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Study on damage in carbon fiber reinforced plastic drilling using step cutting mechanism drill
Journal Pre-proof Study on damage in carbon fiber reinforced plastic drilling using step cutting mechanism drill Zhen Yu, Changping Li, Xinyi Qiu, Ki Moon Park, Tae Jo Ko PII:
S0925-8388(20)30421-7
DOI:
https://doi.org/10.1016/j.jallcom.2020.154058
Reference:
JALCOM 154058
To appear in:
Journal of Alloys and Compounds
Received Date: 23 December 2019 Revised Date:
24 January 2020
Accepted Date: 25 January 2020
Please cite this article as: Z. Yu, C. Li, X. Qiu, K.M. Park, T.J. Ko, Study on damage in carbon fiber reinforced plastic drilling using step cutting mechanism drill, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154058. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Author contributions Zhen Yu: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Data Curation, Writing - Original Draft, Visualization. Changping Li: Visualization, Writing: Review & Editing. Xinyi Qiu: Writing: Review & Editing. Ki Moon Park: Investigation, Writing: Review & Editing Tae Jo Ko: Conceptualization, Resources, Writing - Review & Editing, Visualization, Supervision, Project administration, Funding acquisition
Study on damage in carbon fiber reinforced plastic drilling using step cutting mechanism drill
Zhen Yu a, Changping Li b, Xinyi Qiub, Ki Moon Park a, Tae Jo Ko a * a
School of Mechanical Engineering, Yeungnam University, 214-1 Dae-dong, Gyeongsan-si,
Gyeongsangbuk-do 712-749, South Korea b
College of Mechanical and Electrical Engineering, Hunan University of Science and Technology,
Xiangtan, 411201, China * Corresponding Author / E-mail: [email protected], TEL: +82-53-810-3836, FAX: +82-53-810-4627
Abstract
A new multi-groove drill bit is proposed for drilling carbon fiber reinforced plastic (CFRP). Grooves are proposed with different grinding parameters on the double-point angle drill bit. The parameters are the angle between the grinding direction and the bit shaft, as well as the depth of the groove. The purpose is to reduce the thrust force during drilling and reduce the defects at the inlet and outlet of hole. The thrust force, exit burrs, and defects such as delamination were investigated. The multiple grooves on the cutting edge can change the cutting angle of the cutting edge to reduce the thrust force at the outer layer. They also reduce the burrs on the hole surface and effectively avoid tearing at the edge of the hole exit. Thus, it improves the surface quality of the outlet of the drilled hole.
Key words: CFRP; Drilling; Multi-groove; Defects; Burr
1. Introduction
Carbon fiber reinforced plastic (CFRP) is widely used in various fields due to its excellent mechanical properties, such as high strength, high rigidity, and high corrosion resistance [1]. For CFRP, drilling is one of the important machining processes for holes to join parts. Due to the anisotropy of CFRP, many defects such as burrs and delamination are generated during machining [2]. These defects can cause a reduction in the service life of the CFRP and affect the quality of the assembly [3]. Delamination is one of the most serious problems in the drilling process of composite materials, which leads to a reduction in the quality of the resulting holes[4,5]. Gaugel [6] found that CFRP was drilled with delamination at the entrance (peel-up) and exit (push-out) of the hole. It was found that the outlet delamination of the hole was related to the thrust force generated during the drilling process[7]. For a given tool and material, the thrust force depends on the geometry of the drill bit and the cutting conditions. Some CFRP drilling experiments have been studied for the thrust force and torque generated during the drilling process[7–9]. Lazar and Xirouchakis [10] analyzed the cutting load distribution of the thrust force and torque curves when drilling CFRP using three different drill bits. It was found that the thrust force is mainly affected by the feed rate and tool geometry rather than the spindle speed. Tsao and Hocheng [11] proposed a comprehensive critical thrust force model for circular loading and central concentrate loading. The results show that the critical thrust forces of special drill bits (saw drill, candle stick drill, and core drill) with circumferential drilling torque are lower than that of a drill with central concentrated drilling torque (twist drill). Hocheng and Tsao [12] observed that the delamination caused by drilling a hole is directly related to the thrust force. When the thrust force is lower than the critical thrust, the delamination can be negligible. Therefore, reducing the thrust force in CFRP drilling is
important for reducing the delamination of the hole exit. A large number of studies have investigated the delamination of a hole exit. Zarif Karimi [13] concluded that the quality of a machined hole is highly dependent on the drilling parameters and can be improved by changing the processing parameters [14]. However, with specific cutting parameters, the geometry of the drill bit is an important factor that affects the quality of the drill hole. Yu [15] studied a helical groove on a double-point angle drill bit to achieve secondary removal of the exit burrs and, reduce the tear of the burrs at the hole exit, and inhibit delamination. Díaz-Álvarez [16]studied different drill point angles on bio composite materials. It was proved that the smaller the drill point angle is, the smaller the thrust force is generated, and the smaller the hole exit damage is caused by the machining. Feito [17] compared three different drill bits (a brad center, step drill, and reamer drill). It was found that the reamer drill showed the best results in terms of efficiency and delamination. Qiu [18] used a step drill bit and the ratio of the primary drill diameter to the secondary drill diameter to drill CFRP with different cutting parameters. The experimental results showed that the ratio of the diameter of the primary drill bit to the diameter of the secondary drill bit has a great influence on the hole exit delamination. Tsao and Hocheng [19] studied the effect of the length of the main cutting edge of a drill bit and the diameter of the hole on the delamination. They derived the optimum range of the length of the main edge relative to the diameter of the drill bit. Su [20] proposed a step-controlled drill bit to reduce delamination of the CFRP at the hole exit, and it had good performance in reducing thrust force and damage. These studies show that a smaller cutting edge angle and stage drilling can inhibit tearing and reduce the delamination of the hole exit. Many researchers have proposed an appropriate geometry for drill bits to reduce or eliminate uncut fibers and delamination caused by tearing at the hole exit of CFRP composites. However, there has been no
research on improvements based on conventional drill bits to achieve reduced thrust force and minimize delamination. Therefore, it is necessary to develop a drill bit that minimizes delamination while preserving the advantages of conventional drill bits. In this study, a new drill bit is proposed with a groove on the edge of the drill bit so that the grinding direction is parallel to the flank face of the drill bit, and the relief angle is the same. This design increases the effect of increasing the number of cutting edges, and it is possible to reduce the cutting edge angle and do stage drilling. The thrust force of the multi-groove drill bit was analyzed, and the mechanism of reducing the burr and decreasing the delamination is discussed.
