Most dial indicators have a total measurement range of 2.5 revolutions of the needle. Indicators that allow the needle to go around only once are comparatively rare but offer a distinct advantage for shop-floor inspection applications.The added range in the traditional dial indicator was useful many years ago when machine tool accuracy demanded a broad measurement range to help machinists "creep up" on a specification. Nowadays, though, measuring tool suppliers recommend that an indicator be chosen so that the tolerance range for the parts being measured should cover between one-tenth and one-quarter of a single needle revolution. This provides a large enough tolerance zone to read easily, and leaves more than enough area on the dial to see what's out-of-tolerance. It's a rare occurrence when anyone actually bothers to read a gage if a part is more than a half-revolution out of tolerance. Fig. 1
Two and a half revolutions are simply unnecessary for most comparative measuring applications, and sometimes they're a real liability. Considering how quickly the needle swings on an indicator, it's not surprising that machinists occasionally miss a revolution. As shown in Figure 1, a measurement that is a full revolution out of tolerance can appear to be exactly on spec to an operator who is distracted -- or poorly trained, poorly supervised, or hurried. Errors may occur through simple inattentiveness, or through an absolute misunderstanding of how to set up and master the gage.There is at least one documented instance in the aircraft industry where an entire run of oversize parts passed through inspection, and was assembled into components, which were subsequently installed in subassemblies. It is not documented what happened to the machinist/operator or his supervisor when this costly error was discovered. But the situation has surely been repeated in other companies and other industries.Aside from better operator training, there are a couple of ways to minimize this problem. One solution is to use indicators with revolution counters -- little accessory dials that show the operator how many times the main needle has swung around. The problem here is that an operator can still ignore or misinterpret the tiny rev counter. Indicators with rev counters are actually better suited to long-travel applications, such as where the spindle must clear an obstruction on the part, or where measurements are absolute, as opposed to comparative.
A better solution is the one-revolution indicator, as shown in Figure 2. These have the same range per revolution as comparable indicators (in other words, the needle moves the same distance for a given amount of spindle travel). They also have the same amount of spindle travel, so they can be used on parts that are just as far out-of-spec. But in a one-rev indicator, the needle stops moving after one complete revolution (actually, a bit less: usually 340° to 350°), coming to rest in a "dead zone," even if the spindle keeps traveling.One-rev indicators always have balanced dials, with zero falling between an "over" side and an "under" side: they don't allow continuous clockwise or anti-clockwise readings. The needle of a one-rev indicator cannot come back to zero. It can't even travel a full 180° from zero, so there is virtually no way that "over" can be confused with "under," or that out-of-tolerance can be construed as in-tolerance. For less sophisticated operators, or anyone who performs quick, repetitive part inspections in a production environment, one-rev indicators can eliminate a major cause of misreadings. Applications include snap gages, bore gages, and many other comparative–type inspection gages.The only liability of one-rev indicators, relative to dial indicators, is that more sophisticated users will give up the capability to measure broader ranges of part variation -- for example, when an operator needs to know just how far out of tolerance a part is. Conventional multi-rev dial indicators still have an important role to play where greater measuring flexibility is required.
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