Machinability of Polymeric Composites and Future Reinforcements—A Review

This paper reviews the machinability and mechanical properties of natural fiber-reinforced composites. Coupling agents, operating parameters, as well as chemical treatment effects on natural fiber-reinforced composites’ machinability are also reviewed. Moreover, the impacts of fibers’ physical properties on the machinability of the composite are mentioned. Fiber volume fraction (Vf), fiber orientation as well as chemical treatment effects on mechanical properties are also defined. Conclusively, the effect of fibers’ physical properties as well as mechanical properties is described. It was discovered that chemical treatment of natural fibers improved their compatibility with the matrix by removing their surface tissues, increasing the roughness average (Ra), and reducing moisture absorption. Also, the Orientation of the fiber plays an important role in controlling the mechanical properties of the composite. Moreover, some physical properties of the fibers, including quality of fiber distributed in the matrix; fiber size, length, and diameter; moisture absorption; porosity and the way fibers break during compounding with the matrix, were found to affect the mechanical properties of the composites formed.


Introduction
Polymeric composites nowadays are gradually being utilized in some industrial applications for replacing metals as well as metal composites, which is a positive development [1] [2]. For example, some metal components of aircraft were also replaced with carbon/epoxy composite components. The use of fiber/polymer composites in marine applications has also been discovered; numerous metal parts are also being replaced with fiber/polymer composites in ships. According to Hung et al. [3], suitable tools in a conventional machining procedure are also required for machining metal matrix composites. Synthetic fiber composites, including carbon fiber and glass fiber composites, on the other hand, present a significant challenge due to the fibers' refractoriness, extreme hardness, as well as inert nature to machinability in terms of their bearing strength, material removal rate, thrust force, cutting power, Ra (roughness average), and delamination size.
To ensure the effectiveness of the machining procedure, this is essential that the cutting parameters, as well as the tool, are chosen correctly. Nevertheless, several researchers reported that over the past few years [9]- [20], the machining parameters' effect for example V f , fiber orientation, drill diameter, speed, and feed have a significant impact on such materials' machinability. It has been found, however, that less work was done on natural fiber-reinforced composite materials' machinability for machining power, cutting pressure, tool life, cutting power and force, material removal rate, cutting forces, Ra, along with tool wear than has been done on synthetic materials' machinability. It is now widely recognized that synthetic fibers are extremely rough materials, which can cause important damage to machining tools as well as machines. Fibers are hair-like class materials that can be separate or continuous. It can be categorized into 2 main types: synthetic and natural. Natural fibers, namely plants, minerals, and animals, come from 3 major sources. Natural plant fibers have been used for protection and warmth for the first time more than 5000 years ago. In particular, because of the rise in synthetic fibers, the natural fibers sector lost the majority of its market share after World War II [7] [8] [21]. Synthetic fibers are having a serious negative impact on process machining and manufacturing, including high energy consumption, non-recyclability, high cost, non-renewability, along with high abrasiveness, CO 2 emission, along health problems when inhaled. Such limitations were greatly utilized by the supporters of natural fibers [21] [22] [23] [24].
