Gauges are usually regarded as either fixed-type or deviation-type instruments. Fixed-type gauges are used to indicate whether a given dimension is larger or smaller than the standard. They may be of hard steel, soft steel, or glass. Sometimes chrome plating or tungsten-carbide coatings are used to prevent wear.
Plug, ring, snap, and limit gauges are fixed gauges usually made to satisfy special requirements. To check the accuracy of a hole, a cylindrical bar (plug gauge) with highly finished ends of different diameters is used. If the hole size is correct within tolerable limits, the small end (marked “go”) will enter the hole, while the large end (“not go”) will not. Ring gauges for checking the dimensions of cylindrical parts also utilize the tolerance principle, with “go” and “not go” sections. A snap gauge is formed like the letter C, with outer “go” and inner “not go” jaws, and is used to check diameters, lengths, and thicknesses.
Flush-pin gauges have one moving part and are used to gauge the depth of shoulders or holes. Form gauges are used to check the profile of objects; two of the most common types are radius gauges, which are packs of blades with both concave and convex circular profiles that are used to check the radii of grooves and corners, and screw-thread pitch gauges, which are blades with triangular serrations spaced to correspond with various pitches, or numbers of threads per inch or per centimeter.
Gauge blocks, also known as Johannsson blocks, after their inventor, came into significant industrial use during World War I. They are small steel blocks, usually rectangular, with two exceptionally flat surfaces parallel to each other and a specified distance apart. They are sold as sets of blocks that can be wrung together in increments of ten-thousandths of an inch to gauge almost any linear dimension. Angle-gauge blocks can be put together to measure angles.
Deviation-type gauges indicate the amount by which the object being gauged deviates from the standard. This deviation is usually shown in units of measurement, but some gauges show only whether the deviation is within a certain range. They include dial indicators, in which movement of a gauging spindle deflects a pointer on a graduated dial; wiggler indicators, which are used by machinists to center or align work in machine tools; compactors, or visual gauges; and air gauges, which are used to gauge holes of various types. Very precise measurements may also be obtained by the use of light-wave interference, but the instruments that do so are referred to as interferometers.