Thoughts on rafts

Erik asked me for details of how I do the rafts so here are my thoughts.

I am not totally happy with the way they are currently and keep fiddling about with them. At the moment they hold well and give a good flat surface on the bottom of the object, but they can be difficult to remove. I use a blunt penknife to remove them.



I use the long blade to remove the raft from the bed and the object. The smaller blade is handy for clearing out strings from internal areas.

It needs to be not too sharp otherwise it tends to cut into the object, or the bed, rather than prizing them apart, or scraping off strings.

I have developed a thick callous on my thumb while making the Darwin parts, and I frequently stab myself, another reason for not having it too sharp!

I think most people use more sparse rafts than I do. They will be easier to remove, but I find that gives a ribbed base on the object.

My rafts are orthogonal to the axes and the infill is at 45°. I find that convenient because you can tell where the raft ends and the object begins.

The bed I use for ABS is, I think, Foamex PVC foam board. It is a solid dense foam 3mm thick, not the type that is soft foam laminated with paper. I glue it to a piece of wooden floor laminate with Evostick contact glue. Now that Evostick has gone solvent free / water based I find it takes much longer to dry than it states on the tin. Blowing it with a gentle breeze from a fan makes it dry much faster.

Even when it is glued down, I find the warping force is strong enough to lift the edges, so I have a frame around the edge that is screwed down. The foam board is reusable over and over again. I only have to replace it when I have had an accident that makes the raft impossible to remove (head too low, or temperature too high).

Other people have reported good results with Acrylic sheets and of course an ABS sheet will work. I have yet to try these. For HDPE I use a PE-LLD chopping board from Ikea that is 10mm thick.

I find that I need at least three raft layers. I.e. each of the three layers has a definite function.

The first layer of the raft has to stick well to the bed but still be peelable. It also has to be thick enough to cope with bumps and troughs that develop on the bed with use and slight errors in the z-calibration.

The filament diameter I use for the base layer is twice the nozzle diameter or 0.8mm, whichever is the biggest. The height of the head is 0.7 times that. The pitch of the zigzag is 3 times the diameter. The head is relatively low, so that it gives a wide filament pressed against the bed. It is widely spaced so that when the bed has a bump it can spread further without merging. It is extruded at the maximum rate that the extruder will do, which works out at only 4mm/s with 1mm filament through a 0.5mm nozzle. The temperature is 225°C for ABS and 215°C for HDPE. The other layers of the raft are extruded at 240°C to bond strongly to the layer below.

The middle layer's function is to give the raft some strength and bridge the ridges created by the bottom layer. The filament diameter is 1.5 times the nozzle aperture. The height of the nozzle above the layer below is again 0.7 times the nozzle. The pitch is 1.2 times the diameter, so that gives closely packed threads that tend to merge.

The top layer aims to present a flat platform to the object but still be discrete threads so they can be picked off one by one. The diameter is the same as the nozzle and the height is 0.8 times that. The pitch is 1.8 times the diameter.

The raft is cooled back to room temperature with a fan before the object is placed on it. The first layer outline of the object is done at half the normal speed (8mm/s for ABS, 4mm/s for HDPE) and at a temperature of 215°C for ABS and 230°C for HDPE. The first layer infill is done at full speed and at 195°C for ABS and 205°C for HDPE. The rest of the object is 240°C.

One annoying thing is that when I peel the raft most of the top layer is left stuck to the object not the middle layer. This is despite the fact that it was bonded to the middle layer at a high temperature, and to the object with a low temperature. I think the reason is that the contact area is 100% against the bottom of the object, but the top of the middle layer is quite wavy so has less contact area. I think adding another dense layer between the middle and the top will fix that, but waste more time and plastic. It is on my very long list of things to try.

As Erik suggested, small objects do not need to be bonded as strongly to the raft as large objects. Something else I mean to try is some logic like this: -

If the length or the width is > 30mm and the height > 5mm then use a strong raft else a weaker one.

