Study on tool wear and cutting performance of CFRP for inclined angle milling

Carbon fiber reinforced plastic (CFRP) has been widely used in aviation, aerospace, automotive, and other fields due to its advantages of high specific strength and high specific modulus. However, as a typical anisotropic material, CFRP has a more prominent problem in processing, that is, tool wear, which is easy to form machining defects such as burrs, tearing, and delamination. In this paper, the comparative experimental study on inclined angle milling and spiral milling of CFRP was carried out. The milling axial force, tool wear, quality of hole entrance and exit, and micro-morphology of the hole wall under the two milling methods were analyzed. The results show that compared with spiral milling, the axial force of inclined angle milling is relatively small, and the fluctuation is relatively smooth. The wear of inclined angle milling tool end edge is mainly concentrated in the connection area of end and side edges (CAES), and the wear of side edge is mainly concentrated in the tip of rhombic tooth, while the wear of spiral milling tool end edge and side edge is relatively uniform. As the number of holes increases, the wear morphology of inclined angle milling end edge changes from a small area of discontinuous crescent shape to a large area of continuous triangular, while the wear morphology of spiral milling end edge changes from a long strip to a large area of parallelogram. With the increase of tool wear, the holes by inclined angle milling produce fewer burrs and tearing, there are some defects such as fiber fracture residues and cavities on the surface of the hole wall, while the surface of the hole wall by spiral milling shows a large amount of chip adhesion and pit.


Introduction
Carbon fiber reinforced plastic (CFRP) has good fatigue strength and specific tensile strength and is widely used in aviation, aerospace, and another field [1][2][3].Due to the high brittleness and poor thermal conductivity of CFRP, tool wear is prone to occur during the hole-making process [4,5], which leads to a decrease in the hole quality and the occurrence of processing defects such as delamination, tearing, and burrs [6].The processing quality of CFRP has become an important factor affecting the safety and reliability of the aviation structural parts [7].Compared with conventional drilling methods, spiral milling has less axial force, and it can reduce the contact area between the chip and the hole wall, which facilitates the chip discharge, burr, and fiber delamination reduction [8][9][10].Therefore, spiral milling has gradually become an alternative processing method to conventional drilling [11,12].In addition, in recent years, a brand new low-damage inclined angle milling method for CFRP has received continuous attention from the industry.
The research on spiral milling of CFRP mainly focuses on the cutting force and surface quality of hole-making.Wang et al. [13] established a cutting force model for spiral milling considering the fiber cutting angle, and the accuracy of the model was verified by experimental data.Shang et al. [14] established a cutting force model for spiral milling and carried out an experimental verification of the model accuracy, which optimized the cutting parameters and improved the hole quality.Wang et al. [15] conducted spiral milling experiments on CFRP and discussed the relationship between hole quality and cutting force; at the same time, they also conducted spiral milling experiments for CFRP single-layer and multilayer plates to analyze the influence mechanism of cutting parameters, different workpiece materials, and tool wear on the cutting force [16].Li et al. [17] carried out a study on the modeling and prediction of surface morphology and surface roughness in the spiral milling process.Qin et al. [18] determined the influence of process parameters on the delamination factor and obtained the optimal process parameters for hole-making of CFRP based on the systematic spiral milling experiments.
Different from spiral milling, the existence of a certain inclination angle during inclined angle milling can decrease the centrifugal force generated during the machining process, reduce the contact area between the tool and the workpiece, reduce the vibration caused by the eccentric mass imbalance, and ensure gentle cutting in the entrance and exit areas of the machined holes, which is conducive to improving the hole quality of CFRP [19][20][21].Hosokawa et al. [22] carried out an experimental study on inclined milling of CFRP with a high helix angle end mill, which improved the integrity of the CFRP machined surface compared with an ordinary end mill.Gao et al. [23] conducted a comparative experimental study between inclined angle grinding and spiral milling of CFRP and found that inclined angle grinding had smaller axial forces and lower cutting temperatures.Wang et al. [24,25] carried out a comparative study between inclined spiral milling and spiral milling in terms of kinetics and machined surface quality during hole formation, which showed that inclined spiral milling can reduce drilling forces and avoid zero cutting speed and can obtain high-quality holes with good hole wall roughness and high aperture accuracy.Pereszlai et al. [26] carried out an experimental study on inclined angle milling of CFRP and GFRP with uncoated end mill and obtained the influence of inclination angle and pitch on axial cutting force.
In summary, inclined angle milling can effectively reduce the axial force, improve the machining surface integrity and aperture accuracy, and improve the quality of machined holes.Its study is also mainly focused on the comparison of machining quality with spiral milling, and at present, there is relatively little research on the tool wear mechanism in the process of inclined angle milling.Therefore, in this paper, the milling of CFRP holes was conducted by using rhombic tooth milling tools to carry out the study of tool wear mechanism.The comparative experiments of inclined angle milling and spiral milling of CFRP were designed.The milling performance of the two machining modes was compared and analyzed from the aspects of milling axial force, morphology and mechanism of tool wear, and hole surface quality, which provided a reference for the further application and promotion of the inclined angle milling CFRP technology.

