In order to move from defect prevention, you must be able to document your process capability and the accuracy of your machine tools. To do this, inspect them to a nationally recognized and accepted standard, such as ISO 230 or ASME B5.54. Both call for a ballbar and laser interferometer to be used with a recommended procedure for checking machine tool accuracy.

The purpose of these standards is not to specify an accuracy the machine must meet, but to find out what accuracy level it can meet—its process capability. The part print dictates the accuracy your machine must have to make good parts—where to set the accuracy bar. Testing tells you how high your machine can jump. As long as your machine can top the bar, you have process capability.

Test and calibration technology are now available—and affordable—to enable shops to ensure the accuracy and health of their machine tools. Plants and large shops increasingly maintain their own laser interferometers and electronic levels, while rental equipment and diagnostics services are commercially available to small shops from various sources and competitively priced.

Telescoping ballbars are readily affordable by virtually any shop and provide a fast, 15-minute check-up for prevention and diagnosis in maintaining machine accuracy. The ballbar test allows precise assessment of machine geometry, circularity and stick/slip error, servo gain mismatch, vibration, backlash, repeatability and scale mismatch. Some ballbar software provides diagnosis of specific errors in accordance with ISO 230-4 and ASME B5.54 and B5.57 standards, then provides a plain-English list of error sources rank-ordered according to their overall effect on machine accuracy. This allows maintenance people to target those factors that need the most attention.

Periodic ballbar testing enables trend tracking of machine performance. Preventive maintenance can be scheduled before a machine drifts out of process capability. The industry trend is to calibrate the machine on need, not time. There is no reason for maintenance to pull a perfectly good machine out of production for calibration. Let the ballbar and the accuracy of your parts determine when something has gone awry. In the meantime, run production!