A Different Look at Rules of Friction
The second group of forming modes contains the Punch Radius test (flat-bottom-punch radius) and Hemispherical Rotation test (punch stretching). In contrast to the first group, the initial material and lubricant in the deformation zone remain in contact
|Fig. 1—Three consecutive Draw Bead Simulator tests in Fig. 1A show nonconstant pulling force for sheetmetal with no lubricant. In Fig. 1B, a small amount of poor rust-preventive liquid eliminates the pull-load variability.|
Other lubrication studies have provided useful press-shop data. One misconception is that not adding a lubricant at the press shop is running the part dry. If the part were truly dry, the steel would be colored red. The rust-preventive solution added as the final step at the metal processor is a lubricant—though most likely one with poor friction conditions. Application of better-quality lubricants is possible depending on supplier capabilities.
A completely dry (no solutions or lubricant added) electrogalvanized coated steel was tested using the Draw Bead Simulator. Is this a good reproducible condition for forming? Studies have shown that forming without any lubricant is highly variable. Fig. 1A contains pull loads recorded for three consecutive dry electrogalvanized steel strips. Note the wide fluctuations in pull load. In contrast, Fig. 1B shows how the pull loads become very consistent (more robust) when the same electrogalvanized steel is tested with a thin coating of very poor lubricant.
Lubrication studies by Dr. Harmon Nine, formerly of GM Research, show that one of the two surfaces in the fiction couple (die or sheetmetal) has to be rough and the other smooth. If both are rough, the asperities of each surface interlock, causing a high friction force, welding and galling. If both surfaces are very smooth, no lubricant remains in the work zone under high pressure and heat. One smooth surface (usually the die) allows the asperities of the other surface (workpiece) to slide with minimum resistance. The rough surface brings lubricant into the deformation zone. This suggests that forming a highly polished sheet requires a roughened die surface or a dry barrier lubricant that maintains complete separation between the die and the sheet.
Mr. Norbert Izworski, formerly of Ford Motor Co., conducted a very interesting study with two lubricants—one with a high COF and one with a low COF. With the two lubricants placed one on top of each other, which lubricant controls the forming operation? The results showed the lubricant placed on the sheet first (in the valleys) controls the process. The die pushes the second or top lubricant out of the deformation zone. Under further pressure, the asperities of the sheet begin to deform and decrease in height. This forces the first lubricant in the valleys between the peaks to form a lubricant layer over the deformation zone and control friction during deformation.
The above studies provide a logical, but not surprising, look at how lubricants function. Yet, lubricants can make or break many a stamping. Even worse, successful achievement of the dimensional consistency requirements imposed today on stampings depends on the correct choice and use of lubricants. Unfortunately, lubricants all too often are a commodity chosen only by price. MF
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