| Summary: | 碩士 === 國立中興大學 === 機械工程學系所 === 101 === The main goal of this thesis is to explore and compare two different system models of boring bar containing external vibration absorbers for analyses of the stability lobes and dynamic responses during the machining process. The boring bar is modeled as a shaft that is made of isotropic or composite materials. The vibration absorbers contain masses, springs and dampers and do not rotate with the boring bar. Two different finite element models are used to represent the boring bar, one whose displacement fields are expressed in terms of inertial coordinate systems, and the other in terms of the rotating coordinate system fixed to the boring bar. Obtaining the kinetic and strain energy expressions and also the work done on the shaft by the cutting force as well as by the force that the vibration absorber exerts, the Hamilton’s principle then can be applied together with the finite element method to derive the equations of motion of the boring bar system in terms of both coordinate systems.
In the analysis of the stability lobes of the boring process, the gyroscopic matrix in the equations of motion is omitted. Next the displacements of shaft are transformed into modal coordinates by modal analysis. A simplified couple system model consisting of a single vibration mode of boring bar and vibration absorbers is used to determine the stability lobes using method in Altintas [14]. To examine above stability lobes obtained from the simplified system model, Newmark method is also employed to integrate the original coupled equations of motion to determine the stability lobes from transient responses of the system.
Adopting above methods, both the steel and the composite boring bars are analyzed where the stability lobes and transient responses of two different boring bar models are compared. The results indicate that for isotropic boring bars the stability lobes predicted by both models are very close. However, for the case of the composite boring bar there exist some discrepancies between these two models. One also finds that the stability lobes determined using the simplified system model and by integrating the original system model using Newmark method are closer to each other than those shown in reference [17].
Finally, the influence of the tangential cutting force Ft is studied. Through analyses of systems using Newmark method, it is found that by considering force Ft in additional to Fr, the stability lobes become much lower than that without Ft. Such results, however, could not be verified using the simplified stability model presented in this thesis.
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