I have many salvaged dc motors just laying around in my garage. They
often produce too high rpm/ low torque for many application. So I decided do something about this problem. I just bought my first 3d printer (prusa i3 clone) so this was quite easy problem to fix. I only got PLA plastic available so the heat was bit a problem but if you follow this guide you can make it work too. I also made a test wheels so your next robot can move around :)

Step 1: Assembly Video

All steps are included in this video. There's some test setups in 5.40 mark.

Step 2: Technical Details and Materials

This gearbox has ratio of 1/16. With this 12v motor it can lift around 3,5kg (straight from the shaft.) 2 washers should place between each gear (see details in video). It reduces heat between plastic parts when 2 washers is rubbing each other. Adding lots of bearing grease between everything is essential. Depending your printer tolerances you might need drill out some holes bigger. Motor mount should be standard size.

My print settings with slicer.

Gears