Page 56 - MetalForming September 2012

Tooling by Design By Peter Ulintz
Progressive-Die Strip Evaluation
Tooling Technology
The most important step in designing a progressive stamping die is developing the strip lay- out. Strip layouts directly impact die size and initial die cost, as well as costs for die maintenance and repair. The strip layout affects press selection and the costs for press maintenance and repair, as well as initial stamping cost, in-process reliability, dimensional accuracy of the finished part and the cost of poor quality.
The strip layout serves as a master plan that determines—and restricts— nearly every decision made during die design. The main steps to design a strip layout:
1) Input data, typically consisting of a 3D model of the finished product. 2) Flatten the blank through use of unfolding software or metalforming-
simulation codes.
3) Nest the blank, to optimize mate-
rial consumption and establish carrier location(s), coil width, pitch distance (progression) and part orientation in the strip.
4) Plan the sequence to determine the operations conducted at each sta-
Peter Ulintz has worked in the metal stamping and tool and die indus- tries since 1978. He has been employed with the Anchor Manufacturing Group in Cleveland, OH, since 1989. His back- ground includes tool and die making, tool engi- neering, process engi-
neering, engineering management and product development. He is vice-president of the North American Deep Drawing Research Group. Peter speaks regularly at PMA seminars and confer- ences and maintains the website, www.Toolingby- Design.com. The site serves as a web-based source for the transfer of modern metalforming technology and the advancement of “Performance- Based Die Engineering Strategies.”
Peter Ulintz pete.ulintz@toolingbydesign.com www.toolingbydesign.com
“Solutions without data are nothing more than guesses.”
tion, including idle stations.
For any given part design, numer-
ous variations of strip layouts can be proposed. The final layout largely depends on the designer’s personal experience or opinion, and the cus- tomary practices of the press shop. Often a team of experienced and skilled individuals from various engi- neering and manufacturing disciplines will work together to make the deter- mination.
Don’t Guess; Gather Data
A friend of mine frequently says, “Solutions without data are nothing more than guesses.” That same phi- losophy can be applied to strip lay- outs: Selecting a progressive-die process based on a consensus of opin- ions is nothing more than a group’s best guess.
Financial business decisions often rely on multiple sources of data, such as financial statements, financial ratios, and forecasting and investment analy- sis. But mission-critical technical deci- sions such as the selection of a strip lay- out, which can affect a business’s bottom line for years, often are based on a collection of opinions.
At a recent PMA METALFORM sym- posium on advanced high-strength steels (AHHS), I presented a talk titled, Performance-Based Die-Engineering Strategies. These strategies rely on knowledge-based systems rooted in science and mathematics, rather than on traditional experience-based sys- tems that rely on individual and group experiences. A knowledge-based sys-
tem is particularly important for pro- cessing AHSS stampings, since experi- ence-based systems either are nonex- istent or woefully insufficient.
Another shortcoming of experience- based systems: They are dominated by fears, especially the fear of repeating past mistakes and the fear of the unknown. These fears work their way into our strip layouts and compromise the die design.
Let’s consider how performance- based engineering strategies can be applied to progressive-die strip lay- outs. In order to select the best strip lay- out from the several available options, the designer should compare and rank each layout using a relevant scoring system. Among the factors that influ- ence die cost and quality, four have been proposed as most critical (by Lin and Sheu, authors of Knowledge-based Sequence Planning of Shearing Oper- ations in Progressive Dies, Interna- tional Journal of Production Research, 2010). They are:
• Station number factor, Fn
• Moment balancing factor, Fb
• Strip stability factor, Fs
• Feed height factor, Fh
An evaluation score (Ev) then can be
computed using these four factors and their corresponding weighting fac- tors—wn, wb, ws, wl:
Ev =(wn xFn)+(wb xFb)+(ws xFs)+ (wh x Fh)
The designer or process engineer selects the four weighting factors (from 10 to 100) based on how much each factor contributes to the strip evalua- tion. A higher score indicates better efficiency in cost and production.
For example, presses for progres- sive dies usually require large beds to accommodate multiple worksta- tions. More stations require a wider press bed, and a longer die results in
54 MetalForming/September 2012
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