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A dial indicator is a measuring instrument used in many industries where precision is necessary, such as manufacturing, designing, and scientific businesses. The indicator is a spring-loaded spindle, or plunger, connected to a clock-faced dial. As the spindle moves in and out of the indicator, a needle on the dial indicates the measurement of the distance, indicating the depth of the depression. To use the instrument, an operator lifts the indicator's spindle with a lever, slides the item he wants to measure under it and lowers the spindle. If the part varies from the reference, the indicator’s dial registers the deviation. On some indicators with a larger range, a smaller clock face within the larger clock face tracks each revolution of the larger dial.
Other names for a dial indicator include an indicator, a clock indicator, a dial indicator gauge, a dial gauge, a probe indicator, or a comparator. Dial indicator sizes vary. Generally, indicators have a range of 0.250 inches to 2 inches (6.35 mm to 50.8 mm) and give measurements in increments of 0.001 inches (0.0254 mm). Some specialty dial indicators measure in the range of around 0.015 inches (about 0.25 mm) while others go up to 12.0 inches (about 30.5 cm). Graduation markings on the dial can vary from 0.00005 inches to 0.001 inches (about 0.001 mm to 0.01 mm) in typical indicators.
Digital dial indicators have digital readouts instead of the clock-face readout. The digital technology makes it easy to switch between imperial and metric measurements. Some have the capability to transfer the measurement data to a computer.
Dial indicators are not accurate unless mounted to a base. The typical indicator holder is a magnetic-based stand that has an arm, or arms, to hold the dial indicator in a variety of positions. Sometimes engineers design unique mounts to support the instruments for special jobs. Having the indicator secured to a rigid base or stand helps the user in two ways. First, the instrument is more accurate without user interference. Another advantage of a base is that it gives the spindle a master part or reference point against which to measure the item. Usually a machinist will establish a reference point by rotating the large hand of the dial to zero before they measure a depth.
Dial indicators are used in a wide variety of industries. In manufacturing, professionals use dial indicators during machine set-up to center stock and determine the accuracy of lathe or mill alignment. Production machinists use indicators to monitor the depth of holes, keyways, and ledges. Many quality control technicians use them to check variation in machined part tolerance. In laboratories, scientists and technicians use them for numerous situations where a small measurements need to be recorded. In automotive repair shops, they help mechanics fit new discs in disc brakes, where the tolerance can be as low as 0.0019" (0.05 mm). In home woodworking shops, crafters can use them to set up their lathes and saws, and monitor drilled hole depths.
When buying a dial indicator, a consumer needs to consider its usage. Since coolants, oil, water, and other contaminants can ruin the instrument, most machinists buy waterproof and dust-proof dial indicators. Better indicators have jeweled movements, which offer a better lifespan. A buyer needs to consider if gauge accessories, such as stands, extra contact points, and replacement parts, are available.
Dial test indicators are similar to dial indicators. Typically, they have a smaller measuring range and are more precise. Instead of having a spindle, they have a levered arm with a balled tip that moves up and down. In many situations, a dial test indicator can do measurements that a dial indicator cannot do. Usually, a machinist has a set of both instruments.