Working on a Wiki page update about that, but in the meantime, I use 1 mm diameter music wire bent on a 1/4" arbor, resulting in a spring with a 10 mm outer diameter. 4 active coils per bolt works well.
The maximum pulling force you can get out of this extruder is directly related to the compression force you get on the filament - stronger springs will give you more pull. 1 mm springs got me up to 16 kg of force in testing, which works out to over 3000 psi - seems like plenty.
Here's a photo of some earlier springs with 5 active coils - that was a bit tight, so I made the newer ones with only 4 active coils. Same max force, just a higher K value.
I don't think friction is relevant when teeth are engaged. The force just has to be enough to push the teeth to their full depth into the plastic. The pull is then limited by the shear strength of the plastic.
I don't get anything like the force Wade does. The plastic shears off in the teeth first.
It's not far off, although I agree with Nophead, in that I don't think friction is the only thing that matters here. Granted, my hobbed teeth are a lot smaller than his, and it's probably a combination of the two.
I tried looking up the yield strength of springs based on the dimensions I have, but it's not an easy thing to calculate. The numbers I came up with were either too high, or too low.
I made those springs by stretching some tension springs past their yield limit, creating a compression spring, and then compressing them until their coils touched again. Neglecting friction and a lot of other stuff, that should mean that those springs are near their yield limit when they're at full compression (ie, the coils are almost touching).
So, I just stretched an extra bit of spring between a scale and a vise, and noted the point at where the spring began to stretch non-elastically - that was 11 kg of force (tension). So I should be able to get about 11 kg of force in compression out of those, give or take a few kg since I was eyeballing this and trying to feel the yield point. Note that the spring K value changes with the number of active coils you have, but not the maximum force. So your 8 kg might actually be a bit low, but it's definitely in the ballpark.
16 kg of extruder pull is not required - I was just tired of having my extruders slip, and wanted to make sure I had more than enough pull. 4 kg or so should be plenty of pull, especially if you don't extrude really fast, or have a well designed hot end on your extruder.
Edited 2 time(s). Last edit at 05/12/2010 01:50AM by Wade.