| 摘要 |
PURPOSE:To uniform expansion from an opening periphery to side faces by forming curved faces which are gradually expanded in curved shapes to the outside as tending to the center portions from both ends of their vicinities of the sides forming the opening periphery to be symmetrical in mutual opposite sides. CONSTITUTION:In individual sides 7 which from the opening periphery 6 of an opening face 2, curved faces 5 are formed which are gradually expanded in curved shapes to the outside as tending to the center portions from both ends or their vicinities of the sides 7 (adjacent sides 7). These curved faces 5 are formed to be symmetrical in mutually opposite sides 7, and the centers of the radius (R)s of the curved faces 5 are placed on lines passing through the centers of individual sides 7. Accordingly, the individual sides 7 forming the opening periphery 6 are gradually expanded in curved shapes to the outside with tending to the center portions from both ends of their vicinity and so are prevented from being curved inward at the center portions of individual sides 7. It is thus possible to uniform expansion (b) from the center portions of the individual sides 7 forming the opening periphery 6 to the side faces 4. |
