Modern Machine Shop

AUG 2018

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Page 34 of 316

MMS AUGUST 2018 32 QUALITY GAGING TIPS Measurement Tools Less Is More with Dial Comparators GEORGE SCHUETZ | COLUMNIST Why use a dial indicator with more than the needed range? A dial comparator offers less risk of misreading the dial without the cost of an electronic gage. Most dial indicators have a total measurement range of at least 2.5 revolutions of the needle, as per American Gage Design (AGD) specifications. Additional range in the traditional indicator was useful many years ago, when machine tool accuracy demanded a broad measurement range to help machinists "creep up" on a specification. Now, gaging suppliers often recommend that an indicator be chosen so that the tolerance range for the parts being measured covers between one- tenth and one-quarter of a single needle revolu- tion. This provides a large enough tolerance zone to read easily and leaves more than enough area on the dial to see what is out-of-tolerance. It is a rare occurrence when anyone actually bothers to read a gage if a part is more than a half revolution out of tolerance. Two and a half revolutions are simply unnec- essary for most comparative gaging applica- tions, and sometimes they are a real liability. Considering how quickly the needle swings on an indicator, it is not surprising that machinists occasionally miss a revolution. On a standard dial indicator, a measurement that is a full revolution out-of-tolerance can appear to be exactly on spec- ification to an operator who is distracted, poorly trained, poorly supervised or hurried. There is at least one documented instance in the aircraft industry in which an entire run of oversized parts passed through inspection and were assembled into components that were installed in subassemblies. It is not documented what happened to the machinist/operator or his supervisor when this costly error was discovered. However, the situation has surely been repeated in other companies and other industries. If the recommendation is to use an indicator that covers the tolerance band with between one-tenth and one-quarter of a needle revolution, why use a gage with more range than needed? This is where the dial comparator can be a viable replacement for dial indicators, especially when measuring to very high resolutions (such as those approaching 20 micro inches/0.5 micron). Any analog device has range versus resolution con- straints, and the dial comparator is no exception. It has higher resolution than the dial comparator, but also much less range. However, this is the specific advantage we want in our measuring processes. Limited range is not a major factor in very high- tolerance work, and it can even prevent measure- ment errors like those described in the case above. Dial indicators are not normally thought of as providing a high-resolution readout. Their high-confidence zone typically ranges to 0.0001 inch/2 microns. Manufacturing dial indicators to the highest possible standards results in only modest improvements in resolution. This is because the substantial number of parts in a dial indicator generate a build-up of tolerances. The high amplification required for ultra-precision measurements tends to magnify these errors, The dial comparator enables relatively high-resolution measurements without the cost of an electronic gage and without the risk of error associated with a dial indicator.

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