Production issues

Since the beginning of the year HydraRaptor has been fairly reliable. I optimistically thought that I could get the 100 hours of printing done in a week. I set one build off before I go to bed and another before I set off for work. I use these slots to print the large parts, in multiples if possible. I use the evenings and weekends to print the many smaller parts while I am around to remove them.

I have a script that can print multiple copies of the same object. It works out how many can fit on the bed and spaces them out so that the head can get in between them. It then prints them, one at a time. I have to remember to tweak the script whenever I change the extruder shape. It simplistically works out the bounding box of the object and then uses the object height to decide how fat the extruder is at that height and spaces the objects so the extruder clears their bounding box. A more sophisticated approach would be to do collision detection between 3D models of the extruder and the object. That would get them a bit closer together in a lot of cases, but I don't consider the gain worth the extra complexity.



I could get a lot more on the bed if all the objects were printed one layer at a time. The easiest way to do that is to load them all into a CAD program, place them like a jigsaw and then join the bases with a very thin membrane that is too thin to actually print, but makes them into one object so they can be sliced together. The reason I don't do that is that the chance of a breakdown in a build that would take tens of hours, possibly days, is too high at the moment. Also, if it did go wrong it could waste a lot of plastic. The objects would also end up with a lot more string on them as the extruder has to flit between them on every layer. Perhaps when the system is more reliable, and ooze control is better, I will switch to this approach.

Right from the start things did not go to plan and in the end it took two weeks to complete the build and used almost all of my free time during those two weeks. Not something I would choose to do again until I can get it much less labour intensive.

The first time I made these parts the extruder kept breaking due to parts wearing out: the flexible drive cable, the JB-Weld, the 6V GM3 motor, the PTFE barrel and the bearings. Having eliminated all those causes, the breakdowns were more due to human error and bad luck.

This is the extruder I used: -



A recurring problem I had was the extruder jamming due to the heatsink getting too hot, allowing the plastic to melt inside causing the plug effect I have detailed before. It can then no longer be pushed forward, so I have to remove the pump parts and pull it backwards while it is hot.

I have disassembled it and reassembled it so many time that the M3 threaded rods started to lose their threads. I keep meaning to make a wider extruder with M5 bolts but never get round to it. Once the extruder has broken you can't print new parts so you have to fix it some other way, then you don't need the new parts! I should really keep a spare working extruder.

When the threads had stripped, rather than replacing them, I swapped the bottom two wingnuts for threaded brass spacers. They have a much longer thread engagement area, so get round the problem. They are also blind, so they only go on so far. I found with them fully on, I got the right spring tension for ABS using a pair of M4 nuts as spacers. For HDPE I added a second nut each side. Being able to reset the tension consistently each time I put it back together was a big bonus.

The reasons it got too hot were various, but fundamentally it needs a better heatsink or a fan to give more margin between the running temperature and the glass transition of the plastic. The new extruder controller I have built but not tested yet has a second fan drive and a second thermistor input to allow the cool zone to be regulated. A simpler solution in the case of HydraRaptor would be to make the extruder base / clamp out of aluminium. That would conduct the heat to the z-carriage, which is all aluminium, so could dissipate hundreds of watts .

I eventually tracked down the first reason for over heating to a bad four pin connector in the wires to the heater and thermistor. They are rated at 3A in the Maplin catalogue, but I have had problems with them on the extruder controller before and I am only putting 2A through them. The connections go high resistance for no apparent reason. If it is the heater connection then the heater cools down and the connector gets hot. I have had one de-solder itself from the board.

This time the thermistor connection intermittently went high resistance causing a low temperature reading. I also had the extruder motor stop during a build, again it appeared to be due to its connector failing. I don't know why they do this. If I re-seat them then they work for a while and then fail again. Perhaps they are not rated for the number of insertions they have had due to constantly rebuilding the extruder for 2 years! I have built my new controller with Tyco connectors rather than these unbranded ones.

