In industries where FRP composites are used as structure components, a vast number of holes are required for the assembly of FRP composites. Due to the structure of these materials, drilling parameters, geometry and material of cutting tool, and environmental factors, severe defects happen in the drilled zone and between layers. These defects in composites are seen as fiber-matrix interface separation, fiber breakage, and delamination. The presence of one or more of these defects in composite significantly reduces the strength and fatigue life of the material., The drilling parameters must be selected appropriately to keep these problems at a minimum level. The effects of feed rate and cutting speed on thrust force profile and maximum thrust force in the drilling of GFRP composites having different woven types are given in Fig. 3–4, respectively.
The thrust force mainly depends on cutting speed and feed rate. As seen from Figs. 3 and 4, when the spindle speed and feed rate were augmented, the the thrust force increased in the drilling of all woven types. When UD composites were drilled at 100 m/min cutting speed and 0.05, 0.1, 0.15 and 0.2 mm/rev feed rate, thrust forces were obtained as 248, 307, 336 and 388 N respectively. The thrust forces were measured as 123, 185, 223 and 234 N respectively, in the drilling of ± 45° composites at the same drilling parameters. Compared to UD composites, the thrust forces were found to be 57.7%, 62.3%, 61.9% and 59.8% less in the drilling of ± 45º composites, respectively. When cutting speed is increased to 25, 50, 75 and 100 m/min at a constant feed rate of 0.1 mm/rev, thrust forces for UD composites are 200, 242, 291 and 307 N, respectively, and thrust forces for 0°/90° composites are 196, 215, 245 and 297 N, respectively and thrust forces for ± 45° 159 composites are 120, 150, 159 and 185 N, respectively. The thrust forces obtained in the drilling of ± 45º composites are 60%, 62%, 55% and 60% of the thrust forces obtained from the drilling of UD composites, respectively.
In the drilling of GFRP composites, the lowest and highest thrust forces are obtained from the woven type of ± 45º and 0°/90°, respectively. The lowest thrust force was obtained as 96 N from the drilling of the composite having ± 45° woven type at cutting speed of 25 m/min and feed rate of 0.05 mm/rev. The highest thrust force was obtained as 391N from the drilling of the composite having 0°/90º woven type at cutting speed of 100 m/min and feed rate of 0.2 mm/rev. It may be the reason for obtaining the lowest thrust force during the drilling of the composites having the ± 45º woven type is that the fiber orientation is parallel to the plane of principal shear stress. For the 0°/90° woven type, the fiber orientation is parallel to the principal plane stress. Therefore, the thrust force obtained is higher.
The change in the thrust force profile can be divided into four major regions according to the movement of the drill bit in the workpiece. These regions stage in the thrust force are given in Fig. 5. The first zone is the time that passes up to beginning the drilling operation of the cutting edge of the tool, and thrust force increase quickly by starting from zero because the cutting tool in this region performs a scraping process instead of a cutting process.
The second region is where the cutting edge of the tool starts to a cutting process. The rate of increase in the thrust force in this region has a less slope when compared to the first region. In the third zone, the cutting edge performs completely cutting process, and the drill bit is into the workpiece. Thrust force is slightly higher than the second zone, and the slope of the curve is similar to the second zone. The fourth region is where the cutting tool tip starts to run out of the workpiece, and the drilling process is completed. The thrust force in this zone begins to decrease gradually, and the drilling process has been completed. As seen in Fig. 5, there are fluctuations in the thrust force indicated with a and b. These fluctuations have occurred from contact to fiber and epoxy of the drill bit. While the thrust force suddenly increased with the contact of the drill tip with the fiber, it has suddenly decreased with the contact of the drill tip with the epoxy.
The effects of the feed rate and cut speeds on the cutting temperature during the drilling of GFRP composites with UD, ± 45º and 0°/90º woven types have been experimentally investigated, and the results are shown in Fig. 6.
It was observed that the temperature observed between the cutting tool and the workpiece slightly decreased with increasing feed rate. It was thought that this decline caused by a decrease in contact time between the cutting tool and workpiece. On the other hand, the cutting temperature increased with increasing cutting speed because the friction between the cutting tool/workpiece increased. The maximum temperatures were obtained between 150 °C and 250 °C during the drilling of GFRP composites. Sorrentino et al.  found similar temperatures at the drilling of GFRP composites. The effect of drilling parameters on cutting temperature was obtained similarly for all woven types. The maximum temperatures obtained from the drilling of GFRP composites having UD, ± 45º and 0°/90º woven types at a cutting speed of 100 m/min and a feed rate of 0.05 mm/rev were 188 °C, 200 °C and 190 °C respectively. The minimum temperature was obtained as 118 °C by drilling of the GFRP composites having ± 45º woven type at a cutting speed of 25 m/min and a feed rate of 0.2 mm/rev.
The quality of the drilled surfaces plays an important role in the assembly line. Good surface quality considerably improves fatigue strength, corrosion resistance and friction life. One of the properties, which determine surface quality, is surface roughness. Surface roughness is significantly affected by drilling parameters. For this reason, appropriate machining parameters must be selected in order to minimize the surface roughness in the machining process. The effect of cutting speed and feed rate on surface roughness in the drilling of GFRP composites having different woven types is given in Fig. 7.
