Depth gages are among the simplest of gages, typically consisting of an indicating device mounted through a reference bar or plate. Though they may be simple, depth gages are used in thousands of critical applications to measure the depth of holes, counterbores, slots, and recesses, as well as heights or locations of some features. They are especially common in the tool and die industry. Like other hand tools, they have undergone a gradual change from mechanical scales to digital wonders.
The first depth gages consisted of a simple rule with a sliding perpendicular beam as the reference. The scale was set into the hole and the slide squared up with the reference surface. The depth of the hole was then read directly from the scale itself. This was a simple tool, requiring the operator to employ good judgment and proper technique.