Holes not round


I am a relatively new user of CNC. I am trying to bore some holes on MDF, but they are not coming out round. Any ideas of where the problem lies? Belt tension? Machine not square? Deflection of the tool?

Some details:
Machine: Shapeoko XXL
Router: Dewalt
Toolpath created using HSMworks
Code sender: Carbide Motion
Feed: 20in/min
Plunge: 5in/min
RPM: 20000 rpm
Cutter: Carbide #201 (0.25")
Hole diameter: 0.375"
Holes are on an MDF base that has been faced in the Shapeoko.

Are other things working ok? Wheels, belts? Calibrated already?

With a really deep plunge, 20in/min for feed to make the cut might be too much. How deep is the hole?

Hi, Mike.

Thanks for your reply.

Other things seem to be working fine. Belts are stressed enough I think, and I just tightened the eccentric nuts, so V-wheels should be fine.
I haven’t calibrated the machine yet (you mean setting the number of steps per mm, right?)

The hole is 0.6" deep. The cutter comes, plunges and bores the hole leaving a bit of axial stock, then does a finishing pass starting with another plunging. You think lowering the feed would solve the problem?

I don’t think there’s that much flute on that bit, cutting that depth all at once in going to need both a slower feed, and probably more than one pass to cut less than a flutes-length at a time.


I had a similar issue and found my v wheels on z axis had play in them. Check by grabbing the router head and push and pull, if you feel any play, tighten the v wheels. Fixed mine so far so good. Been running fine for several months now.

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