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Effect of drill geometries in drilling of glass fiber reinforced plastic (GFRP) athigh spindle speed
V Krishnaraj*, Department of Mechanical Engineering, PSG College of Technology,Coimbatore 641 004, India
Abstract
High speed machining is now recognised as one of the key manufacturing
technologies for higher productivity and throughput. Drilling experiments were
conducted with drill geometries, namely standard twist drill, double cone drill, Zhirov-
point drill, and multi facet drill, using wide range of spindle speed, and feed rate. Thrust
force, delamination and surface roughness were measured and studied in the test
trials. The analysis of variance (ANOVA) is employed to investigate the drilling
characteristics. From the experiments it is found that standard twist drill and double
cone could be used successfully at high spindle speed and low feed rate since the
cutting force is less (thrust force and torque recorded a very low value). The special
geometry improves the quality of the hole further, especially Zhirov point drill (with
surface finish values of 4-5m). Multifacet drill is found superior as for as the
delamination value is concerned
Keywords:High speed drilling; Drill geometries; Thrust force; Delamination; Surface
Roughness; ANOVA
*Corresponding author:[email protected]
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1. Introduction
Drilling is one the major machining operations which is carried out on fiber-
reinforced composite materials owing to the need for components assembly in
mechanical structures. For example, over 100,000 holes are made for a small single
engine aircraft, in a large transport aircraft millions of holes are made mostly for
fasteners like rivets, bolts etc. The quality of the drilled hole can be critical to the life of
the joints for which the holes are used. Aspects of the hole such as
waviness/roundness of its wall surface, axial straightness and roundness of the hole
cross-sections can cause high stress on the joints, leading to its failure [1]. There are
many problems encountered when drilling fiber-reinforced composites. These
problems include delamination of the composite, rapid tool wear and fiber pullout [2-4].
The delamination of composites is main concern and its presence will reduce the
strength against fatigue, results in a poor assembly tolerance and affects the
composites structures integrity [5]. Cheng and Dharan [6] used fracture mechanics
approach to analyze the delamination of fiber-reinforced materials. They cited that
thrust force is the main cause for delamination and predicted the critical thrust force
above which delamination is initiated. Tagliaferri, Caprino and Diterlizzi [7,8] studied
the effect of machining parameter and tool conditions on the damage, finish and
mechanical properties of fiber-reinforced composite materials and cutting mechanism
in drilling. Chen [9] carried experimental investigation on carbon/epoxy composite and
recommended that the high speed and low feed rate are key factors for producing
delamination free and good surface finish holes. Increasing the cutting speed will
certainly increase production rate. Another possible benefit of increasing the cutting
speed is the reduction of cutting forces. It has already been found that increase of
cutting speed may decrease the cutting force when cutting aluminum [10]. If increasing
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the cutting speed can reduce the thrust force, the delamination may be overcome.
Mustapha Elahachimi, Serge Torbaty, and Pierre Joyot [11, 12] developed a
theoretical model to predict thrust force and torque in high speed drilling in terms of
geometric features of the drill, cutting conditions and the properties of the machined
material. Lin et al. [13, 23] studied the effects of increasing drilling speed on the thrust
force as well as other drilling characteristics on carbon/epoxy composites and
unidirectional glass fiber-reinforced composites. They concluded that drill wear is the
major problem at high spindle speed. Piqute et al. [14] carried out a study of drilling
thin carbon/epoxy laminates with two types of drills, a helical drill and a drill of special
geometry and concluded that both drills lead to damage at the entrance in wall and exit
of the hole, with the exception of special geometry drill which is possible to cause a
significant reduction in the final damage. Delamination is one of the serious concerns
in drilling holes in composite materials at the bottom surface of the workpiece [22].
Quite a few references of the drilling of fiber reinforced plastics report that the quality
of cut is strongly dependent on drilling parameter as well as drill geometry.
Some of the key solutions for successfully machining composites include high
spindle speeds, light or shallow cuts. In this work, an attempt is made to study the
effects of higher spindle speed in drilling of woven fiber GFRP with different drill
geometries. The results of the experiments are presented in this paper.
