How come my circles are not perfectly round?
When you are machining circles on a CNC and your circles are not coming out perfectly round, it is commonly caused because there is some backlash developing on the machine. Backlash is a tiny amount of play that appears over time and is noticed when a drive changes direction.
When running in a positive direction, the gears/lead screw are engaged with the lead screw nut on the gantry, but when the stepper motor reverses direction there is a tiny space between the lead screw and the lead screw nut that has to be made up before there is positive engagement in that direction. After many, many hours of use or if your machine is not properly maintained and lubricated, then this backlash can develop.
If this is happening to you, please download this document: Stepcraft UCCNC Backlash Compensation.
This will show you how to setup backlash compensation in UCCNC so that you can “dial” out any backlash and go back to having perfect circles again.