An investigation into the influence of helix angle on the torque-thrust coupling effect in twist drills

This paper presents an experimental investigation into the torque-thrust coupling effect in twist drills. A set of cross-coupling stiffness coefficients (Kft and Ktf) are defined. The influence of helix angle on torsional and the cross-coupling stiffnesses is studied. The coupling interaction (as measured by Kft and Ktf) is strongly influenced by the helix angle and has a distinct maxima around 28°. Also, the magnitudes of the coefficients increase parabolically with drill diameter. The pure torsional stiffness also has a maxima around the same helix angle. Under axial restraint, the torsional stiffness increases by 10 to 25% owing to the coupling effect. The larger the torque-thrust interaction, the larger is this increase in torsional stiffness. The findings of this investigation is proposed as a probable basis for the traditional popularity of a 28 to 30° helix angle for drills.