When auditing a MIM supplier's quality system, one of the most important but often overlooked elements is their measurement system capability — quantified by GR&R (Gauge Repeatability and Reproducibility).
GR&R acceptance criteria per AIAG MSA 4th Edition:| % GR&R | Assessment | Decision |
|---|---|---|
| < 10% | Excellent | Acceptable for any feature, including critical/safety-related |
| 10-20% | Marginal | Acceptable for non-critical features; may need improvement for critical characteristics |
| 20-30% | Poor | NOT acceptable for any tolerance-based inspection — measurement system must be improved before production release |
| > 30% | Unacceptable | Measurement system cannot distinguish between conforming and non-conforming parts |
| Feature Type | Measurement Tool | Typical GR&R Expectation | Notes |
|---|---|---|---|
| Linear dimensions (±0.05-0.10 mm) | CMM | 8-15% | CMM is the most repeatable method for MIM parts |
| Hole position (±0.05 mm) | CMM | 10-18% | Affected by datum reference selection |
| Wall thickness (±0.05 mm) | CMM or ultrasonic | 12-20% | Thin walls are challenging to measure repeatably |
| Surface finish (Ra) | Profilometer | 10-20% | Dependent on probe radius and cutoff length |
| Density (Archimedes) | Balance + immersion | 2-5% | Highly repeatable when procedure is followed |
| Visual inspection | Human operator | 15-30% | Always the least repeatable — consider automated vision |
- Do you perform GR&R studies on all gauges used for critical dimensions?
- What is your GR&R acceptance threshold? (< 10% for critical, < 20% for non-critical is industry standard)
- How often are GR&R studies updated? (Annually, or after any major gauge repair or change)
- Can you share GR&R results for my part's critical dimensions? (A qualified supplier will have this data)
Per AIAG MSA standards: GR&R < 10% is excellent and acceptable for any feature. GR&R 10-20% is marginal — acceptable for non-critical features but should be improved. GR&R > 20% is unacceptable. When evaluating a MIM supplier, request GR&R data for gauges used on your part's critical dimensions.
If your part has tolerances of ±0.05 mm and the supplier's CMM has a GR&R of 15%, that means 15% of the tolerance band is consumed by measurement variation alone — this is acceptable but marginal. For tighter tolerances, a higher-performance CMM or a different measurement strategy is needed.