According to the invention, the pin (6) is rotated by the motor (5) via the belt (10) and the pulley (11) that is precision-held on the fixed bushing (13). It comprises a pin arm (16) whose solid rear portion (20) rotates in the bushing (13) and is coupled by screws (18), and the centring part (19) at the inner area of the pseudo universal-joint disk (17) whose outer area is coupled to the pulley (11). A second pseudo universal-joint connection is provided between the rear portion (20) of the pin arm (16) and the front portion thereof (21), which is connected to the clamp-holding bushing (7) in which the clamping collet (8) is mounted.