Thread classes for product threads, and by extension the gages used to inspect them, can become a bit of alphabet soup. Some find the requirements confusing. The confusion generates questions and assumptions. If “2A” is for an external thread, why does it apply to the thread setting plug? Why am I unable to order a class “6H” “M” series metric thread ring gage? What is wrong with using a class “6g” setting plug gage to measure internal product threads?
To answer these questions, we must first have a basic understanding of the product threads we are making. This also requires knowledge of industry standards that govern the product threads and the selection of measuring tools. This selection is based on the product thread requirements. The product dimensions dictate the gage dimensions and tolerances. (i.e.: unified inch screw threads per ASME B1.1-2003 are inspected with gages manufactured per ASME B1.2-1983.) Once we understand what we need to measure, we can develop or purchase tools to insure the mating product threads will assemble. The geometry of the thread form or design can make this challenging.