31 is solidal to the first bush 32 also screw 31 translates and pushes the shaft 30 axially so that the bolt element 38 disengages from the holes 28 of the disk 27 and the end of shaft 30 causes the cam followers detaching lever 39 to rotate around the pin 40 by this rotation the lever 39 comes in contact with a lug 49 of the cam follower 44 and causes cam follower 44 to detach from the cam pile 20 rotating it around the pivot 51 continuing with the rotation of the knob 24 the plane 47 of the groove 42 comes in contact with the pins 43 this coupling causes the rotation of the laid down u shaped element 33 around the axis of the shaft 30 this rotation via the free ends 35 of element 33 the toothed pulley 37 and the hollow shaft 26 is transmitted to the gear 22 which is coupled to the rack 21 of the bridge support 19 the rotation of the gear 22 causes an axial displacement of bridge element 19 and as a consequence of the cam pile 20 until the desired cam is brought into correspondence with cam follower 44 the toothed pulley 37 via a belt 52 displaces in a known way an indicator 53 fig1 which runs along a plate 54 in order to indicate which cam corresponding to the pattern of plate 54 is in correspondence with the cam follower 44 when the indicator 53 is positioned on the desired pattern the operator releases the selector knob 24 by this maneuver the cam follower detaching lever 39 under the action of a recall spring 55 returns to its position illustrated in fig2 this causes an axial displacement of the shaft 30 and as a.