Numerous researchers are now working on replacing synthetic fibers with natural fibers to comply with new environmental regulations as well as petroleum resource depletion [7] [25] [26] [27]. In 2006, the "United Nations' Food and Agriculture Organization declared 2009 the international year of natural fibers". This year's campaign was focused on enhancing global awareness of natural fiber and market demand [28] [29] [30]. Due to their higher characteristics, like renewability, abundance, low cost, non-toxicity, low weight, non-abrasiveness, biodegradability, low environmental effect, as well as CO 2 emissions, many efforts have been made over the last few years (2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020)(2021) to use natural fibers (nature fibers) to enhance polymer composites [31] [32] [33] [34] [35]. In addition, natural fibers are a commodity based on prices, causing less damage to the equipment and molding, a better finish, flexibility, and high energy consumption (bending instead of fracturing) [7] [20] [36]. Furthermore, the special surface micromorphology and low-cost benefits of each fiber, which also grew rapidly around the world to the detriment of synthetic products, have attracted the most significant benefits to the market for natural fibers [37] [38] [39]. However, the "low melting point, poor wettability, low resistance to moisture absorption, and lack of good interfacial adhesion" that results in debonding with age, have been identified as the main disadvantages of natural fibers that make them a less attractive option [35] [39] [40].
Most of the studies that have been done on natural fibers in polymeric composites were done for investigating natural fibers' effects on the electrical, thermal, dynamical, tribological, mechanical, physical properties of polymeric composites. Fiber composite's mechanical properties depend on several parameters, for example, composite structure, fiber's mechanical and physical properties, fibers' V f , fiber's surface characteristics (for interfacial adhesion), as well as fiber orientation. The impacts of every parameter on composites' mechanical properties must therefore be discussed in detail.
The fact that most Polymer matrices are lacking in good adhesion is the major problem in natural fiber-reinforced composites' processing. This can prevent natural fibers' hydrophilic nature from interlocking [10] [59] [60] and can thus adhere to hydrophobic matrixes. For preventing this, the fibers' surface must be adhesively modified for improving composite machinability [61] [62] [63] [64]. Khan and Kumar [10] examined the short agave fiber-reinforced epoxy composites' machinability for fiber-matrix interaction for alkali-treated as well as untreated (5%) fibers. It was discovered that the treated composites had rough surfaces that resulted in improved interlocking among the matrix and fiber and raised the contact area. The untreated composites, on the other hand, were found to have voids. Valadez-González et al. [61] revealed qualitatively identical findings to those obtained by the authors in examinations of the influence on fiber strength of the matrix bonding of reinforced composites with natural fiber-surface treatments.
Athijayamani, Thiruchitrambalam [55] investigated the "machinability of an alkali-treated natural fiber (sisal and roselle) hybrid polyester composite in terms of weight loss." 10% NaOH (sodium hydroxide) solution treatment was applied to fibers for varying periods (2, 4, 6, and 8 hours). Gradual weight loss has been seen as the wear test time (4, 8, and 12 minutes) was increased, and composite samples treated with alkali outperformed the untreated composite samples in terms of wear performance. This could have occurred as a result of the alkali treatment, which removed moisture from the fibers and showed rising results in the interfacial bonding strength between matrix and reinforcement, as per a few researchers. Nam, Ogihara [54] examined the alkali-treated as well as untreated "(5% NaOH) coir fiber-reinforced poly (butylene succinate) biodegradable" composites' machinability in terms of the Ra (roughness average). It has been reported that treating the fiber with 5% alkali for 72 hours increased the Ra along with cellulose amount exposed on the fiber surface, which in turn strengthened the mechanical connection between matrix as well as fiber and resulted in improved machinability. Kabir, Wang [65] reported similar results for the o alkali treatment's effects on Ra of hemp fiber-reinforced sandwich composites, and these findings are qualitatively the same as to report.

