The document discusses composite materials for use in machine tool beds to reduce vibrations during machining operations. It presents an experimental study comparing the vibration response of machining two composite materials - glass fibre epoxy and glass fibre polyester. The results show that with an optimum number of composite layers, vibrations can be decreased significantly. However, more than the optimum number of layers causes an abrupt increase in vibrations. Effective damping is achieved when the composites are properly fixed to the bed and workpiece to avoid additional slip vibrations. Damping ability is higher when the matrix phase density is lower for the same fibre phase.

1. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 4, April (2014), pp. 09-26, © IAEME
9
DYNAMIC STABILITY OF MILLING MACHINE BED USING NATURAL
FIBRE COMPOSITE
1
Riadh Hasan Hadree, 2
Dr. Mohammad Tariq, 3
Dr. Fouad Alwan Saleh
1
Ministry of transport, General Maritime Transport Company, Republic of Iraq
2
Assistant Professor, Department of Mechanical Engineering, SSET, SHIATS-DU, Allahabad
3
Assistant Professor, Mechanical Engineering Department of Al-Mustansiriya University Iraq
ABSTRACT
Composites materials find wide applications in all fabricated products. The application of the
composites in the machine tool technology is to reduce the vibration and chatter during machining
operations. Recently developed composite is granite particles in epoxy matrix. It has high strength,
good vibration damping capacity and frictional characteristics. It is used as machine tool beds for
some precision grinders. In order to eliminate application limitations imposed by ordinary materials;
composite materials may be extended to other commercial utilizations which require durable services
at unconventional environmental conditions. Use of composite materials reduces the vibrations of the
system as desired which is justified from the experimental observations. With increase in number of
layers of composites at an optimum level the vibrations are decreased considerably. Abrupt increase
in vibration amplitude has also been observed with increase in number of layers of composites above
an optimum limit interposed between the table and work piece. Extensive experiments with different
layer combinations along with sandwich plates are carried out to determine vibration response of
work specimen with specified machining parameter, i. e., depth of cut and feed rate. The experiments
are duly carried out at small feed, low depth of cut and low cutting speed to primarily investigate the
scope of damping phenomenon in composite materials. This paper presents the experimental work of
the machining of the two different material composites i.e. Glass fibre epoxy polyester epoxy
material. The results obtained are compared with respect to each other. Effective damping can be
obtained only by proper fixation of the composites to the bed and the work piece. With improper nut
and bolt joint there is a danger of additional slip vibrations between the plates. With same fibre
phase, lower the matrix phase density more is the damping ability.
Keywords: Fibre Composites, Milling Machine, Dynamic Stability.
INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING
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ISSN 0976 - 6480 (Print)
ISSN 0976 - 6499 (Online)
Volume 5, Issue 4, April (2014), pp. 09-26
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1. INTRODUCTION
1.1 Milling Operation
Milling is the process of cutting away material by feeding a workpiece past a rotating
multiple tooth cutter. The cutting action of the many teeth around the milling cutter provides a fast
method of machining. The machined surface may be flat, angular, or curved. The surface may also
be milled to any combination of shapes. The work piece is mounted on the table with the help of
suitable fixtures. The desired contour, feed and depth of cut for the job are noted down. A suitable
milling cutter for the specified job is selected and mounted on the arbor. The knee is raised till the
cutter just touches the work piece. The machine is started. By moving the table, saddle and the knee,
for the specified feed and depth of cut, the desired job may be finished. The machine may then be
switched off. The different methods of milling are
1) Up milling
2) Down milling
Chatter vibrations have damped out on a slotted table Radial Drilling machine using
composite structure as a substitute for the base of the work piece [10]. The signal and RMS
amplitude, frequency and time period of vibrations are recorded for different number of layers.
Mechanical characteristics of an epoxygranite beam against the conventional materials such as cast
iron and steel with reference to constant stiffness have been studied [5]. It is observed that, for same
stiffness, the epoxy granite structure offers a considerable weight reduction, along with high
damping characteristics. New solution with the guide rail assembly technology is presently in use on
the basis of a guideway system consisting of a body and a milling table. The dynamics was compared
with the use of a frequency response function which had been determined in an impulse test. The
proposed solution is characterised by a higher dynamic stiffness, which may directly influence the
precision of the machined surfaces [6]. Properties among conventional materials have been taken.
Among the major developments in materials in recent year are composite materials, as they offer
several outstanding properties as compared to several conventional materials [8]. Hybrid welded
steel bed have been proposed as a replacement for cast iron as a machine tool bed material and the
results showed that the static and dynamic characteristics were superior to cast iron. The
advancements in machine tools to maximize the production by increasing spindle speeds have caused
vibration in machine tools [11]. 3D CAD model for the base line and the optimized design has been
created by using commercial 3D modeling software CATIA [7]. The 3D FE model has been
generated using HYPERMESH. The analyses were carried out using ANSYS and Design
Optimization is done with the help of Optistruct. The novel application examples of composite
structures to components for the robots, machine tools and automobiles considering the stiffness
design issues of composite structures. Mechanical design can be classified into stiffness design and
strength design. In the stiffness design, the stiffness or deformation of members is concerned, and the
enhancement of dynamic characteristics such as natural frequency or damping capacity of members
or systems is also important [2]. Vertical and horizontal slides were constructed of a large CNC
machine by bonding high modulus carbon-fibre epoxy composite sandwiches to welded steel
structures using adhesives. These composites structures reduced the weight of the vertical and
horizontal slides by 34% and 26%, respectively and increased damping by 1.5 to 5.7 times without
sacrificing the stiffness. The effects of transverse shear deformation on the modal loss factors as
well as the natural frequencies of composite laminated plates by using the finite element method
based on the shear deformable plate theory are discussed [12].

