annular surface 21a formed by means of the rolling tool 17 becomes greater than the outside diameter d 1 of the friction surface 21 formed by press forming process furthermore although the radius of curvature of the curved inner surface of the boundary portion 9a between the flat annular surface 21a and the connecting portion 9 is reduced the strength of the boundary portion 9a is increased since the metallographic compactness and the hardness of the boundary portion 9a is increased in the rolling head 12 39 shown on the rightside in fig2 the rolling tool 17 39 of the rolling mechanism 14 39 is formed in the shape of a roller and the housing 15 39 and the pressing element 18 39 are modified so as to be adaptable to the shape of the rolling tool 17 39 the use of the rolling head 12 39 reduces the distance of reciprocation of the operating arm 13 the single point rolling is a method of increasing the metallographic compactness and the hardness and improving the flatness of a surface of a metal plate causing plastic deformation of the surface by pressing with a predetermined pressure and rolling a rolling tool on the surface of a metal plate this method is capable of providing an effect which is comparable with an effect which is caused by a process commonly called as coining effect the front.