Situation
A new engineer measured a part with calipers: 10.023mm. Five minutes later, he measured it again: 10.031mm. Suspecting he made a mistake, he asked a quality control colleague to measure it: 10.018mm.
The three readings differ by a maximum of 0.013mm. Your specification is ±0.05mm, which might seem acceptable.
However, if your Cpk calculation shows a process capability of only 0.95... is it truly a process problem, or does the measurement tool's inherent error account for a large portion of it?
Plain Language Explanation
Gage R&R breaks down the "total observed variation" into three parts:
- True Part Variation (Part-to-Part): You want this to be large.
- Repeatability: Same person measures the same part, different results each time → Instrument issue.
- Reproducibility: Different people measure the same part, different results → Operator issue.
%GRR = (Measurement System Standard Deviation ÷ Total Standard Deviation) × 100%
If %GRR = 38%, it means that 38% of the "observed variation" is noise introduced by the measurement tool, not actual part variation.
Practical Assessment
Standard Design for Conducting Gage R&R:
- 10 parts (should represent the process variation range, not solely good parts within spec)
- 2–3 operators
- Each person measures each part 2–3 times
- Randomization: Do not let operators know previous measurement results
| Main Cause | Diagnosis | Countermeasure |
|---|---|---|
| High Repeatability | Instrument instability | Replace with higher precision instrument, calibrate, anti-vibration |
| High Reproducibility | Inconsistent operator technique | Standardize SOP, fix measurement position, use fixtures |
How InsightFab Does It
To run Gage R&R in JMP and Minitab, you first need to design tables, input the part/operator/replicate structure, and then interpret the ANOVA output. InsightFab allows you to upload a standard CSV format and directly provides traffic light diagnostics and improvement recommendations.
Golden Quote
"Performing Gage R&R before process improvement is like performing delicate surgery while wearing foggy glasses—you think you see clearly, but you don't."