Bit was 2 flute ball endmill 3.175mm (1/8"), speed 1500 mm/min, started with 3000 rpm, up to 10000, did not make difference. Machine is DIY aluminum cnc router, spindle is chinese 2.2kW.

But why only on one side, and not other? Any ideas? And I get similar thing when carving hardwood with parallel passes (this is happeningClick image for larger version.  Name: DSC_0304.jpg  Views: 214  Size: 244.1 KB  ID: 27554 even since I built machine).