## 3 Results and Discussions  
### Analysis of Force Change and Amplitude Effect during Reaming  
As shown in Figure 4, the motion of CR includes the rotation and axial feed motion of the reamer, while UVR is formed by applying longitudinal ultrasonic vibration to the motion of CR. In the reaming process, the reaming edge plays a main role. The reaming edge is long and only part of it is involved in cutting. When reaming CFRP/Ti stacks, it can be divided into six stages as a whole. As shown in Figure 5, stage I is the articulation stage and the beginning stage of reaming, during which the reamer edge contacts the CFRP surface. Stage II is the steady-state reaming stage of CFRP, in which the axial force of reaming gradually increases and enters the stable stage. Stage III enters the transition zone, which is the reaming stage of CFRP. Because of the difference of performance between the titanium alloy and CFRP, the reaming axial force rises sharply and enters the reaming stage of the titanium alloy. Stage IV is the steady-state reaming of the titanium alloy, in which the reaming axial force first rises and then enters the stable stage. Stage V is the end stage of titanium alloy reaming, in which the axial force of reaming decreases and the reaming stage approaches the end. Stage VI is the end stage of laminated reaming, in which the reaming axial force decreases to zero, the reamer edge hinges on the laminated holes, and the reaming process ends. Figure 4: Diagram of reaming: (**a**) conventional reaming (CR); (**b**) ultrasonic vibration reaming (UVR). When the feed speed is 25 mm/min (CFRP) and 10 mm/min (titanium alloy), the tool speed is 1400 r/min, the amplitude of ultrasonic vibration is 6 \(\upmu\)m, and the change of thrust force in reaming is collected. Time-domain signal analysis of reaming force is an effective method to reflect the real-time state of tool-workpiece interaction. Figure 6 is the cutting force curve of CFRP/Ti stacks during CR. It can be seen from the diagram that the thrust force in reaming presents five different stages. In stage A-B, the reamer gradually hinges into the CFRP layer, and the thrust force gradually increases. In the B-C stage, the reamer cutting edge is completely immersed in the CFRP layer material, i.e., in the stable cutting stage of the CFRP layer, and the cutting force remains stable. During the A-C stage, the titanium alloy is not involved in cutting, in which the titanium alloy acts as a simple support. After point C, the axial force decreases to some extent (stages C to C1), which is caused by the existence of voids between the titanium alloy and the CFRP layer material. Starting at point C1, the material is reamed from the reaming edge to the cutting edge, and the axial force increases sharply until the cutting edge is fully penetrated into the titanium alloy (stage C1-D). It can be seen that the axial force in reaming increases to 5-6 times that in the reaming of CFRP in a short time. Titanium alloy is an elastic plastic material with high tensile strength and low thermal conductivity, so its reaming force is relatively high. In stages D-E, the stable reaming stage is attained for the titanium alloy. In stage E-E1, the cutting edge of the reamer is gradually reamed out of the titanium alloy, and the axial force of reaming force decreases rapidly. At F, the reamer reaches the bottom of the hole. At the end of reaming, the axial force decreases to zero. Figure 5: Schematic diagram of reaming process (I) Start of reaming (II) Cutting CFRP layer (III) End of CFRP reaming (IV) Reaming titanium alloy layer (V) Tool cutting edge contact interface (VI) End of reaming. Figure 6: The thrust force of (**a**) CR and (**b**) UVR. Figure 6 shows the thrust force curve obtained by UVR. It can be seen from the picture. UVR causes the large vibration of the reaming force.