Page 24 - MetalForming April 2010

Tooling Technology
Stuart Keeler (Keeler Technologies LLC) is best known worldwide for his discovery of forming limit diagrams, development of circle grid analysis and implementation of other press shop analysis tools. Stuart’s sheetmetal 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. His most recent project is technical editor of the AHSS Application Guidelines—Version 4.1, which now is available for downloading free from www.worldautosteel.org. Keeler Technologies LLC
P.O. Box 283
Grosse Ile, MI 48138
Fax: 734/671-2271
E-mail: keeltech@comcast.net
Last month this column addressed the importance of using data to define the forming problem, create a measurable end goal, and track progress toward the solution. Experience encountered several years ago with a similar stamping having a similar prob- lem is not very useful nor reliable for troubleshooting a problem with a cur- rent stamping. This month a case study illustrates how data and experience can complement each other when applied at the proper time during troubleshooting.
Consider a stamping nearing com- pletion of final die tryout at a die-build company. The steel supplier reviews the stamping to determine the most robust steel for the job. After review, the sup- plier sends two sample coils labeled A and B having identical type/grade and price but different processing. The die builder must run both coils and report back to the steel supplier which coil creates the more robust stamping.
In our case study, both coils could show the same result—no breakage— without data provided from the stamp- ing. One might have to run thousands of stampings before noticing some dif- ference between the two coils. Howev- er, good troubleshooting techniques are available where a single stamping taken when the die is hot and produc- tion conditions have stabilized can yield significant information. Evaluating the first stamping after press startup does not represent a normal production environment.
In our case study, circle grids were electroetched into the surface of the steel. From the ellipses (deformed cir- cles), the long axis represents the largest positive stretch and is called major stretch. For a line of ellipses through the most critical zone of the stamping,
major stretch values are measured and plotted against their location in the stamping. These values are shown as a solid line for both coil A and B (see the illustration). Knowing the properties of both coils allows one to determine the forming limit curve for each coil.
From the forming limit curve one obtains the maximum allowable stretch before the onset of local necking and tearing of the sheetmetal. The maxi- mum allowable stretch (edge of the deformation cliff ) is represented by the dashed line in each graph. Subtracting major stretch in the stamping from allowable stretch from the forming limit curve provides the safety margin. If the safety margin is negative (when the actual stretch is greater than the allow- able stretch) as observed for one loca- tion for steel B, some percentage of necking and failure is expected. These failures usually result when the pro- duction variables increase the actual production major stretch. A larger neg- ative safety margin means a higher per- centage of stampings are expected to neck and fail. A safety margin of 10 strain percent or more (as observed for steel A) is desired to allow for produc- tion variability.
This illustration of the test results was presented to a group deciding whether to ask for steel A or steel B. Since you the readers have seen the data, which steel would you pick—A or B? Surprisingly, half of the group chose steel A and half chose steel B. Their choic- es were based on different past experi- ences in troubleshooting steel stamp- ings. However, each group presented good logical and valid reasons why their steel would make more robust stamp- ings. Each group highlighted the pros for their steel and the cons for the other
THE SCIENCE OF FORMING Troubleshooting—Data Versus Experience
STUART KEELER
Stuart Keeler’s next seminar is “Higher Strength Steels—Solving the Problems” scheduled for June 23 in Grand Rapids, MI. Check www.metalforming.com for this and other seminars.
22 METALFORMING / APRIL 2010
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