PhD Thesis Presentation
When machining thin walled workpieces like turbine blades, the parts are prone to be bent by the cutting forces due to its low rigidity. The workpiece deflection during the machining process of the thin-walled workpiece is a critical issue that is likely to jeopardize the finish surface quality. Traditionally, in order to control the machining error caused by deflections, conservatives machining parameters are adopted as constants for each cutter locations which are determined by simply takes the weakest spot as the reference. As the residual thickness left on the thin-walled part surface usually cannot be removed with a single cut, multi-pass machining is necessary to be used by removing the residual thickness with several rounds of cuts. For a given thin-walled workpiece, the primary objective in this thesis research is to develop optimal process planning strategies for five-axis machining including both end milling and flank milling aiming at improving the machining time efficiency (reduce the total machining time) while maintaining the acceptable machining errors.
For five-axis end milling, the optimization to achieve this goal is divided into two parts. Firstly, the optimization of machining parameters for thin-walled workpiece is accomplished by a novel variable depth-of-cut machining strategy for semi-finish and finish machining based on the workpiece deflection constraints, which strives to maximize the depth-of-cut locally for each CC point while respecting a calibrated threshold of the normal cutting force which is the major cause of the workpiece deflection. The maximum allowed depth-of-cut of each CC point is determined adaptively according to the calibrated maximum allowed of the thin-walled workpiece. The multi-pass semi-finishing tool path is generated by offsetting the finishing tool path with the computed variable depth-of-cut.
Next, we integrate the optimization of finishing tool path and machining parameters together to further improve the machining efficiency of the multi-pass machining for the thin-walled workpiece. A five-axis tool path generation method is developed to automatically generate a finishing tool path that reduces the total machining time of the multi-pass tool path expanded from the generated finishing tool path for an arbitrary thin-walled workpiece. A cutting area potential field on the given part surface of a thin-walled workpiece is first established to indicate the principle feed direction for any point on the part surface. Based on this field, the finishing tool path is planned through the iso-scallop height tool path expansion scheme.
For the flank milling of thin-walled workpieces, a new multi-pass adaptive machining parameters optimization method for flank milling of thin-walled workpiecse is developed that generates the multi-pass tool path from an input finishing tool path, with the optimized radial depth-of-cut and feedrate assigned to each cutter location. Both radial depth-of-cut and feedrate are simultaneously optimized by solving a minimization problem formulated based on the cutting force model subject to the deflection constraints and machine’s kinematical constraints.
Finally, computer simulations and physical cutting experiments are reported to clarify the improvements achieved in machining efficiency by the proposed strategies.
(Supervisor: Prof. Kai Tang)