Hello,
I have build CNC, when I calibrate it with gauge indicator (10mm lenght, 0.01mm resolution) for example: go 3mm Y or X axis, gauge indicator shows 3.01-3.02mm. then go -3mm, indicator shows 0.01 or 0.02mm. Z axis is more accurate, resolution and repeatability is less than 0.01mm. For X axis I use 2 motors and two SFU 2005, Y axis same SFU 2005 ballscrew, for rails I use linear block rails. When I mill squares or corners, everything looks good, but when I mill circle - It looks awfully. there is pic of circles: https://imgur.com/a/lJfTn circles diameter 5 and 7 mm

While CNC is milling circle, X and Y axis half of circle start moving with velocity and almost stops, then again start with velocity and stops. So as you can see in the picture, circle consists of two parts. I use mach 3, I try a lot of different cad's and cam's, but I have same problem. Any thoughts?