# Surface Quality of Staggered PCD End Mill in Milling of Carbon Fiber Reinforced Plastics
## 4 Effects of Fiber Orientation on the Machined Surface
### Surface Topography of the Machined Surface
#### 4.1.2 Surface Topography  
Figure 4a is the surface topography when \(\theta\) = 45\({}^{\circ}\). The carbon fiber material easily undergoes bending fracture and shear fracture, and the two kinds of fracture modes coexist. The fracture of the carbon fiber is perpendicular to the fiber axis, which is 45\({}^{\circ}\) relative to the machined surface, and simultaneously, the top of the fiber fracture is slightly broken under the squeezing of the tool flank face. The resin matrix of the carbon fiber has a typical shear failure, and the sheared matrix is pressed into debris adhered on the exposed fiber fracture surface, as shown in Figure 4b. As the resin is heated and softened, it is extruded by the flank and coated on the surface fibers of the machined surface, which results in a smooth machined surface. However, when milling the CFRPs along the fiber direction, burrs on the edge of the machined surface are found, as shown in Figure 5. During milling, the surface fibers are bent by the milling force. Due to a lack of external support, the fibers are debonded under the milling force [19; 20]. The stiffness of the carbon fiber is decreased, and the carbon fibers are pushed to the outside, so that they cannot be cut off by the mill over time. With the milling process, the debonded fibers are rebounded, and surface burrs are formed. The machined surface topography is shown in Figure 6 when \(\theta\) = 90\({}^{\circ}\). Serious resin coating is seen in Figure 6a, and only slight fiber bareness is found, which shows a machined surface with straight-forward fiber features. Because 90\({}^{\circ}\) is the critical angle of the straight-forward and reversal machined surface, the carbon fiber in the cutting layer can easily undergo bending deformation. However, due to high stiffness of the carbon fiber in this direction, the bending deformation is small. After the shear fracture of the matrix, the interface is also broken by shear stress. A smooth machined surface with a small section of bare fiber appears, as shown in Figure 6b. The machined surface is further crushed under the extrusion of the tool flank, forming a fiber fracture perpendicular to the axial  
Figure 4: Machined surface topography when \(\theta\) = 45\({}^{\circ}\). (**a**) Resin coating and fiber baring (SEM micrograph); (**b**) Chip adhesion (SEM micrograph). direction. The fracture has no obvious fracture characteristics, and the broken fibers are scattered on the top of the fracture. The matrix is squeezed by the tool flank and adheres to the top of the fracture. Meanwhile, the matrix is softened by heating and is coated on the fiber surface, as shown in Figure 6.  
Figure 7 is the machined surface topography when \(\theta=135^{\circ}\). It is seen that there are bare fiber bundles on the machined surface, while many pits appear on the roots of the fiber bundles, as shown in Figure 7a. It is also observed that the lamination gaps appear in the root of fiber bundles, which is caused by the rebounding of the laminated carbon fiber. Therefore,  
Figure 8. There is almost no intact filed fiber remaining. Tensile fracture topography along the axial direction is seen. Due to workpiece clamping, fiber ply and tool vibration, the fiber cutting angle cannot be completely zero [21; 29]. The actual milling is machining with a small fiber cutting angle. When the surface springback fiber is squeezed and rubbed by the tool flank, a tensile fracture of the carbon fiber will occur on the machined surface. The resin matrix is extruded and may break to be patchily distributed in the fiber gap. The outer surface of the fiber is smooth, and only a little chip adheres on the fiber, as shown in Figure 8. This is because the position of the maximum bending stress of the layered fiber is above the cutting plane, and the position of the fractured fiber is higher than that of the machined surface. In the milling process, the fiber on the surface is pulled out to form pits, or the fiber protrudes from the surface, and the fiber may be separated from the matrix.