Quality Scan: Respect Your Dial Indicators
Prior to World War II, you would not have found many dial indicators in the major gaging-manufacturers' catalogs. But during the war it became important to be able to use the custom indicators supplied by various manufacturers interchangeably, in all sorts of fixtures.
So the government promulgated the AGD (American Gage Design) standard, which mandated such things as the size of the dial, diameter of the plunger, etc. As manufacturing engineers embraced that standard, indicator sizes, styles, and accessories ballooned to the point where users today have access to
of variations. With a little imagination, you can measure just about anything with dial indicators.
A wide variety of tasks, including layout, inspection, and quality control use all sorts of indicators. The major indicator classes are: mechanical dial indicators; electronic indicators; lever-style test indicators; and back-plunger dial indicators. The vast majority of the total indicator populations, perhaps three-quarters of them, are mechanical dial indicators.
Companies such as ours manufacture dial indicators to fine watchmaking standards with jeweled bearings. Precisely finished gears and pinions and hardened stainless gear trains are available. The result is a gage that can measure to 50 millionths of an inch (0.00127 mm), if necessary. The contact point is attached to a spindle or rack. Movement is transmitted to a pinion and then through a train of gears to a hand that sweeps the dial of the indicator. As a result, a small movement of the contact is transformed into a larger, observable, and measurable movement of the hand on the dial.
Manufacturers supply dial indicators with either balanced or continuous dials. A continuous dial starts at zero and, during measurement, the hand moves clockwise until it reaches its stopping point to indicate a specific distance. Instead of taking a micrometer or caliper, holding the tool in your hand, and opening and closing it, you can quickly slide it under the indicator and determine the height of your component.
Balanced styles, typically used for inspections, measure positive or negative deviation from a zero point that's preset using a standard, typically a part master or gage block. With a dial indicator, you can see whether a dimension is in or out, and by how much. Using plastic indicators to mark the tolerance limits on the dial makes the decision faster for inexperienced users.
Wouldn't electronic indicators be faster, easier to use, or more accurate? Actually, no. With a dial you don't have to stop and think about whether the measurement is within the tolerance band. As for accuracy, dial and electronic indicators rely on the same internal mechanisms, so neither has an advantage. If you want to automatically capture data to some electronic system, or quickly switch your indicator from English to metric, then by all means go with electronic indicators. Otherwise, in most applications, the dial indicator works best.
I sometimes get calls from customers who want to know why their dial indicator didn't come with a booklet telling them how to use it. I tell those customers that such a booklet would be as thick as an encyclopedia.
For example, put two indicators on a V-block and you can measure the height and diameter of a soft drink can in a halfsecond. Try that with a CMM.
Want to know if a paper cup has the proper circumference to hold a lid securely and not spill scalding coffee on drivethrough patrons? I've seen a device that uses a wire loop operated by a pushbutton mechanism to transfer measured deviation to a dial indicator. Because the round loop applies uniform pressure around the outside of the cup's lip, the measurement is distortion-free and accurate.
I know of a foundry that measures hot pieces of steel using long tongs which roll out on the molten billet and clamp onto its edges. At the cool end of the device, a dial indicator translates the tongs' movement into a width measurement. In a matter of seconds tongs roll out, click to measure, the gage reads "good," and the billet comes out. Nothing could be simpler.
Be kind to your dial indicators and they will serve you well. Handle these instruments with care. Lubricate them lightly. (Too much lubrication is a conduit for dirt.) Most of all, apply your imagination.
This article was first published in the June 2010 edition of Manufacturing Engineering magazine.