Over cooked it

When I was printing 24/7 for about 5 years I never had a problem with filament absorbing moisture because the heat from the machines kept the rooms they were in hot and dry and the filament was stored in the same rooms as the printers. In fact in winter it was so dry I had to buy a long conductive ESD mat that runs the length of my workshop and wear ESD ankle straps to prevent getting sparks off everything I touched.

Since I retired I print more sporadically and tend to go away in winter and leave the heating at only 12°C. I still didn't have a problem with the white ABS that I got from Germany for kit production. It was unusual in that its natural colour was white instead of cream and it didn't smell much while printing. I used to find that I might get one or two bubbles in the skirt round the object but after that the rest of the print was fine. It appeared that filament sitting in the extruder from the last print would absorb some moisture over time but the rest on the spool that hadn't been melted did not, no matter how long it was left.

I also got some black ABS from the same German company and that is totally different. It bubbles very badly and needs to be dried. It also smells like ABS when it is printed. I never got good results from printing it, so I put off using it for years until I ran out of white. I then decided to tackle the moisture problem. Inspired by RichRap's heated dry box, I designed a parametric heated dry box that I could tailor to fit any size spool.

When I made the Dibond version of Mendel90 I noticed the dummy load resistors for the ATX PSU ran a lot cooler than they did on the MDF version. I came to realise that Dibond makes quite a good heat spreader even though the aluminium layers are only 0.3mm thick.

I also had lots of 47 Ω 50W TO220 resistors from various heated bed iterations that didn't go too well. Since this doesn't need to get very hot or need much power I thought it would be a good way to use them up.

I have a parametric box in NopSCADlib that is made from Dibond panels and printed brackets that can be scaled to any size that fits my CNC mill, so it was easy to wrap that around my large 2.4kg spools and add 12 resistors along three sides. The spool runs on 608 ball bearings between penny washers, there is a thermistor to monitor the temperature and a small fan to stir the air around.

I ran the resistors in series from half rectified mains to give a total wattage of 51W. I earthed all the panels and covered all the connections with heat shrink sleeving but it wouldn't pass any safety standards as the wiring isn't double insulated. Safe enough for me though as I don't need to put my hands inside it but I wouldn't recommend it. 

For my smaller 1kg spools I use 9 resistors and wire them in parallel for 12V operation. That gives 28W, which is enough for the reduced surface area and much safer.

To reduce the energy consumption I obviously needed to insulate the outside of the box, so I suspended it inside another slightly larger box and filled the gap with cotton wool. It is a lot nicer to work with than fiberglass or rockwool and quite cheap in the quantities needed. It is also just the right thickness to fill the gap, which is dictated by the corner fixing blocks. Even without the cotton wool the air gap gave quite good insulation. The only connection between the inner and outer cases is a few printed standoffs and the wires and filament exit guide.

It is completed by a hinged double door made of acrylic sheets with a hygrometer and thermometer module in the inner door. An arduino Leonardo with an LCD, some buttons and a couple of MOSFETs controls the temperature and the fan and keeps a record of the heater duty cycle. Again parts I had to use up.

So basically I created a 3D printed fan oven! 

The base of the outer box is extended, so it bridges the frame stays of my Mendel90s after removing the spool holders. Four fixing blocks stop it sliding off but are only screwed to the base, not the printer., so it can be lifted off and they then act as feet.

The mains version is controlled by a stand alone Arduino thermostat I had previously built to control a beer fridge. Another reason the first version was mains operated.

I have parts to make a third one the same as the second, after that I will probably make it headless with an ESP8266 and an I2C temperature and humidity sensor.

To dry the ABS filament I set the temperature to 80°C for a few hours and then left it at 50°C, even when the printer is not in use. The relative humidity in the box drops to about 19%. In the room it is about 60%. When I was printing 24/7 it used to be well below 50%.

As well as stopping bubbles it improves surface finish making it more glossy, makes bridges pull tighter, completely stops nozzle ooze at the end of a print and even reduces the ABS smell while printing to almost nothing.

