Helical milling for making holes on carbon fiber-reinforced polymer

Carbon fiber-reinforced polymer (CFRP) is widely used in various fields due to its excellent properties. However, machining defects such as delamination and burr are still the key issues constraining high-quality hole making on CFRP. This research takes the helical milling process to make holes on CFRP and two kinds of cutting conditions (dry cutting and jet cold air-assisted cutting) are explored. Tool wear, cutting force, delamination, and surface morphology are taken as the main indices to be analyzed. The influences of key cutting parameters (spindle speed n, axial feed per tooth fza, and pitch ap) on cutting force and delamination are investigated and optimized. It turns out that the jet cold air condition is more suitable for helical milling of holes on CFRP with better qualities. The holes with smaller delamination factor and smoother surface are successfully fabricated based on the optimized cutting parameters.


Introduction
Carbon fiber is a potential reinforcing material. It has the advantages of high specific strength, high specific stiffness, good thermal conductivity, electrical conductivity, thermal stability, and corrosion resistance. It has developed into the fourth largest aerospace structural material after aluminum, steel, and titanium [1]. The processing of assembly holes required for mechanical connection is an indispensable part of aircraft assembly. In the hole processing, hole wall damage often occurs due to improper operation method and tool wear. Especially for anisotropic carbon fiber-reinforced polymer (CFRP) material, the hole processing is much more difficult than traditional metal materials [2]. Comparing with traditional drilling processing, the helical milling adopts completely different processing methods. The helical milling process includes three basic motions: the main motion with the milling cutter axis as the rotation axis, the rotation motion of the tool around the hole center, and the feed motion along the axis. This special movement mode determines the advantages of the helical milling process, and eccentric processing is beneficial to improve the hole quality [3,4].
Pereszlai et al. [5] carried out inclined helical milling experiments on CFRP and glass fiber-reinforced polymer (GFRP) using uncoated carbide tools. The influence of inclined angle and pitch on axial cutting force were analyzed and discussed. The experimental results showed that the inclined angle had significant influence on the cutting force and burr. The inclined angle was suggested to increase as far as possible within the scope of the study. Amini et al. [6] conducted experimental studies on CFRP using tungsten carbide tools. The effects of process parameters such as cutting speed and feed rate on hole quality such as hole diameter, roundness, cylindricity, and surface roughness were discussed. The experimental results showed that higher cutting speed could improve the hole roundness and cylindricity. But it had limited influence on the surface roughness. The cutting speed was more important than the feed speed in the helical milling process. Lower feed speed could obtain high hole quality. Geier et al. [7] developed a method to monitor and diagnose helical milling process by using digital image processing of uncut fiber features. The effectiveness and reliability of the method were verified by experiments. The holes processed by monitoring method had less uncut fibers than those by fixed process parameters. Gaugel et al. [8] studied tool wear and delamination damage by using PCD tools and coated carbide tools. The results showed that tool wear and delamination damage were correlated and that PCD tools were more suitable for highquality drilling due to their long life. Wang et al. [9] investigated hole quality using two types of drills with controlled feed rates in order to reduce delamination damage during drilling. The experimental results showed that proper tool geometry and feed rate resulted in better hole quality, and the step drill had less delamination damage compared to the twist drill. Shu et al. [10] designed a special bit with a new thin-walled structure and compared it with a conventional bit to investigate the relationship between drilling force, drilling temperature, delamination, and process parameters. The experimental results showed that the special drill bit was superior to the conventional drill bit in these aspects, and changing the front angle of the drill bit was beneficial to improve the hole quality. Jia et al. [11] proposed a new step drill structure to reduce delamination and burr by step diameter ratio, and performed experimental verification. The experimental results showed that this new structure could improve the hole quality and effectively reduce the damage such as burrs and delamination. Wang et al. [12] developed macroscopic and microscopic models to reveal the cutting mechanism of CFRP. The experimental results verified the feasibility of the models and that fiber orientation had an important effect on the CFRP removal mechanism. Li et al. [13] used coated milling cutters and uncoated milling cutters to perform helical milling holes on the Ti/CFRP laminated structure. The results showed that the uncoated milling cutters had the best cutting performances, the lowest cutting force, the highest hole quality, and the smallest tool wear. Wang et al. [14] drilled holes in CFRP/Ti laminated structure, determined the influence of traditional drilling and helical milling on the geometric accuracy of holes, and studied the changes of aperture and roundness at different positions. The results showed that the geometric accuracy of the helical milling laminated structure in the titanium alloy layer hole was better, the roundness of the CFRP layer hole was better, and the roundness of the transition zone was smaller. Yang et al. [15] proposed a new anti-helical milling method to suppress delamination, which reduced the deformation of uncut material by increasing material stiffness. The effectiveness of the method was verified by experiments, which neither increased the existing delamination nor produced new delamination. Wang et al. [16] studied the cutting performance of CFRP helical milling and discussed the influence of cutting parameters on cutting force by experiments. Then, the relationship between processing performances and cutting parameters was established by nonlinear fitting method. The cutting parameters of CFRP helical milling were optimized based on multi-objective genetic algorithm, and the effectiveness of the optimal cutting parameters was verified by experiments.
From literature review, there is still a large space needing to be filled to fabricate high-quality holes on CFRP. Therefore, the effects of dry cutting condition and jet cold air condition on helical milling of holes on CFRP are studied and compared. Then, the cutting parameters are investigated and optimized. Finally, holes with high qualities are fabricated based on the research.

