Chapter 7
- Designing for Stamping Production
Design Guidelines for Drawing
The following recommendations apply to the design of drawn parts:
Shapes are virtually unlimited because of the ability to perform multiple draws. Round is the easiest to draw, followed by square with adequate corner radii. Irregular shapes and those that combine two basic shapes into one are much more difficult and costly to produce.
Radii should be as generous as possible to facilitate drawing. Normally, the punch radius and die radius (R1 and R2 in Figure 26) should be a minimum of four times the material thickness.
The part radius (R3 in Figure 26) should be a minimum of six times the material thickness with suitable drawing-quality material. Small parts may require larger radii. Generally, the larger the radius, the easier and less costly it is to draw the shape.
Figure 26. Generous radii are recommended for drawn shapes. For minimum internal radii: R3 = 6T (material thickness); R1 = 4T.
Depending on the height of the drawn parts, corners can be designed to a certain degree of sharpness by successive drawing operations that progressively reduce the corner radii (Figure 26).
Additional Considerations
Burr direction should be indicated on the part drawing. Otherwise, for flat parts, burrs are assumed to be on the "near side" of the drawing. On formed parts burrs are assumed to be on the inside of the form. Some slide forming equipment allows the placement of burrs on either side of the form.
Squareness (angularity). Normal variation on formed 90° bends is usually ±1°.
Feature distortion is more likely to occur when various design feature like holes or slots are too close to an edge, form, or each other.
Die marks are most apparent in drawn parts, which undergo much more deformation than formed ones (Figure 27). Likewise, grippers, if required to hold down a part, may result in significant marking on the part surface. Other types of tooling may also cause marking.
Figure 27. Die marks characteristic of drawing.
Flatness should not be over-specified. Requiring flatness of less than 0.003 in./in. (0.031 mm/cm) may require a secondary operation at added expense.
Slide Forming
Slide forming equipment is used for the production of wire forms, formed stampings and assemblies as shown in Figure 28.
Equipment capabilities range in material sizes from the smallest to 1Ž2" (12.7 mm) diameter wire and strip sizes to 3Ž16" (4.76 mm) thick and 8" (203.2 mm) wide.
Fourslide machine. A machine, either horizontal or vertical, used to fabricate formed metal stampings and wire forms, usually by the action of four forming slides acting upon a stationary mandrel or center tool. See Figure 29.
Multiple slide machine is similar to the four-slide, capable of producing complicated stampings that require features including tapping, coining and assembled contacts. See Figure 29.
Rotary slide machine is a vertical forming machine with the ability to place several forming slides radially around the center tool to produce intricately formed stampings and wire forms. See Figure 29.
Linear forming machine is a vertical forming machine with the ability to place several opposing slides arranged in a linear fashion on both the front and back sides of the tooling area providing the ability to produce very complicated stampings and assemblies. See Figure 29.
Machine Operation
The basic slide forming machine operates as follows: (1) wire or strip stock is pulled off a stock reel by a positive grip feed; (2) the material is advanced through a press section or attachement; (3) a blank is then severed from the strip by the cut-off unit; (4) the metal is bent around the mandrel by the action of a number of cam driven forming tools; (5) a stripping motion positively clears the part from the mandrel.
The relative simplicity of tooling and the economic advantage of the slide forming process can be observed by comparing the fourslide method of production with the progressive die method illustrated in Figure 30.
Advantages of the Slide Forming Process
Figure 31 and Figure 32 illustrate two major advantages of the slide forming process: (1) the bends are perpendicular to the grain direction, resulting in a stronger part; and (2) in many instances there is no need for a carrier, reducing material requirements. Other advantages include the ability to adjust or rework individual slide mounted tools, control of burr direction and the ability to include value added operations such as threading or assembly. Wire as well as strip stock can be formed with equal ease.
Limitations
Limitations of the process include the lack of high press tonnage, and the inability to complete deep drawn shapes.
 
Figure 29. Examples of slide forming equipment.
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Excerpt taken from Design Guidelines for Metal Stampings and Fabrications -- 2nd Edition copyright © 1995 Precision Metalforming Association
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