Screw drive to push filament?

Has anybody seen, or tried, using a screw thread to push the filament into the hot end? I read through a lot of back threads and thought I'd seen where someone had tried this early on, but now I can't find anything. Maybe I'm just dreaming, but I thought I'd ask.

I'm trying to remote the extruder drive motor for my legostrap and such a beast, if it exists, would make my life much easier .

thanks.

Back in the dawn times Adrian did one up:
http://reprap.org/wiki/RepRapOneDarwinThermoplastExtruder

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-Sebastien, RepRap.org library gnome.

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Remember, you're all RepRap developers (once you've joined the super-secret developer mailing list), and the wiki, RepRap.org, [reprap.org] is for everyone and everything!

SebastienBailard Wrote:
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> Back in the dawn times Adrian did one up:
> [reprap.org]
> xtruder

Thanks. I knew I'd seen one someplace.

Now to see if I can adapt it or use some concepts from it...

patbob Wrote:
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> Now to see if I can adapt it or use some concepts
> from it...

One problem is that it rotates the filament, so the filament spool needs to be able to rotate in two axes. Also, it is slow extruding and difficult to control accurately without an encoder, and is somewhat complex. These are the reasons the move was made to a stepper driven pinch wheel extruder. If you can use the idea to make a better extruder though, go for it!

NewPerfection Wrote:
> If you can use the idea to
> make a better extruder though, go for it!

Slow probably won't be a problem for me -- my linear steppers aren't exactly speed demons so my cartbot is on the slow side as it is.

I'm planning to adapt the ideas rather than use the design directly. I'm sure I won't improve on anything, but I'll probably learn something along the way.. with any luck, maybe even something useful to others

Screw drive has the best grip: [hydraraptor.blogspot.com], particularly for soft plastics. It also has the advantage over a pinch wheel that the amount of plastic fed is simply the thread pitch times the revs. With a pinch wheel it depends on how far the plastic sinks in to the teeth, so harder plastic feed a little faster.

It isn't slow to feed, simply a higher gear ratio, so you just run the motor faster. The torque is lower, so you can probably direct drive with a stepper.

The down sides are that it is very mechanically inefficient because of the friction involved in cutting a thread into the plastic. It also makes dust. As stated above it tends to make the filament rotate and each rotation reduces the feed. The force on the half bearings is high and brass does not last long, so you need to do something like this: [hydraraptor.blogspot.com].

Reprap moved to pinch wheel and it generally considered better.

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[www.hydraraptor.blogspot.com]