chapter 2 literature review -...
TRANSCRIPT
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CHAPTER 2
LITERATURE REVIEW
2.1 INTRODUCTION
Machining of miniature parts by micromachining is quite different
from traditional machining of parts of identical materials. The difference is in
the mode of chip production, the order of specific cutting pressure
encountered and surface integrity of machined parts. For better understanding,
a detailed literature survey has been carried out and presented in this chapter.
A brief review of micro and nano scale cutting experiments, ultraprecision
machines, metrology in micromachining and workpiece materials employed
in the past studies is also included. The emphasis of this literature review is
on experimental studies concerning cutting forces, size-effect, chip geometry,
surface morphology, machining concepts, tool wear, process modeling and
optimization.
2.2 MICROMACHINING PROCESSES
Micromachining processes have been limited to the machining of
simple features such as holes and slots in work pieces. However, with the
development of micro devices of size in the millimeter to sub-millimeter
range, demand for more complex miniaturized and high precision parts /
shapes is accelerating. This calls for development of miniaturized machining
processes to achieve the complex features. The processes include Micro-
Turning, Micro-Milling, Micro-Grinding, Micro-Electro Discharge
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Machining, Micro Wire-Electro Discharge Machining, Micro-Electro
Discharge Grinding and Micro-Electro Chemical Machining (Azizur Rahman
et al 2005).
2.2.1 Microscale Machining Issues
There is a number of issues in microscale machining which
fundamentally differ from macroscale machining and influence the basic
mechanisms of processing, resulting in changes in the chip-formation, and
dynamics of processing such as cutting forces, vibrations and process
stability. The generation and subsequent character of the resulting machined
surface is mostly dependent on them (Liu 2004). The fundamental issues are
discussed below.
2.2.2 Ultra Precision Machine Tools
Early development of ultra precision machine tools was largely
geared towards machining of large-scale optical devices. Precision diamond
turning machines are used to machining optical elements, glasses and related
parts.
In recent years, multi-axis control ultraprecision machining centers
with varying degrees of freedom are commercially available. They are used to
produce small workpieces with complex geometries and microscale patterns
and texture such as molds and dies for CD pickup lenses, contact lenses,
fresnel lenses, etc., driven by increasing marked trends in consumer products.
The efficient fabrication of these components is a matter of concern / interest
for miniaturization and integration of consumer products along with the rapid
development of micro and optical electronics (Weck et al 1998).
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Currently available multi-axis controlled ultraprecision machining
centers are in fact a progressive developmental form of traditional machine
tools. These ultraprecision machine tools can be classified into several types,
based on the type of positioning mechanisms used which include a screw-
based system driven by a rotary motor, linear motor drives, and a ball screw
or aero hydrostatic screw-based system. With respect to the table actuation
mechanism, two common configurations include the roller guide system or
aero hydrostatic slides in order to traverse the table with low friction and high
straightness. Bearing for rotational elements are similar to those found in the
table traverse mechanism.
Young-bong et al (2005) developed a PC-based 5-axis micro
milling machine, which can be used for machining micro- sized parts, and be
easily constructed at low cost. The micro milling machine presented in this
paper is mainly composed of commercially available micro stages, an air
spindle and PC-based control board. An effective method for initializing the
spindle position is proposed. Test results of the micro milling machine are
presented, which include machining of micro walls, micro columns and micro
blades. The constraint can be the thrust force in axial direction due to air-
spindle.
Furukawa et al (1986) built a machine using alumina-based
ceramics for the structural members owing to their high rigidity and thermal
reliability and surface-restricted type aerostatic slide ways to avoid friction.
Takeuchi et al (2000) have developed a 5-axis ultraprecison milling
machine using non-friction servomechanisms for the creation of 3D
microparts with translational resolution of 1 nm, rotational resolution of
0.00001 degree, and slideway straightness of about 10 nm in 200 nm. The
ultraprecision machining center employs aerostatic guideways and coreless
linear motors to provide non-contact, high resolution drive mechanisms
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achieving 1nm motion accuracy. To ensure thermal stability, alumina
ceramics were used for structural components.
Shamoto et al (2003) have developed an elliptical vibration milling
algorithm in order to achieve additional machining precision over that of
other ultraprecision machines. The elliptical vibration milling machine used a
double spindle mechanism to generate circular vibratory motion of the cutting
tool, which resulted in improved surface finish, even with a diamond tool on
ferrous materials. Diamond on ferrous materials calls for restricted constant
temperature; this can be achieved by minimizing the contact length / duration.
This is similar to thread whirling process.
Hara et al (1990) developed a high stiffness microcutting machine
with dynamic response up to 2 kHz. The contact between tool and the work
piece was detected through a piezoelectric actuator positioning system, and a
two axis micro-pulse system controller. Werkmeister and Slocum (2003)
developed a mesoscale mill using wire capstan drives, ball-screw splines, and
an air bearing spindle with an integral Z-axis.
2.2.3 Significance / Role of Micro-Structure
In micro-scale machining, the relationship of the cut geometry to
the workpiece microstructure is also markedly different from macro-scale
machining. In micro scale machining, where chip loads may range from
submicron levels to a few microns and depths of cut may be in the range of a
few microns to 100 µm, the cut geometry and the grain sizes of the workpiece
material are now comparable in size. As a result when cutting ferrous
materials, for example, the cutter engagement may be completely in ferrite,
then pearlite, thereby significantly altering the cutting mechanisms and
associated process response, e.g., forces and surface roughness.
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Vogler et al (2001) have noted significant frequency content in the
experimental cutting force signal at wavelengths equal to the average grain
size of the material being cut. Microstructural effects in micro scale cutting
necessitate quite different assumptions to be made concerning underlying
material behavior during micromachining and have precipitated the need for
new modeling approaches to account for such effects.
Apart from this the significance of orientation / direction of grains
in the machinability have also been reported. Thus it should be inferred that
the amount of material removed may be relatively smaller compared to
macro-scale machining, the work material will be subjected to higher order of
stressing in micro-machining. This can affect the structural integrity of the
material. Thus care has to be exercised in controlling the size-effect by proper
selection of cutting geometry and cutting condition.
2.3 EXPERIMENTAL STUDIES IN MICROMACHINING
2.3.1 Size Effects in Micromachining
Micro-cutting is characterized by very small amounts of material
removal with uncut chip thickness values varying from few microns (5 to
10 ) to several hundred microns. At these length scales of material
removal, the well-known size effect phenomenon is expected to be prominent.
In machining, the size effect is typically characterized by a non-linear
increase in the specific cutting energy (or specific cutting force) as the uncut
chip thickness is decreased. Thus it should be inferred that through the
amount of material removed may be relatively smaller compared to macro-
scale machining, the work material will be subjected to higher order of
stressing in micro-machining. This can affect the structural integrity of the
material. Thus care has to be exercised in controlling the size - effect by
proper selection of cutting geometry and cutting conditions.
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Kopalinsky and Oxley (1984) have conducted turning tests on plain
carbon steel of chemical composition 0.48% C, 0.3% Si, 0.13% S, 0.8% Mn
and 0.019% P. The cutting tool used was black ceramic indexable tip with
-5º rake angle and 2º clearance angle. The cutting edge radius of the tool was
ground
which was the smallest value of uncut chip thickness used in their tests.
A cutting speed of 420 m/min was used. The result shows a clear nonlinear
scaling effect in the specific cutting energy with decrease in uncut chip
thickness. In micro-machining, feed rate (un-cut chip thickness) of around 0,
294 times the edge radius (maximum values) has been reported. So despite
the feed rate being larger than the edge radius, the specific cutting energy
varied stochastically.
Schimmel and Endres (2002) investigated the effect of tool edge
geometry on cutting forces in orthogonal cutting with cutting tools of
different edge radius. Orthogonal cutting experiments were performed on
materials such as pure zinc, cast iron and Al-2024 at a cutting speed of 56.4
m/min, with carbide tools having edge radii ranging from a few microns to a
few hundred microns. Their results also clearly show the nonlinear scaling
effect in the specific cutting energy with decrease in uncut chip thickness.
Furukawa et al (1988) also reported the presence of size effect in
the specific cutting energy over an uncut chip thickness ranging from 0.5 to
tion of micro-cutting of different materials including
Aluminium alloy, Oxygen free copper, germanium, fluorite (CaF2) and acryl
resin. The aluminium alloy is considered to be isotropic in a macro sense.