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1.2 Damping mechanism in composite materials
Damping mechanisms in composite materials differ entirely from thos
metals and alloys. The different sources of energy dissipation in
(a) Viscoelastic nature of matrix and/or
(b) Damping due to interphase
(c) Damping due to damage which is of two types:
(i) Frictional damping due to slip in the unbound regions between
(ii) Damping due to energy dissipation in the area of matrix cracks, broken
(d) Viscoplastic damping
(e) Thermoelastic damping
Structural damping
Rubbing friction or contact among different elements in a mechanical system causes
structural damping. Since the dissipation of energy depends on the particular characteristics of the
mechanical system, it is very difficult to define a model that represents perfectly str
The Coulomb-friction model is as a rule used to describe energy dissipation caused by rubbing
friction. Regarding structural damping (caused by contact or impacts at joins), energy dissipation is
determined by means of the coefficient of r
Damping in Machine Tools
Damping mechanisms in composite materials differ entirely from those in conventional
metals and alloys. Damping in machine tools basically is derived from two sources
damping and interfacial slip damping. Material damping is the damping inherent in the materials of
which the machine is constructed. The magnitude of material damping is small comparing to the total
damping in machine tools. A typical damping ratio value
0.003. It accounts for approximately
the contacting surfaces at bolted joints and sliding joints. This type of damping accounts for
approximately 90% of the total damping.
2. MATERIALS AND METHODS
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an
oscillatory system, particularly the harmonic oscillator. In mechanics, friction is one such damping
effect. In engineering terms, damping may be mathematically modeled as a force synchronous with
the velocity of the object but opposite in direction to it. If such force is also proportional to the
velocity, as for a simple mechanical viscous damper (dashpot), the force F
velocity v by F= -cv, where c is the viscous damping coefficient, given in units of N
Fig 1
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976
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Damping mechanism in composite materials
Damping mechanisms in composite materials differ entirely from those in conventional
metals and alloys. The different sources of energy dissipation in fibre-reinforced composites are:
a) Viscoelastic nature of matrix and/or fibre materials
(c) Damping due to damage which is of two types:
) Frictional damping due to slip in the unbound regions between fibre and matrix.
(ii) Damping due to energy dissipation in the area of matrix cracks, broken fibre
n or contact among different elements in a mechanical system causes
structural damping. Since the dissipation of energy depends on the particular characteristics of the
mechanical system, it is very difficult to define a model that represents perfectly str
friction model is as a rule used to describe energy dissipation caused by rubbing
friction. Regarding structural damping (caused by contact or impacts at joins), energy dissipation is
determined by means of the coefficient of restitution of the two components that are in contact.
Damping mechanisms in composite materials differ entirely from those in conventional
metals and alloys. Damping in machine tools basically is derived from two sources
damping and interfacial slip damping. Material damping is the damping inherent in the materials of
which the machine is constructed. The magnitude of material damping is small comparing to the total
damping in machine tools. A typical damping ratio value for material damping in machine tools is
. It accounts for approximately 10% of the total damping. The interfacial damping results from
the contacting surfaces at bolted joints and sliding joints. This type of damping accounts for
f the total damping.
2. MATERIALS AND METHODS
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an
oscillatory system, particularly the harmonic oscillator. In mechanics, friction is one such damping
eering terms, damping may be mathematically modeled as a force synchronous with
the velocity of the object but opposite in direction to it. If such force is also proportional to the
velocity, as for a simple mechanical viscous damper (dashpot), the force F may be related to the
is the viscous damping coefficient, given in units of N
Fig 1: Mass spring damper system
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
, © IAEME
e in conventional
reinforced composites are:
and matrix.
fibres etc.
n or contact among different elements in a mechanical system causes
structural damping. Since the dissipation of energy depends on the particular characteristics of the
mechanical system, it is very difficult to define a model that represents perfectly structural damping.
friction model is as a rule used to describe energy dissipation caused by rubbing
friction. Regarding structural damping (caused by contact or impacts at joins), energy dissipation is
estitution of the two components that are in contact.
Damping mechanisms in composite materials differ entirely from those in conventional
metals and alloys. Damping in machine tools basically is derived from two sources--material
damping and interfacial slip damping. Material damping is the damping inherent in the materials of
which the machine is constructed. The magnitude of material damping is small comparing to the total
for material damping in machine tools is
of the total damping. The interfacial damping results from
the contacting surfaces at bolted joints and sliding joints. This type of damping accounts for
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an
oscillatory system, particularly the harmonic oscillator. In mechanics, friction is one such damping
eering terms, damping may be mathematically modeled as a force synchronous with
the velocity of the object but opposite in direction to it. If such force is also proportional to the
may be related to the
is the viscous damping coefficient, given in units of N-s/m.

