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1. ORIGINAL CONTRIBUTION
A Study of Drilling Behavior of Unidirectional Bamboo
Fiber-Reinforced Green Composites
M. Roy Choudhury1 • K. Debnath1
Received: 22 February 2019 / Accepted: 23 December 2019
The Institution of Engineers (India) 2020
Abstract In this study, the biodegradable composite
(unidirectional bamboo fiber/polylactic acid) was devel-
oped by means of film stacking technique in a hot com-
pression molding setup. The drilling characteristic of the
biodegradable composites was experimentally studied by
varying different factors. The effect of three different input
factors such as feed (8, 16, and 22.4 mm/min), speed (710,
1400, and 2000 rpm), and drill geometry (8-facet, dagger,
and slot drill) on the drilling forces (thrust force and tor-
que) was studied.
The signals of drilling-induced forces were found to be
different for the different drill geometries studied. The slot
drill induces minimum forces (thrust force and torque)
while making a hole in the composites among all drill
geometries. The experimental results reveal that drilling-
induced forces reduce at high spindle speed and low feed.
Keywords Natural fiber Polylactic acid Drilling
Drill geometry Thrust force Torque
Introduction
In the past decade, the applications of biodegradable
materials like natural fiber-reinforced composites were
tremendously increased. Researchers extensively utilized
these materials to manufacture innumerable engineering
products due to their adequate mechanical properties. Some
applications of composites reinforced with natural fiber in
the automotive are trunk panels, door panels, headliners,
etc. [1]. Another benefit of using biodegradable composites
is the elimination of the waste disposal problem [2]. These
materials have the ability to resolve into water and carbon
dioxide by the enzymatic activity of the living organism. In
biodegradable composites, both the fiber and polymer are
biodegradable in nature. In recent times, one of the
biodegradable polymers with good mechanical properties
commercially emerging is PLA. The renewable sources
like sugarcane, tapioca roots, etc., are used to derive PLA,
whereas biodegradable natural fibers like kenaf, hemp,
bamboo, flax, jute, ramie, etc., have replaced many syn-
thetic fibers due to their high mechanical properties and
low in cost. Bamboo fiber is the most easily available fiber
worldwide among all the natural fibers. The bamboo fiber
comprises of cellulose of 73.83%, lignin of 10.15%,
hemicellulose of 12.49%, pectin of 0.37%, and aqueous
extract of 3.16% [3]. Due to the high percentage of cellu-
lose, this fiber possesses structural properties. Research on
bamboo/PLA composites revealed their mechanical and
thermal behaviors [4–13]. Fazita et al. [3] stated that
bamboo fiber-reinforced PLA composites can be used for
biomedical, structural, and packaging applications.
Making of the hole by traditional drilling is a vital
operation for the final assembly of the composite parts by
bolt and nuts, rivets, etc. However, drilling of composite
parts causes significant damages to the part. The various
modes of damages observed in the hole drilled on natural
fiber composites are (a) peel-up and pushdown delamina-
tion, (b) fiber pullouts, (c) bending of exposed fibers,
(d) chipping, (e) spalling, (f) matrix burning, and (g) micro-
cracks formation [14, 15]. These damages result in weak-
ening the composite parts. The anisotropic and inhomoge-
neous nature of the composites is the main cause of the
formation of these types of damages. Also, the generation of
M. Roy Choudhury
mridusmitaroychoudhury19@gmail.com
1
Department of Mechanical Engineering, National Institute of
Technology Meghalaya, Shillong 793003, Meghalaya, India
123
J. Inst. Eng. India Ser. C
https://doi.org/10.1007/s40032-019-00550-w

2. a higher drilling force (thrust force and torque) results in
damages to the machined surface [16, 17]. The majority of
work published to study the drilling characteristic of poly-
mer composites is focused on the composites reinforced
with synthetic fibers (glass, carbon, etc.). Debnath et al. [18]
investigated the drilling performance of carbon/epoxy
composites. Carbon/epoxy composites are non-biodegrad-
able as both the matrix and fiber are non-biodegradable. It
was estimated that 7% of total reported work investigated
the drilling performance of composites reinforced with
natural fiber [1]. Debnath et al. [14] studied the drilling
behavior of nettle/polypropylene composites which is par-
tially biodegradable. Though nettle fiber is biodegradable,
the matrix used to develop the composite is polypropylene
which is non-biodegradable. Similarly, the drilling behavior
of the other partially biodegradable composites like sisal/
polypropylene [19, 24], coir/polyester [20–23], sisal/epoxy
[24], nettle/epoxy [25], flax/polypropylene [26], and sisal/-
glass/vinyl ester hybrid composite [27] was also studied by
the researchers. But only a handful of work is accessible on
the study of drilling performance of fully biodegradable
composites where both the fiber and matrix are biodegrad-
able in nature. Bajpai et al. [1] studied the drilling behavior
of fully biodegradable green composites such as sisal/PLA
and grewia optiva/PLA composites. The drill geometry was
found to be the most affecting factor on the induced forces
and drilled hole quality. However, negligible literature is
found to investigate the drilling characteristic of bamboo/
PLA green composites.
