Shapeoko 5 Pro losing z zero

All of a sudden my shapeoko 5 pro is losing the Z zero on projects at random. The sequence of how I set up and check the z zero is as follows:

  • Initialize the machine (bit setter is active so it checks x,y,z limit switches then runs up to the bit setter & checks the bit length. I always start with the correct bit in the spindle)
  • Load new file (I’ve also tried manually setting the z axis 0 position before loading the file i’m going to run. it doesn’t seem to make a difference)
  • hit “Jog” and then manually set the Z zero position at the top of my workpiece. When I click on the Jog button it wants to remeasure the tool. I’ve tried allowing it to measure the tool and i’ve also tried skipping it at that point - either way it doesn’t make a difference for eventually losing the Z axis 0. I am careful to only zero the Z axis as i’ve inset some sections of PVC so that i can repeatedly get my workpieces in the exact same position. Doing this means that i don’t have to constantly reset the X or Y axis and so far i haven’t had any problems with losing the X or Y positions.
  • Back on the “Run” menu i select “Quick Actions” and click “Plunge to 1” above Z Zero". I use a set up block to confirm that the bit stops exactly 1" from the top of my workpiece.
  • I then start the job. Carbide Motion asks that i load the correct bit (which i always have loaded in the spindle to begin with). It measures the bit using the bit setter and then jogs over to begin cutting. It’s at the point of the initial cut that The problem of the z axis z having come undone becomes apparent. Sometimes the bit gets buried in the workpiece, sometimes it starts going through its movements 2" above the workpiece and sometimes it begins cutting at the correct depth.

This problem of the CNC forgetting the z 0 position happens roughly 1/3 of the time is start a project. When it does happen. I’ll go back to the jog menu and reset the Z 0 position. When i get the bit set correctly and go to the menu to actually set the zero I can tell exactly how far off the zero became by the value currently listed for the Z axis. When it’s operating correctly and I go back to reset the 0 position, that position shows up as “0.00”. However, when it’s lost the z position the value will be equal to the amount of error that’s crept into the Z axis 0 position. For instance, when the bit get’s buried I’ve seen that value as high as “0.81” and when the bit is hovering inches over the workpiece i’ve seen that value as low as “-3.0”.

I’d appreciate some help to understand what could be introducing this error.

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I’ve only had this happen several times before I realized it was static discharge causing the problem.
I was closing the door to my enclosure as it was measuring the tool during the program execution, and the metal hinge connecting the plexiglass door was bumping the motor mount.

I made it a habit to check the Z position before the tool measure, and again after. Since the first tool was already measured during initialization, the value should be the same. If the value changed, I would re-zero the Z, and all was then good.

Another common cause is disruption of movement in the Z axis. If the axis stutters, pauses, grinds, or tries to go higher than the physical limit, it can cause the machine to lose steps. It’s not a closed system, so the controller tells it go to a position/distance and then just assumes it made it to that position. It has no way to check. Anything interrupting that motion could cause the axis to be out of position.

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Thanks for the suggestions but my cnc isn’t in an encloser and there’s nothing coming into contact with the z axis motor. It’s also not having trouble with stuttering or anything like that. The Z zero doesn’t change once the motor is running, the problem is always the initial cut right after setting the Z zero position. I am very careful to always verify the z zero position before starting any program but even so it quite often changes the Z zero once the program starts.

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