4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 4, April (2014), pp. 09-26, © IAEME
12
An ideal mass-spring-damper system with mass m (kg), spring constant k (N/m) and viscous
damper of damping coefficient c (in N-s/ m or kg/s) is subject to an oscillatory force and a damping
force,
Fୱ ൌ െkx (1)
Fୢ ൌ െcv ൌ െc
ୢ୶
ୢ୲
ൌ െcxሶ (2)
Treating the mass as a free body and applying Newton's second law, the total force F୲୭୲ on the body
F୲୭୲ ൌ ma ൌ m
ୢమ୶
ୢ୲మ ൌ mxሷ (3)
F୲୭୲ ൌ Fୱ ൅ Fୢ (4)
Calculation of material damping of Glass fibre polyester
In this reinforced composite material, there are different matrix and fibre phases:
Reinforcing material --- E-Glass fibre Matrix ---polyester
Ef = 80Gpa Em = 3.5Gpa
υf = 0.22 υm = 0.25
Gf = 32.7Gpa Gm = 1.4Gpa
ηmE & ηfE are taken 0.05 and 0.01 respectively [12].
Applying all the stresses, assuming for a cycle of operations in a system of forces, the net
material damping taken from [12]
Calculating material damping of Glass fibre epoxy
Reinforcing material --- E-Glass fibre Matrix ---epoxy
Ef = 80Gpa Em = 3.5Gpa
υf = 0.22 υm = 0.25
Gf = 32.7Gpa Gm = 1.4Gpa
2.1 Energy balance approach
The loss factor η is commonly used to characterize energy dissipation, due to inelastic
behaviour, in a material subjected to cyclic loading. Assuming linear damping behavior η is defined
by Vantomme as;
η ൌ
ଵ
ଶπ
∆୵
୵
(5)
where ∆W is the amount of energy dissipated during the loading cycle and W is the strain energy
stored during the cycle. Now consideringηଵ
, ηଶ
and ηଵଶ
:
ηଵ
– normal loading in fibre direction of UD lamina ( longitudinal loss factor)
ηଶ
– normal loading perpendicular to fibres (transverse loss factor )
ηଵଶ
—inplane shear loading (shear loss factor)