In this study, the drilling characteristic of bamboo/PLA
composite was evaluated experimentally by analyzing the
thrust force and torque. The effect of three different input
factors such as (a) feed, (b) speed, and (c) drill geometries
on the drilling-induced forces was studied and analyzed.
The statistical analysis of the experimental results was
performed to find the significance of each factor.
Materials and Method
Development of Green Composites
The long and loose form of bamboo fibers was imported
from Andhra Pradesh, India. Natur Tec Company based in
India provided the pallets of PLA (density = 1.24 g/cm3
).
The melting and glass transition temperature of the PLA
are 170 C and 58 C, respectively. The bamboo/PLA
green composite was fabricated in a hot compression
molding setup by means of film stacking technique. The
raw materials required for fabrication were dried for
dehumidification. A compression molding setup was
developed on a universal testing machine (UTM). The heat
required for melting the polymer was obtained by a heater
placed below the lower mold. The required pressure for the
compression molding was controlled by UTM. Five num-
bers of PLA films having a thickness of 1.5 mm were
obtained by molding setup, as shown in Fig. 1. During the
development of the film, the applied pressure was 3 MPa
Fig. 1 Fabrication of PLA/
bamboo composites
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3. and the temperature maintained to the melting temperature
of PLA. The mold was held under pressure for two hours
and allowed to cool before removing the film. These five
films were stacked alternately between four layers of fibers.
The fiber layers sequence in the stack was [0/45/
90/- 45]. The adhesive tape of high-temperatures resis-
tance was used at the edges of the fiber to constraint the
movement of the fibers during the layup process, as shown
in Fig. 1. The stack was then pressed in the molding setup
at 6 MPa and 180 C. The thickness of the composite plate
was 6 mm. The fiber weight fraction in the developed
composites was 18.37%. The developed composite speci-
men used for drilling is shown in Fig. 2.
Drilling Setup
The drilling operations were conducted on a milling
machine under normal environmental conditions. The
fabricated composite plate was clamped on a dedicated
fixture. Three levels of feed and speed and three types of
drills were considered as input factors. The input factors
and their corresponding levels are presented in Table 1.
The diameter of the solid carbide drills under investigation
was 8 mm. The responses such as thrust force and torque
were obtained by a dynamometer on which the fixture is
mounted that holds the specimen. The data acquisition
software that is installed in the computer was used to
analyze the force signals. The drilling setup and the drill
geometries considered in the present experimental study
are shown in Fig. 3. The different angles of the drills are
given in Table 2.
Fig. 2 Composite specimen
used for drilling
Table 1 Level of the input parameters and their values
Factors Level 1 Level 2 Level 3
Feed (mm/min) 8 16 22.4
Speed (rpm) 710 1400 2000
Drill geometry 8-facet Dagger Slot
Fig. 3 Machining setup and
drill geometries
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4. Results and Discussion
Investigation of Drilling Force Signals
The effect of the input factors on the drilling forces during
making hole in the developed composites was studied by
conducting full factorial design. Table 3 shows the exper-
imental data of induced forces at the different settings of
factors based on factorial design. Figures 4 and 5 depict the
captured signals of the induced forces. A significant vari-
ation in the pattern of signals was observed for different
drill geometries. This shows that the construction of the
drills exerts a significant influence on the drilling forces.
