Modelling of an ultrasonically assisted drill bit
thesisposted on 18.02.2011, 10:03 authored by Peter Thomas
Ultrasonically Assisted Drilling (UAD) has received great interest in the past few years by both academia and industry. The technology has demonstrated a multitude of advantages over conventional drilling technology although its industrial employment has been mainly thwarted by inconsistent results and lack of development; a better understanding of the underlying dynamic process is required. This work commences with pertinent background information prior to delivering a critical and comprehensive literature survey on current UAD technology. From the literature survey, a key area requiring improvement is isolated, anda novel strategy is developed to address this. Standard high-speed steel drill bits are excited ultrasonically in their axial (longitudinal) direction and their vibration characteristics are recorded with respect to their tips longitudinal and torsional velocities. It is revealed that a pronounced vibration mode conversion or coupled motion occurs in the drill bit's structure. This has not previously been measured or documented and prompts a critical change in the design philosophy of UAD vibration systems. It also highlights the shortcomings of employing traditional drill bits and promotes the development of modem alternatives. Ultrasonically excited (standard) drill bits were employed to cut steel samples. During the UAD process, the excitation frequency was swept over a range of frequencies and the drilling reactions were recorded. The swept excitation signal employed is novel and unique to this work. As a result, clearly optimal regimes are revealed. A critical discussion is performed, and the workpieces that result from steady state UAD are evaluated with respect to their physical properties, and mechanisms that occur during UAD are proposed. Knowledge of these mechanism supports future UAD system development. A three-dimensional finite element modelling methodology is developed to further understand the UAD system's vibration characteristics. Simulation results show similar characteristics to the experimental data obtained although inconsistencies have been identified. Further work is necessitated to fully identify the numerical models developed with the experimental system. Routes by which this can be achieved are defined. The modelling techniques employed may be expanded to incorporate non-linear cutting forces, and are suitable for the development of a new generation of UAD technology.
- Mechanical, Electrical and Manufacturing Engineering