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Machinability Analysis of Delamination and Thrust Force in Drilling of Pure and Added GFRP Composites

The aim of this work is to define the cutting conditions that allow the drilling of added glass fiber reinforced epoxy composite materials by taking into consideration the exit delamination factor, thrust force and the optimum combination of drilling parameters. The experiments were carried out under two cutting parameters such as cutting speed and feed rate for three levels each. Taguchi experimental design is used to reduce the excessive number of experiments. The experiment design was accomplished by application of the statistical analysis of variance (ANOVA). Correlations between cutting speed/feed rate and the various machining parameters were established to optimize cutting conditions. These correlations were found by quadratic regression using response surface methodology (RSM). Multiple regression analysis (MRA) was also employed to establish parametric relationships between the experimental parameters and the machinability outputs consisting of delamination and thrust force. The machinability refers to the relative ease or difficulty under certain cutting conditions. Therefore, it is very important to understand the factors affect the machinability and to evaluate their effects. Machinability of GFRP composites was enquired. It is aimed to evaluate the machinability of these materials. A machinability index has been developed in current study.


Introduction
The glass fiber-reinforced plastic (GFRP) composites have been widely used in various industries in recent years due to their excellent properties, such as light weight, high modulus, high specific strength, and good corrosive resistance. Drilling is frequently used in assembling this type of composite components due to the need of the fastener mechanical connection. The drilling of GFRP composites is quite different from that of ordinary metals and their alloys and is often accompanied with multiple damages such as tool wear, rough surface finish, burr, tearing, fiber pull-out, thermal damage, matrix crack and delamination. The most serious damage is surely delamination that can happen both at the entrance and exit of the laminate, and generally refers to the peel up delamination at the entrance surface and push down delamination at the exit surface [1][2][3][4]. A large number of researches have carried out corresponding studies on drilling composites using traditional twist drill.
Khashaba et al. studied on the machinability of GFRP composites and investigated the effect of cutting parameters on thrust force and delamination. They concluded that an increase of the cutting speed and the feed rate leads an increasing of delamination and also as the feed rate increases, the thrust force and delamination increase. It was shown that a high feed rate of drilling causes a crack around the exit edge of the hole. The next phase of this study is the investigation of the effect of tool wear on thrust force. Results indicated that an increasing of tool wear at high cutting speed and feed rate causes a rising of thrust force [3,4]. Uwe Heisel et al. investigated the influence of the point angle of a drill tool and increased cutting speeds on machining forces and drill hole quality. They determined that elevated point angles result in increased feed force while the drilling torque stays almost constant [5]. Rajamurugan et al. modeled that the effect of drilling parameters on delamination of GFRP composites by using response surface methodology. Thus, delamination became predictable according to selected cutting parameters [6]. The drilling parameters are important for minimization of delamination damage and the thrust force plays a critical role in influencing the size of the delamination zone. Chye Lih Tan et al. attempted an analytical study of drilling characteristics for composite material. Results of the analytical study indicate that the delamination damage can be lightened if the applied thrust force is lower than the critical thrust force value. The applied thrust force and delamination damage from experimental results were used to validate the proposed model [7]. Mohan et al. applied the Taguchi technique and response surface methodology on GFRP composites. The major objective of this study is to find out the factors affecting delamination and optimizing the process parameters for minimum delamination [8]. The first analytical model to determine the critical thrust force was developed by Hocheng and Dharan [9]. Hocheng and Tsao also developed a series of comprehensive analytical models of critical thrust force leading to the onset of push-out delamination for various drill bits and compared with the model of conventional twist drill bit [10,11,12]. Khashaba et al. [13] found that the effect of cutting speed on thrust force in drilling woven-ply GFRP composite laminates varied with tool wear. It was illustrated the effect of drill bit pre-wear on the thrust force at different cutting speed. They observed that the cutting speed has insignificant effect on thrust force in drilling GFRP composite laminates with fresh drill bits, while thrust force increased noticeably with increasing cutting speed when using pre-wear drill bits. Khashaba et al. investigated the effect of machining parameters (feed, cutting speed and drill diameter) on the thrust force and machinability of woven glass fiber-reinforced epoxy (GFRE) composites [14]. In a study conducted by Davim et al., a statistical approach was handled to identify the most appropriate cutting parameters to realize drilling operations on carbon-fiber reinforced thermoset materials. They put forward an approach through Taguchi's experimental analysis along with the multi-purpose optimization [15,16].
Palanikumar et al. [17] performed drilling experiments in GFRP composites to study the influence of the drilling parameters on push down delamination. They disclosed that feed rate was the main factor which had the greatest influence on push down delamination, followed by rotational speed. Latha and Senthilkumar [18] revealed that feed rate and drill diameter were the main factors which impacted the push down delamination in drilling of GFRP composites. The cutting speed showed only limited effect on push down delamination in drilling of GFRP composites. Kilickap et al. [19] revealed that the peel up delamination and the push down delamination increased with the increase of feed rate and cutting speed in drilling of composites. Palanikumar et al. [20][21][22][23] analyzed in detail the influences of machining parameters on thrust force and delamination in drilling GFRP using candle stick drills. Experimental results showed that the drilling performance in composites could be improved by optimizing the drilling parameters. Candle stick drills have been recognized as advantageous tools for the reduction of thrust force and delamination damage in drilling composites. Liping Liu et al. carried out Drilling experiments of glass fiber-reinforced plastic (GFRP) composites and finite element simulations. Three candle stick drills with different drill tip geometries and one twist drill were compared in terms of thrust force, peel up delamination, and push down delamination [24]. Ozturk et al. investigated the effect of drlling parameters on the thrust force and machinability of added glass fiber-reinforced epoxy (GFRE) composites [25]. In this study, the effects of cutting speed and feed rate on thrust force and delamination in drilling of the materials including pure glass-fiber epoxy, SiO2 /Al2O3 added reinforced epoxy were studied by means of an experimental design. To achieve this goal, a mathematical model was created by applying the response surface methodology. Using the response surface methodology and Taguchi method together, the significance levels and contributions of machining parameters to the thrust force were determined. Machinability of three composite materials was investigated. To determine the effects of drilling parameters on machinability parameters such as thrust force and delamination, machinability index has been created and compared machinability of pure and added GFRP composites.

