Optimizing drilling parameters for fly ash-filled hemp/epoxy composites: investigating drill bit geometries, feed rates, and spindle speeds

This article investigates the drilling behavior of hemp fiber-reinforced epoxy composites, both unfilled and fly ash-filled. Drilling performance tests were conducted at varying feed rates (8, 16, and 24 mm min−1) and spindle speeds (700, 1400, and 2100 RPM) using three drill geometries: parabolic, 8-facet, and dagger. The fly ash-filled hemp/epoxy composite showed significant reductions in maximum thrust force (25.09%, 19.92%, and 21.43%) and torque (80%, 79.87%, and 59.25%) when drilled with parabolic, 8-facet, and dagger drill bits, respectively, compared to the unfilled composite. The maximum drilling temperature reached 90.6 °C during cutting and decreased to 71.7 °C post-drilling. Overall, the fly ash-filled composites demonstrated superior performance in terms of drilling-induced stresses and damage.


Introduction
Constant advances in science and technology have raised the need for natural resources, particularly the fossil fuels from which plastics are manufactured, resulting in a significant increase in worldwide plastic waste generation [1].According to the United Nations Sustainable Development Goal 13 (SDG 13), global emissions need to be reduced to half by the year 2030 to limit global warming to 1.5 °C above pre-industrial levels.Global aim is to promote mechanisms for raising capacities for effective climate change related planning and management.Therefore, there is an urgent need to decrease the consumption of plastics to cut down the emissions related to their extraction and production.This requirement of reducing plastic usage can be achieved by replacing conventional polymeric products with composite materials filled with various natural-organic fillers.Different naturally available fibers having primary constituents' lignin and cellulose, namely sisal, coconut (coir), jute, pineapple leaf, ramie, kenaf, abaca, banana, oil palm, flax, wheat straw, bamboo, and sun hemp, have emerged as promising organic reinforcements capable of replacing synthetic or traditional reinforcements materials in polymer composites [2,3].These composites filled with nature-derived fibers represent a sustainable and environmentally friendly use of resources due to their renewable and biodegradable properties.Moreover, these reinforcing fibers possess impressive specific characteristics, including stiffness [4], impact resistance [5], flexibility [6], and modulus [7], making them highly appealing for various industrial applications [2,[8][9][10].In industrial applications, one crucial operation involves the fabrication of holes for the final assembly of the different parts of a product.Hole-making operations are often performed using conventional drilling machines due to their widespread availability.However, machining of natural fiberreinforced polymer composites results in various complexities, including delamination issues such as fiber pull-out, fiber pull-in, matrix burning, chipping, spalling, microcracks, and a reduction in composite strength [11][12][13].These challenges arise when the induced forces and temperatures during drilling are high compounded by the anisotropic, inhomogeneous, and hydrophilic nature of natural fibers.Numerous researchers have established that induced forces and temperatures during drilling are highly influenced by the operating parameters, namely feed rate, speed of spindle, and geometry of drill.These researchers correlated induced forces and temperatures with the quality of the drilled hole for various combinations of input parameters [9,[14][15][16][17][18][19].Rajaraman et al [20] studied the effects of operating parameters on the delamination behaviour of epoxy/banana/kenaf hybrid composite.The delamination showed an increasing trend with feed rate and a decreasing one with spindle speed.Debnath et al [11] investigated the drilling characteristics of polypropylene composites reinforced with nettle fiber and found that combinations of low feed rates, high spindle speeds, and parabolic drills significantly influenced induced forces.From the study of Bajpai et al [21], it was revealed that the performance of grewia optiva and sisal fiber-filled polymer composites during conventional drilling depended largely on the geometry of the drill bit.The type of geometry adopted for the drills significantly affected the drilling-induced