Experimental setup
The machining center CarverPMS23_A8 is selected for the experiments, and the three-dimensional force measurement system (Kistler 9257B) is used to measure the cutting force, as shown in Fig. 1. The tool is double-edged PCD milling cutter with the diameter of 4 mm. It has high hardness and strength, and high wear resistance, suitable for machining CFRP material. The detailed parameters of the milling cutter are shown in Table 1. The experimental material is T300 series CFRP, and its size is 80 mm × 50 mm × 5 mm. The material properties are shown in Table 2. During the cutting process, the tool continuously cut the material from the matrix material to produce a large amount of chips. The chip dust is dispersed in the air and easily enters the machine tool components, which affects the accuracy of the machine tool. At the same time, the chip dust is easily inhaled into the human body to endanger human health. Therefore, the dust removal device is set up to clean CFRP chips in time during the cutting process. Two cutting conditions are applied and compared. The dry cutting condition is without any additional cooling medium under the room temperature. The jet cold air condition is with additional low-temperature gas cooling of the cutting area using an adjustable cold air gun with a gas flow rate of 283 L/min and a cooling temperature of -5 °C.
In the experiment, the flank wear width of the milling cutter is selected as the evaluation index of tool wear, which refers to the distance from the outer edge contour of the milling cutter to the boundary of the wear area. Before using Quanta 250 scanning electron microscope to observe tool wear, PL-S40 ultrasonic cleaner is used to clean with 30 min and then dried with blowing air. The surface morphology of the hole is observed and analyzed by USB 200 digital tool microscope. The tool wear, cutting force, and hole diameter in the delamination are measured 6 times, and the obtained experimental data are averaged.

Experimental design
In the process of helical milling, the rotational speed of milling cutter n is usually much larger than the orbital speed n g , so it can be approximately considered that the cutting speed v is proportional to the spindle speed of milling cutter n. Therefore, the cutting parameters of helical milling include spindle speed n, axial feed per tooth f za , and pitch a p . Undeformed chip formation is always changing from the beginning to the steady helical milling state and then the undeformed chip formation remains unchanged. Figure 2 shows the undeformed chips at the steady state, where D t is cutter diameter [17].
The experimental design selects three important influence factors, and each factor selects four levels. According to the number of factors and the number of levels, the L 16 (4 3 ) orthogonal table is selected without considering the interaction between factors. The designed orthogonal experimental table is shown in Table 3.

Tool wear
The helical milling is a semi-closed machining. A large amount of heat generated during the cutting process is not easily taken away by chips and tools, which is easy to cause tool wear. Tool wear not only affects hole quality, but also affects processing efficiency and increases processing cost [18][19][20]. After the experiment is completed, the tool wear is observed, and the tool wear forms under the two cutting conditions are shown in Fig. 3. Figure 3 presents the tool wear forms under two cutting conditions, where the main wear forms are micro-chipping,    [17] corner rounding, and adhesion. Under jet cold air condition, the corner rounding is relatively serious, and the cutting edge is slightly chipped. Helical milling is an intermittent cutting, and the cutting edge is subjected to continuous alternating loads. Due to the brittle nature of PCD material, micro-chipping is prone to occur at the cutting edge, especially when the temperature gradient is large. The corner rounding is mainly from micro-chipping and the frictions between the tool and the workpiece. In addition, the chip is easy to bond on the tool surface under the combined thermal-force actions.
In the process of CFRP helical milling, the flank and transverse edge of the tool have different degrees of wear, and the flank wear is the main wear form [21]. Therefore, the flank wear is measured and analyzed. After the experiment is completed, in order to further determine the influence of two cutting conditions on tool wear, the trend of tool wear changing with the hole number is recorded as shown in Fig. 4.
As shown in Fig. 4, with the increase of the hole number, the tools under both cutting conditions appear different degrees of wear, and the wear values show an upward trend. On the one hand, with the increase of the hole number, the scratching effect on the tool flank is continuously increased. In addition, during the helical milling process, there are more carbon powder particles scratching the tool resulting in increased tool wear. On the other hand, due to poor thermal conductivity of the matrix, more and more cutting heat accumulates. This further increases the tool wear.
Compared with dry cutting condition, tool wear is more serious under jet cold air condition. With the increase of the machined holes, the cutting temperature of the tool in helical milling continuously increases resulting in a large change in the temperature gradient of the cutting edge in jet cold air cutting condition. According to Fig. 3, the micro-chipping and corner rounding is relatively serious under jet cold air condition. Therefore, the tool occurs to wear more than the tool under dry cutting condition.

