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.

Pulling power

There are a lot of extruder drive methods kicking about at the moment, so I decided to evaluate a few by measuring the amount of force they can apply to the plastic before it slips. Rather than build complete extruders, I just made mock ups of the final drive and measured the force they could apply to a spring balance.

My first test was using a splined shaft as a minimalist pinch wheel. This was inspired by Adrian Bowyer's knurled design. I wanted to try it because you can get steppers with splined shafts, so it would be a ready made solution rather than needing a knurling tool and a lathe. I used a 4mm splined shaft that I had lying around. Being that small means the torque required to turn it is quite modest.



I mounted it between ball bearings, which were a press fit into a plastic housing: -



The Meccano gear is just acting as a knob at the moment. I pressed the filament onto it using a skate bearing acting as a roller.



This is my sophisticated test set-up: -



I wind the knob by hand until the filament slips and observe the maximum force for each type of plastic. I noted that tightening the screws past the point where the splines are fully sunk into the filament does not increase grip, it just flattens the plastic more and needs more torque.

The results were: -

PCL	2.5Kg
HDPE	3Kg
ABS	5Kg
PLA	7.5Kg

Not surprisingly the grip gets better with the harder plastics. Unfortunately PCL and HDPE need quite a lot of force to extrude, so this drive method is not really good enough for them. A larger diameter shaft should give more grip due to a larger contact area and possibly deeper splines.

The next method I tried was Zach's pinch wheel drive using a square tooth timing pulley.


This needs much higher torque, but gives a much better grip, particularly with the softer plastics. As you might expect the torque is very uneven as the pulley moves from tooth to gap to tooth.

With HDPE, it pulled out of the chuck before slipping, so I switched to an alternative connection to the spring balance.



The results were: -

PCL	4Kg
HDPE	10Kg
ABS	8.5Kg
PLA	>8Kg

PLA slipped from the chuck and snapped when using the alternative coupling, so the true figure is probably higher, but it far exceeds the force needed to extrude PLA anyway.

HDPE has shot up the ranking because although it is quite soft and very slippery, if you can get a grip on it, then it is very tough.

Only PCL is marginal compared to the force needed to extrude it.

Zach uses a bigger opposing wheel, so maybe that would give a bit more contact area.

The next thing I tried was the original screw feed design, to get a benchmark, as that is what I have been using so far. It can feed all four plastics reliably, the only problems I have had with it are that the bearings wear out after 100's of hours of use, easy to fix by using ball bearings.

The implementation I used for the test has phosphor bronze bearings and a stainless steel screw. Rather than use threaded rod and try to fasten a nut on the end, I used a hex head bolt. Long ones don't come with enough thread, but you have to run a die over it anyway to sharpen the thread.


It is very hard work tapping stainless steel. For my first attempt I made the mistake of turning the top bearing land before tapping, so that I didn't mar the thread in the lathe chuck. Even though I made the land 3.5mm diameter rather than 3mm, the torque required to tap it actually twisted the shaft where it was turned down. My nice polished bearing surface became a dull and wrinkly spiral!

The other half of the drive is made from HDPE. I think this is a big factor in making it work well as the HDPE is very slippery and doesn't seem to wear much.



A self tapping screw secures the PTFE insulator in the clamp.

The other crucial modification is to angle the screw so it bites gradually at the bottom by spacing the top with two M3 washers and only having two very strong springs at the bottom. Here it is under test: -



The grip was too high for my chuck, and the coupling shown above kept snapping PLA, so I made a brass coupling.



This has a 5mm bore that narrows to a 3mm hole in the bottom. I melt the end of the filament to a blob and feed it through the top.

The results were: -

PCL	9Kg
HDPE	>12Kg
ABS	>12Kg
PLA	>12Kg

My spring balance has a maximum reading of 12Kg.

So the screw drive has dramatically more pulling power. It is however, very mechanically inefficient. A lot of torque is wasted by the friction cutting the thread. This can be reduced by shortening the amount of thread engaged. I plan to try it with an opposing roller instead of the HDPE filament guide.

The threaded drive does do more damage to the filament, but the only downside of that seems to be that some dust is produced. The lower filament has been chewed by the timing pulley.



Two other drive methods I plan to try are a knurled shaft and Andy Kirby's worm wheel. That looks like it might have similar grip to a thread, but without as much friction. A lot harder to make though.

Constipated Extruder

My "New Year" extruder, which is the one on HydraRaptor that I use to build things, stopped working while building the first layer of an object. That is the lowest temperature layer, so the plastic is at its most viscous.



I couldn't get it to work again, so I removed the drive and tried pushing the filament by hand. I couldn't shift it. I measured the temperature of the molten plastic with a thermocouple and it was correct, so I deduced that the nozzle must be blocked. I removed the nozzle and when I pushed the filament this came out: -



It is dark and glassy looking. No idea what caused it, but it seemed to have blocked the nozzle. I cleared it out with a drill and reassembled it. I took the opportunity to measure its performance with my "lead kebab" test jig.

Even though this extruder has a 0.3mm nozzle and no taper in the PEEK insulator, it works better than the tapered PEEK extruder with a 0.5mm nozzle.



The most notable difference is that this one has a much bigger heater chamber, so perhaps a smaller heater bore melts the plastic quicker.

I got this interesting graph of flow against force, averaging over five runs of 20mm : -


I think the steep part of the curve is where the flow through the nozzle dominates the force required and the first part is where the plug friction dominates. The point where I operate it is right on the knee of the curve. I suspect adding a taper would straighten it out, but I don't want to strip down my only working extruder to prove that.

So I don't know what caused the blockage, but it is the second time I have had an extruder block, so it goes to show that a detachable nozzle is advisable.