forces and consequent induced damages.Debnath et al [22] emphasized that the performance of natural fiber composites during drilling depends on the machining factors and geometry of the drills apart from the polymer used.Babu et al [13] applied the Taguchi optimization technique to determine optimal conditions for factors influencing damage and tensile strength of the polyester composites reinforced with unidirectional hemp fiber after drilling.The study revealed that lower feed rate and higher cutting speed was the optimal combination of parameters for achieving the maximum tensile strength (residual) and restricting the delamination to its minimum.The spindle speed has the greatest statistical impact on the drilling temperature, followed by drill geometry, according to research on the drilling of polymer composites containing natural fibers [23].The literature survey clearly indicates that studying drilling parameters is crucial, as they significantly impact hole quality, as well as the induced forces and temperature.Different research reported drilling investigation of particle-mixed fiber reinforced polymer composites.Huang et al [24] studied the mechanism of chip adhesion during drilling of ceramic particles filled glass fiber reinforced polymer (GFRP) composites.The results revealed that the efficiency of the dissipation of heat and extraction of the chips can be enhanced by using forced cold air.Researchers have studied natural filler materials in polymer composites alongside fiber reinforcement.These natural fillers are intended to enhance tensile strength, elastic modulus, and yield stress, while significantly reducing yield strain [25,26].Saravanakumar et al [27] delved into the drilling of composites comprising polyester resin, banana fiber, Alumina, and eggshell fillers.Employing response surface methodology, the authors have optimized drilling parameters, including cutting speed, feed rate, and drill bit shape, with a focus on impel force, torque, and delamination effects.Kumar and Jena [28] have studied drilling of glass epoxy composites adding clam shell powder and cenosphere as a filler.The study aims to assess the impact of drill diameter, spindle speed, and feed rate on delamination factor and surface roughness during drilling of GFRP composite plates containing 0%, 10%, and 20% clam shell powder filler.Mahakur et al [29] aimed to evaluate the drilling performance of composites containing 5 wt% Corchorus olitorius (jute) filler.Experimental tests were conducted to explore the impact of tool bit diameter, cutting speed, and feed rate on various responses, including thrust force, surface roughness, delamination peel-up, and delamination push-out.Raja et al [30] have analysed delamination during drilling of natural fiber-reinforced hybrid polymer composites crafted from neem, banyan fibers, and an epoxy matrix containing sawdust fillers.Aluminosilicate-rich fly ash is a discarded by-product of thermal power plants running on energy derived from coal, and its proper disposal is a significant environmental concern.Adding fly ash as a filler in polymer composites can enhance mechanical properties such as flexural modulus, tensile modulus, and stiffness [25,26,[31][32][33][34][35].Aluah et al [36] have shaded light on how synergistic combinations of fly ash and eggshell powder as a filler can enhance the performance of Class G cement, benefiting both the oil and gas industry and the environment.Saraswati et al [37] studied the mechanical properties of glass-sisal-epoxy hybrid composites with fly-ash/graphene fillers.The authors have found that the composite with 60 wt% E-glass, 0% sisal, and 5 wt% graphene exhibited the maximum tensile strength of approximately 282 MPa.
From the above literature survey, it is inferred that, although numerous studies have been conducted on natural fiber-reinforced composites, there is negligible research on the drilling behaviour of fly ash-filled polymer composites reinforced with natural Fibers.Given the importance of fillers in polymer composites, this research investigates the drilling behaviour of epoxy composites reinforced with hemp fibers, including both unfilled and fly ash-filled variants.A comparison of the damage incurred during the drilling of these two composite types was also conducted.Furthermore, Statistical and variance analysis were performed to determine the significant parameters that influence drilling temperature.

