Z axis fails

Hello there,

I've been using LinuxCNC for a couple of years for my Chinese 6040 3 axis CNC machine. honestly it never worked flawless but recently cut into some weird problem with my Z axis.

I'm using Fusion360 for CAD/CAM and exporting a NFC file for the LinuxCNC it a different computer.
when i open the file in LinuxCNC and zero the axes by "Touch off" for every axis (is there any difference if I'm using "Toll touch off"?).
After pressing Play i suddenly have Z height error claims the Z axis settings will exceed limits, also there is no physical truth in this claim. Also sometimes after hitting Play the Z indication suddenly changes by 12mm or so and often i don't catch it on time and the bit crushes into the part and massing over my job.

Any tips?
is it the driver? LinuxCNC setup? other?

Could it be something in the g-code?
With Fusion it could be G53 Z0 if your machine has Z0 defined in an unconventional way.
Or G43 if you don't normally run that code.
Or maybe G28.
Or maybe it's not the g-code at all.

On my router I delete all the above codes. It's just home the machine, then G54 touch-offs and run. No tool changes. I'm not familiar with 6040. Are you using tool length offsets ?

It looks like G53 is used here. Could it be the problem?

It's not a problem if you have run G53 in the past without a crash. G53 moves in machine coordinates unlike most g-code.
G53 G00 Z0 crashes when Z zero is not defined at or near the top of travel. In other words after a machine is homed Z travel should be in negative space if the DRO is set to display machine position.

I'm just making guesses based on the possibility that you don't usually use Fusion or have changed F360 post processors.
When using a machine without repeatable tool lengths I strip CAM created g-code to it's minimum.

The G43 will load your tool length offsets. So you probably have an offset present in your tool library after hitting tool touch off.
I use the touch off button after jogging to my work 0,0,0 position so g54 becomes 0,0,0 if you're working in g54.
For simple parts I don't use the tool touch off if I'm not using multiple tools. I just program for one cutter and re-zero the z in g54.