Abstract: A chuck for a machine tool is disclosed which comprises a cylindrical scroll board with a scroll groove being formed on one end surface thereof and having three radial slots formed therethrough at equi-angular intervals, three slider blocks radially movably received within the radial slots and each provided on the under surface with a number of arcuate grooves so as to be in communication with the scroll groove and on the radially inner end with an engaging element, a cup-shaped cover board adapted to rotatively receive therein the scroll board and provided on the annular cover plate constituting the bottom thereof with three radial slots at equi-angular intervals, three claws radially movably received within the radial slots of the cover plate and each provided on the under surface with a number of arcuate projections so as to be in mesh with the scroll groove, and a manipulating member axially movably received within a bore formed in the scroll board centrally thereof and adapted to be moved by an actuating