Fabrication of composites
Hemp fibers, sourced from local suppliers, were first sun-dried for eight hours and then cut into 150 mm lengths.The epoxy resins and hardener (Make: Araldite) were also procured from local suppliers.Two types of composites were fabricated namely, fly ash-filled hemp/epoxy (FAHE) composite and unfilled hemp/epoxy composite.The fabrication of the hemp fiber-filled epoxy composite was done using a hand lay-up process.A mold with dimensions of 180 × 180 × 4 mm was prepared, and a releasing agent was applied to facilitate easy removal of the mold after curing.The mixture, consisting of hemp fiber, epoxy, and hardener, was placed into the mold.After removing the trapped air from the uncured composite using a roller, the mold was sealed and the temperature inside the mold was fixed at 30 °C for 24 h.A constant load of 30 kg was applied to the mold containing the mixture of hemp, epoxy resin, and hardener.To fabricate the FAHE composite, fly ash was mixed with the hemp fiber, epoxy, and hardener mixture, and the same procedure was repeated.A ratio of 30:70 for the hardener and the epoxy was maintained for the investigation.The existing relevant literatures [23,35] were considered for the selection of weight percentage of the fiber and the filler.Moreover, preliminary studies were conducted by a trial-and-error approach considering two main criteria: the surface quality of the laminate and its machinability under certain standard machining conditions.For example, it was observed in the preliminary study that a very high amount of fillers percentage may lead to agglomeration which is an undesired outcome.Such characteristic of the laminate will also hinder its efficient drilling.Hence, an optimum percentage of the fiber and filler were selected for the present study to facilitate efficient machining of the composite laminates.For unfilled hemp/epoxy composites, fibers contribute a weight fraction of 11.11% of the total weight of laminates.For FAHE composites, fibers and fly ash contribute a weight fraction of 15.38% of the total weight of laminates.The amount of fly ash and fiber used for fabricating the composites were 10 grams and 30 grams respectively.The properties of raw materials use for the study has been given in table 1.The schematics of the fabrication process of the composites has been given in figure 1.The thickness of the composites has been maintained as 4 mm.The size of the unfilled hemp/epoxy composites and FAHE composites obtain by this process is 180 × 180 × 4 mm.Three pieces of laminated composites were fabricated for each type of the composites (unfilled hemp/epoxy and FAHE).However, for performing the drilling experiments single specimen with the best finishing quality was selected for each type of the composites.

Experimental set-up
A vertical milling machine was used for the conducting the experiments of drilling (Manufacturer: Batliboi Ltd, Model: BFV5).To hold the composite specimens securely, a specialized fixture was fabricated.The complete experimental setup, the drilled laminates, and the drill bits under investigation are depicted in figure 2. All experiments were performed without the use of any backup supports and were conducted under dry conditions.The drilling operations on the composite laminates utilized three different types of drill geometries, with three distinct levels of feed and speed chosen as input parameters (table 2).The operating parameters and their levels were fixed based on the machine's constraints and previous study on the relevant area [23].The geometrical features of the drill bits are shown in figure 3. It is worth noting that all drill bits had the same diameter of 8 mm and were constructed from solid carbide.All the tests were repeated thrice in order to enhance the accuracy and minimize the effect of errors in the results.

Design of experiments
It is an established fact that three most influential factors during hole fabrication by conventional drilling are feed rate, spindle speed, and tool geometry.These three critical parameters significantly impact the drilling performance and the quality of the fabricated hole.In this present investigation, these three parameters have   been specifically chosen to examine the drilling behaviour of the prepared composites.To conduct the experiments, an orthogonal array 'L 9 ' was selected.The optimal parameter settings were determined using the Signal-to-Noise (S/N) ratio.The primary objective of this study was to minimize the drilling-induced temperature.Consequently, the 'smaller the better' criteria was employed for both forces and temperature.
To determine the optimum values of all the machining factors and gauge the relative importance of the process parameters affecting drilling temperature, variance analysis (ANOVA) was carried out.ANOVA also provided insight into the percentage contribution of each process parameter to the drilling temperature.