Experimental Method
The samples used in the experiment consisted of pure glass-fiber pure epoxy, SiO2 and Al2O3 reinforced composite GFRP composites. The dimensions of the sample materials are 4x65x165 mm. The mechanical properties of the samples are provided on Table 1 and experimental set-up is given in Figure 1. Mazak Variaxis 500-5X machining center was used to perform the experiments, and the thrust force was measured by Kistler 9257B type dynamometer. In the drilling operation, K10 carbide solid drill with 118 0 point angle and 6 mm diameter was used as cutting tool. The thrust forces for different cutting parameters were recorded according to experimental design by using Dynoware software. In order to calculate delamination factor at entrance and exit of holes on workpiece, the relation is given as Where Dmax is maximum diameter of the delamination zone and D is the diameter of hole. The scheme of delamination is indicated in

Response Surface Methodology
To model the process, implementation of experimental tests is required to find the relationship between responses and independent variables. An important step in response surface modelling is to define an appropriate approximation for the actual relationship between the response and the set of independent variables. A response surface is an analytical function such as a polynomial that relates the behaviour of response variable to several independent variables. After the machining parameters and the response function are identified, the relationship between the response and independent variables are modelled [26]. In the mathematical model, the relation between cutting parameters and delamination factor is stated as follows: In this equation;ρ1 and ρ2 are the coefficients of the parameters while Ci is a constant coefficient. Equation (3) and (4) are the first order and the second order polynomial models respectively.
In this equation, Y I is the estimated response depending on the first and the second order equations, while y is the real response. The coded variables of cutting speed and feed rate are x1 and x2, experimental error is ε and the estimated values of related parameters are b0, b1, b2, b11, b22 and b12.

