| 摘要 |
PROBLEM TO BE SOLVED: To form a bell-mouth of sufficient height by burring processing while using a rolled steel plate of minimum required plate thickness.SOLUTION: A bell-mouth is formed by successively implementing a prepared hole formation process, a burring process, and a diameter expansion process while using a rolled steel plate as a workpiece. In the prepared hole formation process, a prepared hole (71) is formed on the workpiece. Here, peripheral edge raised portions (81 to 84) are respectively formed on parts of 45° direction, 135° direction, 225° direction, and 315° direction around a center (CP) of the prepared hole (71) from a reference straight line (BL1) in parallel with the rolling direction of the rolled steel plate through the center (CP) of the prepared hole (71). In the next burring process, a cylindrical portion is formed by implementing the burring process on the workpiece. In the diameter expansion process, a projecting end portion of the cylindrical portion is formed into the trumpet shape. |
