Z Axis Trouble

[pro70z28]

The last time I used the CNC (ShopBot PRT96) I was cutting stencils out of PVC. After cutting a letter or two the bit wasn't going all the way through the material. After a few more letters the bit would not even reach the material, as if the stepper motor was loosing track of it's location. I can run the Z motor all the way up and down with the buttons on the screen and the stepper motor runs smooth and quiet top to bottom. Yet when I zero it and start the job, it once again comes up short of going all the way down the longer it runs.
Any ideas? Is this a stepper motor issue (hardware) or is there a software set up out of whack? I, just a few months ago, upgraded the control box with a new board and it has been performing flawlessly since then.
Thanks for your help.

[Gary Campbell]

My guess is you are suffering from static "creep". I am assuming that you are using a dust collector. Run one of the problematic files again (no material on table) without the DC running and see if the Z returns to its original zero surface when commanded (MZ,0)

[Gary Campbell]

In case your problem is static, here is some info that may help: https://youtu.be/zKzjhp-yDkE

[pro70z28]

No dust collection was used for this job. I'll take another look at it and bring back more info.