Germanium is difficult to finish precisely because of its high hardness and
brittleness. Fluorite is a single crystal used for ultraviolet ray components, and
is not very hard but is very brittle. Acryl resin is a soft amorphous material
used for optical components. A single crystal diamond tool with 0º rake angle
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and 2º ~3º relief angle was used to cut all those materials with a cutting speed
of 6 m/min. It is also reported that nonlinear effect exists.
Lucca et al (1994) have experimentally determined that the
shearing process could not account for all of the observed energy when
machining OFHC (oxygen-free, high-conductivity) copper at small values of
depth of cut. They showed that the ploughing and elastic recovery of the
workpiece along the flank face of the tool plays a significant role when
machining with chip thickness values approaching the edge radii of the
cutting inserts. They have noticed that the specific cutting energy required to
machine at very low chip-thickness values could not be explained by the
energy required for shearing and for overcoming friction on the rake face of
the tool. But the significance of ploughing under these conditions was used to
explain the increase in the cutting energy.
Lucca and Seo (1991) have studied the effect of single crystal
diamond tool edge geometry rake angle, edge radius on the resulting cutting
and thrust forces and specific energy in ultraprecision orthogonal flycutting
on TECU® copper. Both the nominal rake angle and the tool edge profile
were found to have significant effects on the resulting forces and energy
dissipated over a range of uncut chip thicknesses from 20 m to 10 nm. When
the uncut chip thickness approaches the size of the edge radius, the effective
rake angle appears to determine the resulting forces. At small uncut chip
thicknesses, the effective rather than the nominal rake angle indicates the
direction of the resultant force as well as its magnitude.
Several efforts have been made to explain and predict the size
effect in microcutting. Most of the explanations offered to date can be
classified as follows:
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1) Material strengthening due to factors that vary with the uncut
chip thickness
2) Sub-surface deformation of the workpiece material
3) Tool edge radius effects
4) Energy required to create new surfaces via ductile fracture.
This is applicable to brittle materials. Application of
hydrostatic pressure can lead to crack-free upsetting and
dislodging of upset lumps (ductile machining) of ceramics
2.3.2 Surface Texture Production
The need to create surfaces of exceptional accuracy and quality for
micro-components is the driving force behind research into surface generation
in micro and nano scale machining. An improved understanding of the effect
and dominant mechanisms that govern surface generation in micro and nano
scale machining aids in the fabrication of micro-components with ultra-
smooth functional surfaces and highly precise dimensions, which essential in
many electronics and optics applications. Specific applications include micro-
scale fuel cells, micro-holes for fiber optics and micro-molds for optical
lenses, mostly concerned with geometric / shape precession.
Nakayama (1997) suggested that the quality of the surface finish
generated in micro and nano scale machining can be attributed to the
inaccurate motion of the cutting tool relative to the workpiece, as well as the
presence of a built-up edge. The inaccuracies of the cutting tool’s motion can
be eliminated through a combination of the use of higher precision machines
and designing a more rigid experimental setup. In addition, built-up edge can
similarly be avoided by (i) selecting mutually non-adhesive materials for tool
and work material, (ii) machining the work material at cutting temperatures
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above the recrystallization temperature of the work material, (iii) using a high
rake angle (> 30º), (iv) maintaining a sharp cutting edge and (v) machining at
very high cutting speeds. Use of higher cutting temperature and higher cutting
speed can lead to problem associated with machining dynamics; use of higher
rake angle restricts the tool material to almost use of diamond only. The
pursuit of better surface finish has promoted continued investigation in
surface morphology in micro and nano scale cutting. The constituents of the
work materials and the crystallographic orientation of the work material are
other factors found to have an influence on the surface finish of the machined
surface.
Experimental investigations by Eda et al (1985) on single point
diamond machining of aluminium and copper alloys, within the undeformed
chip thickness range of 2 -
is influenced by the alloys and constituents of the work material and the
deformation of the crystal boundary and separations. Alloyed particles that
were cracked and fractured by the tool during the cutting process and voids
observed on the machined surface supported this deduction. In addition, it
was verified that the machined surface roughness values are close to the
theoretical roughness values, in conformance with the form of the diamond
tool. The smoothest surface finish achievable on pure aluminium workpiece
in this investigation was reported to be 50 Å. The surface finish of the work
material is also influenced by the crystallographic orientation of the work
material (Sato et al 1991; Moriwaki et al 1993; To et al 1997). Especially
machining of aluminium alloy, the resultant stated surface could have yields
finer Ra value; but the texture will not prevent any lay pattern.
Sato et al (1991) and To et al (1997) described similar findings -
that surface roughness and flatness are affected by the cutting direction in
machining single crystal aluminium. Sato et al (1991) have reported that when
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the single crystal aluminium is machined along the [0 1 1] direction,
corresponding to its sliding direction, the surface finish produced has the
lowest roughness values. Alternatively, machining perpendicular to the
sliding direction along the [1 2 1] direction generates a surface finish with the
highest roughness values. Sato et al (1991) together with To et al (1997) have
concluded from their respective investigations that controlling the
crystallographic orientation of the work material during the machining
operations is effective in improving the surface finish.
Moriwaki et al (1993) have described similar findings in machining
single crystal copper. However, they further highlighted that the influence of
crystallographic orientation on surface roughness is significantly reduced
when the undeformed chip thickness is reduced. Moriwaki et al (1993) have
suggested that the improvement of the quality of the surface finish at small
undeformed chip thicknesses was because the surface was not generated at the
grain boundaries. In micro-machining, the material removal is not by
deformation and shearing as in macro-machining; mostly the work material is
plastically upset ahead of the cutting edge and the upset-lumps would be
dislodged in the form of chips. This upsetting results in over flattening of
surface asperities and consequent good finish. Normally meeting of this grain
boundaries will be at an angle (depression) contributing to the roughness. So
when they are under hydrostatic pressure, this grains may deform, resulting in
changes in the grain boundary meeting and subsequent roughness.
Lee and Cheung (2001) presented and experimentally verified a
dynamic surface topography model used to predict the local variation of
surface roughness in diamond turning of crystalline materials. The model
incorporates the micro-plasticity theory, theory of system dynamics and
machining theory to account for materials induced vibration in ultra-precision
machining. The model predicts both the magnitude and the effect of
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materials-induced vibration to provide quantitative estimates in the local
variation of the surface roughness caused by these material induced
vibrations.
Surface generation in the micro-end milling process was studied by
Vogler et al (2004). The surface roughness was found to be strongly affected
by the tool edge radius and significantly by the feed rate. It was observed that
for the 2 m edge radius as the feed rate was reduced to a certain value, the
surface roughness started to increase, indicating that an optimal feed rate
exists that will produce the smallest surface roughness value which was
attributed to the minimum chip thickness effect without any dwelling of the
tool wedge. Larger feed rate (in proportion to edge radius) will work with
lower order, effective negative wedge and less straining.
Experimental studies on microburr formation in milling aluminium
and copper were carried out with a range of chip loads, tool diameters and
depths of cut by Lee (2002). Different types of burr formation in micromilling
and conventional milling such as flag-type, rollover-type, wavy-type, and
ragged-type burrs were observed. At low cutting speeds, bending of the chip
is more dominant than fracture. As the cutting edge exits from the workpiece,
the chip rolls over forming a burr. In addition, a large tool edge radius-to-chip
load ratio causes rubbing and compression (up setting) instead of cutting and
generates more burrs. As the depth of cut and feed rate increased within the
studied range, the burr size was found to increase.
Schmidt et al (2002) investigated the influence of material structure
on the surface quality in micromilling. In the case of mold fabrication where
highly wear resistant materials are often used, the material has to be heat
treated before micro cutting to achieve reasonable surface finish.
