z axis increases height with every pass

[cropwell]

Doddy, you are right in your first example, but if you are cutting a 10mm pocket in 1 pass you will cut an 8mm pocket and retract to Z5 and stop. but if you work the example of a 10mm pocket with 2mm DOC and 1mm stall slippage you will get a 5mm pocket and retract to Z8 at the end, but never cut air, just a shallower DOC.

Anyway. I would be interested to know if the OP saw the cutter moving around way above the original material surface, or just above the cut level. It still points to a stepper / driver wiring issue. Or maybe condensation messing up the control signals.

My experience of tool slippage is that it works its way downwards out of the collet before it snaps due to excessive forces. This has happened to me twice, and one of those times it went through the spoil board to the bed.

[Neale]

If the machine has always done this (is it fairly new?) then it could be as simple as wrong polarity dir signals to the stepper. In effect, the direction change signal is happening at the wrong point of the step pulse and you gradually get a drift in direction on that axis. Not an uncommon problem when a machine is first set up. There is quite a number of threads talking about this but it's late and my brain is a bit too frazzled to give chapter and verse here.

On normal profile cutting, you generally don't see the problem as there are fewer direction changes but if you are doing 3D carving and, in effect, lose one step in Z every direction change, then it builds up. That's how I found exactly the same problem on my own machine, except that in my case, the drift was downwards and potentially more damaging...