80 mm 3jaw chuck A chuck is a specialized type of clamp used to hold an object with radial symmetry, especially a cylinder. In drills and mills it holds the rotating tool whereas in lathes it holds the rotating workpiece. On a lathe the chuck is mounted on the spindle which rotates within the headstock. For some purposes (such as drilling) an additional chuck may be mounted on the non-rotating tailstock.

Many chucks have jaws, (sometimes called dogs) that are arranged in a radially symmetrical pattern like the points of a star. The jaws are tightened up 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.[citation needed] Many jawed chucks, however, are of the keyless variety, and their tightening and loosening is by hand force alone. Keyless designs offer the convenience of quicker and easier chucking and unchucking, but have a lower gripping force to hold the tool or workpiece, which is potentially more of a problem with cylindrical than 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.

Chucks on some lathes have jaws that move independently, allowing them to hold irregularly shaped objects. A few chuck designs are even more complex, involving specially shaped jaws, higher numbers of jaws, quick-release mechanisms, or other special features.

Magnetic and vacuum chucks are also made, with typically flat surfaces against which workpieces or tools are firmly held by the pressure of their respective force.

To chuck a tool or workpiece is to hold it with a chuck, in which case it has been chucked. Chucking individual slugs or blanks on a lathe is often called chucking work. In bar work or bar feed work the stock protrudes from the chuck, is worked upon, then parted off (cut off) rather than sawn. Automatic lathes that specialize in chucking work are often called chuckers.