At the end of a print I retract an extra 1mm and turn off the heater before moving Z back to the top. Without the dryer I used to get 10 to 20mm ooze out of the nozzle as the extruder cooled down. I had always assumed this was due to gravity but it is in fact due to moisture turning to steam pushing the filament out. When it is dry the surface tension must be sufficient to stop any flow due to gravity.

I was used to snapping off the ooze before I start a build, it had become an unconscious action, but now there is nothing to remove. It will be a big advantage when I make a multi material machine as there should be no ooze from the idle extruders while another is being used.

The reduction in smell was a complete surprise. Before I dried the black ABS it seemed to smoke as it came out of the nozzle. I now realise that was water vapour and it must carry off some volatile products. It now hardly smells at all when printing but if I stick my nose in the dryer it does smell of hot plastic, even though it is only at 50°C.  I guess the moisture it has driven off carries some VOCs with it.

About a year ago I got some wood coloured ABS in the UK which was even more affected by moisture than the black, so that is when I built the second smaller dry box, around October. After drying it printed very well. I made this replacement handle for a curtain puller with it.

After switching the dry boxes off when we went away for the winter I turned them on again in March and they have been on ever since. I last printed with the brown plastic at the end of May and it was fine but when I tried to use it yesterday it has gone super brittle. The coil has a very strong heat set, something I don't like about 3mm filament on 1kg spools, and bending it straight now causes it to snap.

I was trying to use it on my original MDF Mendel90 that is now encased in a box with a chamber heater. The filament feeds from the top of the box rather than through a PTFE tube that runs all the way to the extruder. The part that was outside of the dry box for about seven weeks was still ductile but had moisture in it. When I pulled the dry part from the box through it just snaped, so I can no longer print it on that machine. 

It does still seem to work on my Dibond machines that do have the PTFE tube all the way to the extruder. That keeps some of the coil's curve and doesn't require it to be fully straightened. 

And the printed objects don't seem to be brittle at all. I haven't done any proper strength tests but a quick test bending a small 100% filled block with pliers it bent and got white bruises like ABS normally does.

So keeping brown ABS at 50°C for a few months seems to completely denature it but a melt cycle seems to restore it. I have heard of PLA going brittle on the spool but nobody seems to know for sure what causes it. PLA is also somewhat brittle but ABS isn't at all. Presumably the long polymer chains must get shorter somehow and then reform when melted. I am not sure if the temperature is the problem or if it is too dry. I have read there is an optimum moisture level for processing plastic at, rather than as dry as possible, not sure why.

The black ABS hasn't gone brittle yet and it has been in its heated box for longer.

My prototype MDF Mendel90 runs with a chamber temperature of 45C and I have noticed that the printed parts it is made of seem to become brittle over time. The extruder runs a lot hotter of course and the Wade's block tends to crumble after a few years and needs to be replaced regularly. They last a lot longer on my unboxed machines. 

I also think ABS shrinks over time, even at room temperature. I made a test print with some holes in it a four years ago when I was having an interesting discussion about Polyholes with Giles Bathgate. I tested it with plug gauges but as I didn't have a case for them I left them standing in it on a shelf near a north facing window. When I went to use them for my Horiholes test I found they were stuck in it much tighter than I remembered and the plastic has yellowed slightly.

I made a case for them with a screw top using my thread utility in NopSCADlib, printed in the black ABS.

So now I have dropped the temperature of my dry boxes to 30°C as surely that won't degrade the plastic much more than in a hot room without sunlight. That vastly reduces the power consumption of course. 50°C needed about 50% duty cycle but 30°C is only 4%, only a shade over one Watt. The hygrometers are still reading 19% after a day at the lower temperature. I find that odd because, for a given air water content, reported relative humidity should increase as temperature falls.

So I think I will try initially drying new plastic at 80°C overnight and then reducing to 30°C for long term storage from now on and see how that goes. A good measure of whether it is dry enough is the complete lack of ooze at the end of the build.

Will it burn

I always intended to put lots of different tool heads on HydraRaptor but after being a milling machine for a while it got stuck as a 3D printer until I started making Mendel90 kits and then it sat gathering dust.

Back in 2009 I bought a 1W 808nm infra red laser diode to experiment with but I never got around to trying it out until recently.