Research on the experimental equipment
In order to compare and analyze the machining performance of inclined angle milling and spiral milling of CFRP, the motion analysis of the two hole-making methods was first carried out.As shown in Fig. 1, both inclined angle milling and spiral milling are composed of feed motion, rotation motion, and revolution motion.While the inclined angle milling tool rotates around its own axis at n z speed, it makes a conical swing revolution around the central axis of the machined hole at n g speed and feeds in the direction of the central axis of the machined hole.Eccentricity adjustment is achieved by adjusting the inclination angle between the rotating axis and the revolution axis.While the tool rotates in spiral milling, it also carries out feed and rotation motions along the axis of the central part of the machined hole.The eccentricity is adjusted by moving the tool rotation axis parallel to the revolution axis [27,28].The inclined angle milling hole diameter D is determined by the tool diameter d, the distance L between the tool tip and the intersection of The hole diameter D of the spiral milling is determined by the tool diameter d and the eccentricity e between the tool axis and the hole axis, as shown in Eq. 2.
In the inclined angle milling and spiral milling processes, the relationship between the hole diameter D and workpiece thickness H determines the quality of the hole.When the hole diameter meets the requirements shown in Eq. 3, the hole can be machined to meet the quality requirements.
In order to better carry out the comparative experimental study of inclined angle milling and spiral milling CFRP, the tool inclination during inclined angle milling is equated to the workpiece inclination.The equivalent result is shown in Fig. 2.
Currently, the machining hole diameter in inclined angle milling needs to meet the requirements of Eq. 4.
During the experiment, in order to avoid the vibration caused by the tool overhanging too long and thus affecting the machining quality, the distance between the tool tip and the intersection of the inclined axis and the hole axis is taken to be 24 mm, and the diameter of the tool is selected to be 6 mm.When the tool eccentricity is e ≥ (d cos θ)/2, the material near the center axis of the hole will not be removed by the tool.Therefore, it is assumed that when the hole diameter (1) reaches 12 mm by inclined angle milling, the inclination angle of the workpiece is calculated to be 7.12°.To this end, the inclination angle of the machining experimental equipment is designed to range from 0 to 7°, and the diameter of the hole that can be machined by the tool ranges from 6 to 11.89 mm.To facilitate the subsequent comparative analysis of the quality of holes processed by inclined angle milling and spiral milling, the inclination angle of inclined angle milling is selected as 3°, and the eccentricity of spiral milling is 1.26 mm.
Figure 3 shows the inclined angle milling and spiral milling experimental equipment.The experimental equipment mainly consists of workpiece fixture, workpiece adjustment block, revolving table, stepper motor, supporting foot seat, and angle adjustment seat.To accurately derive the size of holes in inclined angle milling and spiral milling, the central origin of the experimental equipment was calibrated through the calibration slot on the revolving table before the experiment began.At the same time, the angle adjustment seat was equipped with three waist-shaped slots b, c, and e by rotating around point a to adjust the machining angle.The experimental equipment needed to be combined with the machine tool to carry out the comparative experiments of inclined angle milling and spiral milling.The inclined angle milling achieved eccentricity adjustment by adjusting the angle of the equipment and matching the machine tool to the calibration the origin.The eccentricity of the spiral milling was adjusted by the machine tool, and the hole diameter of the inclined angle milling and spiral milling was kept consistent during the machining process.The experimental equipment controlled the stepper motor drive system to achieve real-time adjustment of the equipment revolution speed, and spindle feed and rotation motion required for the two holemaking processes were provided by the machine tool.

