An analytical approach to assess the variation of lubricant supply to the cutting tool during MQL assisted high speed micromilling

Abstract

Abstract Minimum Quantity Lubrication (MQL) assisted micromilling showed promising results in improving machinability. However, no past work is available on the adequacy of MQL oil flow rate, and its variation with process parameters during micro end milling. A higher oil flow rate can undesirably increase the volumetric disposal rate of cutting oil to the environment without appreciable change in machinability. Accordingly, this article explores whether a given supply of MQL lubricant is abundant, adequate or deficient for the process for various spindle speeds and axial depths. An analytical model is developed by considering the geometry of the actual rounded-edge of the micro end milling tool and elastic regain of the workpiece material, and interrelating these with the characteristics of the MQL jet and oil droplets. For fixed nozzle angles, the interaction between oil droplets and micro end milling tool rotating at varying speeds is further explored. It is observed that 6 mL/hr MQL oil flow rate is abundant for 15,000 rpm spindle speed, adequate for 25,000 rpm, and deficient for higher speeds. The roles of abundant and deficient MQL oil supply on surface quality and burr formation in micromilling of poly(methyl methacrylate) (PMMA) samples are also investigated by comparing with the corresponding results obtained in dry cutting. It is concluded that higher axial depth of cut primarily does not cause lubricant deficiency, but such deficiency is perceived only when speed exceeds a certain limit. Proper lubrication can effectively reduce the bottom-surface roughness of the machined slot. Also, the difference between top-burr width during dry and MQL assisted micromilling in the up milling side decreases with an increase in the oil deficiency. Finally, it is concluded that either the speed should be kept low or the flow rate should be increased to produce an improved lubrication effect at higher axial depths.