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Analysis of thrust force and characteristics of uncut fibres at
non-conventional oriented drilling of unidirectional carbon
fibre-reinforced plastic (UD-CFRP) composite laminates
Norbert GEIER1*, Tibor SZALAY2, Márton TAKÁCS3
1,2,3 Budapest University of Technology and Economics, Department of Manufacturing Science and
Engineering, H-1111 Budapest, Műegyetem rkp. 3., Hungary
* Corresponding author’s e-mail: geier@manuf.bme.hu, mobile phone number: +3614362641
Abstract Carbon fibre-reinforced plastic (CFRP) is an often-used structural material in the high-tech
industries, like aerospace, wind turbine, sport, automobile, robotics and military. Due to both the
growing application area of composites, and the advanced construction requirements, the used
thickness of the CFRP plates increases, and the necessity of drilling holes on the sides of the plates
(normal II direction) become even more important. Many researchers studied the machinability of UD-
CFRP using numerous drilling experiments at the normal I direction. However, drilling experiments at
normal II and axial directions were not published yet. The main objective of the present study is to
analyse and discuss the influence of a non-conventional drilling direction on hole-quality parameters
and on the thrust force. Drilling experiments were carried out in unidirectional CFRP at non-
conventional drilling direction, based on central composite inscribed design. Influences of feed rate
and cutting speed were analysed using response surface methodology (RSM) and analysis of variance
(ANOVA) techniques. Characteristics of uncut fibres were analysed using digital image processing
(DIP). The results have proved that the effect of the cutting speed is more significant when drilling
UD-CFRP at the non-conventional drilling direction than at the conventional one. Furthermore, the
specific feed force (kf) in the case of the non-conventional drilling direction was more than three times
higher than the kf in the case of the conventional one.
Keywords CFRP, Machinability, Optimisation, Thrust force, Uncut fibres
Nomenclature
Ac (%) – Area factor
d (mm) – Tool diameter
Ft (N) – Thrust force
kf (N/mm2) – Specific feed force
n (rev/min) – Rotational speed
s* (same as the mean) – Standard deviation
vc (m/min) – Cutting speed
vf (mm/rev) – Feed rate
x (mm) – Lengths of the longest uncut fibre
α (1) – Significance level
θ (°) – Fibre cutting angle
ϕ (°) – Fibre orientation angle
1. Introduction
Carbon fibre-reinforced plastic (CFRP) is an often-used construction composite material in the
aerospace, marine, humanoid-robotics, sport, wind turbine and automobile industries due to its specific
mechanical properties, damage tolerance and corrosion resistance [1–4]. In order to meet the micro
and macro geometrical requirements of the CFRP parts, mechanical machining of these materials are
often necessary. However, the machining behaviour of CFRP is difficult because of the non-
homogeneity, anisotropy of the material, and the intensive abrasive wear-effect of the carbon fibre-
reinforcements on the tool.
Mechanical joining CFRP components is often preferred by rivets and bolts [5], the most common
machined features are therefore through and stage holes [2, 6]. By applying improper machining
technologies, several geometrical errors could occur: like delamination, fibre pull-outs, uncut fibres
and matrix burnings [7]. These errors often cause loss of strength of CFRP components, and often
result in additional machining operations and costs. Researchers and tool-manufacturers offers a wide
range of solutions to decrease geometrical errors by applying (i) back-up support plate [8], (ii) special
tool geometry [5, 9] (iii) special tool patch [10], etc.
The structure of CFRP materials is highly non-homogeneous and the mechanical properties are
anisotropic because of the two components of the composite: the matrix and the fibre-reinforcements.
Because the mechanical properties of the CFRP are not homogeneous, the machinability properties are
therefore highly effected by the cutting directions. Many researchers [11–17] use the fibre cutting
angle (θ - angle between the vector of the cutting speed and the fibre orientation) as an independent
variable to describe the influence of the cutting direction on the machined surface-quality. However,
this one-dimensional factor is insufficient for distinguishing the difference between the possible
drilling directions, as demonstrated in Fig.1.
Fig. 1 Schematic drawings of possibly drilling directions in UD-CFRP: (a) normal I direction, (b)
normal II direction and (c) axial direction.
As can be seen in Fig.1, three different drilling types can be determined based on the orientations of
the axis of the tool and of the fibre-reinforcement: (i) normal I (conventional) direction, where the axis
of the tool is perpendicular to the axis of the fibres, and both of the axis are in the Y-Z plane, (ii)
normal II direction, where the axis of the tool is also perpendicular to the axis of the fibres, but the
axis are in the X-Y plane, and (iii) axial direction, where the axis of the tool is parallel to the axis of
the fibres.
