This project was initiated to study the effects of the sheet metal on the process of punching circular holes with viscoelastic tooling. Two different kinds of aluminum sheet metal, each with four thicknesses were used in the experiments. The mechanical properties of the various sheets were evaluated with particular attention being paid to the mechanical anisotropy so that this could be related to the failure during the punching operation. A new high pressure retainer was designed and built for pressurization of the viscoelastic press pads. This along with existing equipment and auxiliary apparatus for the detection of secondary fracture was used to study the primary and secondary fractures. As a result of the analysis of the primary and secondary fractures a useful prediction equation was developed. The prediction equation, with limitations, can be used to estimate the pressure necessary to cause complete fracture when punching holes in the aluminum sheet metals. The secondary, fracture analysis yielded contour-type plots from which the suitability of certain combinations of the process variables can be determined. These plots show that for certain combinations complete fracture cannot be obtained with practical pressure ratios and with other combinations an optimum punching condition can be established. Suggestions for future research on viscoelastic tooling have been presented.