I bought it on eBay for £292, which seems very expensive now, but the seller claimed it has a spot size of only 13um x 120um. That would give a power density of 640 W/mm², assuming a rectangular spot. In comparison a 40W CO₂ laser with a round spot of say 0.25mm would give a power density of 815 W/mm², so I expected to be able to cut through wood and plastic a few mm thick with it.

Inconveniently, the anode of the diode is connected to the case. 

It came with a driver board that takes 11-18V and a TTL enable signal and produces a constant current drive.

It is all a bit last century with through hole components and a relay. I looked at the switching waveform and found that the relay added an 8.2ms delay and there was a 2.95ms rise time.

The blue trace is the enable signal and the yellow trace the output voltage.

The two TO220 devices had their markings ground off but it was trivial to trace the circuit and work out what they are: a 7810 10V regulator and an LM317 variable regulator wired as a 1.25A constant current source.

Laser didoes are very easily destroyed by overshoot transients of even a few micro seconds duration, so most of the circuit seems to be to avoid those. R2 and C3 seem to be to stop inductive spikes from the relay getting onto the 10V rail. R4 and C6 are probably to filter any relay contact bounce but they also make the rise and fall times very slow. D1 is a mystery because it can never be forward biased, so might as well not be there.

I hacked the PCB and reconfigured the circuit to replace the relay with a MOSFET, speed up the edge rate and added a big red LED to warn me when it was on. I have a pair of Thorlabs LG9 safety glasses to protect my eyes.

Here is the new switching waveform: -

This time the yellow trace is the enable signal. The blue trace is the current waveform measured with the hall effect current sensor mentioned in my last post. The small delay turning on is while the output capacitor charges enough for the diode to start conducting. The rise and fall times are now less than 1ms which seems more reasonable.

The forward voltage of the diode is about 2.2V at 1.25A giving a power dissipation of 2.75W and an efficiency of 36% assuming the output is 1W. I mounted it on an old PC CPU cooler which was complete overkill.

I made a rough estimate of the thermal resistance of the heatsink with the fan on by attaching a 50W resistor that has the same case style as the laser. The heatsink itself is about 0.23°C/W and the case of the resistor a little more, 0.48°C/W in total. So the temperature of the diode casing will rise by less than 1°C.

These dashes were made by waving a random piece of black plastic (most likely ABS) in front of it while the 100 Hz test waveform above was driving it.

With continuous power it makes deep scars.

Holding it steady I was able to slowly drill all the way through the 1.75mm thickness but it left a ring on the surface. The exit hole was clean though. By all accounts ABS doesn't laser very well.

With these rough manual tests I established the focus length was about 35mm, which I needed to know to be able to design a mount for HydraRaptor, so that I could position it relative to the Z probe to give me auto focus.

I also established it has no effect at all on white paper because that reflects red light and this is near IR, so it will behave mostly the same as red light. It also had no effect on some Kapton (polyimide) film because that is transparent to red light. With near IR you can only cut materials that absorb the red end of the spectrum. If they are transparent or reflective to red they are unaffected. In contrast, CO₂ laser light is far infra red with a much longer wavelength and that is absorbed by most things including optically transparent materials like glass and clear acrylic and white materials like paper.

I designed HydraRaptor in 2D and that was all it needed at the time because it was made from flat sheets of MDF and had no 3D printed parts. In order to be able to add new parts to it I decided to re-model it in 3D in OpenSCAD.

I mounted the heatsink on a printed bracket that aligns the laser with the centre of the table and also supports a radial blower and duct for air assist. That is a jet of air that blows the smoke away from the cut and the lens.

I made a steel plate bed to protect the XY table and allow me to hold down the work piece with magnets. An L shaped bracket made from DiBond allows repeatable alignment with the back left corner of the bed. I used that corner to allow oversized sheets to hang over the front right where there is maximum clearance.

The hole in the corner of the L is needed because an internal corner would otherwise have a radius equal to the tool radius that cut it.

The first task was to find the exact focal point and I did that by burning a line of spots from different heights into the paint surface of an off-cut of DiBond and looked for the smallest one. I have hundreds of these off-cuts from making Mendel90 kits and because the paint is a thin layer on top of aluminum it seems like a good way to measure the spot size.