Many researchers studied the machinability of UD-CFRP (unidirectional CFRP) using numerous
drilling experiments at the normal I direction [16, 18–24]. However, drilling experiments at normal II
and axial directions were not published yet, because CFRP plates are normally thin, the axis of the
machined holes are therefore perpendicular to the largest surface of the CFRP plate. However, the
development of high-tech technologies and construction requirements initiated the appearance of thick
CFRP plates. E.g. (i) the maximum thickness of CFRP parts in aircraft structures is 25-40 millimetres
[25, 26], (ii) industrial and robotics industries use thick construction CFRP plates, like SUZENA the
humanoid robot (as can be seen in Fig. 2.). The importance of the present research is the industrial
need for the machining of these thick CFRP plates in different directions (e.g. normal II direction of
Fig.1). This paper also gives a detailed summary about the possible measuring methods regarding the
evaluation of the machined structures.
Fig. 2 SUZENA the humanoid robot has load bearing thick CFRP plates. AUTOMATICA 2016
Exhibition in Munich, Germany.
Many researchers investigated the influence of machining conditions, such as cutting parameters, tool
geometry, material properties, cooling systems, preliminary-manufacturing methods and other
environmental features on the thrust force [6, 7, 12, 13, 17], [23, 27], on the tool wear [16, 28] and on
the hole quality characterized by delamination [21, 24], fibre pull-outs, uncut fibres [12, 13], [18, 29],
surface roughness [7, 13, 29] when drilling CFRP at normal I direction. This research focuses on
investigating the effect of drilling direction of normal II and process parameters (cutting speed and
feed rate) on thrust force and on the characteristics of uncut fibres.
Merino-Pérez et al. [23] investigated the influence of cutting speed on thrust force in the case of
conventional dry drilling of CFRP using uncoated tools. They and other researchers [6, 7, 13] showed
that the effect of cutting speed is not significant during drilling CFRP at normal I direction. It was
found that the feed rate has the most significant influence on the thrust force, followed by the number
of holes and the structure of the CFRP composites [2, 7, 11]. Wang et al. [30] conducted high-speed
drilling experiments in CFRP, and decreased the thrust force with the application of pre-drilled pilot
holes. They suggested a technology, which could solve the problem of uncut fibres, but no results are
CFRP
CFRP
published yet. Li et al. [17] carried out orthogonal cutting experiments in UD-CFRP and investigated
the effect of the fibre cutting angle on the chip formation mechanisms. They classified the chip
formation mechanisms into four types. Furthermore, they showed that the fibres can be cut properly,
when the fibre cutting angle is in the range of 90 and 180 degrees. Wang et. al [22] showed that the
occurrence of uncut fibres depends on the fibre fracture mode. Therefore, it is necessary to understand
the interaction of fibres with the cutting tool. They, as well as [17] pointed out that the fibre fracture
mode is significantly influenced by the fibre cutting angle (θ), by the rake angle (γ) and by the cutting
edge radius (rβ). In the case of θ<90°+γ, the fibres are pushed by the cutting tool. However, in the case
of θ>90°+γ, the fibres slip on the rake face of the cutting tool. Furthermore, larger rε causes bending-
dominated and smaller rβ causes crushing-dominated fibre fracture mode. Thus, smaller cutting edge
radius is preferable when machining fibre-reinforced polymer composites. In the case of small cutting
edge radius (compared with the depth of cut) five different chip formation processes can be defined,
according to Sheikh-Ahmad [31]. These five types depend on the fibre cutting angle and the rake
angle, as can be seen in Fig. 3.
Fig. 3 Cutting mechanisms at orthogonal machining of unidirectional fibre-reinforced composite
materials (k shows the direction of the fibres) [32]
In the case of positive rake angle and fibre cutting angle of 0° (Type I), the cutting mechanism is
delamination- and bending-dominated, as can be seen in Fig. 3(a). The main cutting force is
influenced significantly by the strength of the connection between the fibres and the matrix. However,
in the case of negative rake angle (Type II), the rake face of the cutting tool compresses the fibres and
causes them to buckle. Chips are usually small and discontinuous; the cutting force is usually lower
than in the case of chip formation process of Type I. In the case of fibre cutting angle of 0-90° (Type
III and IV), compression-induced shear and interlaminar shear fracture are dominated. Furthermore,
the cutting resistance of the fibres significantly influences the main cutting force. Cutting mechanism
of Type III is observable at any rake angle, as can be seen in Fig. 3(c) and (d). Furthermore, cutting
edge crushes the fibres, but do not delaminate or bend them. The chip thickness is often greater than
the depth of cut when the fibre cutting angle is around 135° (Type V). In this case, the cutting tool
bends, then crushes the fibres, as can be seen in Fig. 3(f). In the case of drilling UD-CFRP, all of the
introduced chip formation types are observable according to Wang et al. [22].