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Paulo et al (2009) have studied the machinability of PA 66
polyamide with and without 30% glass fiber reinforcing, when precision
turning at different feed rates. Four different tool materials were tested:
Chemical Vapor Deposition Diamond (CVDD) coated carbide,
Polycrystalline Diamond (PCD) and ISO grade K15 uncoated cemented
carbides with (K15-KF) and without (K15) chip breaker. Dry micro-turning
experiments were carried out. The specific cutting force decreased as feed rate
was elevated and presented values comparable to metallic alloys;
nevertheless, the PA66 polyamide presented a threefold increase in specific
cutting pressure compared with the PA66-GF30 composite. Moreover, within
the cutting range tested, the surface roughness of the reinforced polyamide
was shown to be insensitive to changes in the feed rate. This trend was not
observed for the polyamide, whose roughness increased with feed rate. The
PCD tool gave the lowest force values associated with best surface finish,
followed by the ISO grade K15 uncoated carbide tool with chip breaker when
machining reinforced polyamide. Continuous coiled micro-chips were
produced, irrespectively of the cutting parameters and tool material employed.
It is seen that coated cutting tools do not perform with flank wear prove
machining environment.
Jiwang et al (2003) studied the performance of diamond cutting
tools during single point diamond turning of single-crystal silicon substrates
at a machining scale smaller than 1 µm. They found that the tool wear could
be generally classified into two types: micro-chippings and gradual wear, the
predominant wear mechanism depending on undeformed chip thickness. The
tool wear causes micro-fracturing on machined surface, yields discontinuous
chips and raises cutting forces and force ratio. Experimental results also
indicate that it is possible to prolong the ductile cutting distance by using an
appropriate coolant.
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2.3.3 Metrology in Micro-Machining
Umeda (2005) has conducted survey on measurement technology
related to micromachining and found that measurements of material
properties, force and displacement dynamics and shape in fabrication at the
micro level were the most interesting. As the scale of features and machined
parts decreases, the resolution of techniques used to measure and quantity
these parts must increase of course. Also the dimension of the features to be
measured may call for non-contact techniques
Howard and Smith (1994) modified conventional AFM technology
to cover long ranges of surface metrology. They used a precision carriage and
slide way mechanism to cover about 20 mm of travel and the AFM force
probe, which utilizes the repulsive atomic force, to generate the surface
contour.
In many cases, microparts include inside features such as pockets,
holes and channels. No technology exists to measure such features. Hence,
Masuzawa et al (1993) developed a vibroscanning method to measure the
inside dimensions of micro-holes. This method is limited only to conductive
materials because it uses a sensitive electrical switch by contacting a vibrating
micro-probe onto workpiece. Kim et al (1999) added another probe utilizing
contact by bending of the probe.
Miyoshi et al (1996) have developed a profile measurement system
using inverse scattering phase retrieval method. The system was able to
conduct in-situ measurement of a surface profile with submicron accuracy.
The tests on symmetric and non-symmetric fine triangular grooves showed
promising results in reconstructing measured profiles. Use of wave length for
measurement of depth related surface profile characteristics (Rt) is similar in
concepts.
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Many researchers developed a precision Coordinate Measuring
Machine (CMM) device with micron or submicron level resolution, but they
were not sufficient for present levels of micromachining capability. Further
developments improved the resolution up to few tens of nanometer and
finally, Jager et al (2000) developed a 3D-CMM with a resolution of 1.3 nm
using a probe and laser interferometers with angle sensors for guiding
deviation.
Cao et al (2003) have developed a three dimensional micro-CMM
for precise three dimensional micro-shape measurements. For this, they also
developed a 3D opto tactile sensor for the probe using a silicon boss-
membrane with piezo resistive transducers which can simultaneously measure
deflections of the probe and force in three dimensions. The system consists of
two stage measurements; coarse and fine measurements with a resolution up
to 1.22 nm and uncertainty less than 100nm. The use of membrane based
sensor is to contain / restrict the force of measurement on fragile features.
Ostuka et al (2003) demonstrated that ductile and brittle cutting
modes could be detected by use of an AE sensor and tool force measurements,
and ductile-mode cutting for brittle materials were achieved.
2.3.4 Process Modeling
Material behavior, friction characteristics and tool geometry are
incorporated into the process models with the aim of better describing the
complex nature of micro and nano scale cutting operations. Experimental
studies are subsequently conducted to verify the applicability of the finite
element or molecular dynamics models developed. Some of the notable
findings from these modeling studies are summarized as follows:
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Kim et al (1999) have analyzed the effect of the tool edge radius on
the cutting process using the finite element method. The model is based on an
Eulerian formulation with tools of finite edge radius and a rigid-viscoplastic
workpiece material. The cutting forces obtained from their finite element
simulation are found to be in good agreement with their experimental data.
They therefore concluded that the major cause of size effect is the tool edge
radius and its relative proportion to uncut chip thickness.
Woon et al (2009) performed Finite Element Analysis (FEA) of
micromachining using the arbitrary Lagrangian-Eulerian (ALE) method. The
assumptions made in modeling of conventional machining as the undeformed
chip thickness a is very much larger than the tool edge radius r, by at least
three orders of magnitude is certainly not appropriate for micromachining
when a approaches r in the micro scale. In this regard, the differences
between conventional machining and micromachining are believed to be
originated from the great size differences between a and r. They have
investigated the chip formation mechanism and its corresponding stress states
of AISI 4340 steel with finite element method (FEM) using the ABAQUS
suite of software coupled with the ALE method. They showed that chip is
formed through material extrusion under a critical a/r < 1. The changes in
chip formation behavior are driven by intense deviatoric and hydrostatic
stresses that are highly localized around the deformation zone. The onset of
such chip formation mechanism is signified with a constant changing negative
effective rake angle that becomes stable in a later stage when chip formation
reaches a stable tool-chip contact length. This depends on the a/r ratio and
feed rate.
Liang et al (1994) employed the FEM to analyze the influence of
the crystallographic characteristics of the material on the micro-cutting
process. The analysis indicated that grain orientation has a significant effect
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on the yielding cutting force for both aluminium and copper. The cutting
force also becomes a minimum when cutting is performed along the (1 1 1)
plane when compared with the (0 0 1) and (1 1 0) planes. Furthermore, the
yielding cutting force also changes at the grain boundary of polycrystalline
materials (for fcc materials).
In the study of tribological phenomena in nano scale machining
using MDS, Maekawa et al (1995) reported that friction and tool wear exert
the same influence in nano-cutting as that observed in macro-scale cutting.
Komaduri et al (1998) investigated the effect of tool geometry in
nano scale cutting using MDS and reported that the tool edge geometry has
significant influence on nano scale cutting. The tool edge geometry is found
to have significant influence on the cutting and thrust forces, force ratio,
specific energy and the sub-surface deformation.
Kim et al (1999) have proposed a FEM technique to predict the
stress and temperature distribution in micro-scale machining of oxygen-free-
high-conductivity copper. The results indicated that the temperature effect is a
very important factor to be considered in micro-scale cutting process due to
its influence on the flow stress distribution. The cutting force and flow stress
were over-predicted when the temperature effect was neglected.
Liu and Melkote (2004) presented a strain gradient based FEM
technique to predict the size effect in orthogonal cutting. The analysis showed
that strain gradient strengthening has minimal effect on the distribution of
temperature, effective plastic strain and effective stress within the workpiece.
However, strain gradient strengthening led to higher effective stress in the
deformation zones and the finished surface and lower plastic strain in the
primary deformation and secondary deformation zones. Furthermore, the
strain gradient effect also led to higher cutting temperatures.
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2.4 MICRO-MACHINING - CHIPLESS PROCESSING
Traditionally machining is classified as chip forming and chipless
machining. Among the chipless machining, electro discharge machining is
widely practiced. Development in electrode discharge machining in the micro
and nano-scale mostly concentrated with wire erosion technique.
2.4.1 Wire Electrode - Consideration
Ranganath et al (2005) studied the performance of wire electrodes
under the varied machining conditions, machining different materials at
different working conditions like voltage and intensity of machining pulse.
They reveal that coated electrodes show better performance capability
compared to the single component wires with respect to the surface finish
obtained. However coating material picked up by the work piece can affect its
performance. This practice has been done for deposition of brass, copper,
diamond impregnated on work piece.
Pham et al (2004) discussed some of the recent developments in
micro EDM (wire, milling and die - sinking) and the main issues affecting the
performance. They focused on planning of the edm process and the electrode
wear problems. They categorized wire edm as micro wedm when the wire
diameter was down to 0.02 mm.
Schacht et al (2004) explained the importance of wire impedence.
