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. And runout from one diameter to the next is easily measured with a V-block and test indicator.
These are just a few of the inspection methods used by shops everywhere, every day, to inspect machined parts. All are relatively simple to perform, require little investment or training, and no quality engineer would argue with their validity. There’s just one problem: they’re slow. What’s the sense in taking ten or fifteen minutes to measure a part that needed but a fraction of that time to produce? Further, what about the documentation and subsequent archiving of inspection results? In a paperless world, old-fashioned metrology comes up short.