Worm drive

I have spent a long time trying to make an extruder that is reliable, performs well and is cheap and easy to make. My last design fits most of those criteria but I have doubts about how long it will last because I am putting a lot of torque through the plastic gears of the GM17 gearbox. These doubts were heightened when a tooth snapped in a GM3 gearbox that I have been using for a long time.

I decided to make a new extruder for HydraRaptor concentrating on performance and reliability. I have tried to pull together all the results of my experiments to pick the best solution for each part of the design, regardless of cost and ease of building. The result is a "no compromise" design that has taken me a long time to make. Hopefully it will be reliable so that I can move on to exploring other things.

The design criteria for an extruder for HydraRaptor are a bit different from Darwin. The weight of the extruder is far less important because it is a moving table machine (rather than moving head). The z-axis is a big slab of aluminium so I don't need a heatsink or fan, I can just conduct the heat away.

I found that the best form of traction is a "worm pulley". Screw drive has slightly more grip on softer plastic but is far less mechanically efficient. It also has the nasty habit of making the feedstock rotate in some cases and also generates dust.

The pulley can impart in excess of 100N force on the filament before it slips, so to have the grip as the limiting factor we need a motor that can provide that amount of torque. The pulley has a radius of 6.5mm so that equates to 0.65Nm. I could do that with direct drive off a NEMA23, but even with micro stepping a single step is quite a lot of filament: 13mm × π / (200 × 8) = 0.025mm. That doesn't seem much but 0.5mm filament comes out 36 times faster than its 3mm feedstock goes in, so that is almost 1mm extruded per step. That seems way too big for accurate control to me, so some gearing is necessary.

A worm gear is attractive because it gives a big reduction in one step so I came up with this arrangement: -



The pulley is on a 4mm splined shaft supported by two ball bearings. The gears are Meccano gears which are readily available. I couldn't find any other metal gears at reasonable prices. I had to drill out the worm wheel to fit the motor shaft. I filed flats on both shafts to allow the grub screws to grip.

This bearing cover holds the bearings in place and guides the filament: -



The assembly is clamped together by M5 hex head bolts that are captive in the plastic.



You can see the top of the stainless steel pipe that the filament feeds into. It has an aluminium outer sleeve to conduct the heat away from the transition section, rather than a heatsink. More on that later.

A skate bearing is used as a roller to apply pressure to the filament: -



A piece of M8 studding forms the axle. It is held in place just by friction. The bearing is centralised by cheeks on the plastic which are clear of the moving part.

The pressure is applied by springs and M5 wingnuts: -



The nuts on the bearing cover prevent the roller from meeting the drive pulley when there is no filament. That allows filament to self feed easily simply by inserting it into the hole in the top.

I measured the performance by attaching a spring balance to the filament and measuring the force at which the motor stalled for a given current: -



The motor is a NEMA17 rated at 0.3Nm holding torque with two coils on at 2.5A. The reduction ratio is 40:1, so I expected to only need about 0.637 / 40 to give a 100Nm pull. I was disappointed to find that I needed 1.5A to pull 10Kg.

With sinusoidal micro stepping drive the holding torque will be 0.7 times the two coil on value. I.e. 0.21Nm @ 2.5A, so 0.126Nm @ 1.5A. The torque from the pulley is only 0.016Nm assuming a reduction of 40:1, so the worm drive is only about 13% efficient if I have got my calculations right. Before I greased it, it was only half as efficient, so worm gears certainly waste a lot of effort in friction. The article here says they are between 98% and 20% for ratios 5:1 to 75:1, so I am probably in the right ball park. There will also be some friction in the bearings and pull out torque will be a bit less than holding torque, even though it is only rotating slowly.

So it reaches the target torque but with far less efficiency than my version with the tiny motor and the GM17 gearbox.

The other disappointment is that is is quite noisy, even when micro-stepping. That is simply because the z-axis couples any vibration to the wooden box behind it that then amplifies it. I
am tempted to fill it with something to dampen it down.

So this half of the extruder seems to perform, and it should be reliable because there is not much to wear out, except perhaps the worm gears, that is where most of the friction is and they are only made of brass.

I will test the bottom half of the design tomorrow.

Fast extruder

I put together my new extruder controller, the worm pulley drive mechanism with the GM17 tiny stepper hack and the stainless steel extruder with heatsink and ducted fan to make possibly the most complicated extruder design yet!



You can see a better view of the drive mechanism fitted on another extruder base here: -



Here is a reminder of what the heater assembly looks like: -



The heatsink is cooled by a tiny fan. When run from 12V it is very noisy and way too powerful. With my new controller I can run it with PWM just a bit faster than its stall speed. That keeps the noise down and still gives more cooling than needed. I attached a thermistor to the heatsink by gluing it into a crimp tag with J-B Weld.



