Research On Drilling CFRP Laminate With A Thin Woven Glass Fiber Surface Layer Using Plane Rake Faced Twist Drill

The delamination produced during drilling CFRP will affect its structural strength seriously. Delamination is closely related to the thrust force during drilling, which is closely related to the tool, so it is particularly important to choose the tools with appropriate geometric structure. Many scholars used tools with different geometric structure to drill CFRP, and then conducted the drilling damage analyses and drilling mechanism researches. It finally came to a conclusion that drill with special structure had certain advantage compared with common twist drill in the drilling process. In this paper, a new type of plane rake faced twist drill was used to drill the CFRP laminate with a thin woven glass fiber surface layer. Experimental results showed that plane rake faced twist drill along cutting edge had a constant reference rake angle value, which caused the plane rake faced twist drill generated smaller thrust force and less drilling damage than the common twist drill. As the reference rake angle of plane rake faced twist drill increased, the thrust force and drilling damage decreased. It was revealed the inhibition of the thin woven glass fiber surface layer on the drilling damage at entrance and exit. Finally, it was proposed that when plane rake faced twist drill was used to drill CFRP laminate with a thin woven glass fiber surface, 46° reference rake angle should be selected.


Introduction
resistance. Therefore, carbon fiber reinforced polymers have been widely used in many high-tech industries. In order to increase the thrust-to-weight ratio of civil aviation aircraft, the use of composite materials was increased in the fuselage to reduce its weigh while considering the strength, thereby improving economy. Carbon fibre reinforced plastics (CFRP) can and will in the future contribute more than 50% of the structural mass of an aircraft [1][2][3].
While carbon fiber reinforced polymer composite has various advantages, the cutting mechanism is quite different from traditional material cutting mechanism due to its anisotropy and high wear resistance. Therefore, the manufacturing processing of carbon fiber reinforced polymer composite has become the main reason for its limited application [4]. For traditional drilling carbon fiber reinforced polymer composite, there is damage such as interlayer delamination and uncut fibers, matrix melting, fiber cracks and so on, which make it particularly difficult to analyze its drilling performance [5]. In the composite material manufacturing industry, traditional drilling is the main method for making composite material holes. Holes need to be drilled during assembling the aircraft structure. The quality of assembly mechanical joints such as bolted connections, rivet connections and pin connections are highly dependent on the quality of drilling [6]. fibers as well as delamination could be minimized [7]. The dagger drill (one-shot drill) was a special double point angle twist drill with four straight grooves. Compared with common twist drill, it had bigger thrust force. However, it exhibited better drilling performance than common twist drill. In other words, the damage area of burrs and delamination factor were lower [8]. The step drill was used to drill a pilot hole with a smaller diameter drill bit firstly and then a larger diameter cutting edge was used for reaming, so that the delamination effect of the chisel edge could be minimized, thereby achieving smaller drilling damage. Qiu et al. [9] used twist drills and step drills to conduct drilling experiments on CFRP.

Geometrical analysis of drilling tool
The plane rake faced twist drill is shown in The plane Pf of different points on the cutting edge is the same and unchanged. As the radius increasing, the helix angle of the common twist drill gradually increases, and the plane Ph changes, so that the angle between the plane Pf and the plane Ph in the normal plane Pn increases at each point. It means that the reference rake angle gradually increases along the cutting edge.
When the common twist drill is grinded to a plane rake face twist drill, the plane Ph of each point on the cutting edge coincides, so that there is a constant reference rake angle at different radius of the main cutting edge.
The width of the plane rake face of grinding is subjected to certain restrictions. On the one hand, when the width of the ground plane rake face is over large, the strength of cutting edge will decrease and the life span of tool will be sharply reduced. On the other hand, if the width of the plane rake face is too small, the cutting performance is not excellent enough. Therefore, the width of the plane rake face is determined by considering the contact length between the tool and the chip, rather than significantly reducing the cutting edge strength. For smooth chip disposal, the width Y of the plane rake face should be larger than the maximum tool-chip contact length hmax [13].
where t is the undeformed chip thickness.
In the lip region of a drilling operation, t can be given by [14] 2 cos sin f represents the feed per revolution, which is related to the speed and feed. P represents the half drill point angle. ε represents the velocity angle.