Taguchi Method
The number of experiments increase depending on the number of processing parameters. In order to reduce the number of experiments, Taguchi method is used to analyze the results. This method combines three methods; orthogonal experimental design, signal-noise (S/N) ratio and variance analysis (ANOVA). Orthogonal experimental design is used to create a speicific design of parameters for different levels by scanning all parameter space with minimum number of experiments. The results obtained from the planned experiments according to orthogonal experimental design are analyzed by transporting them into S/N ratio. The S/N ratio is used to measure performance characteristics of required values. The S/N ratio is identified depending on three major performance characteristics such as "(S/N)SB, the smaller-the better", "(S/N)LB, the larger-the better" and "(S/N)NB, nominal-the best". "(S/N)SB, the smaller-the better " are stated with the equation of (5). Here yi indicates the result measured in experiments, n indicates the number of experiments.

3.1.RSM Modeling for Thrust Force
The modelling was accomplished through mathematical and statistical methods to search for the thrust force as the dependent variable. The parameters of cutting values were identified at five different levels and these are provided on Table 2. In current study, 12 experiments were performed based on rotatable centred composite design. Five level for each variable was determined which is given in Table 2. Experimental plan in Table 3    Second order mathematical models were obtained for three different materials by means of RSM modelling by using the experiment plan. Coded variables are used in equations. Second order mathematical models for Pure GFRP/epoxy, Al2O3 added GFRP/epoxy, SiO2 added GFRP/epoxy are given by equations (8), (9), and (10) respectively.

Fig.3
The F-ratios calculated for each of the three materials are bigger than F-table values; the models with 95% reliability are adequate. Table 4 contains the obtained significant parameters. As "calculated F-ratio" is larger than F-table value for pure GFRP/epoxy, cutting speed (x1) and feed rate (x2) were found to be significant in drilling operation. As "calculated F-ratio" is larger than F-table value for both SiO2 added GFRP/epoxy and Al2O3 added GFRP/epoxy materials, feed rate (x2) is observed to be significant.

Optimization of Thrust Forces
The objective function and constraints of the problem is given below. The objective function is aimed to be minimized.

RSM Modelling for Delamination
The modelling was accomplished through mathematical and statistical methods to search for the delamination factor as the dependent variable. The cutting parameters were identified at three different levels and these are provided in Table 2.
In current study, 12 tests based on rotatable centred composite design, three levels for each variable, were conducted. The experimental plan and levels given in Table 4 were used to create second order RSM model for three different composite materials. Specimens were drilled according to the defined plan and delamination factors were recorded. Second order mathematical models were obtained for three different materials by means of RSM modelling using the experiment plan data given in Table 2. Coded variables were used in equations. Second order mathematical models of delamination factors at hole entrance for pure GFRP/epoxy, Al2O3 added GFRP/epoxy, SiO2 added GFRP/epoxy were given with equations (11), (12), and (13) respectively.
Ydfen3 = 0,28162 + 0,03795x1+ 0,07760x2 -0,01792 2x1 2 -0,02518 x2 2 + 0,02142 x1x2 (13) It is seen that the values of delamination factors for pure GFRP/epoxy composites are lower than for Al2O3 and SiO2 added GFRP composites. The linear effects of feed rate are bigger than cutting speed for three composite materials. The quadratic effects of cutting speed and feed rate are important for all three materials.
It is seen that the values of delamination factors for pure GFRP/epoxy composites is lower than for Al2O3 and SiO2 GFRP/epoxy composites. The linear effect of the cutting speed is smaller than the linear effect of the feed rate, but the linear effect of the cutting speed on the delamination factor at the hole exit is greater than the effect on the delamination factor at the hole entrance. The quadratic effects of cutting speed and feed rate are important for all three materials.

Optimization of delamination factors
The objective function and constraints of the problem is given below. The objective function is aimed to be minimized.
While for all three composites materials the minimum delamination factors at hole entrances and hole exit were obtained with the smallest feed rate, the values of cutting speeds indicate the change from minimum to maximum.

Taguchi Analysis for Thrust Force
In the present analysis, the L9 orthogonal array was used. The data obtained from the experimental plan designed with Taguchi method are shown in Table 6.

