Micrometers measure micro distances. Micro distances are small enough to make a typical ruler or tape measure inadequate. Micrometers are used in machining and mechanical engineering. These fields require precise measuring due to the nature of what they are creating—devices that will not work properly if not made exactly to specifications. Micrometers are also called micrometer screw gauges and are sometimes found in the form of a caliper. A caliper measures distances of opposite but symmetrical sides of a shape and they can look a bit like large tweezers. Micrometers can more accurately measure objects that have shape, like ball bearings, and objects that are very small in diameter, like wire. Accuracy range is typically 1/100 of a millimeter.
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Parts of a Micrometer
Most micrometers look like a screwdriver with a C-shaped frame that drops down and away from the handle (which holds the spindle) and connects back up with the tip (anvil). The parts are called the frame, the anvil, the sleeve, the thimble, the spindle, the thimble lock and the screw. Micrometer frames are heavy, thick metal in order to hold the anvil and the sleeve (barrel) in alignment to one another with minimal changes in flexion. The sleeve is the linear scale or measuring device. The thimble is used to turn the spindle toward the object. The screw sits inside of the sleeve because it is what is threading the spindle towards the object. Once the spindle is tightly held against the object, the thimble lock is used to lock the measurement in place.
Reading a Micrometer
The spindle and anvil are not connected, meaning there is space between them to hold the object. As the thimble is turned, it moves the spindle closer to the object. Once the spindle butts against the object, pushing it against the anvil, the sleeve is checked for the final measurement. The measurement is read via the scale which is clearly marked or imprinted on the surface of the metal. A micrometer may have its measurements marked in an inch scale, a metric scale or a vernier scale. Inch scales have 40 threads for each inch. Each turn moves axially 0.025 of an inch. A metric scale contains 2 threads for every millimeter so that one turn is equal to 0.5 of a millimeter. Vernier scales are usually added as an additional scale to either a metric or inch scale. The vernier scale allows for an even more precise reading because it measures the number of hundredths of a millimeter the object might be.
Accuracy of Micrometers
Micrometers are checked for accuracy and calibrated by measuring known objects, meaning objects whose exact measurements are universally known. These objects are called gauge rods or blocks and they meet certain established standards. If the micrometer does not return the exact known measurement of the gauge block, then the micrometer is not accurate and must be calibrated before it is used again.