Method for fastening piercing nut to stacked body of plate-shaped workpieces

A fastening method for fastening a flat column-shaped piercing nut to a stacked body in which a plurality of plate-shaped workpieces are stacked, the piercing nut having a center provided with a cylindrical portion, a circumferential cavity being formed on a peripheral area of the cylindrical portion, the method comprising:

• a first clinching step of fastening the piercing nut to a first plate-shaped workpiece by locating the piercing nut on one surface side of the first plate-shaped workpiece in a state of being opposed to an opening side of the circumferential cavity, driving a clinching die with a tubular distal end surface from another surface side of the first plate-shaped workpiece to punch out the first plate-shaped workpiece in a circular shape, and extending a peripheral area of the punched portion in the circumferential cavity of the piercing nut;
• and
• a second clinching step of stacking a second plate-shaped workpiece on the one surface side of the first plate-shaped workpiece to which the piercing nut is fastened in the first clinching step, subsequently, driving a clinching die with a tubular distal end surface from the side in which the second plate-shaped workpiece is stacked to punch out the second plate-shaped workpiece in a circular shape, and extending a peripheral area of the punched portion on an outside of the extended portion of the first plate-shaped workpiece in the circumferential cavity of the piercing nut.