A Review on Factors Affecting Machinability and Properties of Fiber-Reinforced Polymer Composites

ABSTRACT The natural, glass, and carbon fiber reinforced polymer composites are currently being replaced conventional materials due to their lower specific weight and superior strength properties. Natural fiber-reinforced polymer composites (NFRPCs) have grown in importance in real world applications recently due to a growing focus on the environmental and sustainability elements of engineering materials. The difficulty of machining FRP composites, which results in dimensional errors, poor product quality, and material damage, is due to their inhomogeneity, ease of deformation, and anisotropic nature. Moreover, this review gives an insight regarding recent developments and challenges that will help for upcoming researchers. The non-homogeneous properties and insufficient ductility of natural and synthetic fiber-reinforced composites have produced fracturing and discontinuous chips during the machining operations. The machinability of FRP composites depends on the constituents present in the composites. More delaminations were found in the natural fiber composites due to improper bonding and less compatibility with the polymer matrices, also the lower heat resistant property of the natural fibers causes serious problems during the machining process. Hence more studies are needed to decrease the thrust force and delamination damages in carbon, glass, and natural fiber reinforced polymer composites.


Introduction
Polymer composites are a type of versatile and high-performance materials formed with a combination of various material phases, in that at least one of the phases, and normally a matrix is made of polymer. The assembly of all these components gives unique mechanical, conductive, and material removal phenomenon of FRP composites is different from those of alloys and metals process mechanisms, and the FRP materials machining is found to be less in the literature the metals and alloys machining. Compared to the metals, the long fibers reinforced polymer composites have a maximum layered structure, with unique properties and their thickness. During the drilling of these layered structures, internal defects occur due to drilling loads and these loads' uneven distributions along with the fiber layers. It was observed that very few review works were found in the literature related to the machinability of FRP composites (Kalla, Sheikh-Ahmad, and Twomey 2010;Sheikh-Ahmad, Urban, and Cheraghi 2012). Hence, this review comprehensively discusses the basic studies on FRP composites machining, the quality aspects of FRP composites machinability, and collects previous literature on the machinability of natural fiber reinforced hybrid polymer composites and synthetic fiber reinforced polymer composites. Also, this review includes the unconventional machining process utilized in FRP composites machining.

Studies on FRP composites machining
The non-traditional machining processes are capable of solving many problems that exist in FRP composites machining, whereas the conventional processes are highly preferred for the reasons of time and cost. However, more research is needed to optimize the machining process to get adequate machining quality results. The failure stress analysis of carbon/epoxy composite indicates the significant effect of cutting speed on the failure stress. The maximum failure stresses were observed with the lower cutting speeds, which does not correlate with microscopic observations (Thakur and Singh 2021). However, surfaces cut at lower rates have rougher surfaces than those cut at high speeds. These surface observations clearly show the roughness measurement alone is not sufficient to analyze the machining damages, and inside propagated damages cause a reduction in mechanical performance Thakur and Singh 2021). The key factor fiber orientation decides the structural integrity of machined epoxy/carbon composites, and subsurface damage will not occur beyond the critical angle of 90°. Usually, the FRP composites offer a bouncing back phenomenon after the machining process, which is attributed to the elastic bending nature of reinforcement fibers. When the orientation of the fibers exists with ≤90° the three deformation zones occur, namely chipping, pressing, and bouncing. If the angle is >90°, more significant fiber bending is observed during cutting. Hence this involves a complicated cutting mechanism. The curing conditions while fabricating FRP composites in the transverse direction significantly affect the mechanical properties, influencing the changing cutting forces in smaller amounts (Kalla, Sheikh-Ahmad, and Twomey 2010;Kangokar Mukesh et al. 2022;Wang et al. 2021). The machinability evaluation phenomenon with cemented carbide tool reveals the better machinability of PEEK material with 30% carbon fiber reinforced composites and unreinforced PEEK than the 30% glass fiber reinforced PEEK composites. The overall evaluation factors indicate the higher machinability of unreinforced PEEK material than the reinforcement with glass and carbon fibers (Davim et al. 2010). The alkaline treated sisal/roselle/polyester composites show better machinability against abrasive wheels than the untreated composites. This was due to forming a strong interfacial bond with higher strength among matrix and reinforcements with the removal of moisture from the lignocellulose fibers. The 5 wt% fiber-reinforced composites with 2 hours of chemical treatment re-bounces in smaller amounts during machining. After the machining process, the 30 wt% fiber contents with 8 hours of treatment show more dimensional accuracy than other combinations (Navaneethakrishnan and Athijayamani 2016). Khashaba et al. (2010) analyzed the influence of drill wear on the machinability of glass fiber/epoxy composites. The machinability parameters were characterized using toque, thrust force, fiber push-outs, peel-ups, and roughness of the drilled hole. The results reveal that the thrust force behavior during drilling was highly affected by the pre-wear drill. This effect was more at a higher feed and cutting speed, which causes an increment in roughness and fibers delamination. The influence of drill wear on thrust force with different feeds and speeds is shown in Figure 2. The thrust force was elevated with higher drill pre-wear and feed; even with the smaller increment in drill pre-wear causes more amount of thrust force which is shown in the graph. When utilizing a pre-wear drill for GFRP composites, the effect is greatest. The thrust force was unaffected by cutting speed when using a new drill. The 30 wt% of glass fibers reinforcement with polyamide (PA66) significantly affected the tooling performance as compared to without  (Khashaba et al. 2010). reinforcement of materials. Using polycrystalline diamond (PCD) tool for the turning of glass/PA66 and neat PA66 gives the lowest forces and slightly higher force values by K15 carbide tools. During the cutting of PA66 material, the K15-KF carbide tool promotes higher forces, and diamond coated tool exhibits maximum forces while machining glass/PA66 composite. The specific cutting forces were decreased with increasing feed rate (Paulo Davim et al. 2009). The experimental machinability analysis of short glass/nylon-6 composite shows that the drill bit tool wear increases with increasing the glass fibers percentage. It is shown that the nylon-6/glass composite offers higher resistance to the drill bit movement with increasing the high modulus glass fibers in the soft nylon-6 matrix. This resistance to drilling bit movement causes tool wear, resulting in the machined composite's dimensional inaccuracy. The observations made from total rig down count (TRDC) against time show the lowest level of cutting forces for 15% glass fiber reinforced nylon-6 composite as compared to the other combinations with 30%, 40%, and 50% glass fibers reinforcement (Jawali et al. 2006). The fiber's contents volume and primary processing were the major contributing parameters that influenced the cutting forces while machining randomly oriented glass fiber reinforced polymer composites. The cutting forces were increased with increasing fiber contents and the depth of the cut. During the composite cutting, an enormous amount of energy was found at higher frequency regions due to burst kinds of cutting force signals due to fiber pull-outs and matrix damage. The stable flank wear was found after hardly one or two minutes of a short GFRP composite cutting process (Kim et al. 2010). (Table 1)