Fundamental geometry on the lips
ε represents the velocity angle, which is defined as: the angle between the work velocity and the normal to a plane parallel to the drill axis containing the lip, projected in a plane normal to the lip. According to the geometric relationship in Figure 2, it is equivalent to the angle between OA and OB, which can be expressed by the following formula: Where ω is the web angle at point M with radius r, W represents the half of the drill web thickness and is given by： i represents the inclination angle, which is the included angle between the normal plane and the cutting speed Vw in the cutting plane. It is given by From formulas (3), (4) and (5), it can be seen that as the increase of radius , the web angle ω gradually decreases. As the decrease of the web angle ω, the velocity angle ε also decreases, which leads to an increase in the normal rake angle. The reference rake angle of common twist drill along the lip can be by [14] ) Where δ represents the helix angle of each point on the cutting edge. The helix angle of common twist drill gradually increases as the radius of the main cutting edge increasing. That is, the reference rake angle increases as the radius of the main cutting edge increasing, and so does the normal rake angle. But when the plane rake faced drill is produced after grinding the curved rake face to a plane rake face, the helix angle δ at each point is equal to the helix angle δ0 at the outsider diameter. At the same time, the reference rake angle γref is also equal to the reference rake angle at the outer corner of lips γref0, that is, δ = δ0，γref = γref0. Finally, the normal rake angle of each point on the cutting edge of the plane rake faced twist drill is larger than that of the common twist drill.

Workpiece material and drilling tools
In this paper, the CFRP laminate with thin woven glass fiber surface layer was similar to that used in literature [15]. The uppermost and the lowermost layers of CFRP laminate had woven glass fibers layers with the thickness of 0.09 mm. The laying sequence is shown in Figure 4 and the size is 75mm×65mm×6.18mm.
The main parameters of internal carbon fiber laminate are shown in Table 1. In this paper, one common twist drill and seven plane rake faced twist drills with different reference rake angle at the outer corner of lips were used to drill CFRP laminate with thin woven glass fiber surface layer. The tools were numbered, as shown in Table 2. The reference rake angle at the outer corner of lips of common twist drill is 30°, and that of seven plane rake faced twist drills are range from 26° to 50°.
Except for reference rake angle at the outer corner of lips, other geometric parameters were the same, such as the half point angle P=60°, diameter d=6mm and clearance angle α=30°. In the process of drilling, the speed was 2000 rpm, and the feed was 100 mm/min.

Damage factor
In this paper, delamination factor was defined as the ratio of maximum diameter (Dmax) of drilled hole surface damage area to the standard diameter of drilled hole (D), which was traditional delamination damage factor (Fa) as shown in Figure 5. The optical microscope was used to accurately compare the delamination factor among the holes drilled by different tools.
The traditional delamination damage factor is shown in equation (8).

