Take this with a pinch of salt, as I have only built my own machine and have relatively little experience with plasma cutters.
After having decided on using stepper motors, my go to thought when deciding on gearing, was what precision do I want per step of the stepper motor.
Assuming 200steps/motor revolution and 25mm per rev, that gives you a resolution of 0.125mm per step. That's a fairly precise machine.
That's just how I approached the problem, although I went for slightly greater precision per step myself. I guess in a simplistic sense the rule of thumb works. It does seem to put the stepper motors in a decent range of operation.
Cheers
Sam
