Self-Induced Vibrations in Metal Cutting

Abstract

Solution of the problem of dynamic stability for the machine tool-cutting process system depends primarily on the assessment of behavior of the cutting process under dynamic conditions. It has been found that under dynamic conditions, apart from force fluctuations due to variations in cutting conditions, additional force fluctuations take place as a result of cyclic variations of the shear angle in the cutting process. Difference in force response of a cutting process to the static dynamic variations of the cutting conditions has been explained by the presence of cyclic variations of shear angle under dynamic conditions. Peaks of the force wave, resulting from dynamic variation of the cutting conditions, are known to be displaced with respect to the originating wave. This displacement has been thought to be due to a time lag of the whole force response; however a sound physical basis for this point of view has not previously been found. The present investigation provides a physical basis for such observations, showing that the displacement of force peaks is caused by the skewing of the force wave by the presence of asymmetric force pulses due to cyclic variations of shear angle. The same event—the cyclic variation of the shear angle—has been recognized to be a sign of instability of the cutting process in itself, resulting in a cyclic chip formation process. Instability of the cutting process in itself has been found to depend mainly on the cutting conditions and not on the dynamic properties of the cutting system. Analytical expressions derived for the frequency and amplitude of cyclic chip formation have been found to be in a good agreement with the results of measurements of these quantities. Study of the effects of dynamic events in metal cutting upon tool life has revealed propagation of fatigue cracks on the wear land. The propagation of the cracks has been found to be in good correlation with the presence of force pulses due to the cyclic chip formation. The way in which the foregoing event affects the tool life has been reconstructed, allowing selection of those conditions which improve tool life.