Tooling by Design



Buying Metal Stamping Dies--Part 2

By: Peter Ulintz

Friday, February 01, 2008
In advance of any RFQ, tooling suppliers and die manufacturers need clearly stated specifications, technical requirements and acceptance criteria that are understood and agreed upon between the seller (tool and die supplier) and the buyer. It is the responsibility of the buyer to set forth such requirements.

A buyer customarily provides a tool and die supplier with a set of die-design standards on behalf of the buyer’s company. This ensures that the types of materials and methods used to construct the tooling are compatible with the materials and methods used in the press shop. For example, some press shops can weld sheared or chipped die edges while others cannot. The company without welding expertise likely needs to have perishable tooling inserts in its dies. Likewise, the company with extensive welding experience views these items as adding both time and expense to die construction. Furthermore, maintaining an inventory of expendable tooling is an expense the latter company may not wish to undertake. By the same token, one of the companies may choose to use only metric fasteners in its tools while the other doesn’t want to see a metric fastener in its shop—ever. Clearly, there is a need for die specifications and standards.

By definition, a standard is an acknowledged measure of comparison for quantitative or qualitative value; a criterion, according to American Heritage Dictionary. But some die standards are written more like work instructions than standards. They detail everything down to the exact size of screw to use, head type, how many, where to place them and even the name of the manufacturer to purchase them from. Other standards are so vague that they simply specify, “The die section must hold up under production conditions.” Where does one find a quantitative value to measure this requirement against?

Originally, this month’s column was intended to establish some guidelines for tool and die standards. The end goal was to provide something between the detailed work instruction and the ambiguous “must hold up in production” standard. As I was writing this I began thinking, maybe we ought to throw all of our die standards away. That’s right, in the trash, every one of them.

Let me explain. My current job responsibilities involve advance product engineering with a primary focus on developing niche automotive components. When a new product program begins, the customer provides each supplier competing for the new business with a set of engineering performance requirements. These requirements may include specifications similar to the following: The new product must fit in the allocated vehicle space (referred to as packaging). When force X is applied at angle W the resulting deflection must not exceed Y. The normal force required to make the movable portion of the product slide must be less than Z. When the product is subjected to temperatures of 100 C, the force required to make the movable portion slide shall be less than two-times Z at room temperature. When subjected to lifecycle testing, the product must not fail below one million cycles. The new product must use standard bearings from the ABC Company.

This represents the birthing process for an engineered product.

It would not be unusual to receive 10, 20, 30 or more requirements, depending on the product’s complexity. Each supplier uses the requirements to create concepts for its own unique design in preparation to bid on the project. Each supplier evaluates the grade of materials to use based on the applied loads, temperatures and the number of stress cycles. Lubricants are selected based on sliding efforts, temperature conditions and material compatibility. Design trade-offs are evaluated in order to meet the performance requirement (e.g. a larger-sized product functions better, but if it does not fit the space allocation it’s unacceptable). The most competitive bid that meets all of the design requirements wins the business.

Now let’s look at how we buy dies. “Here’s a part print and my die design standards. Quote the die out of D2 tool steel (yes, it’s on page 4 in our standards). Heattreat the die sections to the specifications on page 10. Make sure all of the forming blocks have keys and heel blocks, just like it says on page 25. All of the strippers must have removable windows so that the punches can be removed in the press (page 36, I think). The die must fit press number 100 and you must use AAA brand die lubricant. You’ll find the lube in our standard because it’s the only kind we have in the shop. And don’t forget, if you want my business you have to be the lowest bidder and guarantee me that the die will produce the part to print. Oh, and one last thing, do you think I could get a quote back by tomorrow?”

Similarities and differences between these two scenarios are remarkable.

First, the similarities. Both the automotive component and the die are manufactured products; both have well-written and explicit requirements; both are being competitively bid; both are unique one-of-a-kind designs, even though a similar design may exist for each; each is subjected to stresses and deflections; both have sliding mechanisms; both require high-temperature performance; both require lubricants; both must fit into pre-allocated spaces (vehicle packaging and press bed); and both require the use of standardized products.

But the real story is in the differences. The automotive product is purchased based on meeting a set of performance requirements. The die is purchased based on meeting a set of design standards, totally independent of performance.

You may be thinking that performance requirements like these are unnecessary. You’re confident that your company’s tooling standards, if followed properly, ensure that the required performance is met for any die that is built. Well if that’s true, then the majority of the tools you purchase are over-engineered (and over-priced) because your standards were written to accommodate extreme, worst-case conditions. Does your application really require D2, or would less expensive, more easily welded tool steel suffice? Are you sure that you want to accept the finished die based on making a part to print? Dimensional compliance does not ensure that the manufacturing process is stable. Excessive thinning can be occuring even as the part checks dimensionally correct. When the part splits open in service, will your customer be satisfied that the die produced a part to print?

The problem with most die standards is that they are function-based. That is, the design requirements are based on the function (operation) of the die. There are procedures, materials and heattreatment specifications for draw operations, another set of requirements and specifications for piercing and blanking, and still other requirements for forming, coining and flanging operations.

I often make reference in my columns and seminars that we must replace our traditional die-designing practices with die-engineering practices in order to remain competitive. Engineering is based on science, and mathematics and is driven by data and analysis. The following example illustrates this difference:

An engineered die would have performance requirements similar to the engineered automotive product. Again, there could be 20, 30, 40 or more requirements for any given tool. Here is a small sampling: Tipping moments must be calculated for all tools; the tipping moments shall not exceed X in.-tons. Snapthrough forces (negative tonnage) shall be calculated and compensated for in each tool; the snapthrough forces shall not exceed negative-Y percent. Expended flywheel energy shall not exceed 8 percent per hit on progressive dies and 20 percent per hit with single-hit deep-draw tooling. Tool-steel selection shall be based on achieving the following minimum requirements under the agreed-to operating conditions: 50,000 hits between die sharpening and 150,000 hits between radius polishing on form steels. Pierce-punching pressures must be calculated for each tool; when punch-head pressures exceed 20,000 psi, hardened backing plates must be used. Under no circumstances shall punch-head pressures exceed 40,000 psi. Pierce punch-point pressures must not exceed 60 percent of the punch body’s compressive strength. The final stamping must have a minimum of 2-percent biaxial stretch throughout the panel. Thinning strains must not exceed 20 percent in the critical zone identified on the part drawing.

I refer to theses as “Performance-Based Die Standards.” These requirements truly fit the definition given earlier for the term “standard”: An acknowledged measure of comparison for quantitative or qualitative value. Each requirement can be measured, compared and evaluated. These requirements not only ensure the integrity of the tooling but also the press equipment by addressing tipping moments, snapthrough forces and flywheel energy requirements.

It may not be long before die-performance requirements like these begin emerging. If you want to get a head start, check to see how your current die standards stack up against performance-based die standards. Oh by the way, there’s a trash can in the corner. MF


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