Displacements analysis of self-excited vibrations in turning

The actual research deals with determining by a new protocol the necessary parameters considering a three-dimensional model to simulate in a realistic way the turning process on machine tool. This paper is dedicated to the experimental displacements analysis of the block tool/block workpiece with self-excited vibrations. In connection with the turning process, the self-excited vibrations domain is obtained starting from spectra of two accelerometers. One three-axes accelerometer is placed on the tool and one unidirectional accelerometer is placed on the front bearing of the spindle. The existence of a displacements plane attached to the tool edge point is revealed. This plane proves to be inclined compared to the machines tool axes. This plane contains an ellipse that is the place of the points of the tool tip displacements. We establish that the tool tip point describes an ellipse. This ellipse is very small and can be considered as a small straight line segment for the stable cutting process (without vibrations). In unstable mode (with vibrations) the ellipse of displacements is really more visible. A difference in phase occurs between the tool tip displacements on the radial direction and on the cutting one. The feed motion direction and the cutting one are almost in phase. The values of the long and small ellipse axes (and their ratio) shows that these sizes are increasing with the feed rate value. A weak growth (6%) of the long and small axes ratio is obtained when the feed rate value decreases. The axis that goes through the stiffness center and the tool tip represents the maximum stiffness direction. The maximum (resp. minimum) stiffness axis of the tool is perpendicular to the large (resp. small) ellipse displacements axis. The self-excited vibrations appearance is strongly influenced by the system stiffness values, their ratio, and their direction. FFT analysis of the accelerometers signals allows to reach several important parameters and establish coherent correlations between tool tip displacements and the static-elastic characteristics of the machine tool components tested (see Bisu 2007).