Due to the skin-effect, impedence depends on the frequency of the current
signal, especially for ferromagnetic wires, such as steel wire. Coatings will
prove to be primordial to prevent the machining speed from dropping
significantly.
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Tosun et al (2003) studied the effect of cutting parameters on size
of erosion craters (diameter and depth) on wire electrode experimentally and
theoretically investigated in WEDM. The experiments were conducted under
the different cutting parameters of pulse duration, open circuit voltage, wire
speed and dielectric fluid pressure. Brass wire of 0.25 mm diameter and
AISI 4140 steel of 0.28 mm thickness were used as tool and work piece
materials in the experiments. It was found that increasing the pulse duration,
open circuit voltage and wire speed increases the crater size, whereas
increasing the dielectric flushing pressure decreases the crater size. The
variation of wire crater size with machining parameters is modeled
mathematically by using a power function. Increasing dielectric flushing rate /
process can maintain shorting - free inter electrode gap and reduced cavity is
attained.
Puri et al (2003) have investigated the variation of geometrical
inaccuracy caused due to wire lag with various machine control parameters.
They carried out an experimental investigation based on the Taguchi method
involving thirteen control factors with three levels for an orthogonal array L27.
The main influencing factors are determined for given machining criteria,
such as: average cutting speed, surface finish characteristic and geometrical
inaccuracy caused due to wire lag. Also, the optimum parametric setting for
different machining situations have been found out and reported in this paper.
They used die steel as work piece and brass wire of 250 µm diameter as tool
electrode.
Nihat Tosun et al (2003) studied the effect of cutting parameters on
wear of wire electrode in WEDM. The experiments were conducted under
different settings of pulse duration, open circuit voltage, wire speed and
dielectric fluid pressure. Brass wire of 0.25 mm diameter and AISI 4140 steel
of 10 mm thickness were used as tool and work piece material. The level of
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importance of the machining parameters on the MRR was determined by
using Analysis of Variance (ANOVA) method.
Williams et al (1991) presented the results of investigation into the
characteristics of WEDM generated surfaces. They have concluded from the
Scanning Electron Microscope (SEM) photographs that the higher peak
current results in a rougher surface. Energy Dispersive Spectrometry (EDS)
revealed that some amount of wire electrode material from WEDM gets
deposited on to the work piece surface. The machining experiment was on D2
tool steel with stratified wire 0.25 mm diameter was used as electrode.
Prohaszka et al (1996) have investigated the effect of wire material
on the machinability in WEDM. During experiment, they used negative
polarity-wires of pure Magnesium, Tin, and Zinc and of diameter 0.5 mm
were used as the electrode and deionized water was used as a dielectric and
non alloyed steel as a work piece. The parameters pulse cycle time, discharge
time, and open circuit voltage as the fixed parameters. They reveal that
magnesium provides high MRR than other two electrodes and coated
electrodes provide better MRR.
Hewidy et al (2005) have analyzed the WEDM parameters through
Response surface methodology. Peak current, duty factor, wire tension and
water pressure were taken as input parameters. Volumetric metal removal
rate, wear ratio and surface roughness (Ra) were chosen as responses. They
have conducted the experiments on inconel 601 as a work material and thin
brass CuZn 377 wire with 0.25 diameter as an electrode. The experiments
were performed based on 24 factorial with central composite second order
ratable design.
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2.4.2 Process - Performance - Metal Removal Rate, Surface
Roughness
Lee et al (2001) have studied the effects of input machining
parameters such as the gap voltage, electrode polarity, electrode materials,
peak current, pulse duration, pulse interval and flushing, on the machining
rate and workpiece surface roughness. The output parameters are material
removal rate, relative wear ratio and surface roughness. The workpiece
material used in this study was tungsten carbide. Graphite, copper and copper
tungsten, were used as electrode material. They found that tools with negative
polarity give higher material removal rate, lower tool wear and better surface
finish and high open-circuit voltage is necessary for tungsten carbide due to
its high melting point and high hardness value. This is also due to working on
negative polarity.
Puertas et al (2004) have investigated the influence of machining
parameters pulse intensity, pulse time, and duty cycle on surface roughness,
electrode wear and material, while machining of WC-Co composites. The
mathematical models were obtained using design of experiments. The good
surface finish in case of tungsten carbide can be obtained through low values
for both intensity and pulse time. The most influential factor for MRR was
pulse intensity followed by duty cycle and pulse time.
Kuang-Yuan Kung et al (2009) have carried out powder mixed
electrical discharge machining of cobalt-bonded tungsten carbide. The
parameters considered were discharge current, pulse on time, grain size, and
concentration of aluminum powder particle for the machinability evaluation
of MRR and EWR. The response surface methodology was used to plan and
analyze the experiments. They found that machinability can be improved
through using small grain size aluminium particle powder mixed dielectric
fluid. It is surprising to add conductive particles in dielectric medium in
30
erosion process, certainly the plane generation (pyrolysis) will be affected.
Also such particles may lead to short the electricity. Further to keep them in
suspension is a problem.
Kanagarajan et al (2008) have studied the effectiveness of the EDM
process with tungsten carbide and cobalt composites evaluated in terms of the
material removal rate and the surface finish quality. The second order model
developed by regression analysis was used for optimization. The inputs
considered for this study were current, pulse on time and flushing pressure.
The optimization technique utilized was Non-dominated Sorting Genetic
Algorithm (NSGA-II).
Nakaoku et al (2007) have investigated the basic characteristics of
Micro edm of sintered diamond of four types with particle sizes of (1, 3, 10
and 20 µm). They used electrode material tungsten and edm oil. They have
also, tested tungsten carbide alloy for comparison. They revealed that
machinability of smaller particle size is better than larger particle sintered
diamond and machining speed increased with decreasing particle size. There
is no significant difference in machining speed between SD and tungsten
carbide, when an RC circuit with 10-3300 µF of discharge capacitance and
0.2 H of residual inductance. Diamond being not a good conductor, the
sintering medium should have played a significant role in erosion; with coarse
particle due to erosion of the binders, a skeleton matrix of diamond will
result, reducing the mrr. With finer particles, wire erosion of binder, small
diamond particles will be dislodged. This accounts for higher machining
speed into finer particles.
Gadalla et al (1989) have compared the surface produced by the
pulse type and capacitance type. The surface produced by the pulse type
power supply gives higher surface quality. Pulse type gives controlled
impulse of spark energy as per servo-feed electrode gap; this gives good
31
surface quality. Debris analysis showed that metallic vapors are formed and
condensed to hollow non-crystalline spheres. They found that electro
discharge machining of WC-Co composites produces roughness and hardness
comparable to a low speed diamond saw and yields much higher removal
rates.
Fleischer et al (2003) studied that with the development of the
WEDG (Wire-Electrodischarge-Grinding) it became possible to produce very
small electrodes or products like ejection pins or cores for mould inserts.
A new field for the WEDG is the production of milling tools for micro-cutting
to produce these milling tools in tungsten carbide with CNC-controlled EDM
machines. This research has shown the potential of the machining of micro-
cutting tools with a diameter smaller then 100µm, which is at the moment the
size of the smallest commercial milling tool in tungsten carbide—by micro-
EDM.
Ramakrishnan et al (2005) studied a multi response optimization
method using Taguchi’s robust design approach for WEDM operations.
Experimentation was planned as per Taguchi’s L16 orthogonal array. Each
experiment has been performed under different cutting conditions of pulse on
time, wire tension, delay time, wire feed rate and ignition current intensity.
Three responses namely material removal rate, surface roughness and wire
wear ratio have been considered for each experiment. Heat treated tool steel
was used as the work material for experimentation. It was identified that the
pulse on time and ignition current intensity has influence more than other
parameters considered in this study. Pulse on time and ignition current
decides the spark intensity; however pulse-off (delay time) can also play a
significant role especially with MRR and surface integrity (HAZ).
Manna et al (2005) optimized the material removal rate, surface
roughness, gap current and spark gap when machining particulate reinforced
32
Al/SiC metal matrix composite using robust design of experiments. They used
L18 mixed orthogonal array to determine the S/N ratio, and an analysis of
variance and F test values were used to indicate the significant machining
parameter affecting the machining performance. They have also developed
mathematical model for the machining characteristics using Gauss elimination
method.
