Z zero changed mid job

I was cutting a pocket in acrylic on my XL (bought 11/16) when about 20 minutes in it suddenly started cutting through the acrylic and several mm into the wasteboard. Originally the Z zero was paper thickness above the stock, but after I canceled the job, zero was about 13.5 mm below where it was originally set. There were no retractions or other upward movements at the time. I would expect skipped steps or a slipped pulley to cause it to cut too high, rather than too low. A false trigger on Z home would cause it to try plunging 80mm below the original position. On the piece that was inadvertently cut out, I can see an edge where it ramped down to the new mistaken depth. It wasn’t a sudden plunge.

Could this be a bug with Carbide Motion, or is there something else that could cause Z to change to below the stock? A slipping pulley could do it on retraction, but there were no travel moves in the section were the error occurred.

If you are using the G55-G59 settings and do not use them correctly, I have noticed after the spindle move so farm from your original zero position that is set as G54, the machine will sometimes reset to the G55-G59 positions that are stored without any reason. I am not sure what g-code sender you were using but you may investigate to make sure this is not what is happening.

When I had inadequate belt tension, on some cuts, the machine would work itself deeper — it seems to have a harder time retracting and holding position when being pulled down by cutting forces.

Please check the belt tension and pulley set screws.

The belt and pulley are tight. When the motor is energized, I can’t move it, and the flat of the shaft is still aligned with a set screw. I wonder if the Z motor is shutting off after some amount of time after the last movement. I’ll give this command a try and see if it makes a difference- $1 255. That is supposed to set the motors to always stay energized.

I am currently using Carbide Motion for control, and Fusion 360 for CAM.

It might be in the way that the F360 post processor ends the job. I noticed that it moves to the G28 defined positions. On my Nomad this appears to be against the homing switches. I don’t know what it would do on a Shapeoko and whether CM intercepts it or passes it along.

If this turns out to be commanding a move that is 13.5mm higher than your machine can go, you’ll lose steps. Without a limit switch, you’d probably hear it though.

Edit: party->post

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