Improvement of surface grinding process performance of CK45 soft steel by minimum quantity lubrication (MQL) technique using compressed cold air jet from vortex tube

Consumption of cutting fluids imposes high costs on industries. Furthermore, they contaminate the environment and are harmful to human health. Minimum quantity lubrication technique (MQL) is an attractive alternative approach to reduce cutting fluids consumption, improve efficiency of cutting fluids at machining zone and use of harmless fluids. However, this technique faces cooling limitation in grinding. The first purpose of this study is a comprehensive investigation of heat transfer mechanism in MQL technique by its temperature numerical simulation. The second purpose is improvement of cooling ability of MQL air jet by using a simple and inexpensive vortex tube. To do this end, a system was designed and manufactured to measure the convection heat transfer coefficient of different conditions of cold air with MQL (CAMQL). The results of convection heat transfer tests show 95% share of compressed air in heat transfer. Also, the results indicate that air pressure is a more important factor than temperature in cooling process at high thermal power. The results of temperature numerical simulation show that by increasing pressure, the increasing rate of convection heat transfer coefficient decreases. Also, the temperature cooling ability of colder air with lower pressure is perceptible at low thermal power. In surface grinding of CK45 soft steel, CAMQL in comparison with dry and conventional fluid cooling leads to significant reduction of tangential grinding force and friction coefficient. But in general, except in the case of optimum condition which has the highest heat transfer coefficient, surface finish is worse. This is attributed to low heat transfer coefficient of gases at lower pressures.