5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976
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2.2 Two phase model
Fig. 2: Matrix
Longitudinal loss factor ( ) is calculated by the following method: (loading in direction 1)
The total energy dissipated comprises the sum of that lost in the fibres and matrix. These
amounts are proportional to the fractions of elastic strain energy stored in the fibres and matrix
respectively;
i.e.
and are the loss factors for fibres and matrix, associated
Using the expressions for the strain energy,
From equations (8) and (12)
Transverse loss factor ( ) is calculated: (loading in direction 2)
As before, is expressed as
The strain energy contributions are derived in an analogous manner as for
assumption that the same transverse stress
development leads to
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976
6499(Online) Volume 5, Issue 4, April (2014), pp. 09-26, © IAEME
13
(m) and fibres (f) are connected in parallel
) is calculated by the following method: (loading in direction 1)
The total energy dissipated comprises the sum of that lost in the fibres and matrix. These
ounts are proportional to the fractions of elastic strain energy stored in the fibres and matrix
(6)
are the loss factors for fibres and matrix, associated with tensile lo
sing the expressions for the strain energy,
(7)
(8)
(9)
is calculated: (loading in direction 2)
(10)
The strain energy contributions are derived in an analogous manner as for , b
e transverse stress σ2 is applied to both the fibres and the matrix. This
(11)
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
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) is calculated by the following method: (loading in direction 1)
The total energy dissipated comprises the sum of that lost in the fibres and matrix. These
ounts are proportional to the fractions of elastic strain energy stored in the fibres and matrix
tensile loading.
but now with the
is applied to both the fibres and the matrix. This

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Shear loss factor (ηଵଶ
) is calculated: (loading in shear direction)
ηଵଶ
ൌ η୤ୋ
୛౜
୛
൅ η୫ୋ
୛ౣ
୛
(12)
where η୤ୋ
and η୫ୋ
are the loss factors for fibres and matrix associated with shear loading. The strain
energy fractions are worked out in the same way as for ηଶ
, as it is assumed that the shear stresses on
the fibres and matrix are the same. This leads to:
ηଵଶ
ൌ
η౜ృୋౣ୚౜ାηౣృୋ౜୚ౣ
ୋౣ୚౜ାୋ౜୚ౣ
(13)
The coefficients ηfE, ηmE, ηfG, ηmG are calculated from the graphs given in the book “Damping
of materials and members in structural mechanics” by Benhamin J. Lazan. These graphs are plotted
with E’ & E”, G’ & G”.
2.3 Calculation of material damping of Glass fibre polyester
In this reinforced composite material there are different matrix and fibre phases:
Reinforcing material (E) Glass fibre Matrix (polyester)
Ef = 80Gpa Em = 3.5Gpa
υf = 0.22 υm = 0.25
Gf = 32.7Gpa Gm = 1.4Gpa
From graphs given in the book “Damping of material and members in structural mechanics”
by Lazan ηmE & ηfE are taken; ηmE = 0.05, ηfE = 0.01
ηଵ
ൌ
η౜ు୉౜୚౜ାηౣు୉ౣ୚ౣ
୉౜୚౜ା୉ౣ୚ౣ
ηଵ
ൌ
଴.ଶଵଽ଻ହ
ଵ଼.ସ଻ହ
ൌ 0.0118
ηଶ
ൌ
η౜ు୉ౣ୚౜ାηౣు୉౜୚ౣ
୉ౣ୚౜ା୉౜୚ౣ
ηଶ
ൌ
ଵ.଴଴଻଻
ଵ଼.ସ଻ହ
ൌ 0.0545
ηmG = 0.01, ηfG = 0.015
ηଵଶ
ൌ
η౜ృୋౣ୚౜ାηౣృୋ౜୚ౣ
ୋౣ୚౜ାୋ౜୚ౣ
ηଵଶ
ൌ
଴.଴଼଺ଷ଻
଼.ସ଼ଷ
ൌ 0.0101
Applying all the stresses, assuming for a cycle of operations in a system of forces, the net
material damping according to [12] is given by
ηୟ୴
ൌ
ηభାηమାηభమ
଺଴
ൌ 0.0012
2.4 Calculating material damping of Glass fibre epoxy
Reinforcing material (E) Glass fibre Matrix (epoxy)
Ef = 80Gpa Em = 3.5Gpa
υf = 0.22 υm = 0.25
Gf = 32.7Gpa Gm = 1.4Gpa

