MSA 3 — Procedure 3 (Automated Measuring Systems)
MSA 3 is an MSA 2 for automated measuring systems, e.g. coordinate measuring machines (CMM), optical measuring machines or measuring robots. Since automated measuring systems are not influenced by different operators, only one operator (the measuring device itself) is required. Instead, the focus is on the repeatability (Equipment Variation) of the measuring system across a larger sample.
Overview
Purpose and Field of Application
MSA 3 is used when the measuring system works automatically and the operator has little or no influence on the measurement result. Typical applications:
- Coordinate Measuring Machines (CMM) — CNC-controlled 3D measuring devices
- Optical Measuring Machines — Camera systems, laser measurement
- Measuring Robots — Automated inline measurement stations
- Automated Test Stands — Series-accompanying measuring equipment
Since comparability between operators (AV) is not relevant in these systems, this variance component is eliminated. The analysis focuses entirely on the repeatability (EV) of the measuring system.
Distinction from MSA 2
| Property | MSA 2 | MSA 3 |
|---|---|---|
| Field of Application | Manual measuring systems with multiple operators | Automated measuring systems (measuring machines) |
| Operators | 3 operators | 1 operator (automated) |
| Parts | 10 parts | 25 parts |
| Measurements per Part | 3 measurements × 3 operators = 9 | 6 measurements × 1 operator = 6 |
| Variance Components | EV + AV + Interaction | EV only (no AV, no interaction) |
| Focus | Repeatability + Reproducibility | Repeatability only |
| ANOVA Model | Part + Operator + Part×Operator + Residual | Part + Residual (simplified) |
Typical Procedure
- Select 25 parts that cover the tolerance range
- Define reference values and tolerance limits
- Measure each part 6 times with the automated measuring system
- Enter measurement data into my8data
- Execute calculation and evaluate parameters
Info: MSA 3 uses 25 parts instead of 10 (as in MSA 2), since automated systems can efficiently process larger samples and this increases the statistical validity of the repeatability assessment.
Input
Master Data
Before entering data, define the master data:
| Field | Description | Note |
|---|---|---|
| Operator | Designation of the measuring system / measuring machine | e.g., "CMM-001" or "Zeiss Contura" |
| Reference Value | Known true value of the characteristic | Must be determined by higher-order measuring equipment |
| Upper Tolerance Limit (USL) | Upper specification limit | According to drawing |
| Lower Tolerance Limit (LSL) | Lower specification limit | According to drawing |
Info: In the "Operator" field for MSA 3, enter the designation of the automated measuring system — not the name of an operator, since there is only one "operator" (the machine).
Enter Measurement Data
The data table has 6 columns (Measurement 1 through Measurement 6) and 25 rows (parts):
| Column | Description |
|---|---|
| Measurement 1 | First measurement of each part |
| Measurement 2 | Second measurement of each part |
| Measurement 3 | Third measurement of each part |
| Measurement 4 | Fourth measurement of each part |
| Measurement 5 | Fifth measurement of each part |
| Measurement 6 | Sixth measurement of each part |
Each row corresponds to one part. All 25 parts are measured 6 times each.
Tip: The table allows insertion of additional rows if more than 25 parts are to be measured. Use Copy & Paste (
Ctrl+C/Ctrl+V) to transfer measurement data from Excel.
Warning: Ensure that all measurements are performed under identical conditions (same clamping, same measurement program, same environmental conditions). Only this way can the repeatability of the measuring system be correctly assessed.
Notes on Experimental Procedure
- Identical Measuring Conditions: All measurements should be performed with the same measurement program and the same settings.
- Randomization: Measure the parts in random order to detect systematic drift effects.
- Clamping: Each part must be reclamped for each repeat measurement to reflect the real measurement process.
Parameters
Variance Components
Since MSA 3 involves only one operator (the measuring machine), the variance decomposition is simplified:
| Variance Component | Description | MSA 3 |
|---|---|---|
| EV (Equipment Variation) | Variation due to the measuring device (repeatability) | Calculated — main parameter |
| AV (Appraiser Variation) | Variation due to different operators (reproducibility) | Eliminated (always 0) — only 1 operator |
| Interaction (Part × Operator) | Interaction between part and operator | Eliminated — only 1 operator |
| PV (Part Variation) | Variation between parts | Calculated |
| GRR | Overall measuring system variation | = EV (since AV = 0) |
ANOVA Results
The ANOVA for MSA 3 uses a simplified model without operator and interaction effects:
| Source | Description |
|---|---|
| Part | Variance component due to differences between 25 parts |
| Residuals | Variance component due to repeat measurements (= Equipment Variation) |
Info: Compared to MSA 2, the rows "Operator" and "Part × Operator" are eliminated from the ANOVA table. The ANOVA model is thereby simpler and more directly interpretable.
Parameter Overview
| Parameter | Description | Evaluation |
|---|---|---|
| %EV | Share of device variation in tolerance | The smaller, the better |
| %GRR | Total share of measuring system variation (= %EV for MSA 3) | ≤ 10 %: capable, 10–30 %: marginally capable, > 30 %: not capable |
| %PV | Share of part variation | Should represent the largest share |
| ndc | Number of distinguishable categories | ≥ 5: sufficient |
Evaluation Criteria
| Evaluation Range | Criterion | Action |
|---|---|---|
| %GRR ≤ 10 % | Measuring system capable | Measuring system is suitable for the intended purpose |
| 10 % < %GRR ≤ 30 % | Marginally capable | Improvements recommended — optimize measurement program, check clamping |
| %GRR > 30 % | Not capable | Check measuring system, calibrate or replace |
Diagrams
MSA 3 provides two diagrams:
- Boxplot by Parts — Shows the variation of 6 measurements per part. Helps to identify whether certain parts have greater variation than others.
- GRR Evaluation Diagram — Bar chart with variance components (EV, PV) and color-coded evaluation.
Info: Compared to MSA 2, the diagrams "Interaction Plot" and "Boxplot by Operator" are absent, as these are not meaningful with only one operator.
Tip: If repeatability (EV) is too high, check the following possible causes: wear on the probe, inaccurate clamping, temperature fluctuations, vibrations, or an outdated measurement program.