April 10, 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.
Determining the best die geometry to produce multicontoured formed parts can be difficult. Developing die geometry for a formed part with simple axial symmetry is easy compared with die geometry for a nonuniform contoured part.
Examples of difficult part geometries are automotive underbodies, contoured body panels, oil pans, and frame rails. Most tall contoured parts require an abundance of stretch and flow to obtain the part geometry. A full understanding of the drawing and stretching process is necessary to produce these parts. In addition, it's important that the process engineer has a good understanding of all tooling factors required to make such complex geometries.
Most tool and die shops produce difficult contoured part shapes with a forming simulation, such as finite element analysis (FEA). FEA provides some of the necessary data necessary to make die geometry changes.
However, keep in mind that FEA provides results for only the process and geometry that were programmed into it. Forming simulation software does not design the die for you; it only provides the results of what you have already designed. It's up to the process engineer and die designer to determine the starting die geometry. Once this geometry is established, it now can be tested using FEA.
I will take you step by step through the development process. However, each step does not have to take place in the order shown.
A die can't be designed effectively without this critical data. I have been in situations where my client asked me how to make a part but the only data he could give me was that it was made of steel.
What kind of steel? How thick is it? His answer? "We'll figure that out later. Right now I just want to know if it can be made." I told him I couldn't make a decision without more data on the steel. Thousands of material types are being stamped today, and understanding your metal's behavior when it's stretched and drawn is critical for success.
Some metals require annealing after forming, while others may be formed multiple times without annealing. Many metals stretch well, while others stretch very little. Some metals are hard and abrasive, and others are soft and ductile. Various metal types exhibit a lot of springback, while others have very little. And some metals may not be able to take a certain shape in a conventional stamping process, so they may require a different process, such as casting or forging.
Each metal has its own behavior, and the die geometry should be created with respect to this behavior. If you don't know the metal type or behavior, find out before you attempt to develop a die geometry and process. Basic data you need to know are metal type, thickness, elongation, chemistry, and hardness.
Most of this data can be found in a metallurgical handbook or online. Also, keep in mind that the metal being formed also is going to affect your judgment with respect to the tool steel or die materials necessary to form the part. Don't attempt to develop a die without this data. Figure 1 shows a variety of materials formed into difficult contoured geometries. Each metal can affect the die geometry.
Take the time to fully analyze a part's shape to understand its function. If the part is a section of a larger assembly, analyze how it attaches. Look for areas that can be changed to make it easier to manufacture but won't affect its function.
In the automotive industry, the sooner you can present a product concession, the better chance you have of getting that particular product change. As time goes by, you'll find that getting an engineering change is comparable to passing an act of Congress. This is because most of the performance data already has been collected and passed. Key tests, such as a crash test, may require repeating if a part is altered.
Work with your customer on the process. If the customer has made similar parts in the past, it may be able to assist you with creating the die geometry and process. This collaboration can save time, frustration, and money.
Tolerances. Small differences in tolerance will affect the blank size, number of operations needed, and the die geometry. For example, for a 15-inch by 17-in., noncritical contoured part, the general form tolerance is ±2 mm (profile of surface) and tolerance on the trim line is ±4 mm. A great deal of tolerance, depending on part geometry, may allow you to produce the part from a fully developed blank.
In other words, the starting blank's profile will represent the finished part profile. However, if the tolerance is reduced to ±0.5 mm and the trim tolerance to ±1 mm, the part must be trimmed after drawing and most likely will require a second form die to achieve the necessary tolerances.
Flatness. Part flatness is one of the most difficult geometric features to obtain in a conventional stamping process. Other tolerances to consider are parallelism, cylindricity, concentricity, angularity, straightness, and perpendicularity. Also look for items such as burr direction and burnish or shear lengths.
In the next issue we will continue examining tolerances and how they affect the forming process. In later issues I'll address the development of items such as binder shapes, addendum, and blanks. Length of line analysis, draw bead and bar placement, and many other elements also will be discussed.
Until next time ... Best of luck!
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