Frequently Asked Question

Measurement capability

Metrology is the science of obtaining measurements that are traceable, suitable for the tolerance, and stable in the intended environment. Select resolution, accuracy, repeatability, reproducibility, fixture, datum, temperature, and sampling together. A measurement system that is noisy or biased can make a stable process appear unstable or hide a real defect.

σ_gage is measurement-system variation and σ_process is process variation. The acceptance rule must be defined by the organization and application risk.

Worked example

If gauge variation is 0.012 mm and process variation is 0.060 mm, %GRR = 20%. This should trigger a review of resolution, fixturing, operator technique, and the intended tolerance before capability results are accepted.

Engineering check

For Metrology, SPC, and Production Control, record the material condition, tool or fixture identity, process settings, measurement method, sampling plan, and reaction rule. Confirm that the measurement system is capable of resolving the requirement and that the process is stable before using capability or first-article data to justify release. Reconfirm the limits after a material, tool, or route change.

Traceability note

For Metrology, SPC, and Production Control, maintain a visible link between the requirement, design or process characteristic, risk entry, planned evidence, measured result, and approval record. Identify the configuration and revision used, record deviations and open actions, and state who accepts residual risk. When evidence changes, update the affected requirement, risk analysis, verification or validation plan, and manufacturing control rather than editing only the summary status.