Method of burring

A method of burring comprising:

• a first step of forming, in a metal plate, a bottomed sleeve portion expanded into a die hole of a die by means of a punch which is inserted into the die hole, the die hole being formed such that a predetermined gap is formed between an inner peripheral face of the die hole and an outer peripheral face of the punch so that the gap has a width larger than the thickness of the metal plate, the die hole having a peripheral circular projected edge having a projected edge face whose width is smaller than the thickness of the metal plate, and
• a second step
• i) swaging the bottom of the sleeve portion of the metal plate between the punch and the bottom face of the die hole to render the bottom of the sleeve portion thinner such that the material of the sleeve portion bottom is caused to flow to a peripheral wall portion of the sleeve portion positioned in the gap between the inner peripheral face of the die hole and the outer peripheral face of the punch, thereby thickening the peripheral wall portion of the sleeve portion, and
• ii) swaging the metal plate at the top peripheral portion of the sleeve portion adjacent to the rising peripheral wall of the sleeve portion, between the circular projected edge of the die hole and a stepped face of the punch such that the material of the metal plate swaged therebetween is allowed to flow to the peripheral wall portion of the sleeve portion positioned in the gap between the inner peripheral face of the die hole and the outer peripheral wall portion of the punch.

A method of burring comprising:

• a first step of forming, in a metal plate, a bottomed sleeve portion expanded into a die hole of a die by means of a punch held for vertical movement with a vertically movable upper blank holder in cooperation with the die, the die hole having a peripheral circular projected edge having a projected edge face whose width is smaller than the thickness of the metal plate, the die hole being formed such that a predetermined gap is formed between an inner peripheral face of the die hole and an outer peripheral face of the punch so that the gap has a width larger than the thickness of the metal plate, the die being held for vertical movement with a vertically movable lower blank holder so that the metal plate is held between the die and the lowered upper blank holder, and
• a second step of
• i) swaging the bottom of the sleeve portion of the metal plate between the punch and the bottom face of the die hole to render the bottom of the sleeve portion thinner such that the material of the sleeve portion bottom is caused to flow to a peripheral wall portion of the sleeve portion positioned in the gap between the inner peripheral face of the die hole and the outer peripheral face of the punch, thereby thickening the peripheral wall portion of the sleeve portion, and
• ii) swaging the metal plate at the top peripheral portion of the sleeve portion adjacent to the rising peripheral wall of the sleeve portion, between the circular projected edge of the die hole and said upper blank holder moved together with the punch, such that the material of the metal plate swaged therebetween is allowed to flow to the peripheral wall portion of the sleeve portion positioned in the gap between the inner peripheral face of the die hole and the outer peripheral wall portion of the punch.

A method of burring comprising:

• a first step of forming, in a metal plate, a bottomed sleeve portion expanded into a die hole of a die by means of a punch held for vertical movement with a vertically movable upper blank holder in cooperation with the die, the die hole having a peripheral circular projected edge having a projected edge face whose width is smaller than the thickness of the metal plate, the die hole being formed such that a predetermined gap is formed between an inner peripheral face of the die hole and an outer peripheral face of the punch so that the gap has a width larger than the thickness of the metal plate, the die being held for vertical movement with a vertically movable lower blank holder so that the metal plate is held between the die and the lowered upper blank holder, and
• a second step of
• i) swaging the bottom of the sleeve portion of the metal plate between the punch and the bottom face of the die hole to render the bottom of the sleeve portion thinner such that the material of the sleeve portion bottom is caused to flow to a peripheral wall portion of the sleeve portion positioned in the gap between the inner peripheral face of the die hole and the outer peripheral face of the punch, thereby thickening the peripheral wall portion of the sleeve portion, and
• ii) swaging the metal plate at the top peripheral portion of the sleeve portion adjacent to the rising peripheral wall of the sleeve portion, between the circular projected edge of the die hole and a stepped face of the punch such that the material of the metal plate swaged therebetween is allowed to flow to the peripheral wall portion of the sleeve portion positioned in the gap between the inner peripheral face of the die hole and the outer peripheral wall of the punch.