Z axis work zero Problem

Hello I wish to report a problem I am having and hope to find a resolution.

Machine Specs: 500x500mm X-Carve with grbl and gshield, nema 17 stepper motors
Software: Easel.
purchased: Feb 2016

material: plywood
Bit: 1/8inch 2 flute upcut.
Feedrate: 1020mm/min (40IPM)
Depth of cut: 0.7mm (0.0027")

Cut a few parts no problem, but noticed I had cut into the waste board. was unusual as I recently leveled off the wasteboard.
Cut some more parts and noticed that the first path seemed to cut deeper then my settings, I noticed this firstly by the noise. There rest of the passes seem fine, based on the noise.
I measured my parts were I hadn’t cut through all the material and noticed depth was wrong (too deep)

Ran a test cut of 1mm (0.04") deep. Cut came out at around 2mm (0.08")
Ran a test cut of 0.1mm (0.004") deep. Cut came out at around 1mm (0.04")

I ran these tests multiple times. Restarted the Machine and homed the machine a few times.
To origin the Z above the work piece I use paper under the drill bit. Accuracy of my Z origin to date has been really good.

Belts look good, I’ve checked of the bearings and nuts, tightened them if they needed to, gave it a good clean.

I Don’t think steps are wrong, has been working fine previously,
It just seems that the work Z is off. first pass cuts too deep. I thought perhaps because I use one computer to work on the drawings on easel, then a laptop to connect to the machine and run easel; it may cause some conflicts, but test cuts were new projects created with the machine connected.

Has anyone had this issue occurring before?

Have you made sure you selected the correct z axis lead screw in machine setup? Not sure if you need to setup the machine on both comps or if it’s automatic saves the options in your easel profile

Yes I check that I had the right screw, I went through the setup procedure again to see if there was anything like that but no luck. I think I’ll have to pull the Z axis mount off and give everything a good clean and bring it make to spec.

Does the steps of the servo perhaps degrade over time? settings of grbl haven’t changed but maybe the board or servos need periodic tuning due to aging/wear?

Since the Z deviation is not consistent I think you are experiencing lost steps.
You have the Gshield, have you performed any current adjustment of the motor drivers?
What is your GRBL setting / value for “Step idle”? It should read 255 (255=always on)
Earlier I had a Z axis that was so smooth it would drop of its own weight.

Also, the set screws might need a check in case these are slipping (secure with blue Loctite)

Also most, if not all, stock is not uniform in thickness so the first pass may be off by the cut depth + thickness variation of stock.

I had this happen and it was my Z axis steps/mm being wrong. If you modify your Z axis setup including the belt tension, it will change your Z axis steps/mm.