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Circles have flats


Apr 17, 2016
Hi all, looking for a bit of help here. I have a diy cnc mill, when I’m trying to cnc round posts I’m getting lots of visible flats that are making up the round post. I’m using fusion 360 and I tried both adaptive clearing and pocket to cut out the test pieces.
I have attached some images of what is happening. The cnc has servo motors and is set to 10000 steps on a 10mm ball screw.


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quality of circular interpolation is one of the best metrics for testing industrial level cnc milling machines. Look up ball bar testing. Your "circular" posts are a sort of ball bar test and I suspect you have found your diy cnc wanting. A possible work around could be a boring head flipped around to cut an OD, your circularity would then be dependent on your spindle rather than on software and multiple steppers/servos.
Hi all thanks for the reply’s. As far as I can tell there is no or very little backlash, I did do a finishing contour finishing cut with .4mm. I have attached the start of the G code I used


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The post in the front right smells of bad CAM. Not enough steps generated.
The post in the front left with the smaller flat size may be a symptom of something else! Look at the flat back wall of the cavity the post is in. It has scallops just about the same size as the "flats" on the post.
You have other problems to fix before you worry about circular interpolation. Look at the straight wall behind the left post. Bad cutter, holder runout, spindle bearings, something is hosed. The left post looks just like the back wall. As for the other posts, are you absolutely certain your roughing routine didn't nick them?