A ball-peen (also spelled pein) hammer, also known as a machinist's hammer, is a type of peening hammer used in metalworking. It is distinguished from a cross-peen hammer, point-peen hammer or chisel-peen hammer 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, such as striking punches and chisels.
The original function of the hammer was to peen riveted or welded material, which makes it as flexible as the surrounding metal. Today, the ball end of the hammer is used to cut gaskets, expand and shape the free end of copper roves, light rivets, and "set" rivets (which completes the joint).
Variants include the straight-peen, diagonal-peen, and cross-peen hammer. These hammers have a wedge-shaped head instead of a ball-shaped head. This wedge shape spreads the metal perpendicular to the edge of the head. The straight-peen hammer has the wedge oriented parallel to the hammer's handle, while the cross-peen hammer's wedge is oriented perpendicular. The diagonal-peen hammer's head, as the name implies, is at a 45° angle from the handle. They are commonly used by blacksmiths during the forging process to deliver blows for forging or strike other forging tools.
Head materials 
Ball-peen hammers have two types of heads: hard-faced and soft-faced. The head of a hard-faced hammer is made of heat treated forged high-carbon steel or alloy steel; it is harder than the face of a claw hammer. The soft-faced hammers are made from brass, lead, tightly wound rawhide, or plastic. These hammers usually have replaceable heads or faces, because they will deform, wear out, or break over time. They are used to prevent damage to a striking surface, and are graded by the weight of the head.
- Audel, Theodore (1962), Audels new mechanical dictionary for technical trades, Theodore Audel, p. 54.
- Cavette, Chris, How hammer is made, retrieved 2008-12-19
- Benford, p. 36.
- Szykitka, p. 435.
- Benford, Tom (2006), Garage and Workshop Gear Guide, MotorBooks/MBI Publishing Company, ISBN 0-7603-2312-7.
- Szymborska, Walter (2004), The Big Book of Self-Reliant Living: Advice and Information on Just About Everything You Need to Know to Live on Planet Earth, Globe Pequot, ISBN 1-59228-043-9.