After I had established the focal point I then needed to establish how big the spot is. I have a microscope and a graticule slide but it was too hard to align it by hand. The alternative method I came up with was to make a line of spots 0.1mm apart so that I could compare the spot size with their pitch and use the ratio to work out the size.

As you can see the spot isn't quite aligned with the outer case of the laser. The size works out at 0.16mm by 0.07mm. This is a lot bigger than the 0.12mm x 0.013mm advertised and only gives a power density of 90 W/mm². The bright area in the middle where it looks to have cut to full depth is 0.036mm wide.

Laser spots don't have well defined sharp edges. An ideal laser has a Gaussian intensity distribution which falls off  away from the centre asymptotically to zero. The beam diameter is sometimes defined as where the intensity drops to half the maximum and other times where the intensity drops to 1/e² ≈ 13.5%. So my power density calculations are somewhat naive.

The beam starts off long and thin because it comes out of the edge of the chip die. The cleaved edges form the two parallel mirrors. Whereas I think of lasers having a parallel beam, the beam from a diode laser diverges at tens of degrees. And it diverges faster in the axis at right angles to the die than it does in the axis parallel to the die. So although it starts out wide and short it ends up tall and thin.

Not only that, but the beam also has astigmatism. That is: the point that the beam diverges horizontally from is further back then the point it diverges vertically from. So focusing it to a round spot requires tricky anamorphic optics. Mine just has a plain lens that is rotated in a screw thread to adjust the focus, so it can't correct the elliptical beam shape.

My next experiment was to work out what travel speed I can engrave at. This will be different horizontally and vertically because the energy density applied to the material will depend on the area swept out as well as the power and time. This will make motion planning interesting as the speed will need to vary depending on the slope of a line and so will the kerf compensation. Alternatively the laser could be mounted on a rotary axis to keep it pointing along the axis of travel for maximum detail. That would need a very accurately aligned axis though to avoid the spot wandering as it rotates. A round spot would be a lot easier to deal with!

I engraved a 5x5mm crosshatch with each line at a different speed. Speed reduces from left to right and bottom to top. The speeds are 5mm/s, 5/2mm/s, 5/3mm/s. ... 5/13mm/s.

By looking at the cross over points one can tell if the maximum engraving depth has been reached or not. So it needs go as slow as about 0.5mm/s horizontally to not show the vertical lines.

Note that it never goes deep enough to reach the aluminium skin. It looks bright but when I check for conductivity with a mulitmeter I have to scratch away the white layer to get contact. I think there must be white primer underneath the black paint and that reflects the laser, stopping further ablation.

Here is a 5x5mm rectangle engraved with a raster of lines overlapping 50%.

I don't know what gives it an apparent texture.

While doing these tests it soon became apparent that I needed fume extraction because removing even a tiny amount of paint smelt unpleasant. I thought I might get away without it for shallow engraving as there are many open frame laser engraving machines on the market. My first attempt was to add an 80cfm fan close to the edge of the bed that sucks air and blows it down a 1" pipe that I hang out of the window.

It produces quite a powerful suction and this reduced the smell but not enough. I switched to tests on balsa wood because I thought it would be less toxic. I have a lot of 2mm sheets left over from the early days of RepRap when it was used as a bed material for PLA before better options were discovered.

It still smelt very smokey, so I decided to make an enclosure. I had always intended to do this. Way back when I bought the laser, I also bought some brushed aluminum DiBond sheets big enough to make a cover but ended up using most of them for other things. I did have two left to make the front and top and some black off-cuts from Mendel90 production long enough to make the sides.

The width of HydraRaptor is 511mm and that is bigger than the X axis of my CNC router, which is 450mm. By hanging it over the side of the bed so it just cleared the gantry I was able to route one side at a time. I made some tooling holes in the corners of the door cutout that allowed me to turn it around 180° but maintain accurate registration. If it had been 1mm wider it would not have fit the router!

