## 4 Experimental Analysis  
In this paper, experimental research was carried out to explore the cutting mechanism and the influence of ultrasonic vibration on the cutting force of the ultrasonic-assisted bidirectional cone spiral incremental milling hole. The VDL-1000E high speed milling  
Figure 7: Upward ultrasonic vibration-assisted cone spiral cutting trajectory. NC machining center produced by Dalian Machine Tool Plant was used in the test. The spindle speed range was 45-8000 rpm, and the feed speed range was 1-10,000 r/min\({}^{-1}\). The ultrasonic vibration equipment used in the test was USBT40 ER32, an inductive rotary ultrasonic vibration processing tool handle independently developed by Tianjin University. The piezoelectric force sensor Kistler 9139 AA three-way dynamometer was used to collect and record the milling force. Figure 8 shows the test site. After the hole-making process was completed, a dust mask is worn and a vacuum cleaner is used to vacuum the powder-like chips generated during the cutting process. 1. Test tool  
In this test, a bi-directional conical ultrasonic milling cutter was selected, and a reverse milling structure was added, as shown in Figure 9a. That is, a reverse cutting edge with gradual deformation was added at the top of the milling cutter edge for reverse finishing of the hole wall of the carbon fiber composite materials. The tool base material is a YG8-type hard alloy with a diamond coating. The diameter of the bottom edge is 4 mm, the maximum diameter of the top diameter of the peripheral edge is 6 mm, the axial length of the edge is 10 mm, the number of teeth is 6, the rake is 6\({}^{\circ}\), the relief angle is 12\({}^{\circ}\), the spiral angle is 30\({}^{\circ}\), and the taper is 11.4\({}^{\circ}\). 2. Experimental material  
In this test, a one-way CFRP sheet was used for ultrasonic vibration spiral milling. The type of carbon fiber composite was T700, and the matrix material was epoxy resin. The selected sheet sizes were 200 mm in length, and 110 mm in width and in height. The cutting parameters are as follows in Table 1.  
Figure 8: Experimental setup. Figure 9: Tools and materials for testing: (**a**) test tool; (**b**) CFRP. Analysis of cutting force  
The cutting force curve can be obtained through the test. As shown in Figure 10, \(F_{x}\) and \(F_{y}\) gradually increase as the cutting tool enters the cutting state and as it enter the first stage of stable cutting, at which the bottom edge and the peripheral edge participate in cutting. The cutting length of the peripheral edge gradually increases, and the cutting force gradually reaches the peak. The bottom edge is then out of the cutting process, cutting into the second stage. At this time, only the edge participates in the cutting, with the edge axial downward feed, the cutting volume gradually reduced, and the cutting force also monotonically changing with a decreasing trend. When the top of the peripheral edge exits the cutting process, the cutting force drops to the lowest point, which is the third stage. At this time, the cutting force is not zero, because there is a pressure foot in the experimental system. In the process of reverse cutting, the material removal is small, which verifies that during the above mechanism analysis, there may be an elastic rebound, burr and other processing allowance, under the action of reverse cutting edge extrusion, ultrasonic ironing, and the finishing machining.