Thrust force analysis
The rake face of common twist drill was changed from curved to plane through grinding, resulting in the generation of plane rake faced twist drill. The main cutting edge of plane rake faced twist drill could be regarded as many micro-elements of inclined cutting. The inclination angles at each point of plane rake faced twist drill were the same as that of common twist drill before grinding [14]. The forces of plane rake faced twist drill and common twist drill were similar. As shown in Each tool was tested twice under the same parameters and the average value of two test results was calculated. The thrust force value for each experiment was the average of thrust force when the main cutting edge was steadily drilling.
The typical thrust force signals recorded during drilling is shown in Figure 7. Fig. 7(a) is a typical thrust force signal during drilling with a common twist drill whose reference rake angle at the outer corner of lips is 30°. Figure 7(b) is a typical thrust force signal during drilling with a plane rake face twist drill whose reference rake angle at the outer corner of lips is 30°.
Comparing the plane rake faced twist drill with common twist drill, it could be seen from At the same time, plane rake face twist drill had a smaller thrust force peak B2 than common twist drill. There were two main reasons for this.
On the one hand, the chisel edge (near the main cutting edge) could be eliminated through grinding. During this process, it did not reduce the actual length and change the rake angle of chisel edge, so it is usually called "point relieving". This phenomenon had been proved to have a significant effect on reducing peak thrust force [14]. On the other hand, compared with common twist drill, plane rake faced twist drill had an larger reference rake angle at the same position of the main cutting edge. The increase of reference rake angle could reduce thrust force, and it could also improve the cutting performance of the tool during drilling CFRP laminate with thin woven glass fiber surface layer. For plane rake faced twist drills with different reference rake angles, the thrust force and the thrust force decreasing amplitude are shown in Figure 8. The decreasing amplitude is calculated by the following formula:  In this paper, the thrust forces of common twist drill and plane rake faced twist drill were analyzed when their reference rake angle at the outer corner of lips was 30°. It could be seen from Figure 8 that the plane rake faced twist drill had a smaller thrust force than the common twist drill, and the thrust force reduced by 34.3%. It could be seen from Figure 9 that the plane rake faced twist drill had smaller push down delamination factor than the common twist drill with the same reference rake angle of the outer corner of lips. The main reason for the smaller push down delamination factor of plane rake faced twist drill was that its reduction of thrust force caused the decline of pushing effect. In addition, the cutting performance of main cutting edge was improved, which led to less drilling damage. It showed that plane rake faced twist drill had the characteristic of low drilling damage.
It could be seen from Fig. 8   force reduces from 25% to 1%. It showed that the reduction of thrust force was more limited as the reference rake angle increased to a certain value. As shown in Figure 9, as the reference rake angle increasing, the push down delamination factor decreased significantly when the reference rake angle was not more than 46°.
A small push down delamination factor was obtained when the reference rake angle was 46°.
When the reference rake angle was 50°, the reduction of push down delamination would be limited. In addition to, the plane rake faced twist drill with too large reference rake angle always has a risk of tool wear. It means that it is not necessary to blindly select tools with large reference rake angle in order to get small delamination damage.

Comparison of damage between common
twist drill and plane rake faced twist drill In order to describe the damage around drilling entrance and exit more accurately, the entrance and exit was divided into 4 regions, each of them was corresponding to a quarter circle. The fiber cutting angle θ is defined as the angle between cutting speed direction and the fiber orientation of unidirectional carbon fiber layer near the thin woven glass fiber layer, as shown in Fig. 10. At first, the four regions at entrance were numbered in Fig. 10(a), and the numbers of four regions at exit in Fig. 10(b) were corresponding to that in Fig. 10(a). When the fiber cutting angle is 0°< θ <90°, it is defined as along cutting region, such as regions I and III in Figure 10(a). When the fiber cutting angle is 90°< θ <180°, it is defined as against cutting region, such as regions II and IV in Figure 10 Table 3 shows the surface morphology of drilling entrance and exit of seven plane rake faced twist drills and one common twist drill.
The region division of drilling entrance and exit in Table 3 corresponds to that in Figure 10. The 0° fiber orientation in Table 3 is defined the fiber orientation of unidirectional CFRP layer near the thin woven glass fiber layer. It could be seen from Table 3 that the burrs distribution at the drilling exit typically exhibited symmetrical regional characteristic on the hole periphery, which might be caused by the fiber-orientation symmetry relatively to the hole center [16]. The direction of burrs existing at the drilling exit was the same as the fiber orientation of unidirectional CFRP layer near the thin woven glass fiber layer, which was similar to the burrs distribution at the drilling exit when drilling unidirectional CFRP [17]. It showed that the damage of drilling exit and entrance in this paper was mainly affected by the unidirectional CFRP layer near the thin woven glass fiber layer.
The plane rake faced twist drill with 30° reference rake angle was obtained by grinding the rake face of a common twist drill with 30° reference rake angle at the outer corner of lips.
At the outer corner of main cutting edge, the plane rake faced twist drill and common twist drill had the same reference rake angle and geometries. Along the main cutting edge of common twist drill from drill bits center to its outer corner of lip, the reference rake angle gradually increased from a negative value to a positive value, and the contribution of thrust force generated by main cutting edge to total thrust force decreased. In particular, the thrust force generated by main cutting edge close to outer corner was negative, which was upward peeling force [18]. Compared with common twist drill, plane rake faced twist drill had an larger reference rake angle at the same position of the main cutting edge. Therefore, the entire main cutting edge of plane rake faced twist drill could generate larger peeling force than that of common twist drill. As the same time, as the increase of reference rake angle, the cutting performance of the entire main cutting edge of plane rake faced twist drill would be improved. Table 3 for drilling entrance, it could be seen that plane rake faced twist drill with 30° reference rake angle had smaller peeling delamination than common twist drill.

