[RotarySMP]

Thanks for the reply on the gearbox housing and X ball nut space. What ball screws have you chosen?

I use Turbocnc, any my knowledge of Mach is theoretic, as I have been considering changing to it.

Having RPM fluctuations within a 2% band does not sound to me like it is a problem. It will depend on what you are trying to make, but I doubt that a <2% error on a thread will be noticeable for most practical applications.

If you try to make a 4 foot long lead screw for an telescope mirror polishing machine, then it is probably unacceptable.