Page 22 - MetalForming January 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 last seven columns (June through December 2009) described the differ- ent stages of deformation as the mate- rial transforms from a blank to the com- pleted stamping. This transformation may be successful or a horrendous failure in terms of stamping shape, thinning or abil- ity to hold water. Different types of defor- mation are related to specific mechanical properties of the material. Understanding these relationships assist in part design, troubleshooting production problems and transitioning from one alloy to another.
While the sequence of the seven columns coincided with increase in defor- mation, the explanations were rather extensive. To complete the series, this month lists the key points from each col- umn. This list can be used several ways: a refresher about each column, an index for going back to learn more about a specif- ic topic, or knowledge checklist.
Understanding the Behavior of Sheetmetal—June 2009
Place a blank in the die and lower the punch to the blank. Then force the punch into the blank by a small amount. The blank will stretch and/or compress elastically.
• Increasing or decreasing the inter- atomic spacing causes elastic deformation. • This elastic bond between atoms is strong, causing a rapid increase in stress for a small amount of strain. The rate of stress increase is called Young’s modulus. • Approximate Young’s modulus: 30 million for steel, 10 million for aluminum. • Unloading the blank releases the elastic stresses and causes the blank to
spring back to its original shape.
• If a permanent geometry was cre- ated by the deformation, some elastic stresses may be trapped and unable to go back to zero. These stresses are resid-
ual stresses that can change during sub- sequent operations on the stamping.
More About Springs and Springback— July 2009 (Available online only)
• Two types of springback are mean (average) and variable.
• Mean springback usually can be cor- rected by tool design, such as preforming, over-bending, over-crowning, etc.
• Variable springback requires improve- ment towards constant process control.
• Three ways to minimize springback:
1) Change the bad elastic stresses into good elastic stresses.
2) Minimize the magnitude of the elastic stresses.
3) Mechanically lock in the elastic stresses.
What is Yield Strength?—July/August 2009
• At some value of elastic stress called the yield strength, a new mode of defor- mation—slip systems—become acti- vated. Slip systems are discontinuities in the atomic structure that can move through the sheetmetal and cause plas- tic (permanent) deformation.
• Various methods can be used to raise material yield strength, such as smaller grain size, replacement of atoms with larger or smaller atoms, and inclusion of extra atoms. This thermal-mechanical strengthening creates interference to the movement of the slip systems.
• The amount of elastic stresses and the accompanying degree of springback increase as yield strength increases.
Why Work Hardening is So Important— September 2009 (Available online only)
• Most alloys become stronger as they deform. This strengthening is called work hardening.
• Without work hardening, all defor-
THE SCIENCE OF FORMING
Sheetmetal Deformaton—Review of Key Points
Next seminar: Columnists Stuart Keeler and Peter Ulintz (Tooling by Design) are major speakers at the PMA Deep Draw Semi- nar scheduled for January 26 in Chicago, IL.
Check www.metalforming.com for this and other seminars.
20 METALFORMING / JANUARY 2010
www.metalformingmagazine.com
STUART KEELER