Modelling of ultrasonically assisted micro drilling

Micro drilling has been applied in the interconnection and precision manufacturing industries extensively. As a promising machining technique, Ultrasonically Assisted Drilling (UAD) has become increasingly popular in both academia and industry in recent years. In this thesis, modelling techniques and experiments for Ultrasonically Assisted Micro Drilling (UAMD) are investigated. Representative work on modelling of micro drills and UAD has been documented and categorised. Existing gaps in the literature are identified and the aims of this research are formulated. Using the Finite Element (FE) technique, a hybrid model is developed to realise modelling for the whole drill bit without compromising the computation efficiency, even when the drill has a complicated geometry (small diameter flute, multiple step shanks, etc). A specific drill model (Φ0.3 mm diameter, 2 step shanks) is chosen for a case study in order to evaluate the model. The hybrid tool shows sufficiently accurate results and impressive computation efficiency in the evaluation. For vibration modelling, force modelling and experimental work, a standard Φ1 mm drill with 1 step shank is used across the chapters. First of all, FE analysis is conducted on the whole drill and normal modes are solved with boundary condition as fixed simply supported. A 2 Degree-of-Freedom (DOF) model is then built considering rotation and the ultrasonic excitation to solve the transverse vibration with boundary conditions consistent with the FE model. The asymmetric geometric characteristics of the drill bit are taken account of through using the first two fundamental modes in the FE model. Potential parametric resonances are discussed in the numerical simulation. Other vibration characteristics are also discussed with varying parameters such as ultrasonic frequency, ultrasonic amplitude and rotational speed. In order to extend the vibration model, a nonlinear thrust force model has been developed for incorporation into the 2 DOF model. The force model considers ultrasonic parameters, feed rate, material properties and the nonlinearity of the UAMD process. Force reduction during the UAMD process is explained qualitatively with the model and a full range of feed rates have been simulated to study their effect on the force reduction. The limitations of this model have also been explained. A high speed UAMD system was designed to examine the effects of key parameters. Experiments with different ultrasonic frequencies, amplitudes and rotational speeds were conducted and the influences of these parameters on thrust force were investigated. With the thrust force data from these experiments, a correlation study to the simulation results based on the force model is carried out. The study identifies the limitations on the current one dimensional force model and leads to recommendations for the further development of the force model. Further work is identified for both modelling and experiments, and the present models can be expanded to suit the research and development of UAMD techniques.