PURPOSE: To prevent scratch, etc., at the inside corner of bending at bent part of pipe by notching the part of the inside corner of bending of a mandrel so as not to be brought into contact with the inner wall surface of pipe.
CONSTITUTION: A mandrel 1 is constituted of a shank 11, first, second, third mandrel 12, 13, 14. An outside diameter of the shank 11 is about equal to an inside diameter of the pipe to be bent. An outside diameter of plural disk mandrel 12, 13, 14 is about equal to or a little bit smaller than an inside diameter of the pipe to be bent. The shank 11, mandrel 12, 13, 14 are connected through spherical bearing 12a, 13a, 14a. The part corresponding to bending corner 12b, 13b, 14b of mandrel 12, 13, 14 are notched not to be brought into contact with the inner wall surface of pipe. In this way, at pulling out the mandrel 1, the outer peripheral surface of mandrel is not brought into contact with the inner wall surface of pipe of contact with the inner wall surface of pipe of bending part.
OKUYAMA TETSUHIRO
KAGOSHIMA EIICHI
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