Ballscrews v. acme screws

[Jonathan]

1) All the C7 screws I've got have been <0.001" backlash... even after a fair bit of use. Also, I'm not sure, but I reckon the delrin nuts will be a lot more 'springy'.
Either way for a mill your best off using two ballnuts with a spring in between to eliminate backlash (google it).
2) No, luckily that's not right. The stepper motor torque is vector sum of the motor winding currents, so 1.41 (root 2) for full step and 1.00 (as sin^2(x)+cos^2(x)=1) for a microstep drive. However that doesn't apply now as for over 10 years microstepping drivers have used morphing, distorting the waveform from sine/cos to closer to square to achieve the full torque.
3) Advantage for ballscrew as the higher pitch means screw angular velocity is much less, so the inertia and thus the energy the motor has to impart into the screw is a lot less. May not be that significant compared to the relatively high coefficient of friction of dovetail slides, but definitely is on a machine with rolling bearing slides.
4)Ballscrew definately wins ... hardened metal rolling versus hard steel *sliding* on plastic!

I still have ACME screws on my mill that I converted to CNC a few years ago ... it works fine, fast enough, but the backlash is really annoying so I'm going to convert it to ballscrews asap. When I've decided what diameter screws are best and will fit.