Punching
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[edit] Metals
Punching in metal fabrication is the process of using a machine to press a shape through a sheet of metal and into a die to create the desired shape in the metal. This is most commonly done by use of a turret, a computer numerical controlled machine that houses tools and their corresponding dies in a revolving indexed turret. These machines use hydraulic, pneumatic, or electrical power to press the shape with enough force to shear the metal.
The shape is formed by pressing the material against a die with a huge force. The shear forces generated between the material and die separate the material into the desired shape. The desired shape is not obtained, however, as burred edges and rough surfaces are formed. These edges and surfaces must be further processed until the desired shape is achieved.
The punch force required to punch a piece of sheet metal can be estimated from the following equation:
F=0.7tL(UTS)
Where t is the sheet metal thickness, L is the total length sheared (perimeter of shaped hole), and UTS is the ultimate tensile strength of the metal.
Die and punch shapes affect the punching process. The punch force increases during the process as the entire thickness of the material is sheared at once. A beveled punch helps in the shearing of thicker materials. Beveling reduces the force at the beginning of the stroke. However, beveling a punch will disort the punched shape because of lateral forces that develop. Compound dies allow multiple shaping to occur. Using compound dies will generally slow down the proccess and are typically more expensive than other dies. Progressive dies may be used in high production operations. Different punching operations and dies are used at different stages of the operation on the same machine.
Other processes such as stamping, blanking, perforating, parting, drawing, notching, lancing and bending operations are all related to punching.
[edit] Plastics
Punching in plastics fabrication usually refers to the removal of scrap plastic from the desired article. For example, in extrusion blow molding it is common to use punching dies to remove tails, molding flash (scrap plastic) and handle slugs from bottles or other molded containers.
In shuttle machinery, the containers are usually trimmed in the machines, and finished containers leave the blow molding machine. Other blow molding equipment, such as rotary wheel machinery, requires the use of downstream trimming. Types of downstream trimming equipment include detabbers for tail removal, rotary or reciprocating punch trimmers, and spin trimmers.
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