Design of the experiment
In this paper, the experimental study of CFRP milling was carried out on VDL-1000E high-speed milling center, and the maximum spindle speed of the machine was 8000 r/ min.The experimental system and the experimental equipment after clamping were shown in Fig. 4. The workpiece material was T700 unidirectional CFRP laminate, composed of carbon fiber and AG-80 epoxy resin, and the size of the workpiece was 200 mm × 100 mm × 5 mm.The tool selected was an integral carbide rhombic tooth end mill produced by Xiamen Golden Egret Special Alloy Co., Ltd, and the model was SD200-CN12-06015.The tool was coated with ultrafine grain diamond (U-DIA) coating.The coating hardness reached 80 Gpa, and the coating grain size was 10 to 500 nm.The coating surface was smooth, with high hardness, high wear resistance, low friction coefficient, and other characteristics.The tool was 60 mm long and 6 mm in diameter, with the side edge of the tool staggered at a double helix angle of about 15° and 40°, and the cutting zone of the side edge was 15 mm long.A Kistler 9139AA three-way piezoelectric dynamometer was used to collect the cutting force data during the experimental process.The design of the experimental parameters for inclined angle milling and spiral milling was shown in Table 1.
The machining parameters shown in Table 1 were selected to carry out the experiments of inclined angle milling and spiral milling, and the axial forces of the two milling methods were compared and analyzed.To study the wear morphology and mechanism of the tool in the hole-making process, the fixed experimental parameters revolution speed 100 r/min, spindle rotation speed 3500 r/min, and spindle feed speed 7.6 mm/min were used to carry out the comparative experiments.The tool was removed after every 10 holes in the machining process, and the quality of the entrance and exit of the hole was observed with a MARS-1230-23U3C industrial camera.The wear morphology and elemental composition of the tool and the micro-morphology of the hole wall were analyzed by the SU3500 scanning electron

Milling axial force
Figure 5 compares the axial forces generated by inclined angle milling and spiral milling at different feed speeds, revolution speeds, and rotation speeds.As the spindle rotation speed increased, the axial force of both machining methods showed a certain degree of reduction, but the axial force of inclined angle milling was reduced by about 60% compared with spiral milling.With the increase in revolution speed, the axial force of both methods decreased and then increased, and the axial force of inclined angle milling was reduced by about 50% compared with spiral milling.With the increase in axial feed speed, the axial force of both tools increased gradually, and the axial force of inclined angle milling was reduced by about 40% compared with spiral milling.
Figure 6 shows the comparison of axial force between spiral milling and inclined angle milling under the milling parameters of spindle rotation speed 3500 r/min, revolution speed 100 r/min, and axial feed speed 7.6 mm/min.The machining process was divided into five stages, namely, A(I) no-load stage, B(II) entrance cutting stage, C(III) steady cutting stage, D(IV) exit cutting stage, and E(V) withdrawal stage.At the B(II) stage, the tool started cutting into the CFRP workpiece, and there was a significant difference in axial force between the two methods.The rise in axial force was greater for spiral milling than for inclined angle milling.The axial force of spiral milling increased from 0 to 50 N, while that of inclined angle milling increased from 0 to 16 N, and the cutting time for inclined angle milling was slightly longer in this stage.The reason for this difference was that the end edge of the tool in spiral milling was fully involved in cutting, resulting in a sudden increase in axial force.However, there was a certain angle between the tool and machining hole axis in inclined angle milling, which was firstly cut slowly by the tool tip until the end edge and part of the side edge were fully entered to reach C(III).This progressive cutting effectively reduced the axial force.At the stage C(III), the axial forces under both types of milling methods were at a relatively stable level.At this time, the axial force decreased by about 60% for inclined angle milling compared to spiral milling.When the workpiece was machined to the D(IV) stage, the remaining uncut material reduced the axial support stiffness of the tool.The axial force gradually decreased during the slow removal of the workpiece by the tool, and the axial force became zero when the unprocessed material was completely removed.The above analysis found that inclined angle milling can produce smaller axial force in the holemaking process, and the fluctuation of axial force was smaller than that of spiral milling.