The 8-facet drill generates more forces followed by dagger
and slot drill. The thrust force is decreased to 22.6% and
40.9% when dagger and slot drill was used when compared
one-on-one with the 8-facet drill at 8 mm/min and
1400 rpm. Similarly, the torque decreased by 33.2% and
44.5% for dagger and slot drill. The slot drill was found to
produce a lesser amount of force as only two peripheral
cutting edges come in contact with the composite specimen
during drilling. Due to this, the engaged area of the com-
posite specimen with the drill reduces. The indentation
effect renders by the slot drill is also relatively less than the
other drills. Moreover, the slot drill does have more
clearance which facilitates quick ejection of formed chips
during drilling. The holes manufactured by the three dif-
ferent drills are presented in Fig. 6. Fiber breakage and
delamination around the hole are visible in all holes. The
8-facet drill produces more damage during the making of a
hole. On the other hand, damage to the hole produced by
the slot drill was found to be relatively less. This is because
the induced forces are less during drilling with a slot drill.
Many researchers established that the damage is directly
proportional to the forces induced during drilling [1]. The
higher induced forces result in more damage in and around
the drilled hole.
The fiber-reinforced composites are subjected to shear
and tensile stresses due to the generation of forces during
drilling. Due to these stresses, cracks first initiated at the
lower modulus region (polymer) and spread to the high
modulus region (fiber) [18]. Damage at the interlaminar
layers is a common phenomenon that can be observed in
the case of fiber-reinforced composites which is also called
delamination. Chen [28] established a nonlinear relation-
ship between delamination and drilling-induced forces.
Delamination is classified as peel-up and pushdown
delamination. The peel-up delamination causes the sepa-
ration of plies from the topmost layers of the composites.
This is due to the forces induced during drilling push the
plies through the flute of the drill as the drill enters the
composite [29]. As the drill travels through the thickness of
the composites, it pushes the last lamina due to high
induced forces. As a result, the separation of the last lamina
from the adjacent upper lamina occurs [29]. This type of
delamination is called pushdown delamination and is more
significant than that of peel-up delamination [30]. Peel-up
delamination occurs mainly due to higher induced thrust
force while pushdown delamination is due to higher
induced torque [14]. At high drilling-induced forces, the
cutting edge of the drill deteriorates. This result in fracture
of the fibers and thus the tips of the fibers are exposed to
the inner surface of the drilled hole. Hence, high induced
Table 2 The dimension of drill bits under investigation
Drill
geometry
Point angle
()
Lip clearance
angle
Helix angle
()
8-facet drill 118 28 27
Dagger drill 30 70 0
Slot drill 0 – 40
Table 3 Full factorial design for drilling-induced forces
Sl.
no.
Feed (mm/
min)
Speed
(rpm)
Drill
geometry
Thrust
force (N)
Torque
(N-cm)
1 8 710 8-facet 69.61 78.62
2 8 710 Dagger 81.41 41.52
3 8 710 Slot drill 48.00 39.38
4 8 1400 8-facet 69.78 59.27
5 8 1400 Dagger 53.99 25.97
6 8 1400 Slot drill 41.18 25.89
7 8 2000 8-facet 48.89 46.97
8 8 2000 Dagger 30.48 25.46
9 8 2000 Slot drill 27.48 18.49
10 16 710 8-facet 72.72 95.81
11 16 710 Dagger 96.04 45.09
12 16 710 Slot drill 49.19 41.66
13 16 1400 8-facet 76.80 93.16
14 16 1400 Dagger 45.20 33.25
15 16 1400 Slot drill 68.12 43.05
16 16 2000 8-facet 44.30 52.15
17 16 2000 Dagger 42.60 30.38
18 16 2000 Slot drill 32.04 29.13
19 22.4 710 8-facet 60.90 147.3
20 22.4 710 Dagger 97.78 67.33
21 22.4 710 Slot drill 57.56 50.98
22 22.4 1400 8-facet 75.82 123.8
23 22.4 1400 Dagger 74.72 60.09
24 22.4 1400 Slot drill 57.14 50.12
25 22.4 2000 8-facet 65.25 73.72
26 22.4 2000 Dagger 62.44 49.21
27 22.4 2000 Slot drill 52.40 40.88
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5. Fig. 4 Thrust force signals (feed: 8 mm/min and spindle speed: 1400 rpm)
Fig. 5 Torque signals (feed: 22.4 mm/min and spindle speed: 2000 rpm)
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6. forces also result in the unevenness of the drilled surface
[30].
