KEVOS® MAKEOS · Quality
Guard-Banded Acceptance Decision (Uncertainty Budget, Accept, Reject or Indeterminate)
Given my gauge’s measurement uncertainty, can I accept this part from its reading, must I reject it, or is it too close to the limit to decide?
Acceptance decision for a measured part that allows for measurement uncertainty: the uncertainty budget from calibration, resolution, repeatability, Abbe error and temperature, the expanded uncertainty and test uncertainty ratio, and the guard-banded acceptance zone (as the measurement uncertainty budget tool), then the deviation of the reading from nominal compared with the guard-banded and the tolerance limits to accept, reject or flag it as indeterminate.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Nominal size | mm | Nominal size of the feature (mid-tolerance). |
| Tolerance band (total, symmetric) | mm | Total tolerance band, placed symmetrically about the nominal. |
| Measured size | mm | Size measured on the part. |
| Uncertainty contributors | ||
| Calibration uncertainty of the gauge (expanded, k = 2) | µm | Expanded uncertainty from the gauge’s calibration certificate. |
| Resolution of the gauge | µm | Smallest step the gauge displays. |
| Repeatability (standard deviation of repeat readings) | µm | Standard deviation of repeated readings on the same part. |
| Abbe offset | mm | Distance between the measuring line and the scale line (Abbe offset). |
| Tilt of the moving element | arcsec | Possible tilt of the moving jaw or slide. |
| Length measured | mm | Length being measured. |
| Part expansion coefficient | µm/m·K | Expansion coefficient of the part. |
| Uncertainty of the temperature difference | K | Uncertainty in the temperature difference between part and gauge. |
| Coverage factor | Coverage factor for the expanded uncertainty (2 for about 95%). | |
| Least test uncertainty ratio | Least ratio of tolerance to measurement uncertainty you accept. | |
Method and formulas
- Standard uncertainties: calibration U ÷ 2, resolution ÷ √12, repeatability, Abbe and temperature errors ÷ √3; combined by root sum of squares; expanded = k × combined (as the measurement uncertainty budget tool).
- Test uncertainty ratio = tolerance ÷ 2U; acceptance zone = ±(tolerance ÷ 2 − U) about nominal.
- Accept inside the acceptance zone; reject outside the tolerance; otherwise indeterminate.
- Example: U = 5.9696 µm, so accept within ±19.03 µm; the part is 12 µm from nominal.
Limitations
- Symmetric tolerance and uncorrelated contributors.
- Simple guard band of one expanded uncertainty; your standard may use a different rule.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Decide whether a borderline part passes
- Set guard-banded acceptance limits on an inspection plan
- Show a customer how the decision was made
Who it is for
- Inspectors
- Quality engineers
- Metrologists
What you get
- Uncertainty budget, expanded uncertainty and test uncertainty ratio
- Guard-banded acceptance zone
- Accept, reject or indeterminate
Interpreting the result
- Indeterminate parts need a better measurement, not a coin toss: re-measure with a more capable gauge.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 709–735. Topic reference; no source tables or recommended settings are reproduced.
- ISO/IEC Guide 98-3 Uncertainty of measurement (GUM)
WORKOS tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
