# Process Parameters Optimization of Thin-Wall Machining for Wire Arc Additive Manufactured Parts
## 6 Results and Discussion  
The two strategies were tested on the finishing phase of the test-case to assess their performances. During strategy B machining, chatter occurred, as was clear from the sound during cutting and the surface quality of the product. Strategy B was defined to minimize forced vibrations assuming a chatter-free condition; however, adopting such a solution leads to the selection of spindle speeds more prone to chatter vibrations, as explained in the previous section. On the contrary, no chatter was detected during the strategy A machining cycle: the selection of the suitable spindle speed avoided the occurrence of the phenomenon.  
To further investigate the results, airfoils surfaces were acquiring by the CMM and analyzed. Both pressure and suction sides were scanned using a 2 mm spatial step along length and height. First, form errors of the surfaces respect to the nominal NACA 9403 were studied. Mid-surfaces were computed and compared with the reference camber line and results are shown at three heights (Figure 13): Tip is close to the upper part of the airfoil, Mid is in the middle and Base is close to the fixed root part but  
Figure 11: Harmonics weights for optimization strategy B.  
Figure 12: Optimal spindle speeds for optimization strategy B.  
higher because of measurement limits. Both strategies could accurately reproduce the camber line with very small deviations with respect to the reference (less than 0.1 mm). Indeed, this behavior is in line with the model results: almost the same level of vibrations in the same point of the camber line on the two sides of the airfoil (pressure and suction) was predicted, and therefore no significant modifications of the airfoil camber line were expected.  
On the contrary, significant thickness differences are expected along the camber line, caused by static deflection and forced vibrations. To assess this effect, the thickness of the product was computed at the different levels and compared to the desired one Figure 14. The results show a higher thickness at the borders, especially at the trailing edge, increasing from the base to the tip. This is due to compliance of the system, causing workpiece deflection under the cutting force effect. This deflection increases where tool-workpiece relative displacements are higher, i.e., at the trailing and leading edges and at the tip, producing an increased thickness. Moreover, in Figure 14 on the right, errors on both sides are shown, highlighting similar errors between the two strategies and the same level of error on both sides, as predicted.  
Figure 14: Thickness at different Z-level (**a**) airfoil thickness (**b**) error on suction (solid line) and pressure (dashed line) sides.  
Figure 13: Camber line at different Z-level (**a**) camber line (**b**) camber line error.  
The thickness error along the entire surface is presented in Figures 15 and 16. The results present the thickness error trend, higher at the two edges and increasing the height. It is interesting to point out that the thickness error is very similar between the two strategies. This could be explained by the fact that strategy A allowed us to avoid chatter vibrations at the price of working at the resonance of the system, thus increasing forced vibrations and causing thickness errors. Strategy B was selected to reduce forced vibrations, but chatter vibration occurred, leading to a significant increase in cutting forces, which cancelled the optimized speed selection benefits. Moreover, static deflection is not affected by the two strategies and could be the main factor responsible for the deviations that increase at the leading and trailing edge because of structural stiffness reduction. The main difference between the two thickness error maps is roughness: strategy A produced a smoother surface, while strategy B presents higher waviness and roughness caused by the occurrence of chatter.