Mechanical Characteristics of Natural Fiber Polymer Composites
Natural fibers were utilized in the past in certain industries that dealt with door inner panels, seat backs, as well as roof inner panels [16]  respectively [74]. Mallick [69] in his book, summarized the jute fibers, flax sisal, and hemp's mechanical properties, giving overall information regarding the fibers' elongation and modulus as well as tensile strength. As additional information and analysis sources, the following sections examine the chemical treatments' effect on the fibers and composites' mechanical properties, including fibers' orientation and volume fraction.

Effect of Chemical Treatment
This is recognized that natural fibers have a problem when it comes to their compatibility with a matrix of any kind (interfacial adhesion  (2 -8 hours) in alkali (10%) on the hybrid polyester composites' mechanical properties. According to the findings of this study, raising the immersion duration resulted in a 59 percent and a 47 percent rise in composite's flexural strength and tensile strength, respectively. These were because of the significant enhancement in fibers' interfacial adhesion with a matrix that occurred as a result of the removal of the outer layer of tissue from the fiber surface following the procedure. Throughout tensile loading, this increased the fiber's adherence to the matrix as well as allowed it to carry a portion of the load. However, this was discovered that the untreated fibers have been found unable to support the load. As previously reported [48], similar results were obtained when short agave fibers had been treated for alkaline as well as utilized to reinforce epoxy composites. This is possible to argue that the increased duration of the treatment caused damage to the fibers' tissue, resulting in a weaker interfacial adhesion between matrix along with fiber.
A study conducted by Kabir et al. [65] observed the hemp fiber-reinforced sandwich composites' mechanical properties had been treated in 3 distinct chemical ways (alkalization, acetylation, and salination). According to the findings, fibers treated with alkali and pre-soaked in 8% NaOH had more tensile strength (111.05 MPa) as compared to untreated fibers (84.06 MPa) (Figure 1), and this has been because of the removal of lignin, hemicelluloses, along with other cellulosic constituents from the fiber by the treatment. As a result, the fiber as well as the matrix adhered to one another more effectively. The use of saline, on the other hand, resulted in a fiber surface that was coated. The adhesion quality between the matrix and fiber was lowered as a result of this. Lastly, acetylated fibers exhibited a brittle surface on their outer surface. In chemically treated natural fibers' case, numerous authors have reported similar findings [37] [39] [69].
Edeerozey, Akil [52] investigated some impacts of treating kenaf fibers with NaOH at various concentrations (3 percent to 9 percent) on fiber's tensile strength. The researchers concluded that 3 percent NaOH has been ineffective at removing impurities. The most effective concentration for treating the fiber has been 6 percent NaOH, which completely removed the impurities from the fiber. Alawar, Hamed [41] studied the date palm tree fibers' elongation at break, mechanical properties, tensile strength, young's modulus, subjected to various chemical treatments (0.3 -1.6 percent HCl and 0.5 -5 percent NaOH). This has been shown to enhance the surface morphology of fiber surfaces by cleansing fiber surfaces of contaminants with a 1 percent NaOH treatment. In comparison to the raw fibers of 200 MPa as well as the young module, the tensile resistance raised to 800 MPa, up to 160 GPa compared with raw fibers of 8 GPa (Figure 3).
On the other hand, the surface morphology was observed to be distorted and the the last, the treated fibers presented higher resistance to degradation (42 percent) than the raw fibers (35 percent). Usually, increased concentrations of HCl weakened the fibers. Moreover, Table 1 summarizes some of the recent works that study the effect of chemical treatment of natural fibers on mechanical characteristics of natural fiber polymer composites.      [80] irrespective of the marked influence of fiber processing on composites' mechanical properties, as stated in the last section. Valadez-Gonzalez, Cervantes-Uc [56] investigated the effect on mechanical properties (flexural as well as tensile strength) of henequen fiber-reinforced high-density polyethylene of the fibers (longitudinal or transverse) and treatment with 2 percent HAO (HDPE). In terms of modules of elasticity, tensile strength, as well as flexure strength, this paper demonstrated that the longitudinal orientation had approximately 90 -95 percent higher mechanical properties than transverse orientation ( Figure 4). Fiber's longitudinal orientation, combined with chemical treatment, increased the fibers interlock in a composite that resulted in a better transversal mechanical characteristic than those in the untreated fibers. Brahim and Cheikh [13] investigated the effects of polyester composite's mechanical properties on fiber orientation (0˚ -90˚) of unidirectional Alfa fibers (modulus of elasticity and tensile stress). The fibers were extracted from the Alfa plant by soda procedure whereby the plant stem has been heated for 2 hours with a NaOH solution of 3% at 100˚C temperature and then bleached with a NaClO solution of 40% for 1 hour. The composite's mechanical properties (modulus of elasticity and tensile stress) have been found weakened with the increase in the angle of orientation from 12.3 to 5 GPa and from 150 to 18 MPa. This may be described by the fact that if the fibers have been orientated in the direction of load (0˚) this would cause the load to be distributed along the entire fiber's length. Moreover, if load directions had not been parallel to fibers, the load did not spread throughout the whole fiber, thus reducing the value. Jacob, Thomas [82] examined the impact of the orientation fiber (0˚ -90˚) on the tensile strength of the natural rubber composite sisal/oil palm hybrid fiber.
They found that when the fibers are longitudinally oriented (0˚), the maximum tensile strength (8 MPa) is achieved. By contrast, tensile strength is reduced to 4 MPa when the angle for the orientation of fibers has been raised to 90˚. When the fibers are lined up (longitudinally) in a force direction, stress is transferred evenly. Moreover, they cannot distribute a lot of stress when fibers are aligned (transversely) with load direction.
De Albuquerque, Joseph [15] examined the fiber orientation's effect on the jute roving reinforced polyester composites' tensile strength. This has been observed that tensile strength is directly proportional to the fiber content in the transversal direction because, with the rise in fiber content from 0 percent to 30 percent by weight, tensile strength also raised from 40 MPa to 65 MPa. On the other hand, tensile strength is inversely proportional to fiber content in the longitudinal direction as tensile strength reduced from 40 MPa to 10 MPa with the same rise in fiber content. Such results may have been because of the obstacles presented by the longitudinally orientated fiber, which inhibited the stress distribution throughout the matrix, therefore causing a higher stress concentration along with inadequate mechanical properties.

