## 1 Introduction  
Carbon fiber reinforced plastic (CFRP) has excellent physical properties such as light weight, high strength, and corrosion resistance, and thus it is widely applied to aviation, aerospace, automobile, and so on [1; 2; 3]. However, its poor thermal conductivity, low inter laminar strength, and anisotropy [4] have easily resulted in the occurrence of damage such as burrs, tears, and delamination, as well as fast tool wear and failure in the machining [5; 6; 7]. Meanwhile, both the single type of cutting tools for cutting CFRP as well as the serious problems of the tool durability and processing damage on the production site make it essential to study the cutting performance of new-structure cutting tools. Tool wear is a problem in the machining of reinforced matrices, and it has been observed in advanced coated cutting tools that cause a surface roughness increase and/or the reduction of surface functionality, because of the deteriorating effects of worn-tool/substrate interaction [8; 9; 10].  
In order to improve the machined surface quality of the CFRP and the tool life, [PERSON] et al. [11] investigated the tool wear characteristics in milling CFRP with cemented carbide cutters under dry and chilled air machining conditions. [PERSON] et al. [12] analyzed the mutual effect of the cutter and material on each other in milling carbon fiber composites, based on which a mathematical model was proposed to improve the tool life. [PERSON] et al. [13] established a model of tool wear and cutting force while milling CFRP with diamond-coated cutters. [PERSON] et al. [14] carried out a study on designing a diamond-coated double helical milling cutter and on establishing a cutting force model. [PERSON] et al. [15] established an orthogonal cutting force predicting model with fiber orientation, cutting depth, and rake angle being parameters. [PERSON] et al. [16] used finite element method to research the cutting force, chip formation mechanism, and processing damage in milling unidirectional carbon fiber composites. [PERSON] et al. [17] conducted theoretical and experimental research on milling CFRP materials, and found that the proper increase of tool inclination can reduce the fiber protruding defects. [PERSON] et al. [18] studied the effect of cutting parameters on the machined surface integrity of CFRP, and found that the damage mechanism and deep damage degree of the CFRP materials were closely associated with the fiber direction. [PERSON] et al. [19] carried out an experimental study on milling CFRP, and found that the cutting force under high-speed and low-feed cutting conditions was smaller, while the machined surface quality was ideal.  
In conclusion, the current related research mainly focuses on the machining mechanism and the surface quality in milling CFRP, and the types of cutting tools are mostly of straight-tooth or dextral structure. The staggered helical milling cutter consists of alternately arranged continuous cutting edges, while the rhombic milling cutter is composed of parallel segmented cutting edges. Both of them include sinistral and dextral cutting edges, and show excellent performance in restraining processing damage such as tears, burrs, and so on [20]. In order to promote the popularization and application of the new-style cutting tools for machining CFRP materials, in this paper, a comparative experimental study was carried out on milling CFRP with staggered helical milling cutters and rhombic ones. The cutting performance of the two styles of milling cutters was evaluated by analyzing the tool wear morphology, machined surface quality, and cutting force. This has provided theoretical support for the design of cutting tools and the improvement of machining quality in milling CFRP.