A Primer On Dial Indicators
Also known as dial gages, clocks, comparators or just indicators, dial indicators are widely used as basic gages for measuring linear dimensions. This device is useless by itself; it needs to be attached to a fixed base or a stand so that the tip of the spindle is at a specific height against either a master or reference part.
Director of Precision Gages, Mahr Federal Inc.
Also known as dial gages, clocks, comparators or just indicators, dial indicators are widely used as basic gages for measuring linear dimensions. This device is useless by itself; it needs to be attached to a fixed base or a stand so that the tip of the spindle is at a specific height against either a master or reference part. An operator then lifts the spindle with a lever, slides a part to be measured under it and lowers the spindle back down. If the part length varies from the reference, then the operator will see the deviation on the indicator’s dial.
A dial indicator is generally mounted by the stem or by a lug on the back. In the United States (where AGD standards are the norm), indicators are usually mounted by the back. The lug determines the mounting position when an indicator is held by the back. Lug mounting can take different forms, including centered and off-centered lugs, post back with a handle, screw back, adjustable bracket back, rack back with a dovetail bracket and magnetic backs that directly attach to a machine.
Holding an indicator by the stem, however, allows the axis of the dial indicator to be easily adjusted square to a measured surface. This style of mounting also makes it easier to adjust the holding position as needed. A disadvantage of this method is the fact that the stem is a crucial part of the indicator. There is minimal clearance between the stem and the spindle, which must move through the stem with as little friction as possible. The stems of indicators intended to be mounted this way are usually made of hardened steel. However care must be taken, as clamping too tightly may still deform the stem.
For easy zeroing to the master setting, the bezel and dial of the indicator are designed so that they can be rotated to align the “0” of the dial with the hand. They can be clamped or locked in place using the bezel clamp. This ensures that the zero position will not be changed during measurement.
A lifting lever can be mounted atop the indicator to raise the spindle and insert a workpiece, thus allowing the user to move the spindle up and down several times to test for repeat and stability.
One of the internal design characteristics of dial indicators is that the spring-powered spindle puts wear and tear on the gear mechanism and bearings each time the user releases the lifting lever and the spindle springs back into place. The amount of wear depends on the range of the indicator and its construction. As a result, the gear system must be tough enough to withstand these repetitive shocks. Therefore, hardened stainless steel components are a must.
Measuring accuracy is also affected by friction on the bearings. To reduce friction, the contact area between the axis and bearings is made as small as possible. However, this increases the load on the bearings. Even though the actual load is quite small, indicators generally employ jeweled bearings that are tough and produce low friction.
Dial indicators will suffer damage when subjected to oil, water or corrosive materials. Oil contaminants can solidify and adhere to the innards of the dial indicator. When the device is subjected to water coolant, as in grinding, the spindle may draw in the watery metal chips as it moves up and down. To lessen this problem, a protective boot can be added to the spindle, or a waterproof indicator may be necessary.
Another potential problem with dial indicators is the possibility of misreading a dial when the pointer turns more than one revolution. As a solution, one-revolution dial indicators have a red-colored dead zone, in which the pointer has no meaning.
Finally, temperature variations can affect dial indicator readings as the workpiece—and the indicator stand—expand or contract. With comparison measurement, it is possible to minimize thermal effects on measuring by selecting the stand and zero-setting master to correspond to the workpiece in form, size and thermal expansion rate.
For a dial indicator to work properly, it should be inspected when received. The device should also then be checked periodically during use to ensure that it functions as designed. Major items to monitor include long- and short-range accuracy, hysteresis, repeat and gaging force.
In addition, operators should conduct the following checks before each measurement: 1) make sure that the spindle moves smoothly through its entire range; 2) check to see whether the pointer tip is hitting any portion of the crystal that may cause it to stick; and 3) check the repeatability of the dial reading by measuring the same part in the same place a few times.
Finally, workpieces should be inserted in the same direction; this helps with repeatability of both the indicator and the stand or gage on which the indicator is mounted.