Non-isoparametric tool path planning by machining strip evaluation for 5-axis sculptured surface machining

Abstract

Presented in this paper is a new approach to 5-axis NC tool path generation for sculptured surface machining. Techniques of feasible machining strip evaluation are used for non-isoparametric 5-axis tool path generation. A searching algorithm is proposed to find the parameter increments of adjacent cutter locations along orthogonal path intervals for optimal non-isoparametric path generation. Compared to the use of the smallest path interval by the traditional constant parametric path planning, the proposed methodology can generate efficient tool paths for sculptured surface machining by reducing the redundant overlapping between adjacent tool paths. The proposed methodology includes three steps: (1) evaluating feasible machining strip, (2) solving parameter increments (Δu, Δv) along orthogonal path intervals, and (3) searching for adjacent non-isoparametric cutter locations. The techniques presented in this paper can be used to improve 5-axis machined surface quality and to automate the non-isoparametric cutter path generation for CAD/CAM systems.

Introduction

Sculptured surfaces, which are commonly used in industry, usually have irregular curvature distribution that causes difficulties in machining. The increasing complexity of sculptured surface parts makes the potential benefits of 5-axis milling continuously grow1, 2, 3. Some tool and die makers have found that, by changing from 3-axis to 5-axis milling, efficiency gains of 10 to 20 times could be achieved4, 5. However, 5-axis CAM software is still expensive and often lacks flexibility when specifying the tool path generation for machining. In practice, 5-axis machining suffers from a number of drawbacks, most of which are related to complex tool movements, gouging and tool interference6, 7, 8, 9, 10. Because of the two additional degrees of freedom compared to 3-axis machining, 5-axis machining has brought advantages as well as new problems, such as insufficient support by conventional CAD/CAM systems, highly complex algorithms for gouging avoidance and collision detection between the tool and the non-machined portion of the workpiece, etc.

Parametric curves and surfaces including non-uniform rational B-spline (NURB) are commonly used to describe surfaces of objects being designed. Such objects have typically been rendered by tessellating to bilinear quadrilaterals or triangles. Isoparametric curves across the surface are rendered by tessellating to polylines. These simple piecewise linear approximations are then used to drive numerically controlled milling machines. Adaptive forward difference is an incremental technique proposed previously for rendering parametric curves and surfaces[11]. Hybrid rendering algorithm and integer adaptive forward differencing techniques have been proposed for rendering free-form objects in computer graphics11, 12. Recently, [13] also developed an adaptive-isocurve extraction method for tool path generation of milling free-form surfaces.

The difficulties of sculptured surface machining inherent in the conventional CAD/CAM systems for sculptured surface machining are mainly due to the way in which tool paths are generated14, 15. User interaction is needed to generate the NC part program for sculptured surface machining, which requires considerable checking, verification, and rework16, 17, 18. Manual planning and programming for sculptured surface machining is known to be error-prone and inefficient. These problems must be solved in order that the full advantages of 5-axis machining can be exploited more widely.

Tool paths for sculptured surface machining are traditionally generated by guiding cutters to trace along constant parametric curves on the surfaces19, 20, 21. An example of the conventional constant parametric cutter path generation for sculptured surface machining is shown in Fig. 1. When a cutter moves in parallel trajectories, scallops are created on the finished surface. The distance between the parallel trajectories is the CC path interval, which depends on the local surface shape, cutter size and the allowable scallop height remaining on the surface after machining22, 23, 15, 34, 35. To achieve machined surface quality, the scallops left on the part surface need to be controlled within allowable tolerance. In the constant parametric (isoparametric) path generation methods, tool path distribution is determined by calculating, at each path, the smallest tool path interval and using it as a constant offset in the next tool path19, 4, 24, 20. The reason for selecting the smallest interval as the offset distance is that this makes it easy to define the constant isoparametric offset as the next tool path to satisfy the surface accuracy. One serious problem of this method is the inefficient machining due to the non-predictable scallop remaining on the part surface4, 23, 25. Since the next tool path is generated by the smallest path interval between adjacent tool paths, redundant machining overlap occurs between the two adjacent tool paths, which causes machining inefficiency.

In this paper, we adopt a new approach for the determination of efficient tool paths by using the machining strip evaluation method and a non-isoparametric path generation algorithm, which results in a shorter tool path than the conventional constant parametric tool path methods achieve. Along a tool path, a non-isoparametric adjacent tool path can be found by calculating its orthogonal path interval using machining strip width evaluation. The result of using a proposed search procedure is that the machined area between the new path and the previous one maintains a constant scallop that satisfies the required surface tolerance. This new curve is chosen to be the next tool path, which guarantees no redundant tool motion.

The proposed methodology includes three steps: (1) feasible machining strip evaluation, (2) solution of parameter increments (Δu, Δv) along orthogonal path intervals, and (3) a search for adjacent non-isoparametric cutter locations. The proposed methodology can be used to improve the quality of 5-axis NC complex surface machining. Details of the algorithms are presented in 2 Feasible machining strip evaluation for 5-axis surface machining, 3 Tool path distribution by machining strip evaluation, 4 Solving parameter increments of the searching steps, 5 Searching for the adjacent non-isoparametric cutter locations, 6 Algorithm for generating non-isoparametric tool paths.