A method for internal splitting in a cross-section in an end of a metal plate having any of a rectangular, polygonal, or elliptical shape, or an end of a metal rod having a cross-section of any of a circular, elliptical, rectangular, or polygonal shape, the method comprising:
- securing the metal plate by pinching both sides thereof with a clamping die, or the metal rod by pinching at least four opposed positions on the circumference thereof with a clamping die;
- and
- advancing internal splitting by pressing a splitting punch or a cleaving punch having a blade length of 0.8×W to 1.1×W in terms of an intended internal split width W, or by pressing a splitting punch or a cleaving punch after forming a hole or slot at both ends of the intended internal split width, against a crevice of an internal split for repeatedly two or more times until a split length reaches a specified extent, so that a crack generated by the internal splitting does not spread to the peripheral area of the cross-section of the end of the metal plate or the metal rod
- wherein the splitting punch or the cleaving punch having the blade length of 0.8×W to 1.1×W, or the hole or the slot formed at both ends of the intended internal split width is used for determining the width of the internal slit,
- wherein the clamping die has a clamping margin and an alcove opening on at least one side thereof;
- the clamping margin having an abutting length of 0.1×T or more, where the thickness of the metal plate or the diameter of the metal rod is expressed as T, abuts on each of both sides of the metal plate or the metal rod in the area except such an area as corresponds to the intended internal split;
- and
- the alcove opening is formed on the area except the clamping margin extending to the position same as a tip of the intended internal split from the edge of the end of the metal plate or the metal rod,
- wherein at least one of the clamping die having the clamping margin and the alcove opening is arranged in a place where the end of the alcove opening comes to the position same as the tip of the intended internal split from the edge of the end of the metal plate or the metal rod;
- and
- another side of the clamping die having the alcove opening is arranged in the place where one end of the alcove opening comes to a position same as the tip of the intended internal split from the edge of the end of the metal plate or the metal rod, or another clamping die different from the clamping die having the alcove opening is arranged so as to hold the metal plate or the metal rod at the same position as the end of the metal plate or the metal rod,
- wherein, in advance of each time of each subsequent internal splitting operation of the two or more times,
- a position of at least one side of the clamping die that pinches both sides of the metal plate in a prior operation of securing or that pinches at least four opposed positions on the circumference of the metal rod in the prior operation of securing, is moved along the metal plate or the metal rod in advance by a stroke corresponding to the distance from a position of the crevice of the internal split toward a distal end of a split-desired portion, following by securing of the metal plate by pinching both sides thereof with the clamping die, or by securing of the metal rod by pinching at least four opposed positions on the circumference thereof with the clamping die, and
- wherein, in each interval between the internal splitting operation of the two or more times, pressing of the splitting punch or the cleaving punch is suspended.