How do you measure an internal thread on a part? How do you decide what you need? The thread description and thread data are critical to answering these questions. The thread description states the nominal diameter, threads per inch and thread series for UN Threads. If the thread is metric, it will state the nominal diameter and pitch for M Series Metric Threads. A Class of Fit and any special requirements are required for both the Unified (Inch) and M Series (Metric) threads.
Do you need to report the actual measured values of the thread or do you need a pass/fail inspection? If actual measured values of the thread are required, or you are manufacturing threaded parts to System 22 or 23 (ASME B1.3), or possibly making threads per SAE AS8879 Category 2, you will need variable gages as well as Fixed Limit Go / No-Go (pass/fail) gages to inspect the thread. The starting point, or ending point if pass/fail meets your requirements, is the use of a Go / No-Go inspection method. One of the basic pass/fail inspection methods is to use Fixed Limit Go / No-Go gages. Go / No-Go gages include working thread plug gages for inspecting internal part threads and thread ring gages for inspecting external part threads.
Working thread plug and thread ring gages, in the United States, are manufactured and inspected per ASME B1.2 for Unified Inch Screw Threads and ASME B1.16M for M Series Metric Screw Threads. Gages per these standards, including cylindrical gages for measuring major diameter on external threads and minor diameter on internal threads, meet the requirements of System 21 per ASME B1.3. These standards dictate the use, functions, sizes, and tolerances for the gages. These gages are used in conjunction with the ASME B1.1, Unified Inch Screw Threads and ASME B1.13M, Metric Screw Threads: M Profile part standards. In this article, we will be focusing on the function and use of working thread plug gages to inspect internal part threads.