Study of Cutting Speed Variation in the Ultrasonic Assisted Drilling of Carbon Fibre Composites
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Transcript of Study of Cutting Speed Variation in the Ultrasonic Assisted Drilling of Carbon Fibre Composites
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8/18/2019 Study of Cutting Speed Variation in the Ultrasonic Assisted Drilling of Carbon Fibre Composites
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1 Copyright © 2014 by ASME
Proceedings of the ASME 2014 International Mechanical Engineering Congress & ExpositionIMECE2014
November 14-20, 2014, Montreal, Quebec, Canada
IMECE2014-37046
Study of Cutting Speed Variation in the Ultrasonic Assisted Drilling of CarbonFibre Composites
Aniruddha GuptaWMG, University of WarwickCoventry, West Midlands, UK
Stuart BarnesWMG, University of WarwickCoventry, West Midlands, UK
Iain McEwenSchool of Engineering,University of Warwick
Coventry, West Midlands, UK
Nadia KourraWMG, University of WarwickCoventry, West Midlands, UK
Mark A. WilliamsWMG, University of WarwickCoventry, West Midlands, UK
ABSTRACTUltrasonic assisted drilling (UAD) has been proven
effective for the thrust force reduction as compared to
conventional drilling (CD) for same machining parameters. The
following research was focused on the examination of exit
delamination, machined surface and cutting temperature
measurement in UAD and a comparison to that in CD at the
cutting speeds of 0.942 m/min, 9.42 m/min, 94.2 m/min and
282.6 m/min at a constant feed rate of 0.05 mm/rev in the
through-hole drilling of CFRP material. X-Ray computedtomography (CT) was used to identify the exit delamination,
internal damage, circularity and center deviation in CD and
UAD. A maximum of 82.8% reduction in the center deviation
and 33.2% reduction in circularity of the holes were found
when drilled in UAD as compared to those in CD. Furthermore,
the cutting temperature in the drilling of CFRP has been
measured and compared for both the cases of CD and UAD.
Ultrasonic assistance produced 10°C higher cutting temperature
than that in CD at the cutting speed of 282.6 m/min while at
lower cutting speeds (0.942 m/min and 9.42 m/min), the cutting
temperatures with and without ultrasonic assistance did not
have a significant difference (2°C and 4°C respectively).
1. INTRODUCTIONCarbon fibre reinforced plastics (CFRPs) are attractive as
aircraft and aerospace structural components due to their lighter
weight and higher specific strength as compared to metals [1-
7] which has led to greater weight savings resulting in greater
pay load, longer range and fuel saving in comparison to the
metals [8]. In spite of curing to the final shape, the machining
of composites is required at several stages of the production
e.g. cutting and drilling of holes. The mechanical drilling
process is one of the final processes in the manufacturing of the
composite components [2]. The conventional drilling (CD) o
CFRPs has been proven difficult due to delamination and shor
tool life. According to a study [1], 60% of all the part rejections
in air craft industry is associated with delamination in drilling.
For carbon fibre composites, high thrust force is
considered to be the most detrimental to CFRP drilling due to
delamination and damage inside the holes in a drilling process
[9, 10]. Some researchers have reported that the longer tool life
lower thrust force and segmented and smaller chip lengths have been observed when drilling metals by introducing vibrations in
the drilling [11, 12]. The effects have been observed typically
when the frequency of the vibrations has been kept above
20 kHz because of achieving an intermittent cutting action [13]
Such type of drilling is known as ‘Ultrasonic assisted drilling
(UAD)’ [7, 13, 14]. In the recent years, Makhdum et al. [15, 16
have conducted experiments on CFRP to develop an
understanding for ultrasonic assisted drilling and found an
almost 90% reduction in the thrust force and generation of the
longer chips during UAD of CFRP. However, the effect on the
damage during the drilling was not a focus in their work. A
similar experimental study has also been conducted by
Mehbudi et al. [17] for glass fibre laminate (GFRP) materialThey conducted experiment at a low cutting speed (maximum
cutting speed = 31.4 m/min) and found UAD to be effective in
the reduction of thrust force and exit delamination. Sadek et al
[18] have also reported obtaining zero exit delamination
between 113 m/min (6000 rpm) to 226 m/min (12000 rpm
spindle speed having 0.025 mm/rev feed rate with 6 mm
diameter drill but no images having zero exit delamination were
presented in their work. Also, they used the oscillation
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2 Copyright © 2014 by ASME
frequencies of 30 Hz and 60 Hz which are not in ultrasonic
range.
