The Fundamental Problem in Thread Gaging and Thread Gage Measurement

The aim of this report is to analyze measurements of screw threads and thread gaging against specification requirements. The purpose is to not only communicate proper techniques, methods, and technologies, but also to present this information in a clear and concise manner that can be understood by manufacturers of threaded components, calibration laboratories, gage manufacturers, and advisory committees for screw thread specifications. The primary intent of the studies and information contained in this document is to open up a dialog between all parties involved in thread measurement and gaging to address a fundamental problem that has been discovered regarding adjustable thread ring gages and their applicable specifications. To perform the studies, thread set plugs were analyzed using the traditional super micrometer and 3-wire methods while thread ring gages were analyzed using the traditional set plug methods. These measurement results were then compared to results obtained using the newer technology of contact scanning. The results of these studies have indicated that it is very unlikely that adjustable thread ring gages are capable meeting dimensional specifications. In one portion of this study, a sample of 30 adjustable thread ring gages were measured using the contact scanning method. Of these samples none were found to meet specifications. In another portion of this study adjustable thread ring gages were properly set to the correlating thread set plugs. When measured using the contact scanner method the rings were found out of tolerance to specified requirements. The samples of adjustable thread rings were found out of tolerance for all pitch diameter measurements. After several unsuccessful attempts to adjust the pitch diameters into tolerance while keeping the minor diameters in tolerance, it was concluded that the closer the pitch diameter reading got to meeting the defined specifications, the rings would no longer thread onto the thread set plug correctly. With recent technological advances in measurement and this information being presented along with further studies in process, it is absolutely critical that specifications on thread ring gages be reviewed and reconsidered. It doesn't make any sense to have specifications that can not be achieved.