| The study dealt with three important issues in the gundrilling process, such as coolant volume flow rate and pressure, the critical stress in the gundrill head, and the force system. First, a predictive model was developed to estimate coolant volume flow rate and pressure, which considered the whole coolant annuli as a combined system of serial and parallel pipes. Blasius's equation and hydraulic diameters were used in this model, and good agreements between predicted and measured volume flow rates showed the validity of this model. Second, as a limiting factor, the critical stress in the gundrill head was investigated using a finite element method. A stress analysis showed that the maximum tensile principal stress was located at the drill center, and was a good index that reflected the effects of the coolant hole configuration. It was also found that a kidney-shaped hole was a good alternative to dual round holes in the gundrill head. Third, the force system in gundrilling was expressed with force resultants for both cutting and pad forces. It was found that the locating parameters (angular and radial, respectively) of pad normal and tangential resultants were very important in estimating a net torque. In addition to these three issues, a study was conducted for examining cutting tool edge strength with respect to tool rake angle. Other parameters such as cutting forces and time rate of wear depth were also discussed. It was shown that a fracture criterion could be used as a major criterion for determining an upper-bound rake angle, or a limiting value for avoiding the propagation of pre-existing cracks. As a final topic, a manufacturing issue was handled, that is, how to generate the desired normal rake and relief angle distributions in drill point grinding. Freeform grinding methods were developed that used the standard wheel edge or face. Mathematical representations were made of normal rake and relief angles in terms of the drill geometric parameters. With the developed freeform grinding methods, drills with convex and concave flank surfaces could be generated. |
