MultiCraft
International Journal of Engineering, Science and Technology
Vol. 6, No. 4, 2014, pp. 1-10
INTERNATIONAL
JOURNAL OF
ENGINEERING,
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TECHNOLOGY
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Evaluation of machinability index on milling of GFRP Composites with
different fibre orientations using solid carbide endmill
with modified helix angles
M. P. Jenarthanan1*, R. Jeyapaul2
2
1*
School of Mechanical Engineering, SASTRA University, Thanjavur, INDIA
Department of Production Engineering, National Institute of Technology Trichy, INDIA
*
Corresponding Author: e-mail: jenarthanan@mech.sastra.edu Tel 09952552924.
Abstract
Nowadays, glass fibre reinforced plastics (GFRP) plays a vital role in many engineering applications as an alternative to
various heavy exotic materials. However, the users of FRP are facing difficulties to machine it, because of fibre delamination,
fibre pullout, short tool life, matrix debonding, burning and formation of powder like chips. This research work aims to find out
the machinability in milling process of GFRP laminates manufactured by hand lay-up. A plan of experiments based on Taguchi
was established and the milling was performed with prefixed cutting parameters. An analysis of variance (ANOVA) has been
employed to investigate the effect of cutting parameters on specific cutting pressure and surface roughness. The objective is to
evaluate the machinability of GFRP laminates in function of fibre orientation (15°,60° and 105°) and solid carbide end mill
tools with 25°, 35° and 45° helix angle and to propose a new machinability index.
Keywords: GFRP, Helix angle, fibre orientation, machinability.
DOI: http://dx.doi.org/10.4314/ijest.v6i4.1
1. Introduction
Composites are one of the most widely used materials because of their adaptability to different situations and the relative ease
of combination with other materials to serve specific purposes and exhibit desirable properties. Glass fibre reinforced plastics
(GFRP) composite is considered to be an economic alternative to various heavy exotic materials. Glass-fibre-reinforced plastic
(GFRP), an advanced polymeric matrix composite material, the use of GFRP composites in engineering applications such as
automotive, aircraft and manufacture of space ships and sea vehicles industries have been increased considerably in recent years
due to their light weight, high modulus, specific strength, superior corrosion resistance, high fracture toughness and resistance to
chemical and microbiological attacks (Hull & Clyne, 1996). GFRP composite materials are extremely abrasive when machined.
Thus the selection of the cutting tool and the cutting parameters is very important in the machining process. Fibre-glass is simply a
composite consisting of glass fibres, either continuous or discontinuous, contained within a polymer matrix. The machining of
composite is different from the conventional machining of metal due to the composite’s anisotropic and non-homogeneous nature
(Ramkumar et. al, 2004). Milling is one of the most important machining processes in manufacturing parts made out of FRPs.
However, unlike the milling of metals which is characterized by high material removal rates, milling of FRPs is conducted at much
lower scale. The reason for this is that FRP components are largely made near net shape and any subsequent milling is limited
mainly to de-burring and trimming as well as to achieving contour shape accuracy (Sreejith et. al, 2000). Milling composite
materials are significantly affected by the tendency of these materials to delaminate under the action of machining forces, cutting
force, feed force and depth force respectively (Ferreira et al, 1996 & Paulo Davim & Resis, 2004).
The success of the machining operation is mainly affected by the properties of the workpiece materials. These properties and
other characteristics of the work are summarized in the term “machinability”, with denotes the relative ease with which a material
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Jenarthanan and Jeyapaul / International Journal of Engineering, Science and Technology, Vol. 6, No. 4, 2014, pp. 1-10
can be machined using appropriate tooling and cutting parameters (Groover, 1996 & Paulo Davim et al, 1999).Surface roughness
and specific cutting pressure are two important criteria used for evaluate the machinability of a composite material (Koplev et. al,
1983). The surface roughness and delamination is the characteristic that could influence the dimensional precision, the
performance of mechanical pieces and production costs. For these reasons there has been research and development with the
objective of optimizing cutting conditions, to obtain a desired machinability (Kaneeda, 1989 & Puw and Hocheng, 1995). The
works of various authors, when reporting on milling composite materials, have shown that the surface quality (surface roughness),
machining force and delamination factor is strongly dependent on cutting parameters, tool geometry and cutting forces
(Santhanakrishnan et. al, 1988 and Ramulu et. al, 1994.).
