Effect of Tool Radius and Iso-parametric Discretization of Triangulated NURBS Surfaces on Surface Finish in 3-axis Vertical Finish Machining Using Ball End Mill

Abstract

The precision machining of sculptured surfaces is a challenge. The modern CAD-CAM technology has been explored tremendously for adding ease of machining complex sculptured shapes and indeed these systems has added to the productivity of the modern shop floor. The choice of the proper CNC machining centre, the geometry of the cutting tools, the accurate 3D representation of CAD geometry of the part to be machined and the selection of proper NC toolpath computation algorithm are the key components that counts for successful output which in this case is error free and faster machining of complex surfaces within the required surface finish. For the given input machining conditions the accuracy of the toolpath data greatly influence the quality of the machined surface. The use of ball end milling cutter is preferred for sculptured surface machining. The computation of toolpath data for a ball end mill is well established and it is also a known fact that the use of 3D part data in the triangulated faceted format eases the computation involved for determination of gouge-free toolpath data for CNC machining. Though NURBS surfaces can be used to accurately define the geometric information of the 3D surface models, but it is a challenge to determine the gouge free tool positioning data directly from NURBS surfaces. In many applications the NURBS surfaces are preferred for solid modelling and part representation, but for manufacturing applications the use of triangulated data models can yield gouge-free machining results. Thus, in this work a generalized NURBS code is used to fit a NURBS surface through a given set of data points. Also, the developed NURBS code is used to extract geometric information of the developed NURBS surface in the form of a uniform grid of iso-parametrically located points. The iso-parametric point cloud data is intern used to define a triangulated meshed surface which is further used to generate the gouge-free toolpath data for a ball end milling cutter. The toolpath generation algorithm used for toolpath computation for ball end milling cutter uses the concept of “drop the tool” and “ray tracing algorithm” in a unique way which ensures that the user defined scallop height is achieved by computing the adaptive side-step values for various machining passes for a zig-zag toolpath pattern with a uniform feed forward spacing of tool motion. In this work the mathematical model used for development of this algorithm is presented along with the validation of the results from the developed algorithm for faceted freeform, concave and convex surfaces. Further this algorithm is used to identify the relationship between the tool dimensions, iso-parametric triangulation of NURBS surfaces and the surface finish that can be achieved within a reasonable number of machining passes. The results from the algorithm shows that the smaller tool radius helps achieve lower scallop heights while the change in iso-parametric spacing for triangulated meshing of the NURBS surface does not play a role in reducing the scallop height when subjected to a condition that a minimum side-step value has to be ensured to keep the maximum number of tool passes in overall toolpath data under control. The overall procedure of the developed toolpath algorithm for triangulated NURBS surface models is presented in this thesis work along with results obtained for various analytical simulation studied during this work.