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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.