Patent ID: 8893360

Claim:
A method for manufacturing a motor vehicle door hinge from a steel plate blank having predetermined thickness, width, and height using processes including cold heading and punching, the method comprising: a cold heading step of forming, at one end of the blank in the width direction, a cylindrical bulging portion having a bulged cylindrical shape bulging in the thickness direction so as to have a horizontal cross section of a circular shape or an elliptical shape and extending in the height direction by cold heading; a shaft hole forming step of forming a shaft hole that passes through the cylindrical bulging portion along the shaft axis using a first punch and a first die, the shaft hole allowing a hinge pin to pass therethrough; and a shaft hole finishing step of punching the shaft hole formed in the shaft hole forming step from the end at which machining of the first punch ends using a second punch and a second die; wherein a height of the cylindrical bulging portion of the shaft hole formed in the shaft hole forming step is twice or more a diameter of the shaft hole, and wherein the first punch used in the shaft hole forming step has a top end having a shape of a cone with a cone angle in the range from 70° to 120°, and wherein the first die has an inner wall having a gap relative to an outer circumferential surface of the cylindrical bulging portion of the blank, and the size of a gap volume formed by the outer circumferential surface of the cylindrical bulging portion and the inner wall is set so that when the first punch punches the cylindrical bulging portion of the blank, a hole portion formed from the end at which machining of the first punch starts to a predetermined length position does not produce a punched slug and the cylindrical bulging portion bulges outward and a hole portion from the predetermined length position to the end at which the machining of the first punch ends produces a punched slug to be ejected, and wherein the second die has a shape substantially the same as the shape of the first die, and wherein the second punch has a top end having a shape of a truncated cone or a cone with a cone angle in the range from 70° to 120° and a largest diameter that is larger than that of the first punch by 0.1 mm to 0.3 mm.