PURPOSE: To regulate the axial direction of a heating range by providing an annular magnetic material which is interposed while covering a part of the outer peripheral surface, bottom surface and inner peripheral surface of a high frequency heating coil.
CONSTITUTION: A high frequency heating coil 1 consists of an annular coil part 2 and an annular magnetic body 3. The magnetic body 3 consists of an outer peripheral wall 30 covering the outer peripheral surface of the coil part 2, a bottom wall part 31 covering the bottom surface 21 and an inner peripheral wall 32 covering a part of the inner peripheral surface 22 along the axial direction p. Both ends of a blank tube 9 are supported and a bulged part forming objective range L is heated with the high frequency heating coil 1. While the blank tube 9 is heated, a load is applied to the axial direction with a pressing means, and the bulged part 93 is formed on the blank tube 9. The high frequency heating coil 1 has the ring like magnetic material 3 interposed with covering a part of the outer peripheral surface 20, the bottom surface and the inner peripheral surface 22. The blank tube 9 is heated and pressed while turning. Therefore, shape crushing on the formed bulged part caused on excessive heating can be solved.
HOSHI KIYOSHI
ONO MASAKI