2. Groove grinding and parameters affecting groove shape
The experimental setup for the groove grinding is shown in Fig. 1. The tungsten carbide (composition: WC 88%, Co 12%) double point angle drill bit was selected as the drilling tool, and a diamond single V-tip grinding wheel was used to grind the groove on the secondary edge of the drill. The grinding experiment was performed with a 3-axis machining center (DAEWOO ACE45). The grinding trajectory of the diamond grinding wheel was parallel to the flank face of the secondary cutting edge, and the angle of the V-tip. (
) was 60°. The jig used for the groove grinding on the drill bit can realize the rotation along the
X and Y directions, and the angle of the groove can be realized by adjustment of the jig. The groove depth can be controlled by the movement of the machining center table. The grinding wheel speed, feed rate, grinding depth, and detailed grinding parameters are shown in Table 1.
Fig. 1 Grinding of grooves on drill edges with a diamond wheel
Table 1 Grinding parameters of grooves Processing equipment
DAEWOO ACE V45
Parameters of grinding wheel
Abrasive grain: diamond Granularity: #325 Body; SM45C Diameter: 150 mm
Conditions of grinding
Feedrate:700 mm/min Rotation speed:1000 rev/min Depth of grinding: 5 µm
3. Analysis of multi-groove drill bit
3.1 Thrust force analysis
The multi-groove drill bit differs from the original double point angle drill, which was purchased from a merchant for cutting fiber. The groove was ground on the secondary cutting edge by a diamond grinding wheel. This multi-groove drill bit has two different angles of the cutting edge ( , ) compared to the original drill bit.
is the primary cutting edge, and
The corresponding angles
and
of the
is the secondary cutting edge, as shown in Fig. 2.
edge and
can be obtained from the angle θ and
using equations (1, 2). (1) (2)
Fig. 2 Model of the multi-groove drill bit
When drilling CFRP composites, the total thrust force is affected by multiple elements (γ, ,
,
etc.) acting on the cutting edge. The effective rake angle is used instead of the normal rake angle in the cutting force determination. As shown in Fig. 2, from the vector diagram of the drill bit, the effective rake angle (γ) of the double point angle drill bit can be obtained from the cutting edge point angle ( ) and the chisel edge angle ( ): tan (tan ∙
!
) (#
1,2)
(3)
The effective rake angle of the groove edge is different from the secondary cutting edge. It can be obtained from the cutting edge point angle and the angle between the projection on the end face of the groove edge and the Y-axis by the following equation: tan (tan ∙ tan
cos
) (#
3,4)
(4)
For a given feed rate, the area of an uncut chip can be calculated as demonstrated by Lazar and Xirouchakis (2013). An increase in the secondary point angle of the drill tip means an increase in the thickness of the uncut chips. Moreover, the reduction in the point angle of the drill tip is usually accompanied by an increase in the length of the edge, and the total area of the uncut chips remains the same at a given feed rate. sin ∆0 2
·
.
(5)
1
(6)
!
sin
.
· ∙
1 !
is the depth of a chip, f is the feed rate, and 2
(7) is the area of angle i (i=1,2,3,4). 3 is the radial
length of the edge of angle 1 projected at the end face, and ∆0 is the length corresponding to each angle. The total area of uncut chips can be obtained by substituting the area corresponding to the angle of each drill edge:
2
4·
.
∑6 6 2
(8)
The cutting forces in the tangential direction (parallel to the cutting velocity) of chip flow and the radial directions of the cutting force and friction are expressed as: 78 (9)
: (9) ∙ 2 ; : < ∆0
(9)
78. (9)
:. (9) ∙ 2 ; :.< ∆0
(10)
781 (9)
:1 (9) ∙ 2 ; :1< ∆0
(11)
where the cutting coefficients for each element at a certain height are different because of the varying helix, effective normal rake angle, and oblique angles according to Armarego [22]. : < , :.< , and :1< are edge constants to be evaluated experimentally. The cutting constants : , :. , and :1 are evaluated from an orthogonal cutting database using the oblique transformations .
Fig. 3 Direction of drilling forces
Each force component is shown in Fig. 3. The components of the elemental cutting force components (8= ,8> , and 8? ) can be evaluated in the x, y, and z directions as follows: 8=
8. sin
tan
8 cos
81 cos
8>
8. sin
tan
; 8 tan
8 cot
81 cot
8?
8. sin
; 8. sin
8 cos
; 81 cot
cot
(12) (13) (14)
The total thrust and torque acting on the drill bit can be obtained through the sum of the various drill edges. The total thrust and torque of the two edges can be expressed as: @ℎ3BCD
2∑ 6 6 (8E )!
(15)
@F3GBH
2∑ 6 6 (8 )! ∙ 3 (9)
(16)
3.2 Damage analysis
Fig. 4 shows the drilling model of the original drill bit and multi-groove drill bit penetrating the last layer fiber at the hole exit. As shown in Figs. 4 (a, b), when the original drill bit drills and penetrates the CFRP workpiece, the downward thrust force (8 ) of the drill bit and the resultant force (8 < ) of the radial cutting force begin to tear the uncut fiber at the hole exit. The range of delamination continues to expand by tearing the uncut burrs. As shown in Figs. 4(c, d), when the groove of the multi-groove drill reaches the hole exit position, the cutting forces (8 and 8 ) of the groove edges of the
and
edge to the fiber exit neutralizes the total radial cutting force ( 8 < ). The cutting direction of
is the hole axis, which is opposite to the cutting direction of the edge
are opposite, and the radial cutting force ( 8 )
. The connection point of the edge
and
penetrates the workpiece, which can be regarded as the end of a drilling stage. The
penetration of each groove can be viewed as a drilling stage. At the end of each stage, there are almost no
burrs in the hole exit, and the range of delamination is extremely small.
