Gauge Capability and Repeatability

The BIS.Net Team BIS.Net Team

Gauge repeatability (precision) is a measure in terms of standard deviations of how close repeated measurements on the same part, by the same operator, are. Gauge repeatability is about close clustering of repeat measurements, not closeness to the target. Closeness to target is about bias. The first two images below show similar close clustering, but even though one is off-target, the repeatability is similar. The third image shows bad repeatability.

To determine Gauge Capability a Cg, and Cgk index is calculated. These indexes are similar in principle to the Cp and Cpk index.

For the Cp and Cpk the process variation is compared to the tolerance range. For Cg and Cgk the repeatability spread is compared to a percentage of the tolerance range. This percentage is commonly 20%, meaning that measurement error should be no greater than 20% of the tolerance