Method for joining together two or several overlaying sheet formed members, and apparatus for carrying out said method and a joint resulting from said method

A method for joining together a plurality of sheets, comprising the steps of:

• providing a punch having a generally cylindrical core portion terminating in a tip at a first end of the core portion, said core portion being surrounded by a circumferential lateral extension extending outward from a second end of the core portion, said lateral extension having an outer diameter;
• providing a die having a horizontally extending surface defining a cavity that has a diameter no smaller than the outer diameter of the lateral extension, and an anvil having a top surface that is adapted to move within said cavity;
• providing an upper sheet and a lower sheet;
• positioning said punch relative to said die, such that said core portion and said cavity are co-axial;
• positioning said sheets between the punch and the die;
• first moving said punch axially toward said die in a first direction, thereby causing said tip to force material of said sheets into said cavity, whereby said material of said sheets is deformed into a cup-shaped portion having a side wall portion and a bottom wall portion;
• moving said anvil towards the plane defining the horizontally extending surface of said die and locking said anvil in a predetermined position relative to the die;
• then
• moving said punch toward said die in said locked position, thereby compressing said material of said sheets between said tip and said anvil such that said bottom wall portion of said cup-shaped portion bulges outwardly;
• wherein during the first moving step, the circumferential lateral extension displaces material of the upper sheet axially in the first direction.