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UBC Theses and Dissertations

Modeling of mechanics and dynamics of boring Atabey, Fuat

Abstract

This thesis investigates the mechanics and dynamics of boring operations. The mechanics of boring operations deal with the prediction of cutting forces as a function of tool geometry, work material properties, and cutting conditions such as feed rate, radial depth of cut, and cutting speed. The dynamics of the process involve the modeling of interactions between the structural dynamics of a long, slender boring bar, with boring process mechanics. Evaluation of forces allows the prediction of static deflection errors, torque and the power required from the machine tool. Evaluation of the dynamic stability of the process leads to the prediction of the chatter vibration free feed rate, spindle speed, radial depth of cut, and tool geometry. The thesis shows that boring forces are strongly dependent on the tool nose geometry, side cutting edge angle, radial depth of cut, feed rate and cutting speed. The chip thickness distribution along the curved edge of the tool is rather complex. The chip close to the nose is thin, and becomes thicker along the curved edge as the radial depth of cut increases. The chip thickness distribution is also affected by the feedrate. It is proposed that cutting forces are modeled as a function of total chip area and cutting coefficients. The chip area is divided into several distinct geometric regions, and the center of each area is identified. Friction and tangential cutting forces are formed at each region. Cutting forces are modeled at each region, and summed up to find the resultant friction and tangential cutting forces. Using an equivalent friction or lead angle, the friction force is projected in the radial and feed directions. This model allows the prediction of cutting forces in all three Cartesian directions. The influence of tool setting errors for boring heads having multiple inserts are also considered in the general model. Several experimental results are compared with the predictions based on the proposed mathematical model. The predictions are shown to have errors varying between 2% and 15%. The proposed model contributes to the improved prediction of boring mechanics. The fundamental mechanism behind chatter vibrations in boring process is also investigated. It is shown that the cutting coefficients, i.e. process gain, and directional factors, are dependent on the feed rate, radial depth of cut, tool geometry, and cutting speed. While the tool geometry and speed may be kept constant, vibrations modulate radial depth of cut, and leads it to be a timevarying process input parameter. This is the fundamental non-linearity in the process, which differs from milling operations. The dynamic process is modeled in both frequency and time domains. However, the process non-linearity varies significantly during the process, preventing the application of classical linear chatter stability laws to the boring process. It is shown that the time domain modeling also suffers, mainly due to the digital integration of a significant number of tool deflection waves left on the boring surface.

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