I found another way for this to occur.Kayvon wrote:It's also possible for the Z alignment to mismatch as a result of bottoming out against the table/workpiece or topping out against the max height of the gantry.
Yesterday, I repeatedly encountered the exact problem you're describing. After some troubleshooting (and mangled spoilboard), I determined the problem was the Do separate last pass option that I was trying out for the first time. (Yes, this probably should have occurred to me right away.)
I had the separate last pass set to 0.001", which was a small enough amount that I didn't think it would cause any problem. Because I was using an upcut bit, the movement of the bit pulls the material towards the router, upwards. Because the material is secured to the spoilboard, this has the effect of pulling the bit/router downwards. Even though the bit was tightly secured in its collet, the half-inch thick material provided enough pull to steadily pull out the bit, even only cutting 0.001".
If anyone else has experience with the Do separate last pass option, I'd love to hear about it. Until then, I'll stay away from using it.