Cutting force
The magnitude and fluctuation of cutting force directly reflect the machining state and affect the surface quality [22]. In the hole processing on CFRP, the cutting force directly affects the hole quality, which is the main factor causing the material delamination around the hole wall, the tearing of the entrance and exit of the hole, the burr, and other major defects [23]. The resultant force is collected as shown in Eq. (1).
The cutting force obtained after processing the data is shown in Fig. 5. Figure 5 shows the cutting force created under dry cutting condition and jet cold air condition. It can be seen from  Fig. 4 that the cutting force under jet cold air condition is larger than that under dry cutting condition. This is because the matrix material of CFRP is a kind of resins, which determine the final performances of CFRP, such as structural strength and structural stiffness. The resin becomes harder after low-temperature cooling, so that the strength of CFRP increase with the decrease of temperature. It will lead to the increase of cutting forces.

Delamination
Among many machining defects, delamination is one that has a fatal effect on the hole quality [24,25]. It refers to the CFRP interlayer stress or manufacturing defects caused by delamination between the composite layer separation failure phenomenon. It causes a decrease in the tensile strength of the CFRP laminates. Under the working conditions of alternating fatigue load, delamination further expands and it will ultimately lead to early termination of service life of CFRP components. The delamination diagram is shown in Fig. 6.
In the process of helical milling, when the tool begins to contact with the workpiece material and the main cutting edge is not fully cut into the workpiece material, the cutting force pushes the removed material into the helical groove. These materials rise along the helical groove surface before cutting, resulting in an upward peeling force. The peeling force separates the unresected region in the upper layer, namely the peeling delamination. On the other hand, when the tool is about to cut out the material, because the number of remaining uncut layers of the material is less and less, if the cutting force exceeds the interlayer bonding strength of the material, debonding occurs between the layers around the exit, resulting in push-out delamination. The delamination occurs in the interlayer region, so it not only depends on the properties of the fiber, but also depends on the properties of the resin [26]. CFRP laminates are delaminated on both the entrance side and the exit side in helical milling. In this study, the entrance side is focused and studied considering the peeling forces.
The ratio of the maximum stratified diameter D max to the nominal diameter D norm of the hole F d is taken as the standard to measure the delamination degree [27], namely the diameter delamination factor, which is referred to as the delamination factor F d . The formula is shown in Eq. (2).
After the experiment, the diameter of the hole processing under two cutting conditions is measured. The delamination factor obtained after processing the data is shown in Fig. 7.  Figure 7 is the delamination factor of each group under dry cutting condition and jet cold air condition. It can be seen from Fig. 7 that the delamination factor of hole processing under jet cold air condition is smaller than that under dry cutting condition, indicating that the delamination phenomenon at the hole inlet is effectively suppressed under jet cold air condition. Under jet cold air condition, due to the large cutting force, the material can be more completely removed and pushed into the helical groove resulting a smaller peeling force than that under dry cutting condition. In addition, the interfacial bond between fiber and resin increases under jet cold air condition. It inhibits the fiber extrusion during helical milling and reduces the fiber crack extension distance in axial and radial directions, improving the delamination. Therefore, the hole surface is smoother and the delamination factor is smaller.
The delamination factor of the sixteenth hole is the largest with the cutting parameters of n = 20,000 rpm, f za = 0.08 mm/z, and a p = 0.2 mm. The delamination factor is 1.145 and 1.148 under jet cold air condition and under dry cutting condition, respectively. The fifth hole has the smallest delamination factor with the cutting parameters of n = 12,000 rpm, f za = 0.02 mm/z, and a p = 0.4 mm. The delamination factor is 1.128 and 1.130 under jet cold air condition and under dry cutting condition, respectively.

