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CNC & Precision Machining

Ballbar Test (Circular Test)

A ballbar test checks a CNC machine's contouring performance by running a circular interpolation path while a telescoping ballbar measures radial deviations. The circular test method is described in ISO 230-4.

A telescoping ballbar is a precision linear displacement sensor with a ball at each end, held between a magnetic cup on the table and another on the spindle. The machine runs circles, clockwise and anticlockwise, around the table pivot, and the sensor records how the actual radius deviates from nominal. ISO 230-4 describes circular tests for numerically controlled machine tools and how to report circular deviation.

Analysis software interprets characteristic patterns in the plot to diagnose errors such as backlash and reversal spikes, servo mismatch between axes, squareness errors, straightness errors, cyclic errors from the drive system and scaling mismatch. Because a test takes minutes, ballbars are widely used for periodic machine health checks, acceptance checks after installation or relocation, and verification after a crash or repair.

A ballbar test checks two axes in one plane at a time and does not replace full geometric calibration with laser interferometers or other instruments. Results depend on feed rate, radius and temperature, so trending requires repeating the same test conditions. Set-up and running follow the instrument maker's instructions and the machine OEM documentation, and are carried out by trained personnel.

Key points

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