The influence of CAD model continuity on accuracy and productivity of CNC machining

Abstract Efficient and productive manufacturing of freeform shapes requires a suitable three-dimensional CAD model at the entrance to the CAM system. The paper deals with the impact of NURBS or B-spline CAD model geometric continuity on the accuracy and productivity of 5-axis ball-end milling of freeform surfaces. The relationships between a different order of CAD model geometric continuity and the quality of the toolpath generated in CAM system is analysed and demonstrated on example of Blisk blade profile. In order to reveal the pure effect of CAD geometry on the quality of the machined surface, linear interpolation of individual positions of CL points is considered and no further smoothing of the toolpath is applied. In addition to the commonly used indicator of toolpath quality represented by the distance of CL points, the discrete curvature of a toolpath in a form of a piecewise linear discrete curve is introduced and its dependence on the curvature and continuity of the underlying CAD model is demonstrated. It is shown that the higher order of CAD model geometric continuity significantly eliminates sharp changes in the distance of CL points and smoothes the discrete curvature of the toolpath. Finally, it is experimentally verified that increasing the continuity of the CAD model from G0 to G3, while maintaining the same cutting conditions, leads to an increase in workpiece accuracy and a reduction in machining time, without the need to smooth the toolpath generated in the CAM system.