The residuals distribution for this analysis is shown in Figs. 9,10 and 11. The normal probability plot shown in Fig. 9 reveals that the residuals follow a normal distribution as the point follows a straight line with few exceptions, which scatters around the line. Figure 10 presents internally studentized residuals for surface roughness which scatter randomly within the range of ±3, and all data points are fitted in the model. Hence, any modification in the design is not required and no experimental value is found which is to be ignored as all values lie within the range. Peak values were found for run 4 and 18 which lie approximately near to ±3 internally studentized residuals.