Measuring and gaging are two fairly distinct forms of dimensional inspection. Measuring is a direct-reading process, in which the instrument incorporates a continuous scale of units, against which the part is compared directly.
Proper care of contact points is one of the basic considerations in gaging. Whether you're using a simple indicator gage or a sophisticated surface finish instrument, much depends on the condition of the sensitive contact point, which is the interface between the gage and the workpiece.
Many of the gaging applications we've considered over the years involve size inspection of a single feature, that is, the diameter of a hole, depth of a groove, height of a gage block, and so on. Many parts, however, contain multiple features that establish dimensional relationships between two or more other parts.