Surface morphology
Surface morphology is a key factor affecting the performances and reliability of materials, which can further reflect the processing quality of workpiece [28]. Based on the analysis in Sect. 3.3, the delamination factors of the sixteenth and the fifth holes are the largest and the smallest, respectively. The surface morphology of the corresponding processing hole is shown in Figs. 8 and 9.
After the experiment, no obvious surface fiber burrs and tears occurred on the entrance surface of the processing holes under two cutting conditions. However, it can be observed in Fig. 8a that there is chip dust adsorption on the surface at the outlet of the processing hole under dry cutting condition. Figures 8 and 9 show that the processing hole surfaces under jet cold air condition are smoother than that under dry cutting condition. This is because as the cooling temperature decreases, the bond strength between the carbon fiber and the resin gradually increases. The fibers are bound more tightly by the resin and are less likely to deform elastically during the cutting process, thus being cut smoothly. This effectively inhibits the generation of tearing and burr defects at the tool entry point, thereby making the surface of the CFRP material smoother. Therefore, the hole quality under jet cold air condition is better than that under dry cutting condition.  Tables 4 and 5.
In the table, K i is the sum of the experimental results corresponding to any level, i is the level, and k i is the average value of the results at any level. The range R corresponding to each column factor is R = max {k 1 , k 2 , k 3 , k 4 } − min {k 1 , k 2 , k 3 , k 4 }. The larger the R value is, the larger the influence of cutting parameters on the index is. The range analysis result of the cutting force is shown in Table 4. According to the magnitude of the range R, it can be analyzed that the influence of the axial feed per tooth f za on the cutting force is significantly larger than that of the pitch a p and the spindle speed n. The axial feed per tooth f za and pitch a p have the same trend to the cutting force. With the increase of the axial feed per tooth f za and pitch a p , the cutting force gradually increases. With the increase of spindle speed n, the cutting force gradually decreases. Considering the influence of various factors on the cutting force, the optimal parameter combination is selected as n = 20,000 rpm, f za = 0.02 mm/z, and a p = 0.2 mm.
The range analysis result of delamination is shown in Table 5. According to the range R, the influence of spindle speed n on delamination is larger than that of the axial feed per tooth f za and pitch a p . With the increase of spindle speed n, the delamination factor decreases first and then increases. And the axial feed per tooth f za and pitch a p have the same trend for delamination. With the increase of the axial feed per tooth f za and pitch a p , the delamination factor gradually increases. Considering the influence of various factors on delamination, the optimal parameter combination is selected as n = 12,000 rpm, f za = 0.02 mm/z, and a p = 0.2 mm.
In order to obtain good hole quality, based on the orthogonal experimental results, each index is analyzed according to the range analysis results. For the cutting force and delamination indexes, the optimal values of the axial feed per tooth f za and pitch a p are f za = 0.02 mm/z and a p = 0.2 mm. The optimal value of spindle speed n for cutting force index is n = 20,000 rpm, and the optimal value for delamination index is n = 12,000 rpm. Considering that the influence of delamination on the hole quality is relatively large, and the influence of cutting force on the hole quality is relatively small, the optimal value of delamination index is given priority, which is n = 12,000 rpm. To sum up, the final optimal parameter combination is n = 12,000 rpm, f za = 0.02 mm/z, and a p = 0.2 mm. The verification experiment is carried out with the optimal parameter combination. The effects of the created holes on CFRP are shown in Fig. 10. The measured delamination factor is 1.125, and the processing hole surface is relatively smooth. The hole quality after optimized verification experiment is good.

Conclusions
In this research, two cutting conditions (dry cutting condition and jet cold air condition) are used to conduct helical milling studies on CFRP. Comparisons have been conducted from the perspective of tool wear, cutting force, delamination, and surface morphology in helical milling of holes on CFRP. The main conclusions are summarized as follows.
1) Both the tool wear and the cutting force are smaller under dry cutting condition than that under jet cold air condition. However, under the latter condition, delamination can be better suppressed and better surface morphology can be achieved. 2) The impact sequence of cutting parameters on the cutting force from large to small is f za , a p , and n. The impact sequence of cutting parameters on the delamination from large to small is n, f za , and a p .
3) The optimal parameter combination is n = 12,000 rpm, f za = 0.02 mm/z, and a p = 0.2 mm under jet cold air condition. Holes with smaller delamination factor and smoother are successfully fabricated on CFRP.

Funding
The research is financially supported by the Shandong Provincial Key Laboratory of Precision Manufacturing and Non-traditional Machining, and the Natural Science Foundation of Shandong Province (ZR2020ME157).
Data availability All data generated or analyzed during this study are included in this published article.

Declarations
Ethics approval Not applicable.