A chuck is a specialized type of clamp used to hold an object with radial symmetry, especially a cylinder. In a drill, a mill and a transmission, a chuck holds the rotating tool; in a lathe, it holds the rotating workpiece. Chucks commonly use jaws to hold the tool or workpiece. The jaws (sometimes called dogs) are typically arranged in a radially symmetrical pattern like the points of a star. Jawed chucks may require a wrench-like device called a chuck key to be tightened or loosened, but other jawed chucks may be tightened or loosened by hand force alone, offering convenience at the expense of gripping force. Chucks on some lathes have jaws that move independently, allowing them to hold irregularly shaped objects. More complex designs might include specially shaped jaws, greater numbers of jaws, or quick-release mechanisms. Instead of jaws, a chuck may use magnetism, vacuum, or collets, which are flexible collars or sleeves that fit closely around the tool or workpiece and grip it when squeezed. Types Jawed chucks Self-centering A self-centering chuck, also known as a , uses dogs (usually called jaws), interconnected via a scroll gear (scroll plate), to hold onto a tool or workpiece. Because they most often have three jaws, the term three-jaw chuck without other qualification is understood by machinists to mean a self-centering three-jaw chuck. The term universal chuck also refers to this type. These chucks are best suited to grip circular or hexagonal cross-sections when very fast, reasonably accurate (±0.005 inch [0.125 mm] TIR) centering is desired. Sometimes this type of chuck has four or six jaws instead of three. Four-jawed chucks are primarily useful for gripping square or octagon material, while six-jawed chucks hold thin-walled tubing and plastic materials with minimum distortion. There are also independent-jaw (non-self-centering) chucks with three jaws, but they offer few advantages and are very rare. There are hybrid self-centering chucks that have adjustment screws that can be used to further improve the concentricity after the workpiece has been gripped by the scroll jaws. This feature is meant to combine the speed and ease of the scroll plate's self-centering with the run-out eliminating controllability of an independent-jaw chuck. The most commonly used name for this type is a brand name, Set-Tru. To avoid undue genericization of that brand name, suggestions for a generic name have included "exact-adjust". Three-jaw chucks are often used on lathes and indexing