# Surface Quality of Staggered PCD End Mill in Milling of Carbon Fiber Reinforced Plastics
## 1 Introduction  
Carbon fiber reinforced plastics (CFRPs) are being extensively used in various fields due to their excellent mechanical properties [1; 2]. As a difficult-to-cut material, machining of CFRP has attracted increasing attention [3; 4; 5]. Due to various machining defects of CFRPs, the machined surface quality of these materials is difficult to control [6], which makes CFRPs difficult to cut. Machined surface quality determines the reliability, wear resistance and service life of CFRP workpieces. The machined surface quality is affected by cutting parameters, cutting tools and cutting angles [7]. Milling is a basic machining operation of CFRPs [8]. Surface quality and machining defects play an important role in CFRP milling [9; 10; 11]. CFRPs are made of carbon fiber layers, and the interlaminar strength is lower than that of the inner layer. When the component force between layers is larger than the interfacial bonding strength, delamination will occur in milling. Meanwhile, the surface fiber can easily be pulled, causing burrs and tears. The main machining defects in the CFRP milling process include surface fiber burrs, surface fiber tears, and interlayer delamination [12; 13]:  
1. Surface fiber burr: In the milling process, the surface carbon fibers will be affected by the axial cutting force outward from the surface [14]. When the axial force is greater than the interlayerbonding force in milling, fiber debonding will occur, which means a detachment from the resin. Bending deformation occurs under a cutting force after fiber debonding. Because there is no material support outside of the surface, the un-cut carbon fiber remains on the machined surface and forms burrs. The burr direction generally follows the fiber direction. 2. Surface fiber tear: During milling, in the inverse fiber direction, the surface carbon fiber is bent and broken by the tool, and the broken crack stretches deep into the material of the surface layer of the workpiece, and a tear defect is formed [15; 16; 17; 18]. If the residual burr is extremely long, it can easily be twined by the cutter tooth and break the fiber. The fracture location is generally deep in the surface of the workpiece, and the tear defect is formed on the surface of the workpiece; the depth is influenced by the tear angle and cutting edge sharpness. As the axial tensile strength of the fiber is greater than the interlayer bonding strength, the fiber layer is separated from the matrix material before tensile fracture, and the tear direction is generally along the surface fiber direction. Tearing of the milling surface is generally accompanied by burr defects, and they have a similar variation tendency. 3. Interlayer delamination of the machined surface: When the interlayer stress exceeds the interlayer bonding strength and fiber bonding strength in the milling process, debonding will occur between the carbon fiber bundles and matrix material, accompanying the deformation of the fiber layer. The deformation of the fiber layer will gradually recover after cutting; however, the delamination defect is permanent as the matrix loses its adhesive capacity. Interlayer delamination defects may occur in any fiber layer of the milling fracture [19; 20]. If they occur on the surface, there will be tear and burrs. Therefore, delamination of the surface layer is the source of the development of tear and burrs. Delamination will directly affect the material strength and fatigue resistance performance. Milling CFRP materials is quite a complex task due to its heterogeneity [21; 22]. In modern industries, components made of CFRP are increasingly used in relation to their dimensional precision as well as their surface quality [23; 24]. Due to these applications, there is a great need to understand the questions associated with the machined surface quality of CFRPs [25; 26]. Considering the machining defects induced by the CFRP machining process, [PERSON] et al. [2] proposed a finite element model for the prediction of machining defects. [PERSON] et al.