Effect of Volume Fraction
It has been shown that the fiber V f (such as the fiber's concentration in composite) has a significant impact on composites' mechanical properties. This was also reported that some increases in the fiber's V f [43] [81] [83] can result in an improvement in the fiber's mechanical properties. Above a specific level, high V f can cause fiber clotting, as well as applied loads, which cannot be distributed Young's Modulus, Gpa consistently that limiting the mechanical properties [70].  Figure 5) and have been closely related to synthetic fiber composite (glass) at similar V f . Yousif [70] investigated the V f effect (0 percent to 60 percent) on polyester composites' mechanical properties from oil palm fruit fibers. Yousif [70] examined the polyester composites' mechanical properties for the impact of V f (0 -60 percent) from oil palm fibers. The conclusion is that the volume fraction was increased by around 110% compared to neat polyester, resulting in the 41%rise in tensile strength. The increase in composite's compression strength to 38% resulted in a 30 percent increase in the V f . Such results had been caused by strain distribution or uniform stress in the composite. But the clotted fiber cannot be distributed equally at higher fiber loading, and the load applied. Likewise, the composite's tensile strength decreased as the fiber V f increased ( Figure 6).    [97]. Due to concentrated mechanical and thermal stresses, fibers are subjected to this mixing process during which they break up and thus lose some of their reinforcing action. However, in the composite's mechanical property, as mentioned by Le Duc, Vergnes [98], the fiber's aspect ratio, that is the ratio between diameter (D) and longitude (L) also plays a significant part.

Recommendations and Main Points Extracted from the Previous Discussions
It was discovered that chemical treatment of natural fibers improved their com- found that longitudinal fiber orientation provided much higher tensile and flexural strengths than its traverse orientation counterpart. It was also reported that mechanical properties could be enhanced by increasing fiber V f [13] [42] [71].
However, V f above a certain level causes fiber clotting, thus the applied loads cannot be distributed evenly, which leads to a decrease in mechanical properties [70]. Therefore, the optimization of a fiber V f value is necessary for strengthen-

Machinability of Synthetic Fiber/Polymer Composites
The synthetic fiber composites' machining, for example, carbon fiber and glass fiber composites, is the main issue due to their extreme hardness, inert nature, along with re-fractoriness to machinability in terms of material removal rate, thrust force, cutting power, Ra, delamination size, as well as bearing strength utilizing a drilling procedure [2] [3]. Moreover, synthetic fibers' abrasive nature causes poor surface finish along with fast tool wear. Therefore, for machining procedures, the appropriate selection of cutting parameters as well as tools is very essential. Furthermore, the machining parameters' effect for exam-ple V f (volume fraction), fiber orientation, drill diameter, speed, and feed influence greatly the materials' machinability (see [ [102]. In the following parts, the effects of working parameters on the synthetic fiber-based composites' machin-ability are discussed.

Effect of the Operating Parameters on Thrust Force
Tsao and Hocheng [90] have researched the impact on machining machinability regarding the thrust force of woven GFRE composites with the use of a driller machine from machining parameters (speed, feed along with drill diameter). The cutting conditions were reported to produce higher than expected thrust strength, but no apparent effect was observed when speed was cut. Khashaba, El-Sonbaty [83] examined the effect, using a drill machine with 2 distinct methods of cemented carbide (K 10) drills of cutting parameters (feed rate and velocity) under a precise cutting pressure on machinability as regards thrust strength of GFRP (Spur & Brad as well as Stub Length). The findings indicate that the thrust forces of two drills were raised with the raising feed rate under the same cutting conditions and that the thrust pressure values of Brad & Spur were lower than the thrust strength box. Ku, Wang [71] have also done similar work, testing a drilling machine with various cutting speeds as well as various feed rates for the same reinforced material. In the case of neat epoxy, the cutting speed from 10m/minute to 45m/minute has noticed a negligible effect on the thrush force. This may be due to heat increases around the edge of the instrument that has demolished the matrix and therefore decreased the resistance force. Nevertheless, there were conflicting outcomes in GFRE composites. With the increase in cutting speed from 5 m/minute to 45 m/minute, the thrust force reduced by 29.4 percent. Moreover, when drill diameter and V f fiber were increasing, the thrust strength increased by 62.5%. Mata, Gaitonde [14] used a drill press to report similar research, material as well as results. The reactions of the thrust force were delayed, and because of the matrix softening to the generated heat, thrust force values were reduced. Furthermore, the speed reduction effect on thrust force has been noticeable.
To examine drilling variables' effect on machinability using tipped-carbide drilling tool on drilling machine, Jacob, Thomas [82] utilized a high V f (66%)

