I don't see the point of wanting a round number. As soon as you've entered it into mach3/EMC you won't need to know about it at all. I've forgotten what the value is per step on mine...

As fragger said, you will almost certainly be using 1/4 or 1/8 micro-stepping anyway which effectively changes the value but doesn't really gain accuracy.

Quote Originally Posted by fragger6662000 View Post
By using pulleys you get
-another source of backlash
-drive flex(belt stretch)
-reduced top speed
-side load on the screw
1) no, unless your pulleys are awful. Mine have not added backlash.
2) negligible from my experience.
3) What! That's impossible to judge without calculating it properly. You need to get the motor running in the optimal region of the torque/rpm curve and to do that you need to get the ratio right which, unless you're very lucky, requires pulleys.
4) Surely the bearings in the mill are going to cope with that...if not add more? I'd be more worried about the bearing on the stepper motor.

On my router I've found that generally increasing the size of the pulley on the stepper motor has increased the top speed, but of course sacrificed resolution. It's less clear cut on a milling machine as it's heavier and the coefficient of friction is greater.