DIAL TEST INDICATORS / LEVER CLOCKS
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A dial test indicator, also known as a lever arm test indicator or finger clock, finger indicator, DTI, lever clock, has a smaller measuring range than a standard dial indicator. A dial 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. Some dial test indicators are bidirectional, some types may have to be switched via a side lever to be able to measure in the opposite direction.
Toolmaker are most commonly to use only a dial test indicator, smaller movements but more accurate set ups. Dial test indicators have a demeaner with quality; brands such as TESA dial test indicators, VERDICT dial test indicators & MERCER dial test indicators are amongst the high end options for toolmakers.
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. Cosine error is discussed in more detail below.
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 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 or by a mini
magnetic base. Also special clamp that grabs an dovetail on the indicator body.
Some instruments may use special holders.
See also MAGNETIC BASES page