Factors affecting machinability/quality aspects
The FRP composites are currently promising materials for developing structural elements in various industries. Even though the FRP composite parts are being fabricated with near-net shape, machining operations such as drilling and trimming are still required to achieve finalized part sizes and fit tolerances. The machining of FRP composite is significantly more varied than that of the traditional metal and alloys machining. The cutting mechanism of most of the homogeneous materials was characterized by bulk materials plastic deformation with the continuous chips, whereas FRP composites cutting mechanism was characterized by non-homogeneity and anisotropic nature (Gao et al. 2016;Wang and Melly 2018;Wang et al. 2021). This anisotropy and inhomogeneity affect the machining process of FRP composites through fibers delamination, pull-outs/peel-ups, and machining parameters. The integrity offered by the machined surface is a basic aspect while determining an FRP composites machinability. Usually, this structural integrity was affected by machined surfaces due to the presence of internal defects in the FRP composites. Therefore, many extensive research works have been done to analyze factors responsible for the machining process and to study the quality aspects of machining (Shahabaz et al. 2021;Thakur and Singh 2021;Zhu et al. 2022). The stiffness and strength of the polymer composites were reduced due to the effect of delamination, which further led to materials failure. The turning, drilling, milling, and other machining operations are more critical due to the delamination factor in polymer composites. This delamination occurs through removal or separation of composite plies, and it is attributed to interlaminar cracks development in the materials. This delamination causes an improper machining process through dimensional inaccuracies, material wastage, and difficulty getting a good surface finish. The damage occurrence during machining depended on cutting parameters and drilling geometry. The geometry of the fresh tools will change along with cutting time because of wear progression, which affects the quality and process of machining (Helmi Abdul Kudus, Ratnam, and Akil 2021; Vigneshwaran et al. 2021).

Properties and machining of fiber-reinforced composites using conventional techniques
Usually, the failure behavior polymer composite occurs not only with anisotropic and heterogeneous structure but also due to the tool's interactions with the materials and machining methods. This heterogeneous structure of polymer composites causes health-related and structural issues such as During the process with a helical drill, the torque was linearly increased with feed rate, and a slight decreasing effect was observed cutting speed. The delamination factor was found to be increased with increasing feed rate and cutting speed conditions, where the significance was more with higher feed rate contribution. By considering the cutting factors, the performance of the "Brad and Spur" drill was more than the helical drill. (Petropoulos et al. 2008) Glass fiber/ polyamide (PA 66) Turning process P20 cemented carbide tool The feed rate exhibits the maximum influence on the surface texture parameters, which is 88.6% of (R t ) maximum peak to valley, 69.1% of (D) fractal dimension, and 91.9% of (R a ) arithmetic mean roughness. Whereas the cutting speed shows a weak effect on texture parameters, that is 5.5% (R t ), 8.8% (D), and 2.8% (R a ). As per the robust design, the process errors were ±8.023, and ±0.071 and ±1.114 μm for (R t ), (D) and (R a ) respectively. (Kechagias et al. 2009) Glass fiber/ polyamide 66 Precision turning K15 cemented carbide tool The 0.4 mm nose radius gives a maximum radial force followed by cutting and feed force. However, with the reduction of nose radius, the cutting force was more than the radial component for the turning of glass/PA66 composite. The reduction of forces might be attributed to decreasing contact area. With the elevation in feed rate, the specific cutting forces were reduced and comparable to metal alloys.

