The cutter is not tapered I hope
Maybe your cutter is not as sharp at the tip as it is on the higher part.
The tip is seeing more cutting action and gets dull earlier.
I would try with another cutter again, to see if the cutter is involved in the problem.
I have cut holes in my old spoilboard, and also on a piece of 3/4" attached on top of the old spoilboard. Same result, so I don't think it's a taper in the Z axis. I'll do another check with an indicator, raising and lowering z while measuring side position.
I'm using a small telescoping gauge and micrometer to measure the holes, also I have a parallel plug gauge (i.e. the precision round dogs). The holes are clearly tapered.
Good ideas, I'll recheck Z for taper in the axis motion.
Timo, I did look at my cutter edge. Nothing I could see (no glints), but when I felt the edge there was a small difference in texture at the end. I had a new bit, still in it's tube and tried it. Same result.
I've improved the taper from about 8 thou to 5 thou by reducing acceleration and feed rate. I was at 50 ips^2 and changed to 20, feed from 100 ipm to 30 ipm. Further reductions did not help.
I also made 15 holes, all the same gcode. I didn't measure every hole, but they vary by a few thou. I've remeasured holes and get within one thou.
I think I'm as close as my machine will get. The 15 holes vary from too tight (which got better with multiple insertions) to OK, I just wish it was better. I'm going with what I've got.
Thanks for the ideas people!
you maybe reaching the tolerance that MDF can hold, while MDF is More Stable than other woods it still changes shape massively compared to metals and some plastics. i will try to run some holes on my phantom s4. it does many small move like your, I suspect I will get the similar results as you. that would mean tool deflection and/or material movement