I switched from the 0.3mm nozzle I have been using recently to 0.5mm so that I could use the same g-code I made the first set of parts from. I can do that because I only use the g-code for tool path information. All the machine settings like temperature and feed rate are in my script.

After the nozzle swap, molten plastic started oozing out of the side of the extruder. The bolts which clamp the compression joint had worked themselves loose after many heat cycles. Changing the nozzle broke the seal and let the plastic out. Tightening them again fixed it.

With the older 0.5mm nozzle I had trouble getting its PTFE cover to stay on. This is essential for making the nozzle wipe work and prevents burnt bits of plastic getting incorporated into the parts leaving brown marks. I had a daft idea of taping it on with Kapton tape. That did not work, but when putting it on I forgot the thermistor wires round back and broke one off.

I had to drill another hole and stick a new thermistor in with Cerastil, a 24 hour job. Worse than that I must have mixed the Cerastil with too little water because it started to come out a day later. Because it was out of sight I did not notice, but the objects started to get very hard to remove from the raft and the raft from the bed. It was only when I broke a hole in the surface of the bed that I realised. So another 24 hours of repairs!

I fixed the PTFE cover with two tiny set screws into indentations in the nozzle.

Other times the heatsink seemed to get too hot for no apparent reason, the hot weather did not help. Generally it failed near the end of a large object, very annoying. In the end I used a mains fan about 1m away to keep the heatsink cool.

Apart from the reliability problems the other issues I had were as follows: -

The corner blocks have hair line cracks through the narrow bits half way up the middle of the vertical edges.



This happened on some of the blocks on my original Darwin and does not seem to matter. I think it was worse this time round because the rafts I used held them down better, leading to less curl up of the bottom corners, so more stress through the edges. I made these with 90% fill, rather than 25%, to ensure they were strong enough. It made absolutely no difference to the cracking but added 8 hours to the print time. It makes the thin bits stronger, but it also increases the warping forces by the same amount.

The rafts I use at the moment are very well bonded. On the up side that reduces warping by holding the object down, but it is quite difficult and time consuming to remove. The only object that pulled away from its raft was the extruder drive block.



This is the worst case in the set because it is both wide and thick. Fortunately it does not need to be flat.

This part came out a bit bockety where it gets thin near the top: -



The problem is the layer is so small it does not have time to cool before the next layer arrives on top. I fixed it by adding some logic which halves the extrusion speed if a layer would take less than 10 seconds at full speed. You can see it completely fixed the problem: -



It was not sufficient for the very thin opto tabs though, so I also cooled them with a fan. Here are the with and without fan versions side by side: -



I won't be printing another set until I get my own Darwin up and running. I hope to double the extrusion speed and reduce the ooze with a stepper driven extruder. The bed is much bigger so perhaps it will only need two batches taking about 1 day each.

HydraRaptor's second child

Back in March I had a visit from Marcin Jakubowski, the founder of Open Source Ecology. He was over here in Manchester presenting at a conference and asked if he could come and see HydraRaptor, as he wants to use RepRap machines on Factor e Farm. Like RepRap, his project also aims to change the world.

He asked lots of questions and made a couple of videos of my answers for his blog, which you can see here.

I volunteered to print a set of Darwin parts to help get Factor e Farm up and running with 3D printing. I was confident that I would have my Darwin running in time to churn out the parts. However, because I spent a lot of time experimenting with extruder designs in an attempt to get something more reliable, I ran out of time and had to print the parts on HydraRaptor.

Here they are, all 109 of them: -



All the parts were printed with 0.5mm filament at 16mm/s with 32mm/s moves. Most were sliced with Skeinforge set to 25% fill and larger objects have double outlines to maintain strength.

