at 10 is closed from its normally open state thereby energizing a relay coil cr36 shown 11 of fig1 by the energization of this relay coil cr36 relay contact cr36 1 and cr36 2 are closed at 63 and 61 respectively while further relay contact cr36 3 at 62 is opened so as to reverse the rotation of the motor m2 in this way the head contact pressure can be maintained always within reasonable limits limit switches ls12 and ls13 are provided at 13 and 14 respectively when either of these limit switches is actuated relay coil cr21 or 22 is energized for rotation of the motor m3 which corresponds to that shown at 45 in fig9 in one or other direction by 34 on 34 or 34 off 34 control of related contact group cr21 1 cr21 2 and cr21 3 or cr22 1 cr22 2 and cr22 2 shown at 64 66 respectively as in the similar manner described hereinbefore in connection with the motor m2 by this control operation for proper movement of lateral slide 70 proper angular positioning of the welding head wh is established limit switches 17 and 18 appearing at 13 and 14 respectively are mounted at a proper mutual distance on the lateral slide 68 and normally held on actuating.