For cases when the feed rate of the cutting tool is not
negligible in comparison to the cutting speed, the rake and
clearance angles are affected [19]. In the case of drilling, where
the feed rate is in the perpendicular direction to the cutting
speed, the feed rate becomes more dominating as compared to
cutting speed in terms of cutting and thrust forces when the
ultrasonic oscillations are imposed on the drill [19]. Because of
these ultrasonic modulations in the feed rate, the effective
normal rake angle of the twist drill at all the points on the
cutting edge varies rapidly (~114° angle variation within
26.9 µs). This research was focused on the effect of the
effective normal rake angle variation on the thrust force,
internal damage of the machined surface of the holes, exit
delamination and the cutting temperature in the ultrasonic
assisted drilling of CFRP material. Four cutting speeds were
used – 0.942 m/min, 9.42 m/min, 94.2 m/min and 282.6 m/min
at a constant feed rate of 0.05 mm/rev and comparison with that
in CD.
2. EFFECTIVE NORMAL RAKE ANGLECALCULATION FOR CD AND UAD
In order to visualize the tool-material interaction in UAD, a
calculation was performed to identify the variation in the
effective normal rake angle during the drilling for a simple two-
flute twist drill. Though similar calculation has also been
performed by Zhang et al. [20] and Wang et al [21] for the
vibration assisted drilling of CFRP materials while calculating
the mean thrust force and torque for CFRP materials, some of
the intermediate steps involving geometrical relationships for
the tool geometry were not clear even in the mentioned
references in their work. Therefore, it becomes essential to
mention the relevant equations used in the present research fordeveloping a fundamental understanding about the variations
generated because of ultrasonic oscillations on the effective
normal rake angle. Figure 1 shows the axial projection of the
chisel and cutting edges with the visualization of the cutting
velocity, feed velocity and the effect on axial rake and
clearance angles with the introduction of the feed velocity in
the work reference system for an arbitrary point ‘i’ of the
cutting edge.
Figure 1: Visualization of the rake and clearance angle in the work
reference system [19]
In order to calculate the effective normal rake angle, the
effect of feed rate on the orthogonal rake would have to be
considered at every point of the cutting edge [19].
As shown in the Figure 1 for the point ‘i’ –
γXW = γXD μ (1
μ = tan−1
( ) (2 =2 (3
Effective feed rate for CD,
Vf = S0N; (4Effective feed rate in UAD,
Vf = S0Naωcosωt (5From the machining theory, [19, 20],
γXD = tan−1 rr tanθ (6The orthogonal rake angle in the work reference system is
given by [19] –
tan =+
(7
Where, tan =
; tan =∅
∅ = cone angle of the twist drill, = sin−1 , Putting the respective values fromEquation 1 to 6 into Equation 7, the orthogonal rake angle for
UAD in the work reference system for point ‘i’ of the cutting
edge in the Figure 1 is –
tan = [ + (
)]+
(8
Hence, the effective normal rake angle in UAD [19]
tan =
(9
The effective normal rake angle variation has been plotted
and compared for the various cutting speeds in Figure 2 for CD
and UAD. It can be observed that the wave form of the
effective normal rake angle in UAD changes from step wave
(Figure 2(a)) to sinusoidal wave (Figure 2(d)) when cutting
speed is increased from 0.942 m/min (50 rpm) to 282.6 m/min
(15000 rpm) because as the cutting speed is increased, the poinof concern on the cutting edge moves a longer distance before
the ultrasonic oscillation completes an oscillation cycle. It can
also be observed that the value of the maximum rake angle in
an ultrasonic cycle reduces from 63.19°, Figure 2(a), to 33.01°
Figure 2(d), when the speed is increased from 0.942 m/min
(50 rpm) to 282.6 m/min (15000 rpm) while the effective
normal rake angle in CD remains at 28.95° at all the cutting
speeds. Also, Figure 2(d) shows that the maximum normal rake
angle in the UAD varies from 33.01° to 26.33° which is close
rc ri
i
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3 Copyright © 2014 by ASME
to the maximum effective normal rake angle in CD at
282.6 m/min (28.95°) cutting speed.
