An experimental and simulation study of chip transportation behaviour in gundrilling
Tnay, Guan Leong
Date of Issue2017
School of Mechanical and Aerospace Engineering
Gundrilling is a machining process to produce deep holes with length to diameter ratios greater than 10. The material removed by the gun drill known as chips have to be evacuated from the cutting zone as fast as possible, in order to prevent the chip clogging. The failure in chip evacuation will lead to intense thermal and mechanical loading. To improve chips evacuation, a thorough understanding of coolant flow characteristics and chip transportation behaviour is necessary. This project aimed to quantify these characteristics and behaviour via experiments and computational fluids dynamics (CFD) simulation study. Studies of the chips flow behaviours in different concentration of fluids were carried out through drop test experiments. The experiments were conducted in vertical square tube filled with the water-glycerine solutions which have properties similar to the coolant properties with concentration of 85 % oil to 5 % oil. From the experiments, terminal velocities of typical gun drill chips at different range of particle Reynolds number were determined to construct the unique drag curve. Based on the findings, a computational fluid dynamic (CFD) numerical model was developed to predict the lateral drag forces with respect to the particle Reynolds number. The simulated results showed good agreement with the experimental findings. With that, the CFD model was then applied to analyse the coolant and chip flow trajectories in the actual gun drill. The chip motions were determined based on the combination of CFD simulation and mathematical calculation. Coefficients of restitution as required to determine the rebound velocities after the chip collide to the wall were obtained via calibration against the experimental data. With that, detailed evaluation on the effects of drill geometries including coolant hole configuration, nose angles and shoulder dub-off angles were conducted. In the analysis, coolant streamlines and pressure drop profiles from the coolant supply to the rake face cutting zone were presented. Extensive set of gundrilling experiments for commercial gun drill designs were conducted, it was found that the tool life of gun drills corresponds with the effectiveness of coolant and chip evacuation. Based on the CFD analysis and experimental results, a new gun drill design that performs better than most commercial design was proposed.