A micrometer is a tool used for the precise measurement of very small objects. A micrometer can measure the depth, length and thickness of whatever object fits between its anvil and spindle. It is commonly used in mechanical engineering and machining applications.
Depending on the type of micrometer available, a variety of distances can be measured. For example, a micrometer can measure the thickness of a wire or a sheet of paper. It could measure the diameter of a hole, the length of the surface of a nail or the depth of a slot. Standard micrometers will measure objects less than one inch in length/depth/thickness.
The correct type of micrometer is required to measure a specific type of distance. To measure the thickness of an object, an outside micrometer is used. These common tools are also known as micrometer calipers. Outside micrometers can measure wires, spheres and blocks. Inside micrometers do the opposite, measuring the distance within something, like the diameter of a hole. If the hole does not have a perfect circular cross section, a bore micrometer can be used. Tube micrometers measure tube thickness, and depth micrometers measure slot or step depth.
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Each micrometer can be fitted with specialized equipment for specific tasks. Because they grasp the object being measured, anvils and spindle tips are the pieces most commonly customized for unique situations. Some micrometers will have multiple anvils to allow for more precise measurement. The anvil can be formed into a disc or v-shape, or form part of a screw thread. Some micrometers come with interchangeable anvils, allowing for various types of measurement.
For measurement using an outside micrometer, an object is placed flush against a micrometer caliper's anvil. The thimble is turned with the other hand, moving the spindle closer to the anvil until it can move no further. When the object is secure between the anvil and spindle, the micrometer is locked so that a measurement can be read and recorded accurately. A standard, one-inch micrometer has readout markings of .001 inch with an accuracy of +/- .0001 inch.
The micrometer is also known as a micrometer screw gauge. The screw is the heart of a micrometer, but is not seen because of its location within the barrel of the instrument. The accuracy of the threadform of the screw determines the accuracy of the micrometer. The screw's thread is simply the ridges felt when touching a screw. The thread is the helical structure moving up the screw, converting torque to linear force.