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η୫୉
ൌ 0.03, η୤୉
ൌ 0.01;
ηଵ
ൌ
η౜ు୉౜୚౜ାηౣు୉ౣ୚ౣ
୉౜୚౜ା୉ౣ୚ౣ
ηଵ
ൌ
଴.ଶ଴ଵଷ
ଵ଼.ସ଻ହ
ൌ 0.0109
ηଶ
ൌ
η౜ు୉ౣ୚౜ାηౣు୉౜୚ౣ
୉ౣ୚౜ା୉౜୚ౣ
ηଶ
ൌ
଴.଺଴଻଻
ଵ଼.ସ଻ହ
ൌ 0.0328
η୫ୋ
= 0.02, η୤ୋ
= 0.015
ηଵଶ
ൌ
η౜ృୋౣ୚౜ାηౣృୋ౜୚ౣ
ୋౣ୚౜ାୋ౜୚ౣ
ηଵଶ
ൌ
଴.ଵ଺଼ଵଶ
଼.ସ଼ଷ
ൌ 0.0198
The net material damping for a single fibre epoxy plate according to Koo & Lee is given by
ηୟ୴
ൌ
ηభାηమାηభమ
଺଴
ൌ 0.0010
For a single plate of glass fibre polyester and epoxy material damping is 0.0012 and 0.0010
For ‘n’ number of plates:
Loss factor for two plates
ηଶ
ൌ ηଵ
୛ଶ
୛
൅ ηଶ
୛ଵ
୛
where W1 and W2 are the weights of the plates
Considering W1= W2 (homogenous plates) ηଶ
ൌ 2ηଵ
which implies ηn = n ηଵ
Table 1: Material damping of the composite layers
Number of plates Glass fibre polyester Glass fibre epoxy
1 0.0012 0.0010
2 0.0024 0.0020
3 0.0036 0.0030
4 0.0048 0.0040
5 0.0060 0.0050
6 0.0072 0.0060
Table 2: Specimen Details
Material No. Name of the Material Cross section (mm)
1 Glass Fibre Polyester 210x210x6
2 Glass Fibre Epoxy 210x210x5

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3. EXPERIMENTATION
3.1 Instrumentation
The following equipment is needed in recording the amplitude, frequency, period of the
vibrations during the machining operation
(1) Power supply unit
(2) Vibration pick-up
(3) Digital Storage Oscilloscope
Fig 3: Digital Storage Oscilloscope of Tektronix 4000 series and Vibration pickup
3.2 Digital Storage Oscilloscope Tektronix 4000 series
Display: 8x10 cm rectangular mono-accelerator c.r.o. at 2KV e.h.t. Trace rotation by front panel
present. Vertical Deflection: - Four identical input channels ch1, ch2, ch3, ch4.
Band-width: (-3 db) d.c. to 20 MHz (2 Hz to 20 MHz on a.c.)
Sensitivity: 2 mV/cm to 10 V/cm in 1-2-5 sequence
Accuracy: ± 3 %
Variable Sensitivity: > 2.5% 1 range allows continuous adjustment of sensitivity from 2-1(mV/cm).
Input impedance: 1M/28 PF approx.
Input coupling: D.C. and A.C.
Input protection: 400 V D.C.
Display modes: Single trace ch1 or ch2 or ch3 or ch4. Dual trace chopped or alternate modes
automatically selected by the T.B. switch.

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3.3 Experimental procedure
The work specimen of 240mm x 240mm x 10mm is a mild steel square plate. Four holes of
18mm diameter are drilled on the specimen at the corners. The glass fibre epoxy and polyester
composite plates are thoroughly cleaned and polished. Plates are fixed on to a bench vice and the
edges are filed to clear off the irregularities. All the plates are made to the exact dimensions for the
ease of the further operations. Four holes are drilled on each plate and these holes are needed to be
coaxial when the plates are placed upon one another and also with the mild steel. All the plates are
carefully made homogenously similar to avoid interfacial vibrations and slipping. The work piece is
then mounted onto the layered sheets of composites and tightly bolted to slotted table of the milling
machine using square head bolts.
Initially six glass fibre polyester plates each of 6mm thickness are placed upon the bed along
with mild steel. A contact type magnetic base vibration pickup connected to a digital phosphor
storage oscilloscope of Tektronix 1000 series is used to pick up amplitude, time period, RMS
amplitude and frequency on the mild steel during the machining operation.
Fig 4: Composite Six Layers under operation
The response signals with respect to amplitude, time period, RMS amplitude and frequency
are recorded and stored on the screen of the storage oscilloscope. Then the numbers of layers are
reduced to four layers and the observations are recorded. In this way, the experiments are repeated by
decreasing the number of layers of various composites. The experiments are conducted for 1, 2, 3, 4,
5 and 6 numbers of layers respectively. The whole process is again repeated using glass fibre epoxy
plates each of 5mm thickness. An Up milling cutting operation with constant feed of 18mm/min and
depth of cut of 0.03mm is performed during all the experiments. An oil-water emulsion made from
animal fat is used as a cutting fluid.

