## 3 Results and Discussions
### Hole Quality
It can be seen that the fiber fracture is rough and uneven, and there are many \"round holes\", which are caused by uneven pull-out height of the fiber due to uneven bending and fracture. Figure 19b shows the morphology during ultrasonic reaming. It can be observed that the surface of carbon fiber fracture is relatively flat and the number of \"round holes\" is less than that of ordinary reaming. It shows that the bending  
Figure 17: Diagram of 15\({}^{\circ}\) fiber cutting angle: (**a**) CR; (**b**) UVR. Figure 18: Diagram of 40\({}^{\circ}\) fiber cutting angle: (**a**) CR; (**b**) UVR. fracture of the fiber is more uniform, and the pull-out height difference is smaller under the ultrasonic-assisted reaming. Figure 20 is the morphology observed at a 120\({}^{\circ}\) fiber angle. The shear fracture of the fibers plays a major role, resulting in smooth and flat surface fracture of most carbon fibers. The surface dominated by fiber shear has almost no secondary surface defects and has the lowest roughness of all fiber cutting angles (Figure 20a). The evenly broken fiber section can be observed when reaming is 4000 times larger than that under SEM. Figure 20b is the morphology observed when reaming under SEM, 4000 times larger. The end surface of the fiber is smooth and flat. The morphology of Figure 21 is observed when the cutting angle of the fibers is around 150\({}^{\circ}\). Figure 21a is the morphology of ordinary reaming. It can be seen that most of the breaks of carbon fibers are transversely broken along the fibers due to the extrusion of carbon fibers by the cutting edge of the reamer. The debonding breakage of fibers and resins also causes some of the outer surfaces of carbon fibers to be exposed outside the cutting surfaces. Fiber ports are unevenly arranged, with many small segments, not completely cut off, connected to the fiber head, resulting in poor surface roughness. Figure 21b shows the morphology during ultrasonic reaming. Similar to ordinary reaming, there are many fiber breaks of different lengths, but these breaks are flatter than with ordinary reaming, and the roughness is significantly improved with ordinary reaming. Figure 19: Diagram of 70\({}^{\circ}\) fiber cutting angle: (**a**) CR; (**b**) UVR. Figure 20: Diagram of 120\({}^{\circ}\) fiber cutting angle: (**a**) CR; (**b**) UVR. Figure 22a is a plot of the inner wall of a common-reaming titanium alloy when the tool speed was 600 rpm and the ultrasonic amplitude was 6 \(\upmu\)m, observed under a scanning electron microscope. It can be seen that there are flat marks on this inner wall; this is due to the fact that there is only feed speed in the axial direction. A portion in the spiral from the MATLAB simulation is an approximate straight line. Figure 22b is a plot of the inner wall of the ultrasonic titanium alloy observed under SEM when the tool speed is 600 rpm, and it can be seen that there are wave scratches on the inner wall, which are due to the added ultrasonic vibration on the axial feed. A portion of the spiral curve taken from the MATLAB simulation plot is an approximate wave line. The actual scratches under both machining conditions are consistent with the MATLAB simulation.