Integration of Drill Torsional-Axial Coupling in Spindle-Vibratory Drilling Head Model for Stability Analysis

Abstract

The drilling of deep holes with small diameters remains an unsatisfactory technology, since its productivity is rather limited. The main limit to an increase in productivity is directly related to the poor chip evacuation, which induces frequent tool breakage and poor surface quality. Retreat cycles and lubrication are common industrial solutions, but they induce productivity and environmental drawbacks. An alternative response to the chip evacuation problem is the use of a vibratory drilling head, which enables the chips to be fragmented thanks to the axial self-excited vibration. Contrary to conventional machining processes, axial drilling instability is sought, thanks to an adjustment of head design parameters and appropriate conditions of use. In this paper, self-vibratory cutting conditions are established through a specific stability lobes diagram. A dynamic high-speed spindle / drilling head / tool system model is elaborated on the basis of rotor dynamics predictions. The model-based tool tip FRF is integrated into an analytical stability approach. The torsional-axial coupling of the twist drill is investigated and consequences on drilling instability are established. Specific stability lobes are established and indicate modifications of self-excited operating zones. This approach allows refining the stability prediction of the global system during a drilling operation.