Almost all machining industries in Sri Lanka use mineral oil–based and synthetic oils as metal working fluids during machining. But their usage has potential for long-term environmental pollution and threat to workers’ health. Therefore, much effort has been focused on research and development of eco-friendly and hazard-free alternatives to mineral oil–based metal working fluids, and the use of vegetable oils is one such alternative. This study was focused on investigating the easy access of vegetable oils in Sri Lanka and most commonly machined metals in the local industry, so that the outcome would be much more beneficial to the local industries. After several tests with 10 candidate vegetable oils, white coconut oil is selected as base oil and the results of a successful questionnaire survey directed to select mild steel and AISI 304 steel as the machining metals. Machining experiments were planned using Taguchi methods, and the variation of the surface roughness was tracked with respect to four variable parameters depth of cut, feed rate, spindle speed and metal working fluid. Two mathematical models were developed to predict the surface roughness and optimize the parameters. Further experiments were conducted to investigate the effects of metal working fluid for smooth- and rough-cut conditions. These experiments revealed that white coconut oil is a better metal working fluid for mild steel, while soluble oil favors AISI 304 steel. Different thermal conductivities of metals cause the coconut oil to act differently while machining two metal types.