Tool wear
The tool wear in the process of inclined angle milling and spiral milling was compared and analyzed.Figure 7 shows the wear morphology of the tool end edge when milling 20 holes.The connection area of end and side edges (CAES) of the milling tool for inclined angle milling had less wear, and the cutting edge was relatively flat (Fig. 7a and b).A small wear band was produced on the flank of the end edge of the milling tool for spiral milling, and the tool showed slight wear (Fig. 7d and e).Only a small amount of the O element was found to be present in addition to the C element by EDS analysis (Fig. 7c and f).At the initial stage of tool wear, the carbon fiber as a cutting hard point would generate friction with the tool continuously, causing slight scratches on the tool.At this time, the scratches on the coating surface of the inclined angle milling tool were smaller than those of spiral milling tool, and the abrasive wear mechanism was dominant.
Figure 8 compares the wear morphology of the tool end edge when milling 60 holes.At this time, the CAES concentrated wear of the inclined angle milling tool was serious, and the tool coating stress was greater than the bonding force between the fiber bundles so that the CAES was gradually worn to appear a small area of discontinuous elongated crescent-shaped wear band (Fig. 8a and b).The wear of the flank coating of the end edge for spiral milling was further enlarged, and a bright and long wear band appeared (Fig. 8d  and e).The high-speed friction between the tool and CFRP increased the cutting temperature weakening the bonding ability between the tool coating and the substrate, which caused the chemical reaction between the C element in the coating and the O element in the air.It was found that the inclined angle milling tool showed a small area of substrate bareness by EDS analysis with the content of the W element of 26.47% (Fig. 8c).In comparison, the spiral milling tool showed oxidative wear with the content of the O element of 15.20% (Fig. 8f).
Figure 9 analyzes the wear morphology of the tool end edge when milling 120 holes.In inclined angle milling, the wear of CAES further expanded, and the wear area began to expand toward the flank of the end edge.A large triangular wear area and a small crescent-shaped wear area began to appear at the flank of the end edge (Fig. 9a and b), while the spiral milling tool showed a large parallelogram-shaped coated wear area (Fig. 9d and e).A large amount of W and Co elements were detected by EDS analysis (Fig. 9c and f), which indicated that both tools were in the late stage of wear.At this stage, the two tools showed serious coating shedding, resulting in the exposure of the tool matrix, and the machining performance began to decline.
Figure 10 shows the wear morphology of the tool side edge when milling 20 holes.The side edge wear of the inclined angle milling tool was mainly concentrated at the    tip of the rhombic tooth (Fig. 10a and b), while the side edge wear of the tool for spiral milling was more uniform (Fig. 10d and e).Through the EDS analysis, it was found that the O element content of the tool for inclined angle milling was 1.98% (Fig. 10c), and the O element content of the tool for spiral milling was 11.07% (Fig. 10f).Thus, the coating wear of the tool for inclined angle milling was dominated by abrasive wear.However, the spiral milling tool underwent slight oxidative wear, and the coating wear of the tool side edge was obvious.Figure 11 analyzes the wear morphology of the tool side edge when milling 60 holes.Small triangular wear bands with severe chip bonding were observed at the tip of the rhombic tooth of the side edge in the inclined angle milling tool (Fig. 11a and b), while a large area of triangular wear bands was observed on the side edge of the spiral milling tool (Fig. 11d and e).It was found that both tools showed coating peeling and matrix exposing through the EDS analysis, which increased the tool wear (Fig. 11c and f).At this stage, the coating shedding and substrate exposure area of the inclined angle milling tool was smaller than that of the spiral milling tool.
Figure 12 compares the wear morphology of the tool side edge when milling 120 holes.A large triangular wear band appeared at the tip of the rhombic tooth of the side edge in the inclined angle milling tool, and the coating was obviously shedding (Fig. 12a and b).The coating of the tool side edge for spiral milling was utterly shedding, and the tool was severely worn (Fig. 12d and e).At this time, the cutting ability of the tool on fiber and resin decreased, and with the increase in cutting temperature, the tool was prone to chip bonding, resulting in severe tool passivation.Through EDS analysis, it was found that both tools were in the late wear stage, and a large amount of W and Co elements appeared (Fig. 12c and f).The inclined angle milling tool showed a large area of exposed substrate at the tip of the rhombic tooth, while the side edge substrate of the spiral milling was completely exposed.At this stage, the machining performance of the tool gradually decreased, but the side edge wear of the inclined angle milling tool was less than that of the spiral milling tool.