Delamination is one of the most critical machining induced error in CFRP according to [33], however,
this macro geometrical error is not examined in this study but the characteristics of uncut fibres. Li et
al. [34] showed that fibre burrs are highly influenced by fibre orientation and fibre cutting angle. Xu et
al. [12] drilled holes in UD-CFRP and investigated the exit burr defects. They observed that the uncut
fibres -along the exit hole circumference- are located at fibre cutting angles between 110° and 150°.
Based on the local characteristics along the hole circumference, they introduced a new indicator: the
burr area (AB), which can be observed in Fig. 8(a). Researchers are using the burr area as a quality-
number in order to minimize uncut fibres along the exit of the hole. S. Gaugel et. al [18] introduced
the area of fibre-overlap, expressed by AFO=Anom-A, where Anom is the nominal hole area of the drilled
hole and A is the free area of the hole (as can be see in Fig. 5). However, these quality-indicators (AB,
AFO) are not (hole diameter-) specific ones, therefore a new specific indicator: the area factor (Ac) is
introduced by the authors. which will be discussed in the next chapter.
The main objective of the present study is a comparative machinability analysis of UD-CFRP drilled
at normal I and II directions. In the present work, numerous conventional drilling experiments were
carried out at normal II direction. The experimental results are analysed and discussed, and even
compared with the results of a previous research work (drilling UD-CFRP at normal I direction) [7].
The rest of the paper is organised as follows: first, the experimental setup, the digital image processing
and material tests are presented. Second, the thrust force and characteristics of uncut fibres are
analysed. Finally, the drilling orientations (normal I and II) are compared and discussed.
2. Experimental setup and evaluation methods
2.1 Experimental setup
UD-CFRP workpiece was made by hand lay-up process as follows: 55 unidirectional carbon fibre
layers (344.4 g/m2) and epoxy resin matrix (FM20 resin and MH3124 hardener in ratio of 100:35
consequently) were placed on a mould and the entrapped air was removed by squeezing rollers. The
workpiece was heated to a temperature of 60° in order to accelerate the curing process. Then, the
specimen was cut by a waterjet machine into 110x25x6 mm pieces. Detailed material properties can be
found in Section 2.3.
Fig. 4 Experimental machining setup: (a) back-up support plate (b) workpiece (UD-CFRP), (c) clamp
and (d) KISTLER dynamometer
The drilling experiments were carried out on a Kondia B640 three-axis CNC machine centre (12,000
rpm, 10 kW). Because of the high abrasive wear-effect of the carbon fibres, the machine centre was
equipped with a NILFISK GB733 vacuum cleaner in order to clear off the small particle chips from
the cutting space. An Ø11.2 (mm) TIVOLY 11400111120 uncoated twist drill with a point angle of
118° was used, without any cooling and lubricating fluid. A back-up support plate was applied in order
to restrain deflection of material at the exit of the hole. A KISTLER 9257BA three components
dynamometer with a sampling frequency of 6 kHz was used to measure the thrust force of the drilling
process. Furthermore, the force-data were collected by KISTLER DynoWare, and analysed by
Microsoft Excel and Minitab software. The images of the exit of the holes were taken by a Dino-Lite
AM4013MT digital microscope (magnification: 10-70x, resolution: 1.3 Megapixel, maximum frame
rate: 30 fps) and analysed using IrfanView and Wolfram Mathematica software. The experimental
machining setup can be seen in Fig. 4. Drilling process-parameters were determined using the central
composite inscribed design. The levels of the factors (feed rate and cutting speed) were chosen based
on previous studies and suggestions by tool producers, which are listed in Table 1.
Table 1 Experimental parameters and their levels
Factors
Levels
-21/2
-1
0
+1
+21/2
vc (m/min) - cutting speed
50
64.6
100
135.4
150
vf (mm/min) - feed rate
70
103.7
185
266.3
300
2.2 Digital Image Processing (DIP)
Digital image processing is a favourable and widely-used technique to analyse geometrical damages in
CFRP [18, 20, 35]. In this study, the quality of the drilled holes is characterized by a specific number,
which is defined as area factor (Ac), and expressed by Eq. (1).
(%)100
nom
cAA
A
(1)
, where A is the clear (white area in Fig. 5(c)) and Anom is the nominal hole area of the drilled holes.
Thus, according to the uncut fibres, higher Ac values indicate better-quality and lower Ac values
indicate worst-quality holes. The border of these areas can be seen also in the figure. Both of them are
defined in this study using the digital image segment process which replaces the pixels of the picture
of the exit of the holes with a number between 0 and 1, then places the values into a matrix. Then, the
matrix is analysed and processed in the following two steps: replacement of white-grey (1-0.5) data
into white (step 1), then replacement of black-grey (0-0.5) data into black (step 2). The digital image
segment process used in this study can be seen in Fig. 5.
