Analysis of droplet behavior on a rotating surface in atomization-based cutting fluid systems for micro-machining (2010)

Atomization-based cutting fluid (ACF) systems have been recently found to be effective in providing both cooling and lubrication for micro-scale machining operations. The ACF systems are preferable over the conventional flood cooling methods as they effectively reduce the temperature at the workpiece-tool interface through evaporative cooling and provide superior lubrication to the cutting zone. While such ACF systems appear to be beneficial, there is a lack of a physics-based understanding of the phenomena underlying cooling and lubrication performance of ACF systems in micro-machining processes.\nThe research presented in this thesis investigates the effect of ACF system parameters and machining conditions on the cooling and lubrication performance in micro-machining processes in order to enable the design of efficient ACF systems. To accomplish this, experiments are first conducted to understand the cooling and lubrication mechanism of ACF systems. The knowledge gained from the experiments is then used to develop a model-based approach to the design of ACF systems for high cooling and lubrication performance in micro-machining.\nOn the experimental front, micro-turning experiments are carried out and the cutting performance evaluated for varying cutting fluids and at different droplet speeds. Micro-turning experiments indicate that a cutting fluid with low surface tension and low viscosity generates lower cutting temperatures whereas a fluid with low surface tension and high viscosity generates lower cutting forces. Since in most machining processes, either the workpiece or the tool is rotating, single droplet impingement experiments are also conducted on a rotating surface using fluids with different surface tension and viscosity values. Upon impact the droplet shape is observed to be a function of both the droplet speed and the surface speed. The spreading increases with increased surface speed owing to the tangential momentum added by the rotating surface. Spreading is observed to also increase with a decrease in fluid surface tension and does not change with the fluid viscosity. It is concluded that a fluid with low surface tension and low viscosity is an effective coolant of the cutting zone, whereas, a fluid with low surface tension and high viscosity is effective for lubrication. Another set of single droplet impingement experiments are conducted on a rotating surface to capture the 3D shape of a droplet upon impingement to aid the model development.\nOn the modeling front, a parameterization scheme is developed to mathematically define the 3D shape of droplet upon impingement. The shape information is used to develop an energy-based model for droplet spreading. The droplet spreading model captures the experimental results within 10% accuracy. The spreading model is then used to predict the cooling and lubrication for an ACF-based micro-turning process. The model captures the cooling and lubrication trends observed in micro-turning experiments. A parametric study is conducted to identify the significant factors affecting the performance of an ACF system. Droplet speed is found to have a dominant effect on both cooling and lubrication performance, particularly, with a low surface tension fluid for cooling and a low surface tension and high viscosity fluid for lubrication.