Test Dial Indicator

A dial test indicator, also known as a lever arm test indicator or finger indicator, has a smaller measuring range than a standard dial indicator. A test indicator measures the deflection of the arm, the probe does not retract but swings in an arc around its hinge point. The lever may be interchanged for length or ball diameter, and permits measurements to be taken in narrow grooves and small bores where the body of a probe type may not reach. The model shown is bidirectional, some types may have to be switched via a side lever to be able to measure in the opposite direction.

These indicators actually measure angular displacement and not linear displacement; linear distance is correlated to the angular displacement based on the correlating variables. If the cause of movement is perpendicular to the finger, the linear displacement error is acceptably small within the display range of the dial. However, this error starts to become noticeable when this cause is as much as 10° off the ideal 90°. This is called cosine error, because the indicator is only registering the cosine of the movement, whereas the user likely is interested in the net movement vector. You can read more about Cosine errors on this forum.

Contact points of test indicators most often come with a standard spherical tip of 1, 2, or 3mm diameter. Many are of steel (alloy tool steel or HSS); higher-end models are of carbides (such as tungsten carbide) for greater wear resistance. Other materials are available for contact points depending on application, such as ruby (high wear resistance) or teflon or PVC (to avoid scratching the workpiece). These are more expensive and are not always available as OEM options, but they are extremely useful in applications that demand them.

Modern dial test indicators are usually mounted using either an integrated stem (on the right of the image) or by a special clamp that grabs an dovetail on the indicator body. Some instruments may use special holders.

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