The Starrett 663 series heavy duty micrometer head has a wide-diameter spindle, stem, and thimble for heavy-duty use, and a vernier scale in metric units for taking precise measurements to 25mm. It also has a knurled ring-type lock nut to hold measurements in place. The micrometer has a plain stem for mounting in a fixture with a split-body clamp or setscrew. The vernier scale is graduated to 0.01mm or 0.001mm depending on the model, and has a satin chrome rust- and glare-resistant finish. The spindle is made of hardened steel to help provide durability, with a flat measuring face suitable for measurement or use as a stop, and is micro-lapped to help provide accuracy.Micrometers are precision measuring instruments that use a calibrated screw to measure small distances. These measurements are translated into large rotations of the screw that are then able to be read from a scale or a dial. Micrometers are typically used in manufacturing, machining, and mechanical engineering. There are three types of micrometer: outside, inside, and depth. Outside micrometers may also be called micrometer calipers, and are used to measure the length, width, or outside diameter of an object. Inside micrometers are typically used to measure interior diameter, as in a hole. Depth micrometers measure the height, or depth, of any shape that has a step, groove, or slot. Micrometer heads are for mounting in fixtures, where they are typically used for measurement, or as stops or feeds.The L.S. Starrett Company manufactures precision measuring tools, metrology and testing equipment, and saw blade products. The company was founded in 1880 and is headquartered in Athol, MA.