I’m machining pockets in polycarbonate and at a certain point the bit plunges straight down without regard to stock size or etc. Theres no issues in the code and it’s not the bit slipping. I’ve had this issue once in the past but it fixed itself after rerunning the gcode. As you can see I assumed that it would fix itself again, but it repeated after a few inches.
Is the tool pulling out of the collet?
Which Z-axis does your machine have?
Do you have to be cutting a slot? Is it an option to do this as a pocket w/ a smaller tool?
I have experienced a similar thing cutting a slot with an upcut bit on a Shapeoko 3 with belt Z. I caused it by selecting soft plastic feeds/speeds instead of hard plastic feeds and speeds and not securing the plastic firmly to the wasteboard. Mainly just too aggressive cutting. But plastic sheets can also tend to flex up in the middle when cutting making the issue even more likely to occur. In my case I think it pulled the Z axis down resulting in lost steps in Z. It was not an abrupt plunge but a rapid increase in depth until it scarred my pretty wasteboard with a nice long arc of increasing depth. I remedied it by slowing down the feed rate, decreasing the depth of cut and securely clamping the plastic down. ( and filling in the arc with wood putty 
).
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I think it might’ve been the tool, I had ruled it out but I think it was just too small to notice the other times. Thanks!
Yeah, flexing was a part of the issue but was remedied fairly easily with more clamps and shallower cutting
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