Art Hedrick Art Hedrick
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Avoiding the Seven Deadly Sins of Stamping: Part 1

January 19, 2024
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Ignoring Material Type, Grade, Thickness

I would like to thank the leaders and staff of the Precision Metalforming Association as well as the editors and staff of MetalForming magazine for inviting me to author the Science of Forming column and to honor its previous author of several years, Dr. Stuart Keeler. I was blessed to have the opportunity not only to learn from Dr. Keeler but to perform a few consulting jobs with him. His research, including the development of forming limit curves, changed the way we understand how sheet metal is formed, even today. Mr. Keeler was—and remains—an icon in the world of metal forming. His knowledge, ability to mentor others and smiling face are sorely missed. As for me, filling the shoes of a man who was once described as an individual who “forgot more about sheet metal than most people know” is both intimidating and flattering. In any case, I will do my best to live up to the standard established by Dr. Keeler.  

When I first started getting involved in training and consulting, my mentor, Mr. Edwin Stouten, used to tell me, “Prior to troubleshooting, designing, processing or consulting on a metal stamping process, it is essential that you answer these two questions: What is the material type and grade that the part is to be made from? What is the material’s thickness?” Knowing these two answers sets the stage for every decision you will make. This includes determining if the part actually can be made, the process steps, the die-design parameters and numerous other factors. 

Making decisions without this pertinent information certainly is a high-risk maneuver. In my career as a consultant, I have witnessed dies be completely designed and built, only for the builders to find out that they are trying to defy the physical limits of the material. It is only after this expensive failure that they spend some time researching the material parameters. This, of course, typically ends up being a dollar short and a day late.

Each year I conduct numerous technical die troubleshooting and maintenance courses for various companies. When I begin to discuss the topic of understanding materials, many of the tool and die makers and technicians tell me that there’s no need to discuss the material because they have no influence on which material is purchased. They typically tell me that they must make whatever comes in the door work.

Keep in mind that although there may be an element of truth to this statement, to do a good, data-based troubleshooting process means that, minimally, we must understand the fundamental behavior differences among material types. It is only through understanding these behavioral differences that we can make good data-based troubleshooting decisions.

For example, troubleshooting a part stamped from aluminum likely will be different than stamping the part from steel. Even though tool and die makers and technicians may have no direct influence on the material purchased, it is important to understand the behavioral differences. Each material to be formed has different behaviors and often requires different methods for troubleshooting and resolving cutting and forming problems.

Material Type, Grade, Thickness Affects…

  1. If the part actually can be made. Keep in mind that just because a part has been designed from a specific type of material and thickness, it is no guarantee that the material has the physical capabilities of taking the desired shape. When sheet metal meets physics, physics always wins. For example, one of my clients attempted to form a closed flat hem on a part using spring steel. Every time he attempted to create the hem, it would break off. Aside from doing something extraneous, such as heating the material, either the product design or the material needed to be changed.
  2. The tolerancing achievable with respect to the product. Some materials present very challenging springback variability and others have quite a substantial thickness variation. These present consistency issues in the metal stamping process. 
  3. The number of steps necessary to stamp the part as well as the shape and geometry of each die station (assuming that the part can be made).
  4. The amount of force needed to cut and form the sheet material. The part geometry, including its size, also is a determining factor. The amount of force indirectly determines the press type and tonnage needed, as well as the accuracy needed from the press. Generally speaking, very thin, light-gauge materials with small part geometries run in high-precision, high-speed presses. Conversely, in some cases, thick, large parts can be formed in less precise, high-tonnage presses. 
  5. The tooling-material type used to build the die. For example, premium grades of powdered-metallurgy tool steels or solid carbide often are used to cut and form high-strength materials such as stainless steel, while conventional grades such as D2 and A2 often are used to form low-strength materials. 
  6. The size of the die sections and how they are secured to the die. For example, if cutting and forming ¼-in.-thick, low-carbon steel, it may be necessary to key all of the die sections to prevent them from shifting on the tool surface.  
  7. The die length, because the material type and thickness influence the number of stations needed.
  8. If the tooling sections must be polished and coated with special coatings such as titanium carbide and other durable, low-friction coatings. 
  9. Die parameters such as the cutting clearances, forming-radii sizes, holding- and stripping-pressure requirements.
  10. The type, chemistry and application methods of lubricants used to facilitate forming. Many ultra-high-strength materials require special additives in the lubricants to prevent die damage and wear. Often, these special additives must be removed using solvents, which adds a secondary post-stamping process.

Summary

Understanding the material type and thickness has a huge influence on many different factors. Trying to design, process or troubleshoot a die without at least basic data presents a significant risk. I’m certainly not saying that everyone must be an expert metallurgist, but we should, at least, have a basic understanding of different material types and their different behaviors. It is only through understanding that we can make solid data-based decisions. Data and understanding are the keys. 

Until next time… Best of luck! MF

Industry-Related Terms: Case, Die, Form, Forming, Run, Stainless Steel, Surface, Thickness
View Glossary of Metalforming Terms

 

See also: Dieology

Technologies: Materials

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