## 1 Introduction  
CFRP (carbon fiber-reinforced polymers) are widely used in the manufacturing of some complex large aircraft components such as the wing, frame, beam and other structural parts because of its superior mechanical properties such as high strength, light weight, corrosion resistance, strong design ability and good fatigue resistance [1]. These structural parts need a large number of connecting holes for assembly or connection, and the quality of connecting holes will directly affect the bearing capacity and service life of the aerospace equipment; thus, the aerospace industry has high quality requirements for aircraft connecting holes [2]. Due to the physical characteristics of heterogeneous and anisotropic CFRP, as well as the characteristics of small fracture strain and low interlaminar bonding strength, fiber pulling, matrix tearing, delamination, burr and other defects often occur during the machining of holes [3, 4], which seriously affect the accuracy and service life of the products [5] and sometimes even increase the secondary processing of the deburring process, greatly reducing processing efficiency and making the processing quality costly and difficult to control. Additive manufacturing is a powerful tool for the aerospace industry. Therefore, how to create an efficient, high-quality and low-cost hole machining of composite materials is one of the hot spots and difficulties in research at home and abroad. Helical milling is also known as orbital drilling or spiral milling; its working principle is to set the eccentric position of the tool and to integrate the composite spiral cuttingtechnology of the axial feed and circumferential feed movement to complete the machining of the hole parts larger than the tool diameter. This way of eccentric processing improves the heat dissipation of the cutting tool and chip removal condition, and it can significantly reduce the process of the axial cutting force [6]. The tool periodically mobilizes and speeds up the hole wall and air heat exchanger, effectively reducing the occurrence of mechanical and thermal damage [7,8] and improving the processing quality of the hole. This leads to inhibition of the layered composite materials, which prolongs the service life of the cutting tools, and which is suitable for a variety of apertures. It also has other advantages and is widely used in CFRP hole technology. Domestic and foreign researchers mainly focus on the cutting mechanism, cutting force modeling and hole quality control. Among them, [PERSON] [9] and [PERSON] et al. [10] showed, through kinematic analysis of the spiral milling holes, that the end cutting edge and the peripheral edge of the milling cutter participate in the cutting of workpiece materials at the same time. In addition, the peripheral edge of the milling cutter cuts intermittently, and the processing form is similar to that of the peripheral milling. The end cutting edge cuts continuously, and the processing form and insert milling are similar. At the same time, the calculation of the undeformed cutting parameters and material removal volume ratio of the milling cutter edge and the end cutting edge shows that, when the machining aperture and tool diameter are determined, the ratio of material volume of the peripheral edge and end cutting edge is constant and is not affected by the machining parameters [11]. Traditional spiral milling holes often adopt the cylindrical milling cutter, material removal is mostly conducted by the bottom edge, a weak blade is the main part that completes the processing of the hole wall [12,13], and the material removal in the process of the cutting tool's weak blade is the focus. It is difficult to guarantee the product quality requirements, as the bottom edge material removal volume is larger, and the axial force is big, which is greater than the critical force of delamination, leading to delamination defects in the hole [14]. Therefore, the finishing processes such as reaming or deburring are required. Based on this, [PERSON] [15] and [PERSON] et al.