OBRABOTKAMETALLOV Vol. 23 No. 3 2021 MATERIAL SCIENCE EQUIPMENT. INSTRUMENTS 7 2 5 Fig. 3. Dependence of surface roughness parameter Rz on cutting speed, coolant and tool wear An experiment with three factors at two levels each (Table 2) was conducted. Full factorial design yields 8 combinations (k = 23 = 8). Based on theoretical factor influence on roughness, the model considered was: Y(Rz) = a + bfzXfz + + bγXγ + bDXD Here are the calculated natural regression coefficients: a = 3.00; bfz = 2.77; bγ = –0.55; bD = –1.08. The parameters of the regression equation were estimated using the ordinary least squares method. Standardized β-coefficients: βfz = 0.17; βγ = –3.33; βD = –1.01. Comparison of the modules of the values of the standardized regression coefficients β allows us to conclude that γ (the angle of inclination of the ball-end tool) is the most influential factor in the formation of the roughness parameter Rz. When the fz factor is fixed, the remaining factors show negative standardized β coefficients (βγ = –0,54; βD = –1,09), indicating a decrease in the response value Rz. The value R 2 = 0.14 indicates that fz plays an important role in explaining Rz, and fixing it significantly reduces the explanatory power of the model. When γ is fixed, the standardized β coefficients (βfz = 2.77; βD = –1.09) show that fz has a strong positive contribution and D has a strong negative contribution. The high R2 = 0.84 indicates that even when γ is fixed, fz and D remain important in explaining Rz. When D is fixed, the standardized coefficients βfz = 2.76, βD = –0.54. This means that when D is fixed, an increase in fz leads to an increase in Rz, while an increase in γ leads to a decrease. The value R2 = 0.75 also indicates that the model with fixed D explains the variation in Rz well, and the factors fz and γ remain important in explaining Rz. The regression model (RM), obtained after conducting an experiment on processing free form surface with a ball-end tool, characterizing the relationship between the roughness value, feed per tooth, diameter and angle of inclination of the tool, expressed by a normalized model has the following form: Rz(RM) = 3 + 2.77fz – 0.55γ – 1.08D – 0.51fzγ – 1fzD + 0.22γD + 0.2 fzγD. Ta b l e 2 Experimental data values No. fz γ D fz∙γ fz∙D γ∙D fz∙γ∙D Rz 1 0.5 50 12 1 1 1 1 3.05 2 0.1 50 12 –1 –1 1 –1 0.13 3 0.1 10 12 1 –1 –1 1 0.18 4 0.1 10 6 1 1 1 –1 0.37 5 0.5 10 12 –1 1 –1 –1 4.33 6 0.5 50 6 1 –1 –1 –1 6.38 7 0.1 50 6 –1 1 –1 1 0.26 8 0.5 10 6 –1 –1 1 1 9.32
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