Coupled force and vibration modeling of drills with complex cross sectional geometries.

Drill bit vibration can have an adverse affect on drilling performance resulting in lobed holes, burr formation and tool breakage. An understanding of drill dynamics may, thus, provide valuable insight into methods of improving the drilling process. The objectives of this research are to model the effects of complex geometry and rotational dynamics on transverse drill vibration and to investigate cutting forces due to drill deflections. An existing transverse vibration model for drill bits is modified to improve its accuracy and utility. The drill is modeled as a rotated fluted Euler-Bernoulli beam. The effects of material properties, web taper, coolant holes and number of flutes on the fundamental frequency are determined. The number of flutes has a substantial effect on drill vibration with three and four fluted drills having natural frequencies 50% greater than comparable two-fluted drills. Gyroscopic moments and rotary inertia effects on drill vibration are found to become important at high rotational speeds which are being approached in manufacturing at the present time. An analytical model for predicting cutting forces in drilling is presented. The model includes the effects of the drill bit's transverse deflections which lead to variations from the mean values in the cutting forces. Simulations for a drill, exhibiting increasing elliptical translational motion, indicate an increase in the ranges of the torque and thrust while maintaining constant mean values. These trends are also observed experimentally. An experimental investigation of the transverse drill motion and cutting forces during drilling is performed. Although predictions from the analytical force model do not compare well with experimental data, the modeling approach and experimental observations provide insights. Transverse drill motion and the cutting forces are shown to exhibit frequencies corresponding to the rotational speed and the transverse vibration of clamped-pinned and clamped-clamped beams. Other dominant frequencies evident in the cutting forces are not linearly related to drill motion. Torsional and transverse drill vibrations appear to be coupled. Finally, the importance of modeling the force variations due to the drill motion is discussed.PhDApplied SciencesMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/129303/2/9423298.pd