Apparatus and method for forming helically welded tubing

Claim 1

METHOD FOR FORMING TUBING WITH A HELICALLY-EXTENDING WELD LINE, WHICH COMPRISES: ADVANCING A METAL STRIP LONGITUDINALLY IN A DIRECTION AT AN ANGLE TO THE AXIS OF THE TUBING BEING FORMED, SUCH ANGLE CORRESPONDING TO THE PITCH ANGLE OF THE HELICAL WELD LINE, WHILE CAUSING PORTIONS ALONG ONE EDGE OF THE STRIP TO HAVE IMPARTED THERETO A TRANSVERSE CURVATURE SUBSTANTIALLY CORRESPONDING TO THE CROSS-SECTIONAL CURVATURE OF PORTIONS OF THE WALLS OF THE TUBING BEING FORMED; THEN FURTHER ADVANCING THE STRIP IN THE DIRECTION OF SAID AXIS, WHILE GRADUALLY IMPARTING A TRANSVERSE CURVATURE TO THE OTHER ORTIONS OF THE STRIP FOR BRINGING SAME GRADUALLY AROUND INTO A POSITION WHEREBY THE OPPOSITE EDGE OF THE STRIP COMES INTO A POSITION OPPOSING SAID ONE EDGE AND WHEREBY TUBING OF CSHAPED CROSS-SECTION IS FORMED; FURTHER ADVANCING THE LATTER ALONG THE DIRECTION OF SAID AXIS WHILE MAINTAINING A HELICALLY-EXTENDING V-SHAPED GAP BETWEEN THE OPPOSED EDGES, WITH THE APEX OF SUCH GAP AT A WELD POINT; AND APPLYING HIGH FREQUENCY CURRENT TO FLOW ALONG THE EDGES OF SUCH GAP IN ADVANCE OF SAID POINT FOR HEATING SAME TO WELDING TEMPERATURE AT SAID POINT AND WHILE APPLYING PRESSURE TO OPPOSITE SIDES OF THE TUBING AT THE REGION OF SAID POINT FOR BRINGING SAID EDGES INTO WELDED RELATION.