Tooling by Design


 

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The Problem with Die Designs

By: Peter Ulintz

Saturday, November 01, 2008
 
An idiom is a term or phrase whose meaning cannot be construed from the literal definition of the words, but has instead a figurative meaning known only through common use. An example is the old English idiom, “There is more than one way to skin a cat.” This figure of speech has been used for decades to describe die design and building methods. It asserts that for any given stamping there are numerous ways to design and build a die to produce that particular part. It also suggests that any design that ultimately “skins the cat” is an acceptable design.

While having merit in the figurative sense, the idiom does not consider the complex interactions of 30 to 50 input variables that influence the typical stamping process. It also ignores the fact that there may be a limited number of design solutions capable of managing these process inputs and perhaps only one of the design options is anywhere near optimal.

In my opinion, there are two significant weaknesses in the way dies are designed today, the first being the fact that they are indeed “designs.”

In the most general sense, a design is imagined, conceived or fashioned in one’s mind. It is creative in nature and often is based on past experience and in many instances fear will play a large role in the final design. The fear of repeating a past failure or bad experience often results in over-design of future dies. Exsentially, a design is someone’s experience-based opinion.

People who create, invent or design have deep, personal and emotional attachments to their creations. If you have ever been critical of someone’s die design or tried to make changes during a design review meeting, you know first-hand how deep the personal attachments can be. When problems arise they generally are dealt with emotionally.

The designing of stamping dies must be replaced by the engineering of dies. There is a big difference. Engineering is based on science and mathematics. The fundamental principles of science and engineering apply across a broad spectrum of problems, not just the ones we have experience with. Critical evaluations and changes are based on sound analysis and data rather than opinions and emotions.

The second problem with die designs is that they are based primarily on die function with little or no regard to die performance.

Every company that manufactures or buys stamping dies has (or should have) a set of die design and build specifications. These documents outline how a tool should be designed, the construction methods to be used, tool-steel preferences, heattreatment and surface coatings desired, component-mounting methods, pressure-system choices, set-block and parallel placements, and numerous other requirements that are based on the die’s function. A good example would be tool-steel selection and heattreatment. Die materials (tool steel) and heattreat specifications will differ among draw dies, trimming dies, flanging dies and perforating dies. In progressive dies, the requirements can vary station to station depending on the operational function at each step.

Here’s the problem: If tools are built to the exact same tooling specifications, why do they perform differently? Some draw tools in the press shop run 100,000 parts before maintenance is required due to die wear, while others struggle to survive through a single production run of 20,000 parts. Why? If the dies are built to the exact same standards, shouldn’t they perform the same?

Here is the answer: Design requirements for function-based die processes consider operational function of the die and ignore die performance. It is generally assumed that current design and build standards, having been updated on a regular basis over the years, will account for most every problem ever encountered and, therefore, if the standards are followed, the die will perform as desired. This approach is seriously flawed because die performance is an assumed result rather than an engineered objective.

Here is a better solution: Replace function-based die designs with performance-based die-engineering strategies. Performance-based die-engineering strategies result in more robust tooling processes and economical stamping methods, and help to minimize or eliminate inconsistent and unpredictable results. Designs are based on engineering performance criteria not unlike the criteria used in automotive product design. For example, if a hinge were needed to simply hang an automotive door on a vehicle, one could come up with a great number of ways to skin that cat. But the automaker specifies performance criteria such as the loads the hinge must support, its fatigue strength, lifecycle performance, crash performance, weight limitations, cost constraints and space allocations, to name just a few. Soon, the number of hinge designs that meet the engineering requirements narrows considerably. In fact, there may be only one design that provides optimal performance, long life and reliable service at an economical cost; the same desirable attributes of any stamping process.

Want to learn more? I am presenting a PMA webinar (an online seminar) on December 9 at 2 p.m. EST titled, “Performance-Based Die Engineering Strategies.” For more information or to register for the webinar, visit www.pma.org/interview/register.tass. MF

 


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