Random Z-Axis plunge

[John S]

It could also be missing steps because the Z axis is too weak / tight or trying to move too fast.
What happens is you are 10mm off the work, everything zeroed and command a Z-10 move which should put it on top of the work.
Then you command Z0.0 which should go 10 mm above the work but because of missing steps it finishes up say 5mm above the work.
When you then command the next Z-10 move it bungs the cutter 5 mm into the work.

Check the Z axis for freedom and binding and also reduce the acceleration and velocity in motor tuning.