GFRC (glass/phenolic woven fabric) and several holes (300). Results have shown
A. Shalwan et al. that the thrust force is rising as the feed rate rises. This could result from an increase in the undeformed chip's cross-section area. A progressive, slow increase of thrust strength with the growing number of drilled trout, usually linked to the steady wear state of the cutting edge, was observed.
El-Sonbaty et al. [95] used drilling to study the effect of two forms of the matrix (thermoset and thermoplastics) on machinability in terms of the thrust force of woven GFRC. This has been found that with the rise in drilled holes, thrust forces also rise for both matrices, however, the increase has been greater in the thermoset composites. The cause of this may have been machining-induced defects, for example, delamination and debonding.
Tsao and Chiu [19] inspected the machinability cutting parameters' effects in thrust force using three types of stepping-core drills (saw, twist, as well as a candle) for laminates used for CFRP (carbon-fiber-reinforced plastic) compositions.
The used cutting parameters were 800 -1200 revolutions spindle speeds per minute, 8 -16 mm/rev feed rates, 5.5 -7.4 mm/mm diameter ratios. The increased performance parameters were found to lead to an increase in thrust force. By reducing the diameter and spindle speed as well as increasing feed rate, the highest thrust force was achieved.
Instead, when spindle speed along with diameter ratio was increased and feed rate was lower, the lowest thrust has been recorded. In addition, it proposes the highest thrust force in the step-core saw. The main cutting parameters were the diameter ratio (1200 RPM drill diameter, had the least impact on thrust force generated by composite. In addition, when a small feed rate has been utilized and smaller chips were produced as well as delamination was reduced. Eichhorn, Baillie [62] investigate the impact on machinability of thin CFRP laminates by utilizing carbide drill (K20) by drilling parameters (feed rate and spindle speed). This was discovered that feed rate is directly proportional to thrust forces. As a result of the rise in the self-generated feed angle, the drill's working clearance angle was reduced significantly, resulting in the drill rubbing against the work material as well as generating higher thrust forces, which may have contributed to this. On the other hand, it has been discovered that thrust forces decrease as spindle speed is increased. The increasing temperature along with spindle speed may cause the composite to soften, which could explain the phenomenon. A brief discussion on the effect of operating parameters on the machinability of the composites based on synthetic fibers is shown in Table 2.

Effect of the Operating Parameters on Surface Roughness
Hocheng and Tsao [91] investigated the impact on machinability of GFRE composites using a drill machine of machinations by using processing parameters (feeds, speeds, and cutting diameters). Ra has been reported to rise at the cutting feed rate, however, there was no apparent impact of cutting speed. Davim

Effect of the Operating Parameters on Hole Accuracy
Many studies, for example [8] [9] [93], have investigated fixed dimensions and geometric tolerances. Nevertheless, the nature of the composites produced few useful outcomes. Hole features can contribute to early failure, particularly in the attachment assemblies. There are various methods to describe the quality of the generated hole [9]. The characteristics of workpieces may be characterized as a geometric error or a series of errors. Errors in hole roundness and holed size have been the most critical criterion highlighted by Jain, Choudhury [9]. The fibers were bent throughout the drilling process as well as broken by the effect of the cutting edge. The fibers tried to reset after the shear failure, which caused a strain around a drill and therefore reduced the size of the drilled hole to below the cutting diameter. This is frequently found when composites are drilled laminated GFRP.
Nevertheless, when GFRP composites with a greater fiber content are drilled, such a decrease has not been seen because the woven fabric, according to Khashaba et al, does not enable fibers to bend much [93]. Synthetic polymeric composites are widely machined, particularly by drilling, to produce riveted. The usage of these polymer compounds by traditional drilling techniques is widely known to be challenging to generate with good quality lights [53]. Highly efficient. Therefore, the most precise holes must be created with costly drilling procedures and specific drill bits.
Beg and Pickering [8] performed several drilling tests with high-speed steel (HSS) drills on woven glass fabric/epoxy. The first and last hole exit micrographs varied substantially. This can be seen clearly that the first hole was more accurately drilled than the last one, and the accuracy of the holes declined as the number of drilled holes rose. This was because of the gradual usage of the device and rising thrust force at the hole exit. Brahmakumar variations in the size of the holes were found. It was discovered that when cutting feeds were low, but spindle speed was high, larger holes were produced.
This may be explained by two things. The accumulated friction heat may have led to an increase in the cutting temperature. Secondly, larger shear pressures might be due to increased resistance to cuts, due to thinner uncut chips.
In  Table 4 is shown the most important findings of the effect of operating parameters on the machinability of the composites based on synthetic fibers.