Fig. 5 Image segment process of drilled CFRP holes: (a) Original image of drilled exit of the hole, (b)
changed white-grey colour using the colour histogram, (c) changed black-grey colour using the colour
histogram. The amount of the white pixels is proportional to the clear area (white) of the drilled hole
(Ac)
step 1
step 2
step 1
step 2
white pixels
A
Anom
a)
b)
c)
2.3 Material properties of applied UD-CFRP
The structure of the applied UD-CFRP composite is highly non-homogeneous and the mechanical
properties are anisotropic, the machinability properties are therefore highly effected by the cutting
directions. In this section, important material properties related to different fibre directions are
discussed.
A detailed mechanical testing was conducted and the following material properties were defined: (i)
tensile strength (in different fibre orientations), (ii) interlaminar shear, (iii) Shore D hardness and (iv)
Charpy impact strength (in different fibre orientations). The schematic drawings of the material tests
can be seen in Fig.6.
Fig. 6 The schematic drawings of the material testing (a) tensile strength (b) interlaminar shear, (c)
Shore D hardness, and (d) Charpy impact strength
Tensile strength (σ) test was carried out on a Zwick Z250 mechanical tensile tester based on the ISO
527-5:1997 standard (v=2 mm/min, T=23°C). Four different setup (fibre orientation angle of 0°, 30°,
60° and 90°) was applied, each test was performed five times. The measured tensile strength results
are listed in Table 2, tensile stress diagrams can be found in the appendices (Fig. 15). As was
expected, higher fibre orientation angle decreases tensile strength. Furthermore, the percentage
difference between the σ0° and σ30° (848 %) is higher than between σ30° and σ60° (124 %) or between
σ30° and σ60° (51 %). This non-linear effect of fibre orientation angle on tensile strength can be a
possible explanation of the non-linear behaviour of experimental data collected through the machining
processes.
Interlaminar shear (τ) test was conducted on a Zwick Z020 mechanical tensile tester based on the
D3846-02 standard (v=1.3 mm/min, T=23 °C, Φ=0°). The test was repeated five times, and the
average was calculated as follows: τ= 19,26 ± 0,76 MPa. Interlaminar shear diagram can be found in
the appendices (Fig.16).
Shore D hardness (SD) was measured using a Zwick H04.3150 tester based on the ISO R 868 / DIN
53505 standard (t=3 s, T=23 °C F=44.48 N). The test was repeated five times, and the average was
calculated as follows: SD= 85,5±1,9. This Shore D hardness is high in the group of composites,
however, the strong wear effect is not caused by the matrix material, but the carbon fibres [13].
Table 2 Effect of fibre orientation angle on material properties of applied UD-CFRP
Material properties
Fibre orientation angle
Φ= 0 °
Φ= 30 °
Φ= 60 °
Φ= 90 °
σ (MPa) ± s* (MPa)
723.00 ± 58,29
76.25 ± 1,94
34.02 ± 1,98
22.48 ± 2,88
C (kJ/m2) ± s* (kJ/m2)
203.18 ± 31.38
33.56 ± 3.44
24.36 ± 0.31
21.08 ± 0.55
Charpy impact (C) test was carried out on a Ceast Resil Impact Junior tester based on the MSZ EN
ISO 179-1 standard (W=15 J, T=23 °C). Four different setup (fibre orientation angle of 0°, 30°, 60°
and 90°) was tested, each test was performed five times. The results of Charpy test are listed in Table
2. The effect of fibre orientation angle on C is significant but not linear, as was showed at the
discussion of tensile strength.
3. Results and discussion
Response surface methodology (RSM) was applied to analyse and optimise the machining process. A
second order polynomial model was applied in this research due to the expected non-linear effect of
analysed factors [3, 7, 36]. The second order RSM model applied in this study is expressed by Eq. (2).
1
1 11
2
1
021 )...,( n
ij
n
ij iij
n
iiiii
n
iiin xxbxbxbbxxxY
(2)
, where Y is the optimisation parameter, xi are the factors, b0, bi, bij and bii are the regression
coefficients of the parameters and ε is a random experimental error. Based on the analysis of the
mathematical model of the process, optimal conditions and effect of the process parameters can be
determined.
The main effects and the significant factors of the analysed process can be determined at a chosen
value of significance level by means of analysis of variance [7, 37, 38]. The null-hypothesis used in
this study is as follows: H0: It can be stated that the analysed factor has no significant effect on the
response variable. The significance level used in this study is α=0.05.
13 drilling experiments in UD-CFRP at normal II direction were carried out based on the CCI design.