Fig. 4 Delamination model of the hole exit
When the drill bit cuts into the workpiece, it is assumed that the width (w) of the groove is the total cutting width. The fiber at the hole exit of the original drill is subjected to the thrust force (8 ) and radial cutting force (8 ). Therefore, the resultant force (8 < ) can be expressed as Eq. (17) according to the drill bit point angle. However, the cutting direction of the groove edge ( ) is the axis of the multi-groove drill bit in this drilling process. The cutting force of the multi-groove drill bit on the exit fiber can be expressed as Eq. (18): 8<
8 C#I
; 8 cos
8<
8 C#I
; 8J cos
(17) ; 8 C#I
8 cos
(18)
Fig. 5 Displacement analysis of the hole exit: (a, c) Original drill bit(T1), (b, d) Multi-groove drill bit(T8)
The thrust force model (Eqs. (17) and (18)) of the hole exit was verified by the fiber displacement of the hole exit. As shown in Fig. 5, the displacement range of the fiber at the exit of the CFRP hole was determined by applying the equivalent drilling load to the cutting edges of the original drill bit (T1) and multi-groove drill bit (T8) drill bits, as shown in Fig. 7. The size of the workpiece is 10*10*6 mm3. The four sides of the workpiece are fixed. The diameter of the drill is 6 mm. The drill bit penetrated the workpiece to a depth of 1 mm, and the maximum displacement of the fiber at the hole exit was analyzed by
Creo 3.0 Simulate. The exit fiber displacement of the drill bit was simulated and analyzed when a thrust force of 100 N and a torque of 20 N·m were applied on the cutting edge. The maximum axial displacements of the original drill and multi-groove drill are approximately 2.89 and 2.40 mm, respectively. As shown in Fig. 5 (a), the thrust force of the original drill bit causes the exit fibers to bend outward, and as a result, the displacement further enlarges with the drill bit cut-in (Fig. 5 (c)). However, due to the constraints of the inner ring and the hole wall of the multi-groove drill bit, the fiber is cut off from the middle of the multi-groove drill bit, as shown in Fig. 5 (b). One side of the cut fibers remained inside, while the other side remained on the pore wall. The inner fibers are not affected during the subsequent cutting process, and the displacement of the outlet fibers is small, as shown in Fig. 5 (d). The displacement of the exit fibers is an important factor for fiber tearing at the hole exit. A torn matrix and resin bond failure cause the exit fibers to lose supporting force for the cutting action. The fiber/resin interface near the fracture first breaks and cracks, so the fibers at the crack begin to lose supporting force. Under the action of the thrust force, these unsupported fibers are more likely to propagate cracks to the substrate and eventually form a matrix tear. The cracks further extend to the CFRP hole exit and enlarge the delamination.
4. Experiment and material
A multi-directional woven CFRP composite with a fiber volume fraction of 0.6 was used in an experiment. The composite was provided by the Korea Carbon Convergence Technology Institute. The angle of the carbon fibers and the orientation of each layer are 0° and 90°, respectively. The mechanical properties of the CFRP composite are shown in Table 2. The material was formed by compression curing
under high temperatures and then cut to obtain a desired size of 90 mm × 90 mm × 6 mm. The experiments were performed on a CNC machine (DMC model SS-600), as shown in Fig. 6.
Fig. 6 Experimental setup
Table 2 Material properties of CFRP Longitudinal (σ1t) and traverse tensile strength (σ2t)
840 MPa
Longitudinal (σ1c) and traverse compression strength (σ2c)
570 MPa
Young's modulus
61.5 GPa
Shear modulus
3.7 GPa
Poisson's ratio
0.3
Density
155 kg/m
Rockwell hardness
70–75 HRB
3
An original drill and multi-groove drill were used in the experiment, and both drills had a diameter of 6 mm. The original drill bits were prepared by SJ Tools (S. Korea). Fig. 7 shows the drill geometry information. The drilling performance of the different drill bits was studied with fixed processing conditions. The experimental parameters were a spindle speed of 3,000 rpm and feed rate of 0.1 mm/rev, and all drilling operations were carried out under dry conditions. Both drill bits are double-point angle drill bits with an initial point angle of 120°, a secondary point angle of 90°, and a helix angle of 30°. The geometry and parameters of the drill bits are shown in Fig. 7. The multi-grooved drills have two grooves on each secondary edge. The groove angle (θ) is the angle between the drill axis and the grinding direction of the groove, and the vertical distance from the tip of the groove to the secondary edge is the depth (D) of the groove. A delamination experiment was done to calculate the delamination factor and repeated five times for each cutting parameter for a total of 40 holes. The cutting force measurement during drilling was also repeated five times for each drill bit.
Fig. 7 Drill design parameters
5. Results and discussion
5.1 Thrust force and torque
In the drilling experiment, the thrust force was measured using a Kistler tool dynamometer (9526C2). Fig. 8 shows a comparison of the thrust force curve of T1 and T8. It shows the relationship between the drilling position and the thrust force. The drilling angle of the multi-groove bit is composed of four angles: the primary edge angle, the secondary edge angle, and the two edge angles of the groove. According to the thrust force curve, the thrust force of the multi-grooved drill bit has five stages. In the first stage, the primary cutting edge of the drill enters the composite material, and the thrust increases sharply due to the larger angle of the primary edge. In the second stage, the secondary edge of the drill bit enters into the composite material. The third stage is the entering of the multi-groove cutting edge, which causes very high fluctuation of the thrust force, which has two explanations (An et al., 2015). First, the removal of the CFRP material is mainly through brittle fracture of the fiber and the fiber/resin separation. Second, when the same layer CFRP material is cut, the cutting angle is switched from the secondary edge ( ) to the groove edge (
and
). The frequent change of the cutting angle causes the thrust force to
fluctuate greatly. In the fourth stage, the drill tip enters into the CFRP composite completely, and the thrust force fluctuates due to the heterogeneity of CFRP. The fifth stage is when the drill bit penetrates the workpiece’s outer layer, and the thrust force has a small fluctuation when the groove edge penetrates the workpiece. The point angle (
) of the groove edge ( ) is zero for the multi-groove drill bit (T8), so stable cutting
occurs for a short time when the groove edge ( ) penetrates the last layer of CFRP. After the connection point of the groove edges (
and
) penetrates the last layer of the CFRP, the thrust force continues to
fall. The thrust force curve of the multi-groove drill bit shows that the number of thrust fluctuations of the fifth stage is consistent with the number of grooves.
Fig. 8 Thrust force measurement in CFRP drilling process (T1 and T8)
The experimental results verified that the drilling thrust force model is related to the angle of groove edge, as shown in Fig. 9 and Fig. 10. The corresponding thrust force of each edge during drilling was measured by a measuring instrument. In addition, the influence of different groove parameters on the thrust force was studied. As shown in Fig. 9, the thrust force of the multi-groove drill bits decreases with an increasing depth of groove from 0 to 250 µm (T4 and T5) at the same groove angle (θ=15°). However, it increases with an increasing depth from 250 µm to 1000 µm (T5, T6, and T7). At a groove depth 250 µm, the thrust force decreases as the groove angle increases from 0 to 30° (T2, T4, and T8), but it increases with increasing angle from 30 to 45° (T8 and T10). The thrust force decreases with increasing angle from 0 to 15° (T3,T5) at the groove depth of 500 µm, while it increases with increasing angle from 15 to 45° (T5,T9,T11). The torque is shown in Fig. 10. The effect of the drill on the torque is consistent with the thrust force.