Quality of hole entrance and exit
Figure 13 shows the morphology comparison of the hole entrance between the inclined angle milling and spiral milling.When the number of holes reached 20, the entrance machining quality of both milling methods was better.When the number of holes reached 60, the CAES of the inclined angle milling tool was relatively worn seriously, which reduced the removal effect of fiber material, and a small number of burrs and tearing damage appeared at the entrance.The end edge of the spiral milling tool was relatively less worn, and fewer tearing and burrs appeared    at the entrance.The entrance quality of spiral milling was slightly better than that of inclined angle milling in this process.When the number of holes reached 120, the entrance quality of the two milling methods decreased significantly.The burrs and tearing damage at the entrance of inclined angle milling and spiral milling gradually increased.However, the hole-making quality under inclined angle milling was slightly better than spiral milling, which was due to the small contact area between the end edge of the inclined angle milling tool and the workpiece.When the fiber at the entrance was cut, the lower layer of the fiber can play a certain supporting role.At this time, the flank cutting edge of the tool near the CAES began to participate in the cutting, which reduced the generation of burrs and tearing defects.The end edge of the spiral milling tool directly touched the workpiece, and the contact area was large, so that the fiber at the entrance lacked effective support, and serious tool wear made the defects more likely to occur.Figure 14 shows the morphology comparison of the hole exit between the inclined angle milling and spiral milling.When the number of holes reached 20, a few short burrs appeared at the hole exit of inclined angle milling, and the burrs were directed toward the central part of the holes, while the hole exit of spiral milling had a tiny area of tearing in addition to a few burrs.At this time, the cutting edge of the tool was relatively sharp, the ability to cut the fibers was relatively good, the pulling effect on the fibers was relatively light, and the quality of the holes produced by both milling methods was better.When the number of holes reached 60, the burrs at the hole exit of inclined angle milling increased and gradually began to close to the hole wall, and a small number of burrs and tearing damage appeared at the hole exit of spiral milling.At this stage, the exit quality of the inclined angle milling decreased as a result of the concentrated wear of the end edge of the tool, so that the length of burrs and tearing defects in the inclined angle milling holes was slightly larger than that of the spiral milling holes.When the number of holes reached 120, many long burrs appeared at the hole exit of inclined angle milling and grew close to the hole wall, and the area of tearing defects gradually expanded.However, the spiral milling exit began to appear such as long burrs, uncut fibers, and tearing defects, and the tearing area along the direction of the burr generation increased.At this stage, the end edge of the inclined angle milling tool was seriously worn, and the cutting edge of the flank of the end edge near the CAES began to participate in cutting, which had a certain inhibition effect on the holemaking defects.However, the end edge and side edge of the spiral milling were relatively seriously worn, so that the quality of holes made in the way of inclined angle milling was slightly better than that in the spiral milling.

Hole wall micro-morphology
Figure 15 shows the micro-morphology of the hole wall when the number of holes reached 20.The fiber fracture on the hole surface of inclined angle milling was mainly coated by resin.The resin matrix was continuous, the fiber exposed was not obvious, and the machined surface was smooth.However, a small number of exposed fibers were found on the hole surface of spiral milling in addition to the resin coating, but the machined surface was relatively flat overall.At the initial stage of hole-making, the processing surface quality of the two milling methods was relatively good, and no obvious defects were found.Figure 16 shows the micro-morphology of the hole wall when the number of holes reached 60 holes.When the fiber direction angle (the angle between the counterclockwise direction of tool rotation and the fiber direction) was 45°, the tool in inclined angle milling broke the fiber bundles through extrusion and bending.The cutting effect on the fibers was gradually reduced, resulting in the existence of pulling phenomenon of the uncut fiber bundles, which caused the fiber fracture irregularity, resulting in the machined surface began to appear defects such as chip adherence, fiber pullout, fiber fracture, and microcrack.At this time, there were phenomena such as chip adhesion, fiber pull-out, and fiber fracture on the spiral milling.Compared with the inclined angle milling, the fiber fracture on the surface of the holemaking was relatively regular.At this stage, the hole wall quality of inclined angle milling was slightly worse than that of spiral milling.
Figure 17 compares the micro-morphology of the hole wall of the two machining methods when the number of holes reached 120.The hole wall quality of both milling methods showed significant degradation.When the fiber direction angle was 135°, defects such as fiber fracture, groove, and cavity appeared on the hole wall surface of the inclined angle milling.This was because the tool passivation was serious, the cutting ability of the fiber was reduced, and the machined surface was squeezed by the flank surface, resulting in a significant fiber fracture effect.Tool wear reduced the bonding between the fiber and the resin, resulting in internal defects on the machined surface.In the spiral milling process, with the wear of the flank, the ability of the tool to effectively cut the fiber was seriously reduced, resulting in chip adhesion and fracture of the fibers on the hole wall surface, accompanied by a large amount of fiber exposure.At the same time, the hole wall surface would produce defects such as the groove and pit, which eventually led to the gradual deterioration of the hole wall quality.In summary, the hole wall quality of inclined angle milling was better than that of spiral milling.