Figure 2: Effective normal rake angle variation in CD and UAD at
cutting speeds of (a) 0.942 m/min, (b) 9.42 m/min, (c) 94.2 m/min
and (d) 282.6 m/min at with feed rate of 0.05 mm/rev having
ultrasonic frequency of 37.22 kHz and 2.9 µm amplitude (peak to
peak) in UAD
In the work of Zhang et al. [20], because of using low
vibration frequency of 130 Hz, they obtained a small difference
of 4° between the effective normal rake angles in conventional
and vibration assisted drilling.
Arola et al. [22] have mentioned about the localization ofthe damage while increasing the rake angle in the orthogonal
trimming of graphite/epoxy composite material. In order to
identify the effect of these rapid variations in the effective
normal rake angle in UAD, the drilling experiment was
performed further.
3. EXPERIMENTAL SETUP AND PROCEDUREDrilling was performed on an ULTRASONIC 65
monoBLOCK® machine with an ultrasonic actuator built into
the tool holder. This machine works on the concept of the
reverse piezoelectric effect, wherein the transmission of the
high-frequency electrical signal to the piezo-elements in the
tool holder to the spindle is executed without contact(inductive). An oscillation is generated in the axial direction
and is superimposed on the conventional tool rotation and feed
rate. This particular machine has a maximum capability of
18,000 rpm spindle speed and 40,000 mm/min feed rate. The
ultrasonic oscillation parameters depend upon the tool – too
holder combination. Once the drilling tool is located within the
tool holder, the optimum oscillation frequency is determined by
the machine. Also, the maximum oscillation amplitude is fixed
The amplitude can be varied from 0% to 100% of the maximum
oscillation amplitude of tool – tool holder combination. In the
present case, the frequency of the oscillation of the tool was
37220 Hz and the maximum oscillation amplitude was 2.9 µm
(peak to peak).The carbon fibre composite material was provided by BAE
Systems. It was a 7.4 mm thick unidirectional stack of -45°, 0°
+45° and 90° orientations consecutively with woven carbon
fibre layer on the top and bottom having bismaleimide (BMI)
matrix. The drilling tool used in the experiment was uncoated
tungsten carbide, 3 flute twist drill having 6.0 mm diameter
66.0 mm overall length and 150° point angle having flat rake
and clearance surfaces. The constant axial rake and clearance
angles (due to flat rake & clearance surfaces) were measured in
the laboratory, (rake angle = 6.40° and clearance angle =
10.56°). This type of drill was recommended from the too
manufacturer for good surface finish, high hole-accuracy
reduced tool wear and longer tool life due to three cutting edges
as compared to 2-flute twist drill. The experiment wasconducted under dry-machining conditions.
The final experimental setup including the drill, the
ultrasonic actuator, dynamometer (Type – 9257B), fixture and
the CFRP strip is shown in the Figure 3. To find out the
reproducibility, the experiment was repeated three times further
with the same drill. It was found that the variation of the values
was similar whereas the particular values were getting higher
due to the incremental tool wear after each drilling.
