Punching

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Punching in metalworking is the process of using a punch press to push a punch through the material and into a die to create a hole in the workpiece. A scrap slug from the hole is deposited into the die in the process. Depending on the material being punched this slug may be recycled and reused or discarded. The hole walls will show burnished area, rollover, and die break and must often be further processed. Punching is often the cheapest method for creating holes in sheet metal in medium to high production.

Process

A punch is often made of hardened steel or carbides. The punch press forces the punch into a workpiece piercing a hole that has a diameter equivalent to the punch. A die is located on the opposite side of the workpiece and supports the edge of the hole created to keep it from deforming during the punch. There is a small amount of clearance between the punch's diameter and the die's. This clearance depends on the workpiece material and various tolerances. The slug from the hole falls through the die into some sort of container to either dispose of the slug or recycle it.

Punching Characteristics

• Punching is the most cost effective process of making holes in strip or sheet metal for average to high fabrication
• It is able to create multiple shaped holes
• Punches and dies are usually fabricated from conventional tool steel or carbides
• Creates a burnished region roll-over, and die break on sidewall of the resulting hole^[1]

Geometry

The workpiece is often in the form of a sheet or roll. Materials for the workpiece can vary, commonly being metals and plastics. The punch and die themselves can have a variety of shapes to create an array of different shaped holes in the workpiece. Multiple punches may be used together to create a part in one step.

Equipment

Most punch presses are mechanically operated, but simple punches are often hand-powered. Major components of this mechanical press are the frame, motor, ram, die posts, bolster, and bed. The punch is mounted into the ram, and the die is mounted to the bolster plate. The scrap material drops through as the workpiece is advanced for the next hole. A large computer controlled punch press is called a computer numerical controlled turret. It houses punches 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.

Forces

The punch force required to punch a piece of sheet metal can be estimated from the following equation:^[2]

F = 0.7tL(UTS)

Where t is the sheet metal thickness, L is the total length sheared (perimeter of the shape), and UTS is the ultimate tensile strength of the material.

Die and punch shapes affect the force during 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 by reducing the force at the beginning of the stroke. However, beveling a punch will disort the shape because of lateral forces that develop. Compound dies allow multiple shaping to occur. Using compound dies will generally slow down the process and are typically more expensive than other dies. Progressive dies may be used in high production operations. Different punching operations and dies may be used at different stages of the operation on the same machine.

Related Processes

Other processes such as stamping, blanking, perforating, parting, drawing, notching, lancing and bending operations are all related to punching.

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.

See also

• Knockout punch

References

External links

• http://www.emachineshop.com/machine-shop/CNC-Turret-Punching/page90.html

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