Patent ID: 8387494

Claim:
A punching method using a punch for forming a through-hole in a workpiece by punching the workpiece using a punch, the method comprising the steps of: making at least a tooth portion of the punch substantially solid, wherein the punch comprises a cylindrical punch body having a hollow bore formed therein, the punch body comprising a tip end portion having a female tapered surface formed in the hollow bore thereof, and a solid punch core which fits nestingly into the hollow bore of the punch body, the punch core comprising a tip end portion having a male tapered surface, wherein the tooth portion comprises the tip end portion of the punch body, and wherein the tooth portion is made into a substantially solid state by aligning the tip end portion of the punch core with the tip end portion of the punch body; punching all of the way through a partition wall in at least one portion of the workpiece with the tooth portion of the punch in said substantially solid state, whereby the punch breaks said partition wall out of the workpiece to form a punched-out portion and create said through-hole in the workpiece; separating the tip end portion of the punch core and the tip end portion of the punch body while the tip end portion of the punch body is extended through the punched-out portion of the workpiece, whereby the tooth portion is made substantially hollow; and separating the workpiece from the punch body after the punching of the workpiece while keeping the tooth portion substantially hollow, thereby to reduce a diameter of the tooth portion by a reaction force received from the workpiece.