Page 95 - MetalForming October 2014

0.01 to 0.03. This dramatic drop in COF occurs because the lubricant completely isolates the workpiece surface from the diesurface;morphologyofbothsurfacesdoesnotimpactthe amount of friction. The COF is generated by the material slid- ing within the dry film.
One of the more informative results of the study can be seen in the accompanying bar graph. Two different electro- galvanized steels were tested with 16 lubricants. For steel C (upper curve) the lubricant codes are arranged with increas- ingCOF.AnyofthelubricantcodescouldbeusedonsteelC with the same friction results. In the lower graph, the lubri- cant codes are arranged for steel D in the identical sequence as for steel C. Forming with steel D would be dramatically sus- ceptible to the lubricant applied. This shows that lubricant performance on one steel cannot be used to predict the performance on another shipment of steel, especially elec- trogalvanized.
Press-Shop Implications
The research study confirms the difficulties reported by stampers when they substitute galvanized steels for bare steels. The COF of galvanized steels, which can be greater or less than that for bare steel, depends on the coating type and the coating process used. Regardless, there is hope that a press shop might find a lubricant that will generate the same COF for their galvanized and bare steels.
The data from the study suggest that reducing the varia- tion in the lubricant COF is more important than the absolute valueoftheCOF.Duringfinaldietryout,theCOFofthelubri- cant used to achieve buyoff is the COF you want to reproduce throughout the entire production history of the part—a high COF should remain high and a low COF should remain low.
The draw-bead simulator defines the output of the lubri- cant interacting with the sheetmetal surface when moving through the draw beads. While important for control of binder flow, many other deformation modes are important to successfully fabricate stamped parts. These include punch stretching, bending, cup drawing, embossing, edge stretch- ing and hole expansion. Some of these deformation modes have their own test device; DBS data do not apply.
My favorite friction-research study, entitled the Forma- bility Criteria for Selecting Metal Lubricants, was presented several years ago at an SAE International Congress. It gave a broader solution to the lubricant-metal surface problem:
“A single lubricant should be selected for use in a press shop. The control of metal flow should be dependent on the design of the tooling, the blank size and contour, the binder force and the draw beads. A change in the lubricant, much less a dab of lubricant on the blank, should not be the method to modify material flow within the tooling.”
Several press shops have successfully implemented this concept. MF
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