I have a cnc system which uses the thread rods, 2 Y motors, x motor and z motor. All connected with the coupler with 2 set screws between the motor and threaded rod. Using vcarve desktop and tiny g. I been using it for about 2 years or so cutting 100's of projects. I was making a project cutting a few small 13mm holes in a piece of wood using 1/4" end mill bit. Each of the holes cut with a flat side. I adjusted the speed rate and direction etc but still get the issue. After reading on the internet. I checked the backlash which seem to be ok. Even replaced them with spares. I checked the coupler set screws and re-tighten them. Too be honest in all my projects this is the first small circle I have done, so I never noticed the issue. It seems to be where the bit enters the circle to start the cut. I slow down the feed rate and plunge rate into the cut as well. Could this be one of the motors issue? Thanks.