Z axis loses zero

I have been using Linux CNC for about 2 months now and have been have reoccurring issues with the Z axis seeming to lose it's zero. Below I have outlined my procedure and what I am observing. This happens nearly every-time I run the cnc (Joe's Hybrid, HobbyCNC controller).

Procedure to Reproduce Issue:
1) Load gcode.
2) Position the cutter head to where axis zeros should be.
3) Use Touch Off to zero X, Y, and Z axis. Z axis Touched Off the top of the material to align with the Z origin in the gcode.
4) Preview/DRO read the axis are at 0.
5) Run program with a partial material depth cut. Say .125" in .75" thick material.
6) LinuxCNC positions the tool, and then proceed to send the cutter well beyond the gcode commanded location (assuming Z axis zero is where I defined it).
7) Touch Off the Z axis again (to the top of the material). Typically I can visually see the tool path move in the Preview window when I do this....even though I am reseting 0 to the exact same location I did back on step 3.
8) Run the program again.
9) Now everything works correctly.

I am at a loss of what I could be doing wrong or what could be going wrong.

The most likely cause is missing steps on your Z axis. Thais can happen from
1. Velocity too high in the ini file
2. Acceleration too high in the ini file
3. Axis binding
4. stepper voltage too low
5. Current setting mismatched on your stepper controller

7. Cable issue controller-driver driver-stepper
8. EM in system at /PlasmaArcStart or VFD run

Sometimes G43 is active after toolchange, and maybe your routine uses G49, cancel tollength, so at 2nd try it works correct.

Incredibly helpful thank you.

Off the cuff the toolchange G43/G49 sounded likely based on what I am seeing.

After playing with it i bit, I now believe I am missing steps. As I ran an engraving program I observed that Z 0 became closer and closer to the table (I zero on the top of the material).

Clarification: Missing steps could be observed through my lead screw not turning while the stepper makes a sort of grinding/pushing noise correct?

Sometime when jogging my Z axis goes up it makes that grinding/pushing noise while the lead screw does not move. It does not do this going down. I can eliminate this behaviour when going up by apply a very minimal pressure up on the spindle with my hand (maybe 2 pounds of force)

Note: When this issue happens during jogging, I have always re zeroed the Z axis. In the past week the amount this has happened has picked up a bit seemingly out of nowhere.

Did you ever resolve this issue? I'm having the same problem. I touch X,Y,Z Axis . The bit hovers over the material but never cuts anything. The X and Y runs through fine. I've cut things in the past with the machine configured the same way but had to reinstall.

From your description, it sounds like a slipping coupler or the motor not having enough torque to lift the weight of the Z axis. Also double check the setup and hold times in the ini file.

sometimes people also use ESTLCAM with this tiny Zbounces at 0,02mm that breaks of the Zprecision
Take that to Zero in your CAM if you got this Zfreemovement present

Not completely solved but getting closer.

1st win, adjusting the eccentric bearings to ensure the Z carriage was going up with as little resistance as possible. Had to add two washers to push the bottom bearings out further from the carriage that the top two bearings.


Still having zeroing issues, but likely is related to the tool change that seems to always be mentioned the first time I run G-Code (from Fusion 360, using the LinuxCNC output option). I need to dig into this more.