Measurements
The measurement of thrust and torque was conducted using a dynamometer (Manufacturer: Kistler, Model: 9257B), to which the fixture was securely attached.This dynamometer was connected to a charge amplification system (multiple channel) (Manufacturer: Kistler, Model: 5070 A) and a system for the acquisition of the obtained data (Manufacturer: Kistler, Model: 5697A1).The software required for the acquisition of the obtained data (Dynoware, Version: 2.3: 5.16, Type: 2825-A-02) installed on a personal computer was employed to capture the force signals.To ensure the detection of minor variations, a sampling rate of 1000 Hz was selected.For the measurement of in situ temperature generated during the drilling process, a thermal-imaging camera (Model-885-2 SET & Make-Testo,) was utilized.Thermographs were acquired at different instants during the drilling process.It is important to assess the highest temperature attained during each of the instant of acquiring the thermograph during the machining operation.To analyse the maximum temperature reached during each stage of drilling, the processing of the obtained thermographs was done.A computer program dedicated for capturing the data (1394) (Version: 2.0 C; TH71 717) was used for this purpose.The thermal-imaging camera exhibit a resolution of 320 × 240 pixels for infrared light, a temperature sensitivity of less than 30 mK at +30 °C with ±2 °C accuracy.The range of temperature was: 0 °C-650 °C.
The damage around the periphery of the drilled hole was assessed to comment on the quality of the holes.To quantify the damage, digital image processing techniques were employed.A Canon EOS 700D digital camera was used for capturing the hole images.The next step of damage analysis was to import the captured images to a publicly available software (ImageJ/version: 1.42).To ensure accurate results and prevent image distortion, the pixel aspect ratio of the images was set to unity.The actual diameter of the drilled hole (8 mm) served as the reference scale in terms of pixels per millimetre for calculating the delamination area.Following that, a line was drawn around the drilled hole to confine the delamination region.This closed curve's area included both the desired hole area and the delaminated area, as illustrated in figure 4. The amount of delamination was measured using a non-dimensional quantity known as the delamination factor (DeF).Equation (1) presents the mathematical expression for the delamination factor.
Where, A del = Delaminated area of the drilled hole A des = Desired area of the drilled hole, and D = Diameter of the drilled hole

