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An study of unit machine operations for tool paths in high speed milling

Authors Fan, Sai Kit
Issue Date 2008
Summary High Speed Milling (HSM) machines are growing in mainstream manufacturing such as machining of dies and molds. The advantages of HSM include reduction in machining time and better surface finish. A typical HSM machine has a heavy tool-carrying spindle assembly with one to three degrees of freedom. Consequently, high inertial forces are generated as the spindle head moves to perform cutting. Machining path planning for HSM machines is the design of the motion of the cutter relative to the part such that the shape is cut n minimum time subject to several constraints. For example, finish cuts limit the thickness of the layer to be machined below a given value. Rough cutting constraints are often guided by the ability of the machine tool to achieve a desired feed rate and cutting speed, without increasing the likelihood of tool failure. Feed rate and cutting speed are related to machine power, chatter, and cutting forces. Tool failure can be induced due to sudden changes in the cutter-workpiece engagement (CWE). Likewise, tool failure as well as machining accuracy is affected by inertial forces on the table as well as the spindle head assembly. All practical machining planning algorithms design tool paths by using some typical patterns of shapes (e.g. zigzag, contour offsets, spirals or trochoids). In the first part of this thesis, we perform an in-depth analysis of all existing path planning algorithms, and point out their strengths and weaknesses by applying them to a series of benchmark part shapes. The objective was to analyze different types of strategy of tool path joining in unit machining operations and their affect on the overall cutting time. In the second part of the thesis, the issue of designing optimized unit shape patterns, in particular spirals and generalized trochoids is examined. In-dept analysis of path patterns based on several common criteria is carried out, including: path length, curvature, and CWE. Two new pattern styles utilizing Cornu spirals (Clothoidal curve) and Spline interpolation are proposed and optimal methods to use them in path planning are introduced. The results show that path length can be minimized by the employing circular arc splines. In the view of curvature change, Bi-Clothoid had the best performance due to its G2 property. On the other hand, Bspline curves are marginally better from the point of view of engagement analysis.
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2008
Language English
Format Thesis
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