| 摘要 |
A wide plate-like material ( 27 ), having thin and thick wall portions in a transverse direction, is prepared by alternately joining one of flat thin plates ( 28, 29 ) and one of thick corner plates ( 30 ) or thick plates ( 31 ). The wide plate-like material ( 27 ) is bent at and along right and left thick corner plates ( 30 ) and formed into a U-shaped structure ( 32 ) which is U-shape in cross section. Separately from the U-shaped structure ( 32 ), a plate-like member ( 33 ) is prepared by butt welding thick plates ( 35 ) to right and left sides of a thin plate ( 34 ). The plate-like member ( 33 ) is welded to a lower side of the U-shaped structure ( 32 ) by laser welding or the like to form a square tubular structure, which is in a square shape in cross section and serves as a major part of an operating arm.
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