Page 25 - MetalForming April 2010

180
Steel A
Allowable
Actual
Allowable
Actual
Steel B
160
140
120
100
80
0
Location
Location
Two steel coils, each processed differently, are evaluated near final die tryout to determine which steel will produce the most robust stampings. As observed, each steel has its own pros and cons.
steel. The disagreement was the result of opposite sets of personal experiences.
Arguments by the group who want- ed steel A:
Our steel is well below the allow- able maximum allowable stretch with a minimum safety margin of 22 stretch percent. Steel B already has one location over the edge of the cliff and another location just ready to fall off. That is a disaster waiting to happen.
Arguments by the group who want- ed steel B:
Our steel has a nice flat distribution of stretch that is more stable. Steel A has a very sharp localized stretch gradient. Sharp stretch gradients are very unsta- ble, and can easily spike to failure. Even an increase and decrease of the spike means the stamping is not robust and can affect springback and dimensional stability.
Finally, one person presented the following argument. If the sharp stretch gradient were not present, everyone would agree steel A was the best steel for production. However, in my experi- ence, something is wrong with that gra- dient. The normal rule is that stretch gradients become worse as they approach the edge of the deformation cliff. The stretch gradient observed in steel B should be much worse than that found in steel A. But that is not the case. When general rules of forming
are violated, one should go back and redo the test or at least recheck the measurements. Note that the sharp stretch gradient is created by only one data point. We need to go back and check the strain at that one location.
The final result? The correct major stretch for the point in question was 104 but was misreported as 124. The value of 104 resulted in a nice flat plateau of low-level stretch, no stretch gradient or spike, and a large safety margin. Steel A was ordered. Why did steel B have such a large amount of major stretch? A post-mortem evaluation found steel B had a rougher surface with a higher coefficient of friction. Less material moved from the binder into the stamp- ing forcing more stretch over the punch.
The most interesting aspect of this case study is that the answer was reached without even knowing what part was under evaluation. Had the part been identified, too many details of past experiences would have further cloud- ed the analysis. Instead, general forming principles created a focused discussion.
This case study added credence to the relative importance of data and experi- ence. Data is most powerful in uncov- ering and defining the problem. Expe- rience is best applied during the search for a solution. Another great sign to hang at your worksite: Any opinion without data is simply a guess. MF
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•NoAir Consumption
The Pax EGD conveyor is an electrically driven, oscillating type conveyor that utilizes a motorized, elliptical gear drive (EGD) to convey parts and scrap out of the press area.
• Elliptical Gear Drive
PATENT PENDING
• Snap-On Tray
Percent, Major Stretch