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Deep Drawing from A to Z

By: Stuart Keeler and Pete Ulintz

Tuesday, July 01, 2008
 
 
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Deep drawing of irregularly shaped panels constitutes one of the most complex metalforming operations. However, when broken down into simple components, such as boxes and cups, deep drawing becomes a much simpler operation to understand and troubleshoot.

In most forming operations, stretching or squeezing sheetmetal into a desired shape produces the finished part. This holds true for bending, flanging, extruding, embossing and coining. In contrast, the objective of a deep-drawing operation is to flow sheetmetal into a die cavity to produce the required shape with minimal material stretching and thinning.

Deep Drawing Explained: The Basics

Deep drawing produces a part from a flat blank via the action of a punch force onto the blank. That force pulls (draws) the blank into a die cavity, causing the flange to compress circumferentially. Blankholder restraining force controls radial material flow into the die cavity.

Simple example of deep drawing
Fig. 1
A rectangular box serves as a simple example of deep drawing (Fig. 1). The box contains two opposite modes of deformation. The corner deformation can be visualized as drawing a quarter of a cylindrical cup. The straight walls connecting the corners deform via the bend-and-straighten mode. When forming the box, a large gradient of circumferential compression exists with high compression in the corners but little or none in the sidewalls. In such cases, multiple restraining forces may be required. Because material forming the sides of the box can flow more freely than material forming the corners, metalformers often add draw beads to the blankholder in these areas to further control and direct material flow. Without draw beads, excess material can flow into the sidewalls of the box to create undesirable buckles and waves.

The most predictable deep-draw process, axis-symmetric cup

Drawing of a round blank into a cylindrical cup
Fig. 2
drawing (producing simplified box corners) has the most known design parameters. Fig. 2 illustrates the drawing of a round blank into a cylindrical cup. As the blank is drawn into the die cavity, the remaining material (flange) compresses in the circumferential direction. If not properly controlled, these large in-plane stresses will cause the blank to buckle or wrinkle. The blankholder must provide sufficient force to prevent buckles and wrinkle formation, while still allowing blank material to flow toward the die cavity. Setting blankholder forces too high results in excessive stretching and thinning, causing the material to neck, tear or split.

Proper Draw Ratio Heads Off Problems

An important design parameter in cup drawing is the draw ratio (D/d), the ratio of blank diameter (D) to punch diameter (d). Fig. 2 shows that the punch diameter equals the inside diameter of the completed cup. This relationship can be evaluated using a standard laboratory test requiring a tool set with a fixed-diameter flat-bottomed punch. Blanks of known diameters are positioned in the tool set and drawn into round cups. The blank diameter is increased in small increments until reaching a diameter that fails to produce a fully drawn cup free of necks or splits. Dividing the largest blank diameter by the draw-punch diameter provides the limiting draw ratio (LDR). Assuming a maximum blank diameter of 8.8 in. and a 4-in.-dia. punch, the LDR would equal 2.2. Simply stated, the maximum ratio between blank and punch diameters is 2.2:1.

LDR, though commonly used in metalforming laboratory testing, is not well known to manufacturing or tool and die shops.

Drawing of a cylindrical cup from a square blank
Fig. 3
More common in these industries: a mathematically related expression known as percent-reduction. To calculate maximum percent-reduction for the example above, use the following formula: 100 (D-d)/D = % Reduction, so that 100 (8.8 – 4.0)/8.8 = 54 percent maximum reduction. Therefore, an LDR of 2.2 and a maximum percent-reduction of 54 percent represent the same amount of maximum cup drawability.

The LDR’s effect can be observed in attempts to draw a cylindrical cup from a square blank (Fig. 3). To achieve the previous LDR of 2.2, blank-edge dimensions are set at 2.2d. However, now all four corners have a draw ratio exceeding the LDR by a significant amount and the cup generates a tear at each of the four corners. To reduce the draw ratio at the corners, metalformers may first miter the corners to make an octagonal blank. This offers improvement but may not be sufficient. The best workable solution: Create a circular blank with a radius equal to 2.2d.

The LDR measured in the laboratory is not the absolute value that must be applied in the press shop. Like all forming operations, the product and die design are just two of many variables in the forming process. The LDR can be tweaked by proper design of punch and die radii based on sheetmetal thickness. The recommended punch and die radii as a function of sheet thickness are shown in Figs. 4 and 5, respectively.

determining proper punch radius
Fig. 4
Determining proper die radius
Fig

Redraws Needed for Larger, Narrower Cups

To achieve a good, robust process, die designers do not engineer ratios right up to the LDR, as doing so makes the process susceptible to other variables. For an LDR of 2.2, die design may be limited for production purposes to an approximate draw ratio of 1.9. Achieving a cup with a greater height-to-diameter ratio (Fig. 6) requires one or more redraw dies.

When redrawing operations are required, neither the shell diameter nor height approaches the final part-print size during early draw stages, so there’s no need to use the part-print radii. In fact,using a larger punch radius in the draw operation offers an advantage because the redraw blankholder fits inside of the drawn cup at this radius. This provides for a larger redraw radius on the blankholder, reducing bending and straightening loads for that operation.

 

 
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