I have a 30x30" cnc router that I designed and built 10 years ago. It uses ball screws x and y. I'm changing my spoil board so it has spots for precision dogs. Cutting the dog holes has turned into a project. I'm getting decent accuracy at the top of the hole, but around 8 thousandths less at the bottom. The holes are about 20mm x 3/4" deep in MDF.
I built this 10 years ago, but I'm not saying this is a new problem. I haven't done precision holes before, so have no idea if this is new or old.
Hole cutting: With a 1/4" straight Whiteside carbide bit (2 flute), each level is cut with 50% stepover. The 3/4" depth is done in 4 layers, 0.2" each. Each layer is cut conventional, with a single additional pass (finish pass). After the hole is cut, there are two additional finish passes done at full depth, one conventional, one climb. The inside of the holes looks pretty clean.
One other thing, my CAM software does not use circular arcs, it uses a lot of x-y positions to cut a circle. They are very nice circles to the eye, no segmentation.
My first thought was bit deflection due to loading, but the final two finish passes should have very little load.
Another possibility is a cutter ground with a small taper. I don't measure anything like that, the flutes are parallel the full length (1" long flutes).
Holes are tapered
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