Micrometer
The micrometer, alongside the caliper, is one of the most common hand measuring tools in machine shops. A micrometer can measure with an accuracy of 0.01, making it suitable for measuring ISO tolerances commonly used in machining. There are several different models of micrometers, with the most common being the arch micrometer and the three-point micrometer. The arch micrometer is designed for measuring external distances, such as the outer diameters of shafts. Arch micrometers are available with a variety of measuring tips, such as curved or round measuring heads, disk micrometers with disk-shaped measuring heads, thread micrometers with interchangeable measuring tips, and arch micrometers with narrowed measuring tips for measuring narrow grooves. The most common sizes of arch micrometers are 0-25mm, 25-50mm, 50-75mm, and 75-100mm, but they are also available in larger sizes. Three-point micrometers are used for measuring the internal diameters of holes and can easily reveal a hole's triangular shape. Internal dimensions can also be measured with two-point internal micrometers and stick micrometers, which can measure the ovality of holes. Other typical micrometers include depth micrometers.
All micrometers are based on a screw thread designed so that the screw moves the movable measuring jaw either one millimeter or half a millimeter per revolution. The micrometer screw based on the screw thread was developed in 1772 by the Scotsman James Watt. The micrometer screw rotates by turning the measuring drum, from which the measurement reading is also taken in analog micrometers. At the end of the micrometer, there is a friction screw, or so-called "ratchet," which ensures the same measuring force is applied during the measurement process. The micrometer frame is always equipped with a plastic insulating section to handle the measuring instrument to prevent the temperature of the user's hands from transferring to the instrument and causing thermal expansion. The measuring surfaces of the micrometer are usually made of either carbide or hardened and ground steel.
Today's digital micrometers are easy to read and can include several advanced features such as wireless data transfer or other functions, like Mahr's Micromar built-in tolerance function, which immediately indicates if the measured value is within the given limits.
Parts of a Micrometer
The parts of an analog micrometer with a vernier measuring scale are:
- Frame
- Anvil
- Measuring surfaces
- Spindle
- Sleeve
- Thermal insulating plate
- Base line
- Sleeve scale
- Drum scale
- Drum
- Adjustment screw (ratchet)