Carbon fiber/ epoxy
High-speed drilling process K20 cemented carbide tool The experimental results reveal that the composite's delamination tendency was decreased with increasing cutting speed. However, it was suggested to use the combination of low point angle and feed rate values to avoid damage. This could be explained as, with the cutting speed increment, the cutting edge process was reduced due to reducing the number of passes at the same region. Hence, this will causes temperature increment and soften the matrix, thus reducing the CFRP composite damage. (Gaitonde et al. 2008) E-glass/epoxy Drilling process

HSS radial drilling tools
The small diameter drill bit generates lower holes deviation and exhibits a good surface finish. To get a minimum surface roughness, the optimal parameters are spindle speed 1400 rev/min and feed rate of 0.02 mm/rev. Whereas to get a minimum hole deviation, the spindle speed of 800 rev/min and feed rate of 0.02 mm/rev were suggested. The feed rate and spindle speed affect the surface roughness by 32.49% and 60.22%. (Manna et al. 2008) (Continued) matrix smearing, delamination, tool life reduction, fiber pull-out, and formation of unhealthy dust during machining (Hwang et al. 2015;Saoudi et al. 2018). The conventional tools have higher hardness and abrasiveness, leading to fiber crushing during machining. Sometimes the machining of complex polymer composite causes the tool's plastic deformation. In the aerospace industry, the components rejection rate was raised to 60% due to delamination failures, and it was reported in the literature. Moreover, the machining at narrow workspace was difficult with conventional tools because of tools spindle size and machining time increment with tool changing duration for worn-out drilling and milling tools (Altin Karataş and Gökkaya 2018;Katnam, Da Silva, and Young 2013).

Synthetic fiber reinforced polymer composites
The surface integrity and finishing-related problems are generally encountered while CFRP composites machining using conventional machining tools ). Due to lower machinability, the CFRP composites machining exhibits various failures, including burrs, sub-surface failure, and delamination (Xu et al. 2022). The lower feed rate reduces the axial forces, which reduces risk that occurs from delamination initiation, which proves better machinability while drilling CFRP composite layers. The influence of tool geometry on the delamination results was also reported, and accordingly, the minimum delamination was obtained for twist drill with a point angle of 120° (Geier, Davim, and The drilling process of sisal/epoxy gives discontinuous chips, and sisal/PP gives continuous chips. The drilling force magnitude linearly increases with increasing feed rate and decreases with the spindle speed for all the types of drilling tools for both composites. The torque and thrust force developed through parabolic drills was low compared to the other two types of tools. (Debnath, Singh, and Dvivedi 2014) Coir fiber/ polyester Drilling process M42 HSS drill bit The regression models were developed for torque, tool wear, and thrust force to predict drilling conditions in coir/polyester composite. The confirmation test for untreated composite shows an average absolute error percentage of 5.8%, 2.7%, and 2.8%, respectively, for tool wear, thrust force, and torque models, respectively, and of 6.5%, 5.4%, and 6.2% for treated respective models. (Jayabal et al. 2013) Sisal/glass fiber Drilling process Brad and spur type drill bits (SC, HSS, and CSC) Using HSS drill tools for drilling operation gives a maximum thrust force for all types of feed rates. It was also observed that the delamination was more in using HSS drill bit, and hence it is not recommended for drilling holes in sisal/glass hybrid composites. The higher feed rate leads to thrust force increment and causes damage to the hybrid composites. (Ramesh, Palanikumar, and Reddy 2014);