I can tell the controller to keep the temperature below a specified level by turning the fan on and off. I set the trip point to an arbitrary 35°C. It will even turn it on when the extruder is idle, much like the radiator fan of a car runs after the engine is switched off. This is needed to ensure PLA will never soften and jam in the cold part of the tube.

I run the tiny stepper motor at about 300mA to keep it cool enough to touch. It will take more current than that but runs very hot. A good design would use a single fan to cool the motor and the heatsink.

I ran the motor with micro stepping, so even though it has a 15° step, that gives 192 steps per revolution. The GM17 gearbox has a reduction of 228:1 giving a massive 43,776 steps per revolution of the worm pulley. That seems a lot, but the diameter of the pulley is 13mm, so one turn is 40.84mm of feed. That gives 1072 steps per millimetre. In comparison I have been using an 816 step shaft encoder and an 0.8mm pitch thread, which gives 1020 steps per millimetre, almost the same.

I started extruding ABS with my usual feed rate of 16mm/s for 0.5mm filament, which is 3.14 mm³ per second. I kept doubling it until it failed, which was 128mm/s if I have got the calculations right. At that point it mostly worked but something was slipping occasionally. I think it was the clutch in the gearbox. Backing off to 64mm/s it works fine. That is four times faster than the GM3 manages with a screw drive. It is too fast for HydraRaptor but I reckon my Darwin could go that fast. I have no idea what the build quality would be like but it would get the time to print one down to about 24 hours.

Here is a video of it spewing out plastic.

Fast Extruder from Nop Head on Vimeo.

It isn't mechanically compatible with HydraRaptor without making a new bracket to mount it on the z-trolley, so I haven't made anything with it yet.

All torque and no traction?

As promised, I have tested two more drive methods. The first was a 13mm knurled wheel that I had lying around. Handily it was on the end of an 8mm shaft, so I just pushed it through a skate bearing and pressed that into a bearing block.



The results were: -

	PCL	5Kg
	HDPE	10Kg
	ABS	12Kg
	PLA	>12.5Kg

Surprisingly, a bit better than the same diameter timing pulley that I tested previously. I did have to set the gap quite small for the softer plastics, so the filament comes out quite squashed, which may cause problems downstream. The torque is much more even than with a timing pulley.

The final test was a threaded pulley made by the method aka47 blogged here. Following Andy's instructions, I milled a 6mm slot into a block of steel mounted in my lathe's tool post.



I removed the lathe's chuck and backplate and mounted a collet directly in the spindle taper for best centering and stiffness.

I used the shank of an M4 cap head bolt as an axle and some oiled steel washers for spacers, rather than PTFE as Andy's recommendation.



The next bit is magic. You put a tap bit in the lathe's chuck and advance the pulley towards it by 0.05mm each time the pulley revolves. This is viewed from above: -



You would imagine that the inner diameter would have to be exactly an integral multiple of the thread pitch, and the same for a knurling tool. Oddly it doesn't seem to matter, and I can't explain why, even having observed it.

My first attempt was with a M3 x 0.6 tap. I got the height a bit wrong but is was still usable.



The inner diameter of the thread is only 2.4mm, so the filament did not sit in it easily. I made another with an M4 x 0.7 tap, which has an inner diameter of 3.3mm. Perhaps the best fit would be M3.5 x 0.6 but I don't have one of those.



I mounted the pulley on the splined shaft that I had tested before and reprapped yet another bearing block.



I picked the pulley inner diameter as 13mm to get comparable results with my previous tests. Ideally it should be smaller to reduce the torque required. For all but the 4mm splined shaft test I had to use a socket wrench to wind the shaft.



This gave the best result of all the pinch wheel tests, but not as good as screw drive on PCL.

	PCL	6Kg
	HDPE	>12.5Kg
	ABS	>12.5Kg
	PLA	>12.5Kg

I tried the M3 pulley and that was better still, raising PCL to 8Kg. Here is a summary of all the tests: -

	PCL	HDPE	ABS	PLA
4mm splined shaft	2.5 Kg	3.0 Kg	5.0 Kg	7.5 Kg
13mm timing pulley	4.0 Kg	10.0 Kg	8.5 Kg	>8 Kg
13mm knurled wheel	5.0 Kg	10.0 Kg	12.0 Kg	>12.5 Kg
13mm M4 worm pulley	6.0 Kg	>12.5 Kg	>12.5 Kg	>12.5 Kg
13mm M3 worm pulley	8.0 Kg	>12.5 Kg	>12.5 Kg	>12.5 Kg
M5 thread	9.0 Kg	>12.5 Kg	>12.5 Kg	>12.5 Kg

The red figures are lower or marginal compared to the force required to extrude 0.5mm filament at 16mm/s.

My conclusion is that the worm pulley is the best pinch wheel drive method. It also does the least damage to the filament. It does require a lathe though. On the other hand, using an M5 hex head bolt, a couple of ball bearings and some RP parts requires no lathe and should have better grip. That is the direction I am going to go.