PURPOSE: To form clam blade safely and simply, and increase a grinding speed to enable preparation of a clam blade making software just with X, Y, and Z line coordinates in grinding by a robot by installing a ring-shaped polishing material on the outer side from the inner diameter surface.

CONSTITUTION: A grinding material made by forming grinding material in ring shape is installed on the outer side from the inner diameter surface. Then the top edge 1 of a tool is brought into contact with the inner surface of the ring-shaped inside grinding stone 12. Because the inner side of the inner side grinding stone 12 forms a ring-shaped recess, the arc at the recess can form an clam blade and swage sections. Also the tool part is held by a robot, and moved as instructed by the robot while contacting on the inside grinding stone 12 to enable forming of clam blade just in one stroke without kneading as before because the recessed surface of the inside grinding stone 12 is used even for both blades.