A chuck is a specialized type of clamp used to hold an object, usually an object with radial symmetry, especially a cylindrical object. It is most commonly used to hold a rotating tool (such as the drill bit in a power tool) or a rotating workpiece (such as the bar or blank in the headstock spindle of a lathe). Some chucks can also hold irregularly shaped objects (ones that lack radial symmetry). In some applications, the tool or workpiece being held by the chuck remains stationary while another tool or workpiece rotates (for example, a drill bit in the tailstock spindle of a lathe, or around workpiece being milled by a milling cutter). Many chucks have jaws, which are dogs that are arranged in a radially symmetrical pattern (like the points of a star) to hold the tool or workpiece. Often the jaws will be tightened or loosened with the help of a chuck key, which is a wrench-like tool made for the purpose.
Many jawed chucks, however, are of the keyless variety, and their tightening and loosening are by hand force alone. Keyless designs offer the convenience of quicker and easier chucking and unchecking, but have the lower gripping force to hold the tool or workpiece, which is potentially more of a problem with cylindrical than, say, hexagonal shanks. Collet chucks, rather than having jaws, have collets, which are flexible collars or sleeves that fit closely around the tool or workpiece and grip it when squeezed. A few chuck designs are more complex yet, and they involve specially shaped jaws, higher numbers of jaws, quick-release mechanisms, or other special features.
Some chucks, such as magnetic chucks and vacuum chucks, are of a different sort from the radially symmetrical mechanical clamps mentioned above. Instead, they may be surfaces (typically flat) against which workpieces or tools are firmly held by magnetic or vacuum force. To chuck, a tool or workpiece is to hold it with a chuck, in which case it has been chucked. Lathe work whose workholding involves chucking individual slugs or blanks is often called chucking work, in contrast to bar work (bar feed work), which is parted off from bar stock. Automatic lathes that specialize in chucking work are often called chuckers.