Frequently Asked Question

Capability as a design input

Design tolerances should be allocated from the functional requirement backward. Identify contributors, sensitivities, distributions, process stability, and measurement capability before selecting a tolerance. A capability index is meaningful only for a stable process and a trustworthy gauge.

USL and LSL are limits, μ is process mean, and σ is within-process variation.

Worked example

For limits 9.90 and 10.10 mm, mean 10.02 mm, and σ = 0.02 mm, C_pk = min(0.08/0.06, 0.12/0.06) = 1.33. Confirm stability and gauge variation before using this to approve the design tolerance.

Engineering check

For Design Capability and Tolerance Allocation, maintain traceability from requirement to risk, design output, evidence, and approval. Record the configuration, acceptance criterion, test or analysis conditions, open actions, and residual risk. A method is not complete when the document is filled in; it is complete when the evidence supports the decision and affected controls are updated.

Traceability note

For Design Capability and Tolerance Allocation, 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.