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What’s A Drum Micrometer ?

A drum micrometer is a single-purpose instrument, used to measure the inside diameter of a brake drum. A drum micrometer has two movable arms on a shaft. One arm has a precision dial indicator ; the other arm has an outside anvil that fits against the inside of the drum. In use, the arms are secured on the shaft by ock screws that git into groves every 1/8 inch(0.125in) on the shaft. The dial indicator is graduated in 0.005-inch increments.
To use a drum micrometer, loosen the lock screws and move the arms along the shaft so that the micrometer can fit inside the drum. Then extend the arms on the shaft until they align with the increments that indicate the nominal size of the drum. To measure a 10.5-inch drum, set one arm at the 10-inch mark on the shaft and the other 10-inch mark, plus four 0.125-inch marks (0.500in.) for the 10.5-inch drum. Then place the micrometer inside the drum and rock it gently on the drum surface until the highest reading is obtained. If the dial indicator reads 0.019 inch, for example, add this reading to the 10.5-inch measurement for a total diameter of 10.519 inches. Always take six to eight measurements around the drum circumference to check for drum distortion.
Metric drum micrometer work the same way except that the shaft is graduated in 1-cm major increments, and the lock screws fit in notches every 2 mm.