I was thinking of a way to remove the Z-axis motor from a RepStrap machine, and instead use the X-axis to ratchet the boom up and down by traveling to its extremetries and 'pushing' a ratchet mechanism.

Pushing the red frame from one end will turn the captive nut by 90 degrees, pushing the other end turns -90 degrees. This would result in a distance of around 0.3mm travel on a standard M8 thread.

It may also be usable as a mechanical stepper motor with the frame driven by a solenoid and the output geared down to required step rotation. The solenoid should be pulsed one way (or the other) until the frame has reached it's limit of travel.

This 'motor' would be expected to be quite slow and noisy, hence this 'motor' is refered to as a 'mungomotor' ;-)

This part is at present theoretical, although I would welcome any comments.