Circles are the most frequently produced machined form. Generated by many different processes-including turning, milling, centerless grinding, boring, reaming and drilling-there are, correspondingly, a wide variety of gaging methods used to measure inside and outside diameters. At the low end, a hole could be measured with a scale or a fixed go/no-go gage. At the other extreme, any number of precision measuring machines, including coordinate measuring machines (CMMs) and optical or vision machines are available. However, in production environments, most inside diameters and outside diameters (ID/ODs) can be accurately measured using one of several varieties of comparator gage.

 

With a variety of gages available for surface measurement, scratch pads are a method of the past for many manufacturers.