Screw thread inserts (STI) require the threaded hole to be inspected prior to inserting the threaded insert or coil. The dimensions and tolerances for the gages used to inspect these threads have been a bit confusing over the past 30 years. This confusion has affected metrology labs, gage manufacturers, and gage end users. The problems revolved around the tolerance for the gage pitch diameters and the calculation of the No-Go gage major diameters. A quick review of the history of the applicable industry standards will help to explain the issues.
Previously, the Mil-T-21309E standard from 1975 was used to define the gages for STI threads. Per paragraph 3.3, “The gages shall be ground and lapped and conform to Handbook H28, except the pitch diameter and major diameter. The pitch and major diameter shall conform to table III for 3B class of thread fit and table IV for the 2B class of thread fit.” Each gage shall be marked “STI” or “Screw Thread Insert.” The problems occur when viewing the previously mentioned tables and the tolerances stated within. The “Note” on both Table III and IV states, “Thread plug gages above heavy line are class “W” gages and thread plug gages below the heavy line are class “X” gages (see Handbook H28). This is where things get interesting.