Strain Wave Gearbox

This is a 3D printable 20:1 strain wave gearbox for a NEMA 17 stepper motor.

Requires:
 20 x 4mm bearing balls (link, can also be harvested from standard 608 bearings)

4 x M3 * 30mm bolts (link)

1 x M3 * 8mm bolt (link)

1 x M3 nut (link)

4 x M3 threaded insert (Inner M3*0.5 Outer M5*0.5: 6mm, link)

I printed the flex gear in Nylon.
Alternatively, PETG or a stiff flexible may work, but I haven't tested those. 
PLA is too stiff and will break, 95A TPU is too flexible and won't transfer torque.
The rest of the parts can be printed in PLA.

For more about stepper motors, and why I made this gearbox, see the video below: