windows 119 equidistantly spaced about its periphery at the same distance from the end these windows coinciding with an inner housing limited by a conical face 129 of the stopper 116 and the chamfered end of a ring 121 fixed in the body 101 a steel ball 122 is placed in front of each of these three windows 119 in the normal closed or tool gripping position of the chuck the balls 122 are held spaced apart from the shaft 106 and from the stopper 116 by a biasing spring 123 this spring 123 is formed by three axially extending equidistant blades having outwardly bent ends which lead into the openings 119 and contact the balls 122 the spring blades which are lodged in grooves 124 of a ring 105 are united by a collar fixed in the ring 105 the balls 122 are pushed by the spring 123 against the inner face 125 of a sleeve 126 rotatably mounted on the handpiece body 101 between two axial stops namely a ring 127 and a shoulder 101 39 of body 101 the inner face 125 of the rotary sleeve 126 has three cam surfaces each corresponding to a ball 122 each of these surfaces comprising a zone a removed from the centre and a zone b close to the centre when as shown in fig3 the balls 122 are located centrally in the zone a under the action of the biasing spring 123 they are spaced apart from the central shaft 106 this corresponds to the closed position of the clamping chuck starting from the closed position by turning the sleeve 126 by 1 6th of a turn in either direction the zones b of the inner wall 125 are.