Airtripper's Bowden Extruder

by Airtripper May 2, 2012
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What size is the bearing on the end of the stepper shaft?

MR105 ZZ Model Miniature Ball Bearing 5 x 10 x 4mm
The bearing is optional and not really required for the MK7 style drive gears because the drive gears have a good grip on the filament without a lot of force.

I'm wondering if moving to an even smaller drive gear, like the mk8 makerbot gear, would make it possible to mod this to successfully feed 3mm filament ?

Are these files also available as stp214 or any other real CAD-Format?

No. The only formats available are those that OpenScad can export to.

I wouldn't trust aluminum to last for this at all. Its too soft.

Quick question about the Mk7 drive gear ...

Reprapworld (Netherlands, Europe) have what they refer to as an "Aluminum MK7-compatible Drive gear":

This looks promising. Has anyone tested it?

Can this be printed with PLA or should it be ABS? Also, if PLA I would also like to know what parameter to change for MK7 gear so I don't have to cut it out manually as stated in another comment.

This part has only been tested with PLA so you're good to go with that.

I've uploaded a new scad and stl file with changes to make the MK7 gear fit better.

See update above.

I had already modified the source for the MK7. I guess we were thinking alike. I pretty much made the same changes. Opened up the end a little for the MK7 and moved the filament holes a little to account for the larger diameter MK7. Seems to be working good.

Nice job! Does it really work without problems with this modelship gear drive? Would be really cool and cheap for sourcing....

I would not recommend using the model ship gear drive, it slips a lot on filament because the gear teeth are very fine and don't wrap round the filament.

One solution is to have the model ship gear drive hobbled and I'm planning to make some kind of jig to do it.

At the moment, I'm using what I'm calling "The Twin Drive Extruder" which has worked very well with the model ship gear drives but I'm still looking to change this to a single drive with
hobbled gear.

I would like to build your nice extruder, but for 3mm filament, and using a Makerbot MK7 drive gear from (which has a diam. of 10mm).

Could you tell me where I need to change values, in your openscad script?


The extruder would work for 3mm filament with a geared stepper fitted. However, significant changes to the extruder OpenSCAD code would be required to convert the extruder from 1.75mm to 3mm filament to avoid having the filament bending round the gear and still have room for the idler pivot.

If you want to have a go yourself at converting the design, most of the code is commented and using the hash symbol in front of a line of code should highlight where that component is in the design after pressing F5 key. Then just adjust the values to suit.

I can't do a 3mm version myself because
I don't have the hardware to build in to the design and only supporting 1.75mm filament at the moment.


I was able to use the original files unchanged with Makerbot MK7 drive gear. I had to manually remove 1mm of plastic to make room for the drive gear, and drilled out the top and bottom filament guide holes a little bit diagonally to allow the filament to curve around the 10mm diameter of the drive gear.

I strongly recommend using 1.75 mm filament because it requires less force to push. Depending on your stepper motor, direct drive may not be strong enough to push 3 mm filament. Also 1.75 mm allows much more accurate extrusion:

10 mm of 1.75 mm filament = 24 cubic mm of plastic
10 mm of 3 mm fila
ment = 71 cubic mm of plastic