How does the ballbar test work?

In theory, if you program a CNC machine to trace out a circular path and the positioning performance of the machine was perfect, then the actual circle would exactly match the programmed circle. In practice, many factors in the machine geometry, control system and wear can cause the radius of the test circle and its shape to deviate from the programmed circle.

If you could accurately measure the actual circular path and compare it with the programmed path, you would have a measure of the machine's accuracy.

This is the basis of all telescopic ballbar testing and of the QC20 ballbar system.