A chuck has a chuck body rotatable about a main axis and a plate displaceable axially on the body between an outer position and an inner position. Springs urge the plate into the outer position. A locking element on the body can move between a blocking position preventing displacement of the plate from the outer position into the inner position and a freeing position permitting such displacement. A plurality of jaw holders angularly spaced about the axis are fixed axially on the plate and each have radially displaceable outer and inner ends. A piston axially displaceable in the body has cam formations engaging the jaw-holder inner ends for radially displacing the jaw-holder outer ends on axial displacement of the piston.