This thing is a coupling for hooking up an M8 motor shaft to a trolley wheel that has a keyed internal shaft. It's then secured in place with a printed M12 nut. This thing is maintained at https://github.com/hyperair/motor-wheel-coupling, and depends on https://github.com/hyperair/MCAD. Updates: 2014/08/17: Make the motor shaft hole render with a flat portion rather than fully cylindrical. This helps spread some of the load across the entire shaft rather than being fully reliant on the setscrew and the joint between the wheel and collar.