A numerical value that designates the degree of difficulty or ease with which a particular material can be machined; originally based on turning B1112 steel at 180 feet per minute (0.9144 meter per second) with a high-speed tool for an index of 100; with replacement of high- speed steels with carbides in turning operations, it has been found that the machinability index of a given material changes with the type of operation and the tool material.