Z Axis randomly starts 6mm high or low

[dandangerous (Dan Dornback)]

What Z-axis are you using: belt, z-plus, HDZ?

I just did a write up on the Z-Plus and failure in the leadscrew nuts:

Z-Plus Leadscrew Nut Replacement - How To Guide

I was having issues with excessive depth of cut that I finally tracked down to a worn leadscrew main nut/anti-backlash nut assembly, documented here: Z-Plus play/backlash question Support was kind enough to expedite repair parts, it took about 20 mins to replace and was easy enough to do. Here are the steps for anyone else with the same problem. This is the assembly that needs replacement. Note that the nuts are plastic (feels like nylon or similar). The main nut is held on with the 2 screws a…

And if you’re on the belt drive, check your pulley set screws:

Pulley set screws - XXL

After dealing with the Y1 pulley set screws backing out numerous times on my XXL and messing up jobs, finally decided to implement regular socket head screws - covered here and other places, but a couple points to add for others with the same problem. The socket head screw size you need is M3 x 8mm, you can get a multi-size box from Amazon for $6 that will be more than enough. You want to use “purple” thread locker, it’s low strength and made for set screws. Several brands out there for ab…

The random 6mm high, 6mm low sounds a lot like a slip in the pulley on the motor shaft.

Also check your motor mount screws regardless of which Z-axis you have.