Plug gages are fixed-limit gages used to verify internal diameters (bore sizes) of manufactured parts. Go and No-Go plug gages provide rapid pass/fail inspection. Calibration measures the external diameter of the plug gage pins and compares to the nominal size and tolerance class.
Inspect Go and No-Go members for wear, nicks, scratches, burrs, or corrosion on the measuring surfaces. Check that handles are secure and identification markings are legible.
Allow the plug gage and measurement instruments to thermally equilibrate at 20 °C (68 °F). Record ambient temperature. Thermal effects on small gages are significant at the micrometer level.
Measure the plug diameter at three locations along the length (near each end and at the center) and at two orientations (0° and 90°) at each location. Use a calibrated outside micrometer with verified anvil flatness or a CMM.
Evaluate the roundness deviation at each cross-section from the two-orientation diameter measurements. Assess taper by comparing diameters along the length of the plug.
Compare current measurements to previous calibration records to track wear trends. The Go member wears faster than the No-Go member due to more frequent use.
Record all measurements, calculate errors relative to nominal and tolerance, and determine pass/fail status. Issue the calibration certificate and apply the calibration label or condemn tag as appropriate.
The measured plug diameter must be within the tolerance for the specified gage class per ASME B89.1.5. For a Class XX plug gage, the diameter tolerance is typically ±0.00002 in (±0.5 µm). Roundness deviation must not exceed one-half the size tolerance.
12 months, or based on usage frequency
One critical mistake is measuring plug gages at incorrect temperatures, particularly when transitioning between temperature zones. Plug gages must be stabilized at 20°C (68°F) per ASME B89.1.5, as thermal expansion significantly affects dimensional measurements—a 1°C temperature difference can cause a 10-12 µm error on a 25mm gage. Another frequent error is using worn or improperly calibrated measuring instruments; micrometers with uncertainty greater than the gage tolerance will invalidate results. Technicians often neglect proper cleaning procedures, leaving oil films or debris that can add 0.5-2 µm measurement error. Incorrect anvil pressure during micrometer measurements is also common—excessive force deforms the gage surface, while insufficient pressure creates air gaps. Finally, many technicians fail to verify gage block calibration before micrometer verification, creating systematic errors throughout the measurement chain. To avoid these issues: implement strict temperature soaking protocols (minimum 4 hours at 20°C), maintain micrometer calibration with uncertainty ratios of 4:1 or better, establish standardized cleaning procedures using appropriate solvents, train technicians on proper measuring force (typically 5-10N), and verify all reference standards before use.
| Issue | Cause | Remedy |
|---|---|---|
| Inconsistent diameter readings across different measurement positions | Plug gage is out-of-round or has surface damage from mishandling | Check roundness using CMM or rotate measurements 90° intervals; if roundness exceeds ASME B89.1.5 limits, remove from service |
| Micrometer readings drift during measurement | Temperature differential between gage and measuring instrument | Allow 4-hour temperature stabilization at 20°C ±1°C for both gage and micrometer before measurement |
| Measurement results exceed uncertainty budget limits | Worn micrometer anvils or spindle backlash | Verify micrometer calibration with certified gage blocks; replace or service micrometer if anvil wear exceeds 0.5 µm |
| Gage appears undersized but customer reports interference | Measurement taken at worn section of gage due to improper positioning | Measure at multiple axial positions per ASME B89.1.5 requirements; document wear patterns and consider gage retirement |
| Roundness measurements show systematic error pattern | CMM probe qualification invalid or measuring force inconsistent | Re-qualify CMM probe with certified sphere; establish consistent probing strategy with documented measuring force |
CalibrationOS streamlines plug gage calibration management through automated scheduling that tracks calibration intervals per ASME B89.1.5 requirements, preventing overdue calibrations that could compromise dimensional accuracy. The system generates comprehensive digital certificates containing all diameter measurements, roundness data, and environmental conditions, satisfying ISO/IEC 17025 Section 7.8 reporting requirements. When plug gages fail acceptance criteria, CalibrationOS initiates automated OOT investigations, prompting technicians to evaluate measurement validity, check reference standards, and assess customer impact per Section 7.10.4. The measurement uncertainty budget feature calculates combined uncertainties from micrometer resolution, temperature effects, gage block uncertainty, and operator repeatability, ensuring compliance with Section 7.6 evaluation requirements. CalibrationOS maintains complete audit trails for all dimensional measurements, environmental data, and technician actions, supporting ISO/IEC 17025 Section 8.4 control of records. The system also manages gage block calibration schedules for micrometer verification, ensuring traceability chains remain valid. Integration with CMM software allows direct data import, reducing transcription errors and maintaining measurement integrity throughout the calibration process.
The standard interval is 12 months, but high-use plug gages (used hundreds of times per day) should be calibrated more frequently — every 3 to 6 months. Usage-based intervals, where calibration is triggered after a set number of uses, are the most effective approach.
Yes, but the micrometer must itself be calibrated with traceable uncertainty better than the plug gage tolerance. For Class XX gages with tolerances of ±0.5 µm, a high-quality micrometer with verified flatness and calibrated gage block verification is essential.
If the No-Go member is out of tolerance (undersized due to wear), it will falsely accept parts that are actually too small. The No-Go member must be replaced or the entire plug gage condemned. You cannot continue using only the Go member.
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