Cotton fiber/ LDPE
Milling process

Tungsten carbide tool
The burr formation was more strongly affected by the cutting speed parameter than the feed rates. The burr formation was higher at higher cutting speeds. The milled cotton/LDPE composite chips were in a short ribbon shape and looked linty due to cotton insertion in the LDPE matrix. The cleanest burr formation was observed at 200 mm/min feed rate and 25 mm/ min cutting speed. (Kuzu and Bakkal 2016) Szalay 2019; Shunmugesh and Panneerselvam 2016). The GFRP composites are extensively popular in various industries because of their higher mechanical properties, less weight, and low cost as compared to other synthetic materials. Even with good control over the composites manufacturing process and capable of producing near net shapes, many machining operations are required to complete the manufacturing process. The employment of various machining operations is difficult in the case of GFRP composites due to anisotropic and heterogeneous character (Saoudi et al. 2018;Thakur and Singh 2021). A finite element model was developed by Calzada et al. to analyze failure mechanism in the machining of CFRP composite, and the failure mechanism was analyzed throughout the formation of the chipping process. This model was able to predict failure mode, machining forces, and characteristics of chipped fibers with the microstructure of fibers orientation. To validate this model, an orthogonal machining experiment was conducted to record the machining response. This comparison reveals the model's accuracy in determining failure mode with fiber characteristics and the machining forces. This model was demonstrated the failure mode of 135° fibers oriented CFRP composite, where the failure mode was changed to crushing from the bending mode. This failure mode simulation is illustrated in Figure 3. When the tool enters the workpiece, it peels the fibers where the matrix-fiber adhesion is less due to interfacial failures under the cutting planes. After peeling, the bending stresses were developed below the cuts, affecting the rest of the workpiece (Calzada et al. 2012). The addition of boron nitride (BN) nanoparticles into carbon/epoxy composite improves the drilling performance and properties of the resultant nanocomposite. The addition of BN particles increases the thrust force and reduces the delamination factor of the nanocomposite. The hole surface finish was good in BN reinforced composite due to improved bending and shear strength between the composite layers. The optimum parameters for these nanocomposites are 90 m/min of cutting speed and 0.005 mm/rev of feed rate (Burak Kaybal et al. 2020). The thrust force on unidirectional CFRP composite was significantly varied with drill rotational positions, and the lower thrust force values were observed at 135° and 315° rotational angles. The rough surface was observed along the drilled hole and throughout the depth. The fiber pull-outs were observed majorly in two regions where the interaction angle between fiber orientation and cutting angle was from 135° − 175° and 315° − 355°. The quality of the whole surface was better at a low feed rate and high cutting speed (Eneyew and Ramulu 2014). The 3D printed CFRP composite shows maximum tool wear at a low feed rate of 23.64 IPM when subjected to the end milling process. The low feed rate machining allows the workpiece to adhere to the tool, and it causes the tool to material upset instead of shear it. To improve the machining performance and to reduce the burr formation, it was suggested to adopt the minimum quantity lubrication technique (MQL). Under the MQL conditions, both tool wear and surface finish were improved. The utilization of post-process machining gives maximum surface finish quality and could be used in end applications (Cococcetta et al. 2020). The decrement in the latter force was the most effective thing while reducing the increment rate of cutting forces owing to tool wear progress during the CFRP composite down-milling process. This consideration is possible through the usage of a two-layer cutting tool, where it reduces the pressing forces and distributes the wear rate at the tool edge, which leads to overall cutting force decrement (Maegawa et al. 2016). The machining of CFRP composites causes flank wear on the carbide tool due to carbon fiber's abrasive and hard structures. The more hardness of fiber causes an increment in cutting forces and results in the wear of uncoated tools during machining. To get the required wear resistance in carbide tools, the diamond coating was done on the carbide tool surface for sufficient wear resistance. By considering the tool life, the carbide tools are in the first position for machining of CFRP composites, especially with fine grain size carbide grades. For getting the higher cutting performance, the polycrystalline diamond tools are preferred or milling of CFRP composites (Ozkan et al. 2019). A different machining approach was done on the CFRP composites under cryogenic conditions. The CFRP composites were subjected to cold temperature while machining for 30 minutes. The cryogenic heat treatment positively affects the CFRPs tensile properties, which is about 3.04% and 3.65%, respectively, for elastic modulus and tensile strength. Hence, the cryogenic approach's cutting forces were higher than other machining approaches. Although, the formation of higher forces under cryogenic machining has lower delaminations. This