2. Experimental procedure
2.1. Work piece material and cutting tool
Woven E-glass fiber reinforced epoxy laminates were prepared by hand lay-up
process. The laminate consisted of 35 layers with a nominal thickness of 9.5 mm and
fiber content (Vf) of 45% by volume was used in this study. Due to glass fiber content,
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the work piece material is very abrasive to cutting tools and chips produced by
machining are hazardous and irritating to the skin and lungs.
Due to high wear resistance while drilling fiber-reinforced materials, micro-grain
carbide ( 10 mm) was used in this investigation [1, 15]. Four different drill
geometries are used to study the effect of high speed on GFRP laminates. One is a
standard twist drill (Fig.1-a) with 118 point angle and 30 helix angle. The second type
is a double cone drill (Fig.1-b), which has two-point angles. The first section is short
and has an included angle of 70 -75 while the second is a longer section with point
angle of 116 -118. The first portion having an included angle 70-75reduces the
chip thickness and improves surface finish. The third type is Zhirov point drill (Fig. 1-c),
which has a triple lip at each cutting edge, an extra rake ground on the face of the lips,
and a split point. The shortest lip has an included angle of 55, the intermediate lip has
a point angle of 70, and the largest lip has the standard point angle of 118.The chisel
edge has been reduced by a slot or groove. Therefore, extrusion action is replaced by
cutting action. This design results in reduced feed thrust compared to conventional
drill, permitting a higher feed rate with an acceptable drill life. The drill life also
improved due to the presence of triple lips. More dimensionally accurate holes can be
produced because of less spindle deflection by the reduction of thrust force [16]. The
fourth type is the multifacet drill (Fig. 1-d). The chisel edge length is only 0.3 mm to
reduce the thrust force. In order to strengthen the reduced chisel edge length, the tip
height is designed to be 3mm with an inner point angle of 135 (2). The large value
of inner point angle is also required to avoid high-temperature generation by heat
transfer in the reduced chisel edge. An arc cutting edge with a radius of 1.0 mm is
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made to increase the rake angle. The arc cutting edge is effective in dividing chip and
drill centering [17].
2.2 Experimental set up
Experiments were conducted using Acumac high-speed spindle (5kW) mounted
on a vertical CNC machine. Fig. 2 shows the experimental set-up. Machining of
laminates was carried out for the following conditions.
Spindle speed: 14,000, 16,500 & 19,000 rpm
Feed rate: 0.01, 0.03, 0.05, 0.08 mm/rev
Geometry: Standard twist drill, Double Cone, Zhirov and multifacet [16, 17]
A Syscon two-component drilling dynamometer (model: SI-674) of strain gauge
type was used to measure the axial thrust force and torque. The proportional charge
output from the dynamometer was fed to a Syscon amplifier (model SI-223D), thus
producing a scaled voltage output signal proportional to the applied load. The thrust
force was continuously monitored and recorded using a digital storage Oscilloscope
(Tektronix Model: TDS 210 with 60 MHz bandwidth, 1GS/s sample rate and 2500
points record length for each channel). Delamination was measured using a scanner
as suggested by Khashaba [21]. Surface roughness of the drilled hole was measured
using surface profilometer with ruby crystal probe (model of Taylor Hobson pneumo
Surtronic 3+).
3. Results and discussion
3.1. Influence of cutting parameters on thrust force
Drilling parameters cause change in cutting forces, which lead to difference in
quality of the holes in terms of surface finish, circularity, delamination, fiber pull out,
matrix cratering, etc. From the experiments it was found that increasing spindle speed
and feed increase the thrust force, especially feed rate, this is because the larger the
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feed rate, the larger the cross sectional area of the undeformed chip will be, the
greater the resistance of chip formation and consequently the greater the axial thrust
force and torque. As can be seen in the Fig.3 Zhirov point drill can be drilled with lower
thrust force for the same operating conditions when compared to other geometries.
