Forming and Flanging Sheet Metal in Stamping Dies
Two Types of Sheet Metal Stamping Dies
There are many different kinds of mechanical stamping dies, and some are designed primarily to form or flange sheet metal. Forming metal is taking a flat or partially formed sheet metal blank and forming it into a desired shape. Flanging metal is the act of swiping sheet metal in a direction contrary to its previous position.
Forming a sheet metal blank into the shape required by design is initially performed in a draw die. Drawing metal into the shape desired can often be performed in a single die (usually the first in the line of dies), but sometimes it requires two, a draw die and a redraw die. Like all sheet metal operations, there are ideal ways to perform specific die operations. Hydroforming, for example, could be a better option than pressure forming, depending on the operational requirements (i.e. part conditions).
Additional forming of specialized areas can be accomplished with pressure forming or flanging in later die operations. What's key to part layout engineers is that the forming is done in such a way as to not wrinkle or deform the metal in any way. Also, if the sheet metal panel is an outer panel, such as the door of a car, additional measures will be taken to ensure the forming and flanging operations don't mark the part. That is, it's undesirable for any die operation to leave marks, dings, or dents on the sheet metal part, especially if the part is to be used where it can easily be seen (i.e. an outer automobile panel).
Forming Sheet Metal
Forming sheet metal is usually done either by drawing the metal, or by pressure forming the metal. When drawing metal, the die engineers must ensure that the metal flows, as it's being formed, in such a way as to not leave weak areas. What the part engineers look for is even metal thickness on the entire finished part. There are ways designers can form metal to keep it from getting weaker or thicker in areas. This is done during the die layout phase of the part manufacturing - usually a group of engineers that decide how best to perform the required operations need to make the part, and in what order these operations should take place. This may also determine the number of dies a die lineup has to make the part.
Fig. 2, at right, shows the most basic concept of a mechanical draw die's lower assemby. The innermost component, the lower draw punch (green), has the desired shape milled onto its 'face'. The lower punch is surrounded by the lower binder (red). The binder also has the desired part shape milled onto its surface. It is on this surface that a sheet metal blank will be placed when the operator (or, optionally, a mechanical or robotic device) loads the part into the stamping press (when the press is open and the binder is in the up, or load, position.
The binder travels - it moves up and down when the stamping press opens and closes. It's encompassed, and held in place by, the lower die shoe (blue). Additionally, the binder is floating - its bottom isn't fixed, rather it rides on a pressure system, whether it be pneumatic or hydraulic. This pressure system provides slight resistance against the downward motion of the stamping press. With the force of the downward motion of the press overcoming the pressure system upon which the binder is set, this force allows a smoother transition for the sheet metal part from flat blank into a part with its deepest area of forming formed smoothly, and without creating weak spots. It's nicely explained like this:
If you were to take a large piece of leather, and you wanted to make a hat out of it, you could place the leather over a spring loaded platform. In this platforms center is a rounded mold, in the shape of a hat. The pressure platform moves up and down around the center mold. So, when you place the leather onto the platform, and then push down on it, while holding it firmly in place, the fabric is stretched over the center mold. Instead of just ripping it downward and perhaps tearing the material, the pressure platform (binder) slows the downward motion, so the forming is more gently achieved.
In Fig. 3, at right, a sheet metal blank is loaded onto a pressure binder. The binder will trap and hold the part when the press closes. The upper die member contacts the binder before the metal is brought down upon the shaping post, or lower punch.
Note the pressure system, shown in blue in Fig. 3, beneath the binder. Air pin systems are available on many types of large stamping presses. On other metal forming operations, hydraulic cylinders may be used. Die engineers must determine what pressure systems are required to best make their part, and often this depends on the stamping press(es) employed.
There are a variety of ways to locate the loaded sheet metal blanks, such as position pins or guides. These devices allow the press loader to place the blank on the binder in the same location each time it's loaded.
For high production draw dies, binder surface that are guided by the draw punch and/or lower shoe, will have wear plates mounted on wear surfaces (two metal surfaces rubbing against each other). These wear plates are lubricated, and provide easy action - they are designed to surpass the performance of a finished or treated cast surface.
Not shown, but still relevant, is the upper draw die casting. Treated steel inserts may be mounted into the upper forming surface for part areas needing special care. In most cases, the part will be formed by one treated cast steel surface. Small self-contained pins can be mounted in this upper surface to press downward as the press opens, preventing the sheet metal part from sticking to the upper die component, after it's been formed.
Die Operation to Flange Sheet Metal
Sheet metal flange operations are usually much simpler than draw operations. Often, part flanges are made in dies that are accomplishing other operations, too. Engineers bear in mind that dies, and their components, are costly. Rather than building one die that'll perform a flange operation, and another die to achieve hole piercing, the two dies could be combined into one, whenever allowable or possible. Part conditions dictate what different operations can be performed in the same die.
In Fig. 4, a sheet metal tab is being formed downward. The length of the tab will dictate how the flange is formed. This tab is carefully designed, and can be trimmed (in a previous die operation) so that there are cuts, or tears, in the tab at intervals, so that the tab can be flanged around corners or curves.
The simple downward motion of the press will flange the tab, but usually a cast steel pressure pad is added to the upper casting. This pressure pad holds the metal while the tab is being flanged down. Steel inserts are added to the lower post and upper casting for high production die lines.
The main concern for die engineers in this operation is the flange break - the actual point where the metal is bending. This bend must be given careful consideration, and will dictate part development changes if it doesn't test well in die tryout. As with all sheet metal operations, metal thickness uniformity is given high scrutiny, and flange operations that cause weakness in the metal must be reevaluated and changes made either to the part design, or to the flange die that's forming the metal.
Standard Die Cam Flange
When a flange operation cannot be performed directly, a standard die cam unit may be employed. This is an inexpensive alternative to, say, adding another working tool to the die lineup. For instance, a die lineup has two trim die operations in it.
The first trim die could be used to rough trim a sheet metal panel all around, but also finish trim a tab. In the second trim die, where the part is being finish trimmed all around, a standard die cam could be mounted so that it forms the previously finished tab into a desired position.
Obviously there would be engineering concerns, such as ensuring the trim steels aren't causing undesirable conditions with the cam's movement, and also that the panel is getting a proper finish trim.
The first trim die should be designed so that it finish trims well to either side of the tab. By doing so, engineers can be sure that the second trim die is completing the finish trim and not leaving any scrap attached to the panel near the tab.
Another option is for die designers to design their own custom cam. Though a more expensive option, this might be desirable if doing so cuts down on the number of dies in a die lineup. Custom cams may also be required if the attached flange steel cannot be easily mounted to a standard die cam.