Thread-rolling head

A thread-rolling head for processing a workpiece and comprising a support member, a bearing member held in front of said support member and having an axis in common therewith, three crank-like support pieces identical to one another and each having a pivot pin, a supporting journal, and a cheek, three bearing bores made in said bearing member and distributed about said common axis, each said pivot pin being situated in a respective said bearing bore, three thread rollers, each said thread roller being borne by a respective said supporting journal, a control member coaxially surrounding said bearing member and having an inner flange, the inner side of said flange forming a control surface, each said cheek resting against a respective section of said control surface, stop means disposed on said support member and on said bearing member, and a spring disposed within said support member, said control surface being so designed that a relative rotation between said bearing member and said control member causes a uniform pivoting of said support pieces away from one another and, said stop means being adapted either to engage with one another and thereby to prevent said relative rotation or to disengage from one another and thereby to enable said spring to bring about said relative rotation, wherein said control member is held unrotatingly on said support member whereas said bearing member is held rotatingly, said stop means being operative between said bearing member and said support member in such a way that during operation, said stop means transmit to said support member a torque transmitted from said workpiece to said bearing member via said thread rollers, a forward displacement of said bearing member relative to said support member caused by said workpiece rendering said stop means inoperative, and said torque, temporarily still acting upon said bearing member, aiding said spring in rotating said bearing member for enabling said pivoting of said support pieces away from one another.