Pick up any shaft or threaded fastener. How are you going to check it? An outside micrometer is most often the go-to tool for measuring diameters. A height gage or drop indicator could be used to check dimensions between part features, while an optical comparator is a good way to inspect thread forms or measure a groove width.

There was a time when choosing a gaging solution for shaft-type parts was a relatively straightforward task. High-volume parts received a custom-built gage and everything else received a manual or automatic “snap-on” or ring gage of some sort. 

The rapid growth of in-line gaging over the last few years has been focused primarily on the use of this approach as a tool for improving quality.