Page 38 - MetalForming December 2013

The Science of Forming By Stuart Keeler
Tooling Technology
Formability Rules from Mother Nature
Punch
Hydraulic cylinder or spring
Rubber Pad
Punch
Die-design handbooks, material textbooks, seminars, home- study courses, magazine articles and columns and a myriad of other sources, written by press-shop experts, professors, researchers and specialists in engineering, metallurgy and other related fields provide a wide breadth of useful information about metalform- ing. The laws of physics also should be included, but textbooks on this subject can be difficult to comprehend. There- fore, I’d like to provide some applicable laws written at a lower level of com- plexity—“The Rules from Mother Nature.”
Rule 1: Unless controlled by out- side forces, sheetmetal will utilize any deformation mode that requires the least amount of total energy.
As a simple example, consider bend- ing sheetmetal over a punch of a given radius (Fig. 1). To meet part print, the material should wrap tightly around the punch or die. However, if it bends like a hinge (kinking), only a small seg- ment of the sheet deforms. Compared to a larger segment of material con- forming to the punch geometry, the bend mode requires less energy. This kinking can easily be detected by com-
Stuart Keeler (Keeler Technologies LLC) is known worldwide for his discovery of forming limit diagrams, development of circle-grid analysis and implementation of other press-shop analysis
tools. Keeler’s metalform- ing experience includes 24 years at National
Steel Corporation and 12 years at The Budd Com- pany Technical Center, enabling him to bring a very diverse background to this column and to the sem- inars he teaches for PMA.
Keeler Technologies LLC P.O. Box 283
Grosse Ile, MI 48138 Fax: 734/671-2271 keeltech@comcast.net
Fig. 1—The bending operation will take the forming mode with the least energy consumption—a bend radius smaller than the punch radius.
paring actual bend radius to the required punch radius. When the work- piece material contains yield-point elongation, with upper and lower defin- able yield stresses and resulting Lüder’s bands, this tendency for independent bending increases greatly.
Rule 2: To change this independent forming mode, one must pay Mother Nature with additional energy added to the forming process.
For the bending problem in Fig. 1, metalformers typically apply a high- durometer rubber pad to the flat blank and force the pad to maintain blank-to- punch contact with a hydraulic cylinder or stiff spring (Fig. 2). The cylinder or
Fig. 2—Forcing the bend radius to main- tain the punch radius requires extra ener- gy, delivered through a stiff rubber pad driven by a spring or hydraulic cylinder.
sidewall curl, twist, global shape change and surface disturbances. Formed channels or beams are highly susceptible to sidewall curl (Fig. 3). Channel A was formed by a specific process designed for steel with 50-ksi yield strength and 65-ksi tensile strength. These numbers provide a rough estimate of the workhardening exponent (n-value).
Now observe the major sidewall curl in channel B. Yield strength equals that of channel A, but its tensile strength is higher. The TS/YS ratio for channel B is higher than channel A, indicating a
spring contributes to the forming process, as required by Mother Nature.
Rule 3: When the forming forces release, the stamp- ing will take what- ever shape it can to minimize residual stresses.
This shape change, called springback, can occur in several forms including angular change,
additional energy
BA
Y.S. = 50 ksi Y.S. = 50 ksi T.S. = 85 ksi T.S. = 65 ksi
36 MetalForming/December 2013
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Fig. 3—The die was adjusted to provide nearly zero springback for material A. When material B was placed in the same die without any process changes, severe sidewall curl resulted. Material B had a higher n-value and more workhardening. Cour- tesy of WorldAutoSteel—AHSS Application Guidelines ver. 4.0.