A ball-peen (also spelled ball-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, diagonal-peen hammer, point-peen hammer, or chisel-peen hammer by having a hemispherical head.
Though the process of peening (surface hardening by impact) has become rarer in metal fabrication, the ball-peen hammer remains useful for many tasks, such as striking punches and chisels (usually performed with the flat face of the hammer). The peening face is useful for rounding off edges of metal pins and fasteners, such as rivets.
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.
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 have heads faced with 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 struck surface, and are graded by the weight of the head and by hardness of the striking face.
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