Summary

This anti-backlash gearbox has two inner cycloidal gears and three outer housings. The outer housing is held still and the inner housing is twisted with a preload from non-stretch braided fishing line. Ratio 24:1
Video at https://www.youtube.com/watch?v=2fDbMWHQbC8
Testing at https://www.youtube.com/watch?v=EE5JBO_b6KY
More at https://hackaday.io/projects/hacker/313303
Square tubes are thin wall 25.4mm (1") Al extrusion with tapped holes.
print two of ab_output_out one of the rest
Tap the encoder bearing holder with M3x0.5
Also required
6x 3mm shaft 40mm long
2 x M3x50mm csk bolts
2x M3 nuts
16x 8x5x2.5 bearings
4x 18x12x4 bearings
1x 12x6x4 bearing
2x 58x45x7 bearings (6809RS)
6mm shaft 70mm long
Motor
Multistar Elite 5010 or 5008 from Hobbyking but they are always out of stock
I'm using the Gartt ML5010 from gogorc now