An integrated approach to finish machining of RP-produced parts

Rapid Prototyping (RP) techniques are being increasingly used in industry to manufacture functional parts or tools, due to the fact that they have many benefits over traditional methods such as making parts with complex shapes and advanced material systems. However, this kind of layered manufacturing approach, decomposing the 3D CAD model into 2D layers and building the part layer by layer, still has practical limitations on surface quality and accuracy. Therefore, CNC machining is often necessary for finishing rapid manufactured parts to an appropriate accuracy and surface finish for many applications. For most RP techniques, the shrinkage levels and the optimum building conditions are material and process dependent, thus pre-processing of the 3D model is also often necessary prior to its RP fabrication. An investigation was performed to develop techniques for the effective integration of CAD, RP and CNC milling to improve accuracy in RP-produced tools and parts. The work focuses on the issues of original STL file preprocessing and STL-based automatic tool path generation for CNC machining. An OpenGL-based STL viewing module was developed in the integrated software system to effectively display the 3D model and tool paths. A unique offset algorithm was developed to add “skin” to the original STL file, in which, vertices, instead of facets, are offset. The magnitude and direction of each vertex offset is calculated using the weighted sum of normal vectors of adjoining facets. Considering both accuracy and machining efficiency, the machining strategy of adaptive raster milling of the surface, plus hole drilling and sharp edge contour machining, is applied to finish the RP parts and tools. The detailed algorithms, including sharp edge detection, drilled hole recognition, and tool path generation routines for raster milling, sharp edge contouring and hole drilling, were developed. An integrated software package was developed using Visual C++ to implement the developed algorithms. Finally, experiments were successfully performed to prove the feasibility of the presented procedure for finishing RP parts, and to verify the developed pre-process and automatic tool-path generation algorithms. The experimental results also demonstrated some of the advantages of the developed finishing strategies and algorithms