Steps 3 and 4
May 8, 2007
Determining the best die geometry to produce multicontoured formed parts can be difficult. A full understanding of the drawing and stretching process is necessary, as well as a good understanding of all tooling factors to make complex geometries.
Editor's Note: This is first installment of a three-part series that discusses die development for producing nonuniform, contoured parts by breaking the process down into eight steps. Part I covers the part material, its form, and its function. Part II discusses length-of-line analysis and tip angle. Part III discusses unfolding the part, blank holder shape, addendum features, and virtual die tryout.
The previous column discussed designing dies for contoured parts from critical data, provided by finite element analysis. It also walked you though the initial steps you need to take to develop such a die design — Step 1: Determine Metal Type and Grade, and Step 2: Study the Part. Now we move on to Steps 3 and 4.
In an ideal drawing die, regardless of part geometry, the same amount of metal will flow inward from the blank holder around the entire draw punch perimeter. Keep in mind that while this is ideal, it's extremely difficult and usually results in a great deal of wasted material.
A length-of-line analysis is a process of measuring the linear distance through different part areas. It's often performed using tape, string, or a rotary map measuring tool. Determining the linear distance from area to area reveals two important things. First, it shows how much metal will be consumed from one part area to another. This data can be recorded and used later to help develop a die addendum. Second, it often reveals the need for more than one forming operation.
For example, a typical oil pan usually is made using two or more forming operations because of the severe difference in part depth from one area to another (see Figure 1). Another reason is that part depth differences are right next to each other. Severe changes in draw depth over a small distance often require more that one operation.
To successfully make the deep portion of the oil pan, a great deal of metal must flow inward. Unfortunately, because the shallow area of the pan is right next to the deep end, it also will receive nearly the same amount of metal, resulting in severe wrinkling or double metal in the transition area between the deep and shallow portions.
If a metal restrictor such as a draw bead is used to reduce wrinkling, it also reduces flow to the deep area. The result is a split next to a wrinkle. This is an extremely difficult geometry to make in a single conventional forming die. The best way to solve these problems is to partially form the deep portion so it's flush with the top side of the shallow portion. If the punch is completely surrounded with the blank holder, metal isn't allowed to wrinkle. The second drawing die forms the remainder of the depth.
Knowledge and experience in metal flow, draw ratio theory, and stretch distribution are helpful for this step. To aid in this process, I use part models. A part model gives the process engineer a true feel for the product in its entirety. Creating a part model also may reveal features you didn't know existed. It's hard to get a good feel for part geometry from a 2-D part print.
This model can be an early prototype or a plastic foam, wood, or plastic model made by stereolithography. If you have 3-D software, a physical model may not be necessary. Try to tip the part to maximize punch contact with the blank while designing the majority of its features. Look for negative angles and how rotating the part to make these features affects the punch contact surface areas.
To reduce part forming severity and to maximize punch contact, it may be necessary to obtain its negative features by rotating the part in a secondary operation or using cam slides. For parts that have Class A surface requirements, look for features that must remain "neutral" on the product. These product features often are called character or feature lines. A classic example of this type of feature is the crisp, clean depression along an automotive door (see Figure 2).
Feature lines must go into the part near the bottom of the press stroke while there is no movement of material on the forming punch. To achieve this, the addendum must be balanced on both sides of the feature line. In other words, the amount of restraining force created with the blank holder and items such as draw beads must be balanced with respect to the centerline. If the force is greater on one side than the other, the metal may move on the punch while the feature line is being formed. This will result in a surface defect.
Keep in mind, this does not mean that the feature line must remain in the center of the forming punch. However, the forces created during forming must be balanced. In addition, look for critical features that must have a Class A surface. These features must be protected by not allowing a great deal of metal to flow over the feature. Also consider how much metal will be wasted. Try also to tip the part so it can be direct-trimmed. Cams, although often necessary, add a great deal of cost to the tool and require more maintenance.
For some parts, the amount of waste generated to create a nice-looking part may be more costly than using less material and accepting a less visually appealing part. A classic example of these visual sacrifices can be seen on some inner structural automotive parts.
Although I am attempting to describe the process of developing a forming die in a step-by-step method, each of the processing steps is interactive. Changes in each step may affect a judgment made in a different step. A good process engineer looks at all of the steps and understands this interactivity. In the next issue, I will begin discussing the process of unfolding the part and developing the blank holder shape.
Until next time ... Best of luck!