This is because in the Zhirov drill the chisel edge has been replaced by a slot,
therefore extrusion action is replaced by cutting action. The Zhirov-point drill also
produces more dimensionally accurate holes (hole deviation within 10m) because of
less deflection in the spindle through a reduction of the thrust force.
At lower feed rate (0.01 mm/rev) standard twist drill, double cone and multifacet
generated more or less same thrust force (around 20 N). This value is very less when
compared to drilling at normal spindle speed (around 50 N). For all the drill geometries
and cutting parameters the torque values are between 0.1 Nm to 0.2 Nm. Not much
variation in the torque values are recorded within the range examined.
Table 1 shows the results of the analysis of variance with the thrust force in
GFRP material by considering drill geometry as a factor [18-20]. From the analysis of
Table 1, we can observe that the feed rate factor (P= 63.13 %) has statistical and
physical significance on the trust force followed by drill geometries (P=27.98 %). The
spindle speed factor (P=2.76%) on thrust force does not present percentage of
physical significance of contribution because P(percentage of contribution) < Error
associated. Notice that the error associated to the table ANOVA for the thrust force is
approximately 6.13%.
3.2. Influence of cutting parameters on delamination factor
Delamination near the exit side is introduced as the tool acts like a punch,
separating the thin uncut layer from the remainder of the laminate. The entry hole
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produced was neat for all the geometries. However, the fiber pull out at exit was more
in the case of twist drill and Zhirov drill. Multifacet drill produced clean cut holes at the
exit side of the laminate. This is because the cutting mechanism of a multifacet at the
last ply is like a trepanning with knife-edge. Therefore, exit hole was neat and fuzzy
free. A button like chip was ejected at the exit side of the laminate while drilling using
multifacet drill.
The delamination was evaluated in terms of delamination factor. The
delamination factor is the ratio of maximum diameter (Dmax) of the damaged zone to
the actual hole diameter (D). Fig. 4 shows the relationship between the delamination
factor and drilling parameters. It is concluded that delamination factor increases with
feed rate and spindle speed. Fig. 5 shows the hole machined in the drilling process for
standard twist drill, double cone, Zhirov, and multifacet drill respectively. Multifacet drill
presents better performance than other drill geometries. The special characteristic of
the drill is the extreme sickle-form design of the cutting edges. This pre-stresses the
fibers in the direction of pull and separates them in the direction of thrust. This results
in a clean cut with a smooth surface. The delamination is less compared to other drill
geometries.
From the analysis of Table 2, we can observe that the feed rate (P=91.14%)
and the drill geometry (P=5.85) followed by spindle speed have statistical and physical
significance on the delamination factor obtained, especially feed rate factor.
3.3 Influence of cutting parameters on surface roughness
After the drilling test, the quality of hole at entry and exit has been examined.
The surface roughness (Ra) was evaluated as per ISO 4287/1. For each test 3
measurements over drilling surfaces were made. Fig. 6 shows the effect of drill
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geometry on surface finish. The value of surface roughness increases with the feed
rate, and decreases with the cutting speed. Zhirov drill produced good surface finish
(4-5m) at lower feed rate and the circularity of the hole was also good (measured
using CMM, the values were within 10 m). The outer most lip produced thin chip
which improves the finish of the hole. Multifacet and double cone also generated good
surface finish at lower feed rate when compared to standard twist drill.
From the analysis of Table 3, we can observe that the feed rate (P=45.27%)
and the drill geometry (P= 28.33) have statistical and physical significance on the
surface finosh obtained, especially feed rate factor. The spindle speed factor (P=2.52
%) on surface roughness does not present percentage of physical significance of
contribution because P (percentage of contribution) < Error associated. Notice that the
error associated to the table ANOVA for the surface finish is approximately 23.88%
[18-20].
4. Conclusions
In drilling of composites, high spindle speed and low feed rate improves the
machinability aspects within the range examined. The cutting force is less (thrust force
and torque both recorded a very low value). The special geometry improves the quality
of the hole further, especially Zhirov point drill. Standard and double cone was found
suitable for producing more number of holes at high spindle speed and low feed rate.
