To measure friction on the die-workpiece interface, sheet specimens of soft copper were first subjected to plastic compression in plane strain, and then one of the platens was slid in the direction of sheet width. With lower viscosity lubricants and lower sliding speeds, depressed flat portions and discrete oil pits were evident on the worked surfaces. With lubricants of higher viscosity under pressure and higher sliding speeds, “microscopic plasto-hydrodynamic interactions” operated to diminish oil pits and to increase the thickness of boundary films on the flat portions. The frictional stress increased almost logarithmically with the product of viscosity under pressure and sliding speed.
Sheet specimen of soft copper were subjected to plastic compression in plane strain, and then, to measure friction, one of the platens was slid in the direction of sheet width. Combined effects of die surface roughness and oil film thickness on friction are shown as the relations between friction and relative oil film thickness (ratio of mean oil film thickness to die surface roughness). With relatively rough dies, friction decreases with increase in sliding speed, and a corresponding decrease in conformance of the workpiece to the die was observed.
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