Page 38 - MetalForming September 2011

The Science of Forming
By Stuart Keeler
Do We Need Formability Engineers?
Tooling Technology
Metalforming is becoming more complex as new technology provides more options. Once, high- strength steel commonly had 40 to 80 ksi yield strength. Today, stampers deal with steel with 150 to 180 ksi yield strength. Springback (proportional to yield strength) has doubled or tripled at the same time that customers demand greater dimensional consistency and multiple stampings are being combined and formed as one piece. Many areas of stampings now are deprived of material flow from the blank and must be formed by the more restrictive stretching mode. We also see growing use of barrier (dry) lubricants, which can reduce the traditional coefficient of friction of liquid lubricants by a factor of four and change deformation modes to create stampings with less thin-out and more safety margin.
The list of improvements (and restrictions) is extensive. The accompanying schematic shows the cycle from stamp- ing design to successful production. Each stage of the cycle requires specialists (and even experts). The critical question: Who in the shop understands the interaction among the stages, to explain to the structural engineers, for example, why their required change in a bend radius will require a more expensive type of steel or a compensating design change else- where in the stamping?
Consider this example—a stamped part experiencing sporadic splits. Troubleshooting the problem requires several different procedures to gather the required data—from the process and the stamped part—in order to determine exact- ly what is happening and, more importantly, why it is hap- pening. Among the typical procedures and tools used: Circle grids, forming-limit diagram, ultrasonic thickness gauges, laser thermo guns, statistical data analysis, meaningful data plotting and logical solution presentation, each of which requires a unique skill set for effective troubleshooting.
Difficult problems rarely result from a single input variable. One needs to examine the effect of two or more interacting variables. Considering the average stamping has some 40 or
Stuart Keeler (Keeler Technologies LLC) is best known worldwide for his discovery of forming limit diagrams, development of circle grid analysis and implementa- tion of other press shop analysis tools. Stuart’s sheet- metal forming experience includes 24 years at National Steel Corporation and 12 years at The Budd Company Technical Center, enabling him to bring a very diverse background to this column and the many seminars he teaches for PMA.
Keeler Technologies LLC
P.O. Box 283 | Grosse Ile, MI 48138 Fax: 734/671-2271 keeltech@comcast.net
more input variables, the problem becomes quite challeng- ing. Perhaps the most important metalforming tool devel- oped to understand these interactions among input variables is the virtual press shop—computerized die tryout, com- puterized process control, etc. This allows one to study how stampings will react at each forming stage before cutting the first die. What-if scenarios can quickly be tried, and some soft- ware modules will even design die components or flag parameters that will lead to breakage.
While a virtual press shop program can be run by an FEA specialist, metalformers must train people to establish the process parameters and interpret the analysis output—a formability engineer (FE). This person should have extensive training in metalforming, and an engineering degree provides an excellent background. However, the practical training should be done by experienced industrial personnel using the language of the press shop. Courses must address die and press components, forming processes, lubricants, stamping design procedures, material properties, troubleshooting, CAD, etc.
A very important aspect of training is to not only under- stand how things work but why they work. The learning depth for any one subject need not be extremely deep. The understanding must be sufficient for the FE to connect the dots among all stages in the schematic. Knowledge of addi- tional reference material will allow solving more specialized problems.
Illustrative Job Functions of the FE
• Coordination of formability parameters. Say the struc- tural engineer increases the minimum yield-strength spec- ification for a steel stamping from 40 to 60 ksi. The increased minimum yield strength reduces the maximum expected n-value (work-hardening exponent) for the steel. The stamp- ing will have less allowable stretch and sharper localization of strain gradients. In this case, the FE obtains production data on the current severity of the stamping and determines the impact of reduced stretchability. After creating a list of possible solutions, the FE calls a meeting of affected parties— typically the stamping designer, die designer, production engineer and, hopefully, the end user. Meetings continue until consensus and final buy-in are achieved. The FE continues to track stamping severity as modifications are made and pro- duction resumes.
• Training of inhouse personnel. The FE keeps up to date on new alloys, lubricants, heattreating technology, trou- bleshooting procedures and other advances. After translat- ing information into an appropriate format for a specific audi-
36 MetalForming/September 2011
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