Ballbar Basics—Running around in Circles

A ballbar testing system has three main components: the ballbar, a precision magnetic mount that attaches to the machine table, and a second precision magnetic mount that attaches to the machine spindle or spindle housing. The ballbar is, as its name implies, a telescoping bar with precision balls at each end. Transducers within the bar detect the in and out movement of the telescoping components. After attaching the mounts to the table and spindle, the balls of the sensor bar are kinematically located in the magnetic cups on the face of each mount. This arrangement enables the ballbar to measure minute variations in radius as the machine follows a programmed circular path around the mount on the machine table.

The basic ballbar test calls for the machine to perform two consecutive circles—one in a clockwise direction, the other in a counterclockwise direction. In theory, a machine with perfect positioning performance would move in a circular path that exactly matched the program circle. In reality, many factors in the geometry of the machine structure, the capability of the control system, wear in the moving components of machine axes and so on, cause variations in the radius of the test circle. These variations result in a path that deviates from the perfect, programmed circle.

As the sensors in the ballbar capture the variations in the radius and transmits them wirelessly to the laptop, the ballbar software records these deviations, displays them in comparison to a perfect circle and analyzes the results as a report that can be viewed at the machine immediately after completing the test procedure.