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4. EXPERIMENTAL RESULTS AND DISCUSSION
Milling operation is carried out to a depth of 3mm while increasing the number of composite
plates from one to six. A contact type magnetic base vibration pickup connected to a digital phosphor
storage oscilloscope of Tektronix 4000 series is used to pick up amplitude, time period, RMS
amplitude and frequency. Finally Mild steel plate alone is machined with no layer under it. Figure 4
shows the experimental set up.
Table 3: Experimental Frequency and Amplitude data for Glass fibre polyester
S. No. Depth of
cut (mm)
Feed rate
(mm/min)
Number of
layers
Signal
Amplitude
(mV)
Time
Period
(µs)
Frequency
(KHz)
RMS
Amplitude
(mV)
1 0.03 18 1 55.4 812.2 1.232 15.3
2 0.03 18 2 35.7 716.7 1.786 11.6
3 0.03 18 3 24.4 413.8 1.154 6.80
4 0.03 18 4 49.8 516.3 2.661 8.92
5 0.03 18 5 53.3 903.6 2.982 10.3
6 0.03 18 6 58.1 231.2 3.465 12.4
Fig 5: Vibration signal for single layer of Glass fibre polyester plates

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Fig 6: Vibration signal for two layers of Glass fibre polyester plates
Fig 7: Vibration signal for three layers of Glass fibre polyester plates

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Fig 8: Vibration signal for four layers of Glass fibre polyester plates
Fig 9: Vibration signal for five layers of Glass fibre polyester plate

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Fig 10: Vibration signal for six layers of Glass fibre polyester plate
Table 4: Experimental Frequency and Amplitude data for Glass fibre epoxy
S. No. Depth of
cut (mm)
Feed rate
(mm/min)
Number of
layers
Signal
Amplitude
(mV)
Time
Period
(µs)
Frequency
(KHz)
RMS
Amplitude
(mV)
1 0.03 18 1 35.0 864.2 2.728 7.98
2 0.03 18 2 22.4 517.1 1.311 5.23
3 0.03 18 3 42.7 674.2 1.354 8.92
4 0.03 18 4 52.3 231.4 1.722 10.86
5 0.03 18 5 62.8 717.2 2.556 13.41
6 0.03 18 6 68.3 412.4 2.290 17.7

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Fig 11: Vibration signal for one layer of Glass fibre epoxy plates
Fig 12: Vibration signal for two layers of Glass fibre epoxy plates

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Fig 13: Vibration signal for three layers of Glass fibre epoxy plates
Fig 14: Vibration signal for four layers of Glass fibre epoxy plates

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Fig 15: Vibration signal for five layers of Glass fibre epoxy plates
Fig 16: Vibration signal for six layers of Glass fibre epoxy plates

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4.3 Discussions
The above tables show the variation of signal amplitude with respect to number of layers for
different combinations of composites. It is observed that when the numbers of layers are increased,
the signal amplitude has decreased for both the composites to a certain extent and then increased
abruptly. The maximum amplitude is obtained when no composite material was used indicating that
the presence of composite materials decreases the vibration amplitude and increases the counter
vibration characteristics of the system. This shows that with increase in the plates the damping can
be increased but only to a certain limit and it would have a negative effect with much of progress.
Hence optimum level of plates is to be decided to profitably damp out the vibrations. This optimum
number of plates is different for glass fibre polyester and glass fibre epoxy.
Table 5: Optimum no. of plates and Height of machine too bed
Type of composite Optimum no. of plates Height of the machine bed
Glass fibre polyester Three (6 + 6 + 6) = 18mm
Glass fibre epoxy Two (5 + 5) = 10mm
CONCLUSIONS
Use of composite materials reduces the vibrations of the system as desired which is justified
from the experimental observations. With increase in number of layers of composites at an optimum
level the vibrations are decreased considerably. Abrupt increase in vibration amplitude has also been
observed with increase in number of layers of composites above an optimum limit interposed
between the table and work piece. The results obtained are compared with respect to each other. Out
of the two materials, signal amplitudes obtained are less for Glass fibre epoxy material. Therefore, it
can be concluded that Glass fibre epoxy material can be used for machine tool structures to reduce
the undesirable effects of vibrations. Effective damping can be obtained only by proper fixation of
the composites to the bed and the work piece. With improper nut and bolt joint there is a danger of
additional slip vibrations between the plates. Hence a proper and intact joint is necessary. On the
contrary the optimum number of plates is decided and a single plate of optimum thickness is used as
the bed material. The density of the matrix phase plays an important role in damping the vibrations.
With same fibre phase, lower the matrix phase density more is the damping ability. Though both the
thermosets polyester and epoxy have same material properties like Young’s modulus, Rigidity
modulus and Poisson’s ratio, epoxy has more damping ability than polyester because of its low
density.
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