Effect of the Operating Parameters on Cutting Force Using Drill Machine
Khashaba, El-Sonbaty [83] examined the cutting parameters' effects for drilling (cutting feed as well as speed) on the processability of precise cutting strength in  Table 5. Consequently, reinforced PEEK machinability is more costly than nice PEEK.

Effect of the Operating Parameters on Delamination Using a Drill Machine
Due to its inherent characteristics, polymeric composites are regarded preferable to be used as structural components [53]. However, composites with low drilling efficiencies and unwanted drilling-induced delamination are seen as challenging to machines [11]. Due to various types of damage, drilling holes are possible. These may be divided into 4 types: delamination, damage related to temperature, geometric defects, delamination at drill entry [72]. Delamination or interlaminar cracking in polymer composite materials is regarded as one of the major types of damage. The major difficulty related to drilling composite fiber-reinforced materials is examined. The Commission is responsible for the refusal of around 60% of aviation industry components [73]. Delamination may be split into two groups: peel-up delamination that happens at the entry along with push-out delamination around the perimeter of the drilled hole, which takes place at the outlet surrounding the drilling hole periphery.
Delamination may lead to decreased material's structural integrity, low assembly tolerance, stiffness, and gradual degradation; it might result in a longterm loss of performance [54]. Delamination generally refers to the layer's separation into the composite through cell migration procedure that may be described further as motion by an external force, such as drilling of certain cells in the composite. The rise in delamination causes the Polymer Composite Structure to become stiffer, degraded, and eventually fail. Several approaches for measuring lamination after drilling composites have been utilized. These photographs are digital [108] [109] [110], S-Can [10], and a microscope shop. a series of GFRP composite twist drilling tests have been performed utilizing traditional drilling and vibration-based twisting for comparison of the delamination created by both procedures [13] [58]. Both approaches found that delamination was observed, although conventional drilling caused more delamination as compared to vibration-assisted drilling. This may be produced by a rise in thrust force when composite laminates are used to conventionally drill.
In terms of the machinery size delamination of the woven GFRE composites Bras, Hassan [79] employing cemented carbide drill, examined the influence of machinery parameters (speeds, feeds along with drill diameters). Delaminationfree holes have not been found. Push-out and Peel-up delamination of all machining parameters were evident in the pictures. Increased delamination via an increased feed rate, spindle speed, and drilling diameter. This occurred because the cross-section of the non-deformed chip grew, resulting in a higher thrust strength and a heat build-up near the tool edge, that degraded the strength of the matrix.
Ciftci, Turker [111] have achieved the same results with drills of various kinds (candlestick, twist along with saw drills) with thick carbon FRP. Ultrasonic scanning photos indicated that the delamination took place with all forms of drills, but more delamination was shown by the twist drill than by the saw drill and candlestick. It was because the cutting blades of the drills were different. Similar results have been published [13]; in this work, drilling techniques for hole formation have been employed for short GFC composites using 2 standard highspeed drilling machines.
Harper, Turner [100] investigated the impact of three prepreg types with woven shapes or unidirectional on holes drill (3750) as well as feed rate (0.2 -0.4 mm/rev) together with the influence on the machinability of CFRP's cutting feed rate at the level of delamination. First, due to the proximity to the net shape manufacturing processes, the CFRP composites require fewer machining operations than the conventional materials. However, in terms of components and integrity, machinability issues were evident. Second, when the feed rate was 0.4 mm/rev, the longest instrument life has been found. Thirdly, usual wear on the drill head was evident in the SEM pictures, and for the majority of the holes drilled at a higher input rate, a catastrophic error was detected. A brief discussion Effect of the operating parameters on machinability in terms of delamination is shown in Table 6.