Fig. 9 Thrust force of drill bits
Fig. 10 Comparison of torques of 11 different drill bits
5.2 Damage of hole exit
The thrust force generated by the axial operation of drilling tore the fiber at the hole exit. Moreover, the torque produces outward shear stress that causes radial tear damage of the hole exit fiber. When the effective force (Eqs. (17) and (18)) acting on the fiber is less than the elastic force of the fiber, the drill
edge of the original drill bit cannot effectively cut off the fibers, resulting in burrs and tearing of the outlet surface again by the burrs in subsequent feeding. Fig. 11 shows the hole exit morphology of the original drill bit and multi-groove drill bit. The pictures were taken by a high-speed camera (Phantom Miro C110). The thrust force of the original bit tears the fibers downward, resulting in delamination. In addition, the drill’s cutting edge cannot effectively cut off the burrs caused by tearing at the exit, and the margin edge of the drill can further expand the delamination by tearing through the burrs again, as shown in Fig. 11 (a). As shown in Fig. 11 (b), the multi-groove drilling process can be divided into three states. In the first stage, the primary and secondary edges of the drill bit penetrate the workpiece. In the second stage, the groove of the
edge penetrates the workpiece. In the third stage, the groove of the
edge penetrates
the workpiece. When the third stage of drilling is finished, there is no downward tearing of the outlet fibers by axial force to form delamination. Moreover, when the multi-groove drill penetrates the workpiece’s outer layer, the displacement of the fibers in the hole exit is much smaller than with the original drill bit. The thrust force and the tearing of fibers at the hole’s final forming stage are minimized, which reduces the delamination. When the drill bit penetrates into the workpiece, the thickness of the uncut material of CFRP composites decreases gradually, and the workpiece’s resistance to axial and radial forces becomes worse. The drill edge cannot effectively cut off the fibers, and the exit fibers tear with the feeding of the drill bit, forming initial delamination of the hole exit. The tear depth of the original drill bit and the multi-groove drill bit at the hole exit is shown in Fig. 12 for when the fiber angle is 0°. When the original drill penetrates the hole outlet, the thrust force is much larger than the adhesion force between the fibers due to the lack of effective support at the hole exit, resulting in a large tear depth. The
grooves on the multi-groove bit are distributed on the secondary edge, so stage drilling motion is formed between grooves. The grooves increase the number of cutting edges. Therefore, the groove edge ( ) of the multi-groove drill bit penetrates the last fiber layer, which can be viewed as stage cutting. As shown in Fig. 11(b), when the connection point
edge and
edge penetrates CFRP, the fibers of the hole outlet are
all cut. The total resultant force of the drill edge acting on the outlet fibers is less than the adhesion force between the fibers, and the tear depth is not produced at the hole outlet of the multi-groove drill bit as shown in Fig. 12.
Fig. 11 Damage of exit hole: (a) T1: Original drill, (b) T8: multi-groove drill
Fig. 12 Damage depth of exit hole: (a) T1: Original drill, (b) T8: Multi-groove drill
Fig. 13 Delamination of exit hole: (a) T1: Original drill, (b) T8: Multi-groove drill
Delamination or burrs at the hole exit were observed with an optical microscope (Sometech SV-55), as shown in Fig. 13. The delamination and burrs of the original drill bit (T1) and the multi-groove drill bit (T8: groove angle of 30° and depth of 250 µm) at the hole exit are shown. It can be observed that the original drill bit has no burrs in the first hole, and the burr was removed by tearing because the cutting edge is very sharp, as reported by Yu[15]. With the increase of the number of holes, burrs appear, and there are noticeable tear marks at the junction between burrs and the edge of holes. The tearing of burrs further enlarges the delamination at the hole exit. However, the grooves result in a stage drilling mechanism of the multi-groove drill bit, and the radial cutting forces of the two edges of the grooves are in opposite directions. The first hole of the multi-groove bit (T8) has almost no delamination. As the number of holes increases, tiny burrs are generated in the fiber direction of 0°, with which the tear of the fibers at the hole exit is much smaller than that at 90°. Compared with the original bit, the delamination of the multi-groove drill bit has little influence at the hole exit and can be neglected.
Fig. 14 Delamination factor of hole exit with different drill bits
The delamination of the hole exit was evaluated by the delamination factor (8K ) [24], which is based on the largest diameter of delamination ( 8K
LM= )
of the damaged area:
NOPQ NRSO
（19）
As shown in Fig. 14, the maximum diameter of the delamination was measured using an optical microscope. The delamination factor 8K of the hole exit of all drill bits was compared. The original drill bit had the largest delamination factor, while that of the multi-groove bit differed under different groove parameters. The delamination of the original drill bit was greater than that of the multi-groove drill bit. The delamination of the multi-groove bits tended to decrease as the depth increased at an angle of 15°. At a groove depth of 250 µm, the delamination decreased as the angle increased. At a groove depth of 500 µm, the delamination decreased as the angle increased, and minimum delamination occurs at 30°. Therefore, the multi-groove bit can effectively reduce the thrust force, eliminate the formation of burrs, eliminate
delamination damage caused by the burrs, and significantly improve the surface quality of the hole.
6. Conclusions
In this study, a multi-groove drill bit was developed by forming grooves on the secondary cutting edge. The grooves can reduce the thrust force during drilling. Through experimental comparisons between the original and multi-groove drill bits, the following conclusions are drawn: (1) The groove can change the cutting mechanism and cutting direction of the drill bit. Stage drilling is realized by forming stages between grooves. (2) Compared with the original bit, the multi-groove bits with different parameters can reduce the drilling thrust force. The T8 drill bit has the lowest thrust force and is 37.45% smaller than the original drill bit. (3) The multiple grooves realize a stepping mechanism when the drill bit penetrates into CFRP. When the groove edge penetrates into the last layer, the
edge cuts in the direction of the axis and can
counteract the force that results in cutting outward. Moreover, the fibers at the hole exit can be completely cut off, and holes with less delamination damage can be obtained.
Acknowledgments
This work was supported by the Technology Innovation Program (10053248, Development of Manufacturing System for CFRP (Carbon Fiber Reinforced Plastics) Machining), which is funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).
