0,100)
                  state = s_cool
          #
          # Draw the time lines
          #
          if m_state == s_preheat:
              if temp >= preheat_temp:
                  graph.plot([(t,10), (t,temp)], "green")
                  m_state = s_soak
          elif m_state == s_soak:
              if temp >= melt_temp:
                  graph.plot([(t,10), (t,temp)], "green")
                  m_state = s_reflow
          elif m_state == s_reflow:
              if temp < melt_temp:
                  graph.plot([(t,10), (t,temp)], "green")
                  m_state = s_cool

          t += 1
      hydra.init()

  except:
      hydra.init()
      raise

oven_cook(profile)

This is the first board I soldered with it: -

All the joints were good. I had a few solder balls and some bridging but that was due to not getting the right amount of paste on each pad. I will be working on a solder paste dispenser soon!

I need to do some more testing to see if the arbitrary algorithm will work with large and small boards and with inner planes, etc. It relies on the overshoot being fairly constant, although with leaded solder you have some leeway.

I also want to play with PID to see if I can get a more general solution. The problem I see is that PID does not look into the future, so will always overshoot somewhat, which is exactly what you don't want. I think rather than using the angular profile, that is impossible for the oven to follow, I would have to put in a rounded curve, such as the one the oven actually follows now, as the control input.

Reliable extruder at last?

2009-12-21T22:46:00.007Z

... well only time will tell but I have now fixed all the teething problems on my "no compromise" extruder.

The first problem was it was leaking plastic. I simply tightened the thread about another quarter turn while hot. The problem started when I had to dismantle it to replace the first resistor that I damaged. When I put it back together I didn't get it tight enough as it is difficult to judge when full of plastic and hot. The seal relies on the fact that the relatively sharp edge of the stainless steel tube can bite into the softer aluminium. It seems to work when tightened enough.

The other problem was that the motor would skip steps in the middle of a build for no apparent reason. It seems the amount of force required to extrude varies wildly for which I have no explanation, but I did find some mechanical issues that were reducing the torque available.

I noticed the gear would always be in the same position when the motor skipped. I found that the grub screw was catching on the bearing housing. You would expect it just to grind the PLA away, but PLA is very hard, so it would take a very long time to do so. I increased the clearance around the wheel hub and also around the moving part of the ball bearings.

Another issue was that both the worm and the gear were slightly off centre on their shafts, so when the two high points coincided they would bind. The hole in the Meccano gear is slightly bigger than the 4mm shaft it is on, not sure why. The hole I drilled in the worm is 5mm but the MakerBot motors have imperial shafts about 4.75mm, so that was even more eccentric. Added to that was the fact that the motor bracket has a slight warp to it angling the shaft down a little. All these things conspired to make it stiff to turn once per revolution. I fixed it by tightening the bottom motor screw tight and slackening the top two a little. That was enough to reliably extrude PLA. Making the motor holes into slots would make things less critical.

Although the extruder was working reliably for PLA I wanted more torque in reserve, so I switched to a higher torque motor more suited to my driver chip. The Lin motor I was using was rated at 0.3Nm holding torque for 2.5A, but my controller can only manage about 1.5A without some better heatsinking. I switched to the Motion Control FL42STH47-1684A-01 which gives 0.43Nm at 1.7A. So at 1.5A I have gone from 0.18Nm to 0.4Nm, i.e. doubled the torque and also got the right shaft diameter to fit the hole I drilled in the worm.

The only downside is that it is bigger and heavier, not really an issue on HydraRaptor.

To give it a thorough test I printed off a couple of Mendel frame vertices.

These take about 2 hours each with 0.4mm filament, 25% fill, double outline at 16mm/s, infill at 32mm/s. Six are needed in total.

I still have to test it with HDPE and PCL., I know it works with ABS.

Motoring on with the A3977

2009-12-13T21:41:00.005Z

Previously I have blogged about how to set up the Allegro A3977 driver chip to suit a particular motor: -

hydraraptor.blogspot.com/2009/07/lessons-from-a3977
hydraraptor.blogspot.com/2009/08/motor-math
hydraraptor.blogspot.com/2009/08/mixed-decay-mixed-blessing

Most boards I have seen using the A3977 and similar chips just have a current adjustment, with all the other values fixed. Unless you strike lucky this is not going to allow accurate microstepping because the off time and PFD need to be adjusted to suit the motor and supply voltage.

A while ago Zach sent me samples of the prototype V3 stepper controller kits and the NEMA17 motors used on the MakerBot. I made up the board using my SMT oven (pizza oven controlled by HydraRaptor, more on that later).

It works well, but the initial component values are not optimum for the motor, so I decided to make a test bench from the older prototype board that I have been experimenting with. I RepRapped a chassis for it with a panel to mount some switches to vary the timing components.

