FIGURE 1. An undeveloped blank starts off as a basic shape such as a square, rectangle, trapezoid, or other shape that can be created using straight-line cuts. (

Editor’s Note: This is the fourth in a series of articles presenting the fundamentals of stamping die design and construction.

In my previous installment, I discussed the importance of selecting and designing the strip carrier for a progressive die. Now let’s look at the basic steps needed to make a given part geometry. The method used to determine the steps necessary to make a part is called processing the part. These processing steps are critical to the success of the entire stamping operation.

The first thing to determine when processing a part is the style and type of blank that will be used to make it. The blank is the starting piece of sheet metal that will eventually be formed and cut into the desired part shape or profile.

Three basic types of blanks are used in sheet metal stamping, and all three can be used with transfer, progressive, and stage tooling.

An undeveloped blank (see Figure 1) starts off as a basic shape such as a square, rectangle, or trapezoid that can be created using straight-line cuts. The advantage to using an undeveloped blank is that you won’t have to purchase a special blanking die to cut out a special shape; you can use a simple shearing die.

Keep in mind that using an undeveloped blank is not conducive to all metal forming operations, such as deep drawing. The extra material outside of the forming punch can sometimes create a resistance to flow, resulting in part failures such as splitting. Engineers typically choose undeveloped blanks if the material has a great deal of stretchability, and limited inward flow of metal will be required to make the desired part geometry.

A semideveloped blank (see Figure 2) is shaped to use just enough material, so that little material is left to trim away later. This type of blank is popular when the trim tolerance on the part is very small. A semideveloped blank will require slightly more material to produce a piece part than a fully developed blank.

A blank also can be undeveloped in certain areas and partially developed in other areas. This allows the process engineer to hold tight tolerances in areas of the part that require it and save material and the cost of additional cutting operations in areas that have greater tolerance.

A fully developed blank (see Figure 3) often is used when there is a reasonable amount of profile tolerance in the finished part and the geometry is fairly simple. The blank is shaped so that after it is formed, it will not require any additional trimming operations. In other words, the net shape of the part will be established.

Because of normal sheet metal variables such as small differences in mechanical properties and thicknesses and inconsistent frictional values, using a fully developed blank often will result in minor inconsistency in the part’s edge dimensions. If the part requires precision edge location, then secondary trimming usually is required. But when no additional trimming is required, no scrap is produced, so a fully developed blank might result in material savings.

Determining the forming process typically requires a lot of metal forming experience. The general guideline is to establish all the forms on your part first and trim and pierce the part in secondary processes.

When deep forms in the sheet metal are required, deep drawing will likely be necessary (see Figure 4). In drawing, the flow of material into a cavity or over a punch is controlled. Items such as oil filters, kitchen sinks, and cookware are formed using deep drawing. The process typically requires a semideveloped blank and sometimes multiple operations, known as redraws or reductions. If drawing is performed in a progressive die, it will most certainly require a stretch web. (For more information, see “Stamping Die Essentials: The process layout.”

If the shape on the part is fairly shallow and the features are not deep, embossing may be the forming method of choice (see Figure 5). Embossing can be defined as the stretching of metal into a shallow depression. To achieve embossing, the metal must have enough stretchability to make the part shape, while drawing uses flow and stretching to make the part shape.

Bending is defined as deforming the material along a straight-line axis, while flanging is bending the metal on a curved axis. Take care when designing a die to create a stretch flange, as there is a higher probability of the metal splitting when it’s formed. Typically, metals with high formability or elongation are the best candidates when creating a stretch flange.

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