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