Theory of torsional chatter in twist drills: Model, stability analysis andcomposition to test

The mechanism of torsional chatter in drilling differs qualitatively and qu antitatively from other types of chatter In this paper we show that torsion al chatter can he explained by the torsional-axial coupling inherent in a t wisted beam; the beam "untwists" and extends in response to an increase in cutting torque. Based on a model of this mechanism, predictions of stabilit y boundaries and chatter frequencies are derived by frequency domain analys is, and confirmed by numerical simulation and experimental tests. The effec t of the torsional-axial coupling is opposite that of traditional cutting i n that an increase in cutting forces leads to axial extension and greater c hip load, Because of this sign difference, the limiting depth of cut is gov erned by the positive real part of the frequency response function, which e xplains the unexpected fact that torsional chatter occurs below the natural frequency of the tool.