As seen in Fig. 7, higher surface roughness was obtained at high feed rates. This situation was the same for GFRP composites with all woven type. Low surface roughness was obtained from the feed rate of 0.05 mm/rev, while high surface roughness was obtained from the feed rate of 0.2 mm/rev. Furthermore, the surface roughness increased with increasing the cutting speeds. When 0/90 composites were drilled at 25 m/min cutting speed and 0.05, 0.1, 0.15 and 0.2 mm/rev feed rate, the surface roughnesses were obtained as 2.46, 2.56, 2.65 and 2.71 µm respectively. The surface roughnesses were measured as 4.02, 4.24, 4.35 and 4.55 µm respectively, in the drilling of ± 45° composites at the same drilling parameters. Compared to ± 45º composites, the surface roughness were found to be 61%, 60%, 61% and 60% less in the drilling of 0/90 composites, respectively. Low surface roughness was obtained from the cutting speed of 25 m/min, and high surface roughness was obtained from the cutting speed of 100 m/min. The lowest surface roughness was obtained from the composites having the 0°/90º woven type as 2.46 µm, while the highest surface roughness was obtained from of the composites having the ± 45º woven type as 6.16 µm. The reason for the high surface roughness in high cutting speed is the fact that during cutting of the GFRP composite material, the cutting edge is faced with different fiber angles at every instant of the cutting process. The interaction angle between the cutting direction and the fiber orientation varies along the periphery of each layer of the composite, depending on the position of the cutting edge and fiber direction. This situation caused the bad surface quality because the cutting process carried out as the break, stretch and twist the fiber in the drilling of GFRP composites. Depending on the woven type, the lowest surface roughness was obtained from composites having the woven type of 0°/90°, while the highest surface roughness was obtained from composites having the woven type of ± 45º.
In the drilling of the GFRP composites, deformations occur in the entrance and exit of the hole. The maximum diameter of these deformations is measured, and the delamination factor is calculated using Eq. (1). The effect of drilling parameters such as cutting speed and feed rate on the delamination factor occurred; the entrance and exit of the hole are given in Fig. 8.
It can be seen that the delamination factor increased with increasing the feed rate and cutting speed. The lowest delamination factors were obtained 1.05, 1.12 and 1.1 respectively at the drilling of GFRP composites having UD, ± 45º and 0°/90º woven types at a cutting speed of 25 m/min and a feed rate of 0.05 mm/rev. Maximum deformation occurred during drilling operations at 0.2 mm/rev feed rate and 100 m/min cutting speed. The deformation factor formed in the drilling GFRP composites having UD, ± 45º and 0°/90º woven types in these drilling parameters is 1.6, 1.89 and 1.71, respectively. At high feed rates, the cutting tool is found to not cut some of the fiber in the composite material. Due to the high thrust force generated by the increase in the feed rate, these uncut fiber are pushed in the direction of feed. This situation leads to severe deformations between the layers and especially in the last few layers of the composite. Krishnamorthy et al.  pointed out that because of the increase in the feed rate increased the contact between the workpiece and the cutting tool, the thrust force was high. Due to the low value of thermal conductivity of epoxy (about 0.363 W/mK), since the heat generated during the cutting process was not quickly thrown out, the heat generated accumulated around the drill bit, and this situates destroyed the stability of the matrix. The increase of temperature softens the matrix and reducing the bond strength between the fiber and the matrix; thereby, it leads to an increased deformation.
In the drilling of GFRP composites having three woven types, the effect of the delamination factor on the tensile strength of the sample are given in Figs. 9 and 10, respectively.
As seen in Fig. 9, the tensile strength value decreased with the increasing delamination factor. The maximum tensile strength of the GFRP composite having UD woven was obtained as 166 MPa at 1.05 deformation factor value, while the minimum tensile strength was 120 MPa at 1.6 deformation factor value. The minimum tensile strength is 72% of the maximum tensile strength. The maximum and minimum tensile strengths of the GFRP composite having ± 45º woven type at 1.1 and 1.71 deformation values were determined as 38 MPa and 27.5 MPa, respectively. In GFRP composites with 0°/90° weaving type, the maximum tensile strength was determined as 249 MPa at 1.12 deformation factor value and the minimum tensile strength was determined 211 MPa at 1.89 deformation factor value. The drilling parameters and heat play an important role in the delamination factor in the drilling of GFRP composites. Due to the low value of the thermal conductivity of the matrix, the high temperature causes the matrix/fiber interface bond to weaken. Since the weakness of the interface bond causes the fibers to pull through the matrix, the tensile strength decreases. Tao et al.  reported that the formed delamination in the drilling of CFRP composites reduced the tensile strength of composite parts and that this reduction was approximately 15 %. This decrease in strength is mainly due to premature failure of the outer layers due to the incapability of the damaged interface to transfer stress from layers to layers.
The woven type of fiber has an important effect on the mechanical behaviour of the composites. As seen in Fig. 10, in the drilling of GFRP composites, the fiber woven types have revealed the size of delamination. As discussed previously, the feed rate is the most critical parameter that influences the delamination that occurs during drilling and, therefore, the tensile strength.