Variance analysis for Thrust Force of GFRP/Epoxy
Within the scope of Taguchi method, the variance analysis for GFRP/Epoxy and response tables was performed. In drilling GFRP/Epoxy materials, minimum drilling parameters are feed rate of 0.05 mm/rev and cutting speed of 50 m/min that are obtained according to "the smaller-the better" rule for thrust force. Feed rate displays the highest effect on thrust forces. The contribution of feed rate is 91.3% and the effect of cutting speed is 2.8%. The average value for minimum thrust force is 256.2 [19].

Variance Analysis for Thrust Force of SiO2 Added Material
Variance analysis for SiO2 added GFRP/Epoxy composite material and response tables was performed. In drilling SiO2 added composite materials, feed of 0.05 mm/rev and cutting speed 50 m/min are obtained as minimum drilling parameters for thrust force according to "the smaller-the better" rule. Feed rate displays the highest effect on thrust forces. The effect of feed is 89.9% and the effect of cutting speed is 5.32%. The average value for minimum thrust force is 621.41 N [19].

Variance Analysis for Thrust Force of Al2O3 Added Material
Variance analysis for Al2O3 added GFRP/Epoxy composite material was performed. In drilling Al2O3 added materials, feed of 0.05 mm/rev and cutting speed of 50 m/min are obtained as minimum values for thrust force according to "the smaller-the better" rule. Feed rate displays the biggest effect on thrust forces. The effect of feed is 82.03% and the effect of cutting speed is 12.85%. The average value for minimum thrust force is 618.27 N [19].

Application of Taguchi approach for Thrust Forces of Three Composite Materials
If we take material as the third parameter, orthogonal array in Taguchi method turns into the state in Table 7. Table 8, on the other hand, displays average force-loss function and S/N ratios.    In the application of Taguchi method, when the material is taken as variable, the analysis of variance for the three materials is given in Table 9 and the response table in Table 10.
ANOVA was used to determine the percentage contribution and optimum combination of drilling parameters. The results of ANOVA of the raw data or mean of trust force and the results of ANOVA of S/N ratios is given Table 9. The percentage contributions all the drilling parameters and materials were quantified at the last columns of both of the results in the Table  9. Both of the results suggest that the influence of feed rate on trust force is very much larger than that of the influence of cutting speed.
From Response Table for mean and Response Table for the S/N ratio in Table 10 show that trust force is minimum at first levels of cutting speed, feed rate and material. It means that minimum values of cutting speed and feed rate and the use of pure epoxy GFRP composite material give the minimum value of thrust force. In this analysis, composite material shows the greatest effect on the thrust force. While thrust force has a minimum value for pure epoxy/GFRP, thrust force has maximum value for SiO2 added GFRP. It means that pure epoxy/GFRP has the best machinability. The second great effect on the trust force was obtained for feed rate while the minimum effect was identified for cutting speed. The minimum thrust force was determined for epoxy according to the rule "the smaller-the better".
Middle and high values of thrust force for Al2O3 and SiO2 were obtained respectively. It was observed that the thrust force for Al2O3 and SiO2 are close to each other. Feed rate displays less effect than material on the thrust force. The minimum value of thrust force was obtained at 0.05 mm/rev feed and 50 m/min cutting speed values. It was seen that the influence of material on thrust force is 59%, while the influences of feed rate and cutting speed are 34%, and 4% respectively.

Taguchi Analysis for Delamination
In the Taguchi analysis, the average value of experimental response and its corresponding signal to noise ratio (S/N) of each run can be calculated to analyse the effects of the machining parameters. However, S/N ratio was chosen for the Taguchi analysis because S/N ratio represents both the average and variation of the experimental results. In the current analysis, L9 orthogonal array was used. The data obtained from experimental plan, which is designed through Taguchi method, is given for the entrance region in Table 11 and for the exit region in Table 12.

