Abstract

Need for increased productivity has motivated interest in drilling equipment for high speed and high precision drilling. However, long drills exhibit aggressive and self-excited vibrations known as whirling motions. These vibrations deteriorate the outcome drilled surface qualities, i.e., finish, geometrical dimensioning and tolerances. Deep hole boring is the most complex and expensive drilling operation in terms of expertise needed, operating conditions and size. The present research is to study the whirling vibrations of the boring bar workpiece system in the BTA deep hole boring process. After a review of the literature, a set of objectives is outlined for the thesis. The objectives are to formulate an analytical model, which will include the self-excited motion caused by the internal cutting forces in the Z and Y directions and the active suppression forces used to counteract the cutting forces. This model will be used to carry out simulation studies on the whirling vibrations of the boring bar-workpiece system and to carry out some experiments in order to validate the analytical model. In view of the large number of machining parameters that influence the system dynamics and the resulting surface finish, geometrical dimensioning and tolerancing on the workpiece, a fuzzy logic model is proposed for off line modeling. Such a model will be used to examine the results obtained in the experiments and to predict the best parametric combinations that will provide the required surface finish. This will constitute an open loop control of the vibrations. An active vibration suppression is carried out to reduce the whirling motion of the boring bar-workpiece system, which in turn will improve the surface finish. Two electrodynamic shakers are used in Z-Y plane to overcome the displacement of the boring bar while drilling. The experimental investigations are carried out in the deep hole boring laboratory. Instrumentation involving proximity pickups, oscillator, amplifiers and demodulator are used to measure the displacement response and the orbital motion of the boring bar-workpiece system, and two electrodynamic shakers to suppress the motion. The surface roughness, straightness and circularity are measured which are related to the whirling vibrations and the machining parameters.