Circles turn in to ovals. Need some suggestions

I’m trying to cut a 2.5inch circle from 1/4 ply wood. The end result is always oval shaped, measuring about 2.52 to 2.43.

1/8 single flute
3mm doc rough pass at 400 mm/min climb milling leaving 0.3mm for finish pass
Finish pass at full depth 150 mm/min conventional
Fusion 360 CAM

Things I’ve tried:
A various amount of setting changes with no real change
Examined gcode to confirm that it’s actually producing a 2.5 inch circle
Tightened up the core a little when noticing some minor slop. No change
Checked trucks for slop and tightened. No change.
Checked steps. 100mm command = 100.0125mm for x and y. Seems about dead on.
Checked belt tension. Seems ok.
Checked pulleys. Didn’t notice any slipping under load. Will have to disassemble enclosure to access them all and put a wrench on the screws to confirm tight.

What else am I missing? I have a hard time believing deflection is my issue here with a finishing pass. I would think any deflection during roughing would have been cleaned up with finishing. I’ve gotta be missing something. I haven’t checked squareness since I assembled the thing but it was square when I put it together. It’s also got dual end stops and the Rambo 1.4 board.

Have you tried 1mm depth per pass? I would imagine that bit is flexing at that depth of cut, I only take 2mm passes with a 1/4” bit. On my MPCNC.

The feeds are still low, so that’s not the problem.

With the MPCNC it’s usually a grub screw that is loose, so the first motor drags the second one behind.


took apart my enclosure so i could get a closer look at everything. I loosened all of the belts and checked all the pulleys. no loose screws there. i took some time to sand the rails with 120 grit to get some of the scale of the EMT that i neglected to do from the beginning while i had it apart.

And while i was sanding the rails… one of my x axis rails started moving. DOH! The top clamps for my x rail never got tightened enough. i could rack the whole assembly a little bit with that rail loose. Tightened it up, re tightened the belts and ran a new part.

now im getting circles that are ± 0.01. I think i can live with that.

/sigh im so dumb.


No, You just educated a bunch of new builders! Thanks.