Analysis of Drilling-Induced Forces
The effect of three different drilling input factors (feed,
speed, and drill geometry) on the induced forces (axial
Fig. 6 Image of holes produced by different drills under investigation
Fig. 7 Mapping of drilling forces a thrust force and b torque
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7. thrust and torque) was experimentally studied. The map-
ping of induced forces was also carried out as it gives a
general idea of the influence of input factors on the
responses. In these maps, the magnitude of forces was
divided into five different color zones (Fig. 7). The oper-
ator can easily select the appropriate range of drilling
factors to obtain the minimum drilling-induced forces.
From the mapping, it can be noticed that the dispersion in
thrust force values with the input factors is more than the
torque values. The thrust force and torque values are
highest at the higher feed and lower speed. Figures 8, 9, 10
and 11 give the graphical representation of the response of
induced forces with the variation of the level of input
factors. Figures 8 and 9 depict that the drilling-induced
forces tend to increase with the feed of the drill. It can be
noticed that the thrust force increased by 51.1% when feed
increased from lower level to higher level at 2000 rpm
(Fig. 8). There is an increase in 48.2% of torque when feed
increased from lower level to higher level at 710 rpm for
the dagger drill (Fig. 9). The cutting of composites
becomes difficult for the drill at a higher feed as it comes in
contact with the large thickness of the uncut chip and
consequently induced higher forces. Moreover, drilling-
induced damages like delamination, intralaminar cracks,
and high-density micro-failure zone occur in and around
the drilled hole due to the higher impact of the drill at
higher feed rates. The decreasing pattern of graphs with the
speed reveals that induced forces decrease with an increase
in speed (Figs. 10 and 11). There is a decrease of 62.5%
and 38.6% in thrust and torque when speed changes from
lower level to higher level at a constant feed of 8 mm/min
for the dagger drill. At high speed, heat generation due to
friction at the drilling interface is prominent which results
in the softening of the polymer. The softened polymer aids
ease removal and hence leads to the generation of lower
forces during making hole [31]. The lower speed of the
drill implies low strain rates and longer machining time.
This leads to higher drilling forces as compared to the
higher speed. The same responses of drilling-induced for-
ces are obtained during drilling of bio-composites by
Debnath and co-workers [24].
Statistical Analysis
The regression analysis plays a vital role in every statistical
analysis to explore and model the relationship between the
known and unknown variables. The regression analysis was
carried out to find the relationship between the output
20
30
40
50
60
70
8 16 22.4
Thrust
Force
(N)
Feed (mm/min)
8-facet
Dagger
Slot drill
Fig. 8 Response of thrust force with feed for the speed of 2000 rpm
20
40
60
80
100
120
140
160
8 16 22.4
Torque
(N-cm)
Feed (mm/min)
8-facet
Dagger
Slot drill
Fig. 9 Response of torque with feed for the speed of 710 rpm
20
30
40
50
60
70
80
90
710 1400 2000
Thrust
Force
(N)
Spindle Speed (RPM)
8-facet
Dagger
Slot drill
Fig. 10 Response of thrust force with speed for a feed of 8 mm/min
0
20
40
60
80
100
710 1400 2000
Torque
(N-cm)
Spindle Speed (RPM)
8-facet
Dagger
Slot drill
Fig. 11 Response of torque with speed for a feed of 8 mm/min
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8. responses and input variables. Equations 1–6 were
obtained by performing a regression analysis for all three
types of drill geometries. These regression equations can be
used to obtain the induced forces for any feed and speed
values. Analysis of variance (ANOVA) is an important part
of regression analysis. It gives the knowledge of the most
affecting factor and relative contribution of each input
factor in terms of percentage to obtain the responses. The
basic principle of ANOVA is to examine the differences
among the means of the populations by testing the amount
of dissimilarity within each of these samples. ANOVA of
