## 3 Results and Discussion  
### Analysis of Tool Wear  
When the cutting speed was 100 m/min, for the two styles of milling cutters, both of their wear processes consisted of coating wear and coating shedding. In the initial milling, the tool wear was mainly the coating wear on the flank face. When the milling length was 12 m, the tool wear morphology of the two cutters, shown in Figure 3, mainly resulted from the effect of hard abrasive particles in carbon fiber materials. Figure 1: The test site layout. Figure 2: Two styles of milling cutter, (**a**) the rhombic milling cutter and (**b**) the staggered helical milling cutter. As shown in Figure 3a, for the rhombic milling cutter, the flank face of its dextral cutting edge demonstrated a uniform wear band, with the flank wear VB being about 30 \(\upmu\)m, while at the tool tip of the rhombic teeth a triangular wear band occurred, mainly because only a part of sinistral cutting edge near the tool tip was involved in the milling. Nevertheless, as a result of the existence of the sinistral cutting edge, a chip-drain slot formed between the rhombic teeth, which was helpful for chip removal and heat dissipation. As shown in Figure 3b, for the staggered helical milling cutter, its flank face also showed an obvious wear band, with its flank wear VB being about 49 \(\upmu\)m, due to the fact that the cutting edge of the staggered helical milling cutter was continuous, and the larger cutting load per cutting edge length resulted in a more significant cutting force and higher cutting temperature, and furthermore, it accelerated the flank wear. As the tool wear accelerated, the diamond coating on the flank face gradually got thinner, and the adhesion between the coating and the cemented carbide substrate decreased obviously, because of the effect of the frequent impact load and friction. For the staggered helical milling cutter, when the milling proceeded up to 33 m, cracks occurred on the coating at the cutting edge because of the effect of fatigue wear, and eventually, coating shedding was caused. At this moment, the tool wear transformed into the coating shedding from the coating wear. While for the rhombic milling cutter, coating shedding did not occur until the cutting proceeded up to 42 m. When the milling length reached 44 m, the two styles of cutting tools were both in the stage of coating shedding, as shown in Figure 4. Here, the flank face of the rhombic milling cutter demonstrated a small area of coating shedding, and the substrate material at the cutting edge was already exposed. Nevertheless, the cutting edge contour did not change obviously, and the cutting result was relatively good. By contrast, the staggered helical milling cutter showed a large area of coating shedding, and then formed an irregular tear wear band. At this point, the exposed cemented carbide substrate was already involved in cutting and fell off to a serious extent. As a result, the cutting edge changed into an irregular crescent shape from the original helical one. Figure 4: The tool wear morphology when the milling length was 44 m, (**a**) the wear morphology of the rhombic milling cutter and (**b**) the wear morphology of the staggered helical milling cutter. Figure 3: The tool wear morphology when the milling length was 12 m, (**a**) the tool wear morphology of the rhombic milling cutter and (**b**) the tool wear morphology of the staggered helical milling cutter. With the progression of milling, the diamond coating fell off more and more seriously. For the rhombic milling cutter, the cutting edge became obviously blunt, and thus it could not cut off the fibers effectively, which increased the heat generated in the cutting process.