Here are some stats: -

		Build time	Plastic volume	Quantity required	Total build time	Total plastic	Weight	Cost	Percentage of total
Corner bracket @ 90%		02:44:44	29.1 cc	8	21:57:49	233.1 cc	291 g	$5.83	29%
Diagonal tie bracket-chris		00:27:00	4.8 cc	20	09:00:06	96.4 cc	120 g	$2.41	12%
Bed corner		01:32:06	15.5 cc	4	06:08:25	62.1 cc	78 g	$1.55	8%
Z-motor-bracket-chris		01:23:12	14.6 cc	4	05:32:47	58.2 cc	73 g	$1.46	7%
X motor bracket		03:56:35	37.2 cc	1	03:56:35	37.2 cc	46 g	$0.93	5%
X-carriage		03:51:39	40.2 cc	1	03:51:39	40.2 cc	50 g	$1.00	5%
Y housing		00:56:36	9.9 cc	3	02:49:47	29.8 cc	37 g	$0.74	4%
Extruder drive block		02:30:44	26.5 cc	1	02:30:44	26.5 cc	33 g	$0.66	3%
X idler bracket		02:28:06	25.4 cc	1	02:28:06	25.4 cc	32 g	$0.64	3%
Y motor bracket		01:51:06	19.6 cc	1	01:51:06	19.6 cc	25 g	$0.49	2%
Bed constraint		00:43:20	7.5 cc	2	01:26:39	15.1 cc	19 g	$0.38	2%
Bed clamp		00:21:39	3.7 cc	4	01:26:36	14.7 cc	18 g	$0.37	2%
Extruder base		01:13:19	13.1 cc	1	01:13:19	13.1 cc	16 g	$0.33	2%
Z-coupler-airpax		00:14:21	2.6 cc	4	00:57:26	10.2 cc	13 g	$0.26	1%
Opto bracket @ 50%		00:19:00	3.1 cc	3	00:56:59	9.4 cc	12 g	$0.23	1%
X-belt-clamp		00:10:46	1.9 cc	5	00:53:50	9.5 cc	12 g	$0.24	1%
Wiper-diagonal-bracket		00:43:50	7.6 cc	1	00:43:50	7.6 cc	9 g	$0.19	1%
Wiper-brace		00:13:24	2.3 cc	3	00:40:11	6.9 cc	9 g	$0.17	1%
X-constraint-bracket		00:38:10	6.6 cc	1	00:38:10	6.6 cc	8 g	$0.17	1%
Pulley		00:12:35	2.2 cc	3	00:37:44	6.7 cc	8 g	$0.17	1%
Bolt plug		00:04:36	0.8 cc	7	00:32:11	5.8 cc	7 g	$0.14	1%
Tall foot		00:14:27	2.6 cc	2	00:28:54	5.3 cc	7 g	$0.13	1%
Y motor coupling		00:25:02	4.5 cc	1	00:25:02	4.5 cc	6 g	$0.11	1%
Z-adjuster-housing		00:24:12	4.1 cc	1	00:24:12	4.1 cc	5 g	$0.10	1%
Short foot		00:11:21	2.1 cc	2	00:22:42	4.2 cc	5 g	$0.10	1%
Fan base		00:22:29	4.0 cc	1	00:22:29	4.0 cc	5 g	$0.10	1%
Y belt clamp		00:03:43	0.7 cc	4	00:14:50	2.6 cc	3 g	$0.07	0%
Fan-leg		00:15:48	2.8 cc	1	00:15:48	2.8 cc	4 g	$0.07	0%
X-motor washer		00:15:27	2.8 cc	1	00:15:27	2.8 cc	3 g	$0.07	0%
Z-flag-slider		00:13:00	2.3 cc	1	00:13:00	2.3 cc	3 g	$0.06	0%
Bearing 360 run		00:02:47	0.5 cc	4	00:11:09	2.0 cc	3 g	$0.05	0%	HDPE
Extruder PCB holder		00:09:45	1.7 cc	1	00:09:45	1.7 cc	2 g	$0.04	0%
Z-opto-flag		00:08:45	1.6 cc	1	00:08:45	1.6 cc	2 g	$0.04	0%	Black ABS
X-carriage-bearing		00:08:39	1.1 cc	1	00:08:39	1.1 cc	1 g	$0.03	0%	HDPE
Y-opto-flag		00:07:43	1.4 cc	1	00:07:43	1.4 cc	2 g	$0.03	0%	Black ABS
Bearing 360 jam		00:02:49	0.5 cc	2	00:05:38	1.0 cc	1 g	$0.03	0%	Black ABS
X-opto-flag		00:04:43	0.8 cc	1	00:04:43	0.8 cc	1 g	$0.02	0%	Black ABS
Wiper-lever		00:04:26	0.7 cc	1	00:04:26	0.7 cc	1 g	$0.02	0%
Z-flag-clamp		00:03:20	0.6 cc	1	00:03:20	0.6 cc	1 g	$0.01	0%
Circlip		00:01:26	0.3 cc	2	00:02:53	0.5 cc	1 g	$0.01	0%
Bearing 180-x		00:02:38	0.5 cc	1	00:02:38	0.5 cc	1 g	$0.01	0%	HDPE
Bearing 180-z		00:02:03	0.4 cc	1	00:02:03	0.4 cc	0 g	$0.01	0%	ABS
				109	74:28:04	778 cc	973 g	$19.47	100.00%

