Consider the four corners of any deep-drawn box: If the sides of the box were removed, the remaining corner radii could be assembled into a cup (Fig. 1). Subsequently, the corners of the box will form in a manner similar to cup drawing. The corners will be compressive on the material moving toward the die radius and tensile on the material that has been drawn over the radius. But material flow into the sides and ends of the box is bending and straightening. This puts the material between the corners-the side walls and flange areas-in tension. Since drawing stresses in the corners are relieved by allowing some excess material to flow sideways into the straight section, larger drawing ratios are achievable for box corners than for true cups of equal diameter.
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