Research On Feasible Tool Orientation Computation For NC Machining Of NURBS Surface

With the improvement of NC technology, 5-axis machine tool is widely used in machining industry. Free-form surface machining makes the potential benefits of 5-axis machining continuously grow, but it suffers from more complex interference than 3-axis machining. How to eliminate the interference problems is an extremely important issue. Therefore, in this paper some key geometric operations such as interference check and elimination, feasible tool direction judgment and optimal tool gesture selection are mainly discussed for machining of complicated part that expressed by NURBS surfaces.First, with consideration of the difficulty of manipulating NURBS surfaces especially in the process of subdividing and trimming surface, a NURBS surface is converted into several Bezier surfaces by knot insertion algorithm. Then the given Bezier surfaces are further refined within the tolerance requirement. Thus, the issue of multiple intersection points can be avoided, and the proposed method also improves the controllability on NURBS surfaces. Additively, for avoiding gouge and nonlinear computation in rough machining, an approximated model that can envelope the real machined part is established using convex hull property of Bezier surfaces. Then the tool paths for rough machining are generated from the approximated model instead of the real part. The simulation results of rough machining the bike seat have demonstrated the feasibility of the proposed method. Based on division and refinedness algorithms, a two-phase approach to judging interference is presented for 5-axis surface machining. In the first phase, quick feasibility checking is performed based on visibility analysis. Collision is checked quickly with visibility cones of the point, used the condition that visibility orientation is feasible possibly. Visibility orientation is not always feasible orientation for tool radius, so it is possible that there still exist collisions. In the second phase, detailed feasibility checking is proposed based on the distance from refined Bezier surfaces control points to tool axis. The tool orientation is corrected. The two-phase improves the check efficiency, and visibility cones can eliminate indetermination of tool orientation. At last, in the paper, general formula describes tool, and using differential geometry, the condition for free curvature collision is computed in NC machining. According to minimize machine tool librations and smooth the tool path, optimal tool orientation is computed.