The authors presented a machining system based on an industrial robot for foamed polystyrene materials. The developed robotic CAM system provided a simple interface without using any robot languages between operators and the machining robot. In this paper, a robotic preprocessor is reviewed for the machining robot to convert STL data into cutter location source data called CLS data. The STL originally means Stereolithography which is a file format proposed by 3D Systems and recently is supported by many CAD/CAM softwares. It is also known as Standard Triangulated Language in Japan. The STL is widely spread to rapid prototyping using a 3D printer which is a typical additive manufacturing system. The STL deals with a triangular representation of a 3D surface geometry. Then, triangulated patches generated from smoothed point cloud data (PCD) is described. The process allows to reconstruct 3D digital data of a real object written with STL format from original PCD with noise for reverse engineering. The effectiveness and usefulness of the proposed system are demonstrated through actual machining experiments.