was owing to chip breakability improvement and the formation of low forces in the axial direction because of cryogenic coolant. These coolants make the workpiece brittle and avoid thermal damages on machined surfaces (Morkavuk et al. 2018). The comparison of delamination factors for CFRP composite under dry, cryogenic, and MQL conditions reveals that the cryogenic drilling has maximum delamination at the hole entry and exit sides. The comparison of delamination factors under these three conditions at the entry and exit sides of the hole is illustrated in Figure 4. In comparison to dry drilling, the cryogenic drilling exhibit below 10% delamination at the hole entrance. In the case of the MQL condition, the delamination was 2% higher than the measured values at dry conditions at the hole entrance. When the delamination at the exit side was concerned, the delamination magnitude was found to be 16% higher in cryogenic conditions than the dry drilling due to a higher magnitude of forces (Nagaraj, Uysal, and Jawahir 2020).
During the machining of flax fiber-reinforced polymer composites, the flax fiber orientations significantly affect the machined surface state. The fiber orientation with an angle of 45° gives the best machinability with low surface roughness, effective fibers shearing, and lower cutting energies due to friction and shearing. This could be related to the higher shear stiffness involved in 45° fiberoriented samples. The 90°-fiber orientation shows maximum cutting energies due to flax fibers transverse deformation during machining. Whereas the 0° fibers-oriented composite has maximum surface roughness because of random contact position among flax fibers and cutting tool edge within the composite (Chegdani et al. 2020). Irrespective of radial and axial force during the helical milling of CFRP composites, the cutting forces increased with the depth of cut and feed rate. The radial forces were high at higher rotational speeds. The tool wear was obvious during helical milling of CFRP composite due to higher cutting forces; additionally, the cutting forces are higher in multidirectional CFRP milling than unidirectional CFRP milling. The entry delamination is less while using the sharp cutting tool, and the delamination was enlarged with reduction of tool sharpness (Wang, Qin, and Li 2015). With the increase of MWCNTs loading in CFRP composite, both torque and thrust forces were decreased due to stiffness improvement in MWCNTs loaded CFRP nanocomposite. The surface roughness value of the hole was increased due to more severe damages with higher feed rates. However, the roughness decreased with increasing spindle speed due to matrix softening at high temperatures generated between chip-tool interfaces. Moreover, the addition of MWCNTs into the polymer matrix reduces surface roughness due to the lubrication behavior of MWCNTs at the chiptool interface, also minimizes fibers debonding and matrix pull-out (Kumar and Singh 2019). The depth of cut was the most significant influencing factor on the CFRP laminate machinability. The lowlevel cutting temperature, cutting force, surface roughness, and cutting energy were attained at a small depth of cut (equivalent to chip thickness) and higher cutting speed. The cutting temperature was exceeded the glass transition temperature at full diameter tool usage, which causes machining ineffectiveness without coolant use in these conditions. Due to lower cutting conditions, the internal damage and delaminations were absent at the machined edge (Sheikh-Ahmad and Mohammed 2014).
The machining of GFRP composite generates enough heat required to cause thermal damages to the epoxy matrix. The 300 mm cutting length produces a cutting temperature of more than 160°C. With the generation of lower cutting forces and increased thermal conductivity, the graphene-filled GFRP composite laminate generates lesser heat during milling and leads to lower cutting temperature. The modified GFRP laminate has lower cutting forces and decreases the roughness through the grooves making happened due to the chip removal process. Overall, the addition of graphene improves the machinability of GFRP composites (El-Ghaoui, Chatelain, and Ouellet-Plamondon 2019). The 3000-rpm spindle speed reduces the thrust force than 2000 and 1000 rpm during the sisal/glass polymer composite drilling. The increment in feed rate results in a higher load on the drill bit, which causes more thrust forces during the machining process. Moreover, the drill diameter also shows an increment trend as like of feed rate. Hence the higher spindle speed, lower drill diameter, and feed rate are the best optimum conditions for drilling the sisal/glass polymer composite (Manickam and Gopinath 2017). The maximum thrust force of 55 to 70 N was observed while drilling glass/PP composite using HSS drill compared to solid carbide (SC) and tipped carbide (TC) drill, which was due to more tool wear and less hardness of HSS drill. The marginal thrust force variation was observed at solid carbide and tipped carbide tool performance. The delamination factor value varied between 1.8-1.9 for the HSS drill at 0.15 mm/rev of feed rate, whereas for solid carbide and tipped carbide tools it was recorded between 1.6-1.8. This delamination factor reduction was attributed to the drilling process constrained with thrust forces, also it might be due to high working temperature with carbide drill tools. The detailed comparison of delamination factor and thrust force is illustrated in the graphs shown in Figure 5 (Mudhukrishnan, Hariharan, and Palanikumar 2020).