Variance analysis for delamination of GFRP/Epoxy
Variance analysis for pure GFRP/Epoxy composite material was performed. In drilling GFRP/Epoxy materials, at the hole entrance, the drilling parameters are feed rate of 0.05 mm/rev and cutting speed of 70 m/min that were obtained according to "the smaller-the better" rule for the minimum delamination. Feed rate displays the highest effect on delamination factors. The contribution of feed rate is 74,87% and the effect of cutting speed is 13,27 %. Variance analysis for pure GFRP/Epoxy composite material for delamination at the exit was performed. In the drilling of GFRP / Epoxy materials, at the hole exit, according to the "smaller-better" rule, optimum drilling parameters were obtained as the feed rate of 0.05 mm/rev and cutting speed of 70 m/min for minimum delamination at the hole exit. Feed rate shows the highest influence on delamination factors. The contribution of feed rate is 91.72% and the effect of cutting speed is 4.13%.

Variance Analysis for delamination of Al2O3 Added Material
Variance analysis for delamination factor at the hole entrance of Al2O3 added GFRP/Epoxy composite material was performed.
In the drilling of Al2O3 added composite materials, feed of 0.05 mm/rev and cutting speed of 90 m/min were obtained as minimum drilling parameters for delamination according to "the smaller-the better" rule. Feed rate displays the highest effect on delamination. The effect of feed is 65.878% and the effect of cutting speed is 15.21 %.
Variance analysis for delamination factor at the hole exit of Al2O3 added GFRP/Epoxy composite material was performed. In drilling Al2O3 added composite materials, feed of 0.05 mm/rev and cutting speed of 50 m/min are obtained as minimum drilling parameters for delamination factor according to "the smaller-the better" rule. Feed rate displays the highest effect on delamination factor. The effect of feed is 81.423 % and the effect of cutting speed is 16.889 %.

Variance Analysis for delamination of SiO2 Added Composite Material
Variance analysis for delamination factor at the hole entrance of SiO2 added GFRP/Epoxy composite material was performed. In drilling SiO2 added GFRP/Epoxy composite materials, feed of 0.05 mm/rev and cutting speed of 50 m/min are obtained as minimum values for delamination factor according to "the smaller-the better" rule. Feed rate displays the biggest effect on delamination factor. The effect of feed is 76.66 % and the effect of cutting speed is 19.03 %.
Variance analysis for delamination factor at the hole exit of SiO2 added GFRP/Epoxy composite material was realized. In drilling SiO2 added GFRP/Epoxy composite materials, feed rate of 0.05 mm/rev and cutting speed of 50 m/min were obtained as minimum values for delamination factor at the hole exit according to "the smaller-the better" rule. Feed rate displays the biggest effect on delamination factor. The effect of feed is 76.66 % and the effect of cutting speed is 19.035 %.