The chassis is one of the biggest parts I have made, not in volume, but in overall expanse. It warped a little, despite being PLA, heated bed coming soon!

The switch on the left must be at least 20 years old and the one on the right more than 40 but they both still work fine. I save all this junk and eventually it comes in handy.

I also have potentiometers on V_ref and PFD, so together with a bench PSU and a signal generator I can vary every parameter.

I knocked up a label on a 2D printer, it's so much easier to make this sort of thing than it was when the switches were born!

Zach has updated the board to have four preset potentiometers to make it fully adjustable. There are test points to allow the pots to be set to prescribed values with a multi-meter.

Vref and PFD can be measured as a voltage, but the two RT values have to be set by measuring resistance with the power off. My multimeter seems to give accurate readings of these despite them being in circuit. A good tip is to measure the resistance with both polarities and if it reads the same either way round then it is most likely the chip is not affecting the reading.

So here is a list of motors and optimised settings: -

MakerBot Kysan SKU1123029 NEMA17

This is the motor that MakerBot use for the axis drive on the Cupcake, details here. It is actually a 14V motor, so is not ideally suited to being driven from a 12V chopper drive. You normally want the motor voltage to be substantially lower than the supply.

You can't run it at its full current because the duty cycle would tend to 100%. With a fixed off-time, the on-time tends towards infinity and the frequency drops into the audio range. In practice I found the maximum current at 12V was 0.3A, any higher and the microstepping waveform was distorted on the leading edge due to the current not being able to rise fast enough.

To maintain the sinusoidal waveform at faster step rates requires the current to be lowered further, 0.25A gives a good compromise. It is not a bad idea to under run steppers anyway, otherwise they can get too hot for contact with plastic.

I used the minimum values for CT and RT, i.e. 470pF and 12K to keep the chopping frequency as high as possible, so that it is outside of the audio range. Not only is this a good idea to keep it quiet when idling, but also you want it much higher than your stepping frequency, otherwise they beat with each other.

The values give a minimum frequency of ~17kHz @ 0.3A and a maximum of ~150kHz on the lowest microstep value. 17kHz is not audible to me, but younger people might be able to hear it. There is still some audible noise at the point in the cycle when both coils have similar currents and so similar high frequencies. The beat frequency, which is the difference of the two, is then in the audio range. It isn't anywhere near as loud as when the chopping is in the audio range though.

I can't see any spec for the maximum switching frequency although a couple of parameters are given at less than 50kHz. I suspect 150kHz is a bit on the high side, which would increase switching losses, but with such a low current compared to the rating of the chip I don't think it is a problem.

One problem I had initially was that the switching waveform was unstable. It had cycles with a shorter on-time than required, which let the current fall until it then did a long cycle to catch up. The long cycle gave a low frequency that was back in the audio range.

I think it was a consequence of the motor needing a very short off-time in order to be able to have the duty cycle nearly 100%. The current hardly falls during the off period, so a little noise due to ringing can trigger it to turn off too early. It is not helped by using the minimum blank time. I fixed it by putting 1uF capacitors across the sense resistors.

The PFD value is best set to 100% fast decay with this motor.

It works better with a 24V supply. The full 0.4A current can be achieved (but it gets much hotter of course) and it maintains microstepping accuracy at higher step rates than it does on 12V.

MakerBot Lin SKU4118S-62-07 NEMA17

This is the NEMA17 that MakerBot used to supply. It is at the opposite extreme compared to the one above, i.e. it is a very low voltage motor, only 2V @ 2.5A. As mentioned before, this causes a couple of issues: -

The inductance is so low that the ripple current is significant compared to the lowest current microstep, causing positional errors. OK at 2A, but gets worse with lower currents.
It is difficult to get 2.5A from the A3977 without it overheating. The PCB layout has to be very good. The datasheet recommends 2oz copper and four layers. 2A is no problem and that is the maximum with the 0.25Ω sense resistors fitted to the board.

At 2A the motor runs at about 40°C, so just about OK for use with PLA. The chip gets a lot hotter, about 77°C measured on the ground pins.

I used a value of 56K for RT and 2.1V on PFD. To some extent the optimum PFD value depends on how fast you want it to go.

Motion Control FL42STH47-1684A-01 NEMA17

This is the recommended motor for the Mendel extruder, details here. After buying a couple of these a friend pointed out that Zapp Automation do the same motor with dual shafts for about half the price!

This is a high torque motor so it is longer and heavier than the previous two NEMA17s. Electrically it is in the sweet spot for the A3977 with a 12V supply. The A3977 can easily provide the full current and the switching frequency doesn't have wild fluctuations or drop into the audio range.

