Ball-peen hammer

A ball-peen hammer is a type of peening hammer used in metalworking. It is distinguished from "point-peen" and "chisel-peen" hammers by having a hemispherical head. Though the process of peening has become rarer in metal fabrication, the ball-peen hammer remains useful for many tasks.

The original function of the hammer was to "peen" riveted or welded material so that it will exhibit the same elastic behavior as the surrounding material. Specifically, striking the metal imparts a stress at the point of impact which results in strain-hardening of that area. Strain hardening raises the elastic limit of a material into the plastic range without affecting its ultimate strength. A strain-hardened material will not deform under the same low stresses as a non-hardened material. Ultimately, Most metals can be "worked" by such methods that they lose all of their ductile characteristics and become strong but brittle.

The ball-peen hammer is also used to expand and shape the free end of copper roves, light rivets and similar, "setting" the rivet in place to complete the joint. Peening is also the method by which steel drums are formed and tuned.

A ball-peen hammer is also known as an engineer's hammer or a machinist's hammer and may be graded by the weight of the head.