Gauge R&R Type 1 Study.
Quick measurement system analysis (MSA) — measure one master part 25–50 times with one operator and one gauge. Computes Cg and Cgk capability indices to determine if the gauge is fit for the tolerance.
Cgk = (0.1 × Tolerance − |Bias|) / (3 × s)
%EV = 6s / Tolerance × 100
Pass criteria: Cg ≥ 1.33 and Cgk ≥ 1.33
Acceptance criteria
| Cg / Cgk | Verdict | Action |
|---|---|---|
| ≥ 1.67 | Excellent | Use as is — gauge well within capability |
| 1.33 – 1.67 | Capable | Acceptable per AIAG MSA — use for production |
| 1.00 – 1.33 | Marginal | Acceptable for non-critical features only — investigate |
| < 1.00 | Not capable | Replace gauge or tighten the measurement procedure |
What is Gauge R&R (Repeatability and Reproducibility)?
Gauge R&R is a Measurement System Analysis (MSA) study that tells you how much of your observed variation comes from the measurement system itself versus the actual part variation. If your gauge is contributing too much noise, your Cp/Cpk numbers are lying to you.
Cg and Cgk Formulas (AIAG MSA)
The AIAG MSA 4th edition uses these formulas for gauge capability:
Cg = (0.2 × Tolerance) / (6 × s)
Cgk = (0.1 × Tolerance − |Bias|) / (3 × s)
Where s is the standard deviation of repeated measurements on a reference standard, and Bias = (mean of measurements) − (reference value).
Pass / Fail Criteria
- Cg ≥ 1.33 AND Cgk ≥ 1.33 — Gauge capable. Use for production.
- 1.00 ≤ Cg or Cgk < 1.33 — Marginal. Non-critical features only.
- < 1.00 — Not capable. Replace gauge or upgrade procedure.
Worked Gauge R&R Example
You're verifying a digital caliper used to measure a 50.00 mm shaft (tolerance ±0.020 mm, so Tolerance = 0.040 mm). You measure a calibrated reference (50.000 mm) 25 times.
- Mean of measurements: 50.0008 mm
- Standard deviation: 0.0014 mm
- Bias: 50.0008 − 50.000 = 0.0008 mm
- Cg = (0.2 × 0.040) / (6 × 0.0014) = 0.95 — fails!
- Cgk = (0.1 × 0.040 − 0.0008) / (3 × 0.0014) = 0.76 — fails!
The gauge resolution is too coarse for this tolerance. You'd need either a tighter tolerance or a higher-resolution instrument (digital micrometer or air gauge).
Common Causes of Gauge R&R Failure
- Insufficient resolution — gauge readability should be 1/10th of tolerance.
- Temperature variation — steel parts grow ~12 µm/m/°C. Critical for tight specs.
- Operator technique differences — clamping force, alignment, reading angle.
- Gauge wear — anvils, jaws, and reference surfaces wear over time.
Workflow After Gauge R&R
Once your gauge passes MSA, you can confidently run a Cp/Cpk capability study. Document the MSA results in your PPAP submission as Element 10. For aerospace, attach the study to your AS9102 Form 3. Verify any final dimensions with measurement uncertainty analysis for high-precision parts.