thrust force (Table 4) shows that p values are less than 0.05
at 95% confidence interval for spindle speed and drill
geometry. But p value is slightly higher than 0.05 in the
case of feed. It can be stated that spindle speed and drill
geometry are significant factors as compared to feed in
obtaining the desired thrust force. All the p values in the
ANOVA for torque (Table 5) are smaller than 0.05. This
indicates that all three factors are significant. R2
value
reveals the fitting of experimental data to the model. In
other words, it is also called the coefficient of determina-
tion. The value of R2
always lies between 0 and 100 per-
cent. R2
value closer to 100% indicates the closeness of
fitted values in the model to the observed value. This
implies that the model explains the variability of response
data closer to mean. The value of R2
was found to be
Table 4 ANOVA for thrust force
Source DF Adj SS Adj MS F value P value %Contribution
A: Feed 2 993.6 496.8 3.31 0.057* 11.42
B: Spindle speed 2 3009.3 1504.7 10.03 0.001* 34.60
C: Drill geometry 2 1694.7 847.3 5.65 0.011* 19.48
Residual error 20 2999.6 150 34.48
Total 26 8697.2 R2
= 65.51%
DF Degree of freedom, Adj. SS adjusted sum of square and Adj, MS adjusted mean square
*Significant at 95% confidence level
Table 5 ANOVA for torque
Source DF Adj SS Adj MS F value P value %Contribution
A: Feed 2 5239 2619.4 16.71 0.000* 21.54
B: Spindle speed 2 3291 1645.5 10.50 0.001* 13.53
C: Drill geometry 2 12,648 6324.2 40.35 0.000* 52.02
Residual error 20 3135 156.7 12.89
Total 26 24,313 R2
= 87.11%
(a) (b)
0
5
10
15
20
25
30
35
40
Feed Spindle
speed
Drill
geomtry
Error
Percentage
contribution
(%)
Drilling input paramters
0
10
20
30
40
50
60
Feed Spindle
speed
Drill
geomtry
Error
Percentage
contribution
(%)
Drilling input paramters
Fig. 12 Percentage
contribution of drilling input
parameters for obtaining
a thrust force and b torque
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J. Inst. Eng. India Ser. C

9. 65.51% and 87.11% for thrust for and torque, respectively.
The fitting of the experimental data for induced torque is
adequate as R2
value is closed to 100% while R2
value for
thrust force is very low. The relative effects of each factor
on induced forces are graphically presented in Fig. 12.
Thrust force ð8-facetÞ ¼ 75:70 þ 1:018 A 0:01938 B
ð1Þ
Thrust force ðDagger drillÞ ¼ 75:77 þ 1:018 A
0:01938 B ð2Þ
Thrust force ðSlot drillÞ ¼ 58:93 þ 1:018 A0:01938
B
ð3Þ
Torque ð8-facet drillÞ ¼ 78:47 þ 2:292 A0:02064 B
ð4Þ
Torque ðDagger drillÞ ¼ 34:86 þ 2:292 A0:02064 B
ð5Þ
Torque ðSlot drillÞ ¼ 30:56 þ 2:292 A0:02064 B
ð6Þ
Conclusions
The major findings of the present experimental investiga-
tion of drilling characteristic of green composites are
summarized below:
• The signals of the drilling-induced forces are different
for different drill geometries. This indicates that drill
geometry significantly affects the drilling behavior of
developed green composites.
• The slot drill produces minimum drilling-induced
forces among all the drills (8-facet, dagger, and slot
drill). The thrust force was decreased by 22.6% and
40.9% and torque was decreased by 33.2% and 44.5%
when dagger and slot drill is used for producing hole as
compared to 8-facet drill at a lower level of feed and a
medium level of speed.
• The drilling-induced forces reduce at high spindle
speed and low feed. The thrust force and torque were
increased by 51.1% and 48.2% when feed is increased
from lower to a higher level at a constant speed using
dagger drill. Similarly, the thrust force and torque were
decreased by 62.5% and 38.6%, respectively, when
speed is increased from lower to a higher level at a
constant feed.
• From the ANOVA, it was observed that spindle speed
and drill geometry are the two factors that significantly
affect the thrust force, whereas all the three factors
were found to be significant for obtaining the torque.
The percentage contribution of the spindle speed was
highest followed by drill geometry and feed in
obtaining thrust force. The factor that has the highest
percentage contribution is the drill geometry in obtain-
ing torque followed by feed and spindle speed.
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