A micrometer ( /maɪˈkrɒmɨtər/ US dict: mī·krŏm′·ĭ·tər), sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw used widely for precise measurement of small distances in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as dial, vernier, and digital calipers. Micrometers are often, but not always, in the form of calipers.
The topmost image shows the three most common types of micrometer; the names are based on their application:
- Outside micrometer (aka micrometer caliper), typically used to measure wires, spheres, shafts and blocks.
- Inside micrometer, used to measure the diameter of holes.
- Depth micrometer, measures depths of slots and steps.
 Specialized types
Each type of micrometer caliper can be fitted with specialized anvils and spindle tips for particular measuring tasks. For example, the anvil may be shaped in the form of a segment of screw thread, in the form of a v-block, or in the form of a large disc.
- Universal micrometer sets come with interchangeable anvils, such as flat, spherical, spline, disk, blade, point, and knife-edge. The term universal micrometer may also refer to a type of micrometer whose frame has modular components, allowing one micrometer to function as outside mic, depth mic, step mic, etc. (often known by the brand names Mul-T-Anvil and Uni-Mike).
- Blade micrometers have a matching set of narrow tips (blades). They allow, for example, the measuring of a narrow o-ring groove.
- Pitch-diameter micrometers (aka thread mics) have a matching set of thread-shaped tips for measuring the pitch diameter of screw threads.
- Limit mics have two anvils and two spindles, and are used like a snap gauge. The part being checked must pass through the first gap and must stop at the second gap in order to be within specification. The two gaps accurately reflect the top and bottom of the tolerance range.
- Bore micrometer, typically a three-anvil head on a micrometer base used to accurately measure inside diameters.
- Tube micrometers have a cylindrical anvil positioned perpendicularly to a spindle and is used to measure the thickness of tubes.
- Micrometer stops micrometer heads that are mounted on the table of a manual milling machine, bedways of a lathe, or other machine tool, in place of simple stops. They help the operator to position the table or carriage precisely. Stops can also be used to actuate kickout mechanisms or limit switches to halt an automatic feed system.
- Ball micrometers have ball-shaped (spherical) anvils. They may have one flat and one ball anvil, in which case they are used for measuring tube wall thickness, distance of a hole to an edge, and other distances where one anvil must be placed against a rounded surface. They differ in application from tube micrometers in that they may be used to measure against rounded surfaces which are not tubes, but the ball anvil may also not be able to fit into smaller tubes as easily as a tube micrometer. Ball micrometers with a pair of balls can be used when single-tangential-point contact is desired on both sides. The most common example is in measuring the pitch diameter of screw threads (which is also done with conical anvils or the 3-wire method, the latter of which uses similar geometry as the pair-of-balls approach).
- Bench micrometers are tools for inspection use whose accuracy and precision are around half a micrometre (20 millionths of an inch, "a fifth of a tenth" in machinist jargon) and whose repeatability is around a quarter micrometre ("a tenth of a tenth"). An example is the Pratt & Whitney Supermicrometer brand.
- Digit mics are the type with mechanical digits that roll over.
- Digital mics are the type that uses an encoder to detect the distance and displays the result on a digital screen.
- V mics are outside mics with a small V-block for an anvil. They are useful for measuring the diameter of a circle from three points evenly spaced around it (versus the two points of a standard outside micrometer). An example of when this is necessary is measuring the diameter of 3-flute endmills and twist drills.
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