Abstract: An indexing mechanism for the rotatable element of a machine tool wherein a shaft is coaxial with the element and has formed thereon a helicoidal groove. A control member is engaged in said groove and moves alternately therein to transmit an oscillating rotative movement to the shaft to effect go-and-back movements along the groove in a direction parallel to the axis of rotation of the rotatable element. A control cam effects go-and-back movements which are synchronized with the movements of the control member. A driving member is mounted on the shaft for rotation therewith and is capable also of axial movement between one position in which it is rigid with the element and a second non-rigid position. The axial movement of the driving member is controlled by the cam. Accordingly, during the go movements of the control member and of the cam, the driving member is rigid with the element.