Four-jaw chuck

A chuck is a specialized type of clamp. It is 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 .

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. 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 lower gripping force to hold the tool or workpiece, which is potentially more of a problem with cylindrical than hexagonal shanks. Some lathe chucks have independently moving jaws which can also hold irregularly shaped objects (ones that lack radial symmetry). 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 or bar feed work. In bar work the bar protrudes from the chuck, is worked upon, then cut off by a lathe tool (parted off) rather than being sawn off. Automatic lathes that specialize in chucking work are often called chuckers.

...
Wikipedia