INTERNAL VARIABLES EFFECTS IN PUNCH FRICTION CHARACTERIZATION

Despite the complexity and importance of friction, most current simula tions of metal-forming processes use relatively simple friction models such as the Amon-tons-Coulomb constant friction coefficient. It has b een pointed out that simple models are not capable of capturing the in fluence of process variables such as geometry, speed, and surface topo graphy on friction. A realistic friction model should include the inte rnal variables such as lubricant film thickness, tooling roughness, an d workpiece roughness. Irt the present research, the punch friction te sts which use a tensile strip experiment to simulate the stretching of sheet over a punch corner radius in a typical draw die are used to me asure the effects of internal variables on friction in various stretch ing conditions. The measured friction coefficients increase with lower stretching speed and decrease if lubricant is applied at the interfac e between workpiece and cylindrical pin. Theoretical friction modeling , which includes the different lubrication regimes range from thick fi lm, thin film, mixed regime and boundary regime, are presented. Numeri cal methods have been used to solve the governing differential equatio ns with the known initial boundary conditions obtained from the experi ments. The theoretical prediction shows the same trend as the experime ntal measurements.