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Measuring tips for the Dial Indicators

A dial indicator is not, by itself, an independent measuring tool; to be of any use, it must be combined with some other apparatus. In a machine shop or at a quality control inspector’s station, it will most likely be used as a comparator—it will measure the dimensional difference between some reference object and the workpiece being measured.
In automotive mechanical work, the dial indicator needs, at least, something that holds it in place while some other type of comparative measurement is taken. An example of this might be the measurement of the end play of a crankshaft or camshaft in its bearings. Here, the comparison is between one position of the shaft and another. In such a situation, the indicator is most likely secured to a magnetic base, itself clingling to the engine block, though other types of base are available. To connect the indicator to the base, the most usual arrangement involves a stout rod or “base post?projecting from the base, plus a crossbar that is secured to the base post by a swivel clamp, allowing the crossbar to be positioned at virtually any height and angle. For attachment is usually built with a projecting lug in the back, with a projecting lug on the back, with a 1/4-inch hole through it. Care should be taken to keep all the “overhangs?to a minimum, to eliminate as much spring in the setup as possible.
Bear in mind that the only thing a dial gauge actually measures is the movement of its own plunger. If the plunger is not exactly aligned with the work, an erroneous reading will be given. The situation is not as critical as you might think, however. First, recall that the accuracy of an AGD dial indicator is ±one dial division. Second, let’s assume that the instrument is marked off in 1-thou increments, and that its total range is 0.125 inch. Even if the dimension being measured (or, more correctly, the difference between the positions being compared) is near the gauge’s limiting range, the “cosine error,?as it is called, that arises from a misalignment of 5 degrees—quite apparent to even an unskilled eye—will be just 4/10 of 1 percent of 0.125 inch, or less than half a thou. Clearly, straight by “eyeball?is generally quite good enough, except when something closer to 0.0001 inch accuracy is needed, in which case the parallelism between the plunger and the work may need to be confirmed with a machinist‘s square.
Once the work is arrange in place, and with (in our example) to the crank or camshaft pushed to one extreme of its end travel, the worker will typically preload the dial indicator about one quarter-turn, so the pointer is approximately at the ?2 o’clock?position, then the instrument is zeroed. (It is not necessary to fiddle about endlessly with the setup, trying to get the pointer to line up exactly with the “Zero?on the dial; the dial face is attached to the surrounding movable bezel, and so can be turned.) When the shaft is pushed in the opposite direction , taking up the end display the total travel.
Some dial gauges come with a “shockproof ?mechanism that greatly reduces the risk of damage from a sudden sharp impact, by allowing some internal components to disengage themselves momentarily if the foot moves too suddenly. Still, dial gauges are delicate instruments(more so than micrometers ) and while they do not need to be handle like fine china, their accuracy and life are not likely to be enhanced by ham fistedness.
Obviously, you should take care that it does not get dropped, and the contact foot on the plunger should not be allowed to slam against the work. On side-plunger (bottom plunger) indicators, the plunger extends through the diameter of the case, and the protruding stump can be grasped (once its protective cap has been unscrewed) to manually raise the plunger and so control its descent into the workpiece. A valuable accessory is a finger lift attachment extending to one side.
It also important to avoid driving the plunger hand up against its end stop . One way to prevent this , and to avoid imposing violent blows to the mechanism, is to arrange things so that the plunger moves away from the scale during the measurement, rather than toward it. In the above example, it would be preferable to have the shaft pushed fully toward the gauge before the instrument is brought in contact, preloaded one full turn, and zeroed. Then tapping on the shaft would tend to unload the gauge.
In another situation, the dial gauge might be used to measure, say, the “runout?of a shaft. In this case, assuming the usual 2-plus turn instrument, the gauge would be loaded about 1 1/4 turns, leaving 1 full turn in either direction available for gauging “plus?and “minus?variations. The shaft should be rotated slowly; never attempt to use a mechanical dial indicator to measure any fluctuating value (value lift, radial or axial runout of a pulley, etc.) on a running engine ! The amplification of movement that takes place in the instrument’s internal gear train would destroy the tool. Besides, there is no early way you could read the wildly flailing pointer. With the use of some very sophisticated (and expensive) circuitry , certain electronic instruments, such as occur when the runout of a circular part is being checked while the part is rotating at quite a high speed. But within our price range , any digital electronic instrument is likely to have internal circuitry that requires a little time to “decide?on the value being measured. If the value is constantly changing, as it is when visualizing and measuring runout in this way, the value doesn’t stand still long enough for the instrument to do the calculation. The result would be either a blank stare from the gauge or a meaningless flickering of numbers. A mechanical gauge is often preferable to an electronic one for this sort of measurement of a value that changes, say, once per second.