Z-axis keeps moving slightly up during long carves

[Testaccount]

I do have the upgraded X-Carve.
During longer carves the Z-axis keeps moving upwards. I starts losing details in v-carves until the bit doesn’t touches the wood at fine details anymore.
Before starting the job, I plained the piece of wood with a 35mm plain bit, so the wood is perfect parallel to the Z-axis.
You can the see different hole sizes in the pic, they roughly 30min apart and at least 1mm smaller than the first ones.

My first thought was the bit is pushing deeper into the router, but its not.

Any idea?

[DennisHuddleston]

Hey Testaccount, I don’t know if you have checked this or not, but the most common reason that this problem is hapening is the bit is not tight enough in the chuck. You might remove the bit, CLEAN the chuck (sawdust builds up), and reinstall the bit making sure that it is tight.