Define macro and micro surface error and its measurements
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Explanation:
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Explanation:
The machining of free form surfaces is one of the most challenging problems in the field of metal cutting technology. The produced part and machining process should satisfy the working, accuracy, and financial requirements. The accuracy can describe dimensional, geometrical, and surface roughness parameters. In the current article, three of them are investigated in the case of the ball-end milling of a convex and concave cylindrical surface form 42CrMo4 steel alloy. The effect of the tool path direction is investigated and the other cutting parameters are constant. The surface roughness and the geometric error are measured by contact methods. Based on the results, the surface roughness, dimensional error, and the geometrical error mean different aspects of the accuracy, but they are not independent from each other. The investigated input parameters have a similar effect on them. The regression analyses result a very good liner regression for geometric errors and shows the importance of surface roughness.
Keywords: free form milling, ball-end milling, geometric tolerance, surface roughness
1. Introduction
The machining of free form surfaces is a focused problem in the mould and die industry and in power plant turbines. During the manufacturing the surface (Nat: +1/−1 = convex (CX)/concave (CV) surface) has important effect on the radius (R_CMM), cylindricity (Cyl) and the surface profile error (SPE), but has no effect on the surface roughness (Rz).
Figure 9
Figure 9
Main effects plots of the measured parameters. (a) Main effects plot for the radius; (b) Main effects plot for the Rz surface roughness; (c) Main effects plot for cylindricity; (d) Main effects plot for surface profile error.
The milling path direction (MilDir = A) has an effect on all four parameters. In case of radius (measured by CMM), the values decrease with the changing of milling direction, but comparing with Figure 5, it comes from source of average value. The decreasing of the radius of the convex surface is larger than the increasing of the radius of the concave surface. The milling path direction has larger effect on the surface roughness (Rz), the cylindricity (Cyl), and the surface profile error (SPE).
Based on these data regression analyses were performed. The nature of the surface (Nat), the milling path direction (MilDir) and the surface roughness (Rz) were considered for the linear regression generation. In the case of cylindricity (Cyl) the value of R2adj is 95.6%, so Equation (1) can describe the cylindricity with good accuracy, as shown in Figure 10. The measured and estimated values are close to the ideal state (continuous red line).
Figure 10
Figure 10
The measured and the estimated values of the cylindricity.
Cyl = 0.0128 – 0.0116 Nat + 0.000145 MilDir + 0.00232 Rz (1)
Predictor Coef SE Coef T P
Constant 0.01283000 0.00646500 1.98 0.094
Nat −0.01163250 0.00097420 −11.94 0.000
MilDir 0.00014485 0.00006073 2.39 0.054
Rz 0.00232300 0.00153100 1.52 0.180
S = 0.00307985 R-Sq = 97.1% R-Sq = 95.6%
As Figure 11 shows that the convex surfaces have smaller cylindricity error than the concave surfaces. The geometric error and the surface roughness have a close relationship. The milling path direction has an important effect on the geometric error, but the effect of the parameters of the machining process (like the tool diameter, width of cut, the feed speed and the cutting speed) have to be considered. Therefore, the accurate estimation of the surface roughness of free form milling is an important research topic.
Figur