Circle problems

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?

You have a lot of backlash, or your screw or nut is loose.

when measuring with gauge indicator backlash is 0.02mm, so 20um. I bought new ballscrews. Sad, I will need to buy new ones. But that circle milling per two half's, how I can make that circle mills without any stops at half way?

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when measuring with gauge indicator backlash is 0.02mm

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from the look of the cuts, I'd say you measured wrong, or something came loose.
Can you post the g-code you used?

backlash or losing steps on the Y, but with the flattened section and the ends not meeting most likely as Gerry has said backlash

hello,
I think I found problem, today I will try to fix it and then I will post results.

Thanks for help, after ger21 say about backlash I started to search where can it be. And I found it, ball screws support bearings were not fixed correctly , so I fix it and results: https://imgur.com/a/EElbO

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Originally Posted by Patuc

Thanks for help, after ger21 say about backlash I started to search where can it be. And I found it, ball screws support bearings were not fixed correctly , so I fix it and results: https://imgur.com/a/EElbO

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excellent, well done