Gage accuracy depends on product info. Thread form plating changes pitch diameter. Compensate for buildup to make correct pre-plate allowances. 60° UN threads change pitch diameter by 4x plating thickness.

One of the more frequent questions we receive is: I am tapping a ¼-20 UNC 2B internal thread with an “H13” tap, what gage do I need to inspect the threaded holes in my parts after tapping?

Taper Pipe Threads are one of the more challenging threads to produce and measure. The ASME B1.20.1-2013 Pipe Threads, General Purpose (Inch) is the current standard that covers both the part thread specifications and the gages for NPT, NPSC, NPTR, NPSM and NPSL threads.

Cylindrical ring gages have a number of inspection purposes. They are used as Go/No-Go gages to inspect the outside diameter of a shaft or rod. They are masters for dial bore gages, internal micrometers and coordinate measuring machines (CMMs). 

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. 

Selecting gages and other types of measuring tools can be challenging. It does not matter if you are an experienced or novice gage user, tool crib manager, lab manager or buyer. 

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 measurement of the depth of a hole sounds straightforward and simple. In many cases it is. In others, there are design features of both the part and the gage that must be considered to insure an accurate measurement and a properly assembled or functioning part. 

External thread inspection can seem confusing and, in some cases, frustrating. There is a wide variety of inspection tools available for external threads.

How do you measure an internal thread on a part? How do you decide what you need?