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armstrom

Idea for a modified extruder (no more bowden tube!)

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I don't yet have a UM but in my research I noticed a fair number of people having issues with the extruder design. Also, it seems the current design doesn't allow for filament retraction very well. My idea is rather than try to fix the bowden tube design simply eliminate it. Move to a system where the filament is pulled into the hot end directly like on repraps. Now, I ALSO don't want to give up the fast print speeds and additional stability that comes with the very light weight printer head. Obviously, mounting a stepper on the flying print head will ruin that... So.. Don't!

I think a good compromise would be to use a flexible drive shaft (like the flex shaft that comes with most dremel tools). Keep the heavy stepper and gear reduction mounted to the back of the box (but rotate it so the shaft points up). Then run a flex shaft up in the same basic location that the bowden tube runs in the current design. Then you mount a light-weight right-angle gearbox (1:1) on the print head to adapt the flex shaft (coming in vertically) to rotate the knurled bolt. This will add a bit of weight to the head, but not nearly as much as adding a stepper motor. The bowden tube can be replaced with an oversized tube to route the filament.

The right angle gear could be something as simple as an off-the shelf drive like this one:

http://www.amazon.com/Huco-332-31-2-Z-M ... pg__header

or you could piece something together out of a printed housing, some bearings, shaft and a set of bevel gears.

So the power transfer would go like this: Stepper->Gear Reduction->Flex shaft->right-angle gearbox->Extruder bolt.

An alternative would be to mount the stepper at the top of the box with the shaft pointing toward the front. Then the flex shaft would only have to bend 90 degrees rather than a full 180. A small bracket could be hung off the back to move the stepper as far rearward as is reasonable to reduce the minimum bend radius the shaft is subjected to.

I'm curious if anyone has ever considered a modification like this to get the best of both worlds? (lightweight head without a bowden tube). And if anyone tries it before I get a chance a little mention would be nice ;)

-Matt

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Assuming you mounted the motor in the back right corner (where the current motor is) with the shaft up, then transiting the head from X0 Y0 to X200 Y0 would rotate the flex shaft inside the flex shaft housing ~10-15 degrees. So, the normal XY motion of the gantry would have a extrusion/retraction effect on the filament which, I think, might be a lot of work to back out in software. You might be able to do it with an inverse kinematic model, but that would add a lot to the slicing overhead.

I think there are mechanical ways to cancel it out, differentials come to mind. It would be interesting to design a printable version- lots of gears and aluminum torque tubes. The torque tube designs of some RC helicopter tail shafts might be a good place to get inspiration.

Kyle

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Thanks for the feedback. Yeah, I didnt' think about that issue.. hmm.. Well that's the type of feedback I needed! I guess a flex tube will have the same problem no matter where the stepper is mounted. It could be mitigated slightly by centering the stepper vertically above the X/Y envelope but that would only spread the errors out. Hmm... Bummer :)

Edit:

Well... on second thought.. if you keep the axis of the tube rotation parallel to the Z plane then the X/Y movements shouldn't cause additional rotations. It also eliminates the need for the right-angle gear box... however, I'm not sure that a flex tube can bend that sharply.... I'll have to play around with the idea some more.

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This has already been done! I don't have any links handy but I'm sure you can find it in a search or someone more familiar with it can chime in.

The basic problem with it is, those flexible shafts have too much play and depending on start/stop/retraction, it is difficult to model where you actually are in terms of rotation from the one end to the other. People have used laser diodes to track this with software and make compensations, needless to say it removes some of the desired simplicity...

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