## Part Functionality as Effected by Surface Topography
### Machined Surface Topography Characteristic
Based on the tool machining paths and the trajectory equation of the cutting edge relative to the workpiece, [PERSON] et al. [23] developed an iterative algorithm for numerical simulation of machining surface topography in multiaxis milling. [PERSON] et al. [24] proposed a new method to predict the machined surface topography with relative cutting edge trajectory equation. [PERSON] et al. explored the effects of different tool paths on the surface morphology, and the results are shown in Figure 3. By comparing Sa and Rt values, it found that vertical upward is the optimal cutting path [25]. [PERSON] et al. [26] studied the effect of different angle combinations on the surface topography in multiaxis milling. Better surface roughness could be achieved when rotation angles are 0 (positive lead), 60 (combination of positive tilt and positive lead), 90 (positive tilt), and 330 (combination of negative tilt and positive lead). [PERSON] et al. [27] studied the micrometer milling mechanism of Inconel 718 and concluded that the status of the machined surface is determined by the profile of the cutter, the cutting path of the cutter and the flexible deformation of the cutter. Therefore, the author comprehensively considered these three factors to build three-dimensional surface topography and a surface roughness prediction model. Compared with the experimental results, the maximum relative error of the model is 10.9% and the average relative error is 6.8%. The results can be used as a reference for the prediction of surface milling. By combining boundary intersection and mean squared error method, [PERSON] developed the prediction on surface roughness in AISI 1045 steel end milling process. The results showed that the achieved optimum lessens the sensitivity to the variability transmitted [28]. [PERSON] et al. used the dynamic milling force ratio to predict the surface roughness of the ball milling process. The model was verified by experiments. The results show that the model can predict the average surface roughness accuracy up to 92.82% [29]. Also, some other research works were developed to get reasonable surface texture [30; 31]. [PERSON] et al. analyzed the surface texture generation mechanism under different tool paths by using the method of surface topography analysis, and obtained the best surface texture when the workpiece was inclined at 75\({}^{\circ}\) for high speed milling and then determined the optimized machining path [32]. Meanwhile, there may be defect appearing on the machined surface in milling process. Common surface defects are feed mark, tearing surface, and burr formation. Feed marks are produced by the combined effect of tool rotation caused by cutting speed and tool movement caused by feed speed [33]. Feed marks on machined surfaces become more pronounced with increasing cutting speeds and tool wear [34]. Tearing surfaces usually include smeared material, surface microvoids, scratches, and  
Figure 3: Effect of different cutter path orientations on surface topography [25]: (**a**) Vertical upward; (**b**) vertical downward; (**c**) horizontal upward; and (**d**) horizontal vertical downward. groove and pitting corrosion. The typical tearing surfaces is showed in Figure 4. Damage on the machined surface in hard milling process of FGH95 PM superalloy was investigated by [PERSON] and [PERSON] [35]. Their research results showed that several defects appeared on the machined surface at higher cutting speeds. The results are significant for the prediction of component service life. When the tool moves along the feed direction, the workpiece material will flow laterally under the combined extrusion action of the minor flank face and the machined surface causing smearing. The surface microvoids are formed from the carbide particles of the workpiece material, the cutting off of the cutting tools, and the deposition of the chip. The surface microvoids affect the mechanical properties of the workpiece, so the surface microvoids should be avoided in the key components [36].