The measured data - thrust force (Ft), lengths of the longest uncut fibre (x) and the area factor (Ac) -
were processed and analysed using Microsoft Excel, Minitab and Wolfram Mathematica software. The
experimental design matrix, showing the factors, measured values and absolute values of the asolute
percentage error (APE) can be seen in Table 3. The APE, expressed by equation (3) is used to give
the reliability of the developed RSM models.
i
ii
iYYy
APE
(3)
, where yi is the experimental value (measured parameter) and Yi is the RSM-predicted value. The
average APE of the thrust force is 0.17, of the lengths of the longest uncut fibre is 0.07, and of the area
factor is 0.04. The average APE of the thrust force is relatively high, because the RSM model gives an
uncertain prediction at low feed rate regions (at vf=70 mm/min). The possible reason of this difference
could be the higher force caused by friction at low feed rates [7, 39].
Table 3 The CCI design matrix showing actual variables, experimental values and absolute values of
the absolute percentage error (APE) between the measured and predicted values
No. of
test
Factors
Experimental
values
Absolute percentage
error (APE)
vf
vc
Ft
x
Ac
Ft
x
Ac
(-)
(mm/min)
(m/min)
(N)
(mm)
(%)
(-)
(-)
(-)
1
70.0
100.0
63.2
54.8
90.6
0.93
0.02
0.02
2
103.7
64.6
122.0
70.4
89.5
0.22
0.00
0.04
3
185.0
100.0
148.0
46.2
92.5
0.39
0.22
0.13
4
103.7
135.4
104.4
56.7
87.6
0.13
0.02
0.01
5
300.0
100.0
260.2
30.9
95.4
0.07
0.14
0.03
6
185.0
100.0
274.1
61.4
81.8
0.14
0.04
0.00
7
185.0
150.0
229.5
61.7
87.6
0.03
0.04
0.04
8
266.3
135.4
266.3
57.5
84.1
0.07
0.11
0.05
9
266.3
64.6
375.5
53.6
87.2
0.00
0.09
0.00
10
185.0
100.0
239.1
62.9
80.5
0.01
0.06
0.02
11
185.0
100.0
266.8
62.6
74.4
0.11
0.06
0.09
12
185.0
100.0
270.0
64.3
80.9
0.12
0.09
0.01
13
185.0
50.0
370.9
71.6
78.6
0.05
0.02
0.03
3.1 Analysis of thrust force
The thrust force (axial cutting force component) was measured and analysed in order to evaluate the
machinability of UD-CFRP at normal II direction. According to Jia et. al [5], the analysis of thrust
force is necessary in order to understand and describe the removal mechanisms at the exit of the hole.
Discrete Fourier transformation (DFT) was applied to decrease the noise effect. The effect of cutting
speed (vc) and feed rate (vf) on the filtered thrust force can be seen in Fig.7(a) and expressed by Eq.
(4). The goodness-of-fit of the developed RSM based model can be explained by the following
regression coefficients: R2=86.72 %, adjR2= 77.23 %.
cfcfcfcf vvvvvvvvFt 00796.00215.000641.098.324.46),( 22
(4)
From the surface plot of Fig.7(a), it is clear, that the feed rate increases the thrust force. It is well
known that the larger the feed rate the larger the chip-section is, which causes the increase of the
cutting force [37]. The effect of the feed rate on the thrust force can be seen on the main effects plot in
Fig.7(b). The ANOVA table (Table 3) shows that the feed rate has the most significant impact on the
thrust force (F-value: 28.79, P-value: 0.001).
Fig. 7 Effect of cutting speed (vc) and feed rate (vf) on thrust force (Ft): (a) response surface and (b)
main effects plot for Ft
Drilling CFRP at higher cutting speed causes higher cutting temperature, which results in a decrease of
the elastic modulus of the resin matrix, therefore the thrust force decreases in the case of drilling
CFRP at higher cutting speed. The effect of the cutting speed is highly non-linear, as can be seen in
the response surface and on the main effects plot in Fig.7(b). These non-linear effect of the cutting
speed was observable also, when drilling UD-CFRP at normal I direction [7]. The effect of the cutting
speed at drilling UD-CFRP at normal I direction is low (F-value: 0.01, P-value: 0.906) [7], but in this
study the effect of the cutting speed is more considerable (F-value: 6.38, P-value: 0.039). The possible
reason of this difference can be explained considering the different reinforcement directions, from the
point of view of the axis of the cutting tool. Future work is needed in order to find a more detailed
explanation of this difference.
Based on the present research, in the case of conventional drilling UD-CFRP at normal II direction,
the optimal machining process parameters for minimizing thrust force are feed rate of 70 mm/min and
cutting speed of 105 m/min.
