Page 27 - MetalForming June 2010

expansion beyond 100 percent will gen- erate edge cracks that can grow to sub- stantial lengths.
This is only one example of hole expansion data. The exact amounts of edge stretch will be determined by the shape of the punch, the quality of the punching tool, lubrication, forming speed, sheetmetal composition and micro- structure, and other specific test param- eters. With standardized test parameters, comparison data shown in the graph can be very useful and educational.
Another potential problem area is friction. In high school, or now even earlier, the teacher demonstrates meas- urement of the coefficient of friction (COF) by hooking a spring scale to the end of a weight and pulling it across a surface to be measured. The COF is the pulling load divided by the normal force (weight of the block) on the interface. This computation says the COF is inde- pendent of the contact area between the sheet and the weight.
Friction studies about a decade ago showed the COF changed with contact area. The sheetmetal has a rough topog- raphy characterized by a series of peaks and valleys. As the material deforms over the die, the amount of contact area increases as the peaks break off or are bent. The measured COF increases with increasing deformation. There- fore, the laboratory measurements typ- ically represent only the undeformed material. Even worse, the studies showed that the contact area and therefore the COF changed with location in the stamping due to the amount of defor- mation, the local rate of deformation, the interface pressure, the quantity of residual lubricant, the heating of the lubricant and other factors.
In summary, one does not know the instantaneous COF at any location within the stamping with any degree of accuracy. Putting any single number into the virtual metalforming code is a classic case of GIGO.
There is one exception, however. The
newer dry (barrier) lubricants use poly- mer or other coatings to physically sep- arate the workpiece from the die. Now the COF is determined by the interfacial shearing within the lubricant. Even bet- ter, most of these dry lubricants are temperature insensitive. Now a specific COF can be entered into the code for more accurate prediction of final virtual forming. These lubricants also mean that physical press shops will produce more dimensionally stable stampings to meet the more rigid dimensional specifica- tions imposed by their customers.
Another troublesome forming prob- lem is attempting to switch between steel and aluminum in the same die—even if both materials have the same yield strengths. First, the aluminum will have three times the springback of steel because the Young’s Modulus of alu- minum is one-third that of steel. Sec- ond, steel has a positive strain-rate hard- ening exponent. The more quickly steel is deformed, the higher the flow stress becomes during forming. In contrast, many aluminum alloys and other non- ferrous alloys have a negative strain- rate hardening exponent. The faster the metals are deformed, the lower the flow stress becomes. This change in strain-rate hardening greatly affects the distribu- tion of strain and severity of strain gra- dients throughout the stamping. Mak- ing these substitutions not only requires good data but a much deeper under- standing of sheetmetal deformation.
The core problem seems to be the transfer of knowledge from one gener- ation of press-shop personnel to the next generation. The emphasis is most- ly on how to do things—knowledge gained by trial and error experimenta- tion. Rarely do the right press-shop personnel get to understand the knowl- edge needed to transfer correct actions from low-strength to higher-strength materials or between different types of materials. My favorite saying covers it all: Any solution without data is simply guessing. 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