A Primer On Dial Indicators

A Primer On Dial Indicators
George Schuetz, Mahr Federal Inc.

Dial indicators - also known as dial gages, clocks, comparators, or just indicators—are widely used as basic gages for measuring linear dimensions. A dial indicator is useless by itself, and 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 is different from the reference, the operator will see the deviation on the indicator's dial. Test

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. When a dial indicator is held by the back, its mounting position is determined by the lug. Lug mounting can take many 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, but 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 and then 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 on top of the dial indicator to raise the spindle and insert a workpiece. This also allows the user to move the spindle up and down several times to test for repeat and stability.

Minimizing Error

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 every time the user releases the lifting lever and the spindle springs back into place or hits the workpiece. The amount of wear depends on the range of the indicator and its construction. As a result, the gear system must be designed tough enough to resist these repetitive shocks, and hardened stainless steel components are a must.

Measuring accuracy is also affected by friction on the bearings. To help 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 extremely 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, and when an indicator is subjected to water coolant, as used in grinding, the spindle may draw in the watery metal chips as it moves up and down. To help reduce this problem, a protective boot can be added to the spindle, or a waterproof indicator may be required.

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. For example, with a one-rev indicator, once the spindle goes past its usable measuring range, the indicator hand will stop in this dead zone, indicating that this is a non-measurement area and out of the indicator's range.

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.

To ensure that a dial indicator works properly, it should be inspected when received and then periodically during use to ensure that it functions as designed. Major items to check include long and short-range accuracy, hysteresis, repeat and gaging force.

In addition, operators should conduct the following checks before each measurement:
• Make sure that the spindle moves smoothly through its entire range.
• Check to see whether the pointer tip is hitting any portion of the crystal that may cause it to stick.
• Check the repeatability of the dial reading by measuring the same part in the same place a few times.

Finally, workpieces should always be inserted in the same direction; this helps with repeatability of both the indicator and the stand or gage the indicator is mounted on.