Page 20 - MetalForming July 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
The utilization of statistics in the press shop is a powerful tool when applied correctly. Examples include control charts, failure prediction and design of experiments. Unfortunately, severe problems are encountered when used improperly. One example is the use of raw data that is biased, measured improperly or not even related to the analysis being conducted. These data are obvious examples of the underlying theme in our last two columns—GIGO, or garbage in garbage out.
More subtle and extremely danger- ous is the use of good data in a flawed application. The use of statistics cloaks the analysis with a visible scientific component that hides the underlying flaw. In the last two columns, this becomes a slightly different case of GIGO —garbage in gospel out. If the statistics are correct, the output of the analysis must be true. An example of this flawed application is the popular worst-case scenario currently found in many auto- motive industry die-tryout programs.
Some time during die tryout (often at the end) a stamping is made from typical production metal. The blank contains a circle-grid pattern that pro- vides a measurement of major and minor stretch combinations everywhere in the stamping. The mechanical prop- erties of the next blank are obtained from a tensile test. Two especially important properties are the steel thick- ness (t) and work-hardening exponent (n-value). These two properties set the forming limit curve that determines the maximum allowable stretch (engi- neering strain) for that sheet.
The forming limit curve is the upper curve in the graph. The vertical position of the curve for steel is set by the value of FLC0 obtained from the equation:
STUART KEELER
FLC0 = (23.3 + 360t) n/0.21.
The equation shows a small influence of the thickness, but a direct propor- tional relationship with n-value. The actual major-minor stretch values from the formed stamping are plotted relative to the curve to determine the stamping location undergoing the most severe deformation (dot 1 in Graph A). Analy- sis of this diagram indicates the stamp- ing location with the most severe defor- mation is well below the maximum allowable stretch line (FLC) or the edge of the deformation cliff.
Everything in Graph A was done properly. The most severe stretch of the stamping is assessed against the allow- able stretch determined from the actu- al properties of the blank being formed. Many years ago, however, some engi- neers had a bright idea. They argued that the typical stamping plant does not always receive material with typical or average properties. Sometimes the properties are inferior in terms of formability—such as a thinner sheet or a lower n-value. Instead of using the properties of the blank actually being formed, the -3 sigma values are deter- mined for the grade of steel being pur- chased. These values now are inserted into the FLC0 equation to determine the forming limit curve for the worst-case scenario. The thinner sheet and the lower n-value reduce the height of the FLC (Graph B). This procedure is still valid because the FLC in Graph B now becomes the maximum allowable stretch combinations for a blank that has the properties equal to -3 sigma steel.
To complete the analysis, the most severe stretch value (dot 1 in Graph 1) from the formed stamping is trans- ferred as dot 2 in Graph B to deter- mine the severity of the stamping with
THE SCIENCE OF FORMING Worst-Case Scenario—Good or Bad?
Stuart Keeler’s next seminar is “Sheet Metal 101—Under- standing the Metal,” Septem- ber 16 in Cleveland, OH. Check www.pma.com for this and other seminars.
18 METALFORMING / JULY 2010
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