Stamping Die Essentials: The process layout for stamped metal parts
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Editor’s Note: This is the second in a series of articles presenting the fundamentals of stamping die design and construction. Check out the first and third parts of this series.
A process layout can be defined as the steps we expect to use to produce a part successfully. It can be a simple, two-step process for a simple part, or it could require 20 to 30 various steps for a difficult part.
The exact number of steps for a process layout depends on the metal that the part is made of, the complexity of the part geometry, and the geometric dimensioning and tolerancing characteristics.
Keep in mind that a process layout is about more than just being able to make your part shape successfully. You also must determine how to cut the metal and properly dispose of the scrap. Scrap being cut from the part must be shed off the sides of the tool or dropped through a clearance hole in the die shoe, and it must be able to fall freely onto a scrap collection system or directly on the bolster plate of the press. For this reason, only certain areas can include parallels to support the die shoe.
For some part shapes, you might need to add idle stations to the process. These stations perform no work, but they allow more room for larger, stronger tooling sections and necessary die components.
In addition, if you are designing a progressive die, you will need to determine what type of carrier web is best suited to your part’s geometry. To do that, you’ll need to determine if any metal flow is taking place during part forming and if there is any height difference among the die stations. If either of these conditions exists, you will probably need to design a flex or stretch carrier that allows metal to flow into the desired part geometry without upsetting the critical centerline distance between each part, known as the pitch or progression (see Figure 1). If the strip will remain flat throughout the entire progressive die, with no metal flow or up and down movement, you can use a solid carrier (see Figure 2).
A process layout for a progressive die is called a strip layout. This layout defines the process, the type of carrier to be used, how the scrap will be shed, how the part will get carried through the tool, and how it will be ejected from the die. Carrier strength is important, as it must be strong enough to move the part from station to station without buckling, severely flexing, or deforming. Carrier development can be problematic, especially when dealing with large parts made of very thin metal; the metal does not have the stiffness necessary to hold the parts in the carrier stable or to feed without buckling. In such a case, it may be necessary to form strengthening beads or ribs in the carrier web to give the carrier the necessary stiffness.
If the part will be stamped using a fully automated system, such as a transfer system, you must carefully determine how the parts will be picked up and carried through the tool. You also will need to determine if a 2-axis transfer or a 3-axis transfer is best suited to your part geometry. For larger, contoured parts, a 3-axis transfer typically is preferred, as it allows you to pick up the part and place it within the boundaries of gauging or over a pilot pin. You’ll also need to define the pickup height for each die. This is the level at which the fingers on the transfer bars will be engaging with your part. Normally the pickup height for every part is at the same level. Before the tool or die is designed, you’ll also need to determine methods for clamping and moving the parts and for scrap.
Whatever type of tooling you're designing, it will have to fit within the boundaries of a certain press style and size. Before tool design begins, you have to understand the parameters of the press in which the tooling will be run. Each press has a specific bed size, tonnage rating, stroke length, shut height, and drive method. Some presses offer through-the-bolster scrap removal, while others do not. The press conditions most certainly will affect many of the die design parameters.
If you don’t plan for scrap shedding, the press likely will be stopped every few cycles for scrap removal. If you design a die with parallels far apart to allow a large piece of scrap to fall through, those parallels won’t adequately support the die shoes. The better solution would be to think ahead and plan for the scrap to be cut into smaller pieces before ejection from the tool, thus allowing the parallels to be placed properly for support.
As you can see, many details must be clarified before tool design can begin. Your best chance for well-performing dies starts with thorough planning.