To contrive the extension of the length of a workpiece which is bendable and to make the workpiece hard to be scratched.

Angles (set to 55, for example.) formed by the edge parting lines L1, L2 of each mortar 1, 2 (set the angle of the edge to 90, for example.) and edge sides 13, 23 of the outside of a cross section are made larger than the angle (set to 35, for example.) formed by the edge parting lines L1, L2 and edge sides 14, 24 of the inside of the cross section and, in the each mortar 1, 2, the edge parting line L1 or L2 of the mortar 1 or 2 on one side is situated in a position where is shifted toward the edge parting line L2 or L1 of the other mortar 2 or 1 from the center position between the edge parting line L2 or L1 of the mortar 2 or 1 on the other side and the inside edge of a working block 26 or 16.