Table 3 ANOVA table for thrust force (Ft) versus cutting speed (vc) and feed rate (vf)
Source
DF
Adj SS
Adj MS
F-Value
P-Value
Model
5
95573
19115
9.14
0.006
Linear
2
73546
36773
17.58
0.002
vf
1
60202
60202
28.79
0.001
vc
1
13344
13344
6.38
0.039
Square
2
19930
9965
4.76
0.049
vfvf
1
12515
12515
5.98
0.044
vcvc
1
5010
5010
2.40
0.166
Interaction
1
2097
2097
1.00
0.350
vfvc
1
2097
2097
1.00
0.350
Error
7
14640
2091
Total
12
110213
3.2 Analysis of hole quality
The quality of the drilled holes is described in this study by the length of the longest uncut fibres (x) at
the exit of the holes, and by the area factor (Ac). The segmented images of the exit of the holes in UD-
CFRP can be seen in Fig.9. It can be observed from the images that the burr area (where the uncut
fibres occur) is observable at the fibre cutting angle of 110°-160°. Xu et al. [12] have observed the
same phenomena, when drilling high-strength multi-directional (MD) CFRP with PCD twist drill. The
possible reason of the position of the burr area is, that the fibre fracture mode is bending-dominated at
these fibre cutting angles [22], the cutting edge can not cut the fibres properly, because they buckle.
As can be seen in Fig. 9, the hole made by the highest feed rate (5th test) has less uncut fibres that the
others. However, Xu et al. [12] presented the opposite, when drilling MD-CFRP at normal I direction:
the burr area grows with the feed rate. Due to the direction of the drilling (normal II), the cutting edge
gets in contact with the fibre reinforcements in a different way (as can be seen in Fig. 1 and Fig. 12),
therefore the burr area has more separated uncut fibre-groups, as illustrated in Fig. 8(b). The separated
fibre-groups in the burr area can be controlled possibly not just by the cutting process parameters, but
especially more by the ratio of the matrix and reinforcement, when pre-manufacturing the workpiece.
This phenomenon is described more deeply in chapter 3.3 discussion.
Fig. 8 Burr area when drilling CFRP at (a) normal I [12] and at (b) normal II direction (k shows the
direction of the fibres)
Test Nr. 2
Test Nr. 1
Test Nr. 5
Test Nr. 6
Test Nr. 7
Test Nr. 3
Test Nr. 4
Test Nr. 8
Fig. 9 Segmented images of the hole exits in UD-CFRP
The effect of cutting speed (vc) and feed rate (vf) on the longest uncut fibres (x) can be seen in
Fig.10(a) and expressed by Eq. (5). The measured values can be seen in Table 2. The goodness-of-fit
of the developed RSM based model can be explained by regression coefficients of R2=76.98 % and
adjR2= 60.54 %.
cfcfcfcf vvvvvvvvx 00152.000383.0001076.0133.1169.07.111),( 22
(5)
Fig. 10 Effect of cutting speed (vc) and feed rate (vf) on uncut fibres (x): (a) response surface and (b)
main effects plot for x
The effect of the feed rate on the longest uncut fibres can be seen on the main effects plot in Fig.10(b).
The data of ANOVA table (Table 4) prove that the feed rate has the most significant impact on the
uncut fibres (F-value: 6.96, P-value: 0.034), followed by the cutting speed (F-value: 1.58, P-value:
0.249). The effect of the cutting speed is also highly non-linear, as was observed at the optimisation
parameter of thrust force. The interaction terms of uncut fibres are more significant (F-value: 1.73, P-
value: 0.230) than in the case of the thrust force, therefore (i) the feed rate decreases the uncut fibres at
low cutting speed (ii) the cutting speed increases the uncut fibres, after a local minimum point. Other
researchers [12, 20, 35] observed that the feed rate increases the number of uncut fibres and the rate of
Test Nr. 9
Test Nr. 10
Test Nr. 11
Test Nr. 12
Test Nr. 13
the push-down delamination. However, they drilled CFRP at normal I direction. Future work is needed
in order to find the reason of these differences.
Based on the present research, in the case of conventional drilling UD-CFRP at normal II direction,
the optimal machining process parameters for minimizing uncut fibres are feed rate of 300 mm/min
and cutting speed of 88 m/min.