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Highlights
A new drill bit was proposed where the groove edge is ground on the secondary edge. The thrust force model of drilling for the groove edge of the drill bit was analyzed and established. The influence of the groove edge on the material removal mechanism at the hole was explored. The thrust force is less than the critical thrust force, and no burrs are generated. The groove edge contributes to the defect minimization by the stage drilling action.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(20)30421-7
DOI:
https://doi.org/10.1016/j.jallcom.2020.154058
Reference:
JALCOM 154058
To appear in:
Journal of Alloys and Compounds
Received Date: 23 December 2019 Revised Date:
24 January 2020
Accepted Date: 25 January 2020
Please cite this article as: Z. Yu, C. Li, X. Qiu, K.M. Park, T.J. Ko, Study on damage in carbon fiber reinforced plastic drilling using step cutting mechanism drill, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154058. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Author contributions Zhen Yu: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Data Curation, Writing - Original Draft, Visualization. Changping Li: Visualization, Writing: Review & Editing. Xinyi Qiu: Writing: Review & Editing. Ki Moon Park: Investigation, Writing: Review & Editing Tae Jo Ko: Conceptualization, Resources, Writing - Review & Editing, Visualization, Supervision, Project administration, Funding acquisition
Study on damage in carbon fiber reinforced plastic drilling using step cutting mechanism drill
Zhen Yu a, Changping Li b, Xinyi Qiub, Ki Moon Park a, Tae Jo Ko a * a
School of Mechanical Engineering, Yeungnam University, 214-1 Dae-dong, Gyeongsan-si,
Gyeongsangbuk-do 712-749, South Korea b
College of Mechanical and Electrical Engineering, Hunan University of Science and Technology,
Xiangtan, 411201, China * Corresponding Author / E-mail: [email protected], TEL: +82-53-810-3836, FAX: +82-53-810-4627
Abstract
A new multi-groove drill bit is proposed for drilling carbon fiber reinforced plastic (CFRP). Grooves are proposed with different grinding parameters on the double-point angle drill bit. The parameters are the angle between the grinding direction and the bit shaft, as well as the depth of the groove. The purpose is to reduce the thrust force during drilling and reduce the defects at the inlet and outlet of hole. The thrust force, exit burrs, and defects such as delamination were investigated. The multiple grooves on the cutting edge can change the cutting angle of the cutting edge to reduce the thrust force at the outer layer. They also reduce the burrs on the hole surface and effectively avoid tearing at the edge of the hole exit. Thus, it improves the surface quality of the outlet of the drilled hole.
Key words: CFRP; Drilling; Multi-groove; Defects; Burr
1. Introduction
Carbon fiber reinforced plastic (CFRP) is widely used in various fields due to its excellent mechanical properties, such as high strength, high rigidity, and high corrosion resistance [1]. For CFRP, drilling is one of the important machining processes for holes to join parts. Due to the anisotropy of CFRP, many defects such as burrs and delamination are generated during machining [2]. These defects can cause a reduction in the service life of the CFRP and affect the quality of the assembly [3]. Delamination is one of the most serious problems in the drilling process of composite materials, which leads to a reduction in the quality of the resulting holes[4,5]. Gaugel [6] found that CFRP was drilled with delamination at the entrance (peel-up) and exit (push-out) of the hole. It was found that the outlet delamination of the hole was related to the thrust force generated during the drilling process[7]. For a given tool and material, the thrust force depends on the geometry of the drill bit and the cutting conditions. Some CFRP drilling experiments have been studied for the thrust force and torque generated during the drilling process[7–9]. Lazar and Xirouchakis [10] analyzed the cutting load distribution of the thrust force and torque curves when drilling CFRP using three different drill bits. It was found that the thrust force is mainly affected by the feed rate and tool geometry rather than the spindle speed. Tsao and Hocheng [11] proposed a comprehensive critical thrust force model for circular loading and central concentrate loading. The results show that the critical thrust forces of special drill bits (saw drill, candle stick drill, and core drill) with circumferential drilling torque are lower than that of a drill with central concentrated drilling torque (twist drill). Hocheng and Tsao [12] observed that the delamination caused by drilling a hole is directly related to the thrust force. When the thrust force is lower than the critical thrust, the delamination can be negligible. Therefore, reducing the thrust force in CFRP drilling is
important for reducing the delamination of the hole exit. A large number of studies have investigated the delamination of a hole exit. Zarif Karimi [13] concluded that the quality of a machined hole is highly dependent on the drilling parameters and can be improved by changing the processing parameters [14]. However, with specific cutting parameters, the geometry of the drill bit is an important factor that affects the quality of the drill hole. Yu [15] studied a helical groove on a double-point angle drill bit to achieve secondary removal of the exit burrs and, reduce the tear of the burrs at the hole exit, and inhibit delamination. Díaz-Álvarez [16]studied different drill point angles on bio composite materials. It was proved that the smaller the drill point angle is, the smaller the thrust force is generated, and the smaller the hole exit damage is caused by the machining. Feito [17] compared three different drill bits (a brad center, step drill, and reamer drill). It was found that the reamer drill showed the best results in terms of efficiency and delamination. Qiu [18] used a step drill bit and the ratio of the primary drill diameter to the secondary drill diameter to drill CFRP with different cutting parameters. The experimental results showed that the ratio of the diameter of the primary drill bit to the diameter of the secondary drill bit has a great influence on the hole exit delamination. Tsao and Hocheng [19] studied the effect of the length of the main cutting edge of a drill bit and the diameter of the hole on the delamination. They derived the optimum range of the length of the main edge relative to the diameter of the drill bit. Su [20] proposed a step-controlled drill bit to reduce delamination of the CFRP at the hole exit, and it had good performance in reducing thrust force and damage. These studies show that a smaller cutting edge angle and stage drilling can inhibit tearing and reduce the delamination of the hole exit. Many researchers have proposed an appropriate geometry for drill bits to reduce or eliminate uncut fibers and delamination caused by tearing at the hole exit of CFRP composites. However, there has been no
research on improvements based on conventional drill bits to achieve reduced thrust force and minimize delamination. Therefore, it is necessary to develop a drill bit that minimizes delamination while preserving the advantages of conventional drill bits. In this study, a new drill bit is proposed with a groove on the edge of the drill bit so that the grinding direction is parallel to the flank face of the drill bit, and the relief angle is the same. This design increases the effect of increasing the number of cutting edges, and it is possible to reduce the cutting edge angle and do stage drilling. The thrust force of the multi-groove drill bit was analyzed, and the mechanism of reducing the burr and decreasing the delamination is discussed.
