Strange z axis problem

I have had my x-carve for a couple of years, never had a problem until now. I’m cutting a 11.75" disk from .175" acrylic. The acrylic is stuck to a 15 x 15" 1/8" MDF using carpet tape. Everything is clamped down at the corners.

I created the file in V-Carve Pro, and set the cut depth at .18" in two passes. I’m using a .25" straight mill. I export to g-code (using the x-carve post processor, and import the file into Easel, same as I always have done. When the cut executes, after 2 passes, the front of the cut (toward the operator) goes through to the MDF, but the back (away from the operator) is not cutting very deep, only about .035".

I assumed my axes needed calibrating so I ran a test: An 18" square mdf with a grid of lines every 2" , X and Y. I designed them to cut .25" deep. After the cut, over the entire grid, the greatest error was ,03". So why the heck is the acrylic cutting so unevenly? It is definitely very secure, I have a heck of a time getting it off the MDF afterward.

I’d appreciate any help you can offer.

Your spoil board is not true to your machine axes. Attach a sacrificial board (MDF) to the top of the spoil board and surface it true to the machine.

Thanks for your reply, John. But if that were the case why would my test carve be so accurate?

Is there a possibility that you somehow tilted the design in V-Carve Pro before generating the g-code, and just didn’t notice it?

If you are sure there is no looseness in the Z axis up/down positioning (play in the ACME screw and nut?), then the spoil board being not parallel to the machine seems the only thing to question.

And errors can accumulate. Example: a slightly unlevel machine bed + a varying thickness in the MDF spoil board + a slight bending from clamping pressure = cutting depth error???

I suggest you eliminate the possibility this is the cause by clamping the board down first, then surface (level) it, then stick down the acrylic.

If then you have the same issue, it has to be machine related, either mechanical, Easel (not likely), or the gcode (also not likely).

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Thanks - I’ll take a close look at it today. I’ll double check the file also.

I suspect the acrylic and 1/8 mdf are not flat once you have them joined.

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