A new machinability index was proposed by Paulo Davim and Mata (2005 & 2007) for the turning of hand laid up GFRP
materials using polycrystalline diamond (PCD) and cemented carbide (K15) cutting tools. The investigation reveals that the PCD
tool performs well compared to cemented carbide (K15) tool in terms of surface roughness and specific cutting pressure. Fibre
orientation is a key factor that determines the surface integrity of a machined surface and 90° is a critical angle, beyond which a
severe subsurface damage will occur. If the fibre orientation angle is greater than 90°, the three distinct deformation zones appear
namely chipping, pressing and bouncing (Wang & Zhang, 2003). Palanikumar et al. (2006) have also attempted to assess the
influence of machining parameters on surface roughness in machining GFRP composites, it concludes that the feed rate has more
influence on surface roughness and it is followed by cutting speed.
This paper investigates the machinability of GFRP composites through the milling process (machining force) and the workpiece
(surface roughness) in function of Fibre orientation (15°,60° and 105°) and cutting tool (solid carbide end mill with 25°,35° and
45° helix angles). A new machinability index will be established in function of machining force and surface roughness.
2. Materials and experimental procedure
Glass fibre reinforced plastics (GFRP) composite plates made by Hand lay-up method are used for these experiments. GFRP
plates are of 150 mm x 100 mm x 3 mm thick with 12 lay-up with desired fibre orientation (15°, 60° and 105°) are used for the
milling operations. The cutting tool is made up of solid carbide tool of 5 mm diameter. The solid carbide end mill of different helix
angles (25°, 35°, and 45°) are shown in Fig. 1.
The fibre orientations are defined in clockwise with reference to the cutting direction as shown in Fig. 2. The experiments are
conducted based on Taguchi L9 orthogonal array using a CNC milling machine. The CNC milling machine is automated
machinery, and its specifications are given in Table 1. The fixation of the composite material in the machining centre is as shown
in Fig. 3 to make sure that vibrations and displacement are eliminated.
Fig. 1 Solid carbide end mill with different helix angles
Fig. 2 Notation of the fibre orientation with respect to cutting tool movement.
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Jenarthanan and Jeyapaul / International Journal of Engineering, Science and Technology, Vol. 6, No. 4, 2014, pp. 1-10
Table 1 Specification of the CNC milling machine
Type of machine
Make
Table size
Spindle motor power
Spindle speed
Feed
X axis
Y axis
Z axis
Accuracy (Positioning)
Vertical machine centre
Hartford, Taiwan
810 x 400 mm
7.5 KW
60-8000 rpm
1-7000 mm/min
510 mm
400 mm
400 mm
± 0.005/300 mm
Fig. 3 Fixation of GFRP composite plate by using clamps in the machining centre.
•
•
•
•
Work piece fibre orientation angle,
Helix angle,
Feed rate, and
Spindle speed
A detailed analysis has been carried out to fix the lower and upper limits of the factors. Based on the analysis, the upper and
lower limits of the factors are identified and are given below. Previous studies indicated that, the surface roughness increases with
increase of spindle speed. But very high spindle speed was found to cause a large deformation rate of glass fibre and it produces
imperfection in surface and hence the spindle speed has been fixed between 2000 and 6000 rev/min. The fibre orientation angle
plays an important role for deciding the surface roughness. The previous results indicated that fibre orientation angle is one of the
important aspects, which affect the surface roughness. For the present study, the fibre orientation angle considered is between 15o
and 105o. Feed rate is the main parameter, which affect the machining process. The increase in feed rate increases the chatter and it
produces incomplete machining at a faster traverse, which led to higher surface roughness and hence low limit of feed rates are
advisable in machining GFRP composites. In the present study, the feed rate is selected between 0.04 and 0.12 mm/rev. The helix
angle also an another important parameters which affects on machining force and surface roughness and hence three different helix
angles (25o, 35o, and 45o) are chosen in this study. The identified parameters and their levels chosen are summarized in Table 2.
