Page 50 - MetalForming May 2017

The Science of Forming
By Stuart Keeler
Higher Strength Steels Can Cause Unique Shape Challenges
Shape correction always is an active problem as the strength of the workpiece material increases. A channel, shell or other shape created by pulling the sheetmetal from the binder (Fig. 1) will have a gradi- ent of positive to negative elastic stresses from one surface to the other. This causes springback- related sidewall curl.
of the stroke by locking the flange area with either a moveable bead, lock step, another die or some other method. This causes the neutral axis to move outside of the sheet. Further, stamping advanced high-strength steels may require lock beads to prevent sheetmetal flow into the sidewall during post-stretching.
Negative Positive compressive tensile
As the sheet forms around the
corner of the tool, the outer surface stretches. When released the outer-surface tensile stress pulls the material away from the tool; the reverse happens to the inner surface. Upon bending over the radius, the sheetmetal is compressed.
arrow shows the
When the lower left corner was held to the table, the diagonal corner had a gap measuring several inches. Another panel, stamped using the post-stretch process, exhibited straight sidewalls and no gaps between the table and the stamped part.
Fig. 1
Fig. 3 shows a typical panel made with- out any post-stretch. When placed on a granite table, we can see that the sidewalls have turned away from vertical. The white direction of twist of the diagonal corners.
Positive Positive
Typical 2% stretch
Sidewall curl
Diagonal twist
DT
Fig. 2
When released, the inner mate- rial expands and the sheet pushes away from the tool. Both inner and outer surface stresses force the material to move in the same direction.
Post-stretch is used to coun- teract this prob-
lem. This practice changes the positive-to-negative gradient to all-positive elastic stresses across the sheet thickness (Fig. 2). Release of the forming load only causes a reduction in sidewall length.
The sidewall strain is increased 1 to 2 percent at the end
Stuart Keeler (Keeler Technologies LLC) is known worldwide for his discovery of forming limit diagrams, development of circle-grid analysis and implementa- tion of other press-shop analysis tools. Keeler’s metal- forming experience includes 24 years at National Steel Corporation and 12 years at The Budd Compa- ny Technical Center, enabling him to bring a very diverse background to this column and to the semi- nars he teaches for PMA.
Keeler Technologies LLC
P.O. Box 283 | Grosse Ile, MI 48138 keeltech@comcast.net
Fig. 3
A group of formability specialists reviewed this applica- tion, and discussed at length the post-stretch process. One specialist complained specifically about a product that his company stamped—reflectors for huge outdoor lights. Noting that the final shape of the reflector was way beyond specifi- cation, he said he would try the post-stretch process. Six months later, when the group of specialists met again, that same lighting-company specialist asked to talk first. Why? He was excited to let his peers know that the dimensional accuracy of his company’s stamped reflectors now were three times tighter than the specification required. MF
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MetalForming/May 2017
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