-60
-40
-20
0
20
40
60
80
0 20 40 60 80
N o r m a l r a k e a n g l e ( d e g
)
Time (micro seconds)
Normal rake angle in CD and UAD in work reference system
r = 1.0 mm UAD
r = 1.4 mm UAD
r = 1.8 mm UAD
r = 2.2 mm UAD
r = 2.6 mm UAD
r = 3.0 mm UAD
r = 1.0 mm CD
r = 1.4 mm CD
r = 1.8 mm CD
r = 2.2 mm CD
r = 2.6 mm CD
r = 3.0 mm CD
-60
-40
-20
0
20
40
60
80
0 20 40 60 80
N o r m a l r a k e a n g l e
( d e g )
Time (micro seconds)
Normal rake angle in CD and UAD in work reference system
r = 1.0 mm UAD
r = 1.4 mm UAD
r = 1.8 mm UAD
r = 2.2 mm UAD
r = 2.6 mm UAD
r = 3.0 mm UAD
r = 1.0 mm CD
r = 1.4 mm CD
r = 1.8 mm CD
r = 2.2 mm CD
r = 2.6 mm CD
r = 3.0 mm CD
-30
-20
-10
0
10
20
30
40
50
0 20 40 60 80 N o r m a l r a k e a n g l e ( d e g )
Time (micro seconds)
Normal rake angle in CD and UAD in work reference system
r = 1.0 mm UAD
r = 1.4 mm UAD
r = 1.8 mm UAD
r = 2.2 mm UAD
r = 2.6 mm UADr = 3.0 mm UAD
r = 1.0 mm CD
r = 1.4 mm CD
r = 1.8 mm CD
r = 2.2 mm CD
r = 2.6 mm CD
r = 3.0 mm CD
0
5
10
15
20
25
30
35
0 20 40 60 80
N o r m a l r a k e a n g l e ( d e g )
Time (micro seconds)
Normal rake angle in CD and UAD in work reference system
r = 1.0 mm UAD
r = 1.4 mm UAD
r = 1.8 mm UAD
r = 2.2 mm UAD
r = 2.6 mm UAD
r = 3.0 mm UAD
r = 1.0 mm CD
r = 1.4 mm CD
r = 1.8 mm CD
r = 2.2 mm CD
r = 2.6 mm CD
r = 3.0 mm CD
b
a
d
c
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4 Copyright © 2014 by ASME
Figure 3: Final experimental set-up having dynamometer, fixture
holding CFRP strip, ultrasonic tool holder and the 6 mm diameter
drilling tool
4. RESULTS AND DISCUSSION
4.1. Thrust force The thrust force was recorded using a Type 9257B Kistler
dynamometer and the average thrust force was considered as a
reading for each drilling as shown in the Figure 4.
The average was taken between the readings where thethrust force was stable and away from the entrance/exit effects
of the hole. Distance drilled from 2 mm to 5 mm during drilling
was considered for the average for each drilling, e.g. the
average thrust force in the Figure 4 was measured between the
time instances of 120s to 192s for 0.942 m/min cutting speed as
the depth from 2 mm to 5 mm was drilled between these time
instances (considering 3 mm depth travelled by the tool prior to
drilling). The overall average values of thrust force
measurements for each parameter are plotted in the Figure 5.
The thrust force was found to be the maximum at the
lowest cutting speed of 0.942 m/min in CD and UAD, Figure 5.
Unlike reported by Mehbudi et al. [17] and Makhdum et al. [15,
16], thrust force was not found to be reduced because ofultrasonic assistance at every cutting speed. The maximum
reduction in the thrust force because of ultrasonic assistance
was found to be 9.1 N (although a small difference) at
9.42 m/min cutting speed as compared to CD. For the rest of
the cutting speeds, the thrust force was found to be similar in
CD and UAD Also, the minimum thrust force was found to be
at 282.6 m/min cutting speed in both CD and UAD.
Figure 4: Thrust force profile during the drilling of one hole (0.942
m/min, 0.05 mm/rev), average thrust force = 231.8 N
Figure 5: Average thrust force recorded CD and UAD
4.2. X-Ray Computed Tomography (CT)In order to examine the extent of the damage accurately in
the drilled-holes, the non-destructive technique of X-ray CT
scanning has been used and a methodology has been developed
[23]. Each drilled hole was scanned separately by NikonMetrology system XTEK 320. In order to identify the physica
center of the hole, four straight marks were machined around
where the hole was going to be drilled, Figure 6. A 2 mm
diameter, 2-flute, Q-coat tungsten carbide slot drill with flat end
was used to machine the marks. First, the four marks were
machined and then the tool was changed to the drilling tool
before the hole was drilled keeping the work-piece fixed. The
center of the hole was identified by the point of intersection of
the lines joining the midpoints of the machined marks around
the hole through image processing, (Figure 6 (b)).
Figure 6: (a) Straight machined marks and the drilled hole, (b
center identification and the dimensions of hole and marks in mm.
The utilization of this method lies to the quality of the
machined marks around the hole and the identification of the
edges of the slots in the reconstructed data. Hence, severa
parameters were tried and finally, at the cutting speed o
113 m/min and feed rates of 250 mm/min cross-feed and 100
mm/min in-depth feed, the marks were machined with two
passes – first 0.4 mm deep and then further 0.3 mm making the
final depth of the machined marks to be 0.7 mm.