Application of Taguchi Approach by Taking Composite Material as a Variable
If we take material as the third parameter, orthogonal array in Taguchi method turns into the state in Table 13. Table 14, on the other hand, displays average loss function and S/N ratios. In application of Taguchi method, where material is taken as a variable, for delamination factor at hole entrance, variance analysis for the three composite materials is given in Table  15 and the response table is given in Table 16.   To determine the percentage contribution and optimum combination of drilling parameters more accurately, ANOVA was used. The results of ANOVA of the raw data or mean of delamination factor and the results of ANOVA of S/N ratios are given in table 14 and table15. The percentage contributions all the drilling parameters and materials are quantified under the last column of both the tables. Both of the tables suggest that the influence of material on delamination factor is very much larger than the influence of feed rate and cutting speed. It is clear from Table 15 that delamination factor is minimum at first level of cutting speed, first level of feed rate and at second level of material. The S/N ratio analysis from Table 16 also shows the same results that delamination factor is minimum at first levels of cutting speed, feed rate and second level of material. In accordance with that, the minimum delamination factor was obtained for the smallest cutting speed and feed rate in the drilling of Al2O3 added GFRP composite. The material having the best machinability of the three materials was Al2O3 added GFRP composite.
In the application of Taguchi method, when material is taken as a variable, for delamination factor at hole exit, Table 17, displays average loss function and S/N ratios for delamination factor at hole exit, variance analysis for the three materials is given in Table 18 and the  response table is given in Table 19.   It is clear from table 19 that delamination factor is minimum at first level of cutting speed, first level of feed rate and at first level of material. The S/N ratio analysis from table 31 also shows the similar results. To accordance with that, the minimum delamination factor was obtained for the smallest cutting speed and feed rate in drilling of pure GFRP/epoxy composite. When the percentage of contribution are examined in Table 18, it is seen that the effects of material, feed rate and cutting speed on the delamination factor are 23.62%, 57.93% and 7.39%, respectively. In the investigation of the change of delamination factor for the values of three levels of cutting speed and feed rate of three different composite materials in the drilling, it was found that the effect of the material on the delamination factor at the hole exit is larger than effect of cutting speed and less than feed rate.
The best result of minimum delamination was obtained at pure GFRT/epox composite material in according to the rule is "the smallest is better". The minimum value of delamination factor was determined at 0.05 mm/rev feed and 50 m/min speed values.
Due to the effect of delamination on the quality of the drilled surface, delamination is an indicator of the machinability of the material. For this reason, the material which obtains minimum delamination is the material that has better machinability from the three examined materials. Good machinability was obtained in drilling of pure GFRP/epoxy composite. Machinability gradually decreases from pure GRFP/epoxy composite toward Al2O3 added composite and SiO2 added composite materials.

Machinability
The applications of GFRP require high quality drilling surfaces, including dimensional accuracy and surface integrity, using an appropriate tool and drilling parameters. Machinability of Epoxy/GF, SiO2 added-Epoxy/GF and Al2O3 added-Epoxy/GF was investigated. A machinability index established in function of delamination factor and thrust force. L9 orthogonal array that has nine rows corresponding to the number of tests (8 degrees of freedom) with two columns at three levels was chosen for determining machinability index. The plan of experiments is made of nine tests (array rows) in which the first column was assigned to the cutting velocity (v) and the second column to the feed rate (f) .The experimental plan and the chosen cutting parameters is given in Table 2.
In order to analyze the machinability of these materials, delamination factor (Fd) and thrust force (Ft) from experimental data have been obtained. These are given in Table 3 for thrust force and in Table 7 for delamination factor respectively. A new machinability index (MI) is constructed as in Eq. 17. (17) Where, Ft is the thrust force in N, Fd is the delamination factor in mm, α and β are the weight of the parameters. The values of α and β have been obtained to provide a similar contribution for Ft and Fd in MI. The relation between the thrust force (Ft) and the delamination factor (Fd) allows to calculate α =100 and β =1, by taking into consideration of the results of this study. Machinability indexes calculated with the delamination and the thrust force values obtained by using feed rate and cutting speed values according to L9 orthogonal index used in Taguchi analysis are given in Table 32. MI increase with the decrease of feed rate for several cutting velocities and present a maximum for 0.05 mm/ rev. A graphical representation of variation of MI according to material is given in Fig. 4.

