In production shops, the inspection of cylindrical parts (shafts or holes) for acceptance is quickly done using simple GO and NO-GO snap/ring gauges (for checking shafts) and plug gauges (for checking holes). They are mostly simple and easy to comprehend.
In the case of (screw/bolt) threaded parts, thread gauges are used in production shops where the “screws” (externally threaded parts) or the “nuts” (internally threaded parts) are to be quickly checked for acceptance. But when you consider the metrology (the science of measurement) of threaded parts as a whole, more than a couple of thread gauge types are involved; a novice in metrology may become somewhat confused with the terminology of the threaded gauges, their application, and their usage.
In this article, we shall try to understand the various types of thread gauges and their applications. This article deals with thread gauges used primarily in the shop for acceptance gauging of threaded parts.
Before that, a word about thread gauging practice:
In a threaded part, several features are involved – the thread’s major diameter, minor diameter, pitch diameter, pitch (in case of metric threads) or threads per Inch (TPI in case of Standard American Equivalent (SAE) threads), flank angle, etc. Normally, a GO / NO-GO type of threaded gauge will inspect a threaded part based mainly on the pitch diameter. As long as a GO gauge is answered (threads properly without being forced) and a NO-GO gauge is not answered, the threaded part is deemed to be accepted in totality.
But in some specific cases where the thread major diameter (for screws) or minor diameter (for nuts) is critical, separate, plain GO/ NO-GO gauges (as those used for measuring cylindrical parts) are also used in addition to a threaded gauges to determine the acceptance of a threaded part.