Low cost 0 backlash transmission - Ball bearing + trapezoidal screw

[M250cnc]

True, you're not going to be able to put anywhere near as much force on it - and the pressure is much greater due to the tiny contact area. Probably only a couple of mm, not 20, since it's only contacting at two lines (in theory). Could improve this with more bearings.

I was being generous, so its even worse than i thought it was.

Yes it does. Except one thing - the sort of machines we're making are generally not used for heavy cutting (like commercial machines). I think this method excels when you're only cutting woods/plastics so the forces are low. ACME rod would certainly be better, but I just used stainless rod as I know that eventually I will probably buy a ballscrew.

It could be good for a PCB machine where low backlash is going to help with accuracy and the cutting forces are tiny.

To drive a small PCB machine a belt drive like they use on the RepRap machines would do the job better, be easier to implement, would last longer and be far stronger. You also do not need angular contact bearings for the fixed end of the screw either. That is the HD 10mm pitch belts BTW.

Phil