Z axis moves below set zero

[whensparksfly]

It dose it in a few codes I've tried to you have any code I can try

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[routercnc]

In my CAM software I define the top surface of the work as work coordinates Z Zero. Any cuts down into the work are then in the -ve Z direction. So it goes below zero when cutting but that is normal. Is that what you mean?
You can also define the bottom of the workpiece as zero in CAM and then set work Z zero at the bottom of the work (eg the spoil board). Then all cutting is +ve Z

Can you write down each step you are taking as this might help understand your problem? Explain at which point it does something unexpected. Video and screenshots may help if it is difficult to describe

[lucan07]

With a plasma cutter I believe sheetcam is a popular product to use with Mach3 and looks simple to generate plasma code with

[magicniner]

It dose it in a few codes I've tried to you have any code I can try

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You should post some code here with which the problem occurs