Table 2 Process control parameters and their levels
Process parameters
Fibre orientation angle
Helix angle
Spindle speed
Feed rate
Units
o
(degrees)
o
(degrees)
rpm
mm/rev
Notation
Φ
Ө
N
F
1
15
25
2000
0.04
Levels
2
60
35
4000
0.08
3
105
45
6000
0.12
Jenarthanan and Jeyapaul / International Journal of Engineering, Science and Technology, Vol. 6, No. 4, 2014, pp. 1-10
4
3. Measurements
The Output responses considered in this study are surface roughness (Ra) and machining force (Fm). The measurement and
calculation of responses based on the input parameters are described below.
3.1 Measurement of Surface Roughness
The surface roughness value of the machined surfaces is measured in order to analyze the surface finish quality. The surface
roughness of a machined product could affect contact causing surface friction, wearing, light reflection, heat transmission, the
ability for distributing and holding a lubricant, coating, and resisting fatigue. The surface roughness (Ra) was evaluated using
stylus type profilometer Mitutoyo SJ-201. For each test, five measurements were made over milling surfaces, according to Fig. 4.
Considering the number of measurements to be carried out, a programmable technique was used, by previously selecting a
roughness profile, the cut-off (0.8 mm) and the roughness evaluator parameter (Ra) according to ISO.
Fig. 4 Diagram of measurement that were made over each milling surface
3.2 Machining force
Force measurement in manufacturing, especially in machining, is very important. This is because:
•
•
Force measurement can be used for monitoring the tool conditions, and avoiding breakage during the machining process.
Force measurement helps us understand machining process, because cutting force is one of the most sensitive indicators
of machining performance. Both the static and dynamic components of the cutting force contain information concerning
the state of chip formation and the cutting tool.
Force measurement enables engineer to optimize manufacturing process and design proper machining tool.
•
The force measurement was carried out using a Kistler dynamometer. The data acquisition was carried out by appropriate
software called Dynawarekistler. The value of machining force in the work piece is determined using the Eq. 1.
Fm = Fx2 + Fy2 + Fz2
(1)
3.3. Measurement of Machinability Index (MI)
Machinability Index of these GFRP composites have been calculated in terms of specific cutting pressure (Ks) in N/mm²which
has been evaluated from the machining force (Fm), using the following equation (2)
Ks =
Fm
Fm
=
,
S
f xd
(2)
Where, Fm the machining force in N, f the feed rate in mm/rev and d depth of cut in mm. The new machinability index (MI) was
defined with the following equation:
 1  1 
 x 10 3 ,
MI = 

 Ksα  Raβ 
Where, Ks the specific cutting pressure in N/mm2, Ra the surface roughness in mm, α and
α and
β
(3)
β
the parameters weigh. The values of
have been obtained to provide a similar contribution for Ks and Ra in MI. The relation between the Ks and Ra allows to
calculate α = 130 and
β
= 1, for the results of this study.
3.4. Results and discussion
The machinability in this work was evaluated by surface roughness (Ra) of the machined surface of the work piece and
machining force. The results obtained through experiments are presented in Table 3.
