Hammers are identified by the material and weight of the head. There are two groups of hammer heads: steel and soft face. The heads of steel-face hammers are made of high-grade alloy steel. The steel is deep forged and heat treated to a suitable degree of hardness. Soft-face hammers have a surface that yields when it strikes an object. Soft-face hammers are preferred when machined surfaces and precision are involved or when marring a finish is undesirable. For example, a brass hammer can be used to drive in gears or shafts. Technicians should also consider purchasing ball peen hammers in which the head and haft (handle) are forged; the insulation on the handle of these hammers is better at absorbing shock and they last longer in a truck shop environment.