STABILITY OF THE TURNING PROCESS

Abstract

Investigations on the stability problems in turning have mostly been conducted for the case of orthogonal turning without giving due considerations to some of the important process parameters, such as the workpiece dimensions, its comp-liance, the compliances of the head and tailstock centres, system damping, etc. The work embodied in the thesis presents an analysis so as to predict the magnitude and position of the limiting width of cut for chatter free ,turning and applies to the case Of workpieces held between the two centres. The proposed mathe-matical model accounts for the workpiece material, its dimensions and compliance, the compliance of the lathe centres, system damping, cutting tool angles, etc. The natural frequency of vibrations of the in position workpiece has been determined analytically by considering the compliances of the workpiece, the lathe centres and the system damping. The concept of universal machinability index, origi-nally proposed for orthogonal cutting, has been extended to oblique cutting. This accounts for the material characteristics in the proposed analysis. In order to determine the incremental cutting force component, in three dimensional cutting, some of the important existing theories, applicable to orthogonal tur-ning, have suitably been modified. The model for the prediction of the limiting width of cut and its position along the work length has been developed by taking into account the workpiece natural frequency, iv its material, system dampingletc. and arrived at by using dl Alembert, s principle together with the principle of virtual work. Experiments were performed to determine the universal machinaJoility index for 0.27% carbon steel bars. In order to determine the workpiece natural frequency and damping, work-pieces of different proportions were fabricated out of same material. These workpieces were held between the lathe centres and excited with an electrodynamic vibrator and resonance curve in each case recorded. The damping of the in position workpiece (68); was determined using the method suggested by Tobias The limiting width of cut for chatter free machining were obtained from the actual cutting tests for a range of cutting conditions, on the above mentioned material using carbide tools. Results indicate that the workpiece dimensions, system damping, the compliances of the workpiece and lathe centres, etc., have a pro-found influence on the stability limit and its position in tur-ning. This aspect has not been considered in most of the exis-ting analyses. The analytical and experimental results of the natural frequency and the limiting width of out for chatter free machi-ning have been compared and a good correlation between the two exists for workpieces having length to diameter ratio between 10 to 15. Numbers in the parnthes.es pertain to the references given at the end of the thesis in the Bibliography.