2. Groove grinding and parameters affecting groove shape
The experimental setup for the groove grinding is shown in Fig. 1. The tungsten carbide (composition: WC 88%, Co 12%) double point angle drill bit was selected as the drilling tool, and a diamond single V-tip grinding wheel was used to grind the groove on the secondary edge of the drill. The grinding experiment was performed with a 3-axis machining center (DAEWOO ACE45). The grinding trajectory of the diamond grinding wheel was parallel to the flank face of the secondary cutting edge, and the angle of the V-tip. (
) was 60°. The jig used for the groove grinding on the drill bit can realize the rotation along the
X and Y directions, and the angle of the groove can be realized by adjustment of the jig. The groove depth can be controlled by the movement of the machining center table. The grinding wheel speed, feed rate, grinding depth, and detailed grinding parameters are shown in Table 1.
Fig. 1 Grinding of grooves on drill edges with a diamond wheel
Table 1 Grinding parameters of grooves Processing equipment
DAEWOO ACE V45
Parameters of grinding wheel
Abrasive grain: diamond Granularity: #325 Body; SM45C Diameter: 150 mm
Conditions of grinding
Feedrate:700 mm/min Rotation speed:1000 rev/min Depth of grinding: 5 µm
3. Analysis of multi-groove drill bit
3.1 Thrust force analysis
The multi-groove drill bit differs from the original double point angle drill, which was purchased from a merchant for cutting fiber. The groove was ground on the secondary cutting edge by a diamond grinding wheel. This multi-groove drill bit has two different angles of the cutting edge ( , ) compared to the original drill bit.
is the primary cutting edge, and
The corresponding angles
and
of the
is the secondary cutting edge, as shown in Fig. 2.
edge and
can be obtained from the angle θ and
using equations (1, 2). (1) (2)
Fig. 2 Model of the multi-groove drill bit
When drilling CFRP composites, the total thrust force is affected by multiple elements (γ, ,
,
etc.) acting on the cutting edge. The effective rake angle is used instead of the normal rake angle in the cutting force determination. As shown in Fig. 2, from the vector diagram of the drill bit, the effective rake angle (γ) of the double point angle drill bit can be obtained from the cutting edge point angle ( ) and the chisel edge angle ( ): tan (tan ∙
!
) (#
1,2)
(3)
The effective rake angle of the groove edge is different from the secondary cutting edge. It can be obtained from the cutting edge point angle and the angle between the projection on the end face of the groove edge and the Y-axis by the following equation: tan (tan ∙ tan
cos
) (#
3,4)
(4)
For a given feed rate, the area of an uncut chip can be calculated as demonstrated by Lazar and Xirouchakis (2013). An increase in the secondary point angle of the drill tip means an increase in the thickness of the uncut chips. Moreover, the reduction in the point angle of the drill tip is usually accompanied by an increase in the length of the edge, and the total area of the uncut chips remains the same at a given feed rate. sin ∆0 2
·
.
(5)
1
(6)
!
sin
.
· ∙
1 !
is the depth of a chip, f is the feed rate, and 2
(7) is the area of angle i (i=1,2,3,4). 3 is the radial
length of the edge of angle 1 projected at the end face, and ∆0 is the length corresponding to each angle. The total area of uncut chips can be obtained by substituting the area corresponding to the angle of each drill edge:
2
4·
.
∑6 6 2
(8)
The cutting forces in the tangential direction (parallel to the cutting velocity) of chip flow and the radial directions of the cutting force and friction are expressed as: 78 (9)
: (9) ∙ 2 ; : < ∆0
(9)
78. (9)
:. (9) ∙ 2 ; :.< ∆0
(10)
781 (9)
:1 (9) ∙ 2 ; :1< ∆0
(11)
where the cutting coefficients for each element at a certain height are different because of the varying helix, effective normal rake angle, and oblique angles according to Armarego [22]. : < , :.< , and :1< are edge constants to be evaluated experimentally. The cutting constants : , :. , and :1 are evaluated from an orthogonal cutting database using the oblique transformations .
Fig. 3 Direction of drilling forces
Each force component is shown in Fig. 3. The components of the elemental cutting force components (8= ,8> , and 8? ) can be evaluated in the x, y, and z directions as follows: 8=
8. sin
tan
8 cos
81 cos
8>
8. sin
tan
; 8 tan
8 cot
81 cot
8?
8. sin
; 8. sin
8 cos
; 81 cot
cot
(12) (13) (14)
The total thrust and torque acting on the drill bit can be obtained through the sum of the various drill edges. The total thrust and torque of the two edges can be expressed as: @ℎ3BCD
2∑ 6 6 (8E )!
(15)
@F3GBH
2∑ 6 6 (8 )! ∙ 3 (9)
(16)
3.2 Damage analysis
Fig. 4 shows the drilling model of the original drill bit and multi-groove drill bit penetrating the last layer fiber at the hole exit. As shown in Figs. 4 (a, b), when the original drill bit drills and penetrates the CFRP workpiece, the downward thrust force (8 ) of the drill bit and the resultant force (8 < ) of the radial cutting force begin to tear the uncut fiber at the hole exit. The range of delamination continues to expand by tearing the uncut burrs. As shown in Figs. 4(c, d), when the groove of the multi-groove drill reaches the hole exit position, the cutting forces (8 and 8 ) of the groove edges of the
and
edge to the fiber exit neutralizes the total radial cutting force ( 8 < ). The cutting direction of
is the hole axis, which is opposite to the cutting direction of the edge
are opposite, and the radial cutting force ( 8 )
. The connection point of the edge
and
penetrates the workpiece, which can be regarded as the end of a drilling stage. The
penetration of each groove can be viewed as a drilling stage. At the end of each stage, there are almost no
burrs in the hole exit, and the range of delamination is extremely small.