As shown in
The plane rake faced twist drill had not only higher peeling force, but also excellent cutting behavior. The common twist drill had smaller peeling force, but it would result in worse damage at entrance than plane rake faced twist drill due to its bad cutting performance. It indicated that the peeling delamination of drilling entrance was not just affected by the peeling force of the entire main cutting edge, but by the cutting behavior of the entire main cutting edge. At the exit of drilling, it could be seen that plane rake faced twist drill had less overhanging burrs than common twist drill when the reference rake angle at the outer corner of lips was the same. In Figure 9, it further showed that plane rake faced twist drill caused less delamination damage of drilling exit than common twist drill. Therefore, it showed that when the reference rake angle at the outer corner of lips was the same, plane rake faced twist drill could obtain smaller delamination damage of drilling entrance and exit of CFRP with thin woven glass fiber layer than common twist drill . showed that when the reference rake angle was 50°, there was damage in both along cutting region and against cutting region of entrance.
The damage in against cutting region of the reference rake angle 50° was more serious than that of the reference rake angle 42°. angle [20]. Therefore, the drilled surface quality at entrance about plane rake faced twist drill was influenced by the combination of cutting behavior and peeling force. When the reference rake angle was smaller than 42°, with the increase of the reference rake angle of the plane rake faced twist drill, the damage at entrance decreased, the peeling delamination didn't increase as the increase of peeling force. It proved that when the reference rake angle was smaller than 42°, the surface quality at entrance was mainly affected by cutting performance of plane rake faced twist drill and less affected by peeling force. When the reference rake angle was larger than 42°, the damage of drilling entrance increased with the increase of reference rake angle. This phenomenon indicated that even though the cutting behavior of plane rake faced twist drill was improved, it couldn't obtain better surface quality at entrance. On the contrary, the surface quality at entrance reduced owing to larger peeling force. Therefore, when the reference rake angles was larger than 42°, the surface quality at entrance was significantly affected by peeling force. In order to obtain best surface quality at entrance, it was recommended to use a plane rake faced twist drill with a reference rake angle of 42°.

Exit damage analysis
As shown in Table 3 The existence of burrs was associated with axial pushing effect and circumferential cutting behavior at the drilling exit. Hintze et al. [21] pointed out that due to fibers or fiber bundles could repeatedly avoid the tool during its feed motion, thus the fibers or fiber bundles were bent either in the laminate plane or perpendicular to it. The fibers would not be broken if the bending did not reach its transverse fracture strength. Jia et al. [22] believed As shown in Figure 12, the against cutting region was divided into two situations, when the fiber cutting angle θ > 90° + γn, the rake face contacted with fibers firstly, then the fibers were bent and deformed through the action of rake face. Its crack extension was more complicated than that in the along cutting region of exit.
When the drill continues to cut along the hole circumference, the pressure from rake face increased and then the tensile failure of fibers and compressive shear failure occurred. This situation was mainly based on the bending fracture of fibers. When the fiber cutting angle θ<90°+γn, the cutting edge firstly contacted with fiber, and the compressive and shear extrusion fracture was generated in vertical fiber direction.
The fibers were sheared off by cutting edge in local contact area and then the fractured fibers were pushed by the rake face along the circumferential cutting direction. Next, it moved outward along the fiber direction and then was separated from the material to form chips. This situation was mainly dominated by the compression shear fracture of fibers.
From Equation (3), it could be seen that the reference rake angle of plane rake faced twist drill was positively correlated with the normal rake angle. In the along cutting region, due to the fiber cutting angles θ always satisfied θ<90°+γn, so the increase of the reference rake angle didn't change the shear-dominated fiber fracture mode.
It was further showed that the reduction of damage in the along cutting region of drilling exit was mainly influenced by the reduction of thrust force, but not by the fiber fracture mode. The Table 3 showed that the improvement of drilling quality was not significant when the reference rake angle of plane rake faced twist drill was larger than 46°, which was due to serious tool wear of the plane rake faced twist drill with large reference rake angle. The plane rake faced twist drill with 50° reference rake angle could achieve the fiber shearing and chip  showed the drilling exit morphology about the plane rake faced twist drill with 30° reference rake angle, which was corresponding to that in the entrance regions II and III shown in Figure   15. As shown in Figure 16(a), the propagating distance of cracks along the lowermost layer of 0° carbon fiber orientation was longer than the cracks along the uppermost layer of 0° carbon fiber orientation in Figure 15 Compared to the along cutting region of drilling entrance, the thrust force in the along cutting region of drilling exit was much bigger than the peeling force. As shown in Figure 15(b) and 16(b), the cracks propagating distance of the drilling exit was longer than that of the drilling entrance. Therefore, the glass fiber in thin woven layer only had a certain inhibition effect on push down delamination at the drilling exit.

