CAUSES OF RANDOM VARIATIONS OF FRICTION IN SHEET METALWORKING

Studies of lubrication in sheet metalworking are often plagued by rand om variations in the coefficient of friction (f) derived from simulati on tests. Causes of such variations were studied in drawbead simulatio n (DBS) involving roller-die tests for determining the force (D(roll)) required for bending and rebending the strip. A powerful variable is sheet gauge since draw forces increase with the square of thickness. R andom variations in f due to gauge variations can then be eliminated i f local values of D(roll) are used instead of a mean value for an enti re batch of sheets. A sheet mapping scheme allows economical use of ma terial. Specimen width must be strictly controlled and mill oil residu es removed. Any remaining apparently random variations can then be tra ced to true variations in inputs to the tribological system, for examp le, in the surface topography of the sheet, structure of the zinc coat ing, or variations in lubricant residues.