The Science of Forming


 

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Do We Need Formability Engineers?

By: Stuart Keeler

Thursday, September 01, 2011
 

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 stamping 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 elsewhere 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 exactly what is happening and, more importantly, why it is happening. 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 more input variables, the problem becomes quite challenging. Perhaps the most important metalforming tool developed to understand these interactions among input variables is the virtual press shop—computerized die tryout, computerized 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 software modules will even design die components or flag parameters that will lead to breakage.

Schematic showing work units and flow of information needed to form a stamping.
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 understand 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 additional reference material will allow solving more specialized problems.

Illustrative Job Functions of the FE

• Coordination of formability parameters. Say the structural engineer increases the minimum yield-strength specification 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 stamping 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 production resumes.

• Training of inhouse personnel. The FE keeps up to date on new alloys, lubricants, heattreating technology, troubleshooting procedures and other advances. After translating information into an appropriate format for a specific audience, the FE provides training sessions, modifies internal documents, creates e-newsletters and uses other media to keep the metalformer on the leading edge of metalforming knowledge.

• The FE manages a virtual press shop staffed by a tool and die maker, a CAD operator and an FEA simulation operator. The output of the virtual forming analysis for each stamping can become a guide covering stamping design, die buy-off and troubleshooting of production problems.

• The FE participates in an effective troubleshooting production swat team along with a tool and die specialist and a statistical specialist. The team’s primary job: Respond to press lines encountering downtime. Its secondary job: Study the top five stampings returning the lowest profit and implement corrective actions.

Some excellent FEs are active in the metalforming industry. Unfortunately, the opportunities for practical training of these people have been very limited, as most FEs have been self-taught on the job. Training centers will not be established until requests for FEs reach a critical mass. However, press shops will not request an FE until they have heard FE success stories and understand their impact in the pressroom. Hopefully metalforming organizations will begin to work together to undertake this project. MF

 


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