The data provided by CT scanning was reconstructed to a
3D representation of the hole with back-projection
reconstruction algorithms in CT Pro software, provided by
Nikon Metrology. The reconstructed model was exported from
VG Studio Max 2.2 software in DICOM images. The exported
slides had the thickness of a voxel, 3D reconstructed pixel
150
170
190
210
230
250
0 50 100 150 200 250 300
T h r u s t f o r c e ( N )
Cutting speed (m/min)
CD UAD
Drill
Ultrasonic actuator
CFRP strip
FixtureDynamometer
a b
T h r u s t
f o r c e
[ N ]
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5 Copyright © 2014 by ASME
having a thickness of approximately 14 µm. Every DICOM
image was analysed by a MATLAB code that utilizes image
processing tools to provide complete internal information of
each drilled hole.
With the help of the image processing, the exit
delamination and the dimensional metrological results on the
deviation of radius, circularity and distance between actual and
ideal centers (center deviation) were calculated for each layer.
The entrance delamination could not be measured through this
method because the depth of the machined marks was 0.7 mm
and the analysis of the image processing was started once the
center of the hole was determined at the depth of 0.7 mm.
4.2.1. Exit delamination In general, two types of the exit delamination factors have
been considered i.e. the diameter delamination (Dmax/D0) and
the area delamination (Amax/A0) [17, 24]. Where Amax is the
damaged area at the exit and Dmax is the maximum diameter of
the damaged area (Amax) at the exit and D0 and A0 are the
diameter and circular area of the original drilled-hole. As the
diameter delamination implies the whole area to be affectedinside the maximum diameter of the damage whereas the area
delamination signifies the actual affected area due to
delamination as the case of spalling mentioned by Davim at al.
[24], therefore, the area delamination was considered for the
analysis in the present research work.
The damaged area of the exit delamination for each drilled
hole was calculated from X-Ray CT scanning data of the layers
near to the exit of the hole. The maximum area amongst all the
areas of the layers near to the exit was considered as the area
for exit delamination of a particular hole. The area delamination
factors were then calculated by the ratio of the exit delaminated
area and the circular area of the original intended hole for all
the parameters. The respective area delamination factors are plotted in the Figure 7.
The exit delamination at 0.942 m/min cutting speed was
found to be higher in UAD as compared to that in CD because
the mean thrust force at the exit-laminates was found to be
higher in UAD (222.2 N) as compared to that in CD (217.4 N),
even if the average thrust forces at 0.942 m/min are same in CD
and UAD (difference of only 0.5 N). One possible reason for
this could be the ultrasonic impacts at the exit in UAD as the
cutting speed is very slow. The maximum reduction in the exit
delamination because of ultrasonic assistance was found at
9.42 m/min cutting speed in UAD in comparison to that in CD.
One of the reasons for this reduction in the exit delamination
corresponds to the maximum reduction of thrust force (9.1 N)at 9.42 m/min because of ultrasonic assistance, Figure 5. The
exit area delamination at 94.2 m/min was found to be lower in
UAD as compared to that in CD even if the average thrust
forces are same at in both. At 282.6 m/min cutting speed, the
exit delamination was found to be higher in UAD in
comparison to that in CD as also shown in their corresponding
average thrust force data in the Figure 5.
Figure 7: Area delamination factor for all the cutting speeds
4.2.2. Internal DamageThe internal damage was measured by measuring the
maximum radius at each layer of the X-ray CT scanned hole
through image processing in MATLAB. Further, the average
was taken between the drilled depths from 2 mm to 5 mm. Thisaverage value was considered for the analysis for a particular
hole. The average values of the maximum radius for all the
parameters are plotted in Figure 8. It was found that the average
maximum radius (and hence internal damage) was lower in the
case of UAD in comparison to that in CD at all the cutting
speeds except 9.42 m/min. At 9.42 m/min cutting speed, the
internal damage was found to be similar in both the processes
of CD and UAD.