Miller et al (2004) have investigated the effect of spark cycle (pulse
on time and pulse off time) on MRR in wire electrical discharge machining of
four types of advanced materials: porous metal foams, metal bonded diamond
grinding wheels, sintered Nd-Fe-B magnets and carbon-carbon bipolar plates.
Machine slide speed limit and spark on-time upper and lower limits are
identified for machining these materials. It reveals that it is difficult to
machine the metal foams without damaging the ligaments.
Scott et al (1991) used a factorial design method to determine the
optimal combination of control parameters in WEDM. A number of
32 machining settings, which resulted in a better metal removal rate and
surface roughness, were determined from 729 experiments. They constructed
a mathematical model to predict the metal removal rate and surface finish
when machining D2 tool steel material at different machining conditions.
They used zinc coated copper wire as electrode. They found that there is no
single combination of levels of the different factors that can be optimal under
all circumstances. Since MRR and surface roughness require conflictions
conditions, no single combination of levels / factors can give optimal
solutions.
Lin et al (2001) have presented the results of electric discharge
machining characteristics of TiNi shape memory alloys. They found that the
MRR of alloys in the processes significantly relates to the electro discharge
energy mode, involving the pulse current and pulse duration. It also has the
33
reverse relationship to the product of the melting temperature and thermal
conductivity of TiNi SMA’s. Many electro discharge craters and re-cast
materials are observed on the machined surface. The thickness of recast layer
initially increases, reaches a critical value and then decreases with increasing
pulse duration. The hardness of the alloys outer surface reached 750 HV.
Either increasing in melting temperature / thermal conductivity (both
normally oppose) will restrict erosion.
Yan et al (2007) have developed a micro WEDM pulse
discriminating and control system for the identification of gap states, more
precise on-line quantitative pulse train analysis, machining condition
monitoring and process control. They studied the effect of pulse interval,
machining feed rate and work piece thickness on the variations of the
proportion of normal spark, arc discharge and short circuit in the total spark.
They found that a long pulse interval results in an increase of the short circuit
ratio under a constant feed rate machining condition. With pulse interval, mrr
increases resulting in the tendency to shorting
Yan et al (2007) developed a transistor controlled RC type fine
finish power supply for wire-EDM. They developed power supply using anti -
electrolysis circuitry and CPLD-based pulse controlled circuit that can
provide low discharge energy pulses with a frequency of 500 KHz. They have
obtained discharge duration of 150 ns and peak current of 0.7A by adjusting
the capacitance and current limiting resistance in the discharge circuit. The
peak current increases slightly with the increase in pulse on time. A higher
current limiting resistance results in a lower peak control. The developed fine
finish power supply reduces the recast layer, thus eliminating rusting and
bluing in titanium. Anti-electrolysis which normally aims at reverse polarity
(DCRP). This can reduce heating of work piece and consequent relax layer.
34
Neelesh and Vijay (2001) done their research on Performance of
any machining process evaluated in terms of machining rate and surface
finish produced. Higher machining rate and better surface finish are desirable
for better performance of any machining process Various analytical and some
semi-empirical/empirical material removal models (approximately 40) for
different mechanical type advanced machining processes have been
comprehensively and exhaustively studied
Jose Marafona et al (1999) describe an investigation into the
optimisation of the process which uses the effect of carbon which has
migrated from the dielectric to tungsten-copper electrodes. This work has led
to the development of a two-stage EDM machining process where different
EDM settings are used for the two stages of the process giving a significantly
improved material removal rate for a given tool wear ratio.
Fuzhu Hana et al (2000) have developed the system to monitor the
gap distance. By integrating the transistor type isopulse generator with this
new servo feed control system, they were able to obtain a removal rate of
about 24 times higher than that of the conventional RC pulse generator with a
constant feed rate in both semifinishing and finishing. The effectiveness of the
servo feed control proved higher in finishing than in semi-finishing. Normally
constant feed drives assumes constant / fixed electrode wear. With finish
machining, even small variations in electrode wear can affect the
performance; have the need for servo feed control for finishing.
Shinya Hayakawa et al (2003) their aim of the study is to
understand the cause of short-circuiting and to improve the machining rate. A
pipe electrode is used and the working fluid is supplied to the gap space
though a fixed restrictor and the pipe electrode. It is found that the gap
distance at the moment when the short-circuiting occurs is more than several
micrometers. This result indicates that the cause of the short-circuiting is the
35
bridging of the gap space collected by debris. Based on the results, a new gap
control method in which the gap distance is kept constant even when short-
circuiting occurs was examined. If short-circuiting is due to bridging by
debris, a remedy can be change the flush - rate; use of servo controlled feed.
Palmers et al (2004) researched that white layers can be formed
during the surface generation of substrates to be PVD coated. The influence
of different manufacturing processes used in industry and especially the
influence of the possible presence of a white layer, as a consequence of these
techniques, on the final adhesion strength of a PVD coating is not well-
known. In this paper the link between the presence of different white layers
and the final adhesion is discussed and it is found that white layers formed
during grinding cause no adhesion problems. This is in contrast to white
layers formed on wire-electro-discharge machined surfaces, which are in
general thicker and give rise to bad adhesion. First measures are taken to
avoid the negative influence of white layers produced by wire-
electrodischarge machining on the PVD coating adhesion. Also the white
layer which can be a recast / resolidified layer in wedm is of varying
thickness.
Pham et al (2001) researched that, due to the high precision and
good surface quality that it can give, EDM is potentially an important process
for the fabrication of micro-tools, micro-components and parts with micro-
features. However, a number of issues remain to be solved before micro-EDM
can become a reliable process with repeatable results and its full capabilities
as a micro-manufacturing technology can be realised. Special attention is paid
to factors and procedures influencing the accuracy achievable, including
positioning approaches during EDM and electrode grinding.
Shankar Singh et al (2003) studied the effect of machining
parameters such as pulsed current on material removal rate, diameteral
36
overcut, electrode wear, and surface roughness in electric discharge
machining of En-31 tool steel (IS designation: Ti05 Cr 1 Mn 60) hardened
and tempered to 55 HRc. The work material was electric discharge machined
with copper, copper tungsten, brass and aluminium electrodes by varying the
pulsed current at reverse polarity and achieved with copper and aluminium
electrodes.
Wong et al (2002) developed a single-spark generator to study the
erosion characteristics from the microcrater size. Using a simple heat transfer
model, the efficiency at different discharge condition is also deduced. The
average efficiency of erosion, when estimated to be due primarily to melting
or evaporation alone, is found to be up to an order of magnitude higher at
lower-energy discharges than that at higher-energy discharges. Apart from
conductivity, diffusivity may also be significant heat characteristics
influencing erosion rate.
Guoa et al (2003) researched that in WEDM, the vibration of the
wire electrode has a significant influence on the performance and stability of
the machining process. The result shows that the discharge points can be
distributed much more evenly along the span of the wire when an optimum
condition is reached between discharge energy, discharge frequency, wire
tension and wire span. Under such a condition, it is possible that the hazard of
wire breaking can be avoided.
Lim et al (2002) developed the system to measure and control the
dimension of the thin electrode during the tool fabrication process. Different
methods have been investigated to fabricate a thin electrode with the desired
dimension without deflection. The performance of the micro-edm process was
evaluated in terms of the Material Removal Rate (MRR), Tool Wear Ratio
(TWR), and the stability of the machining. Influences of the various operating
parameters of the micro-EDM process, such as the operating voltage, gap
37
control algorithm, and resistance and capacitance values in the R-C spark
control circuit.
Miller et al (2001) investigated that the effect of spark on-time
duration and spark on-time ratio, two important EDM process parameters.
During the wire EDM, five types of constraint on the MRR due to short
circuit, wire breakage, machine slide speed limit, and spark on-time upper and
lower limits are identified. An envelope of feasible EDM process parameters
is generated for each work-material.
Guoa et al (2004) performed the research work, the discharge
points can be distributed much more evenly along the span of the wire when
an optimum condition is reached between discharge energy, discharge
frequency, wire tension and wire span. The modes of electrode vibrations are
quite complex under the action of continuous discharges, the first-order or the
second-order vibration plays an important role. It is evident that the
displacement of electrode relative to the original balance point is not
symmetrical and the backward bending of the electrode results. Significance
of dynamics of electro-sparking is seen.
