Page 64 - MetalForming April 2015

The Science of Forming By Stuart Keeler
Evaluate The Results Before Taking Action
Haste makes waste. This opera- tional rule should be applied to all aspects of business and pri- vate life. If we do things in a hurry with- out analyzing the probable outputs, things will go wrong. Let’s review some other operational rules that are more applicable to industry. Press shops offer a good starting point.
Simultaneously changing all six inputs (A to F) did not save time, but wasted time. If some of the six inputs can be changed with the die still in the press, only one input must be changed at a time. Change input B and run some stampings. Did the change to input B help correct the problem? If so, leave correction B in the die. If correction B made the problem worse, we have learned two useful pieces of information. First, input B is affecting the problem. Second, try reversing what was first
done to input B to see if that helps correct the problem. When changing the
inputs that only can be
duction records show that the cracks never extend into the final stamping, then these cracks are not the cause of the forming problem. Unfortunately, that opinion does not include com- plete data. The sheetmetal surrounding the final stamping develops elastic stresses during forming just as the stamping does. Different crack lengths in the offal change the magnitude of these stresses, modifying the stress state and springback of the final stamp- ing. When attempting to troubleshoot this problem, the evidence has disap- peared down the scrap chute before it can be examined.
3) Solutions without data are real- ly bad guesses. Enough problems fit this category to fill a huge book. For example, machined or welded draw beads are found in a large proportion of dies. These draw beads are placed in the die at build time. During die tryout, the height and shape of the draw beads usually are modified to better control material flow from the binder into the stamping. As production continues, the draw beads wear down and lose control of the blank. Bringing the die back to initial buyoff status often requires extensive repetition of initial die tryout. Why? Because the initial draw-bead configuration was not recorded. Data from a simple pin pro- file gauge would allow a set of tem- plates to be created for restoring the initial draw-bead profiles during future rework.
Production of a box-shaped stamp- ing has been running smoothly for three months. The customer has sent you a simple engineering change to place a dome-shaped embossment in the flat bottom of the box near one corner. The change looks feasible so the die is pulled Saturday morning for the modification. The modified die is placed back in the press Sunday afternoon in
1) Averaging data loses informa- tion. Something happened to a pro- duction stamping, creating increased springback; an embossment does not contact the punch; and the
part exhibits dimensional
variation from the speci-
fications. Six inputs to the
stamping have been iden-
tified as possible causes
and are labeled A, B, C, D, EandF.Thediewas
removed from the press on
Friday night. To save time
all inputs were modified by a crew of diemakers on Saturday. The die was reinserted in the production press on Sunday and new measurements were made of the problem areas. Instead of correcting the problems, the magni- tude of the problems increased. No additional corrections were obvious.
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
“Solutions without data
modified by removal of the are really bad die, only one input change can be made and then the die must be replaced in the press to check the result. Mixing all six input changes before testing the individual result sim- ply produces average data with lost information about which inputs are important and the direction in which
guesses.”
the corrections must be made.
2) Opinions without data are sim- ply guesses. Some attempt to attack a forming problem by assembling a brainstorming session. Possible prob- lem solutions are contributed by atten- dees and recorded via a white board, paper pad or other method. When all contributions have been recorded, vot- ing is conducted to select the top three or five candidates to be investigated. How many persons providing these suggestions have valid data to propose a viable cause? Has anyone bothered to ask the stamping for data relative to the problem?
A simple example: cracks of different lengths in the offal surrounding the final stamping. Often, combinations of long and short cracks are generated from the blanked edge. However, if pro-
62 MetalForming/April 2015
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