Calibration Troubleshooting
Investigate failed inspection results by checking instrument suitability, test points, uncertainty, setup, environment, method, and product condition.
Your Calibration Certificate Passed. Why Is the Part Still Failing
Inspection?
The instrument has a
valid calibration certificate. The calibration result is acceptable. Yet, the
part still fails inspection.
This can seem
contradictory when calibration and inspection are viewed as the same. They are
not.
Calibration checks how
measurement standards match what a measuring system shows under specific
conditions. Inspection checks if an item meets set requirements.
When results conflict,
review the entire measurement process before blaming the instrument.
Calibration and
Inspection Answer Different Questions
Calibration and product
inspection serve different purposes.
Calibration connects the
values from measurement standards to what the measuring system shows in
specific conditions.
Inspection uses
measurements or other evidence to see if an item meets requirements.
A calibration certificate
shows an acceptable result. But, it doesn’t ensure that every measurement taken
during production or inspection will be acceptable.
JCGM VIM:
Calibration
Check the instrument
Start by confirming that the instrument used for the inspection is the instrument identified in the calibration records.
- Confirm the instrument identification.
- Check the calibration status.
- Review the relevant calibration results.
- Check the measurement range.
- Check the instrument condition.
- Review intermediate checks where applicable.
Also consider whether the instrument was configured or used differently from the conditions covered by the applicable procedure.
A calibration record should be interpreted in relation to the measurement task for which the instrument is being used.
Review the inspection method
The next step is to examine how the part was measured.
Review the details that apply to the particular measurement:
- Measurement location
- Positioning
- Alignment
- Reference surfaces
- Contact conditions
- Fixtures
- Measurement sequence
- Operator procedure
A valid calibration record does not remove variation introduced by the measurement application.
If two people use the same instrument differently, or if the part is positioned differently between measurements, the results may not be directly comparable.
Check the specification
Confirm the requirement against which the part is being inspected.
Record the actual tolerance or acceptance requirement. Check that the correct revision of the specification, drawing, customer requirement or documented procedure is being used.
Also confirm that the measured characteristic is the one specified by the requirement.
Do not assume that an expected value is the same as the actual acceptance criterion.
Consider measurement uncertainty
Measurement uncertainty can become important when an inspection result is close to a specification boundary.
Measurement uncertainty describes the dispersion of quantity values attributed to a measurand based on the available information.
If the result is close to the limit, identify the applicable decision rule before deciding whether the item conforms.
Do not apply a universal guard band or acceptance formula. Use the decision rule specified by the applicable standard, specification, customer requirement or documented procedure.
Investigate the part and process
The measurement may be showing a genuine change in the component.
If the inspection result differs from previous results, compare the available measurement records with relevant production information.
Depending on the manufacturing process, useful records may include equipment settings, tooling changes, maintenance activity, material information or other documented process changes.
Do not assume that a production change caused the measurement result. Treat the connection as a finding only when the available evidence supports it.
Likewise, do not blame the measuring instrument simply because the part failed inspection.
Keep the evidence together
A measurement investigation is easier to review when the relevant records are kept together.
Document:
- Instrument identification
- Calibration status
- Calibration results where relevant
- Measured value
- Measurement method
- Measurement conditions
- Applicable specification
- Measurement uncertainty where relevant
- Decision rule
- Final decision
This record allows another qualified person to understand what was measured, how it was measured and why the resulting decision was made.
Inspection investigation checklist
- Confirm the instrument identification.
- Confirm calibration status.
- Review the relevant calibration result.
- Check the measurement range.
- Confirm the inspection method.
- Check the measurement location.
- Review positioning and alignment.
- Check reference surfaces, fixtures and contact conditions where applicable.
- Confirm the applicable specification and tolerance.
- Consider measurement uncertainty for borderline results.
- Identify the applicable decision rule.
- Review relevant part and process records.
- Document the evidence and final decision.
Continue the investigation
For the broader question of why a calibrated instrument can still produce an unexpected measurement, read:
Your Instrument Is Calibrated. Why Can the Measurement Still Be Wrong?
If two calibrated instruments give different readings, read:
Two Calibrated Instruments Give Different Readings. Which One Should You Trust?
If recalibration did not remove the problem, read:
The Instrument Was Recalibrated, but the Measurement Problem Remains
If the result is just outside the tolerance, read:
A Measurement Is Just Outside Tolerance. Should You Reject the Part?
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