Optimization of Surface Roughness in Cryogenic CO2 Milling of Inconel 718 Using RSM

Recent studies have reported the advantages of cryogenic machining in producing superior surface roughness (Ra) when applied in metal cutting. This study shows the application of Box-Behnken Response Surface Methodology (RSM) to analyze the effect of milling parameters; cutting speed, feed rate, axial depth of cut (DOC) and radial DOC on the Ra of Inconel 718 when machined at high speed using PVD carbide coated ball nose inserts. The experimental work was conducted under cryogenic condition using a new cryogenic CO2 cooling system for efficient and consistent cooling at the cutting area. Experimental results revealed that the Ra was in the range of 0.114 to 0.197 μm, which far surpassed the calculated ideal Ra (0.28–0.78 μm) or that achieved by the manual polishing process (≈0.5 μm). Based on ANOVA, the dominant factor affecting Ra was cutting speed followed by axial DOC and interaction between cutting speed and feed rate. Meanwhile, the effect of radial DOC was relatively insignificant. By applying the quadratic model generated, the average error between the predicted and the experimental Ra was found to be 3.44%. Thus, the mathematical model is valid and acceptable to be used as a prediction model in this scope of study. This study also proves the effectiveness of cryogenic cutting using CO2 to produce better surface finished.