THREAD GAGE BASICS

When inspecting a threaded part, there are a handful of features that are involved – major diameter, minor diameter, pitch diameter, pitch, threads per inch (TPI), GO and NO-GO gages. Each of these terms are essential in the inspection process and need to be measured precisely. It is very important to understand what exactly these terms mean and how they are particularly related to the acceptance of a both internal and external threaded parts. Below you will find a brief description of each and how it pertains to the threaded part.

known as diameter of the imaginary cylinder which bounds the crest of an external thread or the root of an internal thread.
known as the diameter of an imaginary cylinder which bounds the root of an external thread or the crest of an internal thread.

the distance between points on adjacent thread forms in the axial plane and on the same side of the axis
a count of the number of threads per inch measured along the length of a threaded part.
These two terms are used to describe if a part is usable or unusable. As stated previously, if a GO gage can correctly and smoothing fit, then the part is considered usable. If the NO-GO gage can successful turn more than two times, then the part is deemed defective

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