A micrometer gauge, often simply called a micrometer or mic, is a finely calibrated measuring tool. Micrometer gauges are routinely used in engineering and mechanical trades to precisely measure small distances or dimensions. Micrometer gauges are made in several configurations, but they all rely on the same basic mechanism to take measurements. The measuring tip, or spindle, is moved a specific distance by turning a calibrated, fine-thread screw. When the spindle contacts the item being measured, the dimension can be read on a dial or digital readout.
Micrometer gauges vary in design, depending on the task for which they are intended. There are many specialized types of micrometers, but the three most common types are the outside micrometer, the inside micrometer and the depth micrometer. As the names imply, each style of micrometer gauge has its own specific purpose.
The most familiar type of micrometer gauge is the outside micrometer. This instrument is sometimes called a caliper micrometer because it usually has a caliper, or "C-shaped" body, and a spindle that moves up or down when the screw is turned. The outside micrometer gauge is most often used to measure the thickness or length of an object or the diameter of a round or spherical item.
An inside micrometer can also have a caliper design, but it also is available with a straight body. Instead of a spindle, the inside micrometer gauge uses measuring pads, or points. These are turned outward and extend out toward the object being measured as the screw is turned. This design makes the inside micrometer perfect for accurately measuring the inside diameter of a hole or other opening.
The depth micrometer usually has a straight-bodied design. This type of micrometer gauge is often sold as a set, with interchangeable steel rods of varying lengths. Adding a rod of the proper length to the spindle of the depth gauge allows the user to measure the distance to the bottom of a hole or groove. In some cases, a depth micrometer gauge can also be adapted to measure the height of a piece, such as a shim or spacer.
Regardless of the style, micrometer gauges that are graduated in inches can easily show readings as small as 0.001 inches, and metric micrometers typically show increments of 0.01 millimeters. For even greater accuracy, some micrometers have an integrated Vernier scale. The Vernier scale gives users ten additional gradations to further pinpoint their dimensions. A Vernier-equipped micrometer gauge can take measurements as tiny as 0.0001 inches or 0.001 millimeters.