August 25, 2009
Continuing his series about sheet metal stamping, tool-and-die expert Art Hedrick focuses on die basics, including die materials and rudimentary maintenance. He also explains the cutting process and what happens to metal when you cut it with a stamping die.
Editor's Note: This series presents an overview of metal stamping. Part I of this series focused on the various careers in the metal stamping industry. Part II discussed stamping materials and equipment; Part III focuses on dies and cutting and Part IV offers more detail about cutting processes. The final installment, Part V, investigates forming methods.
A stamping die is a special, one-of-a-kind precision tool that cuts and forms sheet metal into a desired shape or profile. Dies often are referred to simply as tooling.
Usually only one die is made to stamp out a certain part shape or type. The exception to this rule is when the volume of parts is so high that a stamper needs to run a die continually to meet the quantity requirements. In this case, it may be necessary to create two identical dies. When one die is in need of repair or maintenance, the other die can take its place.
Often dies are designed with inserts to produce many variations on a single part, such as adding or removing holes or achieving slight form changes.
A die basically consists of two halves: a punch and a cavity (Figure 1). Both the punch and cavity components typically are attached to precision guided metal plates call die shoes. The shoes assembled with the die is referred to as a die set. The die set is the foundation on which all of the working die components will be mounted. Die sets can be made from steel or high-strength aluminum (Figure 2).
Although many commercially available components are used in manufacturing dies, most of the die's cutting and forming sections usually are made from special types of hardenable steel called tool steel. Areas of the die that are not intended to cut or form the metal most often are made from low-cost mild steel. Dies also can contain cutting and forming sections made from solid carbide or various other hard, wear-resistant materials. These components will be discussed in greater detail later in this series.
Dies range in size from those used to make microelectronics, which can fit in the palm of your hand, to those 20 ft. square and 6 ft. thick that are used to make entire automobile body sides.
Not all dies are used to form sheet metal. Some dies can cut and form plastic, paper, rubber, and other materials. You probably have seen embossed letterhead or raised impressions on a brochure or marketing piece. These are made with embossing dies. You may have seen pennies smashed into souvenir coins from attractions, such as Walt Disney World, which often contain raised images. These trinkets are made using coining dies. Even the coins in your pocket are made using dies.
The part a stamping operation produces is called a piece part. Certain high-speed dies can make more than one piece part per cycle and can cycle as fast as 1,500 cycles (strokes) per minute.
All stamping dies perform one of two basic operations: cutting or forming. Many dies can handle both operations.
Cutting is perhaps the most common operation performed in a stamping die. During cutting, the metal is severed by placing it between two bypassing tool steel sections that have a small gap between them. This gap, or distance, is called the cutting clearance. The process of metal cutting not only takes a great deal of force, but it also produces a great deal of shock. For this reason, metal cutting is one of the most severe stamping operations. Excessive shock can cause die sections to break, punches to snap, and presses to fail.
Imagine this scenario.If you had a hammer in your hand and I instructed you to set it on a nail and push it into a piece of wood, you most likely wouldn't have a great deal of success. However, if I told you to lift the hammer and strike the nail, it would go into the wood with very little effort. The energy of the hammer falling combined with the shock performed the necessary work.
If you have ever been in a stamping plant that is cutting thick, high-strength steel, you literally can feel the floor shake every time the press cycles. Presses intended for cutting heavy or high-strength materials usually are designed with extra-heavy-duty frames and components that can withstand this tremendous shock. Certain presses even have special dampening units installed to help dissipate and absorb the shock.
First, understand that you sometimes must shift your paradigm or thought pattern with respect to sheet metal. Despite its physical appearance, density, strength, and weight, sheet metal is an elastomer, which essentially means that when it is subjected to a great deal of force, it behaves much like rubbery plastic. Think of it like Silly Putty®. You know the rubbery stuff that came in a red egg that you may have played with as a kid. Some metals are far more rubbery than others. Harder metal, such as high-strength steel, are less like Silly Putty.
In most cutting operations, the metal is stressed to the point of failure between two bypassing die sections or components. To cut metal, the die must first have a cutting punch and a mating section into which the punch enters. The cutting clearance, the distance between the cutting sections, varies with respect to the metal type, thickness, hardness, and desired edge quality.
The cutting clearance often is expressed as a percentage of the metal's thickness. Although clearances can vary from zero to as much as 25 percent of the metal thickness, the most common cutting clearance used is about 10 percent. For example if you were to design a die to cut metal that is 0.050 in. thick, the distance between the upper and lower cutting sections would be 0.005 in.
Excessive or insufficient clearance between cutting sections can produce an excessive burr on the part (Figure 3). To minimize the burr height, the cutting sections not only must have the proper cutting clearance between them, they must be ground periodically to maintain a perfectly square edge. Diemakers and technicians refer to this process of grinding die sections simply as sharpening the section.
After the punch and mating sections have been ground as necessary, they often have to be shimmed back up to their working height. Shimming is the process by which thin sheets of material—typically stainless steel—are placed underneath the ground section in an effort to compensate for the amount that has been ground off. Grinding and shimming are normal tasks in a basic maintenance procedure.
First the metal cutting punch travels downward and hits the sheet metal At this point a great deal of shock is transferred up through the punch. As the punch begins to enter the metal, it deforms it slightly. Remember that metal is an elastomer. Imagine driving your finger into a marshmallow. The amount of deformation is a product of the metal's ductility, thickness, and the clearance between the mating cutting sections.
As the punch continues to travel downward it begins to push out, or cold extrude, the metal. When the metal's shear strength is met, it begins to fracture or break away. Shear strength can be defined as the maximum strength that a metal can withstand when subjected to two bypassing planes.
This process produces a cut edge with a shiny portion referred to as the cut band, or shear zone, and a portion called the breakout, or fracture zone. In most conventional cutting operations, the thickness of the cut band is about 20 to 40 percent of the metal's thickness, with the exception of specialized processes, such as fineblanking.
Conventional cutting processes don't produce a perfectly square cut edge, but rather a smooth, small flat, followed by a slightly angled, rougher surface. This is an important characteristic for product designers to understand, because this angle means that the part's dimensions will differ from one side to another.
Selecting cutting clearances and other engineering guidelines and specialized processes will be covered in greater detail later in this series.
After the cutting punch performs the necessary work, the metal decompresses and has a tendency to stick to the punch. For this reason, a punch stripper is necessary. A stripper is a spring-loaded or urethane component that functions to pull the metal from the punch. Because the sheet metal is interfacing with the cutting punch and the mating die section, these items usually are made from premium, tough, wear-resistant tool steel or carbide.
Part IV of this series will outline the most common cutting processes.