This joint is compact along its rotation axis as is desirable for various robotic arm projects. The gear ratio is 2:1 which provides a good balance between torque and speed.

The joint part prints in place, requiring only M3 bolts to mount the NEMA 17 stepper motor. The tolerances (gaps) can be modified in the OpenSCAD file provided. If you find the joint is stuck after printing, a gentle knock with a hammer may be required to release it.

I have since made a robotic arm based on this design too: https://www.thingiverse.com/thing:4546364

Enjoy :)