The drilling work was conducted for glass/polyphthalamide (PPA) composite demonstrating the delamination factor increment at low feed rate and high cutting speeds. The best-drilled surface quality was obtained at a 0.05 mm/rev feed rate. The GFRP composite surface roughness at lower feeds was affected by glass fibers pullout or fracture; however, it could be controlled with feed rate at high feeds. The SEM observation of drilled surface confirms the existence of fibers pull out, cracks, shearing, and crushing of fibers (Ficici, Ayparcasi, and Unal 2017). When end milling GFRP composite, increasing the feed rate or cutting speed speeds up tool wear and leads to fast cutting tool failure. The machining of 90° fibers-oriented sample makes wear easy to grow and prolongs the tool life as compared to the 0° fibers-oriented sample. The more tool wear in 90° orientation was due to inherent tool/fiber contact. This tool wear progression was effectively monitored with neuro-fuzzy and multiple regression analysis modeling. This modeling shows an acceptable accuracy while predicting the tool wear compared to experimental data (A. I. Azmi, Lin, and Bhattacharyya 2013). Figure 6 illustrates the graph of thrust force v/s time variation with depth of cut during the woven GFRP composite drilling process at 16.3 m/min and a feed of 0.025 mm/rev. At the first stage, the linear increment of thrust force was observed with the entry of the tool edge onto the workpiece. This stage represents the workpiece's elastic loading due to the tool edge's negative rake angle and its zero-center speed. Hence the tool edge extrudes the workpiece material instead of cutting. The second stage involves the entry of the tool edge inside the surface layer. At this point, the peaks are observed due to the stiffness reduction in the composites. The peaks are further increased due to the cutting-edge entry into a new layer and the left uncut layers are observed in decreased trend. The decrement of stiffness owing to the overall cutting of 53% thickness with temperature results in a drop of thrust force by 60% of their maximum peak. Finally, at the fourth stage, the cutting edges exit layer by layer in the workpiece, decreasing thrust force gradually over the tool length (Khashaba and El-Keran 2017). The GFRP composites were subjected to ANOVA, DFA (Desirability Function Analysis), and GRA (Grey Relational Analysis) analysis to study the turning machinability using TiCN/TiN coated tool. The findings of the ANOVA show that the depth of cut, followed by feed rate, had the greatest significant impact on surface roughness, while cutting rates had the greatest impact on tool wear. The GFRP turning process was subjected to multi-response optimization by using GRA to analyze the optimum parameters, and it was found to be 0.08 mm/rev of feed and 50 m/min of cutting speed. To overcome the problems that occurred in optimized multi-response during the GFRP turning process, the DFA was a very powerful tool while setting the optimal parameters to reduce roughness and tool wear, and those are 0.5 mm depth of cut, 100 m/min of cutting speed and 0.08 mm/rev of feed (Raveendran and Marimuthu 2015).
The fiber reinforcement causes a micro-abrasion on the grains of the uncoated tungsten carbide tool during the end milling of the GFRP composite. Severe delamination damages and uncut fibers have offset the tool wear performance across fibers orientation during machining. The end mill cutter tool life was analyzed through Taylor's equation, and it confirms the cutting speed as a governing factor that affects the tool life of the end mill cutter (A. I. Azmi, Lin, and Bhattacharyya 2012). An experimental investigation on drilling glass/PP composite using the "Brad and Spur" tool reveals the major influencing factor on the cutting process: feed rate followed by drill diameter. These two parameters increment causes more thrust forces and increases in spindle speed have no significant variations on thrust forces. The empirical model was employed for thrust force prediction, and this model was best suited for predicting the thrust induced during cutting of glass/PP composite (Palanikumar et al. 2016). The artificial neural network (ANN) model for glass/PA66 composite reveals the higher impact of feed rate on the process outputs. The influencing percentage was over 80% for measured responses, which implies that low feed rate values cause smooth machining operation during longitudinal turning. The cutting speed has a low impact on measured responses, and it gives low cutting forces when used with low levels as compared to the feed rate. The machining parameters interactions were also noticeable when the influencing percentage of measured outputs reached 4% (Fountas et al. 2013). The insertion of CNT buckypaper into glass/epoxy composite shows a good milling performance with lower axial forces. The composite specimen without CNT insertion exhibits 3% higher axial forces than the filled composite. This CNT buckypaper improves the interlaminar strength between the epoxy and glass fibers, which results in lower delaminations during the milling process. Moreover, the CNT buckypaper in glass/epoxy improves the interlaminar shear stress by 25% more than the unfilled composite. Hence, it helps get a good surface finish with optimum machining performance .

