Page 28 - MetalForming July 2009

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, 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 introduced the concept of unit cells, where atomic forces hold the arrangement of atoms in a min- imum energy, stable state. Visualize each atomic force as a tension or compression spring between each pair of atoms with all of the springs interconnected and balanced. Applying external forces to this atomic structure causes the atoms to change spacing within the unit cell. Tensile forces increase the spacing, while compressive forces decrease the spacing. Any displacement of the atoms from their stable state creates additional elastic stresses. The springs undergo an addi- tional expansion or contraction. Remove the external forces and the extra elastic stresses disappear as the atoms return (springback) to their stable position.
If the material has been permanent- ly deformed into some geometrical shape, this stable shape creates a barri- er to the release of the elastic stresses. Some elastic stresses will remain in the part, creating a temporary increased- energy state. These elastic stresses are called residual or trapped stresses. The springs want to return to their lowest energy state but mechanically are pre- vented from doing so. However, any subsequent deformation (offal trim- ming, hole punching, bracket welding,
STUART KEELER
bending, etc.) will cause a further change in shape (springback) that reduces the total energy within the part.
Springback problems usually fall into one of two categories—mean or average springback, and variable springback. Target shooting has the same two prob- lem categories (Fig. 1). One extreme (A) is a very tight grouping with very lit- tle variability. However, the mean (or average) value is not in the center of the target. This can easily be corrected by a mean shift from the current value to the desired value. Often this requires only adjustment of the gun sight or scope. The other extreme (B) is variability that occurs from shot to shot or from one series of shots to the next. This vari- ability often is the interaction of many small variations and cannot be correct- ed by a simple mean shift. Condition (B) in Fig. 1 has zero mean error, but rarely does a shot strike bull’s-eye.
Various forms of springback com- pensation are used in the press shop to shift the mean or average dimensional error of the part (Fig. 1A) from the wrong dimension to the correct dimen- sion. Compensation often begins in the die design by over-bending, over-crown- ing and other changes that allow the part to spring back to part print. This requires the loading of the springs to be absolutely consistent from part to part—both before and after compensa- tion. Unfortunately, most press shops undergo frequent and intentional mean shifts, and then wonder why their parts fail dimensional checks.
“No two presses are identical” is a well-known fact of life in press shops— even new presses built to the same blue- prints. Dimensional mean shifts occur when moving a die from the tryout press to the primary production press.
THE SCIENCE OF FORMING More About Springs and Springback
AB
26 METALFORMING / SPECIAL ONLINE-ONLY ISSUE 2009
www.metalformingmagazine.com
Fig. 1—Sketch A represents a mean shift away from the center of the target and very little variability. Sketch B has no mean shift but extensive variability.