This thesis presents a new method to compute a global visibility map (GVM) in order to determine feasible axes of rotation for 4-axis CNC machining. The choice of the 4th-axis is very important because it directly determines the critical manufacturing components; visibility, accessibility and machinability of the part. As opposed to the considerable work in GVM computation, this thesis proposes an innovative approximation approach to compute GVM by utilizing slice geometry. One advantage of the method is that it is feature-free, thus avoiding feature extraction and identification. In addition, the method is computationally efficient, and can be easily parallelized in order to vastly increase speed. In this thesis, we further present a full implementation of the approach as a critical function in an automated process planning system for rapid prototyping.