UBC Theses and Dissertations
Mechanics and dynamics of milling thin walled structures Budak, Erhan
Peripheral milling of flexible components is a commonly used operation in the aerospace industry. Aircraft wings, fuselage sections, jet engine compressors, turbine blades and a variety of mechanical components have flexible webs which must be finish machined using long slender end mills. Peripheral milling of very flexible plate structures made of titanium alloys is one of the most complex operations in the aerospace industry and it is investigated in this thesis. Flexible plates and cutters deflect statically and dynamically due to periodically vary- ing milling forces and self excited chatter vibrations. Static deflections of the plate and cutter cause dimensional form errors, whereas forced and chatter vibrations result in poor surface quality and chipping of the cutting edges. In this thesis, a comprehensive model of the peripheral milling of very flexible cantilever plates is presented. The plate and cutter structures are modeled by 8 node finite elements and an elastic beam, respectively. The cutting forces are shown to be very dependent on the magnitude of the plate and cutter deformations which are irregular along the helical end mill-plate contact. The interac tion between the milling process and cutter-plate structures is modeled, and the milling forces, structural deformations and dimensional form errors left on the finish surface are accurately predicted by the simulation system developed in this study. A strategy, which constrains the maximum dimensional form errors caused by static deformations of plate and cutter by scheduling the feed along the tool path, is developed. The variation of the plate thickness due to machining and the partial disengagement of the plate and cutter due to excessive static deflections are considered in the model. The simulation system is proven in numerous peripheral milling experiments with both rigid blocks and very flexible cantilevered plates. The self excited vibrations observed during peripheral milling of very flexible struc tures with multi-degree of freedom dynamics is investigated. A novel analytical model of milling stability is developed. The stability model requires structural transfer functions of plate and cutter, milling force coefficients and helical end mill geometry. Chatter vibration free cutting speeds, axial and radial depths of cut, i.e. stability lobes, are predicted analytically without resorting to computationally expensive time domain sim- ulations. The analytical chatter stability model is verified in various peripheral milling experiments, including the machining of plates. The cutting force and chatter stability models developed in this thesis can be used to improve the productivity of peripheral milling of thin webs by enabling simulation and process planning prior to production.
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