At a spindle speed of 16,000rpm and feed rate of 0.01mm/rev its performance is
comparable with (Cutting force, delamination and surface finish) Zhirov and multifacet
drills. Fifty holes were drilled for each geometry and found that the force values are
stable after the initial increase. When the geometries of the different drills were
examined, the chisel edge of the multifacet was found damaged, and chip off was
found in the lip of the Zhirov, where as in standard and double cone uniform wear on
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the flank was found. When the standard drill was subjected to drill wear study it lasted
up to 250 holes.
The feed rate is the cutting parameter, which has influence on thrust force
(63.13%) followed by drill geometry (P=27.98 %). The special geometries contribute to
the thrust force.
The feed rate contribution on delamination is (P=91.14%) high followed by drill
geometry (P=5.58%). The feed rate and drill geometry have contributions on surface
roughness (Ra) (P=45.27% and P=28.33%).
Acknowledgement
The authors gratefully acknowledge Department of Science and Technology,
Govt. of India, (Grant No: III.5(75)/2001-SERC-Engg) for funding this research work.
References
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[15].K.Sakuma, Y.Yakoo, M.Seto, Study on drilling of fiber-reinforced plastic-relation
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(a) (b)
(c) (d)
Fig.1 View showing geometry data of: (a) Standard twist drill (b) Double cone
drill (c) Zhirov-point drill (d) Multifacet drill
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Fig. 2 Photographic view of the experimental set-up
High speed spindle
Strain gauge
dynamometer
Oscilloscope
Dynamometer
out put
AC Inverter
GFRP laminate
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Feed Vs Thrust force for std. twist drill
0
10
20
30
40
50
60
70
80
0.01 0.03 0.05 0.07
Feed rate (mm/rev)
Thrustforce(N)
14000 rpm
16500 rpm
19000 rpm
Feed Vs Thrust force for double cone drill
0
10
20
30
40
50
60
70
80
0.01 0.03 0.05 0.07Feed rate (mm/rev)
Thrustforce(N)
14000 rpm
16500 rpm
19000 rpm
(a) (b)
Feed rate Vs Thrust force for Zhirov drill
0
20
40
60
80
0.01 0.03 0.05 0.07
Feed rate (mm/rev)
T
hrustforce(N)
14000 rpm
16500 rpm
19000 rpm
Feed rate Vs Thrust force for multifacet
drill
0
20
40
60
80
0.01 0.03 0.05 0.07Feed rate (mm/rev)
Thrustforce(N)
14000 rpm
16500 rpm
19000 rpm
(c) (d)
Fig. 3 Effect of feed rate & spindle speed on thrust force: (a) Standard drill
(b) Double cone drill (c) Zhirov-point drill (d) Multifacet drill
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Feed rate Vs Delamination factor for
Standard twist drill
1.07
1.09
1.11
1.13
1.15
1.17
1.19
1.21
0.01 0.03 0.05 0.07Feed rat (mm/rev)
Delaminationfactor
14000 rpm
16500 rpm
19000 rpm
Feed rate Vs Delamination factor for
Double cone drill
1.07
1.09
1.11
1.13
1.15
1.17
1.19
1.21
0.01 0.03 0.05 0.07Feed rate (mm/rev)
Delaminationfactor
14000 rpm
16500 rpm
19000 rpm
(a) (b)
Feed rate Vs Delamination factor for
Zhirov drill
1.07
1.09
1.11
1.13
1.15
1.17
1.19
1.21
0.01 0.03 0.05 0.07Feed rate (mm/rev)
Delaminati
onfactor
14000 rpm
16500 rpm
19000 rpm
Feed rate Vs Delamination factor for
Multifacet drill
1.07
1.09
1.11
1.131.15
1.17
1.19
1.21
0.01 0.03 0.05 0.07
Feed rate (mm/rev)
Delaminatio
nfactor
14000 rpm
16500 rpm
19000 rpm
(c) (d)