The reduction in the average maximum radius because of
ultrasonic assistance was found to be more at 94.2 m/min
(20 µm) than that at 282.6 m/min cutting speed (6 µm) even if
no reduction in the thrust force because of ultrasonic assistance
at these cutting speeds was observed. This happened becausethe effective normal rake angle at 94.2 m/min in UAD (39°)
was more than that obtained at 282.6 m/min in UAD (33°)
Figure 2(c) & (d), leading to a localization and reduction in the
damage as mentioned by Arola et al. [22].
Figure 8: Comparison of the average maximum radius in CD and
UAD
1
1.05
1.1
1.15
1.2
1.25
1.3
1.35
0 50 100 150 200 250 300
D e l a m
i n a t i o n f a c t o r
Cutting speed (m/min)
CD
UAD
3.05
3.06
3.07
3.08
3.09
3.1
3.11
3.12
0 50 100 150 200 250 300
A v e r a g e m a x i m u m r a d i u s ( m m )
Cutting speed (m/min)
CD
UAD
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6 Copyright © 2014 by ASME
4.2.3. CircularityThe circularity of each hole was calculated from the
difference between the maximum and the minimum radius in a
particular layer including the damage of the layer. The average
of the circularity of all the layers between the drilled depths
from 2 mm to 5 mm was considered as the average circularity
of a hole. The lower is the circularity value, the more circular
the hole is and the less damage has been produced. The
circularity values for the various cutting speeds have been
plotted in the Figure 9. It was found that the average circularity
values were lower in the case of UAD as compared to those in
CD at all the parameters except 9.42 m/min cutting speed. At
9.42 m/min cutting speed the circularity was found to be lower
in CD than that in UAD. Also, at 282.6 m/min, the average
circularity for CD was found to be the highest amongst all the
cutting speeds which also corresponds to the highest ‘average
maximum radius’ of the internal damage at 282.6 m/min cutting
speed as shown in the Figure 8.
Figure 9: Average circularity in CD and UAD at various cuttingspeeds
The lowest circularity was found at 0.942 m/min cutting
speed in UAD. A maximum of 33.8% reduction in circularity
was obtained in UAD at the cutting speed of 282.6 m/min in
comparison to CD. Based on the circularity data, it can be
concluded that the holes drilled by UAD at 0.942 m/min cutting
speed have the lowest internal damage as also shown in the
Figure 8. Also, similar average maximum radius, Figure 8, and
higher circularity in UAD, Figure 9, indicate higher internal
damage in UAD at 9.42 m/min cutting speed as compared to
that in CD.
4.2.4. Center deviationDuring the drilling, because of the natural vibrations and
chattering, the drill sometimes deviates from its original center
which is known as deviation of the centers. The ideal center of
the hole was fixed at the point of cross-section of the lines
joining the mid-points of the four machined marks around the
hole as shown in the Figure 6. The actual center of the hole in a
layer was found by the MATLAB itself by identifying a circular
loop. The distance between the ideal and actual centers was
calculated for each layer of a hole. Further, the average of the
center deviations of the layers between the drilled depth of
2 mm and 5 mm was calculated. This average was considered
as the average center deviation of a hole.
The deviation in the centers in UAD was found to be lower
than that in CD at all the cutting speed parameters except 9.42
m/min cutting speed, Figure 10. At 9.42 m/min cutting speed
the deviation was found to be similar (3 µm difference) in both
the drilling processes of conventional drilling and UAD. Figure
10 shows that the highest deviation in the center of the drilled
hole was found at 282.6 m/min cutting speed in CD while the
lowest deviation was found at 282.6 m/min cutting speed in
UAD. A reduction of 82.8% in center deviation was obtained at
282.6 m/min cutting speed.
Figure 10: Average center deviation in CD and UAD at variou
cutting speeds
It can be concluded from the center deviation data that the
drill was more stable in UAD than that in CD at all the cutting
speeds. Some selective cutting speeds in CD (in this case
9.42 m/min) may have similar center deviation as that in UAD
A similar metrological study of circularity, cylindricality
surface roughness and hole oversize was also performed by
Akbari et.al [13] in the drilling of Inconel 738-LC alloy in
UAD. They also found reduced circularity and lower surface
roughness in UAD as compared to those in CD at the hole-
exits. However, their study was limited to the hole-exits only
unlike the current analysis of throughout the hole.
