Patent GB-954752-A (GB)
Title: Improved forming method and apparatus therefor
IPC: B21D26/14
Filed: 1963-02-05
Abstract: 954,752. Stamping; tube expanders; welding by pressure. GENERAL DYNAMICS CORPORATION. Feb. 5, 1963 [Feb. 15, 1962], No. 4714/63. Headings B3J, B3Q and B3R. A method of forming e.g. shaping, embossing or engraving a workpiece comprises passing a current through the workpiece to preheat it and establishing a varying magnetic field of predetermined intensity and shape at the surface of the workpiece to form the workpiece in a desired manner. The means for setting up an alternating magnetic field in a work space 12 to preheat a workpiece 14 positioned therein comprises a primary winding 16 e.g. a water cooled tubular conductor 20, and a secondary winding 18 e.g. a split hollow member of brass or beryllium copper which includes a hollow core portion 38 and a split outer concentric portion 40 comprising sections 50, 52 soldered together and having a spiral aperture 46 into which is screwed the coil 16 having an external tubular insulating layer 48. A radial gap 42 is provided between the work space and the outer periphery of the member 18 and a conical recess 44 is provided at each end of the core 38 such that the axial length of the work space 12 is shorter than the length of the member 18. A H.F. current source is coupled to the winding 16 via T- shaped connectors 36. The core 38 may be insulated from the outer portion 40 in which case the current will be induced in the core portion, and held in position by brass or beryllium copper end plates 54. In an embodiment a charging circuit comprises a capacitor bank 64, a current limiting resistor 74 and switch 72 connected in series with a voltage source 72. A tubular workpiece 14 e.g. 1 inch diameter molybdenum tube having a wall thickness of 0.010 inch is positioned within an asbestos sleeve 62 in the work space, the capacitor bank 64 charged, the switch 76 opened and a switch 34 closed to connect the H.F. current e.g. at 100,000 c.p.s. at 5 kV. for 10 seconds to the coil 16 which induces a current in the winding 18 and hence eddy currents in the workpiece to effect heating thereof. Subsequently a switch 70 e.g. an ignitron or thyratron is closed and a high amperage current, e.g. 50,000 amps. discharged from a 700 ÁF capacitor charged to 8 kV., flows through the coil 16, and is prevented from flowing to the A.C. source by an isolating capacitor 35, to produce a high intensity field around the coil 16 which induces a current within the winding 18 which in turn sets up a high intensity field e.g. 150,000 gauss in the work space at the surface of the workpiece to effect a reduction in the diameter thereof to 0.7 inch. The coil 16 may comprise 8 turns of 0.25 inch diameter copper tube encased in a 0.020 inch thick vinyl tube and coolant passed there through at 0.25 gallon per minute. The operation may be carried out in a vacuum or inert gas. In an embodiment a pair of telescoped tubular members 78, 80 are positioned with their ends in an asbestos shroud 88 through which an inert gas is directed, surrounded by a water cooled coil 82 supported in a fibre glass structure 84 and welded together by a H.F. source and a high amperage pulse source as before. Tubular workpieces may be expanded in a similar manner by disposing them about the primary coil. The coils may be flat, hair-pinned or spirally shaped. Specification 896,923 is referred to.