2.  Precision Gages

a.General.

(1) Practically all shops require measuring or gaging. A machinists will most likely measure or gage flat or round stock; the outside diameters of rods, shafts, or bolts; slots, grooves, and other openings; thread pitch and angles; spaces between surfaces or angles and circles.

(2) The term “gage”, as used in this lesson, identifies any device which can be used to determine the size or shape of an object.There is no significant difference between gages and measuring instruments. They are both used to compare the size and shape of an object against a scale or fixed dimension. However, there is a distinction between measuring and gaging which is easily explained by an example. Suppose that the machinist is turning a workpiece in the lathe and wants to know the diameter of the workpiece. He would take a micrometer, or perhaps an outside caliper, adjust its opening to the exact diameter of the workpiece, and determine that dimension numerically. On the other hand, if he wants to turn a piece of work down to a certain size without frequently taking time to measure it, he could set the caliper at a reading slightly greater than the final dimension desired; then, at intervals during the turning operations, measure, gage, or “size” the workpiece with the locked instrument. After the workpiece dimension has been reduced to the dimension set on the instrument, he would measure the workpiece to the exact dimension desired.