Quality Assurance and Inspection
Core concepts
Quality assurance covers the systems that prevent defects (planning, procedures, calibration, supplier quality) while inspection covers detection (measurement, sampling, testing). Candidates should understand the difference between QA and QC, the role of a control plan, inspection types (incoming, in-process, final), and measurement system analysis.
What to know cold
- Control plan: what is checked, how, how often, and the reaction plan.
- Sampling: attribute versus variable plans; acceptance sampling risks (producer's and consumer's risk).
- MSA / Gage R&R: measurement variation must be small relative to tolerance, commonly judged against a percentage-of-tolerance rule.
- SPC: control charts distinguish common from special cause; control limits are not specification limits.
- Calibration: traceability and recall of out-of-tolerance instruments.
Traps in questions
A classic trap is confusing control limits with spec limits - a process can be in control and still produce scrap. Another is assuming 100% inspection is perfectly reliable; it misses defects and adds cost. Questions may also test whether a gauge that is out of calibration invalidates prior results - it does, pending review.
Rule of thumb
Measurement variation should be a small fraction of the tolerance; a common guideline is Gage R&R under 10% of tolerance is acceptable, 10–30% may be conditionally acceptable, and above 30% is not. When in doubt, improve the measurement system before judging the process.
Worked example
A feature has a 0.20 mm tolerance band. Gage R&R variation of 0.05 mm is 25% of tolerance - conditional; investigate before tightening process controls, because the gauge itself may be generating apparent variation.