Sunday, June 22, 2008


Making the Extruder Extrude

I am now working on getting my rebuilt extruder to extrude, and there seem to be a couple obstacles to perfect extrusion.

On first assembly, it seemed that no amount of tightness (way beyond the "hand tight" suggested) on the silicon tubing produced enough "grip" to push down the filament without slippage.

I plan to take out the silicon tubing as I have seen on another RepRap blog somewhere (I don't remember where), and see if that helps.

Also, the drive shaft does not seem to be rotating with a constant velocity. Instead, certain parts of the rotation are faster, and others are slower. I suspect it has to do with the fact that the rectangular hole in the moulded part that interfaces with the motor's drive shaft is too big. Therefore, certain angles of rotation cause the part to shift on the motor and the resulting rotation to be uneven. Another possiblity is that the axis of the flexible shaft and the axis of the motor are offset (also because of the interfacing piece), causing different amounts of "slack" in the flexible shaft, and therefore different amounts of friction. What do you think? Here's a video of the situation. Try cranking up the sound to hear the differences in the motor wine. Also, any clicking you hear is not the camera, but is that "interfacing part" shifting on the motor's drive shaft.


I plan to test both of these thoughts when I continue. Right now, real-life beckons me. Alas, I must go grocery shopping...


Cut down your motor mount bracket so that you can have a straight shot drive system. Makes a hell of a difference. Take the time now and save yourself the hassle later. Also, lock-nuts for your extruder body. Looks like good work so far!
You don't say which plastic but I think you mentioned HDPE previously.

You need far more pressure for HDPE, ABS and PLA that you can get with some squashed tubing. That may be OK for PCL but from what I saw at Cheltenham I don't think so.

I use springs which are 1.55mm diameter coiled with an outside diameter of about 9mm. They are 24mm long and I compress them about 6mm for ABS. I previously used 1.15mm springs and they were not strong enough for PLA. I think spring force is a fourth power of the wire diameter.

I only use two springs at the bottom and space the extruder apart at the top with some washers so the the thread in the filament is cut gradually.

I think your rotation speed variation is because the flexible coupling is not exactly straight and centered. I used a lathe to line mine up but it only lasts 10's of hours so I ditched it for the direct drive version.
Thanks guys! :) I might go get some springs if this tubing doesn't work out.
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