Study of Cutting Speed Variation in the Ultrasonic Assisted Drilling of Carbon Fibre Composites

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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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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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 )



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 )



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 )



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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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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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


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 )


CD

UAD


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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 )


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 )


CD

UAD


7/11


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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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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Study of Cutting Speed Variation in the Ultrasonic Assisted Drilling of Carbon Fibre Composites

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