Hung-Sung Liu et al (2001) had done their research on the
feasibility of fabricating micro-holes in the high nickel alloy using micro-
EDM. In this work, a two-stage cylindrical cutting tool of high hardness was
first used. The tool was precisely shaped with a first stage (i.e., tip) having a
smaller diameter, and a helically grooved second stage with a larger diameter
by the Wire Electro-Discharge Grinding (WEDG) process. The first stage of
the tool electrode was then used to drill a micro-hole in a plate using
micro-EDM process.
Liao et al (2000) have done their research to overcome wire rupture
in the Wire Electrical Discharge Machining (WEDM) process. A new
38
computer-aided pulse discrimination system based on the characteristics of
voltage waveform during machining was developed in this work. With the use
of this system, a large amount of sparking frequency data during wire rupture
process and under normal working conditions were collected and analyzed.
The voltage wave form (if not controlled) can lead to transient arcing and
consequent rupturing of electrodes.
Prohaszka et al (2002) have done their research to identify the
requirements of the materials used for wedm electrodes that will lead to the
improvement of wedm performance. Experiments were conducted regarding
the choice of suitable wire electrode materials and the influence of the
properties of these materials on the machinability in wedm. The machinability
during wedm was significantly improved with, the proper combination of the
electrical, mechanical, physical and geometrical properties of the wire
electrode. The materials used for the fabrication of wire electrodes must be
characterized by a small work function and high melting and evaporation
temperatures. The wire coated with magnesium /alkaline metals / alkaline
earth metals are found to improve the cutting efficiency compared with that
by Zn coated wire. Enhanced thermal stability means improved performance.
Sadegh Amalnik et al (2001) introduced an intelligent knowledge-
based system for evaluating Wire-Electro-Erosion Dissolution (WEED) in a
Concurrent Engineering (CE) environment and based on object-oriented
techniques. This design description was obtained through a feature-based
approach. Nine different classes of design features are interactively acquired.
The attributes of steel as a workpiece material, copper wire as a tool material,
a single electrolyte solution, one type of WEED machine, dielectric and
machining conditions such as current pulse on- and off-time, and nozzle
distance, are stored in a database. For each design feature, information needed
in manufacturing, such as the machining cycle time and cost, material
39
removal rate, width of cut, maximum and working feed-rate, cutting area, and
operation efficiency were estimated from the data base. Both working of
electrode and removal of material in edm are concurrent phenomenon; have it
as essential to adapt a concurrent appropriate for balanced performance.
Ulas Çaydas et al (2004) have developed an Adaptive Neuro-Fuzzy
Inference System (ANFIS) model to predict the White Layer Thickness
(WLT) and the average surface roughness achieved as a function of the
process parameters. Pulse duration, open circuit voltage, dielectric flushing
pressure and wire feed rate were taken as model’s input features. The model
combined modeling function of fuzzy inference with the learning ability of
artificial neural network; and a set of rules has been generated directly from
the experimental data. The model’s predictions were compared with
experimental results for verifying the approach.
Bonny et al (1999) said that the tribological characteristics of hot
pressed zirconia-based composites containing 40 vol. % of Wc, TiC0.5N0.5
or TiN and surface finished by Electrical Discharge Machining (EDM) were
evaluated by performing linearly reciprocating pin-on-flat sliding experiments
against WC-Co cemented carbide under unlubricated conditions. The
ZrO2-40 vol.% WC grade displayed an undoubtedly better wear resistance
compared to that of ZrO2-40 vol.% TiCN and ZrO2-40 vol. % TiN
composites. The morphology of the worn surfaces and the wear debris was
investigated by Scanning Electron Microscopy (SEM) and X-Ray Diffraction
(XRD), revealing several wear mechanisms such as polishing and abrasion,
mainly depending on the imposed contact load and the material composition.
The composite ZrO2-WC (4% of volume) solidify on WC-Co could have
experiments mostly attrition wear, compared to relatively higher wear
experiments by ZrO2 -TiCN / TiN (4% of volume).
40
Liu et al (2000) presented a new process of machining insulating
ceramics using Electrical Discharge (ED) milling; this process uses a thin
copper sheet fed to the tool electrode along the surface of the workpiece as the
assisting electrode and uses a water-based emulsion as the machining fluid.
This process was able to effectively machine a large surface area on insulating
ceramics. Machining fluid was a primary factor that affects the material
removal rate and surface quality of the ED milling. The effects of emulsion
concentration, NaNO3 concentration, polyvinyl alcohol concentration and
flow velocity of the machining fluid on the process performance had been
investigated. Machining of ceramics under severe conditions is attributed to
dynamics of electro-discharge. Hence conduction of electrode is an important
factor.
Choia et al (2001) investigated the effects of heat treatments on the
surface machined by W-EDM by four different machining methods such as
(i) milling and then grinding, (ii) W-EDM, and (iii) low temperature heat
treatment or (iv) high temperature heat treatment after W-EDM. The resulting
surface roughness was measured and the changes of surface microstructures
were investigated using a Scanning Electron Microscope (SEM) with an
Energy Dispersive X-ray Spectrometer (EDS). In general, heat treatments
after W-EDM improve the quality in terms of microstructures and surface
roughness. In particular, high temperature tempering can remove almost the
entire defects in the thermally affected zone. In order to examine the service
life of a press, on-line experiments with a chain product were carried out and
concluded that the quality of a press die prepared by the method (D) could be
as good as that by traditional manufacturing processes of the method. This
would be so, provided the heat affected zone and re-solidified layer are
uniform thickness.
41
Lauwers et al (1999) investigated the Wire-EDM machinability on
various newly developed electro-conductive ZrO2 ceramic matrix composites.
This investigation was based on design of experiments supported by a
fundamental study of the material removal mechanisms. It was shown that a
variation in grain size of the second phase material significantly influences on
the EDM performance, which can be largely related to the microstructure and
the properties of the developed material. This is the case even with metal
matrix composite.
Cabanesa et al (2001) proposed a methodology that guarantees an
early detection of instability that can be used to avoid the detrimental effects
leading to both unstable machining and wire breakage. The proposed
methodology establishes the procedures to follow in order to understand the
causes of wire breakage and instability. In order to quantify the trend to
instability of a given machining situation, a set of indicators related to
discharge energy, ignition delay time, and peak current has been defined.
Kanlayasiri et al (2000) investigated on the effects of machining
variables on the surface roughness of DC53 die steel machined by WEDM. In
this study, the machining variables investigated were pulse-peak current,
pulse-on time, pulse-off time, and wire tension. Analysis of Variance
(ANOVA) technique was used to find out the variables affecting the surface
roughness. Assumptions of ANOVA were discussed and carefully examined
using analysis of residuals. Quantitative testing methods on residual analysis
were used in place of the typical qualitative testing techniques. Results from
the analysis show that pulse-on time and pulse-peak current are significant
variables to the surface roughness of wire-EDMed DC53 die steel. The
surface roughness of the test specimen increases when these two parameters
increase. Lastly, a mathematical model was developed using multiple
regression method to formulate the pulse-on time and pulse-peak current to
42
the surface roughness. The developed model was validated with a new set of
experimental data, and the maximum prediction error of the model was less
than 7%.
Fuzhu Han et al (2002) described a novel simulation method for
wire Electrical Discharge Machining (EDM) in corner cut of rough cutting.
The simulation system analyzed the wire electrode vibration due to the
reaction force acting on the wire electrode during the wire-EDM. They also
set up a geometrical model between the wire electrode path and NC path, and
investigated the relationship between the wire electrode movement and the
NC coordinated movement. From the simulation system, the wire electrode
path could be obtained when the machining parameters such as the discharge
current, the tension of the wire and the thickness of the workpiece were
known. Simulations of the corner cut in right-angle machining, sharp-angle
machining, obtuse-angle machining were carried out. By comparing the
simulation results with experimental results, the feasibility of the simulation
method was proved. This is essential to avoid positioning over-cuts especially
around corners.
Seiji Kumagai et al (2004) developed a new EDM system using the
electrode, a wire encased in a dielectric pipe, which serves as a jacket,
however the operation parameters of the system were not systematically
optimized. In their study, the optimum width of the current pulse applied, the
optimum distance between the tips of the electrode and the jacket, and the
optimum vibration amplitude of the electrode feed controls were determined.