4.3. Machined surface and SEM analysisIn order to analyze the internal machined surface of a hole
the material was cut diametrically opposite with a diamond
coated precision saw and the internal cylindrical surface of theholes was analyzed in the scanning electron microscope (SEM)
The machined surfaces in both the processes of CD and UAD
“look” similar at all the parameters when observed at a low
magnification of 40 x. The machined surfaces at 0.942 m/min
in CD and UAD are shown in the Figure 11 and Figure 12
respectively. However, at the exit side, fibre pull-out was found
in the conventionally drilled hole, Figure 11. Apart from the
damage in the laminates near exit, the entire machined surface
0
0.05
0.1
0.15
0.2
0.25
0 50 100 150 200 250 300
A v e r a g e c i r c u l a r i t y ( m m )
Cutting speed (m/min)
CD
UAD
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0 50 100 150 200 250 300
A v e r a g e c e n t e r d
e v i a t i o n ( m m )
Cutting speed (m/min)
CD
UAD
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7 Copyright © 2014 by ASME
display similar damage. Also, when observing at a higher
magnification (60,000 x), plastic deformation of the smeared
matrix was found in CD and UAD at 0.942 m/min cutting
speed.
Figure 11: Machined surface in CD (0.942 m/min, 0.05 mm/rev)
Figure 12: Machined surface in UAD (0.942 m/min, 0.05 mm/rev)
At 9.42 m/min cutting speed, regular, sharp cracks were
found all over the smeared matrix in conventionally drilled
hole, Figure 13, while the matrix-softening and edge-rounding
was found at the cracked edges in the matrix of the machined
surface in UAD, Figure 14, at a higher magnification of
100,000x. Also, the crack-ends were found to be round in UADwhich suggest the softening of the smeared matrix, Figure 14.
This kind of softening of the matrix could happen either in the
case of temperature rise or if the property of the matrix would
have been transited from brittle to ductile. Similar transition
from brittle to ductile of CFRP material in UAD was also
reported by Makhdum et al. [7]. They reported larger lengths of
the chips generating in UAD of CFRP material as compared to
that in CD and the reason behind formation of such larger chips
in UAD was asserted as transition from brittle to ductile
behavior. This phenomenon is further discussed at the end o
section 4.5 of this paper.
Figure 13: Sharp and brittle cracks in CD at 9.42 m/min
Figure 14: Softened Matrix in UAD at 9.42 m/min cutting speed
At 94.2 m/min, Figure 15 and Figure 16, the machined
surface in CD was found to have debris on throughout the
machined surface while in UAD, the surface was machined
smooth showing plastic deformation in the smeared matrix. On
the other hand, at 282.6 m/min, clean machined surface was
found in CD, Figure 17, and a lot of debris was found on themachined matrix surface in UAD, similar to the one shown in
Figure 15. One possible reason for generating the debris on the
machined surface due to ultrasonic assistance at 282.6 m/min
could be the intermittent cutting mechanism in UAD; however
there is no evidence to support this argument.
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Figure 15: Debris formation on the machined matrix surface in
CD (94.2 m/min, 0.05 mm/rev)
Figure 16: Clean machined matrix surface in UAD (94.2 m/min,
0.05 mm/rev)
Figure 17: Clean machined matrix in CD (282.6 m/min, 0.05
mm/rev)
4.4. Chip morphologyThe chips were collected after drilling each hole on a
double-sided carbon tape. Further analysis was performed in
SEM after applying the gold coating. In order to have overal
information of the average chip formation, 3 random samples
were analyzed out of every drilled hole.
The chips obtained in both the processes of CD and UAD
were found to be similar. In general, three types of chip-
fragments were found in both the processes –
Large (1.4 mm) fragments of chip
Small (100 µm to 300 µm length) fragments of chip
Small broken fibres (10 to 200 µm length)The large fragments of the chips were obtained a
0.942 m/min and 282.6 m/min cutting speeds in CD, Figure 18.