Table 4 ANOVA table for uncut fibres (x) versus cutting speed (vc) and feed rate (vf)
Source
DF
Adj SS
Adj MS
F-Value
P-Value
Model
5
1040.03
208.01
4.68
0.034
Linear
2
379.39
189.69
4.27
0.061
vf
1
309.13
309.13
6.96
0.034
vc
1
70.26
70.26
1.58
0.249
Square
2
583.93
291.97
6.57
0.025
vfvf
1
352.26
352.26
7.93
0.026
vcvc
1
159.84
159.84
3.60
0.100
Interaction
1
76.71
76.71
1.73
0.230
vfvc
1
76.71
76.71
1.73
0.230
Error
7
311.02
44.43
Total
12
1351.05
Table 5 ANOVA table for the area factor (Ac) versus cutting speed (vc) and feed rate (vf)
Source
DF
Adj SS
Adj MS
F-Value
P-Value
Model
5
201.322
40.264
1.21
0.393
Linear
2
7.677
3.838
0.12
0.892
vf
1
0.142
0.142
0.00
0.950
vc
1
7.535
7.535
0.23
0.648
Square
2
193.287
96.643
2.91
0.120
vfvf
1
192.268
192.268
5.80
0.047
vcvc
1
0.652
0.652
0.02
0.892
Interaction
1
0.358
0.358
0.01
0.920
vfvc
1
0.358
0.358
0.01
0.920
Error
7
232.197
33.171
Total
12
433.518
The data of ANOVA table (Table 5) identified that the cutting speed (vc) and the feed rate (vf) does
not have any statistical (α=0.05) influence on the area factor (Ac). F-values of the independent
variables have significantly lower values than the critical F-value: Fcrit(0.05,1,11)=4.8443, the null-
hypotheses are therefore fail to reject. ANOVA results proved that vf2 has a significant influence (F-
value: 5.80, P-value: 0.047) on the response variable (Ac), so the effect of vf on the Ac is non-linear, as
can be seen in the main effects plot in Fig. 11.
Fig. 11 Main effects plot for Ac (%)
The highly non-linear effect of vf can be explained by the following: (i) higher feed rate results in a
higher chip section, which causes higher cutting force and higher friction according to [7].
Furthermore, higher friction causes higher cutting temperature. The thermal conductivity of epoxy
resin matrix is low (compared with metals), so higher cutting temperature decreases the stability of
matrix material, the fibres can therefore separate from the matrix more easily (fibres are not crushed
but bended). This can be a possible reason that the higher the feed rate lower the Ac. (ii) However, in
the case of higher feed rates (vf >185) Ac is likely to be increased due to the higher cutting temperature,
which decreases the stability of the fibres, too, so they can be crushed more effectively than in the case
of low feed rates. In the future, additional experiments are required to analyse the effect of vf in more
detail (full factorial test, cutting temperature measurement, etc.).
Based on this study, in the case of conventional drilling UD-CFRP at normal II direction, the
machining process is optimal (from the point of view for maximizing the area factor) when both feed
rate and cutting speed are at maximum.
3.3 Discussion
The specific feed force (kf) is a key parameter showing the specific resistance of the material to the
axial cutting force (thrust force, as defined in [40]), as expressed by kf=Ft (fz d)-1, where Ft is the thrust
force, fz is the feed per tooth and d is the tool diameter. Results of previous drilling experiments [7] in
UD-CFRP at conventional direction (normal I) were applied to calculate the specific feed force, andto
make a comparison with the results of this study (Fig. 12.)
Fig. 12 Specific feed force vs drilling directions in UD-CFRP
It can be stated that the difference of the specific feed force values is significant. the specific feed
force belonging to the normal II direction is more than three times higher than the specific feed force
that belongs to the normal I direction. The reason of these differences can be found in the different
geometric composition of the fibres and the geometry of the cutting edge, as can be seen in Fig. 13.
Fig. 13(a) demonstrates, the cutting tool edge meets just one group of reinforcement fibres (on Fig. 13
(a): 1. group) at a certain hole depth. However, the cutting tool edge get in contact with more group of
fibres (on Fig. 13 (b): 7.,6.,5.,4… groups) in the case of drilling at normal II direction, which can be
observed in Fig. 13 (b).
As was mentioned before, (i) the distribution of uncut fibres in the burr area is balanced [7, 12] when
drilling at normal I direction, and (ii) the burr area contains separated uncut fibre groups, when drilling
at normal II direction (as illustrated in Fig. 8.). These can be explained by the fact that at the exit of
the hole (considering the same fibre cutting angle) there is a bigger space between the fibre groups
(rowing) when drilling at normal II direction, than the space between the fibres, when drilling at
normal I direction, as can be seen in Fig. 13. It is clear from the model (Fig. 13.) that decreasing the
distance between the fibre groups (rowing) causes balanced distribution of uncut fibres in the burr
area, too, when drilling at normal II direction.
Fig. 13 Schematic drawings of drilling directions in UD-FRP at (b) normal I and (c) normal II
directions.
Based on the chip formation mechanisms (explained in the introduction), dominant chip formations
can be associated to the different fibre cutting angles when drilling UD-CFRP. As can be seen in Fig.
