Nema34 - DM860A Driver & Mach3

[cropwell]

Is this calculation valid even if the pulley on the threaded rod has not been tapped but is held in place by nut-locknut?

Only if the pulley does not slip, and that arrangement looks as if it will do so at some time.

You are only using threaded rod on X and Y as axles, so the pitch is irrelevant.

I have yet to get my head round the X and Y axis arrangements, but it looks like the belt attached to the axis will move 200mm per turn of the axle, but you are geared down 3: or 4:1 depending on the number of teeth on your drive pulley. So if you are using 15T drive then your resolution is 4 steps per mm or if 20T 3 steps/mm. So I can see why you are hoping to get some positional accuracy using microstepping to increase your resolution.

I have doubts that this current design will be very accurate. You should also look at some way of fixing your pulleys to the axles other than locking nuts, are there no grubscrew holes on the pulleys ?