| 摘要 |
An upper die and a lower die are provided. An unpenetrating hole is formed at an upper face of a metal board with the upper die, so that a protrusion is formed on a lower face of the metal board at a portion corresponding to the unpenetrating hole. A flat portion is formed on the protrusion with the lower die. The unpenetrating hole is punched with the upper die while supporting the flat portion with the lower die to form the through hole. |