14(a), Type I, III, IV and V fibre cutting type chip formations dominate, when drilling UD-CFRP at
the conventional direction. During one rotation of the twist drill, the cutting edge continuously touches
the fibre-reinforcements, as can be seen in Fig.13(a). The reviewed chip formation types therefore
satisfactorily describe the cutting mechanisms. However, in the case of drilling UD-CFRP at the
normal II direction, cutting of matrix has a more significant effect on the cutting mechanisms, because
in certain cases the cutting edge gets in contact only with the matrix, as can be seen in Fig.13(b). Due
to this unique phenomenon, the chip formation is more influenced by the matrix material when drilling
at normal II direction. Fig.14(b) model shows that the matrix plays an important role at the chip
formation. With this model, the higher specific cutting force can be explained in the following way. It
is well known that interrupted machining causes higher mechanical strain, therefore the specific
cutting force will possibly be higher in the case of drilling UD-CFRP at normal II direction.
Furthermore, there is a difference in resistance of matrix material and fibre reinforcement against
cutting, which causes considerable vibrations affecting hole damages and change in thrust forces.
However, the verification of these statements requires further investigation.
Fig. 14 Dominant chip formation types vs fibre cutting angle in the case of (a) normal I and (b)
normal II drilling directions (k shows the direction of the fibres)
Future directions regarding the present research work are as follows: (i) a huge number of holes in the
aircraft industries have to be machined, where the speed is one of the key factors influencing the
manufacturing performance. The future goal is to increase machining performance by increasing the
feed rate. However, it is also important to increase quality of the holes with using more special cutting
tools. (ii) Push-down delamination and uncut fibres at the exit of the hole can be decreased by
applying back-up support plate (as was used in this study, too). However, in the case of industrial
applications, usually this plate cannot be applied due to the huge elements needed to be machined.
Future investigation is required in order to find other solutions for the industry supporting the back
layers of CFRPs. (iii) Drilling process monitoring and diagnostics are suggested in order to ensure the
high machining quality. Cutting force measurement has already been suggested by many researchers
in order to monitor drilling process (tool wear), but the implementation in industrial environment
raises many problems. Based on the present study a future research work regarding the characteristics
of uncut fibres is planned ensuring monitoring and diagnostics of the hole making process.
5. Conclusions
In the present study, machining experiments were carried out in unidirectional CFRP at normal II
direction (where the axis of the tool is perpendicular to the axis of the unidirectional fibres, and the
axis are in the plane of the UD-CFRP plate), using a twist drill in order to analyse and compare the
effect of different drilling directions. According to the present study, the following conclusions can be
drawn:
The results of ANOVA have proved that the effect of the cutting speed is more considerable
when drilling UD-CFRP at normal II direction than in the case of conventional (normal I)
direction. It was also obtained that higher feed rate increases the thrust force.
It was found that the hole machined by the highest feed rate (300 mm/min) has less uncut
fibres than the others do. Furthermore, it was concluded that the feed rate decreases the uncut
fibres at low cutting speeds (50-100 m/min).
Optimal machining process parameters for (i) minimizing thrust force are feed rate of 70
mm/min and cutting speed of 105 m/min; (ii) for minimizing uncut fibres are feed rate of 300
mm/min and cutting speed of 88 m/min; and (iii) for maximizing the area factor, are on
maximum of both process parameters.
It was shown that the burr area has separated uncut fibre-groups when drilling UD-CFRP at
normal II direction, because the cutting edge gets in contact with separated fibre groups in the
case of drilling at normal II direction.
Concerning UD-CFRP, the specific feed force belonging to the normal II direction is more
than three times higher than the value that belongs to the normal I direction. The reason of
these differences can be found in the different geometric composition of the fibre groups and
in the geometry of the cutting edge.
In the future, additional experiments and more detailed analysis are required on the following
topics: (i) influence of cutting speed on the cutting force in the case of different drilling
directions. It is necessary to prove the fact that the effect of vc is larger in the case of normal II
than at normal I direction. (ii) In order to explain more detail the non-linear effect of vf and the
higher specific cutting force (normal II), full factorial drilling experiments in different drilling
directions are planned with cutting temperature and tool vibration measurements.
Acknowledgement The authors would like to acknowledge the support provided by the CEEPUS III
HR 0108 project. This research was partly supported by the EU H2020-WIDESPREAD-01-2016-
2017-TeamingPhase2-739592 project “Centre of Excellence in Production Informatics and Control”
(EPIC). This work was partly supported by the Higher Education Excellence Program of the Ministry
of Human Capacities in the frame of Nanotechnology and Material Science research area of Budapest
University of Technology and Economics (BME FIKP-NANO). Furthermore, the authors
acknowledge to prof. Gyula MÁTYÁSI, Norbert FORINTOS and to András TŐKE for their
participation in the experimental work.
Appendices
Fig. 15 Tensile stress of applied UD-CFRP in different fibre orientations: (a) ϕ=0°, (b) ϕ=30°, (c)
ϕ=60° and (d) ϕ=90°
Fig. 16 Interlaminar shear of applied UD-CFRP, ϕ=0°
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