The door is an acrylic one I recycled from my Mendel case. I might replace it with DiBond to remove the need to wear safety glasses. It is sealed around the edges with rubber sealing strip tape. The enclosure isn't airtight because there are holes for wires to the z-axis driver, etc, but it is under significant negative pressure when the fan is running, so they are not a problem. I cut an 80mm hole opposite the extractor fan to get a good stream of air across the bed.

The enclosure removes any smell in the room while it is engraving wood but the smokey smell remains inside the machine even after a couple of weeks.

I did a larger grid test (50mm x 50mm) to see what speed I could cut through 2mm thick balsa wood but found it didn't matter how slow I went it did not go right through but it did give a wider charred area. The speed is 5 / n mm/s, where n is the line's index.

Here is the underside: -

Just a few pin prick holes and some surrounding char where the slowest lines cross.

I did another test where instead of reducing the speed by the line's index I kept the speed constant at 3mm/s (5 mm/s for y) but repeated the line n times. I found this gave far less char and actually cut all the way through.

So horizontally it took 4 passes at 3mm/s to cut through and vertically 6 passes at 5mm/s. Working out the power energy density as passes * power / speed these are more or less the same, which is odd considering the big difference in beam width.

The next test I tried was to cut out a square using four passes at 3mm/s in both directions.

I was disappointed to find it didn't cut all the way through so I re-ran the grid test above and the laser power dropped to zero and it never lased again. The left edge of the wood was not very straight where I cut it with a knife and it left a small gap that allowed the laser beam to hit the steel plate below. What seems to have happened is the reflection was enough to destroy the diode's mirrors. It still takes the same power but gives no output. This is known as Catastrophical Optical Damage.

Where the beam had previously gone all the way though to the steel it had created black stains so that stopped any reflection. So it looks like I should have painted my steel plate black. I was also lucky that the DiBond didn't engrave down to the aluminium surface as I expect that would be an even better mirror.

So a disastrous end to the experiment!

I have ordered a 2.3W blue laser from China so I will continue experimenting with that when it arrives. I also have a 12W IR fiber laser to play with but that requires a serious power supply and cooling system, so I will get more experience with lower power lasers before I attempt to power that up.

Mendel90 GitHub catch up

I finally found time to update GitHub with some Mendel90 changes that I have had in the works for a long time. The problem with releasing them sooner was that they were all not quite finished and / or would make unintended knock on changes to the kits I was producing. In particular the changes I did to make a Huxley90 in a hurry for the TCT show and the E3D mods kindly contributed by Philippe LUC that conflicted greatly with it, so needed a lot of work to merge.

OpenScad

I also updated to the latest version of OpenScad. The upside was that hull and some of the 2D operations are much faster. I was also able to replace all the calls to minkowski with offset as I was only using it for 2D offsetting. The net result is it is now four or five times quicker to generate the preview and the STL files. The downside is that the 2D sub-system now uses fixed point coordinates but the rest of OpenScad doesn't. This makes it difficult to get 2D and 3D geometry to match up. For example, an extruded circle now has slightly different vertices to a cylinder of the same size. This created a few degenerate triangles requiring that I changed the way I constructed some objects in order to get nice clean STL files.


The solution in the case above was to make the cylinder slightly bigger than the circle used to make the pointer.

On the up side it seems OpenScad has got better at handling unioning exactly coincident faces since I first wrote Mendel90, so I could remove some of my small offset bodges to avoid z-fighting.

Another benefit is that the X end brackets now slice correctly in Slic3r, as the bug that caused internal faces to point the wrong way has now been fixed. Skeinforge doesn't care about face orientation, it just counts edges to work out what is inside and what is outside. Other slicers got confused and filled in the nut cavity.

