Punching aluminum sheet with rubber press pads
Hertz, Philip Barry
Rubber pads can be employed to cut or form sheet metals when acted upon by high pressures in conjunction with suitable tooling. This phenomenon has been incorporated into a production technique known as the Guerin process. It is advantageous in that the manufacturing of the closely mating conventional steel punch and dies can be eliminated, resulting in exceptional press work economies. However, its application has been limited due to a lack of understanding of the process variables. This thesis analyzes the effects of the rubber characteristics and tool geometry upon the punching of circular holes in aluminum sheet by confined rubber press pads. A Hydrostatic Shear Theory was developed to predict the pressure necessary for punching. Seven rubber compounds were physically tested to obtain bases of comparison for analyzing their punching performances. Four diameters of circular dies were used in conjunction with one type of aluminum sheet to study the punching behaviour of each of the rubbers. Experimental results show that punching occurs in two stages of fracture - primary and secondary. For a given tool geometry the criteria for the initiation of primary fracture is a critical volume rubber displacement. Modified empirical equations are derived from the Hydrostatic Shear Theory relating the experimental primary fracture pressure to tool geometry, material resistance, and rubber stiffness as determined compression - deflection tests. The press pad thickness and rate of loading are also found to influence primary fracture. Secondary fracture appears to depend upon the depth of die employed. The effects of lubrication and lamination of the press pads are briefly explored tor both primary and secondary fracture. Additional fields of research directly related to this punching process are indicated.