Still can't cut a good circle

So a few weeks ago, I posted some questions regarding issues I was having trying to cut several holes out of MDF for a worktable replacement. The holes were not cutting the correct size. I’m still trying to figure that out, but in the process, I’ve noticed another issue. The holes are not a circle. They have flattened edges on the left and right sides. Please take a look at these pictures I’ve attached. I’ve cut concentric circles to show the repeatabilty of the issue. first hole is supposed to be 20mm, then the ring around it should be 40mm, then 60mm. But notice the straight edges on the left and right sides of each circle. Also note the entry point, outside the circle on the left of each one. Why could this be happening? The tool path is set to cut on the inside of the line. I’m using a 1/4" end mill at default f/s. Thoughts?

Hello Chris, is there any play in the X axis?

Not sure what machine you have but if the anti-backlash is loose you could end up with flat sides. What happens is lets say the machine is moving to the right and when it gets to the far right of the circle and starts moving left the router won’t actually move left until the play is taken up.

So grab your XZ carriage and try to wiggle it left and right to see if there is some play. Then adjust the backlash to get rid of it.


Thank you for the suggestion. I will give that a try.

That fixed it! The X axis had a solid 1/2mm or so of backlash. As soon as I got that tightened, I was repeatedly cutting round circles. Now if I can just get the measurements dialed in. My circles are still coming out 1/2mm or more too small. If anyone has a suggestion other than adjusting the size in CAD to be larger, I would appreciate it.