Jenarthanan and Jeyapaul / International Journal of Engineering, Science and Technology, Vol. 6, No. 4, 2014, pp. 1-10
5
Test
1
2
3
4
5
6
7
8
9
θ
(°)
25
25
25
35
35
35
45
45
45
Table 3 Values of Fm and Ra in function of the cutting parameters
(Φ) 15°
60°
N
f
Fm
Ra
Fm
Ra
(rpm)
(mm/rev)
2000
0.04
15.27
0.81
26.24
1.08
4000
0.08
20.78
0.95
28.16
1.25
6000
0.12
22.79
1.27
30.14
1.29
2000
0.08
19.25
1.1
24.32
1.62
4000
0.12
21.64
1.18
31.84
1.69
6000
0.04
14.21
0.92
21.19
1.48
2000
0.12
20.79
1.59
32.15
1.82
4000
0.04
13.54
1.31
20.79
1.58
6000
0.08
18.15
1.35
25.62
1.62
105°
Fm
39.24
37.25
59.61
39.25
41.52
29.72
38.62
29.15
19.26
Ra
1.39
1.64
1.72
1.98
2.08
1.68
2.48
2.06
2.12
ANOVA is carried out for the level of significance of 5% (the level of confidence is 95%). Table 4 shows the result of ANOVA
with Fm during the machining of GFRP composite plates. From the analysis it is observed that the feed rate factor and helix angle
factor have statistical and physical significance on the machining force obtained, especially the feed rate. The effect of the factor
spindle speed is practically insignificant. It is also clear that the feed rate factor has a higher influence on the results for the fibre
orientation 15°(51.26%) than the 60° and 105° fibre orientation (47.95% and 46.26%). Table 5 shows the results of the ANOVA
with Ra. From the analysis it is clear that the feed rate factor has a higher influence for the obtained Ra followed by the helix angle
factor. It can be evidenced that Ra is more influenced by the feed rate than the Fm.
Table 4 Evaluation of machinability index for different fibre orientation
Machinability index (MI)
θ
N
f
Test
(°)
(rpm)
(mm/rev)
15°
60°
105°
1
25
2000
0.04
0.561325
0.275237
0.143005
2
25
4000
0.08
0.803202
0.443182
0.255361
3
25
6000
0.12
0.888477
0.601843
0.228228
4
35
2000
0.08
0.736718
0.395955
0.200733
5
35
4000
0.12
0.916382
0.434867
0.270954
6
35
6000
0.04
0.59664
0.248715
0.15622
7
45
2000
0.12
0.707888
0.399911
0.244316
8
45
4000
0.04
0.436417
0.237456
0.129894
9
45
6000
0.08
0.661157
0.375864
0.382061
Table 5 ANOVA for the machining force (Fm) for the three composite materials
Degree of
SDQ
Variance
Test F
Fα=5%
Percentage
Source of
variance
freedom
contribution (P)
15 degree fibre orientation angle
θ (°)
2
6.811 0.123
3.406
6.96
19
36.63
N (rpm)
2
85.803 0.092
0.061 42.902 0.33
19
1.97
f (mm/rev)
2
92.829
0.046
9.74
19
51.26
Error
2
10.14
Total
8
100
60 degree fibre orientation angle
2
9.880
4.940
5.97
19
31.42
θ (°)
2
0.7341
0.0867
0.45
19
2.31
N (rpm)
f (mm/rev)
2
113.989
56.995 0.406
9.11
19
47.95
Error
2
12.812
18.33
Total
8
135.415
100
105 degree fibre orientation angle
4.7814
5.74
19
30.21
θ (°)
2
10.57
0.0912
0.52
19
2.73
N (rpm)
2
0.8502
128.28
64.14
8.79
19
46.26
f (mm/rev)
2
2
13.762
0.522
20.8
Error
100
152.705
Total
8
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Jenarthanan and Jeyapaul / International Journal of Engineering, Science and Technology, Vol. 6, No. 4, 2014, pp. 1-10
Table 4. shows the evaluation of MI in function of the feed rate for different fibre orientation. MI decrease with the fibre
orientation for different feed rates and present a maximum for 15°.It is realized that the GFRP composite with 15°fibre orientation
provides a better MI (average MI=0.7) in comparison to 60°and 105° fibre oriented GFRP composites (average MI=0.38 and
0.19).This is due to the smaller values of Ra and Ks for the 15° fibre oriented GFRP composite.
