Printable Harmonic Drive gearbox MK4 Nextruder

A new type of extruder gearbox: strain wave gear / harmonic drive.
It sort of resembles the cycloidal drive in a way - but it's not the same as the principle of function differs.

Fully printable; internal flex gear must be printed from a flexible material. Precision matters a lot here. When printed from a flexible resin, the extruder was printing nicely for a brief moment. Shortly afterwards, the gear has failed. FDM printed flex gear proven to be extremely resilient but wasn't printing as nicely. The overal gearbox shape is somehow crooked in order to accomodate the 625 bearings I had lying around. If you used smaller bearings, the gearbox might be made to be in line with the NEMA17 motor. But then again, larger it is, less precise the printed parts can be while maintaining the same print qualities (movement linearity). The inner gear has two teeth less, compared to the outer one. Correct extrusion rate achieved at 720 extruder steps per mm of filament with the Nextruder feed gear. Transmission rate is somewhat around 1:20 (forgot the exact figure).

Other hardware required apart from the printed components: 
- Two 625 bearings (MMU2S or MK2 Idler), 
- pancake NEMA17 stepper motor with a long shaft, 
- bolts and nuts, 
- nextruder components: heatsink and feed gear
There is also a suitable 3rd party feed gear available here:
https://store.tonylabs.com/products/nextruder-drive-gear

DISCLAIMER
Consider this to be just a design study / weekend project; definitely not a finished product. Try at your own risk. I shared the project because the design might be useful as an inspiration to other DIY projects as it is:
- pretty simple and mostly printable
- can be extremely cheap to manufacture
- has high transmission rate with very low backlash