Most engineers are afflicted with insatiable curiosity and can’t help but take things apart. However, as dial indicators are highly sensitive instruments, I’d like to help them resist this urge with the following anatomical description. Mechanical dial indicators (comparators) are characterized by a sensitive contact; a measuring system that transfers and magnifies the mechanical motion; and a means of indicating the measurement.
The American Society of Mechanical Engineers (ASME) has the responsibility of preparing standards that encompass the inspection/means of measuring various dimensional characteristics, as well as the instruments used to make those measurements. Most dial indicators used in the United States are built to and used to measure inch specification; they generally do not fall under ISO specifications. Rather, ASME has updated old commercial and American Gage Design (AGD) standards into the most recent ASME B89.1.10-2000. This spec defines many aspects of dial indicators, including the various sizes, resolutions, suggestions for construction and statements about performance.
Getting down to the nuts and bolts, a dial indicator is enclosed by a metal case that protects the instrument’s mechanism. The case also serves as the rigid framework to support the means of amplifying the motion from the contact point to a hand moving against a graduated dial.
Why metal? With all the composites and plastics available, one would think it would be easy to reduce costs by using a composite case. We have to remember that we are in the measurement world here, and we can’t go against certain basic measuring rules. In this case (no pun intended), the case is part of the measuring loop, and we know that temperature is one of the biggest contributors to measurement errors. If the case or any part of the indicator is not affected by temperature the same way as the part or the stand, then small errors will creep into the results.
In order to read a measurement, the often tiny motion of the sensitive contact needs to be magnified. The two most common means of amplification in an indicator involve the use of levers or gears.
Levers are one of the oldest and simplest means of amplifying motion—think of the old bar and fulcrum with the force applied to the short end of the bar. However, used alone, the lever does not provide the accuracy, degree of amplification or sensitivity of other methods. Yet, because of its simplicity, the lever is usually found in combination with gears. For example, it is the basis for that relative of the dial indicator, the test indicator.
Gears are also a type of lever. Gear-train amplification is the most common method used to magnify dial indicator motion because its accuracy meets the requirements of comparison measurement. In use, the sensitive contact is part of a rack in which a series of three to five gears magnifies and transmits the movement of the contact to the pinion gear on which the indicator hand is mounted.
The amount of magnification of an indicator depends on its use and desired resolution. Dial indicators can have gear trains that amplify the movement anywhere from 40:1 to 1,500:1. This can give a dial indicator a discrimination, or resolution, of 0.001 inch to 0.00005 inch. The total travel, or reading capacity, of the indicator commonly ranges from 0.003 inch to 2 feet or more. With long-range indicators, the gear-train drives revolution counters that tell the user the number of times the hand has traveled around the dial, and thus, the range of the indicator’s displacement.
Most manufacturers of indicators exceed the ASME specification and provide indicators that go beyond these ranges or resolutions determined in the specification.
Those are the basics of dial indicator anatomy, but it does not end there. There are literally thousands of modifications and attachments that can customize an indicator to your application. Some are common and employed by the vast majority of users, such as balanced/continuous dials; plus-sign on right or left; and various backs or contact points. More complex configurations include special magnifications and ranges.
Regardless of the final configuration, however, itchy-fingered engineers should bear in mind that the heart of an indicator is based on nothing more than a fulcrum and a stick.