Sunday, April 05, 2009


A gripping read

I have done some work comparing the amount of grip that different extruder drive methods achieve. I measured the amount of force the following devices gave before the filament slipped: -

A 4mm splined shaft.
A 13mm timing pulley
The original M5 thread drive
A 13mm knurled wheel
A 13mm threaded "worm" pulley

The results where as follows: -

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.

More details in my blog and


The worm pulley design is what is in the stratasys machines
This is really useful stuff. Zach started all this with a range of experiments. The most useful thing about his and Nop's (and my) results is that it gives us a range of working solutions that we can use in different designs. For example, I want to do two things in the near future: one - to make a really small extruder, and two - to make one that works with drink bottles cut into strips. These may well need different solutions.
If you want to make really small extruder you might consider feeding it with 0.5-1.0mm filament from a static version of the current extruder. That way the forces get much less, although you do put the plastic through an extra heat cycle.
I have a NEMA 14 that will do about 10 N-cm (, which should give me 4 kgf at 2.5mm radius (which is the stepper's shaft). From your numbers here:

that should just about be enough to work ABS and PCL with a 3mm filament. I'll try it and see.

If it works it should give a very light and compact extruder design indeed - about a credit card in plan view.
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