March 14, 2002
Learning how to get every bit of flow out of your metal while minimizing scrap -- doesn't that sound like a good idea?
Global competition has forced many U.S. stampers to use all means possible to cut costs in their stamping operations. Consumption of costly items, such as lubricants, has been greatly reduced and occasionally eliminated. Special link-drive presses commonly are used now to increase strokes per minute (SPM), resulting in increased part counts without sacrificing the benefits of slower drawing speeds.
Among all of the cost-saving techniques you could employ, maximizing material usage has the greatest impact on final product cost and profit. Most basic types of steel used in drawing operations cost from 20 to 30 cents per pound when purchased in coil form. Scrap goes for about 2 to 3 cents per pound, and the cost of handling it often exceeds its resale value.
This article discusses several ways you can reduce material consumption and scrap in a drawing operation.
Draw beads restrict and control metal flowing into the draw die cavity and over the draw punch by forcing it to bend and unbend before it enters the cavity. They also are used to help obtain stretch in your final product and reduce or eliminate wrinkling, buckling, and oil canning.
Excess material outside the draw punch acts in similar ways to a draw bead. In other words, the more material that is outside the draw punch, the greater the resistance will be to flow into the draw die cavity. This basic principle of the relationship between the blank and the draw punch is called the draw ratio. If this additional material is not part of the final product, it often can be reduced by adding a draw bead in the same area. If we simply reduce the blank's size in this area and don't add a draw bead, wrinkling or buckling in the final part may result. A good rule of thumb is to use the smallest blank possible and avoid restricting the metal flow by adding material. Figure 1 shows this basic principle.
Offset (Step) Beads. Using offset, or step, beads often helps to reduce material consumption in drawing operations.
A step bead works on the same principle as a conventional half-round bead. It forces the metal to bend and unbend before the metal enters the draw die cavity. The main advantage to using step beads is that they help to reduce material consumption.
Conventional half-round beads must be placed farther away from the draw cavity than step beads. This is because the draw bead portion gets extremely thin and weak if it is placed close to the draw punch cavity. This weak area may break during production and result in extensive die damage. To prevent metal from pulling off the draw beads during the drawing process, more metal must be used to obtain the finished part geometry.
Step beads are placed at the edge of the binder opening and can be placed close to the draw cavity without sacrificing strength. This allows you to reduce the amount of material you use (see Figure 2).
Consider lancing in between parts to create blanks instead of actually piercing out material between parts. This allows you to reduce material consumption, as well as reduce the progression amount.
A word of caution: When lancing in an operation that is followed by drawing, you should avoid pushing the lance back together. Doing so most likely will produce a sliver, which can create a lot of problems in a drawing operation. Instead, clear the face of the blank holder and the die face so that the lance is not pushed back together. Also, be careful not to relieve it in functioning areas (see Figure 3). This could upset the metal flow and cause unnecessary wrinkling.
When you design your draw dies, and whenever possible, nest right-hand and left-hand parts together to form a single drawn part. The parts can be separated in a later operation. This reduces material usage enormously. It also helps balance the forces in the drawing operation (see Figure 4).
Keep some of the finished product on the blank holder whenever possible. This also allows you to reduce the amount of material you consume and may reduce the forming severity (see Figure 5). In other words, if you can design your die to keep some of the finished part geometry on the blank holder, you will need less material to create the finished part.
Whenever possible, contour the binder, or blank holder, to the same basic shape as the finished part. This offers you two basic advantages: reduced material consumption and a more uniform punch contact area (see Figure 6). By using a more uniform contact area, you reduce the forming severity of the deepest part of the draw and get a better looking part, as well.
When forming cylinder cups with flanges or cups that need a flat base and have a reasonable amount of trim line tolerance, you should consider using a material with an isotropic grain pattern, such as interstitial-free.
Interstitial-free, otherwise known as nonearing material, can be drawn in a cylinder drawing operation without producing the ears, or scallops, that conventional materials produce. This allows you to produce cups (with a reasonable trim line tolerance) from a smaller blank and without an additional trimming operation (see Figure 7).
Saving even a little material on a single part can result in massive savings, especially if the part volumes are high. Take the time to carefully review your die designs and production methods when you're attempting to save material, and remember: The little bit of material that you saved may be the difference between you and the competitor right behind you.