Thanks for your designs.
In my case the main goal was to design a system that could be easily dismantled, as I have to travel each night to a dark side. Instead of using a gear profile I designed a piece with pockets. To avoid any backlash a small screw can press both pieces.
But your design has given me some good ideas to solve the manual focusing. I have a separate Pegasus Astro Controlller, with a knob to move the focuser without a computer.
I attach three pieces: the telescope side (with the four pockets), motor side and the main piece that connects the motor with the main flange, should Jim need to assemble/disassemble the motor each night.
It has not been tested yet on field.
To tighten the plastic coupling to the shaft I use brass inserts, much better than a plastic thread.
I agree with El Corazon that 3D printing is addictive. I have mine since February, and design with Freecad too. Two month ago I knew nothing on 3D printers.
I have been testing a way to engage the main shaft to the motor. You may need to print a plastic coupling as shown (yellow part is the main shaft and brown part is the new coupling).
I had some slippage problems with the first prototype, but I have redesigned a bigger coupling. Unfortunately I have not tested it due to the COVID-19, as we have to be confined at home.
This way the focuser is not so slow, and the resolution is good enough, in the range of 2 microns/step.
Finally, you cannot manually rotate the focuser, but we'll think about that...