The times and weights are calculated, and don't include the raft time, which is significant, or the time waiting for temperature changes and raft cooling. I weighed the parts on kitchen scales and they came out at 931g, so pretty close to the calculation. The cost shown is on the basis of ABS at $20 / Kg.

I save all the rafts for the day when we get recycling working. I weighed them in at ~ 200g, that is about 20% wastage and will bring the actual printing time up to about 100 hours.



I also wasted 150g in failed prints, for silly reasons, more on that later. It gives a measure of the reliability I am achieving at the moment, i.e. 8 parts failed out of 117 prints so 93% success rate. Of course the bigger the part is, the more chance something will go wrong, so by weight and time it is much worse .



I used plain ABS for most of the parts because it seems to bond better than coloured. I used black for the opto tabs. No guarantee that they will be opaque to IR, but I think black ABS usually is. The green parts are just ones I had left over from experiments.

I made some of the bearings in HDPE as that should be a better bearing material than ABS, lower friction and longer lasting. The black ones are "jam" bearings so I left them in ABS as they want maximum friction.



Some of the parts are my own design. Most significant are the z-axis parts described in the previous post. Here is a list of the other design tweaks, with links to the article describing them:- simplified diagonal tie brackets, X-motor washer, x-carriage bearing and the feet.

Some parts I had never printed before. The Pinch wheel extruder: -



The nozzle wiper assembly has appeared in the latest Darwin release but I can't find any assembly instructions. I leave it as a puzzle for Edward Miller, the guy who is actually going to build this machine.



Similarly the new adjustable z-opto flag assembly: -



I aimed to print these parts over the course of a week, three batches a day, but the machine had other plans and it actually took me two weeks. I will give more details tomorrow.

$8 Z-axis

About a year ago I blogged an alternative Z-axis for Darwin using four tin can steppers instead of one expensive stepper and a belt drive. The only thing missing was a source of cheap motors to make it economically viable. Some time ago Forrest Higgs pointed out a source of cheap 15° motors for $2.50 made by Airpax. I also found them available for $2 at Surplus Shed. That makes a z-axis for $8 possible, which is much cheaper than original motor, let alone the belt.

They are surplus stock, so when they are gone they are gone, but there does seem to be a lot of them around. Unfortunately it costs more than $40 to ship them from the US, so the economics don't look nearly so good this side of the pond.

They are 12V 0.4A per coil, so four wired in parallel will take 1.6A, well withing the 2A capability of the RepRap electronics. They are six wire unipolar motors, but they can also be driven from a bipolar drive by using the red and orange wires as one coil and the green and brown as the other.

The pull in rate seems to be about 200pps, which would give 200 × 15 × 1.25 / 360 = ~10 mm/s with M8×1.25 threaded rod.

