THESIS
2019
xiii, 116 pages : illustrations ; 30 cm
Abstract
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,
conservative machining parameters are adopted as constants for each cutter locations which are
determined by simply taking 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...[
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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,
conservative machining parameters are adopted as constants for each cutter locations which are
determined by simply taking 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
optimized process planning strategies for five-axis machining including both end milling and
flank milling aiming at improving the machining time efficiency to educe the total machining
time while maintaining the acceptable machining errors.
For five-axis end milling, the optimization process 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. It strives to maximize the depth-of-cut
locally for each cutter contact (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 workpiece 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.
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