# Experimental Study on Milling CFRP with Staggered PCD Cutter
## 3 Results and Discussion
### Effects of Tool Wear on Machining Surface Topography  
The anisotropy of CFRP makes it easy to cause problems after machining, such as fiber fracture, fiber exposure or fall off, fiber pulling out from the matrix, debonding and delamination etc. Figure 6 shows the surface topography of the workpiece machined by the staggered PCD cutter under various wear conditions.  
Figure 5: The milling force curves of staggered PCD cutter due to variations in milling length.  
Figure 6: The surface topography of CFRP under various wear conditions: (**a**) 3D machining surface topography (\(L\): 3 m, \(R_{\text{max}}\): 23 \(\upmu\)m); (**b**) 3D machining surface topography (\(L\): 51 m, \(R_{\text{max}}\): 61 \(\upmu\)m); (**c**) 2D contour (\(L\): 3 m, \(R_{\text{max}}\): 23 \(\upmu\)m); (**d**) 2D contour (\(L\): 51 m, \(R_{\text{max}}\): 61 \(\upmu\)m).  
It can be seen from Figure 6a that, when the milling length was 3 m, there was no obvious wear on the cutting edge, the machining surface was relatively smooth in the fiber layer region of 45\({}^{\circ}\) and 135\({}^{\circ}\), a tiny groove appeared in the fiber layer region of 0\({}^{\circ}\), and a few fiber bundles protruded from the fiber layer region of 90\({}^{\circ}\). A corresponding 2D contour of the surface topography was further extracted, shown in Figure 6c. The fact that the overall contour curve fluctuated within \(\pm\)2 \(\upmu\)m indicated that the machining surface quality was relatively satisfactory. It can be learned from Figure 6b that, when the milling length was 51 m, the cutting-edge radius reached 61\(\upmu\)m and the machining surface was still relatively smooth in the fiber layer region of 45\({}^{\circ}\) and 135\({}^{\circ}\), while the local fiber bundles protruded and pulled apart in the fiber layer region of 0\({}^{\circ}\), and fiber bundles were pulled out so seriously as to cause hole defects in the fiber layer region of 90\({}^{\circ}\). Its 2D contour was extracted, as shown in Figure 6d, from which it can be seen that the hole depth was up to 52 \(\upmu\)m, and that the machining surface quality deteriorated gradually.  
In order to further analyze the effects of tool wear on machining surface quality, SEM was used to test the machining surface. Figure 7 displays the SEM topography of the machining surface in various wear states. It can be learned from analysis that the fiber fracture topographies with obvious differences and the area coated with resin appeared on the machining surface with the extension of cutting edge wear zone. As shown in Figure 7a, when the cutting-edge radius was 23 \(\upmu\)m, the cutting edge was relatively sharp, residual resin was not found and the fiber fracture was even on the machining surface, and the machining surface topography was better. It can be seen from Figure 7b that, when the cutting-edge radius \(R_{\text{max}}\) increased to 61 \(\upmu\)m, the CFRP breakage form tended towards fiber bundle extrusion, some fiber particles did not fall off, and the fiber fracture became gradually irregular. In addition, the milling temperature increment heated and softened the resin materials, and to some extent the machining surface was coated with resin under the ironing effect of the cutting tool. The machining surface was composed of irregular fiber fracture and partial resin backfill, and thus the surface quality dropped obviously. Figure 7c shows that when the cutting-edge radius further increased to 76 \(\upmu\)m, the expansion of fiber bundle extrusion and resin backfill resulted in adhesive fiber particles being coated with resin. Thus, the fiber bundles were pulled out or fell off from the matrix for the effect of strong scraping, and such defects as fiber fracture exposure, groove and hole, etc., appeared clearly.