Natural fiber-reinforced composites (NFRCs)
The NFRCs machinability is strongly dependent on the fiber type in the composite and their mechanical, thermal properties. Hence, care must be taken while selecting cutting tools and machining parameters for natural fiber or filler reinforced composite. The relevant parameters selection gives desired integrity and surface quality in the machined NFRC component (Abd-Elhady et al. 2020;Nassar, Arunachalam, and Alzebdeh 2017). Franke (2011) investigated how the cutting edge affected the quality of the drilled holes while lengthy fiber-reinforced thermoplastic composites were being machined. The significant key factors during the drilling of long fibers reinforced thermoplastics were discovered to be feed force and fiber breakup. Bajpai and Singh (2013) studied the drilling behavior of sisal/PP thermoplastic composite. When fibers are under tension while being cut, cutting is made easier and a high-quality surface is produced at the hole wall. Just after drilling procedure, the area of the hole wall that was created was visually inspected; neither uncut nor loose fibers were discovered, and the hole wall's surface quality was satisfactory. When physical and mechanical qualities are not as crucial, it is vital to select the fibers kind which results in drilling-friendly NFRPCs. Different forms of cellulose are found in different types of fibers, and the various characteristics of fibers rely on the fiber plant parameters, including the maturity of the plant, the period of harvest, the weather, its soil, etc. The brittleness and ductility of fibers also affect the quality of drilled hole (Helmi Abdul Kudus, Ratnam, and Akil 2021). The NFRCs possess lower mechanical properties, and their unique characteristics with polymer material led to internal defects (cracks, improper bonding, blisters, and voids) in the composites. Hence these will cause problems when NFRCs are employed for machining processes (Chegdani and El Mansori 2019;Madhu et al. 2022). The orthogonal cutting of flax fiber reinforced PP composite reveals the generation of continuous chips at all cutting conditions. This was inherently correlated to the specific mechanical action of flax fiber within the PP matrix undercutting solicitations. This cutting behavior causes consequent surface damage through uncut fibers extremities and debonding zones. The influence of cutting depth was more on the machined surface, where the cutting forces were increased with increasing depth of cut. The cutting speed effect was significantly reduced because of thermal influence by plastic deformation and friction (Chegdani and El Mansori 2018a). The drilling and tribological behavior of flax/PP composite was affected by coated tool materials (diamond coated, titanium diboride coated, and uncoated tungsten carbide tool). The diamond coating leads to increment in thickness with cutting edge radius and also the intrinsic friction behavior of cutting tool by the alteration of surface morphology. The diamond-coated drill's even though with the maximum response of intrinsic friction by diamond coating. This was due to the multiscale friction mechanism at the tool-workpiece interface and also due to coated surface morphology. This diamond coating declines the flax/PP composite machinability more than TiB2 coating. Figure 7 illustrates the comparison of machined surface SEM images using three types of tools. In that, the uncoated tool shows better machinability than other coated tools, preferably at a higher feed rate. Thus, the elemental flax fibers clear cut was observed on the machined surface, shown in Figure 7(a,d). The torn-off fibers without cutting are observed on the machined surface (Figure 7(c,f)) due to higher specific cutting energy induced by diamond-coated tools (Chegdani and El Mansori 2018a). The cutting tool geometries significantly affect machined surface quality in the case of drilled flax/PLA composite. The changes in drill point angle lead to variations in thrust forces and cause various types of damages in the biodegradable composite. The usage of acute drill geometry improves the drill weakness and wear rates. At higher feeds, the damage caused by the drill point angle at entry and exit of the hole was reduced by 1.93% and 0.76%, respectively (Díaz-Álvarez et al. 2019). The delamination factor was elevated with both feed and speed parameters while drilling banana/epoxy composite. ANOVA analysis of this composite reveals more influence of feed on the delamination factor than the speed, with a significance of 51.73 and 53.13 at entry and exit, respectively. As compared to experimental analysis, the prediction of delamination factor using regression model has an error of 0.678 and 0.357 respectively at entry and exit (Venkateshwaran and ElayaPerumal 2012).
The alkaline treated jute fiber-reinforced composite exhibits reduced push-down delaminations more than the peel-up delaminations during the machining process. The delamination was increased with an increasing feed rate. The GRA Model evaluation was subjected to 16 experiments for the analysis of treated jute composite; out of 16, the 10 experiments gave better machining performance than the untreated composite. As per ANOVA suggestion, the delamination factor was slightly reduced due to alkali treatment, but the fiber treatment influence on the delamination factor was insignificant. The optimum feed rate and speed values are 0.03 mm/rev and 9.42 m/min, respectively, for jute/polyester composite (Sridharan and Muthukrishnan 2013). The machinability studies on kenaf reinforced plastic composite milling report the influence of milling parameters on delamination factor and surface roughness. The optimum parameter values are 500 rpm, 200 mm/min, and 2 mm, respectively, for spindle speed, feed rate, and depth of cut for kenaf reinforced composites. However, the spindle speed does not significantly affect surface roughness, but high spindle speed gives a good, machined surface. Whereas the influence of depth cut was not reported clearly, hence it was assumed to be a good surface finish with a deeper depth of cut (Azmi et al. 2018). An experimental study related to the drilling of rice husk reinforced composite reveals a lower delamination value as compared to glass fiber composite. The minimized delamination value was obtained for 118° drills, and it was well adapted for drilling rice husk/polyester composite. The drilling process with Brad drill causes maximum delamination value than the end mill and twist mill (Syed Azuan, Juraidi, and Wan Muhamad 2012). A new experimental study was conducted by Tran et al. to analyze machinability in polypropylene reinforced with biochar and miscanthus fiber composite. The study reveals that the drill diameter, spindle speed, and feed rate significantly affect the roughness, thrust force, and cutting energy of biocomposites. The emission of ultrafine particles was not affected significantly by drill diameter and machining parameters, while the feed rate has a much more significant effect on particles emission and thrust force as compared to other parameters. The cutting energy was decreased with increasing drill diameter and feed rate, while it was increased with increasing spindle speed. During the drilling process, the concentration of fine particles was reached up to 1300 #/cm 3 and it was recorded 9000 #/cm 3 for ultrafine particles concentration (Tran et al. 2020). The experimental studies on milling of jute and flax fiber-reinforced composite reveal the lower values of roughness, delamination factor, and cutting force for tungsten carbide mill, and these values are higher for HSS end mill. FFRP and JFRP composite milling show a low delamination factor at 0.01 mm/rev of feed rate and 2500 rpm of spindle speed, while maximum delamination factor was obtained for 0.02 mm/rev of feed rate 7500 rpm of spindle speed parameters. Similarly, the lower cutting force and surface roughness values were obtained at 0.01 mm/rev of feed rate and 7500 rpm of spindle speed, while the highest value was found at 0.02 mm/rev of feed rate and 2500 rpm of spindle speed. The closer values of tool vibration amplitudes (Çelik and Alp 2022). The effect of compatibilizer over natural fiber reinforced thermoplastic composite (jute/PP) was reported by Pailoor, Murthy, and Sreenivasa (2021). Regarding fiber orientation and fiber type, compatibilizer had no effect on delamination during drilling process. However, throughout the composites without such a compatibilizer, delamination grew as fiber content did. Thrust force in the longitudinal axis was reduced in composites containing coupling agents. Increased velocity and feed in the composite without a compatibilizer were correlated with thicker, longer, and less curled chips. One of the best methods for enhancing interfacial bonding among bio-fibers and thermoplastics is considered to be the use of a compatibilizer. As a compatibilizer, a wide range of compounds including silanes, isocyanates, and maleated polyolefin have already been investigated. More delamination and decreased thrust force were the results of the compatibilizer addition. The creation of covalent and hydrogen linking with the -OH of jute fiber is responsible for the increased delamination (Aggarwal et al. 2013).

