Had something odd happen last night where the z-zero changed somehow during the cut.
I have a sbp file (part I drew in Fusion360 and wrote out using the HB post processor). When I was using the 1/4" spiral upcut bit it was cutting through the material but when I switched to a straight flute 1/4" bit an ran the same file it did not cut through the material. I did re-zero out the top after the bit change and verified that both a setting of Z=0.75" was 0.75" above the surface and then Z=0 was just touching the surface.
The chucked bits the spiral upcut is about 0.25" longer than the other and it is also possible the sprial upcut is changing zero as well just not as noticeable (never checked it because it was doing what I needed).
Tested and reset it many times assuming that I didn't zero it or was user error. Always the same result (z-zero is higher after the cut).
I am going to simplify it tonight for testing and have it drill a simple hole and see if that works. But figured I would ask in case anyone knows what is going on.