The boss on the back of the motor is a bit bigger than the motors I used before so I have updated the bracket design accordingly. The motors come with a spiral drive screw on the shaft. I could not find a way of getting it off, so I made a new coupling piece that clamps over it. It has a pointer so that it is visually obvious if the motors get out of step with each other.



I have uploaded both of these to Thingiverse. The other parts needed are shown below: -



And this is how they go together: -

Back to Blog

Sorry I haven't posted here for a couple of months. A few people emailed me to see if I was OK, still alive, etc. No sinister reason for not blogging, I have just been on holiday, had a few weekends away, beer festivals, BBQ's, etc, and also visited the British F1 Grand Prix.




I have also been designing a new extruder controller for HydraRaptor with a stepper motor drive. I normally build electronics straight from brain to veroboard, no schematics or planning, I just pick up the parts and solder them in. That is very quick and efficient but does not leave any design record.

I decided I wanted a micro stepping bipolar drive and the only sensible way to do that is with an off the shelf chip. They are nearly all fine pitch surface mount these days so I needed to use a PCB. It is probably 10 years since I last designed a PCB and I have never used Kicad before so it took me quite a while to get it sorted. Now that I have sent the board away for manufacture I can catch up with the blogging.

Tiny stepper torques big

Having calculated that the tiny stepper and GM17 gearbox combination should be able to drive a pinch wheel, I made a lash up to test the theory.

When you have a 3D printer "lash up" is probably not the right term as quite sophisticated parts can be made easily.



Here it is pulling a spring balance with a piece of HDPE filament.



It got to 10 Kg and then the coupling from the GM17 to the 4mm shaft of the pinch wheel let go.



Not surprising given the torque involved and the fact that it was made with 25% fill. I made it again with 100% fill. I can't remember the last time I made a solid part.



It is coupled to the shaft with a hexagonal steel insert drilled out to 4mm and tapped M3 for a set screw onto a flat on the shaft.



With the 100% fill coupler it easily pulled the scale to the end, i.e 12.5Kg. The motor was powered from 8V (to stop it getting too hot) and stepped at 200pps. With a step angle of 15°, the GM17 default gear ratio of 228:1 and a 13mm pinch wheel that gives a feed rate of: -

200 × 15 / 360 × 1 / 228 × 13 × π = 1.5 mm/s.

That would give an output rate of 54mm of 0.5mm filament per second. I think that is comparable to the rates Adrian Bowyer has reported from a NEMA17, but it only weighs about 60g whereas a NEMA17 is about 200g. There are a lot more parts to wear out though, so a NEMA17 may be a better option. Darwin can easily throw 200g about and HydraRaptor is moving table, so the head weight has little relevance.

I have some NEMA17's arriving this week. I tried one from an old disc drive but it didn't have much torque. I don't know if that was because it had aged in the 20+ years I have had it or whether modern motors are much better.

GM17 stepper hack

I have thought for some time that the best thing to drive an extruder with would be a small stepper with a gearbox. The reason being is that a stepper motor has close to zero efficiency when moving slowly. Power is speed multiplied by torque, so as speed increases the efficiency increases until the torque falls away due to inductance. A gearbox allows a much smaller stepper to be used because it can be run faster producing more power.

I had a look for steppers with gearboxes, but they seem to be ridiculously expensive. An alternative idea was to replace the DC motor in a gear motor with a small stepper. I couldn't find one with the correct ratio though until Solarbotics started selling replacement gears for the GM17. They allow the standard ratio of 1:228 to be changed to 1:104 or 1:51.



That makes the GM17 very flexible as they also do a magnetic shaft encoder with an integral H-bridge driver. Great for robotics, but it seems a bit under powered for an extruder.



The motor is about the same size, and has the same shaft, as the tiny steppers I got from Jameco for my first attempt at an alternative Z-axis.

I cut away the plastic cylinder that holds the motor and RepRapped an adapter flange to mount the stepper.



Here it is assembled: -


The small pinion gear is a push fit on the motor shaft, but I found that with the higher torque from the stepper I had to glue it on.