Conclusions
In this paper, one common twist drill and seven plane rake faced twist drills with different reference rake angle were used to drill CFRP with thin woven glass fiber layer. The characteristic of low drilling damage with plane rake faced twist drill compared with the common twist drill was studied. In addition, the effect of plane rake faced twist drills with different reference rake angle and thin woven glass fiber surface on the drilling quality of drilling entrance and exit of CFRP laminate with thin woven glass fiber surface layer were also studied.
It could draw some conclusions.
(1) Compared to the common twist drill, the plane rake faced twist drill could generate smaller thrust force. Due to the constant value of the reference rake angle of plane rake faced twist drill, the increase of thrust force was more stable when the main cutting edge was gradually drilled into the drilling entrance. The thrust force decreased with the increase of reference rake angles, but when the reference rake angles increased from 46° to 50°, the decreasing amplitude of thrust force was only 1%.
Therefore, the reduction of thrust force was limited when the reference rake angle of plane rake faced twist drill increased to a larger value.
(2) The drilling entrance damage produced by plane rake faced twist drill and common twist drill at the same reference rake angle at the outer corner of lips were compared. It was revealed that the drilling entrance damage was not just influenced by the peeling force of the entire cutting edge, also by the cutting behavior of the entire cutting edge. The drilling entrance damage produced by plane rake faced twist drill was less than that of the common twist drill. Compared the push down delamination factor of drilling exit, it showed that the push down delamination factor generated by plane rake faced twist drill was smaller than that generated by the common twist drill. Therefore, plane rake faced twist drill had the characteristic of low drilling damage compared to common twist drill.
(3) When the plane rake faced twist drill was used to drill CFRP with thin woven glass fiber layer, the drilling surface quality at the entrance was influenced by the combination of cutting behavior and peeling force. When the reference rake angle of plane rake faced twist drill was 42°, the best surface quality of drilling entrance was achieved. The existence of burrs in the along cutting regions and against cutting region at drilling exit was the result of combined action of axial pushing behavior and circumferential cutting behavior, and the best drilling quality at exit was achieved with the 46° reference rake angle. As drilling damage at exit had a greater impact on the drilling quality of holes than the drilling damage at entrance. So, when the plane rake faced twist drill was used to drill CFRP laminates with thin woven glass fiber surface, a plane rake faced twist drill with 46° reference rake angle should be recommended. Geometry of a plane rake faced drill.

Figure 2
Geometrical relationship between various ''fundamental'' and speci ed drill angles at the lips of a plane rake faced drill  Layer morphology.

Figure 6
Thrust force distribution of plane rake faced twist drill.

Figure 8
Thrust force values and decreasing amplitude with different plane rake faced twist drills.

Figure 9
Push down delamination factors with different plane rake faced twist drills.

Figure 10
De nition of ber cutting angle Figure 11 Micro morphology at the entrance surface of drilling hole.

Figure 12
Diagrammatic sketch of cutting with different rake angles Figure 13 Micro morphology at the exit surface of tool drilling hole.

Figure 14
Micro morphology tipping of too Figure 15 Micro morphology at the entrance surface of 30° tool drilling hole.

Figure 16
Micro morphology at the exit surface of tool drilling hole