Measurement of lubricant oil film thickness under minimum quantity lubrication milling process

Cutting fluid has been extensively used in the manufacturing fields, especially in the machining areas due to its beneficial outcomes, i.e. the capability to lubricate the cutting process, reduce the cutting temperature as well as to slide away the chips from agglomerated onto the cutting zone. Furthermore, this lubrication approach also has been widely utilised to improve the machining performance such as to minimise the surface roughness of workpiece and enhance the rate of tool life. Traditionally, flood coolant has been used as the lubrication method during the machining process. However, excessive amount of oil being supplied through flood cooling method has caused occupational health hazards, manufacturing cost rising and global environmental damage. To overcome these drawbacks, an improvisation lubrication method known as Minimum Quantity Lubrication (MQL) has been implemented. MQL is an alternative lubrication method for near-dry cutting process using a mixture of compressed air and less amount of lubricant oil. MQL in the machining application proven to minimize friction at the cutting zone, reduce the occupational health hazards as well as lower the manufacturing cost. However, since the usage of oil is very little, the goodness of MQL in the lubrication process highly depends on the ability of the oil to penetrate the narrow cutting. This can be obtained by examining the mechanism of oil while lubricating the cutting zone, where this study is still scarce because most of the past studies were merely focus on the effects of machining or lubrication parameter without showing how the lubricant behaviour related to the cutting performance. The study of fundamental nature of lubricant oil during machining operation is highly important to be observed but nevertheless it is still deficient due to the complexity of experimental setup during the ongoing operation as well as the natural flow of oil cannot be disrupted have caused the limitations to accomplish the study. To cater this issue, an experimental work with the objective to analyse the lubricant oil behaviour in MQL milling process by using a non-intrusive method known as laser induced fluorescence (LIF) was carried out. The study was conducted to investigate the thickness of oil during the MQL milling process under various oil viscosity and cutting speed effects. Four different types of oil, i.e. mixed esters oil, sunflower oil, olive oil, and calophyllum inophyllum oil with dynamc viscosity values at 40˚C of 11.075, 35.260, 35.260 and 63.223 mPa.s , respectively were dissolved into a fluorescent dye, i.e. Coumarin 153 as the working fluid. Concurrently, machining cutting speed of 14.514, 19.514, 24.514, and 29.514 m/min were set to observe the average lubricant oil film thickness along the milling path. Calibration procedure was carried out before conducting the milling operation to achieve a linear relation between emitted light intensity and the lubricant oil film thickness. From the study, it was found out that the average lubricant oil film thickness increased as the oil viscosity decreased. This phenomenon is explained due to the smaller particles of low viscous oil that made it easier to penetrate from MQL nozzle which increased the spreadability of lubricant oil onto the milled path. Meanwhile, it was observed that as the cutting speed increased, the average lubricant oil film thickness also increased. The increment in cutting speed has raised the friction between cutting tool and workpiece which eventually led to providing a good lubrication effects. To clarify the relation between MQL lubricant oil behaviour and the performance of milling process, the measurement of surface roughness of workpiece was analysed. The results showed that the average surface roughness decreases with the increasing of machining cutting speed. This is supported by the findings that confirmed the results of lubricant oil film thickness under the same conditions, indicating that more lubricant supplied on the workpiece gives better surface finishing of workpiece. vi However, it was found that the average surface roughness increased with the decreasing of dynamic viscosity, η. This is contradict with what was found in lubricant oil film thickness. Although the small particles of low viscous oil was easier to penetrate the cutting zone and spread along the milled path, the oil unable to repel the flying chips and thus made the workpiece rough. Finally, the findings from this research study was able to fill the gap of knowledge in clarifying the oil film thickness behaviour on the machining performance