Fixture keys are used to accurately locate fixtures or machine components. Typically, they are assembled into a hole or a machined slot. There are many different types of products. Standard fixture keys are slotted to mount on a specific machine table. Step fixture keys adapt a key-slotted fixture to machine tables with different slot sizes. Interchangeable fixture keys are used to mount any fixture on any machine. They eliminate the need to slot fixture and are suitable for most applications. Reamed-hole fixture keys are designed to fit two different table slots. They can be fastened from above or below with a socket-head cap screw. Locating keys are inserted from above after a fixture is placed on a machine, and then removed after the fixture is fastened. Pallet fixture keys are used to mount tooling plates and blocks on standard DIN pallets. Mill fixture keys are also commonly available. Fixture keys are specified in English measurements such as inches (in.) and fractions of an inch, and in metric measurements such as millimeters (mm). For inch-based fixture keys, table shot sizes include 1/2", 9/16", 5/8", 11/16", 3/4", 13/16" and 1-1/16". For metric fixture keys, table slot sizes include 12 mm, 14 mm, 16 mm, 18 mm, and 20 mm. For both types of products, measurements have lettered designations such as A, B, C and D. With subplate locating keys, two standard diameters match standard fixture-key holes so that one fixture plate can be mounted either on a subplate or directly on a machine table. The subplate hole-pattern may allow mounting for multiple sizes and combination of rectangular tooling plates, either lengthwise or widthwise. Fixture keys are made from grades of steel such as 1018, 1144, 1045, or 12L14. They often heat treated or carburized-hardened, and treated with a black oxide finish. Heat treated fixture keys are subjected to processes such as annealing, austempering, case hardening, conventional hardening, homogenizing, hot isostatic pressing (HIP), martempering, precipitation hardening, shot peening, and solution treating. When low- carbon carburized grades of steel are used, carbon is often introduced into the surfaces of the fixture keys after machining; to a depth sufficient to produce a hardened case that will sustain bearing loads.