Machinability Performance of Natural Fiber/Polymer Composites
This was also observed that machinability of these materials was less important than the machinability of Synthetic Materials for Ra, tool wear and life, cutting strengths, mate-rial elimination rate, cutting force, special cutting pressure, and machining force (Figure 7 and Figure 8). The obtained observation stimulated the work under way for studying the machinability of polymer composites based on natural fiber.

Effect of Treatment on Machinability of Natural Fibers Reinforced Polymeric Composites
The machinability of natural fiber polymer composites' literature can be viewed in the figures mentioned ( Figure 7 and Figure 8). Although the following sections examine key findings of works and identify the main problems because of the available resources for the various work procedures for natural fiber polymer composites. Natural fibers were utilized in composites as reinforcements for decades, but their machinability has been limited by their poor adhesion to most polymeric matrices. Due to their hydrophilic nature, natural fibers harm their adhesion to hydrophobic matrixes, which can result in interlocking between matrix and fiber being prevented [54] [81]. The fiber surface should be changed to enhance adhesion as well as, consequently, the composites' machinability [58] [81]. This can be accomplished by modifying the fiber surface. Mylsamy and Rajendras [20] studied the effect on machinability of fiber-matrix interaction for alkaline treated and untreated fibers by utilizing a frying machine from alkali-treated fibers (5 percent) of short-scale agave fiber-reinforced epoxy composites. The treatment was performed on rough surfaces that resulted in better fiber-matrix interlock and a rise in the contact area. In the untreated fibers, however, a void was observed [45] [50]. To prevent them, a modification of the fiber surface needs to be made in favor of adhesion for enhancing the composites' machinability [58] [81].   According to Athijayamani, Thiruchitrambalam [55], a drilling process was used for investigating the effects of alkali treatment on machinability of a natural fiber (sisal and roselle) hybrid polyester composite in terms of weight loss. The fibers have been subjected to a 10-percent NaOH solution treatment for varying times (2, 4, 6, and 8 hours). With the rise in wear test time (4, 8, and 12 minutes), Gradual weight loss has also been raised, and the alkali-treated composite samples outperformed the untreated composite samples in terms of wear performance. A possible explanation for this is that the alkali treatment removed moisture from the fibers that resulting in interfacial bonding strength between matrix as well as reinforcement.
In their study, Nam, Ogihara [54] used a drilling process to investigate the machinability of alkali-treated and untreated (5 percent NaOH) "coir fiber-reinforced poly (butylene succinate) biodegradable composites" in terms of the Ra. A 5-percent alkali treatment for 72 hours increased Ra along with cellulose amount exposed on the fiber surface, according to some researchers. This improved the mechanical interlocking between matrix and fiber, as well as the composite material's machinability. A brief discussion on the effect of treatment on machinability of natural fibers reinforced polymeric composites is shown in Table 7.

Effect of Fiber Physical Properties
Few papers have reported the parameters, which impact the natural fiber composites' machinability using a milling machine (i.e., the fiber distribution in the matrix, porosity, moisture absorption, diameter, and length of the fiber, along with how fibers break). Mylsamy and Rajendran [48] studied the impacts of fiber length (3 mm, 7 mm, and 10 mm) on machinability for dimensional accuracy of the drilled holes (4 mm, 6 mm, 8.5 mm, and 12.5 mm) utilizing a drill machine to determine the machinability of various fiber lengths. On short-agave agave fiber-enhanced epoxy composites, an HSS drill bit has been utilized at 260 RPM. The 3 mm fiber-long compound with better dimensional accuracy was found to be optimum from the drilled hole test. As the results show, shorter fibers have improved the mobilization and impregnation of fibers. In turn, this resulted in improved workmanship. The effects of the fiber content in the drilled-loom dimensional accuracy of a drill machine were investigated by Athijayamani, Thiruchitrambalam [55] for hybrid polyester composites (5, 10, 20, and 30 wt. percent). Composite specimens with a fiber content of 30 wt. percent showed greater precision in their dimensions than composite specimens with a fiber content of 5 wt. percent. This can be due to the increased number of fibers that helped to maintain the surface of the drilled hole. A brief discussion on the Effect of fiber physical properties on machinability of natural fibers reinforced polymeric composites is shown in Table 8.

