Palm oil as a substitute of oil for cooling media in the milling process: Effect of pressure on surface roughness

The oil-based cooling media is one source of pollution and made from non-renewable resources. The use of cooling in machining with a mist cooling system will reduce the amount of oil consumption in for cooling media, decreasing of oil make the amount of pollutant decrease but still produce pollutant event in small quantity. Replacing oil with palm oil will remove pollutants caused by the use of oil as a cooling media in the machining process. This study sets out to reveal the surface roughness of St 60 steel machined by milling process using different spindle speeds (360, 490, and 720 rpm) and mist cooling pressures (0.5; 1.5; and 2.5 bars). The milling process carried out using up milling with the feed rate of 75 mm/min. The surface roughness was tested using surface roughness tester at 3 different points to obtain the average arithmetical surface roughness value (Ra). The results showed that the enhancement of surface roughness occurred along with the increase of mist cooling pressure, however, it dropped when the pressure reached the value of 2.5 bars. On the other hand, the higher spindle speed applied could reduce surface roughness. These phenomena can be explained due to the less optimum of the spraying process on cutting region when the pressure below 2.5 bars was used. Yet, along with the further addition of pressure, the spraying occurred on the cutting region turned to be more effective due to the better ability of oil mist to penetrate the narrower cutting crevice. The highest average Ra value has resulted in the spindle speed of 360 rpm and the mist cooling pressure of 1.5 bars with the Ra of 2.65 µm. Conversely, the lowest one can be observed on sample cut using the spindle speed of 720 rpm and mist cooling pressure of 2.5 bars with a value of 1.29 µm.