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
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.
CalibrationOS tracks due dates, stores certificates, and generates audit-ready reports.
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