Machining method for burred flat holes in metal plates

A machining method for a burred flat hole in a metal plate, comprising:

• a first step of pressing an overhanging punch having a flat cross-section orthogonal to a pressing direction onto a surface of a metal plate to form an overhang portion that is flat in the cross-section;
• a second step of pressing a perforating punch having a flat cross-section orthogonal to the pressing direction onto a bottom surface of the formed overhang portion to form a planar flat hole portion in the portion;
• and
• a third step of pressing a burring punch onto a peripheral edge portion of the formed hole portion to form a burring by pulling up the portion,
• wherein:
• each of the overhanging punch and the perforating punch has a pair of side surfaces that are parallel to the pressing direction and are positioned at both ends in a longitudinal direction (hereinafter, referred to as a “major axis direction”) of the cross-section and a pair of side surfaces that are positioned at both ends in a width direction (hereinafter, referred to as a “minor axis direction”) of the cross-section;
• and
• difference (7n−2n) between the side surface gap 7n at both ends in the major axis direction of the cross-section of the burring punch and the side surface gap 2n at both ends in the major axis direction of the cross-section of the overhanging punch is set to be smaller than difference (7m−2m) between the side surface gap 7m at both ends in the minor axis direction of the cross-section of the burring punch and the side surface gap 2m in the minor axis direction of the cross-section of the overhanging punch, that is, (7n−2n)<(7m−2m).