When microstepped at 1.7A it gets to about 43°C but the chip only gets to 56°C.

I used 39K for RT and 0V on PFD, i.e. 100% fast decay.

I have high hopes for this motor as a replacement for the one above that is in my extruder currently. It should give me almost twice the torque and has the correct sized shaft, i.e. 5mm. The Lin and Kysan motors both have imperial shaft sizes which caught me out as I drilled the worm gear for 5mm thinking NEMA17 specified that, but it must just be the frame dimensions.

MakerBot Keling KL23H251-24-8B NEMA23

This is the motor I used on my Darwin. It has 8 wires so it can be connected in bipolar serial or parallel. Series has the advantage that the full torque can be achieved with 1.7A which is easily within the range of the A3977. Parallel has one quarter of the inductance so torque will fall off with speed four times slower. To get full torque 3.4A is needed but I found 1A was enough for the X and Y axes. I think Z needs more torque but my z-axis uses different motors so I don't know how much.

An RT value of 56K is fine for currents in the range 1-2A. PFD is best at 0v, i.e. 100% fast decay.

Summary

Here is a summary of the motor specifications :-

Motor	Resistance	Max Current	Voltage	Max Power	Holding Torque	Inductance
LIN 4118S-62-07	0.8 Ohm	2.5 A	2.0 V	10.0 W	0.30 Nm
Kysan SKU 1123029	35.0 Ohm	0.4 A	14.0 V	11.2 W	0.26 Nm	44.0 mH
Motion Control FL42STH47-1684A-01	1.7 Ohm	1.7 A	2.8 V	9.5 W	0.43 Nm	2.8 mH
Keling KL23H251-24-8B Series	3.6 Ohm	1.7 A	6.1 V	20.8 W	1.10 Nm	13.2 mH
MakerBot Keling KL23H251-24-8B Parallel	0.9 Ohm	3.4 A	3.1 V	20.8 W	1.10 Nm	3.3 mH

Here are my suggested settings :-

Motor	Current	Vref	CT	RT	PFD
Kysan SKU 1123029	0.25 – 0.3A	0.5 – 0.6V	470pF	12K	0
LIN 4118S-62-07	1 – 2A	2 – 4V	470pF	56K	2.1V
Motion Control FL42STH47-1684A-01	1 – 1.7A	2 – 3.4V	470pF	39K	0
Keling KL23H251-24-8B Parallel	1 – 2A	2 – 4V	470pF	56K	0

Quality control

2009-12-04T19:54:00.004Z

I RepRapped a doorstop for our new bathroom shower: -

It has a 10mm hole most of the way down and a countersink to take a ~5mm wood screw.

A 2mm self adhesive felt pad covers the screw hole and acts as a shock absorber.

It has a rim around the bottom to prevent it rocking if the base warps or the wall is not flat. To support the bottom of the hole there is a one layer membrane: -

I removed it with a 5mm drill: -

I was quite proud of it but my wife had something more like this in mind: -

I can't print chrome yet, so I will have to go out and buy one, and it has three screws which have to be drilled through the tiles into the wall.

The files are on Thingiverse if you prefer function over form.

Pinchless Extruder

2009-11-30T21:30:00.003Z

While dismantling my extruder for a small mod I accidentally discovered that the worm pulley has so much grip that it will still extrude PLA with the pressure roller removed. It is hard to see on this low quality video but it was extruding 0.4mm filament at 32mm/s.

I will still run it with a roller because it helps to guide it into the tube when self feeding to start a new filament. I also expect softer plastics would need it.

BTW, I have stopped using Vimeo and gone back to YouTube because they added an artificial processing delay unless you pay.

Beefed up bracket

2009-11-20T22:35:00.008Z

When I started reversing my extruder I noticed the motor bracket flexing. Here is a short video showing it in operation: -

BendyExtruder from Nop Head on Vimeo.

It was immediately apparent that I had not made it strong enough.

As the worm gear is about twice the diameter of the threaded pulley the axial force on the motor is about half the force required to push the filament, i.e. a few kilograms. After making a few objects it cracked along the layer where the bearing housing rises out of the flat motor mount.

I designed a new bracket but I was back in a chicken and egg situation with no working extruder to print it. As Erik pointed out you need a Robin Hood / Friar Tuck strategy of having two machines so that one can make replacement parts for the other. I must get my Darwin up and running!

In the meantime I cobbled it back together with some random bits of metal, some tiny G-clamps and tie wraps: -

I made some of the new bracket thicker where I could: 8mm instead of 5mm, which should be ~2.5 times stronger. I also added some ribs and extruded it at 10°C higher temperature.

This one seems solid as a rock, but it did warp a little more. The stronger you make something the more it warps.

Here is a video of it not flexing: -