Figure 18: Large fragment of the chip obtained in CD (0.942
m/min, 0.05 mm/rev)
The short broken chip-fragments with the length between
100 µm to 300 µm were the most common fragments types
obtained in all the cutting speeds in both the processes of CD
and UAD, Figure 19. These types of chip fragments were found
in a large in portion at 282.6 m/min cutting speed in CD and
UAD. Because of the fibre-matrix debonding, the broken loose
fibres were also obtained at every cutting speed in both the
processes of CD and UAD as shown in Figure 21.
At every cutting speed, there was a portion of chips having
plastic deformation in the matrix, Figure 22 and 23. These types
of chips were obtained in a large portion at the cutting speeds of
94.2 m/min and 282.6 m/min in both CD and UAD. Though asmall portion of chips having plastic deformation were also
found at the cutting speed of 0.942 m/min in CD and UAD
Hence, there was a mixture of chips found at every cutting
speed in CD and UAD and in particular, no clear difference was
found in the chips formed in CD and UAD at the current
machining parameters.
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Figure 19: Long and short, broken chip-fragments in UAD at
0.942 m/min cutting speed
Figure 20: Long chip-fragments obtained in UAD at 282.6 m/min
cutting speed along with short and regular chip-fragments
Figure 21: Loose fibres obtained in the drilling of CFRP in CD as
well as UAD (9.42 m/min, 0.05 mm/rev)
Figure 22: Plastic deformation in the matrix of the chip in UAD a
0. 942 m/min cutting speed
Figure 23: Definite chip formation and plastic deformation in the
matrix obtained at 94.2 m/min cutting speed
Makhdum et al. [15] reported generation of larger chips in
UAD in comparison to that in CD whereas in the presen
research there was no difference found in the chips. A possible
explanation for this could be the different ultrasonic oscillation
amplitude and different matrix of the CFRP material; however
the matrix used in their work was not reported.
4.5. Cutting temperature measurementThe cutting temperature is still a point of discussion
amongst the researchers. Some researchers have claimed the
cutting temperature to be higher in UAD as compared to CD
[14]. In the present study, the cutting temperature was measured
using the “surface mount” thermocouple by sticking it between
two CFRP plates as shown in the Figure 24. The other end o
the thermocouple was connected to the thermocouple data
logger connected to a computer. PicoLog software was used for
data acquisition and storage. To avoid damaging the welded
joint of thermocouple, drilling was performed within a distance
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10 Copyright © 2014 by ASME
of 1 mm from the welded joint. The dimensions of the CFRP
plates for temperature measurement were 70 mm x 25 mm x
7.4 mm and the rest of the setup was similar to Figure 3.
Figure 24: Thermocouple setup for cutting temperature
measurement
The cutting temperature variation in CD and UAD is
plotted in Figure 25.The cutting temperature was found to be
11°C higher in UAD at 282.6 m/min than that in CD while at
94.2 m/min cutting speed, it was 20°C lower in UAD than that
in CD. At the lower cutting speeds of 0.942 m/min and 9.42
m/min, the cutting temperature did not show a significant
difference between CD and UAD (2°C and 4°C respectively).
Figure 25: Cutting temperature measurement at various cutting
speeds in CD and UAD1
The glass transition temperature of bismaleimide matrix
lies between 200°C to 288°C based upon the composition [25].
The cutting temperature at 9.42 m/min was 126.56°C in UAD
(
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= axial rake angle in work reference system at point i,= angular deviation of the resultant cutting velocity vector because of the feed velocity at point i,
= dynamic axial clearance angle at point i,= clearance angle in the work reference system at point i = dynamic reference plane for point i; = reference plane in the work reference system for the point i
= cutting velocity of the point i in mm/min; = feed velocity of the point i in mm/minS0 = feed velocity of the drill in mm/rev
r = external radius of the drill;
r i = axial radius of the point i
N = rotational frequency of the drill (rpm)
a = ultrasonic oscillation amplitude of the drill
= angular ultrasonic oscillation frequency of the drill = helix angle of the drill = inclination angle in the work reference system
7. ACKNOWLEDGMENTSThe authors would like to acknowledge BAE Systems
for supplying the CFRP material, SGS Solid carbide tools Ltd.
for supplying the drills and DMG Mori Seiki Ltd. for supplying
the machining center for conducting the experiments in this
work. The authors would also like to thank Mr. Darren Grant, a
WMG technician, for his kind help, support and guidance
during the machining work.
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