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A micrometer (sometimes called micrometer screw gauge) is a precision measuring instrument, used to obtain very fine measurements and available in metric and imperial versions. They use a calibrated screw or thread (found internally on the spindle) for precise measurement of components in mechanical engineering and machining as well as most mechanical trades. They usually come in the form of calipers e.g. opposing ends joined by a frame. Learn how to calibrate and use micrometers correctly in our comprehensive micrometers guide.Micrometers are composed of:Frame – The C-shaped body that holds the anvil and barrel in constant relation to each otherAnvil – The shiny part that the spindle moves toward, and that the sample rests againstSleeve / barrel / stock – The stationary round component with the linear scale on it, sometimes with vernier markingsLock nut / lock-ring / thimble lock – The knurled component (or lever) that tightens to hold the spindle stationarySpindle – The shiny cylindrical component that the thimble causes to move toward the anvilThimble – The component that turns by using a person’s thumb. Graduated markingsRatchet stop – Device on end of handle that limits applied pressure by slipping at a calibrated torqueScrewHow do they work?The precision is determined by the thread pitch of the spindle. The spindle is a very accurately machined screw and the object to be measured is placed between the spindle and the anvil. The spindle is moved by turning the ratchet knob or thimble until the object to be measured is lightly touched by both the spindle and the anvil.Types of Micrometers Outside micrometer: most common type used to measure external dimensions such as the outside diameter of an objectInside micrometer; used to measure inside dimensions like the inside diameter of a hole or tube. There are two types: caliper-type and tubular and rodDepth micrometer: these are used to measure the depths of holes, slots and steps. They come with a variety of interchangeable rods of different lengths so that they can be used to measure a range of depthsAdvantagesProvide very accurate measurements and are considered one of the most accurate types of measuring deviceRatchet speeder helps to provide reliable measurementsIntegrated scales, the sleeve scale and thimble scale are built into the tool, another measuring instrument such as a ruler or tape measure is not requiredDurability and long lasting and unlikely to need replacing or repairing