This optimization provides a -35% higher machining speed and ~40% less
electrode wear than the parameters used in the previous study. This could be
due to electrode in a confined space, minimizing tendency for facilitating
better flushing and reduced electrode vibration.
43
Gwo-Lianq Chern et al (1999) studied burr formation in micro-
machining using micro-tools. The micro-tools employed were fabricated by
micro-EDM using the Wire Electro-Discharge Grinding (WEDG) method in
the micro-EDM/milling machine researchers already developed.
Microtungsten-
fabricated. The simple-shaped micro-tool fabricated is able to perform the
micro-machining operation which is a combination of micro-milling and
grinding processes. Micro-slot and micro thin-walled structure had been
produced on Al 6061-T6 materials successfully. Burr formation in micro-
machining is experimentally investigated and classified into four types:
primary burr, needle-like burr, feathery burr and minor burr. Formation
mechanisms of these burrs and the relationship between their existence and
the machining condition were discussed.
Lauwers et al (2005) studied the WEDM of Si3N4, ZrO2 and Al2O3
based ceramics. To make this ceramic material electrically conducive TiN,
TiC, WC, TiCN were added. The cutting rate and surface roughness have
been measured under different machining conditions. In all rough machined
samples, they found many droplets and micro cracks. There was no evidence
for spalling. Besides the typical EDM material removal mechanisms, such as
melting evaporation, other possible mechanism such as change in composition
and oxidation of the base material occur.
Narender Singh et al (2004) the use of unconventional machining
techniques in shaping Aluminium Metal Matrix Composites (Al-MMC) has
generated considerable interest as the manufacturing of complicated die
contours in these hard materials to a high degree of accuracy and surface
finish is difficult. The objective of this work is to investigate the effect of
Current (C), Pulse ON-time (P) and Flushing pressure (F) on Metal Removal
Rate (MRR), Tool Wear Rate (TWR), Taper (T), Radial Overcut (ROC), and
44
Surface Roughness (SR) on machining as-cast Al-MMC with 10% SiCP
reinforcement. Analysis of Variance (ANOVA) was performed and the
optimal levels for maximizing the responses were established. Scanning
Electron Microscope (SEM) analysis was done to study the surface
characteristics.
2.4.3 Process - Optimisation - Review
Jin Yuan et al (2007) used Gaussian Process Regression (GPR) to
optimize the high speed wire - cut electrical discharge machining process.
They took mean current, pulse on time and pulse off time as input parameters.
Material removal rate and surface roughness were taken as measures of
process performance. They proved by experimental results that the GPR
models have the advantage over other regressive models in terms of model
accuracy, feature scaling and probabilistic variance.
Hewidy et al (2005) developed a model for machining parameters
of wedm for Inconel 601 alloy using Response Surface Methodology. They
developed a mathematical model for correlating the inter relationships of
various wedm machining parameters of Inconel 601 like peak current, duty
factor, wire tension and water pressure on the material removal rate, wear
ratio and surface roughness. Based on the Response Surface Methodology
(RSM), they found that increase in peak current leads to the increase of the
volumetric material removal rate. The increase in water pressure decreases the
tendency for arcing and increases the metal removal rate.
Puri et al (2005) considered white layer as a major flaw on a
workpiece surface machined with Wire-cut Electrical Discharge Machining
(WEDM). In this paper, an attempt has been made to model the white layer
depth through response surface methodology in a wedm process comprising a
rough cut followed by a trim cut. An experimental plan for rotatable central
45
composite design of second order involving four variables with five levels has
been employed to carry out the experimental investigation and subsequently
to establish the mathematical model correlating the input process parameters
with the response. Pulse-on time during rough cutting, pulse-on time, wire
tool offset, and constant cutting speed during trim cutting are considered the
dominant input process parameters whilst the white layer depth is the
response. Normally in electro discharge machining from rough machining
followed by finish / trim machining (micro-machining condition) result in
always nil formation of white layer.
Mahapatra et al (2006) used genetic algorithm, a popular
evolutionary approach, to optimize the wire electrical discharge machining
process with multiple objectives. Discharge current, Pulse duration, Pulse
frequency, Wire speed, Wire tension and Dielectric flow rate were taken as
input parameters. Metal removal rate, Surface finish and Cutting width (Kerf)
have been considered as measures of the process performance. They have
used Zinc coated copper wire of diameter 250µm. Experimental design is
done using Taguchi’s L27 Orthogonal array.
Sarkar et al (2005) have used constrained optimization and Pareto
optimization algorithm for optimization. Pulse on time, Pulse off time, Peak
current, Servo reference voltage, Wire tension and Dielectric flow rate were
taken as input parameters. Cutting speed, surface finish and dimensional
deviation have been considered as output parameters. Brass wire of diameter
250 µm and rectangular titanium aluminide alloy work piece have been used.
Taguchi’s parameter design using six parameters and three levels have been
used. L18 mixed orthogonal arrays table was chosen for DOE.
Ozdemir et al (2005) investigated the machinability of standard
GGG40 nodular cast iron by wedm using different parameters (voltage,
current, wire speed and pulse duration). The variation of surface roughness
46
and cutting rate with machining parameters was mathematically modeled by
using regression analysis method.
Ho et al (2004) reviewed the vast array of research work carried out
from the spin-off from the EDM process to the development of the WEDM. It
reports on the WEDM research involving the optimization of the process
parameters surveying the influence of the various factors affecting the
machining performance and productivity. Their paper has also highlighted the
adaptive monitoring and control of the process investigating the feasibility of
the different control strategies for obtaining the optimal machining conditions.
Wide ranges of WEDM industrial application were reported together with the
development of the hybrid machining process. It discusses these
developments and outlines the possible trends for future WEDM research.
Nihat Tosun et al (2004) used signal-to-noise (S/N) ratio and found
optimum parameter combination for the minimum kerf and maximum MRR.
Open circuit voltage, Pulse duration, Wire speed and Flushing pressure were
taken as input parameters. Material removal rate and kerf are the output
parameters. CuZn37 Master brass wire of diameter 250 µm and rectangular
AISI 4140 steel work piece of dimension 200 × 40 × 10 mm have been used.
Liao et al (2004) introduced a new concept of specific discharge
energy (SDE), which is found to be a material property in WEDM. The
relative relationship of SDE for different materials is fixed as long as these
materials are machined under the same conditions. Materials having similar
values of SDE display very similar machining characteristics if they are
machined under the same condition. By means of dimensional analysis of
SDE, a quantitative relationship between the machining parameters and gap
width in WEDM is obtained. The discharge energy required depends on
melting point, thermal conductivity and diffusivity. So identical consideration
of their properties will require same SDE with identical response.
47
Thillaivan et al (2003) developed optimization procedure based on
Genetic Algorithm (GA), Simulated Annealing (SA) and Continuous Ants
Colony Optimization (CACO) to optimize the machining parameters viz.
Pulse on time, Pulse off time, Peak current, Wire feed rate and machining
speed. The objective functions considered are maximization of MRR and
minimization of SR; they reveal that among their optimization techniques,
CACO is an effective method. They have developed mathematical models for
mild steel specimens. However, the material chosen mild steel will be erratic
in response. Since it is not a graded material for spark erosion.
Liao et al (1997) carried out study on the machining parameters
optimization of WEDM in SKD 11 alloy steel. They used 0.25 mm diameter
brass wire as electrode. According to Taguchi quality design concept
L18 orthogonal arrays table was chosen for conducting experiments. A total of
six machining parameters like pulse on time, pulse off time, table feed, wire
tension, wire speed and flushing pressure were chosen for the controlling
factors and each parameter have three levels and the designed machining
conditions are evaluated in terms of the measured machining performances
like gap width, material removal rate, surface roughness, discharging
frequency, gap voltage, normal discharge frequency ratio. The significant
parameters are found by the Analysis of Variance (ANOVA). By regression
analysis method mathematical models were established. They found that table
feed and pulse on time have a significant influence on the MRR and the gap
voltage, whilst the gap width and the SF are mainly influenced by the pulse
on time.
Shajan Kuriakose et al (2005) used multiple regression model to
represent relationship between input and output variables and a multi-
objective optimization method based on a Non-Dominated Sorting Genetic
Algorithm (NDSGA) was used to optimize Wire-EDM process. A non-
48
dominated solution set has been obtained and reported. Titanium alloy was
chosen as the work material and work piece thickness was kept as 60 mm.