Fig. 4 Effect of feed rate & spindle speed on delamination: (a) Standard twist
drill; b) Double cone drill (c) Zhirov point drill; (d) Multifacet drill
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(a)
(b)
(c) (d)
Fig. 5 Effect of drill geometry on delamination: (a) Standard twist drill; (b) Double
cone drill (c) Zhirov point drill; (d) Multifacet drill
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Feed Vs Surface roughness for standard
twist drill
0
5
10
15
20
0.01 0.03 0.05 0.07Feed rate (mm/rev)
Surfaceroughness(m)
14000 rpm
16500 rpm
19000 rpm
Feed Vs Surface roughness for double
cone drill
0
5
10
15
20
0.01 0.03 0.05 0.07Feed rate (mm/rev)
Surfaceroughness(m)
14000 rpm
16500 rpm
19000 rpm
(a) (b)
Feed rate Vs Surface roughness for Zhirov
drill
0
5
10
15
20
0.01 0.03 0.05 0.07Feed rate (mm/rev)
Surfaceroughness(m) 14000 rpm
16500 rpm
19000 rpm
Feed rate Vs Surface roughness for
multifacet drill
0
5
10
15
20
0.01 0.03 0.05 0.07
Feed rate (mm/rev)
Surfaceroughness(m)
14000 rpm
16500 rpm
19000 rpm
(c) (d)
Fig. 6 Effect of feed rate & spindle speed on surface roughness: (a) Standard
twist drill (b) Double cone drill (c) Zhirov point drill; (d) Multifacet drill
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Table 1ANOVA for thrust force
SourceSum ofsquares
DOF VarianceVarianceratio(F)
F=0.5%Pure sum ofsquares(S')
% ofcontribution
Spindle speed 444.4 2 222.2141 11.562 3.239 405.99 2.76
Feed rate 9352 3 3117.393 162.212 2.846 9294.53 63.13
Drill Geometry 4178 3 1392.614 72.464 2.846 4120.19 27.98
Error 749.5 39 19.21793 903.24 6.13
Total 14724 47 14723.95 100.00
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Table 2 - ANOVA for delamination factor
SourceSum ofsquares
DOF VarianceVarianceratio(F)
F=0.5%Pure sum ofsquares(S')
% ofcontribution
Spindle speed 0.0013 2 0.00065 33.80 3.239 0.001264 1.76
Feed rate 0.0657 3 0.02190 1137.20 2.846 0.065649 91.14
Drill Geometry 0.0043 3 0.00142 73.85 2.846 0.00421 5.85
Error 0.0007 39 1.93E-05 0.000905 1.25
Total0.0720
470.072027
100.00
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Table 3 - ANOVA for surface roughness
SourceSum ofsquares
DOF VarianceVarianceratio(F)
F=0.5%Pure sum ofsquares(S')
% ofcontribution
Spindle speed 30.203 2 15.101 3.477 3.239 21.517 2.52
Feed rate 399.941 3 133.314 30.697 2.846 386.912 45.27
Drill Geometry 255.115 3 85.038 19.581 2.846 242.086 28.33
Error 169.373 394.343 204.117
23.88
Total 854.632 47 854.632 100.00
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FIGURE CAPTIONS
Fig.1 View showing geometry data of: (a) Standard twist drill (b) Double cone drill (c)Zhirov-point drill (d) Multifacet drill
Fig. 2 Photographic view of the experimental set-up
Fig. 3 Effect of feed rate & spindle speed on thrust force: (a) Standard drill (b) Double conedrill (c) Zhirov-point drill (d) Multifacet drill
Fig. 4 Effect of feed rate & spindle speed on delamination: (a) Standard twist drill; b) Double
cone drill (c) Zhirov point drill; (d) Multifacet drill
Fig. 5 Effect of drill geometry on delamination: (a) Standard twist drill; (b) Double cone drill(c) Zhirov point drill; (d) Multifacet drill
Fig. 6 Effect of feed rate & spindle speed on surface roughness: (a) Standard twist drill (b)Double cone drill (c) Zhirov point drill; (d) Multifacet drill
TABLE CAPTIONS
Table 1-ANOVA for thrust force
Table 2-ANOVA for delamination factorTable 3-ANOVA for surface roughness