I am running version R 1.84.000 of Mach 3

I have a mechanical dial indicator on the Z axis and have a m1 pause programed near the start of my part program just after a Z0.0 line.  I religiously check the dial indicator on each pass to mechanically verify that Z is at zero.

On a number of occasions over a few months, I have found the dial to read minus, today minus .004 (this was after running about 15 parts without a problem).  Since I was cutting oversize on this pass, I completed the cut and found the part to in fact be -.004.  I made a .004 correction to the tool offset and and following parts were AOK.

I am sure that it is not a mechanical problem.

Any suggestions would be appreciated.

Thanks, Quin

For starters, you're using a 9-1/2 year old version of Mach3. First thing I'd try is a newer version maybe?

I suspect that you are trying to drive the z-axis a bit too hard.  Have you tried adjusting the motor tuning down just a smidge?  Doesn't seem like you are off that much.

Thanks for the idea.
Cutting back both the acceleration and the velocity slightly did the trick.

Any suggestions on a procedure or test program to set up motor tuning with a margin of safety?

Quin