Finishing of freeform surfaces with an optimized Z-Constant machining strategy

Abstract

Freeform surfaces used in molds and dies industries…etc. are machined on multi-axis CNC milling machines with the objective to achieve a good surface finish and a minimum machining time. For the finishing operation, Parallel Plane, Isoparametric, Z-Constant, etc. machining strategies are used. Z-Constant machining strategy is suitable for machining vertical or slant surfaces and deep cavities. In this strategy, cutting tool removes material on consecutive horizontal parallel planes which increases tool clearances and engagements and hence air cuts and machining times. Commonly, tool path is generated using a linear interpolation which makes tool movements discontinuous which creates vibrations that affect surface finish. To improve surface finish and to reduce machining programs sizes, tool path must be generated using continuous curves. In this paper, for finishing freeform NURBS surfaces on 3-axis CNC milling machines using ball cutters and Z-Constant machining strategy, an approach is proposed for determining the optimum tool avoiding interferences, optimizing the tool path by considering machining and geometric constraints. Finally, generation of One-Way or Zig-Zag tool path in terms of B-Spline curves by finding the minimum control points number producing the best tool positions approximation to achieve a good surface finish and a reduced machining program. This approach reduces the product development cycle, decreases machining times and costs and increases productivity.