Interesting conundrum

Levelled my spoilboard to clean up past gouges, when the board was leveled, taped and CA’d a workpiece to the board. cut a pocket for a bowl, then did an offset to carve the outside of the bowl, as the toolpath was finishing, the bit cut into the spoilboard on about 1/2 the circumference of the bowl, about 0.5mm deep.
Any ideas as to why the bit cut into the spoilboard only on 1/2 of the circle, when everything should have been true to the machine and spoilboard.

Assuming you surfaced it flat using the cnc itself and didn’t “level” with like a bubble level and shims (I’ve seen it happen this way before)
One reason could be that the wasteboard is less rigid than it should be, such that it can flex down in the center and this sort of deflection occurred during the surfacing. If possible you might want to mount the wasteboard to the frame and ensure the frame is supported well in the middle area to the table top below because this is often left floating, which also allows for more easy flexing…

Depending on the bit type used, an upcut bit (causing a lifting of the workpiece and wasteboard) could cause that excess depth cut to occur where the wasteboard is less secure, like the center of the board. Where as the opposite would be expected with a downcut, causing the center to flex downward and the part of the wasteboard that has the half circle cut into it to be along the edges where it’s better secured and not able to flex as much.

Good answer Seth, never occurred to me that I got lift off.
Was using a 3/8ths upcut and was pushing it for speed.
Beginners spoil board, looking fwd to putting in a new one with the errors fixed.

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