| 摘要 |
PROBLEM TO BE SOLVED: To form a swelling part 23 so as to form a circular flat top surface 27 by removing a tip 30 of a hexagonal pyramid. SOLUTION: The swelling parts can be most tightly arranged, and rigidity of a panel can be increased in any direction of the panel. The surface area of the swelling part can be decreased comparing with the case that the top surface is formed in a hexagonal flat surface shape, deformation quantity in plastically machining the swelling part can be decreased, and thickness of the swelling part is uniformed to stably provide the panel with high rigidity.
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