Challenges and future scope
Similar to the abundance of tool materials, the abundance of natural fibers also presents a difficulty when it comes to machining NFRCs since each natural fiber may have a distinct set of machining properties. This is further exacerbated by variations in the binding of the fiber matrix. The heat produced while milling is another important difficulty. Even while it can lessen the thrust force, the heat created leads to melt the polymers and scorch the fiber, which leads to structural failure and fiber peeling. Coolant might be used to lower the heat produced, but this would cause additional issues including moisture uptake by natural fibers, which will cause them to lose some of their qualities. The majority of investigations on CFRP composite machining have demonstrated that it is very difficult to reduce surface roughness. Hence, more analytical modeling and machine learning techniques needs to optimize the machining parameters.

Conclusions
This review article outlined the comprehensive studies on machinability of FRP composites, the factors/quality aspects of machinability, properties, and machining of fiber-reinforced composites using conventional techniques. A lot of research work going on in recent years to increase the machinability of polymer composite to the level of metal-based composites machinability. The cutting forces and damages that occurred in the sub-surface and surface of the composite were dependent on individual characteristics of reinforcements and matrix materials. Normally a higher spindle revolution speed, low drill diameter, a point angle nearer to 1000, lower tool traverse rate, and preferred parabolic tool drill are found to be effective for the drilling process. The maximum machinability for the end milling process may be obtained at a lower tool traverse rate, low cutting depth, and higher spindle speed. The non-homogeneous properties and insufficient ductility of GFRP, NFRC, and CFRP composites have produced fracturing and discontinuous chips during the machining operations. Additionally, research into high speed milling is a potential area since recent experimental findings indicated that faster cutting speeds produce lower process forces, extending tool life, or enabling faster feed rates. A further intriguing possibility is the use of minimal amount lubrication, which may enable increasing the intensity of the use of cutting fluids while potentially extending tool life and minimizing environmental effect. The different materials fracture or failure modes seem to have occurred randomly along the machined surfaces. Recent advancements in drilling of FRP composites are characterized by the advent of orbital drilling procedures, which require longer processing periods but can decrease or eradicate the delamination and heat damaging flaws caused while drilling composites. The tool selection and processing parameters are required to be optimized to get a better-machined surface. To improve the machining parameters, further research has to be done on utilizing analytical modeling and machine learning approach.

Highlights
• Study on machinability of FRP composites, the factors/quality aspects of machinability, properties, and machining of fiber-reinforced composites using conventional techniques. • Machinability of FRP composites depends on the constituents present in the composites.
• Delaminations were found in the natural fiber composites due to improper bonding and less compatibility with the polymer matrices. • Lower heat resistant property of the natural fibers causes serious problems during the machining process.
• The maximum machinability for the end milling process may be obtained at a lower tool traverse rate.

Acknowledgment
The author extends their appreciation to the Deanship of Scientific Research at Majmaah University for funding this work under project No. R-2022-26.

Disclosure statement
No potential conflict of interest was reported by the authors.

Data availability statement
The data used to support the findings of this study are available from the corresponding author upon request.