Conclusion
This paper carried out the comparative experiment of inclined angle milling and spiral milling of CFRP through the designed experimental equipment, compared and analyzed the axial force of milling, the morphology and  mechanism of tool wear, and the surface quality of machined holes according to the experimental results, and obtained the following conclusions: 1. Based on the analysis of the kinematics of inclined angle milling, an experimental equipment based on machine tool assistance to realize inclined angle milling and spiral milling was designed, and real-time adjustment of the revolution motion of the equipment was realized by using the drive system to control the stepper motor.By changing the angle of inclination, the comparative experimental design of inclined angle milling and spiral milling of CFRP was carried out by the equipment.2. The axial force of inclined angle milling and spiral milling showed a similar growth trend, and the difference in axial force increased with the increase of rotation speed and the decrease of feed speed and decreased first and then increased with the increase of revolution speed.Under the same cutting parameters, inclined angle milling had a smaller axial force compared with spiral milling, and the magnitude of the change in axial force was also relatively small.3. The tool wear mechanism under the two milling methods was mainly abrasive wear.The end and side edges of the tool in inclined angle milling were less worn than that in spiral milling, and the wear was mainly concentrated in the connection area of end and side edges, and it was easier for the chip bonding phenomenon to occur.In the late stage of tool wear, a large continuous triangular coating wear area appeared on the end edge of inclined angle milling, while the end edge of the spiral milling tool appeared to have a large area of coating shedding, forming a parallelogram coating wear area.4. When the amount of tool wear was small, the cutting edges of inclined angle milling and spiral milling were relatively sharp, and the quality of holes was relatively ideal.With the increased wear of the tool, inclined angle milling produced fewer burrs and tearing, but a certain amount of fiber fracture residue, cavity, and other defects appeared on the surface of the hole wall.Spiral milling was obviously affected by the sharpness of the cutting edge, and a lot of chip adhesion and pit appeared on the surface of the hole wall.

Fig. 3
Fig. 3 Experimental equipment of the inclined angle milling and spiral milling (a) Experimental system (b) Experimental tool and workpiece clamping

Fig. 5
Fig. 5 Effect of milling parameters on axial force

Fig. 6
Fig. 6 Comparison of axial force between inclined angle milling and spiral milling (a) The end edge in inclined angle milling (b) The magnified area Ae1 (c) The EDS analysis of point Ce1 (d) The end edge in spiral milling ( e) The magnified area Be1 (f) The EDS analysis of point De1

Fig. 7
Fig. 7 The tool end edge wear micro-morphology and EDS analysis when milling 20 holes.(a) The end edge in inclined angle milling.(b) The magnified area A e1 .(c) The EDS analysis of point C e1 .(d) The

Fig. 8
Fig. 8 The tool end edge wear micro-morphology and EDS analysis when milling 60 holes.(a) The end edge in inclined angle milling.(b) The magnified area A e2 .(c) The EDS analysis of point C e2 .(d) The

Fig. 9
Fig. 9 The tool end edge wear micro-morphology and EDS analysis when milling 120 holes.(a) The end edge in inclined angle milling.(b) The magnified area A e3 .(c) The EDS analysis of point C e3 .(d) (a) The side edge in inclined angle milling (b) The magnified area As1 (c) The EDS analysis of point Cs1 (d) The side edge in spiral milling ( e) The magnified area Bs1 (f) The EDS analysis of point Ds1

Fig. 10
Fig. 10 The tool side edge wear micro-morphology and EDS analysis when milling 20 holes.(a) The side edge in inclined angle milling.(b) The magnified area A s1 .(c) The EDS analysis of point C s1 .(d)

Fig. 11
Fig. 11 The tool side edge wear micro-morphology and EDS analysis when milling 60 holes.(a) The side edge in inclined angle milling.(b) The magnified area A s2 .(c) The EDS analysis of point C s2 .(d)

Fig. 12
Fig. 12 The tool side edge wear micro-morphology and EDS analysis when milling 120 holes.(a) The side edge in inclined angle milling.(b) The magnified area A s3 .(c) The EDS analysis of point C s3 .(d)

Fig. 16
Fig. 16 Micro-morphology of the hole wall when milling 60 holes

Fig. 17
Fig. 17 Micro-morphology of the hole wall when milling 120 holes

Table 1
Experimental design of inclined angle milling and spiral milling for CFRP Experimental method Revolution speed n g /r•min −1 Rotation speed n z /r•min −1 Feed speed V f /mm•min −1