Patent ID: 8689598

Claim:
A method of forming a tenon on one side of a metal plate member, comprising the steps of: a) providing a metal plate member having a top surface and a bottom surface each being a flat surface, outer edges defining a periphery of said metal plate member, and a thickness located between said top surface and said bottom surface, the top surface being parallel with the bottom surface; b) providing a first punch having an outer step, an inner step perpendicularly protruded from said outer step and a first recess at the center of said inner step; c) punching the top surface of said metal plate member with said first punch and driving said inner step into the wall of the top surface of said metal plate member, deforming a part of the wall of the top surface of said metal plate member and forming a groove having an outer peripheral wall corresponding to a shape of an exterior of said inner step of said first punch and forming a protrusion in said recess of said first punch, said groove of said metal plate member surrounding said protrusion and said groove and said protrusion are spaced apart from said outer edges of said metal plate member; d) providing a second punch that is prepared subject to the configuration of the surface area of said metal plate member around said protrusion and has a second recess fitting said protrusion and a punch rod axially inserted through and movable up and down relative to said second recess; and e) punching said protrusion of said metal plate member with said second punch and said punch rod extending the height of said protrusion and making said protrusion into a tubular configuration and forming a tenon on the top side of said metal plate member, said tenon having a first end located below the top surface of said metal plate member and a second end protruding outwardly above the top surface of said metal plate member; wherein, when said groove and said tenon have been formed by said punching step c) and said punching step e), except for the part of said top wall of said metal plate member where said groove and said tenon are located, said thickness of said metal plate is unchanged by said punching step c) and punching step e), a portion of said top surface surrounding said groove is unchanged by said punching step c) and punching step e), said bottom surface of said metal plate member is unchanged by said punching step c) and punching step e), and said outer periphery of said metal plate member is unchanged by said punching step c) and punching step e).