## 3 Study on Surface Integrity of Milled Workpiece Surface
### Microscopic Topography of Milled Workpiece Surface
#### 3.2.1 Effect of Milling Parameters on the Surface Roughness of the Milled Workpiece  
The trends of the surface roughness with the milling parameters for different machining methods are shown in Figures 15-18. From the experimental results, the surface roughness for the high-pressure air cooling condition is slightly higher than that of the dry milling condition. Compared with dry milling and high-pressure air cooling, the surface quality of the workpiece is the best under MQL condition. This is because the use of  
Figure 14: The surface topography of the machined workpiece, (**a**) tool wear, (**b**) two-dimensional topography, and (**c**) three-dimensional topography.  
lubricant can well improve the tool-workpiece contact condition and reduce the scraping effect between the workpiece and the milling cutter. In addition, the reduction of cutting temperature reduces plastic deformation and scraping on the milled surface.  
Figures 15 and 16 show the effects of milling speed and radial cutting depth on the surface roughness Sa and Sq, respectively. As the milling speed increases, the surface roughness slowly decreases. With the increase of radial cutting depth, the surface roughness under dry milling condition increases rapidly. MQL can effectively suppress the generation of secondary material deposition. Thus, the surface roughness changes more slowly with radial cutting depth under the MQL condition compared with the dry milling condition.  
Figure 16: The effect of radial cutting depth on surface roughness Sa and Sq. (\(V_{c}=28.27\) m/min, \(a_{p}=2\) mm, \(f_{z}=20\) μm/z).  
Figure 15: The effect of milling speed on surface roughness Sa and Sq. (\(a_{e}=1\) mm, \(a_{p}=2\) mm, \(f_{z}=20\) μm/z).  
Figure 17 shows the effect of axial cutting depth on surface roughness Sa and Sq. From Figure 17, it can be seen that the axial cutting depth has less effect on the surface roughness. With the increase of axial cutting depth, the surface roughness Sa and Sq change hardly. Although the increase of axial cutting depth will cause a corresponding increase in milling force, the milling area will also increase approximately proportionally. Thus, the change of the force per unit milling area does not vary significantly with axial cutting depth. Since plastic deformation is the result of the comprehensive effect of force and heat, the amount of plastic deformation on the milled surface is small. As a result, the machining efficiency can be increased by appropriately increasing the axial milling depth.  
Figure 17: The effect of axial cutting depth on surface roughness Sa and Sq. \((V_{c}=28.27\) m/min, \(a_{e}=0.6\) mm, \(f_{z}=20\) μm/z).  
Figure 18: The effect of feed per tooth on face roughness Sa and Sq. \((V_{c}=28.27\) m/min, \(a_{p}\)= 2 mm, \(a_{e}=1.4\) mm).  
Figure 18 shows the effect of feed per tooth on the surface roughness Sa and Sq. With the increase of feed per tooth, the machining quality of the milled surface becomes worse and the surface roughness increases. Banding bulges and secondary material deposition are the causes of increased roughness of the milled surface under dry milling and high-pressure air cooling conditions. In contrast, the residual feeding tool mark is the primary factor affecting the surface roughness under the MQL condition. From the experiments, the radial cutting depth affects the surface roughness significantly under high-pressure air cooling and dry milling conditions. It is the axial cutting depth that has the least effect on surface roughness in all three environments. Since the lubrication and cooling effect of high-pressure oil mist can reduce the scraping between the milling cutter and workpiece, MQL can effectively suppress the plastic deformation of the workpiece surface. Although the feeding tool marks are obvious, the surface roughness is still significantly reduced and the machined surface quality is improved.