Ballscrews v. acme screws

[Jonathan]

A bit beyond my budget. It would also reduce table travel too much.

Assuming you're getting the ballscrews from linearmotionbearings2008 on eBay the extra ballnut is pretty cheap. I can't remember exactly but I think it's less than $30 for an extra ballnut.

Sorry, I don't understand this but my electrical engineering degree was obtained 44 years ago.

Mine should be obtained in almost 3 years. Looks like I'll have to wait until the final year for the module containing stepper motors, sigh.

I'll have to do more reading! Can you give me a technical reference?

Err...not without using Google which I'm sure you can do. But I will explain a little more fully.

I said originally that the stepper motor torque is the vector sum of the motor winding currents (common sense really). More precisely it is proportional to the phase currents, up to magnetic saturation of course, but that is irrelevant to this discussion. For full step the rated current is applied to both phases, call it 1 amp, so the vector sum of those is:(1^2+1^2)^0.5=2^0.5=1.41
When microstepping a sine wave is applied to one phase, and cosine to the other phase. So the vector sum of those is:
(sin(x)^2+cos(x)^2)^0.5
Using the trig identity sin(x)^2+cos(x)^2=1,
(sin(x)^2+cos(x)^2)^0.5
=(1)^0.5=1

Hence the torque when microstepping would be 1/1.41=0.71 times what you get with a full step drive.

But, to avoid this (and due to other effects such as resonance damping / motor not ideal) the waveforms are not pure sinusoids. They are distorted to be closer to a square wave, and hence you get close to the full torque. Some drivers now revert to a full-step drive at higher speeds anyway, so clearly they must get the full torque.

Hope that helps, or maybe expanding my previous post was just stating the obvious...