The statistical characteristics of the gauge measurement system
The measurement gauge system must be in statistical control, which means that the variation in the measurement system can only be caused by common reasons rather than due to special reasons. This can be called statistical stability.
The variation of the gauge measurement system must be less than the variation of the manufacturing process.
The variation should be less than the tolerance band. The gauge measurement accuracy should be higher than that of both the process variation and the tolerance band. The accuracy of the CMM measurement is one tenth of who is the higher of the process variability and the tolerance band.
The statistical characteristics of the gauge measurement system may vary with the measured items. If in this the case, the maximum variation of the gauge measurement system should be less than the smaller one of the process variability and tolerance bands.
Gauge repeatability: the same evaluator, using the same measuring instrument, multiple measuring the same parts of the same characteristics, obtain the measured value (data) variation.
Gauge reproducibility: the different evaluators, using the same measuring instruments, measuring the same characteristics of the same parts and obtain the overall variation of measuring value.
Gauge stability: the gauge measurement system in a certain period of time to measure the same reference or a single feature of the part, obtain the total variation of measuring value.
When the repeatability variation is greater than the reproducibility, the structure of the gauge must be enhanced in design. Gauge clamping or part positioning way (inspection points) need to be improved, and measuring tools should be maintained.
When the reproducibility variation is greater than the repeatability, the operating methods and data reading methods of the gauges should be strengthened, and the operating standards should be clearly defined or revised. Some fixtures may required to assist the operator to make it more consistent with the use of gauges.
After gauge and clamping fixture calibration frequency repair correction, measurement system analysis should be done and record.