Along the way I discovered that, although OpenScad now has trig functions that are accurate for multiples of 90 degrees, etc., it doesn't use them in rotate, or vertex creation for circles and cylinders. It converts to radians and uses the library trig functions. Degrees can never be represented accurately as radians in floating point because Pi is irrational, not to mention transcendental. To get round this I now override the built in rotate with a user space version that uses the accurate sin and cos degree functions.

module rotate(a)
{
 cx = cos(a[0]);
 cy = cos(a[1]);
 cz = cos(a[2]);
 sx = sin(a[0]);
 sy = sin(a[1]);
 sz = sin(a[2]);
 multmatrix([
  [ cy * cz, cz * sx * sy - cx * sz, cx * cz * sy + sx * sz, 0],
  [ cy * sz, cx * cz + sx * sy * sz,-cz * sx + cx * sy * sz, 0],
  [-sy,      cy * sx,                cx * cy,                0],
  [ 0,       0,                      0,                      1]
 ]) children();
} 

Not surprisingly every STL and DXF file generated is now slightly different numerically but hopefully not dimensionally. I made a stable branch to record the state before these global changes, just in case. GitHub has some excellent image and STL comparison views but unfortunately it gives up if more than a handful of files have changed and there are hundreds in the Mendel90 tree.

Wade's Block

After a few people started to report broken or cracked Wade's blocks I strengthened it a bit around the bearing block. I also made the bearing sockets a bit bigger so there is less stress created pressing them in. Kits from around March 2015 have shipped with this version.

X Carriage

When Philippe LUC created the E3D branch he fixed a few bugs. One of these was that the X carriage top was only 2mm thick, when the design intent was 3mm. This was due to the fan duct using the same variable name. Whoops! I have updated it now in the main branch. I also made the nut traps for the fan bracket screws deeper to allow for longer screws and to allow them to be withdrawn further without loosing the nuts. This makes it easier to remove and replace the duct. Simply removing the washers is an alternative.

E3D Hotend

I temporarily parked Philipp's mods in an E3D branch until I could merge them. I have now updated the master branch to support E3D V5 and V6 hot ends with this one line change to the config file. The generated files for V6 that are different from standard build are in new folders dibond_E3D and sturdy_E3D and I have deleted the temporary E3D branch.

There is no room for the right hand wing nut because it clashes with the hot end's fan. Fortunately the carriage has always had nut traps to allow the screws to be inserted from below. A plain nut above can then be used to secure the extruder.

Primarily the things that change are the Wade's block, the fan duct and the fan bracket. The Wade's block has no extension to avoid losing more Z build height than necessary and a plain screw hole on the right end instead of the hex socket.

The fan duct has to slope downwards to avoid the E3D heatsink. That creates a sloping bridge that is also skewed horizontally. I haven't found a slicer that handles this properly yet, having tried Skeinforge, Slic3r, Cura, Kisslicer and even paid for Simplify3D! I have blogged about their failings in another post here: hydraraptor.blogspot.co.uk/2016/01/a-bridge-too-far. Any other slicers I should try?

Another bug Philippe noticed is that there was almost no clearance between the fan and the belt. Fortunately the belt is twisted so it actually does clear the fan. I have added more clearance as
Philippe did. It makes the fan bracket and fan duct 2mm longer. If you print either from the new files be sure to print both or the duct will be misaligned.

I also improved the internal shape of the duct a bit. From this: -

To this: -

It probably doesn't make a lot of difference but a comparative test of various fans and ducts will be the subject of a later post.

Even with the shortened Wade's block the E3D V6 hot end is 4mm lower and the V5 is a bit longer still. If you retro fit it to an old machine you will lose 4mm Z travel. If you are building a new machine then there are alternative files which add 4mm to the height of the frame and lengthen the Z smooth rods and threaded rods on the bom. That also has a knock on effect on the shape of the spool holders and the dust filter. If you use the larger sheets be sure to get the correct size rods and use the correct spool holder parts to match the frame.

New Lighting Options

I redesigned the lighting system I described here to work with some commonly available LED light strips. These consist of an aluminium PCB strip that slides into an aluminium extrusion with plastic end caps, which I discard. Instead of printing a bar to hang the lights and camera from I now add printed end caps to the light strip and uses those to hinge it from the frame edge clips. I then hang the camera from the strip with its own hinge.

The strips come in 500mm lengths but they can be cut at discrete points between every third LED. They are described as "50CM 5050 SMD 36 LED Warm White Aluminium Rigid Strip Bar Light Lamp" and I bought them from bgood2010 on eBay.

