Page 39 - MetalForming May 2009

Product trim line
Draw wall
Addendum
Draw bar
Fig. 2—Addendum features added on beyond the product trim line.
ing the plan view corner radius reduces forming severity at the draw corner because there is a larger area for the mate- rial to be com- pressed. A small plan-view radius reduces the area in which the material can compress. This significantly
ancing cross-sectional lengths of line and providing a constant depth of draw. Designing draw-die addendum is not a simple task and is too complex to address in detail here. However, when addendum is required it is extremely important that it be designed correctly. Since aluminum is expensive and addendum (which means to add on) requires additional trim material that ends up in the scrap container, shortcuts inevitably occur for the sake of materi- al conservation. Avoid this temptation no matter how much pressure is applied by others to keep material consumption within budget. Nothing short of a total redesign can compensate for a draw die with poorly engineered addendum. When a new die with proper addendum design is built, the added material cost will pale in comparison to the die main- tenance, press downtime and expedited shipping costs incurred with alternative designs that supposedly save money on material costs. MF
using programmable hydraulic pressure systems or other recently developed variable-blankholder technologies.
As mentioned, the corners of a box form in a manner similar to cup draw- ing. This includes material thickening found in the vertical cup wall and the flange remaining on the blankholder. Allowances for thickening must be pro- vided in the draw corners by machining additional punch-to-die clearance in the vertical wall and spotting addition- al in the blankholder (Fig. 1).
Spotting consists of locally grind- ing specific areas on the blankholder to account for nonuniform thickening in the corner flange area to help improve material flow into the cavity. This is usually done by trial and error. Spotting can sometimes be avoided when draw- ing low-carbon steel or other highly formable materials by using gap blocks or standoffs on the blankholder. How- ever, spotting is particularly important when working with aluminum alloys because of their tendency to wrinkle and buckle. Spotting increases the con- tact area and force distribution across the entire blank surface.
Product Geometry
Aluminum-sheet forming depends highly on the part design, including radii, draw depths, wall angles, steps and transitions. These variables interact to affect the cost and quality of the alu- minum stamping. The size of the plan- view corner radius in a box, for instance, has a strong influence on the maximum depth of draw. As a general rule, increas-
increases forming severity in the corner, thereby reducing the maximum depth of draw.
Increasing the length-to-width ratio of a rectangular box also will decrease local forming severity. The increase in aspect ratio allows the blank to draw in faster on the long side as compared to the corner. The difference in material flow rates allows material from the cor- ner to flow laterally into the side wall; thus, providing a corner-assist effect.
Part radii specifications should be selected carefully. Small radii can restrict material flow, localize strains and sub- stantially increase drawing loads. Large die radii reduce bending and straight- ening forces, thus easing the draw. How- ever, overly large radii will decrease the blank contact area between the die radius and the punch face, increasing the ten- dency to form wrinkles in this unsup- ported region. As a general rule, punch and die radii for aluminum will be in the range of eight to 10 times material thickness on the punch and five to 10 times material thickness for the die. Keep punch radii equal to or greater than the die radius to reduce the likeli- hood of the punch breaking through the material.
Difficult and irregularly shaped product geometries often require the use of addendum to improve material flow. Commonly used addendum features include draw walls, draw bars, material gainers, take-up beads, pillows and punch extensions. Addendum features are added beyond the product trim line (Fig. 2) to assist material flow by bal-
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