Chuck (engineering)

Self-centering

A self-centering chuck 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 in TIR) centering is desired.
Sometimes this type of chuck has four or six jaws instead of three. More jaws confer more secure grip (if the work is truly cylindrical) and thin-walled work will deform less. Four jaws are also useful for square bar work.
Independent-jaw (non-self-centering) chucks with three jaws also can be obtained.
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 runout-eliminating controllability of an independent-jaw chuck.
Three-jaw chucks can often be found on lathes and indexing heads.