I got some from another seller and although the eBay picture looked the same the extrusions where actually not as deep. The STL files on GitHub are for 8.6mm deep extrusions and are generated by light_strip = RIGID5050_290. Setting it to Rigid5050_290 generates the clips for 7mm deep extrusion. Other sizes can easily be accommodated as long as they are rectangular. The definitions are here.

Rather than waste the off-cut I mount it above with a second pair of end caps that clip onto the main light strip. These are set back just far enough to avoid the build volume in the unlikely event you print something tall at the back edge of the bed. This is calculated by the model with lots of trig and Pythagoras maths. Set show_rays = true to see this view showing that the camera and lights are pointing at the centre of the bed and the build volume is clear.

Another light strip that can be selected is this one: FSRP3W, discovered by Alzibiff.

Again the end caps are removed and replaced with printed ones that clip into the screw channels in the extrusion. There is no room for the plug so I just solder the wires on.

It looks neater and gives a more diffuse light but is not as bright as the double strip of 5050 LEDs and is more expensive. I bought it from www.ledlightingandlights.com.

The only problem with these light strips compared to my original Sanken ones is that they are unregulated, so they flicker when the bed switches on and off. I described how I fixed that here. I also need to update the mounting for the Raspberry Pi to accommodate the plethora of new Pis that have appeared since my original design.

Huxley90

The Huxley version is scaled down in the same way as the Sells Mendel was scaled to make the Huxley. It has a build volume of 150mm cubed and uses NEMA14 motors, 6mm smooth rods and M3 fasteners for the frame. There is a good photo of it alongside the full sized machine on Ivor O'Shea's blog post.

The NEMA14 motors have about half the torque of the NEMA17s when driven with the same current. The Y carriage and bed have about half the area hence half the mass, so that is about right. Also a NEMA14 has half the mass of a NEMA17, so the X carriage also has about half the mass.

I believe the flex in the middle of the rods is proportional to the length cubed times the weight divided by the bar radius to the power of four. The length of the X rods is almost exactly 75% of the Dibond version and the diameter is obviously 75% as well. The relative flex then boils down to 0.5 / 0.75 = 0.67. So going down to 6mm rods is justifiable as well. Everything scales very nicely physics wise.

As the design is fully parametric shrinking it should have been easy, but because vitamins don't scale perfectly lots of snags arose where things clashed. A typical example was the x_motor_bracket. The NEMA14 motors are smaller but the raised boss around the shaft is the same size. This makes the bracket a different shape and it then needs a support to print it.

Half a truncated teardrop with a crutch!

The heated bed was made with veroboard and coincidentally has the same resistance as a full sized Prusa PCB, so the machine takes the same amount of power but heats up about twice as fast. There is no room on the frame for an ATX PSU, so I used an external XBOX 200W PSU. I couldn't find a spec for the 5V standby rail but it seems to supply enough current to power a Raspberry Pi.

Direct Drive Extruder

The extruder is where the scaling fell down a bit. The original Huxley used a Bowden drive to make the carriage small. I didn't fancy that but I didn't want to have the carriage as big as a geared extruder would need, so I went for direct drive with a NEMA14 and 1.75mm filament. 

The filament needs about one third of the force to feed and a Wade's has roughly 1:3 gearing, so a direct drive NEMA17 is about equivalent. The NEMA14 has half of the torque, so it is a bit under powered. I used the smallest drive pulley I had which was a mini hyena from Laszlo Krekacs' Indigogo campaign. Unfortunately I don't think the small diameter version is available now. I could probably make one from a hobbed bolt if I needed to or hob one from scratch.

It feeds PLA fine at 200C but isn't able to pull it off a spool. I will try a spool holder with a central bearing rather than the rim bearings to see if that is low enough friction. If that doesn't work I might try a powered filament supplier like the one on the first Up printer preserved here. Or maybe even try Bowden drive.

The design is parametric so there is a NEMA17 version suitable for Mendel90. I just need to adapt it for a commonly available drive pulley. It should just be a matter of adding a description here.

It can also use the E3D hot end but that doesn't fit between the bars on a Huxley90.

So that is Github up to date and hopefully correct although I haven't tested a lot of these changes.

I noticed that Blogger is now a lot worse than it used to be. Headings and pictures are now a nightmare.