Behind Every QC Report: The Gages and Instruments That Measure a Screw
A fastener QC report cites numbers; those numbers are only as good as the instruments behind them. Knowing the metrology helps customers read reports critically.
The gage bench
Thread Go/No-Go ring and plug gages decide interchangeability — binary, fast, and subject to wear; calibrated at intervals or they lie quietly. Pitch diameter can also be measured over wires for disputes. Hardness (Rockwell/Vickers) verifies class, with decarb checks on the surface where fatigue starts.
Optical and geometric inspection
- Profile projectors and vision systems capture head geometry, thread angles and radii against overlays — the workhorse for 2D conformance.
- Optical comparators miss nothing below the silhouette line; coordinate machines add 3D for precision screws and custom heads.
- Surface roughness testers verify bearing-face Ra where torque friction lives.
What no bench instrument sees
Internal structure — grain flow, decarb depth, inclusions — needs sectioned metallography, which is why sampling plans section parts periodically rather than trusting surface impressions. Topprecision's QC lab runs gage, optical and metallographic stations with calibration records per instrument, and shares raw measurement data with customer audits on request.
