| Geometrically complicated workpieces are often modeled by compound surface, meshed surface and trimmed surface. In the machining of these parts, the pathopography generated by traditional toolpath planning model can not conform to the surface geometry. Due to the implicit expression of surface, the iso-planar method is still commonly adopted to machine mesh model. Thanks to the predefined parameterization being sacrificed by Boolean operation in CAD stage, the toolpaths generated by iso-planar, iso-parameteric and iso-scallop model, are no longer boundary conformed in trimmed surface and compound surface machining. In these cases, it is hard to guarantee the machining efficiency and quality with respect to discontinuous cutting process and many "go-stop" motions of machine. In practical production, performance of machine is restrained and machining period is delayed in terms of manual scheduling and verification of tool posture and selection of conservative process parameters. So, in this dissertation, the NC machining of complicated surface is studied. The models of toolpath planning, cutting force prediction and feedrate optimization are proposed to improve the efficiency under given precision.Designated mapping based toolpath planning method is proposed. Based on the designated mapping, the parametric relationship between surface guided model and mapped region is built. Cutter contact toolpath is constructed by reversely mapping the path interval based locus in mapped region with area coordinate. Then cutter location path is generated with machine kinematics-constrained and whole faired tool posture. The designated mapping is constructed by linearly combined spring length energy function and conformal mapping function. The principle of original tool axis determination is proposed which is derived from the precious calculation of path interval for the filleted-end milling cutter. The cutter contect point path is constructed with the aid of the area coordinate algorithm, path interval and parameter increment calculation. The special case of spiral type toolpath is discussed in detail. With respect to the original tool axis vector, whole faired tool axis vector is obtained by orientation spline interpolation of tool axis vector of "key point" selected by kinematical constraints of machine. The computer simulation and cutting experiment is operated for verification of the proposed model.Natural boundary mapping based contour offset five-axis toolpath planning model is presented for complex trimmed surface with islands. The natural boundary mapping is defined on the basis of area gradient equation and area of mesh model which can represent the Dirichlet function. With the proposed mapping, natural boundary parameter region can be determined by constraining arbitrary two adjacent boundary points. Starting from boundary of mapped parameter region, the contour-parallel path is construed with offset-modification method based non-local interference regional offsetting, monotone chains and division based globe interference eliminating and tree-like data structure based offset loop connecting. For the contour offset cutting contact path mapped with area coordinate algorithm, a modified machine kinematical constraints criterion is presented with consideration of the influence of sharp change of feed direction. The computer simulation of trimmed surface with multi-irregular islands machining verified our method.Based on the tool-workpiece relative motion, a precious cutting force prediction model is proposed for general milling tool cutting with curve path in five-axis machining condition. According to differential geometry, geometrical model of revolution surface and spiral flute of cutter is constructed and swept surface of flute during cutter moving is modeled with consideration of kinematic structure of multi-axis machine, toolpath and cutting parameters. With swept surface of flute, undeformed chip thickness is calculated which is more accurate than traditional model proved by slot cutting experiment. The detailed calculation of undeformed chip thickness for three-axis and five-axis ball-end cutting case is also derived. For cutting coefficients calibration, a single-cutting experiment based analytical model is proposed and used for aluminum 2024 and 7075-T6. The in-cutting tool region is automatically determined by a modified Z-map model. Validation tests are conducted under different cutting conditions for three-axis and five-axis machining. The comparison between predicted and measured values demonstrates the applicability of the proposed prediction model of cutting forces.An adaptive feedrate schedule strategy is proposed for complex curved surface machining using process constraints and a tool motion analysis based precision cutting force prediction model. The influence of feedrate on surface topography and workpiece performance is first analyzed by surface topography simulation model and parameter defined in ISO 12085:1996. The adaptive feedrate schedule strategy is then proposed. In terms of the cantilever beam model, tool stress model and cutting torque model, key process constraints are calculated with the proposed precision cutting force model. With defining the material removal rate as objective function, the feedrate is scheduled by solving a constrained optimization problem. The machine experiment illustrates that the proposed feedrate schedule strategy is very useful in high precision and efficiency machining of complex curved surface. Finally, the typical marine propeller is machined with the proposed five-axis toolpath planning model and the adaptive feedrate schedule strategy. The experiment results illustrated that presented spiral and contour offset five-axis toolpath are better than two traditional toolpath model in terms of tool path length, machining time, as well as machined surface quality with given machining precision. With the optimized feedrate, the efficiency is significantly improved in machining with spiral toolpath. |
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