Fig. 4 Delamination model of the hole exit
When the drill bit cuts into the workpiece, it is assumed that the width (w) of the groove is the total cutting width. The fiber at the hole exit of the original drill is subjected to the thrust force (8 ) and radial cutting force (8 ). Therefore, the resultant force (8 < ) can be expressed as Eq. (17) according to the drill bit point angle. However, the cutting direction of the groove edge ( ) is the axis of the multi-groove drill bit in this drilling process. The cutting force of the multi-groove drill bit on the exit fiber can be expressed as Eq. (18): 8<
8 C#I
; 8 cos
8<
8 C#I
; 8J cos
(17) ; 8 C#I
8 cos
(18)
Fig. 5 Displacement analysis of the hole exit: (a, c) Original drill bit(T1), (b, d) Multi-groove drill bit(T8)
The thrust force model (Eqs. (17) and (18)) of the hole exit was verified by the fiber displacement of the hole exit. As shown in Fig. 5, the displacement range of the fiber at the exit of the CFRP hole was determined by applying the equivalent drilling load to the cutting edges of the original drill bit (T1) and multi-groove drill bit (T8) drill bits, as shown in Fig. 7. The size of the workpiece is 10*10*6 mm3. The four sides of the workpiece are fixed. The diameter of the drill is 6 mm. The drill bit penetrated the workpiece to a depth of 1 mm, and the maximum displacement of the fiber at the hole exit was analyzed by
Creo 3.0 Simulate. The exit fiber displacement of the drill bit was simulated and analyzed when a thrust force of 100 N and a torque of 20 N·m were applied on the cutting edge. The maximum axial displacements of the original drill and multi-groove drill are approximately 2.89 and 2.40 mm, respectively. As shown in Fig. 5 (a), the thrust force of the original drill bit causes the exit fibers to bend outward, and as a result, the displacement further enlarges with the drill bit cut-in (Fig. 5 (c)). However, due to the constraints of the inner ring and the hole wall of the multi-groove drill bit, the fiber is cut off from the middle of the multi-groove drill bit, as shown in Fig. 5 (b). One side of the cut fibers remained inside, while the other side remained on the pore wall. The inner fibers are not affected during the subsequent cutting process, and the displacement of the outlet fibers is small, as shown in Fig. 5 (d). The displacement of the exit fibers is an important factor for fiber tearing at the hole exit. A torn matrix and resin bond failure cause the exit fibers to lose supporting force for the cutting action. The fiber/resin interface near the fracture first breaks and cracks, so the fibers at the crack begin to lose supporting force. Under the action of the thrust force, these unsupported fibers are more likely to propagate cracks to the substrate and eventually form a matrix tear. The cracks further extend to the CFRP hole exit and enlarge the delamination.
4. Experiment and material
A multi-directional woven CFRP composite with a fiber volume fraction of 0.6 was used in an experiment. The composite was provided by the Korea Carbon Convergence Technology Institute. The angle of the carbon fibers and the orientation of each layer are 0° and 90°, respectively. The mechanical properties of the CFRP composite are shown in Table 2. The material was formed by compression curing
under high temperatures and then cut to obtain a desired size of 90 mm × 90 mm × 6 mm. The experiments were performed on a CNC machine (DMC model SS-600), as shown in Fig. 6.
Fig. 6 Experimental setup
Table 2 Material properties of CFRP Longitudinal (σ1t) and traverse tensile strength (σ2t)
840 MPa
Longitudinal (σ1c) and traverse compression strength (σ2c)
570 MPa
Young's modulus
61.5 GPa
Shear modulus
3.7 GPa
Poisson's ratio
0.3
Density
155 kg/m
Rockwell hardness
70–75 HRB
3
An original drill and multi-groove drill were used in the experiment, and both drills had a diameter of 6 mm. The original drill bits were prepared by SJ Tools (S. Korea). Fig. 7 shows the drill geometry information. The drilling performance of the different drill bits was studied with fixed processing conditions. The experimental parameters were a spindle speed of 3,000 rpm and feed rate of 0.1 mm/rev, and all drilling operations were carried out under dry conditions. Both drill bits are double-point angle drill bits with an initial point angle of 120°, a secondary point angle of 90°, and a helix angle of 30°. The geometry and parameters of the drill bits are shown in Fig. 7. The multi-grooved drills have two grooves on each secondary edge. The groove angle (θ) is the angle between the drill axis and the grinding direction of the groove, and the vertical distance from the tip of the groove to the secondary edge is the depth (D) of the groove. A delamination experiment was done to calculate the delamination factor and repeated five times for each cutting parameter for a total of 40 holes. The cutting force measurement during drilling was also repeated five times for each drill bit.
Fig. 7 Drill design parameters
5. Results and discussion
5.1 Thrust force and torque
In the drilling experiment, the thrust force was measured using a Kistler tool dynamometer (9526C2). Fig. 8 shows a comparison of the thrust force curve of T1 and T8. It shows the relationship between the drilling position and the thrust force. The drilling angle of the multi-groove bit is composed of four angles: the primary edge angle, the secondary edge angle, and the two edge angles of the groove. According to the thrust force curve, the thrust force of the multi-grooved drill bit has five stages. In the first stage, the primary cutting edge of the drill enters the composite material, and the thrust increases sharply due to the larger angle of the primary edge. In the second stage, the secondary edge of the drill bit enters into the composite material. The third stage is the entering of the multi-groove cutting edge, which causes very high fluctuation of the thrust force, which has two explanations (An et al., 2015). First, the removal of the CFRP material is mainly through brittle fracture of the fiber and the fiber/resin separation. Second, when the same layer CFRP material is cut, the cutting angle is switched from the secondary edge ( ) to the groove edge (
and
). The frequent change of the cutting angle causes the thrust force to
fluctuate greatly. In the fourth stage, the drill tip enters into the CFRP composite completely, and the thrust force fluctuates due to the heterogeneity of CFRP. The fifth stage is when the drill bit penetrates the workpiece’s outer layer, and the thrust force has a small fluctuation when the groove edge penetrates the workpiece. The point angle (
) of the groove edge ( ) is zero for the multi-groove drill bit (T8), so stable cutting
occurs for a short time when the groove edge ( ) penetrates the last layer of CFRP. After the connection point of the groove edges (
and
) penetrates the last layer of the CFRP, the thrust force continues to
fall. The thrust force curve of the multi-groove drill bit shows that the number of thrust fluctuations of the fifth stage is consistent with the number of grooves.
Fig. 8 Thrust force measurement in CFRP drilling process (T1 and T8)
The experimental results verified that the drilling thrust force model is related to the angle of groove edge, as shown in Fig. 9 and Fig. 10. The corresponding thrust force of each edge during drilling was measured by a measuring instrument. In addition, the influence of different groove parameters on the thrust force was studied. As shown in Fig. 9, the thrust force of the multi-groove drill bits decreases with an increasing depth of groove from 0 to 250 µm (T4 and T5) at the same groove angle (θ=15°). However, it increases with an increasing depth from 250 µm to 1000 µm (T5, T6, and T7). At a groove depth 250 µm, the thrust force decreases as the groove angle increases from 0 to 30° (T2, T4, and T8), but it increases with increasing angle from 30 to 45° (T8 and T10). The thrust force decreases with increasing angle from 0 to 15° (T3,T5) at the groove depth of 500 µm, while it increases with increasing angle from 15 to 45° (T5,T9,T11). The torque is shown in Fig. 10. The effect of the drill on the torque is consistent with the thrust force.
