Method for manufacturing helically welded tubing

Claim 1

IN A METHOD FOR FORMING METAL TUBING BY HELICALLY WINDING A METAL RIBBON TO FORM SUCCESSIVE CONVOLUTIONS JOINED TOGETHER AT THEIR EDGES BY A HELICALLY EXTENDING SEAM IN THE FORM OF A CAST WELD, THE COMBINATION OF STEPS WHICH COMPRISES: ADVANCING THE RIBBON GENERALLY FLATWISE AND TANGENTIALLY OF A CONVOLUTION OF THE DESIRED TUBING WITH THE EDGES OF THE RIBBON POSITIONED AT AN ANGLE TO THE AXIS OF THE TUBING BEING FORMED, SUCH ANGLE CORRESPONDING TO THE PITCH OF THE DESIRED HELICALLY WELDED SEAM; FORMING THE RIBBON INTO A SUCCESSION OF CONVOLUTIONS WITH ABUTTING EDGES, HEATING SAID EDGES ALONG A LENGTH THEREOF SUBSEQUENT TO THE POINT WHERE SAME ARE BROUGHT TOGETHER, BY APPLYING CONTACTS THERETO AT SPACED APART SUCCESSIVE POINTS THEREON, SAID CONTACTS RESPECTIVELY BEING CONNECTED TO THE TERMINALS OF A SOURCE OF CURRENT OF A FREQUENCY OF ABOUT 50,000 CYCLES PER SECOND OR HIGHER, WHEREBY SAID CURRENT FLOWS FROM ONE OF SAID TERMINALS TO ONE OF SAID CONTACTS, THENCE ALONG ON SAID EDGES TO THE OTHER OF SAID CONTACTS AND THENCE TO THE OTHER TERMINAL OF SAID SOURCE, AND PROVIDING MEANS EXTENDING ALONG CLOSE TO THE MEETING LINE OF SAID EDGES FOR INDUCTIVELY CAUSING SUCH CURRENT TO BE CONCENTRATED ALONG CLOSE TO SAID LINE AND MORE TOWARD THE OUTER SURFACE OF THE TUBING THAN AT THE INNER SURFACE, THE CURRENT BEING SUFFICIENT TO ACT SUBSTANTIALLY TO FUSE A MASS OF THE TUBE METAL HAVING A CROSS-SECTIONAL SHAPE AT THE SEAM SUBSTANTIALLY WIDER AT THE EXTERIOR THAN AT THE INTERIOR SURFACE OF THE TUBING, SUCH MASS BEING ALLOWED TO BECOME CAST IN SUCH SHAPE TO FORM SAID CAST WELD WITHOUT SUBSTANTIAL INTERNAL UPSET OF THE METAL.