Statistical analysis
In accordance with Taguchi's experimental technique, three levels were assigned to each of the process parameters, denoted as A (feed), B (spindle speed), and C (drill geometry), as outlined already in table 2. Consequently, each parameter possessed two degrees of freedom (DOF), corresponding to its level minus one.Additionally, there were two DOF attributed to error.This resulted in a total of 8 DOF in the experiment.Therefore, Taguchi's orthogonal array L 9 was chosen for the current study to investigate the impact of three independent variables.Three different levels of each of the parameters are considered for this investigation.The layout plan of the experimental runs and the measured values of drilling temperatures during the drilling of both the unfilled hemp/epoxy composite and the FAHE composite are presented in table 3. The S/N ratio was computed for each trial number to assess the performance characteristics of the experiment's parameters.The average S/N ratio for each parameter at each level was then determined.The parameter configuration that yields the highest S/N ratio is considered the optimal setting for that parameter at its specific level.There are essentially three types of S/N ratios: larger-the-better, smaller-the-better, and nominal-the-best.In this analysis, the objective is to find the optimal combination of parameters that results in the minimum drilling temperature.Hence, the 'smaller the better' characteristic is employed to calculate the S/N ratios for drilling temperature for both composite materials in the present study.
The S/N ratio for smaller the better is given by the following equation: Where, y -Observed data, s 2 -variation and n -number of observations.The computed S/N ratios are presented in table 3, and the average values of the raw data and S/N data for thrust force, torque, and drilling temperature for both composite materials can be found in tables 4-6.In this study, the optimal configurations for minimizing drilling forces and drilling temperature are determined by selecting the parameters that exhibit smaller mean values in the raw data and larger values in the S/N ratios at each level.These specific values, which represent the smaller mean values in the raw data and larger S/N ratios for each parameter at each level, are highlighted in bold text in tables 4-6.To provide a visual representation, these values are also plotted in mean value graph and S/N ratios graph.The lowest peak in the mean value graph and highest peak in the S/N ratios graph as shown in figures represent the optimal value.Slope changes in the plots of mean values and S/N ratios against different levels of a factor may manifest when transitioning from one level to another.However, abrupt changes in the slope indicate a noteworthy alteration in the experimental conditions.By observing lowest and highest peaks in the figures 5(a) and 6(a) respectively it can be determined that the optimal parameters for minimizing thrust force in unfilled hemp/epoxy composites are achieved with the lowest feed rate (A), the highest spindle speed (B), and the level 3 drill geometry (C).In the case of FAHE composite, the optimal parameters for minimizing thrust force are achieved with the lowest feed rate (A), medium spindle speed (B), and level 3 drill geometry (C), as shown in figures 5(b) and 6(b).The optimal drilling parameters for minimizing torque can be determined by examining the lowest and highest peaks in figures 7(a) and 8(a) for unfilled hemp/epoxy composites, and in figures 7(b) and 8(b) for FAHE composites.The optimal condition information for drill induced temperature can be observed from figures 9(a), (b) and figures 10(a), (b) for both the type of composites.This observation also signifies that the factor exerts a substantial influence on the response.The optimal parameter settings for achieving the lowest drilling-induced forces and drilling temperature for both composite materials are documented in tables 7 and 8. Subsequent confirmation experiments were conducted using these optimal process parameter settings.
Among the three drill bit types, namely parabolic, dagger, and 8-facet, it's noteworthy that the dagger drill bit exhibits the lowest drilling force in both composite materials, while the 8-facet drill bit demonstrates the lowest torque and drilling temperature.The choice of drill point geometry significantly impacts the drilling-induced forces, with these forces primarily being influenced by the phenomenon of indentation.This indentation is   caused by the relatively stationary chisel edge of the drill bit.The chisel edge is estimated to contribute to more than 50% of the thrust force.An interesting characteristic of the dagger drill bit is its smaller point angle of 30°c ompared to the other drill bits.This smaller point angle reduces the indentation effect, as it transforms the chisel edge into more of a point.It can be concluded that dagger drill bit may exhibit the lowest drilling force due to its specific geometry, which allows for efficient chip evacuation and reduced cutting resistance.Conversely, the 8-facet drill bit features additional clearance faces that aid improved chip formation and evacuation and dissipation of heat generated during drilling to the surrounding environment.Also, the designed 8-facet drill bit distribute cutting forces more evenly, resulting in reduced torque and less heat generation during drilling.Thrust forces were reduced by employing higher spindle speeds for the unfilled hemp/epoxy composite and by utilizing medium spindle speeds for the FAHE composite, particularly when employing lower feed rates.This is due to the interaction between cutting parameters and material properties.The selected spindle speeds corresponded to the most effective cutting action for the specific composite being drilled.The presence of fly ash in the hemp/epoxy composite might influence the material's response to different drilling conditions.The torque was also minimized with the utilization of low feed rates and low speeds.Multiple studies have consistently demonstrated that an increase in the feed rate results in higher drilling forces [40,41].The drilling temperature reached its minimum when employing low-speed settings for both composite materials.In the case of the unfilled hemp/epoxy composite, optimal conditions for minimizing drilling temperature are associated with low feed rates, while for the fly ash-filled hemp/epoxy composite, medium feed rates are deemed optimal.It is worth noting that as the drilling temperature increases, the matrix material undergoes softening.This softened matrix is less capable of supporting the cutting force applied to the fibers, as the modulus of the matrix gradually decreases with rising drilling temperatures.A similar observation has been reported by Debnath et al [11] and Choudury et al [19] while conducting drilling investigation on lignocellulosic fiber reinforced composite laminates.

Analysis of variance (ANOVA) for drilling temperature
The primary objective of the Analysis of Variance (ANOVA) is to identify the operating parameters that significantly influence the variation of the quality characteristic under investigation.In the current study, ANOVA was conducted with a focus on the response variable, drilling temperature, and the results are summarized in table 9.The ANOVA analysis was performed at a 95% confidence level, ensuring that the findings are statistically robust and reliable.

