Boundary conformed toolpath generation for trimmed free-form surfaces

This dissertation presents methods for generating boundary-conformed toolpaths for trimmed free form surfaces in CNC machining. Normally, before cutting a free-form surface, the cutter paths must be generated based on the surface information extracted from the design model. In the process of CAD/CAM, predefined parametric surfaces are often subject to the operation of trimming or extending. In such a case, the original isoparametric curves are no longer boundary conformed. Consequently, they are not anymore suitable for being used as natural CNC toolpaths. For the applications requiring certain finish quality or superior aerodynamic effect, it is sometimes desirable to be machined with boundary-conformed or streamline-like toolpaths.; In this thesis, by adopting a 2D reparametrization procedure, a two-phase mapping method to regenerate boundary conformed iso-parametric toolpaths on altered freeform surfaces is presented. Three methods for the 2D reparametrization of trimmed boundaries in parametric space are examined and compared. They are the Coons method, the Laplace method, and a newly developed boundary blending method. Complete algorithms for surface reparametrization are developed. The isoparametric toolpath generation algorithm for discrete parametrized surfaces is given and implemented. The application of this method has been successfully illustrated on a typical trimmed surface, a turbine blade. The results also showed that both the computational efficiency and parametrization robustness are quite good for the newly developed boundary blending method. In addition, it alleviates the uneven distribution problem appeared in the Laplace results.; Based on 2D boundary-conformed parametrization, a practical application to generate toolpaths for machining arbitrarily shaped pockets with or without islands is also presented. The thin wall problem can be eliminated. Also, the tool retractions can be reduced. No search of the self-intersection and loop separation is needed via our approach. The complicated determination of path sequence can be omitted or be simplified. Detailed procedure for generating thin-wall-free pocket contouring is developed. Some implementation results are illustrated to demonstrate its applicability.