Zinc coated brass wire of 0.25 mm was used as electrode for all experiments
and they have studied the phase transformation of Ti64 titanium alloy during
machining by WEDM process and found the formation of a thick oxide layer.
Pulse interval, pulse duration, injection pressure, wire speed and wire tension
were taken as input parameters. They have revealed that pulse interval is the
most important parameter influencing the formation of oxide layer. They also
revealed that coated wires are preferred over uncoated wires to obtain uniform
surface roughness characteristics.
2.5 GREY RELATIONAL ANALYSIS
Ko-Ta Chiang et al (2006) used grey relational analysis to optimize
the WEDM process with the multiple performance characteristics such as the
material removal rate and the maximum surface roughness. Cutting radius, On
time, Off time, arc on time, arc off time, servo voltage, wire feed and water
flow were taken as input parameters. Surface roughness and metal removal
rate have been considered as measures of the process performance. The
experimental design is based on Taguchi’s L-18 orthogonal arrays. The
response table and response graph for each level of the machining parameters
are obtained from the grey relational grade, and the optimal levels of
machining parameters are selected.
Huang et al (2003) applied Grey Relational analysis to determine
the optimal selection of machining parameters for the wedm process when
machining SKD11 alloy steel with brass wire electrode. The Grey theory can
provide a solution for a system in which the model is unsure or the
information is incomplete. Based on Taguchi quality design concept, they
have chosen an L18 mixed orthogonal array table for the experiments. With
both Grey rational analysis and a statistical method, they have found that the
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table feed rate had a significant influence on the metal removal rate, whilst the
gap width and surface roughness were mainly influenced by pulse on time.
Lin et al (2002) have reported the use of the grey relational analysis
based on an orthogonal array and fuzzy-based Taguchi method for optimizing
the multi- response process. Both the grey relational analysis method without
using the S/N ratio and fuzzy logic analysis are used in an orthogonal array
table in carrying out experiments for solving the multiple responses in the
Electrical Discharge Machining (EDM) process. Experimental results have
shown that both approaches can optimize the machining parameters (pulse on
time, duty factor, and discharge current) with considerations of the multiple
responses (electrode wear ratio, material removal rate, and surface roughness)
effectively. Cylindrical pure copper with a diameter of 8 mm and SKD11
alloy steel with diameter of 12 mm were used as tool and work piece in the
experiments.
Lin et al (2002) reported a new approach for the optimization of the
EDM process with multiple performance characteristics based on the
orthogonal array, with the grey relational analysis has been studied. In this
study, the machining parameters, namely work piece polarity, pulse on time,
duty factor, open discharge voltage, discharge current, and dielectric fluid are
optimized with the consideration of multiple performance characteristics
including material removal rate, surface roughness, and electrode wear ratio.
Cylindrical pure copper with a diameter of 8mm was used as an electrode to
machine the work piece of SKD11 alloy steel with a diameter of 12mm.
Grey theory established by Deng (1989) includes Grey relational
analysis, Grey modeling, prediction and decision-making of the system in
which the model is unsure or the information complete. It provides an
efficient solution to the uncertainty, multi-input and discrete data problem.
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The relation between machining parameters and machining performance can
be found out by using Grey relational analysis.
Chiang et al (2006) used Grey relational analysis for optimization
of the WEDM process of aluminium oxide particle reinforced (6061 alloy)
material with multiple performance characteristics. In this study, the
machining parameters namely the cutting radius of working piece, the on time
of discharging, the off time of discharging, the arc on time of discharging, the
arc off time of discharging, servo voltage, the wire feed and water flow are
optimized with consideration of performance characteristic such as the surface
roughness. L18 Orthogonal array was selected for experiments according to
Taguchi method. The grey relational co-efficient is calculated. The average
value of the grey relational co-efficient is the grey relational grade. From
these grey relational grade values, it was found that four operating factors arc
on time, arc off time, on time of discharging, and servo voltage have greater
influence.
2.6 ARTIFICIAL NEURAL NETWORK MODEL - REVIEW
Sarkar et al (2006) attempted to develop an appropriate machining
strategy for a maximum process criteria yield. They developed feed forward
back-propagation neural network model to model the machining process
when machining titanium aluminide alloy. The three most important
parameters - cutting speed, surface roughness and wire offset - have been
considered as measures of process performance. Their model is capable of
predicting the response parameters as a function of six different control
parameters, i.e. pulse on time, pulse off time, peak current, wire tension,
dielectric flow rate and servo reference voltage.
Fengguo et al (2004) created a neural network model for the
increased explosive electrical discharge grinding. A Genetic Algorithm (GA)
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was then applied to the trained neural network model to determine the optimal
process parameter values, in which a Grey Rational Analysis (GRA) is
conducted to determine the weights of two performance characteristics. The
integrated NN-GRA-GA system is used to determine the optimal process
parameters. They took polarity, pulse on time, pulse off time voltage and peak
current as process parameters.
Spedding et al (1997) attempted to model the WEDM process
through the response surface methodology and artificial neural networks and
found that the model accuracy of both was better. The same authors attempted
further to optimize the surface roughness, surface waviness, and speed of the
artificial neural networks and predicted values using a constrained
optimization model.
Tarang et al (1995) used a neural network system to determine
settings of pulse duration, pulse interval, peak current, open circuit voltage,
servo reference voltage, electric capacitance and table speed for the estimation
of cutting speed and surface finish. They formulated a neural network model
and simulated annealing algorithm in order to predict and optimize the surface
roughness and cutting velocity of the WEDM process when machining
SUS-304 stainless steel material.
From the literature survey carried out, the following can be
summarized:
1. Recent literature on micro machining involving chip
production and chipless processing have been reviewed.
2. With chip production processing, it is with seen that in micro-
machining the work material undergoes higher order with
cutting zone.
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3. Also to ensure good machining with controlled surface
integrity with high positive rake angle and clearance angle
called for. This restricts the selection of cutting tool to mostly
diamond.
4. Also in micro-machining due to relatively higher order radial
forces, machining tools of higher rigidity / precision are called
for.
5. Hence as an alternative, one can go in for chipless processing -
Non contact, wire erosion can be a good alternative.
6. The process can meet requirements of precision machining of
intricate shapes and smaller dimensional features.
7. The deviation on wire EDM regarding the process parameters
most of the deviation show pulse on time, peak current and
flushing rate as an important parameter for achieving desired
MRR / Surface Roughness.
8. Owing to the stochastic nature of the wire electrode space
conditions, no single combination of the parameters can give
optimum solutions.
9. Regarding the wire electrode the paper have highlighted
characterization of wire, breakage of wire and coated wire has
been recommended. The study on performance of coated wire
is rather incomplete.
10. Four paper have highlighted the significance of electric source
for wire erosion. Pulse operation is preferred.
11. Regarding process optimization, regular statistical modeling,
artificial neural network modeling and grey relational model
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have been demonstrated. The parameters for optimization are
pulse on time, peak current, flush pressure, wire speed and
electrode gap.
Mostly tool steels, cemented carbides, cast iron and composite
materials have been studied.
2.7 SCOPE FOR THIS STUDY
From the literature survey, it is clear that there is a good scope for a
detailed study on wire erosion process. For better understanding of the
process in application to aero space materials especially there belong to the
transition elements such as titanium and aluminium.
Accordingly the objectives of the study will be to:
Conduct detailed study on wire erosion of materials such as
Titanium, aluminium alloy and tungsten carbide to develop
useful data.
Analyse the data and develop statistical model considering
individual and interactive parametric influence.
The literature has shown that Magnesium / alkaline metals /
alkaline earth metals can be preferred to zinc for coating. But
coated magnesium will lead to oxidation problems and
alkaline metals are comparatively insufficient in erosion
machining with that of zinc coating. Hence zinc coated copper
wire has been chosen.
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With zinc coated, copper electrode conduct wire erosion
studies in cemented WC (die material / and aero space
materials such as titanium alloy and aluminium alloy.
Analyzing the data and developing the useful statistical
models.
Carry out micrographic studies to assess the response of the
materials to wire erosion.
Recommend optimum machining condition for good
machinability.