Page 34 - MetalForming November 2012

The Science of Forming
By Stuart Keeler
Why Should You Converse with Your Stampings?
Friends bring an unconscious patient into the emer- gency room. The ER doctor examines the patient and finds no cuts, broken bones, bleeding or other visual external injuries. Does the doctor send the unconscious patient home because no treatment is obvious? No—instead, he conducts batteries of tests on the patient, in effect asking the body of the unconscious patient a series of questions, to which the body responds with data. This data provides exact information about the body’s condition. The success of this “conversation” depends on the doctor asking the right ques- tions, and having the equipment to decipher the answers. Often the problem does not lie with one single body function; rather it results from the interaction of several functions.
Segue into the pressroom, where a new tool has made its first hit. Behold, the resulting stamping displays no tears, strain gradients, excess springback, faulty dimensions or class A visual defects. Does the tool and die person release the stamping for production? All too often the answer is “yes,” because job one rapidly approaches. However, this is the per- fect time to hold a “conversation” with the stamping.
What questions to ask? While no tears are observed, one might ask if the stamping deformed into the yellow (danger) zone of the forming-limit diagram, where sporadic breakage is expected. To collect the relevant data, we use circle-grid- ded blanks or other surface-strain measurement techniques.
We might also look for steep strain gradients with highly localized thinning, which can cause major changes in elas- tic stresses and result in springback and dimensional changes. An ultrasonic thickness gauge can extract the required data from the stamping.
Are temperatures increasing in areas of severe deforma- tion and causing a change in the effectiveness of the lubri- cant? We can track these temperature changes easily, for the die and the stamping, with a laser thermometer gun.
Looking at the total forming system, more than 50 inputs can affect the final characteristics of a stamped part. Most of
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 Company Technical Center, enabling him to bring a very diverse background to this column and to the seminars he teaches for PMA.
Keeler Technologies LLC
P.O. Box 283 | Grosse Ile, MI 48138 Fax: 734/671-2271 keeltech@comcast.net
these inputs interact with each other. Attempting to measure every input and gauge all of their interactions proves near- ly impossible. Hence, we focus our “conversation” on the out- put of the system—the stamping.
Conducting a continuous conversation with your stamp- ing can form the foundation of virtual forming, where an operator enters into the computer the design for a stamping and its die. The conversation begins immediately, with ques- tions such as:
• Where are your hot spots?
• Do you like the properties of the specified material?
• If I change this blank contour, are you more comfortable? The dialog continues until the stamping and the design-
er reach an agreement. Then the hard die produces an actu- al stamping, restarting the conversation:
• Are you exactly like the virtual stamping?
• Where are you different?
• Why are you different?
Consider this case study illustrating how a conversation
with the stamping can solve a random breakage problem. Under investigation: two locations on the punch, A and B, 3 ft. apart. The two strain distributions, measured at full depth or home stamping, are nearly identical (Fig. 1). Location A appears stable and never fails, while location B experiences frequent breakage. The available data proves insufficient for identifying the problem, or a solution.
Tooling Technology
50
Location Location
40 30 20 10
0
AB
Grid Pattern Centers
Grid Pattern Centers
32 MetalForming/November 2012
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Fig. 1—Stamping has two areas with identical final strain distributions.
Step one is to identify the strain history by asking the stamping for a series of incremental hits or breakdown stampings (Fig. 2). Location A appears to have a constant rate of straining until the very end of the stroke, where the rate of straining decreases. Location B exhibits the opposite behav- ior, as forming begins later in the stroke and then increases its rate of straining as the stroke approaches home depth.
Engineering Strain (%)