| Five-axis machine tool has not only broaden tremendously the processing craft scope, but also improved greatly the machining precision and efficiency thanks to the two additional rotary axes. However, there are still some difficulties in using the five-axis machine tools, such as the five-axis interferences detection and avoidance problem, the five-axis tool path generation problem, the five-axis singularity detection and avoidance problem and the five-axis nonlinear error control problem. Based on the current available relevant theories and literatures, this doctoral thesis is intended to do some basic research studies on the above mentioned problems. The purpose of these studies is to accumulate some experiences and useful algorithms for the development of five-axis computer aided manufacturing (CAM) software. This thesis uses the five-axis tool path planning process as its main line, including the three-axis roughing and the five-axis finishing procedures. And in this thesis, the research focus is especially on the five-axis finishing tool path generation. A salient feature of this thesis is that the interference problem and the singular problem are considered at the beginning of the five-axis tool path generation process; with these considerations, the uniform scallop error and uniform chord error tool path is generated. The nonlinear error that appears in the five-axis machining process is controlled by the workpiece setup position on the working table of the machine tool. The main contribution of the thesis is summarized as follows.Several efficient key algorithms are proposed for the roughing tool path planning process. At first, a fast point-sequence curve (PS-curve) linking algorithm is proposed. This algorithm is based on the quick sort process of the end points of the intersection line segments in the layer-by-layer rouging process, and is used to speed up the identification process of the cutting areas. As there might be some island contours in the obtained cutting areas, a fast island-bridging algorithm is proposed to bridge all the island contours to the outer contours. The output of the algorithm is a curve which has connected all the inner or outer contours. For this curve, a fast curve algorithm based on the profile updating rules and the tree analysis procedure is proposed. This algorithm is used to generate the contour-parallel rouging tool path on each layer. The time complexity of the above three algorithms are all near linear. At last, a novel uncut region clearance strategy is developed to cope with the uncut problem in the path corner or neck areas. This strategy is based on the arc extraction and linking techniques and is used to improve the rouging efficiency.A fast five-axis tool axis admissible area generation algorithm based on the admissible area interpolation process is proposed. This algorithm is used to improve the efficiency of the interference detection process between the cutting tool and the part in five-axis tool path generation process. The algorithm can be briefly described as follows. At first, a few sample cutter contact (CC) points are picked on the input freeform surface S(u, v) according to some certain rules. For each CC point, the tool axis admissible area is calculated with the tool posture equations and the tool-surface intersection detection model. Then the cubic B-spline surface interpolation method is used to interpolate the obtained admissible area. The control points together with the knot vectors are calculated. The output of the algorithm is an expression as the form Ω(w, v). For Ω.(u, v), when the surface point parameter (u0, v0) is assigned, the corresponding tool axis admissible area can quickly be calculated simply by substituting (u0, v0) into Ω(u, v) as Ω(u0, v0).A five-axis uniform scallop error tool path curve generation algorithm based on the cutting normal vector method is proposed. This algorithm can be used for larger cutter and all kinds of part surfaces, and is therefore generic. The basic principle of the algorithm is as follows. At first, grasses (normal vectors) in enough density are planted onto the part surface. The heights of these grasses are set to be the maximum allowed scallop error. When the cutting tool is positioned on the current tool path curve on the surface at the CC point and then moves along the tool path, the grasses that make contact with the cutting edge of the tool are cut short, forming a machining band on the surface. The forward border of the machining band is extracted and is used as the backward border of the machining band of the next tool path curve. This process is recurred until the whole part surface is covered with tool paths. The feature of the proposed algorithm is that less mathematical approximation operations are used; therefore, the generated uniform scallop tool path is more precise and generic.A five-axis uniform chord error off-line cutter location generation algorithm based on the cutting normal vector method is proposed. This algorithm can generate fewer cutter locations on a tool path curve while keeping the machining precision. In the algorithm, the cutter locations are also generated recursively. In each recursion, there are two sub procedures, the initial step calculation and the precise step calculation. The initial step is calculated by using the second order Taylor’s expansion and the circular arc approximation of the local tool path curve. As the Taylor’s expansion is imprecise, the obtained initial step is imprecise. It is used as the input of the second sub procedure. In the second sub procedure, the precise step is calculated using the precise surface machining error evaluation model based on the cutting normal vector method. The proposed algorithm can achieve uniform surface machining quality; meanwhile, the number of cutter locations is reduced. It is especially suitable for high-speed machining.A five-axis non-singular tool orientation optimization algorithm based on the orientation polyline translation in the C-space is proposed. This algorithm detects and avoids the singularities at the tool path planning stage. At first, just after the initial cutter locations are generated, extra information like the corresponding CC points and local coordinate system are stored. Then all the tool axis unit vectors from the current tool path are projected to a predefined C-space, forming an orientation polyline. In the C-space, a taper circle is defined to detect the potential singular problem. If the orientation polyline touches the taper circle, the polyline is translated with a minimum distance to avoid the taper circle. Finally, the obtained new orientation polyline together with the saved local coordinate system information are inversed to calculate the new cutter locations. These new cutter locations are free of singularizes. The feature of the proposed algorithm is that it can keep the number and positions of the original CC points unchanged.A five-axis workpiece setup position optimization algorithm based on the particle swarm optimization (PSO) considering the RTCP (ratational tool center point) interpolation function is turned on is proposed. The algorithm is used to decrease the inevitable five-axis nonlinear error in the machining process. The rotary table (RT) type five-axis machine tool is used as the example machine tool. At first, the kinematic and reverse kinematic equations of such machine tool are established. Then the nonlinear error evaluation method with RTCP interpolation function on is derived. With this, the nonlinear error is expressed as the function of the workpiece setup position on the working table. After that, the improved PSO is used to optimize the workpiece setup position. The purpose is to achieve the minimum machining nonlinear error. The original PSO is improved by canceling the velocity item and adding a mutation item. The feature of the proposed algorithm is that it can significantly reduce the nonlinear error in five-axis machining process with nearly zero cost. And it can be popularized to any kind of five-axis machine tool with any axis configuration. |
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