PURPOSE: To perform smoothly ironing and to prevent surely a crack of flare by forming the taper part and the ring part of a punch end into a continuously formed construction in forming the flare of a fin by an ironing punch.
CONSTITUTION: A semispherical-shape predrawing is provided to the prescribed part of a fin P of hard material by a predrawing punch 14. Successively, a hole is pierced in the semispherical part of fin P by a drawing punch 11 functioning also as a piercing die and a piercing punch 14. Next, the pierced part is ironed between an ironing punch 17 and an ironing die-bush 18, to form a flare Pf. The most front end of punch 17 is rounded along its peripheral face, and is continuously connected to a taper part 30. The base part of taper part 30 is formed into a rounded ring part 31, which is continuously connected to a small-diameter stepped part 32. In this way, the generation of a crack of flare is prevented, because a contacting part between the flare Pf and the punch 17 is restricted only to the vertex of ring part 31.
IIZUKA KENJI
MATSUI TAKAYUKI
OOISHI YOSHITAKA
