DIEVESTIGATION: Designing stamped parts
How material selection affects downstream operations and costs
In his first installment of DIEVESTIGATION, a new column about using research and data to solve stamping problems, tool and die expert Art Hedrick discusses how the metal specified during part design affects downstream operations and costs. What should you consider when selecting the metal?
When you design a part, do you consider how the design affects manufacturing costs? All too often stamped parts are designed to have certain features and tolerances with little consideration given to the effect these features have on the manufacturing process and costs. To optimize cost savings, you must take into account how the design interacts with and affects downstream processes. The failure or success of a stamping operation begins with a good, cost-effective part design.
Metal and Downstream Operations
Among the factors that affect downstream operations and influence cost are the type and thickness of the metal you specify to make the part. The selected material has a direct impact on several aspects of the manufacturing process. So what should you consider before making your decision?
1. The metal's ability to make and shape the part—Just because a part is designed to be made from a certain material, there is absolutely no guarantee that it can be done. Some product designs I have seen made my head spin just looking at them. One example is deep-drawn shapes designed to be made from special armor plating having yield strengths above 200,000 PSI. Another example is high-tensile, hard stainless steel parts 8 in. in diameter and 0.020 in. thick with a flatness tolerance of 0.003 in. TIR. These are just a couple of instances of specifying materials without fully considering the manufacturing process. I'm not saying that these geometries can't be made from these materials, but they most likely can't be made using a conventional sheet metal stamping process.
2. The consistency of the part geometry—Metals with great tolerances within a given specification behave differently when subjected to the various metal cutting and forming processes. The result is inconsistent part geometry.
I find it very interesting that parts frequently are designed with tolerances that are tighter than the metal is held to. In other words, stampers often are expected to create a consistently shaped part using a process in which many of the input variables are inconsistent. I was once taught that whatever tolerance was expected from the end product, the process that makes it must have a tolerance that is 10 times less than the end product.
3. The press type and tonnage needed—Higher-strength materials, such as advanced high-strength and stainless steel, require greater forming and cutting forces. In addition, certain materials, such as stainless steel, are best-suited to slower forming and drawing speeds. Parts with a small hole-to-thickness ratio require a rigid press with very little ram-to-bolster deflection.
4. The type of tool steel needed—Materials such as high-strength and martensitic grades of steel require the die to be made from premium tool steel grades like powdered metals and solid carbide. These premium tool steel grades not only cost more, but require more machining time to produce. Also, certain die components may need to be coated. This also inflates the tooling cost and increases the time necessary to complete the tooling.
5. Tool steel geometry—Parts designed from heavy metal gauges require larger, stronger sections of tool steel. It also may be necessary to key in all tool steel sections to reduce the deflection during cutting and forming the metal. Additional heels often are needed in the tools used to process these heavier materials.
6. The number of stations required to make the part—Certain metal types and thicknesses may require more stations than others. For example, designing a formed part out of high-strength steel may require one or more re qualifying stations or re strike operations.
7. Additional special operations—When using some grades of superalloys, such as INCONEL®, you may need an additional annealing operation between forming stations. This increases manufacturing costs and slows down production. This is why aircraft parts are so expensive.
8. The lubricant needed to form and cut the metal—How does the metal type that the part is made from affect the lubricant used in the tooling? It's simple; higher-strength metals generate more friction than their low-carbon counterparts. As a result, they require lubricants that do not break down or "burn off" during the metal cutting or forming process.
High-strength steel grades often necessitate adding extreme-pressure additives such as sulfur to the lubricant. These additives not only increase lubricant cost, but they often create welding problems during the assembly process. In addition, using bright stainless steel requires the lubricant to have special wetting agents to help it stick to the metal.
9. Additional material needed to make the part—Parts made from high-strength steel, such as dual- and triple-phase steel, often have more springback problems than low-carbon steel. To help reduce the amount of springback, it frequently is necessary to increase the strain or stretch levels in the part. Achieving this often calls for the part geometry to be drawn or stretched into the finished geometry, which may require excess material on the part's perimeter so that it can be stretched over a punch. This extra material later is trimmed away and discarded as scrap.
Making the part from thicker-gauge low-carbon steel can reduce the need for stretching and drawing dies, but keep in mind that the finished part will now weigh more. Also keep in mind that metal is purchased by the pound or ton. Parts made from thicker metals will cost more.
As you can clearly see, the metal used to make the part has a significant effect on the tooling and manufacturing costs. Next time you're choosing a metal for your part design, consider how many factors your decision affects. It all begins with the product designer. A product designer's decisions can make or break a metal stamping process.
In the next installment in this series we will examine part features, determine their true cost, and discover less expensive alternatives for certain features.