Page 38 - MetalForming December 2017

The Science of Forming By Daniel J. Schaeffler, Ph.D.
The More You Know
Optimizing sheetmetal flow to form a stamped part requires understanding the interaction between the sheetmetal, surface char- acteristics and tooling. Consider where we were in the 1970s. Back then, we could choose between seven grades of sheet steel. Most of the steel used on cars did not contain a galvanized coat- ing. Lubrication for many deep-drawn parts consisted of a paste made from animal fat. And the tooling material choices for large stampings? With apologies to Henry Ford, you can have any material you want, as long as it’s D2. If the surface wasn’t up to the desired quality, there was always Bondo and a few more layers of paint. Using just trial-and-error could produce an acceptable part, given that there were a limited number of variables to master and the fact that a threshold for “suf- ficient quality” took comparatively less effort to achieve.
This sounds like ancient history. But is it? Almost half of all tool-and-die workers today are more than 55 years old. When they entered the field in the mid-1980s, they apprenticed under
Danny Schaeffler, with
30 years of materials and applications experi- ence, is co-founder of 4M Partners, LLC and founder and president of Engineering Quality Solu- tions (EQS). EQS provides product-applications assistance to materials and manufacturing com-
panies; 4M teaches fundamentals and practical details of material properties, forming technolo- gies, processes and troubleshooting needed to form high-quality components. Schaeffler, who also spent 10 years at LTV Steel Co., received his Bach- elor of Science degree in Materials Science and Engineering from the Johns Hopkins University in Baltimore, MD, and Master of Science and Doctor of Philosophy degrees in Materials Engineering from Drexel University in Philadelphia, PA.
Danny Schaeffler
Tel. 248/66-STEEL
E-mail ds@EQSgroup.com or Danny@learning4m.com
journeymen who learned the trade in the 1970s or earlier. Ideally, these work- ers now have 30 yr. of good experience. Unfortunately, some of them may very well have 1 yr. of experience 30 times over, repeating the lessons learned from their teachers.
purchase of new servo presses.
Even with these advances, basic principles still apply. Fundamentally, sheetmetal forming involves applying energy to a flat metallic piece to deform it into the desired shape. The energy to make stamped parts transfers from a punch, where the top surface features a contour that mirrors the targeted profile. That energy transfer is a func- tion of how the workpiece responds to deformation, the path the sheetmetal follows in reaching that desired part shape, and the inter- faces between the punch and sheet as well as between the sheet and the rest of the tooling. If the energy applied to the forming system to stamp a part surpasses the amount that the forming system can handle, a split will form in the part. When this occurs, either the energy requirements in part forming must be lowered or forming-system capability must be increased. This is where the interface interaction of lubrication, tooling sur- faces and radii, and sheetmetal prop-
erties all come into play.
We must account for the variability
inherent within the forming system. The specifications to which sheetmet- als are ordered allow for a range of properties deemed to influence form- ing. A coil may measure more than a mile long, but we assume that mill certs taken on one sample from the coil end represent the entire batch. When we run a part and some of the stampings split, our first instinct may be to say that the sheetmetal changed, when in reality change occurred elsewhere in the forming system where we hadn’t measured—think of the unused ton- nage monitors sitting in the corner of the plant. A better job of collecting
Today’s metalforming industry looks quite a bit different than the simpler times of the 1970s. We now can choose from more than 200 grades of sheet steel for automo-
tive applications,
ranging from one-
half to 10 times
the strength of the
steels available
four decades ago.
Properties today
can be engineered to meet the require- ments of specific applications, moving away from commodity sheetmetals. Zinc-coated steels for corrosion resist- ance are commonplace—with some automakers choosing soft pure zinc (either hot-dip galvanized or electro- galvanized) and others using a much harder zinc-iron alloy known as gal- vanneal. New aluminum alloys refined over the last decade are making sig- nificant inroads in more mass-market applications, with most suppliers bringing new capacity onstream.
Class A panels for exposed applica- tions made from these steel or alu- minum alloys must exhibit consistent surface quality such that just 100 microns of paint (half of that being clearcoat) is sufficient. The choice of tooling material, with coatings and inserts as necessary, is made based on the sheetmetal, intended application and part volume. Lubricants with addi- tives are engineered based on the con- tact pressures and temperatures encountered during stamping. Produc- tivity increases and the ability to stamp difficult to form grades help justify the
“Today’s metalforming industry looks quite a bit different than the simpler times of the 1970s.”
36 MetalForming/December 2017
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