Translated Abstract
High precision and high efficiency machining is inseparable from the researches on the performance of CNC machine tools and cutting process. Traditionally, the researches about machine tools and cutting process are usually carried out independently with each other, but with the continuous improvement of the requirements for machining precision and efficiency, these kinds of separated researches cannot meet the actual engineering needs because of its obvious disadvantages in revealing the essence of CNC machining, and it is urgent to consider both of them. Therefore, the dissertation studied the basic principle of mechanics and dynamics in multi-axis machining process, explored the dynamics integration modeling theory and method of multi-axis motion and milling process, proposed the optimization strategies of machining parameters and tool path under multi-constraint for high precision and efficiency, which have great significance for revealing the quantitative relationship and influence mechanism among multi-axis motion, milling process and machining precision and efficiency, breaking through the interaction mechanism between machine tool and machining in theory, and achieving high precision and high efficiency processing in application.
During multi-axis machining of complex curved surface, an accurate milling force prediction model for helix cutter was proposed. Based on the tool position and orientation information, the dissertation described the geometric relative motion relationship of tool-workpiece in spatial space, and with systematic consideration of the cutter runout, the tool-workpiece relative attitude and the motion feed speed, a trajectory surface model of cutting edge in the workpiece coordinate system was derived. By using the linear numerical iterative algorithm, an accurate calculation model of the instantaneous uncut chip thickness of cutter-workpiece engagement was established, and then the tooth start/end cutting angles were further solved, which can accurately characterize the instantaneous cutting parameters under complex cutting conditions. A non-contact identification method for cutter runout was proposed, which realized the accurate determination of cutter offset and inclination. It is found that the cutter runout has an obvious spindle speed-dependent characteristic. Then, a three-step separated calibration method for cutting force coefficients based on the thin-plate milling experiment was presented. The calibration of nonlinear shear coefficients on side edges and cutting force coefficients on bottom edges was successfully conducted. At last, a large number of milling experiments have been carried out, including straight path milling, peripheral milling, and five-axis milling. The results showed that the proposed milling force modeling method has a high prediction accuracy and good generality.
A milling dynamics model with consideration of variable cutter-workpiece orientation and multi-order modal was put forward. Based on the displacement interpolation relationship between the micro cutting disks and the DOF nodes of multi-DOF cutter-workpiece system, a calculation model of the regenerative chip thickness under arbitrary cutter-workpiece orientation was established, and in the dynamics model, the multi-order modal characteristics of the flexible machining system, the non-identical direction characteristics between DOF of nodes and cutter-workpiece engagement coordinate system, and the non-identical direction characteristics of dominated vibration in cutter-workpiece system were all considered. Based on the finite element theory, by using the constitutive model of thin-plate bending element and plane stress element, a numerical method for the dynamics analysis of typical thin-walled workpiece system was proposed, which realized the accurate calculation of the dynamic characteristics of the workpiece considering the material removal process. The dissertation further studied the milling stability with taking the time-varying delays into account, and presented a time domain numerical algorithm for dynamic engagement between cutter and workpiece under steady-state forced vibration. Lastly, a searies milling experiments have been done, in which the effect of multi-order modal characteristics, complex tool-workpiece orientations, cutter runout and milling parameters on the stability and quality of parts were analyzed.
In view of the multi-axis machining process, firstly the harmonic output torque of PM servo motor was analyzed by considering the non-ideal air gap, the asymmetry of driving circuit and the cogging moment, and the motion control model of single axis feed system and multi-axis motion of CNC machine tool was established. Then, the dissertation gave out the dynamics integration modeling method of multi-axis motion and milling process, realized the description of dynamic cutter-workpiece engagement under the condition of non-ideal displacement output of multi-axis motion process, and form the physical simulation of the machined surface during milling process. It is found that a low frequency fluctuation phenomenon would be produced on the machined surface of the workpiece under the interaction effect of harmonic excitatoin of motor torque and milling force, which further explained the forming mechanism of this kind of low frequency fluctuation on the part surface, and pointed out that the harmonic excitation will seriously deteriorate the surface roughness and found out the reason why the measured value of the machined surface roughness is far greater than its nominal value. The study clarified the superposition and coupling relationship between the multi-axis motion error and the milling process induced error, and the mapping relationship with the contour error of the parts. A series of milling experiments with multi-axis machining were carried out. The results showed that the proposed integrated modeling method is an effective approach for the accurate prediction of contour error of parts, in which the necessity and correctness has been verified.
Considering the mechanics and dynamics of milling process, as well as the multi-axis machining performance, the machining parameters and tool path optimization methods for the different stages of roughing and finishing operations were put forward. In the roughing operation of simple outline with constant milling parameters, the optimization objective with maximizing material removal rate was constructed, and the multi-constraint was included, such as parameter feasible region, milling stability, cutting torque and spindle power, and then the high efficiency machining parameters under chatter-free condition was obtained. Similarly, in the finishing operation of simple outline with constant milling parameters, the optimization objective with maximizing material removal rate was constructed, but in the multi-constraint, the machining precision was also taken into account, lastly the high precision and efficiency machining parameters was achieved. For the workpiece contour feature with complex surface, aiming at minimizing the machining error, a global optimization method of tool path based on control points correcting for complex curved surface during multi-axis machining was proposed. In the experimental cases, the tool path optimization for ruled surface in side milling and free-form surface in point milling under the high precision requirements were successfully realized.
Translated Keyword
[Integrated modeling, Machine tools, Milling Process, Multi-axis motion, Optimization]
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