Significant factors
The ANOVA results indicate that feed (A), spindle speed (B), and drill geometry (C) are all significant factors affecting the drilling temperature.This significance implies that changes in these parameters lead to substantial variations in the drilling temperature, which is a critical quality characteristic in the drilling process.The significance of feed rate, drill geometry, and spindle speed can also be confirmed from a previous work on the study of drilling-induced damages on carbon fiber reinforced polymer (CFRP) composite [42].

Comparison of influential parameters
Among the significant factors, spindle speed and drilling geometry emerged as the most influential parameters.This finding suggests that the variation in drilling temperature is more sensitive to changes in spindle speed and drilling geometry than to changes in feed rate.The high influence of spindle speed on drilling temperature can be  attributed to its direct impact on the friction and heat generation during the drilling process.Higher spindle speeds typically increase the cutting temperature due to increased frictional heat.Drill geometry significantly affects the heat distribution and cutting efficiency.Different geometries can alter the contact area and the heat dissipation rate, thereby affecting the drilling temperature.

Percentage contribution
The percentage contribution of each factor to the total variation in drilling temperature was also analysed.Drill geometry was found to have the highest percentage contribution, indicating that it plays the most critical role in altering the drilling temperature.This could be since the geometry of the drill bit affects the cutting dynamics more profoundly than the other parameters.The exact design and shape of the drill bit can influence the way heat is generated and dissipated during the drilling process.A similar observation has been reported by Debnath et al [43] while conducting drilling investigation on lignocellulosic fiber reinforced composite laminates.Following drill geometry, spindle speed was the next most significant contributor.This further underscores the importance of controlling spindle speed to maintain optimal drilling temperatures and prevent excessive thermal damage to the materials being drilled.

Force analysis
Statistical analysis in section 3.1 indicates that drill bit geometry has the highest significant contribution to thrust force.Therefore, in the revised section 3.3, different patterns of thrust force resulting from changes in drill bit geometry have been examined.The values of thrust force along with the torque signals were recorded for all three drill bits investigated at different combinations of feed and spindle speed, as depicted in figures 11 and 12.
It is evident that the force signals generated by the three different drill bits exhibit significant variations, underscoring the considerable influence of drill geometries on drilling-induced forces.However, a similar pattern in torque signals was observed for all drill bits, albeit with varying peak torque values.The recorded signals exhibit substantial periodic oscillations, which can be attributed to several factors, including abrupt changes in cutting angles during the drilling process, the heterogeneous composition of composite laminates, the presence of a continuous matrix-fiber interface, varying thermal conductivity of fiber and matrix materials, and intermittent propagation of fractures.The figures also illustrate that both thrust force and torque are notably lower during drilling of FAHE composite compared to unfilled hemp/epoxy composite, across all types of drill bits.For instance, when using a parabolic drill bit, the maximum thrust force was reduced by 25.09% in FAHE composite compared to unfilled hemp/epoxy composite as depicted in figure 11(a).Whereas the torque for FAHE composite was reduced by 80% as compared to unfilled hemp/epoxy composite while using a parabolic drill bit and this is shown in figure 12(a).The reduction in torque may be attributed to the increased homogeneity of the composite constituents due to the addition of fly ash.One of the common disadvantages encountered during conventional drilling of fiber-reinforced polymer composite is its non-homogeneous composition which leads to undesired levels of induced forces and torques.This difficulty can be minimized to some extent by bringing uniformity in the distribution of the reinforcement and matrix in the composite.Hence, with the addition of fly ash particles to the hemp/epoxy composites, their drilling became easier, and the induced torque was also reduced due to the favourable condition for drilling.With the 8-facet drill bit, drilling in  These results are on the contrary to the previous work on drilling of CFRP [42], where it was found that dagger drill's performance was the worst in achieving a damage-free hole.The hardness of CFRP is higher than hemp/ epoxy composite.Therefore, owing to the small point angle of the dagger drill, the force generated on the hemp/ epoxy composite will be lesser than CFRP.Additionally, the calculated delamination factors were found to be lower in ash-filled hemp/epoxy composite compared to unfilled hemp/epoxy composite, as evident from the figures (figures 11 and 12).The introduction of fly ash into the epoxy composites led to an observed increase in compressive strength of the composite as the quantity of fly ash increases.This enhancement is attributed to the hollow structure of fly ash particles and the strong interfacial energy interaction between the resin and fly ash.The values of DeF for unfilled hemp/epoxy composite and FAHE composite at different levels of speed and feed rate are presented in figures 11 and 12.The DeF for unfilled hemp/epoxy composite drilled by a parabolic drill at a spindle speed of 710 rpm and a feed rate of 8 mm min −1 is 1.397.The corresponding value for FAHE composite is 1.230, depicting a reduction of 11.95%.When the 8-facet drill was used at a speed of 710 rpm and a feed rate of 16 mm min −1 , the DeF for the unfilled composite was 1.130 and the corresponding value for the FAHE composite was 1.062, showing a reduction of 7.06%.The value of DeF for the dagger drill used at a speed of 710 rpm and feed rate of 22.4 mm/min was 1.692 for the unfilled hemp/epoxy composite.The corresponding value for the FAHE composite was 1.379, depicting a reduction of 18.45%.Overall, it can be concluded from the DeF results that the addition of fly-ash to the hemp/epoxy composite has led to the reduction of the amount of delamination in the composites.

