Thread gages verify the pitch diameter, lead, and flank angle of threaded parts. Go and No-Go thread ring and plug gages ensure threaded assemblies will function correctly. Calibration of thread gages requires specialized measurement equipment and is among the most complex gage calibration tasks.
Inspect thread form for damage, galling, burrs, or wear on the thread flanks and crests. Check that the gage is identified with thread size, pitch, class, and Go/No-Go designation. Clean threads thoroughly.
Verify the Go member enters the appropriate master setting plug or ring for the full length of the thread engagement. Verify the No-Go member does not engage more than the specified number of turns (typically 1.5-3 turns).
Measure the pitch diameter using the three-wire method (for thread plugs) or other suitable method. Calculate the pitch diameter from the over-wire measurement using the appropriate formula for the thread form. Record at multiple locations along the thread length.
If measurement capability is available, verify the lead (thread pitch) accuracy using an optical comparator, CMM with thread scanning software, or dedicated thread measuring machine. Verify flank angle is within specification.
Compare current pitch diameter measurements to previous calibration data. Thread gage wear occurs primarily on the Go member flanks and reduces the pitch diameter of thread plugs (increases for thread rings).
Record all measurements including pitch diameter, functional check results, and wear data. Issue the calibration certificate and apply the calibration label. Condemn gages that exceed wear limits.
Pitch diameter must be within the tolerance for the specified gage class per ASME B1.2 or B1.16M. For a Class W Go thread plug gage, pitch diameter tolerance is typically 0.0002 in (5 µm). Functional check must pass: Go enters freely, No-Go does not engage beyond limit.
12 months, or based on usage frequency
The three-wire method uses three precision wires placed in the thread grooves — two on one side and one on the opposite side. The over-wire dimension is measured with a micrometer, and the pitch diameter is calculated using a formula that accounts for wire diameter, thread pitch, and flank angle.
Modern CMMs with thread scanning software can measure pitch diameter, lead, and flank angle of thread gages. However, the CMM's uncertainty for thread measurement must be validated, and the probing strategy must be appropriate for the thread form.
The best wire size contacts the thread flank at the pitch diameter and is calculated as: best wire = 0.57735 × pitch for 60° threads (Unified and Metric). Wire sets must be calibrated and traceable. Using the wrong wire size introduces systematic measurement error.
CalibrationOS tracks due dates, stores certificates, and generates audit-ready reports.
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