[boblon]

It's not backlash. Backlash isn't cumulative, so even if you have 1/4" of backlash, it will still return to zero, provided you return there from the same direction that you used when you first measured.
By moving to a SmoothStepper, I would have though that any Mach3 issues would be eliminated.

What is your acceleration set for in the Z axis? Most of the time, the Z gaining position is due to the acceleration being too high.

Other than that, the only other culprit would be some electronic issues.

Gerry, I agree with everything you've said. My first though on the backlash was that it shouldn't be cululative but with the other guys problems in that area I wanted to confirm it myself.

As far as the Z acceleration I can't give you numbers but I can tell you I tried everying from V=100IPM / A= 15% down to 2 or three percent and then with V= 30IMP and acceleration set from 5 to even lower and lots of places in between. A points the Accel was so low it would drift to a stop. I know that not good/accurate info to go on, but through all of that the error was always similar (I say similar because I just eye-balled it after the run). I would think it should have changed in there somewhere, or at least it's a would be a big clue staring me in the face I don't recognize.

BobL.