The Design features of dial test indicators

A dial test indicator looks much like a dial indicator.Indeed, there are strong similarities and the distinction between the two seems, on the face of it, to be trivial, or at least arbitrary: While a dial indicator uses a rack-and-pinion mechanism to convert a linear movement into pointer rotation, a dial test indicator uses a lever and sector gear arrangement for the same basic purpose. Thus, while the plunger on a dial indicator moves in the plane of the dial face, the stem of a test indicator moves at right angles to that plane, in a slight arc.
This difference in construction proceeds out of the differing ways these instruments are used. A dial indicator , with its potentially longer range, is a versatile general purpose instrument, but the fact that the plunger travels in a straight line toward or away from the gauge makes certain kinds of measurement awkward. By itself, it cannot be used to measure the circularity of an inside diameter being machined on a lathe, for example:While a dial indicator can be adapted for this kind of measurement using a bell-crank accessory, a dial test indicator can be used directly, because its stem moves sideways rather than linearly. These instruments tend also to be more compact than their relatives which further recommends them for such applications. They are often mounted onto a machine even as the work rotating at a good clip. Even for outside measurements, where a plunger-type instrument could be used, the shallow angle of contact between the stem tip (invariably carbide) and the work makes the test indicator much more able to cope with a moving workpiece.
Dial test indicators are also frequently attached to height gauges, to improve the accuracy and convenience of those tools when they are used for comparative measurements.