Conclusions
In this study, the effects of cutting parameters on the thrust force and delamination factor in drilling of pure, Al2O3 and SiO2 added GFRP/epoxy composites was investigated. For this purpose, a series of experimental study were performed according to experimental plan to investigate thrust force and delamination factor. A single indicator of the machinability of composites is also proposed and it was called MI (Machinability Index). Taguchi method and ANOVA were used to investigate effect and contribution of cutting parameters on thrust force and delamination factor in drilling pure epoxy, Al2O3 and SiO2 added GFRP / epoxy composites. The results of this study were summarized as follows: According to the results of Taguchi analysis, feed of 0.05 mm/rev gave the minimum delamination factor for three of the materials and feed rate shown the biggest effect to delamination factor. The effect of cutting speed to delamination is less and the cutting speeds for the minimum delamination were obtained 70 m/min for GFRP/Epoxy and 50 m/min for the other composite materials.
Feed rate is an important parameter and its contributions for GFRP/Epoxy, Al2O3 added GFRP, and SiO2 added GFRP were obtained 91.72%, 80% and 76.66 % respectively. The contributions of cutting speed for GFRP/Epoxy, Al2O3 added GFRP and SiO2 added GFRP were found 4.13 %, 16. 89 % and 19.035 % respectively. When material was considered as a variable, for delamination at the hole entrance, it was found that the effect of material is higher than feed rate and cutting speed on delamination factor. Secondly, the parameter with the maximum effect is feed rate and cutting speed is the parameter with minimum effect. The minimum delamination factor was obtained for the smallest cutting speed and feed rate in the drilling of Al2O3 added GFRP composite. Feed rate displays less effect than material on the delamination factor. For delamination at the hole exit, it was found that the effect of material is smaller than feed rate and less than cutting speed on delamination factor. The minimum delamination factor was obtained by the smallest cutting speed and feed rate in the drilling of pure GFRP/epoxy composite. It was observed that the delamination factors at the hole exit were greater than the delamination factors at the hole entrance. Due to the effect of delamination on the quality of the drilled surface, the material with the minimum delamination was defined as the material with better machinability and the pure GFRP composite was found to be the material with good machinability at the hole exit.
According to the results of Taguchi analysis, cutting speed of 50 m/min and feed of 0.05 mm/rev gave the minimum thrust force for all three materials. The minimum obtained thrust force for epoxy is 256 N, for SiO2 and Al2O3 added reinforced composites is approximately 620 N, which is almost three times bigger. For epoxy and SiO2 reinforced composites, feed is an important parameter and its contribution is 90%. The effect of feed for Al2O3 reinforced composite is 80%. Good machinability is obtained with pure epoxy composite material. Machinability gradually decreases from pure epoxy composite material toward Al2O3 and SiO2 added composite materials. In this study, it was seen that when the material is taken as variable, the effect of the material on the thrust force is higher than the feed and cutting speed. The second parameter with the maximum effect is the feed and the cutting speed is the parameter with the minimum effect. In this analysis, the thrust forces were higher in GFRP / epoxy composites with Al2O3 and SiO2 added compared to pure GFRP / epoxy.
In general, when examining the effect of cutting speed and feed on thrust forces and delamination factors for the three materials, the smallest values of the cutting parameters for all three materials gave the minimum thrust forces and delamination factors. This will increase the cutting time and processing cost in terms of labour. Optimum cutting parameters should be determined by taking an acceptable delamination factor as a constraint. By this way, it will reduce the cutting time and processing cost.
When the second order mathematical models created for three materials are examined, it is seen that the thrust force for pure epoxy composites is lower and the thrust values for Al2O3 and SiO2 are closer to each other. The linear effects of cutting speed and feed increase from pure epoxy composite towards Al2O3 and SiO2 reinforced composite material. It was found that the quadratic effects cause a decrease in the thrust forces for three of the materials and the biggest quadratic effect was observed on SiO2 reinforced composite.
Second-order mathematical models were created by splitting the fractional factorial experimental design into two blocks, thus reducing the number of experiments in the experiment plan.
A new machinability index was established in function of delamination factor and thrust force. According to the machinability index, under the effect of delamination and thrust force parameters, it was seen that pure epoxy composite material has better machinability than the other composite materials. In addition, it was determined that epoxy composites reinforced with Al2O3 and SiO2 additives have been equivalent machinability. The reason of this deterioration of machining performance may be the effect of abrasive effects of additives such as SiO2 and Al2O3 Declarations: Funding: No funding was received for conducting this study. Ethical approval: The article follows the guidelines of the Committee on Publication Ethics (COPE) and involves no studies on human or animal subjects. Not applicable The authors state that this paper is an original work, it has not been published in any journals, and this research does not involve any ethical issues of humans or animals.

Compliance with ethical standards
List of Figures   Figure 1: Experimental setup and CNC machining center.                          Graphical represantation of the second order mathematical models for pure GFRP/epoxy, Al2O3 added GFRP/epoxy, SiO2 added GFRP/epoxy.

Figure 4
A graphical representation of variaton of MI for all three materials.