I can run the stepper up to 1000 steps / second in full step mode, with a 12V constant voltage bipolar drive. The step angle is 15° so that is 2500 RPM! It has very little torque at that speed, but it gets multiplied by the gear ratio of course.

At lower speeds the current increases and the motor gets way too hot at 12V, so it needs to be driven from a constant current drive. That is what I was intending to use anyway.

Jameco state the holding torque as 140 g.cm, so I have calculated the torque after the gearbox, assuming no losses as: -

Ratio	Max Speed	Max Torque
51	49 RPM	0.7 Nm
104	24 RPM	1.4 Nm
228	11 RPM	3.1 Nm

It seems remarkably high as NEMA23 steppers are only about 1 Nm. Note that the max speed is for about zero torque and the max torque is for about zero speed.

I attached it to a screw drive extruder and managed to extrude ABS at a rate equivalent to 0.5mm @ 19 mm/s with a step rate of 800 pps using the 1:51 gears.



So similar performance to a GM3 with these advantages: -

• No brushes to wear out.
• No shaft encoder and PID software.
• No RFI suppressor.
• Only needs step and direction pins on the controlling micro rather than two or three H-bridge controls and two quadrature inputs.
• The output shaft and final gear are one piece, whereas on the GM3 the plastic shaft is on a metal splined shaft that can slip.
• The clutch is one gear back from the output, so gives higher torque before slipping.
  

The interesting thing is that the projected torque figures indicate that it would be able to do a pinch wheel extruder with its original gear set. I will give that a go next.

I think the cost is about the same as a NEMA17. The advantage is it is smaller and lighter, the disadvantage is it would need separate bearings and a coupler. The NEMA17 will go a lot faster, but has less torque.

Threading

Khiraly asked me to explain how I manage to put a thread on stainless steel, so here goes.

Aluminium and brass are fairly easy to thread, but stainless steel is very tough. In order to make it easier you need to use a split die and a holder designed for one.



By tightening the middle pointed screw you can force the die to spread and increase the diameter of the thread a little. That allows you to make a first pass that doesn't cut as deep, so does not require as much force. By loosening the middle screw and tightening the outer ones you can reduce the thread diameter and make a second pass.

Another thing that makes it easier is to use cutting compound to lubricate it. I use Trefolex on Adrian Bowyer's recommendation. It is a sort of green lardy gunk.



To start off you need to align the rod or tube that you are threading orthogonally to the plane of the die. The easiest way to do this is with a lathe. You put the work piece in the headstock chuck and mount the die in a die holder that slides along a bar held in the tailstock.



You then turn the chuck with one hand and the die holder with the other. I use the handy chuck grip that I RepRapped, but a chuck key can be used to turn the chuck in 1/3 turn increments.

You need to go about half a turn forward and then one third of a turn backwards to break the chips off. If you don't it may jam.

When you start you need to feed the die against the workpiece with some force, but once the thread is started it feeds itself.

It is unlikely the chuck will have enough grip for cutting a stainless steel thread from scratch. You may have to file some flats on the stock.

If you don't have a lathe, the next best thing is to put the workpiece in the chuck of a drill press and put the hand die holder flat on the bed. Let the weight of the head press the work into the die and turn the chuck by hand. Once started you can put the work in a vice and spin the die holder.

Using a die to extend the thread on a hex head bolt is much easier because you start on the existing thread and you can hold the head in a chuck or a vice.

Unexpected find

While looking through my collection of salvaged stepper motors I found a couple of NEMA17s. This one came out of the hard drive in the first PC that I bought, an 80286 AT clone for about £1200 in the 1980's.



All the subsequent hard drives I have owned had voice coil head servos, but this one, which was a full height, 5¹/₄", 20MB MFM drive, was built more like a floppy drive with a stepper motor to move the heads.

The motor had a plastic wheel with an endstop on it preventing it making more than one revolution. On removing it I was surprised to find that it was also a resonance damping device.



