December 15, 2009
How are bending, flanging, coining, embossing, stretching, curling, hemming, ironing, necking, and drawing related? They all are common metal forming operations. Find out more about these processes in this final installment of stamping expert Art Hedrick's sheet metal stamping series.
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 focused on dies and cutting and Part IV offered more detail about cutting processes. The final installment, Part V, investigates forming methods.
Bending is one of the most common die forming methods. It can be defined simply as a forming operation in which the metal is deformed or bent along a straight axis. Items such as tabs and channels are created with this process. Creating a U-shaped part by bending is called U forming or channel forming.
Because different materials and thickness have different springback values, achieving the correct bend angle in a bending operation sometimes can be very difficult. Many methods can be used to compensate for springback.
Three of the most common bending methods performed in a die are wipe bending, V bending, and rotary bending. All three are very popular, and each has its advantages and disadvantages.
Wipe bending utilizes an upper forming section, a pressure pad, and a lower forming section. The pressure pad holds the metal tight to the punch while the metal is bent and wiped tight along and over the forming punch.
V bending uses a V-shaped block, a lower block, and a punch shaped like the cavity.
Rotary bending uses mechanical rotary motion to fold the part around the forming punch.
Both compression and tension occur during bending (Figure 1).
Flanging is bending metal along a curved axis. The two basic types of flanges are tension, or stretch, and compression, or shrink. Tension flanges are susceptible to splitting and thinning, and shrink flanges are susceptible to wrinkling and thickening.
Like bending dies, flanging dies use a pressure pad to hold the material while flanging takes place. Flanges are created with a flanging die that wipes the metal between a punch and a lower die section. Because tension flanging stretches the metal, it is best-suited to parts made from metals that have good stretchability.
Both tension and compression occur during the flanging process. Flanges often are used to make a part stronger, stiffer, or more rigid; they also serve as a way to assemble parts (Figure 2).
Embossing can be defined as a process in which the metal is stretched into a shallow depression. Embossing often is used to create lettering, logos, small depressions, strengthening features, textured nonslip surfaces, and other geometric features.
Because stretch or tension is the main deformation mode used to emboss, the metal is subject to excessive thinning or fracturing. For this reason, the depression depth is limited by the material's stretchability and thickness; the emboss geometry; and frictional values (Figure 3).
Coining is a process in which the metal is compressed or squeezed into the desired shape or profile. The coins that you have in your pocket are a classic example of items made by coining.
Coining has several advantages and disadvantages. It can produce sharp, crisp corners; well-defined features; and a brilliant surface finish. Because coining uses compression to form the part, tension failure is unlikely.
One disadvantage is that it takes a great deal of force to coin metal. Special coining presses or knuckle presses are often used. Minting presses are used to make metal currency. These presses apply significant force in the small localized area where the actual coining takes place. Although many different materials can be coined, softer metal such as brass, copper, gold, and silver are best-suited for coining (Figure 4).
Drawing is a process in which the surface area of a blank is displaced by tension into an alternate shape via controlled metal flow. Metal flow can be defined as metal feeding into the cavity. Chances are, if a blank's outer profile changes as the metal is deformed, drawing is taking place.
Drawing is one of the most complicated yet effective means of shaping sheet metal. Some metal stretching does take place during this process, so the key is to try to limit it as much as possible.
In drawing, a punch pulls the metal into the forming cavity. For this reason, drawing can be considered a method that forms the metal in tension. Items such as oil filter pans, deep-formed auto parts, kitchen sinks, cookware, motorcycle gas tanks, and fenders, as well as thousands of other parts, are made using this process.
Almost any metal with reasonable formability characteristics can be drawn. Materials such as spring or ultrahigh-strength steel are not well-suited for drawing (Figure 5).
Stretching is a metal forming process in which the surface area of a blank is increased by tension. Don't confuse stretching with drawing. During drawing, the metal is flowing in the die and the blank is changing shape. During stretching, there is no inward movement of the blank edge.
Stretching dies are very similar to embossing dies with the exception that, unlike most embossing operations, stretch dies use a high-pressure binder to restrict and stop metal flow. A binder is very similar to a drawing pad, except that a binder intentionally restricts the metal from moving inward. Parts such as automobile hoods, roofs, and fenders are often made using stretching dies.
Stretching the metal over a well-finished, smooth punch is a cold-work process that produces a good surface finish suitable for painting and makes the metal dent-resistant. Parts requiring smooth, aesthetically pleasing, defect-free surfaces often are designated as Class A surfaces. Exposed body panels, such as hoods and doors, are classic examples of Class A surface requirements.
Some dies create significant metal stretching along with flow to obtain the finished part shape. These dies often are referred to as stretch drawing operations (Figure 6).
If you have ever looked at a beverage can and wondered how it's made, welcome to ironing. Ironing is a process in which the metal is squeezed and reduced in thickness along a vertical wall. The main purpose of ironing is to increase the height of a vessel and to unify its wall thickness. Ironing also gives the metal a polished appearance.
Unlike drawing or stretching, ironing uses compression to form the metal. This makes it an ideal process to use when the metal is soft but has reasonably poor stretchability, such as aluminum, the material of choice for making beverage cans. Most ironing operations begin as a drawn shell that is later subjected to an ironing process.
Ironing can reduce the walls of an aluminum beverage can to approximately 0.002 in. thick. In addition, because ironing is a cold-working operation, the metal gets harder and stronger each time it is formed, which creates a thin, lightweight, strong, very shiny beverage can.
Gun shells also are made with this process. A gun shell starts as a thick-walled, small brass cup. The cup goes through several ironing operations. With each, it gets taller and taller as the walls get thinner and thinner, while retaining the metal's original thickness at the bottom or closed portion of the gun shell. This portion later is coined and machined into the finished working end (Figure 7).
The reducing/necking process gradually reduces the diameter of the open end of a vessel. Gun shells are a classic example. The smaller necked-down area holds the bullet, while the larger portion holds the gun powder.
Although necking sometimes can be achieved in a die, it is best performed using special rotary spinning machines. This is because metal has a tendency to buckle or wrinkle when forcibly compressed into a smaller diameter. Beverage cans are necked using a rotary process (Figure 8).
Curling deforms metal into a tubular radial profile. Door hinges are a good example of parts that are made using this process.
Curling also is used to smooth edges and add strength and rigidity to a part. Although some curling can be done in dies, like necking, it also can be performed using rotary curling machines, which are ideal for curling round part edges (Figure 9).
Hemming is a process in which the metal is folded over onto itself to create a smooth edge or strengthen the part. Hemming also can be used to join two parts together. An automobile door is a good example of a hemmed product. Although the door consists of many parts, two main parts make up its basic assembly: the inner door and the outer door.
The outer door has a Class A surface, while the inner door does not. Hemming is used to attach the inner door to the outer door. The bent or flanged portion of the outer door panel is folded around the inner door and crimped is such a manner that it does not mark the surface of the outer door, yet sufficiently holds the two parts together.
The three basic hem types are closed, open, and rope. A closed hem is fully shut; an open hem is open or wrapped around another part. A rope or loop hem is used when the larger radius is needed on the part or the metal does not have the ductility to be formed into a closed hem. Designing a closed hem in a high-strength-steel part is asking for trouble as the probability of cracking is high (Figure 10).