Heretofore, a rivet has been widely used to fasten two workpieces such as two steel sheets. In this case, a mandrel having a caulking portion and a shank is disposed in a hollow rivet having a flange and a stem, and this combination of the mandrel and rivet is inserted into a hole formed in the workpieces. Then, a nosepiece fixed to a fastening tool is brought into contact with the flange of the rivet, and the shank of the mandrel is drawn by the fastening tool to deform the stem of the rivet to increase in its diameter. The mandrel is broken at a breaking portion thereof during the drawing of the mandrel shank. Thus, the workpieces are fastened between the flange and the deformed portion of the rivet.
The fastening tool for fastening the rivet to the workpieces is known in the art. The fastening tool comprises an elongated hollow housing, a jaw provided inside the housing movably rearward and adapted to engage with the side surface of the mandrel and hold the mandrel, and a nosepiece adapted to be attached at the front end of the housing and to engage with the flange of the rivet. When the jaw is moved rearward to draw out the front end of the mandrel held by the jaw, the mandrel is broken at a breaking portion thereof, and the rivet is deformed to increase in its diameter by the rear end of the mandrel so as to fasten the workpieces together.
When the workpieces covered by a typical coating are fastened with the rivet in the conventional manner, the coating of one of the workpieces on the side of the stem of the rivet is partially peeled by a force caused when the rivet is deformed to increase in its diameter, and thereby the stem-side workpiece and the rivet can assure electrical conductivity therebetween. On the other hand, no strong pressure is applied on the flange-side workpiece, and thereby the coating of the flange-side workpiece is not peeled. Thus, it has been difficult to assure electrical conductivity between the flange-side workpiece and the rivet.
The present invention has been made to solve the above problem. Therefore, the present invention is directed to assure electrical conductivity between the flange-side workpiece and the rivet as well as between the stem-side workpiece and the rivet.