My z axis is doing queer things. It will begin cutting a part and when it rises to the safe z height and plunges to the next cut it does not go to the material height but cuts air until it eventually counts down the passes calculated in Mach 3 and eventually reaches the material surface when it begins to cut again. It does not cut right through the material because it thinks it has stepped down to the right thickness.
Today it finished a cut and instead of raising to safe z above the material it cut a horrible groove across the material back to the origin point.
I went to SETTINGS in Mach 3 and configured the z axis again for steps. It was accurate. No problem with the stepper motor.
I then dismantled the machine (a 6040 from China with water cooled spindle). I did not find anything binding in the physical workings of the z axis and gantry and the coupling between the stepper motor and the ball screw was secure.
I can only think the problem is in the circuitry of the stepper drivers.

Any ideas out there?

It's a very long shot but it could be that you are loosing steps on the Z axis. Try reducing your Z axis Velocity and Acceleration (in Motor Tuning) by 50 % and save the changes.
Try it and report back on the result.

Tweakie.

I would try something else first before motor tuning. Use the rapid rate override slider and slow down the rapid rate only. Rapids at full speed are when motor torque falls way off and steps can get missed. As you lower the rapid rate the motor torque increases and that may solve your problem. My machine never misses steps, but the other day it did while cutting stainless. Part shape was perfect but location was wrong. I finally realized that I was using the slow speed spindle with a large motor that is much heavier so it missed some steps during a rapid to a new location.

Further to my earlier post on my confused cnc I have an embarrassing confession to make.
My problem had nothing to do with software or the cnc hardware, but the operator's fiddling.
I have been building a dust shoe and in the process I raised the spindle in its clamp to try and get more z height. However, in doing so I also lowered the distance between the bottom of the z travel and so the spindle carriage was hitting the bottom of its reach and the stepper motor was put out of sequence.
When I realised that the problem was solved.javascript:void(0);