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UBC Theses and Dissertations

Mechanics and dynamics of line boring operation with process damping effect Aygün, Adem


Rotating shafts are used in the power train components of aircraft and automotive engines. The shafts are turned on lathes. Engine cylinders and bearing housings are finish machined using boring bars with single or multiple inserts. The cutting forces excite the structural dynamics of the turned shafts or boring bars during machining, leading to a poor surface finish and possible damage to the machined parts. This thesis presents mathematical models of single and multiple point turning/boring operations with the aim of predicting their outcome ahead of costly physical trials on the shop floor. Turning and boring operations are conducted at low angular speeds where the system dynamics is dominated by the process damping mechanism. The dynamic forces are modeled proportional to the static and regenerative chip thickness, tool geometry, and velocities of the vibration. The process damping coefficients, which are dependent on the material, tool geometry, cutting speed and vibrations, are identified from chatter tests conducted at the critical speeds and depths. The structural dynamics of the long boring bars are modeled using the Timoshenko Beam elements in Finite Element model which allows parametric placement of the boundary conditions, such as the bearing supports. The dynamics of the interaction between the cutting process and the structure are modeled. The stability of the operations is solved in frequency domain, analytically when the velocity and vibration dependent process damping is neglected. When the process damping is included, but the periodicity of the dynamic forces is neglected, the stability of the process is solved using the Nyquist criterion. When the periodicity and process damping are considered, the dynamic system is represented by a set of differential equations with periodic, time delayed forces. The stability of such systems, which are found in the line boring of crank and cam shaft housings, is solved in the time domain using an analytical but semi-discrete method. The thesis presents a complete set of solutions in predicting the static and dynamic forces, as well as the critical depths of cuts and speeds to avoid chatter vibrations in single point, multi-point and line boring operations.

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