Compact Bowden Extruder on RepRap Prusa i3 with MK8 Gear.

I had previously built a Compact Bowden extruder for my RepRap Prusa i3, 3D printer that utilized a plain insert filament drive gear. I built it from parts from my stock extruder.
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After a bit of research I determined I wanted to go the route of the MK8 hobbed gear. Why change something that works fine? Because I can. I got mine here.

So then I went to print a new Compact Bowden rig…….here is the one I had used in the photo above for the plain insert drive gear.

Because the MK8 gear is considerably smaller in diameter you have to print out a new Compact Bowden. I found a design for a MK8 gear here. It comes with the caveat that the designer has a gear that measures 8mm outside diameter, and 13mm in length. Also the diameter of the teeth is 6.35mm.

Trouble is the gear I bought is 9mm x 11mm x7mm.

The designer says this in his instructions:

Important: If you're not using the same 13x8mm drive gear as I do, adjust the drive gear dimensions in the OpenSCAD file and export idler and base as separate STL files.”

Yikes. I don’t know how to do that. But I figured it out and now I want to share that. First download OpenSCAD.

Now download his files from the “Download This Thing” button on this page and open the file named


Scroll down to this section on the left. It starts on line number 36.
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Now simply change his dimensions 8, 6.35, and 13 to the actual dimensions of your hobbed gear like so. I probably should have just changed them to 9, 7, and 11 respectively, however I took a micrometer and got the exact dimensions. Change the values then save them then hit the F6 key to compile the new file. Once you do that hit the button (upper right) that says “STL”. That will export a printable STL file.
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The designer says to export the base and idler arm separately but to be honest I couldn’t figure that out. I just printed it on my machine and it fit perfectly.
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Now, that little gear has to spin a lot more than the bigger gear to move the same amount of filament. You have to calibrate your extrusion rate. I am using Marlin firmware. You’ll have to figure it out yourself if you are using something else.

Disconnect the tube going to the extruder and just leave it hanging into space. Have a ruler or dial calipers handy.

you should have a little filament hanging out of the tube. I pulled a little more through here just to illustrate.

Open RepetierHost and go to the Manual Control tab.
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Pull it in to where it is almost perfectly flush.
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Now in Repetier type in the following GCode and hit “Send”.


This allows you to cold extrude. Otherwise your printer wants that plastic melted before it will move it. Retract or push your filament out to where it is flush……

Hit your extruder button and push 50mm out and measure the results. My calipers are in inches so:

50mm equals 1.96 inches

When I did the test I extruded 33.68 mm or 1.32 inches……..not even close.

Now 50 divided by 33.68= 1.484

in my Marlin firmware I had previously set up an extrusion rate of 100.94 for a MK7 gear. Your number very well could be different here. When I first set the MK7 gear up the default value in my firmware was 90.

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Anyway 1.484 x 100.94 = 149.85

That is the value you want in Marlin.

Open your Arduino program and launch the Marlin.INO file. Go to the configuration.h file and scroll way down until you see the picture above. The fourth value is what we’re changing. As I said mine was 100.94 previously. Now change it to 149.85 (or whatever measurement you came up with).

Save the file and hit the upload button.
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Now do your test again and measure your 50mm of extrusion.

BAM. Nailed it. Exactly 50mm of extrusion. Your machine is now calibrated. Hook everything up, heat it up and print.
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