Table 6 ANOVA for the surface roughness (Ra) for the three composite materials
Source of
Degree of
SDQ
Variance
Test F
Fα=5%
Percentage
variance
freedom
contribution (P)
15 degree fibre orientation angle
θ (°)
2
N (rpm)
2
f (mm/rev)
2
Error
2
Total
8
60 degree fibre orientation angle
0.02717
0.01047
0.15847
0.00220
0.46240
θ (°)
2
0.037576
N (rpm)
2
0.003756
f (mm/rev)
2
0.092689
Error
2
0.004356
Total
8
0.456556
105 degree fibre orientation angle
θ (°)
N (rpm)
f (mm/rev)
Error
Total
2
2
2
2
8
0.060827
0.002061
0.092069
0.008290
0.857425
0.03563
0.00523
0.10923
0.01110
6.22
0.47
11.14
19
19
19
32.73
2.47
58.63
6.17
100
0.018788
0.001878
0.103644
0.002178
4.67
0.86
10.69
19
19
19
24.58
4.5
56.26
14.66
100
0.03041
0.00108
0.10347
0.00414
4.39
0.93
10.02
19
19
19
23.11
4.89
52.74
19.26
100
Fig. 5 shows the evaluation of the surface roughness in function of the feed rate, for different spindle speeds. It is clear that the
surface roughness increases with the feed rate and decreases with the spindle speed. For the fibre orientation 15° smaller values of
surface roughness have been obtained.
(a)
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Jenarthanan and Jeyapaul / International Journal of Engineering, Science and Technology, Vol. 6, No. 4, 2014, pp. 1-10
(b)
Fig.5 Effect of factors on surface roughness for (a) 15º fibre orientation angle
(b) 60º fibre orientation angle (c) 105º fibre orientation angle
The evaluation of machining force in function of the feed rate, for different spindle speeds, can be seen in Fig. 6. It is observed that
Fm increases with the feed rate, for different spindle speeds, and the effect of spindle speed is insignificant. In order to obtain a
reduced machining force the GFRP composites with the fibre orientation 15° is essential.
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Jenarthanan and Jeyapaul / International Journal of Engineering, Science and Technology, Vol. 6, No. 4, 2014, pp. 1-10
(a)
(b)
9
Jenarthanan and Jeyapaul / International Journal of Engineering, Science and Technology, Vol. 6, No. 4, 2014, pp. 1-10
Fig.6 Effect of factors on machining force for (a) 15º fibre orientation angle
(b) 60º fibre orientation angle (c) 105º fibre orientation angle
4. Conclusions
This research presents the findings of the parameters of machinability and provides the following conclusions:
1. GFRP composites with fibre orientation 15° gives smaller values of Ra and Ks then the 60° and 105° fibre oriented GFRP
composites.
2. GFRP composites with fibre orientation 15° provide a better MI in comparison to the 60° and 105° GFRP composites.
Therefore, GFRP composites with 15° fibre orientation gave the best overall performance.
3. Feed rate is the cutting parameter that has the highest physical as well statistical influence on Ra and Ks followed by the
helix angle of the end mill cutter.
4. A maximum MI has been obtained for optimal cutting parameters (N= 4000rpm, f =0.12mm/rev) during machining of
15° fibre oriented GFRP composite using the end mill cutter with helix angle 35°.
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Biographical notes
M.P. Jenarthanan is a Senior Assistant Professor in the School of Mechanical Engineering, SASTRA university, Thanjavur, India and received M. E
(Engg.Design) from Government College of Technology, Coimbatore, India in 2005 and received B.E.(Mechanical Engg.) from of Madurai Kamaraj University,
India in 2003. He has published 6 papers in International Journals. He has published 4 papers in the proceedings of the international conferences and 3 papers in the
proceedings of national conference. His research interests include modelling and analysis of machinability of FRP composites.
Dr.R. Jeyapaul is an Associate Professor in the Department of Production Engineering, National Institute of Technology, Tiruchirappalli, India. He has more than
15 years of experience in teaching and research. His current area of research includes Design of Experiments, Taguchi Methodology. He has published more than
fifty papers in referred international journals. He has also presented more than sixty research articles in national and international conferences.
Received February 2014
Accepted March 2014
Final acceptance in revised form March 2014