Welding machine and procedure

Claim 1

IN A PROJECTION WELDING SET-UP FOR A BASE MEMBER IN THE FORM OF A SHEET OF METAL HAVING AN INSULATING COATING AND AN ATTACHMENT FORMED OF A SUBSTANTIALLY FLAT PIECE OF SHEET METAL INTENDED TO BE SECURED TO THE BASE MEMBER FACE TO FACE HAVING A PROJECTION ON ITS FACE WITH A SHARP PRESENTED EDGE CAPABLE OF PENETRATING THE INSULATING COATING AND PROGRESSIVELY COLLAPSING AS FORCE IS APPLIED AT RIGHT ANGLES THERETO, THE COMBINATION COMPRISING FIRST AND SECOND WELDING ELECTRODES ARRANGED OPPOSITE ONE ANOTHER AND HAVING SUBSTANTIALLY FLAT FACES WITH AN AREA SUBSTANTIALLY LARGER THAN THE AREA OF SAID PROJECTION, AT LEAST ONE OF SAID ELECTRODES BEING MOVABLE TOWARD THE OTHER, FORCE TRANSMITTING MEANS FOR POSITIVELY ADVANCING THE MOVABLE ELECTRODE INTO CONTACT WITH THE WORK SO THAT THE BASE MEMBER AND ATTACHMENT ARE BOTTOMED TOGETHER ON THE PROJECTION, AND A STIFF PRELOADED SPRING INTERPOSED IN THE FORCE TRANSMITTING MEANS SO THAT THE FORCE EXERTED BY THE ELECTRODES IN INCREASED FROM AN INITIAL VALUE TO A SUSTAINED HIGHER VALUE SO THAT SAID PROJECTION IS AXIALLY CRUSHED THEREBY CAUSING PENETRATION OF THE COATING AND THEREAFTER SUBSTANTIALLY COMPLETE AXIAL COLLAPSE OF THE PROJECTION, A SOURCE OF WELDING CURRENT CONNECTED TO THE ELECTRODES, MEANS FOR DELAYING THE TURNING ON OF THE WELDING CURRENT UNTIL THE FORCE REACHES A VALUE WHICH CORRESPONDS TO THE SUBSTANTIALLY COMPLETE COLLAPSING OF THE PROJECTION, MEANS FOR VARYING THE CURRENT GRADUALLY FROM A LOW VALUE TO A SUSTAINED HIGH VALUE SO THAT THE HEATING OF THE PROJECTION RESULTING FROM THE CURRENT FLOW TAKES PLACE GRADUALLY WITH THE FORMATION OF A WELDING NUGGET HAVING AN AREA SUBSTANTIALLY GREATER THAN THE AREA OF THE PROJECTION, AND MEANS FOR CAUSING THE CURRENT TO BE TURNED OFF AND THE MOVABLE ELECTRODE SUBSEQUENTLY RETRACTED FOR REMOVAL OF THE BASE MEMBER AND ATTACHMENT FROM BETWEEN THE ELECTRODES.