To suppress the exchange of the top/bottom blades at minimum by forming the front/rear parts of the top blade and the bottom blade of a projecting curved surface of plane symmetry plus top and bottom planes, and a recessed curved surface plus top/bottom planes and setting cross section parts crossing the curved surfaces and the planes to shear parts.

A bottom blade scallop 45 mated with a top blade scallop 27 into a pair is provided with two recessed curved surfaces 45A, 45A' having the same curvature as projecting curved surfaces 27A, 27A' in its front and the rear and planes 45C in the top and the bottom. Cross section parts which are corner parts crossing the respective recessed curved planes 45A, 45A' and the planes 45C are set to shear parts 45D1-45D4. The respective blades 27, 45 have 2-4 shear surface parts so that they can be quickly changed into a new shear part just by replacing it by turning it over or rotating it by 180°.