Effect of Operating Parameters and Conditions
Jayabal, Natarajan [43] studied the impacts on usability of "tool wear for hybrid composites, E Glass and Natural Coir Fiber, utilizing a drill machine", of drilling parameters for example drill bit diameter, feed rates as well as the spindle speed. Feed rate has also been observed to have a more essential purpose than other factors in the instrument wear mechanism because of hole size and the fibers contracted in composites. Between feed rate and drill bit diameter, more effective interaction on machinability was found. The optimal level for achieving a minimum tool wear value (0.2 mm/rv feed rate; 1.503 RPM spindle speed and 8mm diameter) has also been determined. Jayabal et al. [43] studied the drilling parameters' impacts, for example, spindle speed, feed rate, drill bit diameter, on machinability in terms of natural coir fiber, E-glass and tool wear for hybrid composites, utilizing a drill machine. The feed rate was observed to have a more essential purpose than other factors in the tool wear mechanism because of the hole size and the fibers contracted in composites. The negligibility of the impacts on machinability of spindle speed as well as drill bit diameter parameters has been because of composite softness. The most efficient interaction on machinability has been observed to be between feed rate along with drill bit diameter. In addition, the optimum levels for attaining a minimum value have been found for tool wear (0.2 mm/rev feed rate, 1503 RPM spindle speed, 8 mm diameter).

Effect of Coupling Agents
No works have been reported about the coupling agent's effects on natural fiber/polymer composites' machinability. Nevertheless, few works describe the tribological nature of these materials, as well as surface roughness, which is also considered in described studies. The efficacy of many coupling agents, which are utilized for enhancing the interface between matrix along with fiber, in changing the interface by forming a bond among composite components has drawn considerable attention [40] [53] [58]. For example, vinyl tris (2-methoxy) silane and methacryloxypropyltrimethoxy silane are two coupling agents that were utilized in current research for changing the surface of fibers properties. The fiber's compatibility with the utilization of polymeric matrix may also be enhanced using these coupling agents [46] [101] [103].
Demir, Atikler [53] analyzed the impacts of silane coupling agents for polypropylene-luffa fiber, specifically AS am (inopropyltriethoxy silane) and MS (mercapto silane), on machinability as measured by Ra. A silane layer was used to raise the coverage of the fiber surfaces, which resulted in lower Ra values (88 nm and 85 nm, respectively) than untreated composite (138 nm). This was due to the increased coverage of the fiber surfaces by the silane layer.

Conclusions and Future Perspective
Machinability and mechanical properties of natural fiber-reinforced composites were reviewed according to many parameters such as chemical treatment, fiber orientation, volume fraction of fiber, physical properties of fibers and operating parameters. The main findings of this research are concluded in the following points: 1) NaOH concentration influences the strength of the fibers significantly, as well as the interfacial adhesion of the fiber with the matrix. This has been found that by removing surface tissues, decreasing moisture absorption as well as rising the Ra, the natural fibers' chemical treatment enhanced their matrix compatibility. This also improved the composites' mechanical properties. The length of time chosen for the chemical treatment has been a crucial consideration. The mechani-Journal of Materials Science and Chemical Engineering cal properties raised when the treatment continued longer but having risen to maximum, they would then fall. Mostly NaOH chemical has been utilized at various concentrations.
2) For controlling the composites' mechanical properties, fiber Orientation has an essential role to play. For controlling the composites' mechanical properties, fiber Orientation has an essential role to play. It was noticed in separate experiments that longitudinal fiber orientation gives more flexural strengths as well as higher tensile than its counterpart with transverse orientation.
3) According to several studies, increasing fiber V f can improve mechanical properties as well as other properties. Fiber clotting, on the other hand, occurs when V f exceeds a certain level; as a result, the applied loads cannot be distributed properly, resulting in poorer mechanical properties. Thus, to strengthen the composite the fiber V f value must be optimized. 4) According to the findings, certain fibers' physical properties such as fiber's quality distributed in the matrix; diameter, length as well as the size of fiber; porosity; "moisture absorption; and how fibers break throughout compounding with the matrix all have an effect" on the composites' mechanical properties. Furthermore, fiber's aspect ratio (L/D) has an impact on the composite material's mechanical properties. Findings also have revealed that decreasing the length of Kraft fiber-reinforced polypropylene composites resulted in increases in impact strength, tensile strength along with Young's modulus.