Fig. 9 Thrust force of drill bits
Fig. 10 Comparison of torques of 11 different drill bits
5.2 Damage of hole exit
The thrust force generated by the axial operation of drilling tore the fiber at the hole exit. Moreover, the torque produces outward shear stress that causes radial tear damage of the hole exit fiber. When the effective force (Eqs. (17) and (18)) acting on the fiber is less than the elastic force of the fiber, the drill
edge of the original drill bit cannot effectively cut off the fibers, resulting in burrs and tearing of the outlet surface again by the burrs in subsequent feeding. Fig. 11 shows the hole exit morphology of the original drill bit and multi-groove drill bit. The pictures were taken by a high-speed camera (Phantom Miro C110). The thrust force of the original bit tears the fibers downward, resulting in delamination. In addition, the drill’s cutting edge cannot effectively cut off the burrs caused by tearing at the exit, and the margin edge of the drill can further expand the delamination by tearing through the burrs again, as shown in Fig. 11 (a). As shown in Fig. 11 (b), the multi-groove drilling process can be divided into three states. In the first stage, the primary and secondary edges of the drill bit penetrate the workpiece. In the second stage, the groove of the
edge penetrates the workpiece. In the third stage, the groove of the
edge penetrates
the workpiece. When the third stage of drilling is finished, there is no downward tearing of the outlet fibers by axial force to form delamination. Moreover, when the multi-groove drill penetrates the workpiece’s outer layer, the displacement of the fibers in the hole exit is much smaller than with the original drill bit. The thrust force and the tearing of fibers at the hole’s final forming stage are minimized, which reduces the delamination. When the drill bit penetrates into the workpiece, the thickness of the uncut material of CFRP composites decreases gradually, and the workpiece’s resistance to axial and radial forces becomes worse. The drill edge cannot effectively cut off the fibers, and the exit fibers tear with the feeding of the drill bit, forming initial delamination of the hole exit. The tear depth of the original drill bit and the multi-groove drill bit at the hole exit is shown in Fig. 12 for when the fiber angle is 0°. When the original drill penetrates the hole outlet, the thrust force is much larger than the adhesion force between the fibers due to the lack of effective support at the hole exit, resulting in a large tear depth. The
grooves on the multi-groove bit are distributed on the secondary edge, so stage drilling motion is formed between grooves. The grooves increase the number of cutting edges. Therefore, the groove edge ( ) of the multi-groove drill bit penetrates the last fiber layer, which can be viewed as stage cutting. As shown in Fig. 11(b), when the connection point
edge and
edge penetrates CFRP, the fibers of the hole outlet are
all cut. The total resultant force of the drill edge acting on the outlet fibers is less than the adhesion force between the fibers, and the tear depth is not produced at the hole outlet of the multi-groove drill bit as shown in Fig. 12.
Fig. 11 Damage of exit hole: (a) T1: Original drill, (b) T8: multi-groove drill
Fig. 12 Damage depth of exit hole: (a) T1: Original drill, (b) T8: Multi-groove drill
Fig. 13 Delamination of exit hole: (a) T1: Original drill, (b) T8: Multi-groove drill
Delamination or burrs at the hole exit were observed with an optical microscope (Sometech SV-55), as shown in Fig. 13. The delamination and burrs of the original drill bit (T1) and the multi-groove drill bit (T8: groove angle of 30° and depth of 250 µm) at the hole exit are shown. It can be observed that the original drill bit has no burrs in the first hole, and the burr was removed by tearing because the cutting edge is very sharp, as reported by Yu[15]. With the increase of the number of holes, burrs appear, and there are noticeable tear marks at the junction between burrs and the edge of holes. The tearing of burrs further enlarges the delamination at the hole exit. However, the grooves result in a stage drilling mechanism of the multi-groove drill bit, and the radial cutting forces of the two edges of the grooves are in opposite directions. The first hole of the multi-groove bit (T8) has almost no delamination. As the number of holes increases, tiny burrs are generated in the fiber direction of 0°, with which the tear of the fibers at the hole exit is much smaller than that at 90°. Compared with the original bit, the delamination of the multi-groove drill bit has little influence at the hole exit and can be neglected.
Fig. 14 Delamination factor of hole exit with different drill bits
The delamination of the hole exit was evaluated by the delamination factor (8K ) [24], which is based on the largest diameter of delamination ( 8K
LM= )
of the damaged area:
NOPQ NRSO
（19）
As shown in Fig. 14, the maximum diameter of the delamination was measured using an optical microscope. The delamination factor 8K of the hole exit of all drill bits was compared. The original drill bit had the largest delamination factor, while that of the multi-groove bit differed under different groove parameters. The delamination of the original drill bit was greater than that of the multi-groove drill bit. The delamination of the multi-groove bits tended to decrease as the depth increased at an angle of 15°. At a groove depth of 250 µm, the delamination decreased as the angle increased. At a groove depth of 500 µm, the delamination decreased as the angle increased, and minimum delamination occurs at 30°. Therefore, the multi-groove bit can effectively reduce the thrust force, eliminate the formation of burrs, eliminate
delamination damage caused by the burrs, and significantly improve the surface quality of the hole.
6. Conclusions
In this study, a multi-groove drill bit was developed by forming grooves on the secondary cutting edge. The grooves can reduce the thrust force during drilling. Through experimental comparisons between the original and multi-groove drill bits, the following conclusions are drawn: (1) The groove can change the cutting mechanism and cutting direction of the drill bit. Stage drilling is realized by forming stages between grooves. (2) Compared with the original bit, the multi-groove bits with different parameters can reduce the drilling thrust force. The T8 drill bit has the lowest thrust force and is 37.45% smaller than the original drill bit. (3) The multiple grooves realize a stepping mechanism when the drill bit penetrates into CFRP. When the groove edge penetrates into the last layer, the
edge cuts in the direction of the axis and can
counteract the force that results in cutting outward. Moreover, the fibers at the hole exit can be completely cut off, and holes with less delamination damage can be obtained.
Acknowledgments
This work was supported by the Technology Innovation Program (10053248, Development of Manufacturing System for CFRP (Carbon Fiber Reinforced Plastics) Machining), which is funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).
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Highlights
A new drill bit was proposed where the groove edge is ground on the secondary edge. The thrust force model of drilling for the groove edge of the drill bit was analyzed and established. The influence of the groove edge on the material removal mechanism at the hole was explored. The thrust force is less than the critical thrust force, and no burrs are generated. The groove edge contributes to the defect minimization by the stage drilling action.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: