THESIS
2022
1 online resource (152 pages) : illustrations (some color)
Abstract
Multi-axis machining has been widely used in the manufacturing industry and the multi-axis
milling process is one typical machining technique owing to its flexibility and accuracy, which
could be classified as point milling and flank milling. The process planning of multi-axis milling
on complex parts is of great significance to manufacturing efficiency and accuracy. Nevertheless,
the complexities of parts such as the non-height field, deep cavity, non-zero genus features, and
weak structures of workpieces limit the effectiveness of conventional point or flank milling
methods. In this thesis, aiming at these issues, we propose three process planning methods for
multi-axis machining of complex parts.
First, we present a novel volumetric field slicing method for rough and finish machining...[
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Multi-axis machining has been widely used in the manufacturing industry and the multi-axis
milling process is one typical machining technique owing to its flexibility and accuracy, which
could be classified as point milling and flank milling. The process planning of multi-axis milling
on complex parts is of great significance to manufacturing efficiency and accuracy. Nevertheless,
the complexities of parts such as the non-height field, deep cavity, non-zero genus features, and
weak structures of workpieces limit the effectiveness of conventional point or flank milling
methods. In this thesis, aiming at these issues, we propose three process planning methods for
multi-axis machining of complex parts.
First, we present a novel volumetric field slicing method for rough and finish machining on a
part with complex features such as deep cavities, non-zero genus as well as weak structures. For
the roughing operation of point milling, rather than simply taking Z-level parallel planes as
intermediate machining layers, we propose to use curved machining layers, which will eliminate
the severe staircase effect on the in-process workpiece as suffered by the Z-level method, and hence
significantly stabilize the cutting force and reduce the susceptibility to dynamic problems e.g.,
chattering. To facilitate the determination of the desired curved machining layers, an elaborate
algorithm is presented to construct a geodesic distance field embedded in the volume-to-remove
whose iso-surfaces are then naturally used as the machining layers, which are assumed to be smooth and have a uniform thickness. In terms of finishing, aiming at improving the stiffness of the in-process
workpiece, and also facilitated by the prescribed geodesic distance field, we employed a
new machining strategy of alternating between the roughing and finishing processes. Finally,
collision-free tool paths are devised to machine the generated machining layers.
Next, we present a quasi-developable approximation-based multi-pass flank milling method to
improve the efficiency and accuracy on a relatively large complex freeform surface. We
approximate the surface mesh with a series of quasi-developable strips on which efficient multi-pass
flank-milling tool paths are generated. The proposed methodology enjoys generality – it can
handle surface mesh with singular points and concave regions, compared with the conventional
flank milling of a single parametric ruled strip whose application is limited when dealing with
doubly curved surfaces. In specific, we calculate the normal curvature of each tangent direction on
each vertex and select the flattest direction to build a curvature-induced vector field whose local
variation assists the partition of the input mesh using n-cuts. We then cover each segmented patch
with a set of quad strips whose ruling length is ECL, through an elaborate manipulation of scalar
and vector fields and their iso-lines. These quad strips are then further optimized to increase their
developability considering the constraints such as interference-free and semi-positive, forming the
final cutter contact rulings of the flank milling tool path, follows by connecting the paths on each
strip and path to output the G-codes.
Finally, we present a collision-conscious multi-pass flank milling method to balance the
continuity of the tool path and the feasibility of the tool orientation generated on an arbitrary
complex freeform part, even with non-height field or non-genus structures. This method can
automatically generate collision-free multi-pass flank milling tool paths and resolves the greedy-lock
issue caused by the post-check of collision and the oversimplified binary collision condition.
The key to the methodology is a stripification process on the offset surface of the part which
considers the global collision information upfront when the tool path is computed, rather than
avoiding collision by amending an already generated tool path. On the offset surface, each tangent
direction reflects the possible tool axis, and the degree of collision is measured by a real number.
On each vertex, we select an optimal tangent direction that has both a small collision degree and
variation with adjacent directions, forming a smooth collision-conscious vector field. Again, along
with singular points, the local variation of the field assists in partitioning the offset mesh by n-cuts. Each patch is then parameterized by the segmented vector field, whose iso-curves guide the
potential postures of cutter location rulings. The rulings are then optimized sequentially to form a
set of quad strips to serve as cutting passes of the final flank milling tool path, tending to the
constraints of fairness, feasibility, and collision-free.
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