It seems to consist of a brass flywheel isolated from the shaft by a ball bearing, but coupled to it with a viscous fluid, probably some type of oil. I think it behaves like an electrical snubber, which is a resistor and a capacitor in series use to dampen voltage transients. I think this will have an analogous effect on velocity transients.

I found a similar motor in a 5¹/₄ floppy drive, but that was uni-polar whereas this one is bi-polar, and it did not have the damper. It looks like they were pushing the performance of steppers as far as they could before moving to voice coil servos.

I don't know if it still works, it is more than 20 years old and I damaged it a bit removing it from the shaft as it was glued on. I don't think I will need it when driving a high friction, low inertial load like an extruder drive.

Dinosaur?

This may be an evolutionary dead end, with the move to stepper motors and pinch wheels, but I wanted to try a couple of things that have been on my "to try" list for a long time.

The main issue that I have had with the pump part of the original extruder is that the bearings wear out fairly quickly. Both the half bearings themselves and the lands on the shaft. One problem is that being only half bearings, any lubrication soon gets carried away by the plastic.

The best lifetime I have had is with stainless steel bearings and a stainless steel shaft. The downside of a stainless steel shaft is that you cannot solder a nut on to provide the drive. I have found two ways round this:-

1. Use a hex head bolt. For some reason stainless steel bolts never seem to have thread all the way to the top. Since the thread needs to be sharpened with a die anyway, it can be extended at the same time. It is hard work tapping stainless steel though. You need a split die, set to its biggest diameter to start with, and you need cutting compound. The hex head allows you to get a good grip to stop it turning and the original thread makes it easy to start off square.
   
2. Drill through the nut and shaft and insert a pin. If, like me, you break lots of drills then broken drill shafts make perfect pins. I now buy drill bits in packs of five or ten!

I replaced the two half bearings with three ball bearings. At the top is an M5 bearing to take the axial thrust. At the bottom I use two M4 bearings as rollers to take the radial load.



The downside of this arrangement is that you still need to turn a land on the bottom of the shaft. It could probably be done with a file and drill though. It actually works without removing the thread, but I expect it might wear away the rollers.

This design works but there are a few things I would change if I built another: -

I made it compatible with the existing filament guide to avoid having to reconfigure my machine for HDPE. Ideally the screw holes at the bottom end need to move out to allow longer bolts to hold the rollers and the size needs increasing from M3 as the threads strip eventually.

I left clearance to allow the top bearing to be inserted from below, but left no access to the nuts. Consequently it was very difficult to assemble and I had to make undersized nuts.

I used the smallest outside diameter bearings I could find for the given inside diameter. That was a mistake because it is hard not to foul the outer part of the bearing with a washer as the moving part is so small. Star washers seem to just grip the inner and provide enough standoff to clear the outer. I used counter sunk heads to clear the outer face of the rollers. I expect larger diameter bearings use bigger balls, so perhaps have higher ratings.

All easy things to put right with a design iteration.

Another thing I have been meaning to try is the GM17 gearmotor. I have had some for a long time, but without a second shaft, adding a shaft encoder is not trivial, as it is with the GM3. Solarbotics now sell a cheap magnetic encoder that fits inside the casing, making it a more attractive proposition.

To fit the motor in place of the GM3 a new mounting bracket and a shorter version of the shaft coupler is needed.



Here is the completed pump: -



And here it is built up into an extruder: -



I am waiting for the magnetic encoder to come from Canada so I tested it open loop with a couple of bench power supplies.

The GM17 is a bit quieter than the GM3, but not that much when heavily loaded. It extrudes at a similar rate, but the speed seems to vary a lot with load, so it would be useless without closed loop control. It seems to labour and get quite hot at 12V, so I don't imagine its life would be a lot better than GM3. It overruns a lot when the power is disconnected, so it would need a full H-bridge and reverse thrust to get decent stopping.

I still have lots of things to try: stepper drive, a roller instead of the filament guide, an offset screw drive to avoid the rollers.