Analysis of drilling temperature
Statistical analysis in revised section 3.1 indicates that drill bit geometry has the highest percentage of contribution to induced temperature.Therefore, in the revised section 3.4, we have examined the different patterns of induced temperature resulting from changes in drill bit geometry.The temperature generated by the friction between the tool and the workpiece varies when drilling laminated composites made of polymers and natural fillers.In actuality, the temperature during drilling might be higher than the polymer's glass transition temperature.It is a well-accepted fact that with the enhancement of the drilling temperature above the glass transition temperature of the polymer, the overall strength of the polymer laminate decreases.This rise in drilling temperature can influence the dimensional accuracy and surface finish requirements of the drilled hole as well as the tool's longevity.Subsequently, the drilling temperatures and the force signals for the FAHE composites were correlated as presented in figure 13.These signals were recorded under specific input parameters, including a spindle speed of 710 RPM, a feed rate of 8 mm min −1 , and the use of a parabolic drill bit. Figure 13 portrays both the relationship between drilling temperature and force signals and the distinct stages of the drilling process on the FAHE composite, employing the input parameters.The thermograms presented in this figure correspond to the three distinct phases of the experimental work namely the pre-drilling stage (I-II), the drilling phase (II-V), and the post-drilling phase (V-VI).The drilling phase contains three different stages: stage 1 -indentation (II-III), stage 2 -cutting (III-IV), and stage 3 -reaming (IV-V).The second stage, labelled 'II,' marks the conclusion of the pre-drilling phase or the commencement of the indentation phase.During this stage, there was a sudden surge in thrust force due to the action of the chisel on the workpiece either by indentation or extrusion.The machining temperature exhibits a rapid increase, rising from an ambient temperature of 21.9 °C to 43.9 °C.Following the indentation stage, the drilling temperature continued to climb from 43.9 °C to 62 °C.The process of cutting commenced at the instant of the completion of the indentation stage.It persisted till the moment of exit of the cutting lips from the bottommost lamina layer.During the cutting phase, the drilling temperature escalated from 62 °C to 90.6 °C.It is at this stage that the removal of the composite constituents (resin and fiber) typically occurs.By being fully engaged with the composite laminate, the cutting lips aid in material removal.The thrust force achieved its maximum value at this point, as does the temperature readout (90.6 °C).The cutting phase was succeeded by the reaming stage, during which the force signal subsided, and the drilling temperature began to decrease (from 90.6 °C to 72.9 °C).Finally, the postdrilling stage represents the concluding phase where the force signal stabilizes.During this stage, the drilling temperature was observed to decrease from 72.9 °C to 71.7 °C.
All types of drill bits showed a similar pattern of drilling temperature as depicted in figure 14.It is worth mentioning that the drilling temperatures recorded for the FAHE composite were lower than those observed for the unfilled hemp/epoxy composite.For parabolic drill bit the temperature gap between the two types of composites were higher for the cutting and reaming stages as visible in figure 14(a).The temperature gap for

