What is IFIs position on choice of screw thread gaging?
Above all else, threads must fit and function to be acceptable. Thread Acceptability System 21 (Method A) is most practical for all threads, except for Class 3A external threads where System 22 (Method B) is recommended. IFI research investigations prove that System 21 (or Method A) GO/NOT GO limit gages can detect minimum material limits. Also, measurement of pitch diameter as a predictor of fastening performance is a meaningless exercise. Chemistry of fastener material, heat treatment, etc. are the key performance characteristics. Measurement of pitch diameter, on the other hand, is of value to fastener producers for analytical purposes relating to the threading process itself. Thread acceptability System 23 (Method C) should not be used as a routine inspection of threaded fasteners. These requirements should be limited to research and analysis as recommended in FED-STD-H28/20A. When higher quality confidence levels are required, sampling plans should be adjusted or statistical proce