Conclusions
This study aimed to contribute to the realization of the UN's Sustainable Development Goals (UNSDG) by developing natural fiber-reinforced composites.The drilling performance of the fabricated composites, namely, unfilled hemp/epoxy composite and FAHE, was assessed using three distinct drill bits.The key findings of the study can be summarized as follows: • The results of the statistical analysis revealed that the dagger and 8-facet drill bit geometries were optimal for minimizing thrust force, torque, and drilling temperature, respectively, for both types of composites.For the unfilled hemp/epoxy composite, the optimal feed rates were 8 mm min −1 for thrust force and torque, and 16 mm/min for drilling temperature.For the FAHE composite, the optimal feed rates were 8 mm/min for thrust force and torque, and 22.4 mm min −1 for drilling temperature.Additionally, lower and medium spindle speeds were found to be optimal.
• The FAHE composite exhibited a reduction of 25.09%, 19.92%, and 21.43% in maximum thrust force compared to the unfilled hemp/epoxy composite when utilizing a parabolic drill bit, 8-facet drill bit, and dagger drill bit, respectively.Correspondingly, the torque experienced a substantial decrease of 80%, 79.87%, and 59.25% under identical conditions.
• The drilling temperature increased from 62 °C to a peak of 90.6 °C during the cutting stage, coinciding with the point of maximum thrust force.This trend was consistent across all stages of the drilling process for all three types of drill bits.
• In contrast to the unfilled hemp/epoxy composite, the FAHE composite exhibited diminished damage during the drilling process.Additionally, employing low feed rates resulted in reduced torque and thrust force for both composite types.
In general, the fly ash-filled hemp/epoxy composites demonstrated superior performance in terms of drilling-induced forces and damages.Consequently, future research could explore the machining characteristics of fly ash-filled composites incorporating multiple natural fibers.

Figure 2 .
Figure 2. Drilling set up, drilled laminates, and image capturing device.

Figure 5 .
Figure 5. Thrust force main effect plot for mean of raw data for: (a) unfilled hemp/epoxy composite and (b) FAHE composite.

Figure 6 .
Figure 6.Main effect plot for S/N ratio of thrust force for (a) unfilled hemp/epoxy composite and (b) FAHE composite.

Figure 7 .
Figure 7. Torque main effect plot for mean of raw data for (a) unfilled hemp/epoxy composite and (b) FAHE composite.

Figure 8 .
Figure 8. Main effect plot for S/N ratio of torque for (a) unfilled hemp/epoxy composite and (b) FAHE composite.

Figure 9 .
Figure 9. Drilling temperature main effect plot for mean of raw data for (a) unfilled hemp/epoxy composite and (b) FAHE composite.

Figure 10 .
Figure 10.Drilling temperature main effect plot for S/N ratio for (a) unfilled hemp/epoxy composite and (b) FAHE composite.

FAHE
composite resulted in a 19.92% reduction in thrust force and a 79.87% reduction in torque compared to unfilled hemp/epoxy composite, as shown in figures 11(b) and 12(b) respectively.Similarly, when the dagger drill bit was employed, torque and thrust force values were reduced by 59.25% and 21.43% respectively, in FAHE composite compared to unfilled hemp/epoxy composite, as demonstrated in figures 11(c) and 12(c).

Figure 13 .
Figure 13.Thrust force versus time graph with thermograms showing temperatures at different drilling stages.

Table 2 .
Details of operating parameters.

Table 3 .
Design and data acquired from experiments as per L 9 orthogonal array.

Table 4 .
Responses for raw data and S/N data for thrust force.

Table 5 .
Response for raw data and S/N data for torque.

Table 6 .
Response for raw data and S/N data for drilling temperature.

Table 8 .
Optimal setting of the parameters for FAHE composite.
a Significant at 95% confidence level.