Complete Research Paper

1

Capstone project report

On

Effects of Process Parameter on Machining of HCHCR Die Steel By EDM Using

Copper Electrode.

Submitted in Partial Fulfilment of the Requirement for Award of the Degree

Of

BACHLOR OF TECHNOLOGY

In

MECHANICAL ENGINEERING

Under the Guidance of

Jaspreet Singh

DEPARTMENT OF MECHANICAL ENGINEERING

LOVELY PROFESSIONAL UNIVERSITY

PHAGWARA, PUNJAB (INDIA) -144402

2014

Submitted by;

Sumit Kumar(11110667)

Saveen(11104893)

Sohil kumar(11104870)

Shayam Sundar(1104879)

2

CERTIFICATE

I hereby certify that the work which is being presented in the Capstone Dissertation entitled in

partial fulfilment of the requirement for the award of degree of Bachlor of Technology and

submitted in Department of Mechanical Engineering, Lovely Professional University, Punjab is

an authentic record of our own work carried out during period of Dissertation under the

supervision of Jaspreet Singh(17676), Assistant Professor, Department of Mechanical

Engineering, Lovely Professional University, Punjab.

The matter presented in this dissertation has not been submitted by me anywhere for

the award of any other degree or to any other institute. .

Date:28-11-2014

This is to certify that the above statement made by the candidate is correct to best of my

knowledge.

Date:28-11-2014 (Jaspreet Singh)

Supervisor

Signature of Examiner

3

ACKNOWLEDGEMENT

It is with a feeling of great pleasure that I would like to express my most sincere heartfelt

gratitude to Jaspreet singh, Asst. Professor, Dept. of Mechanical Engineering, lovely

Professional University, Phagwara for suggesting the topic for my capstone report and for his

ready and able guidance through out the course of my preparing the report. I am greatly indebted

to him for his constructive suggestions and criticism from time to time during the course of

progress of my work.

I express my sincere thanks to Prof. Ankur Bahel, Head of the Department of

Mechanical Engineering, lovely Professional University, Phagwara for providing me the

necessary facilities in the department.

I am also thankful to all the staff members of the department of Mechanical

Engineering and to all my well wishers for their inspiration and help.

I feel pleased and privileged to fulfill my parents ambition and I am greatly indebted

to them for bearing the inconvenience during my B.Tech. course

Date:

4

ABSTRACT

Electric discharge machining (EDM) is one of the most popular machining methods to

manufacture dies and press tools because of its capability to produce complicated shapes and

machine very hard materials. The intent of the present study is to study the effect of different

input parameters, namely, current, workpiece material, electrode material, dielectric medium,

pulse on time, pulse off time and powder and some their interactions on the MRR, TWR, micro

hardness and surface roughness. The effect of various input parameters on output responses have

been analyzed using Response Surface Methodology ( RSM).

5

Contents

CHAPTER 1 INTRODUCTION .................................................................................................... 9

1.1 PRODUCTION PROCESS ................................................................................................... 9

1.1.1 Raw material extraction ................................................................................................ 9

i) Primary Processes: ............................................................................................................... 9

ii) Secondary processes ............................................................................................................ 9

1.2 MATERIAL REMOVAL PROCESS ............................................................................. 10

i) Conventional machining: ............................................................................................... 10

ii) Non-conventional machining: ........................................................................................ 10

1.2.1 Mechanical Energy (Mechanical Processes): ............................................................... 11

1.2.2 Thermal Energy (Thermal Process): ............................................................................. 11

1.2.3 Electrical Energy (Electro chemical Processes): .......................................................... 11

1.2.4 Chemical Energy (Chemical processes): ...................................................................... 11

1.3 ELECTRICAL DISCHARGE MACHINING ........................................................................ 11

CHAPTER 2 - SCOPE OF THE STUDY

....................................................................................................................................................... 26

CHAPTER 3 OBJECTIVE OF THE STUDY .............................................................................. 28

CHAPTER 4- LITERATURE REVIEW ..................................................................................... 29

4.1 GAPS IN LITERATURE REVIEW ................................................................................... 33

CHAPTER-5 EQUIPMENT, MATERIALS AND EXPERIMENTAL SETUP ......................... 35

5.1 Description of machine / set up ........................................................................................... 35

5.1.1 Dielectric reservoirs pump and circulation system....................................................... 35

5.1.3Working tank with work holding device– ..................................................................... 37

5.1.4 X-y table accommodating the working table ................................................................ 37

5.1.6 The servo system to feed the tool ................................................................................. 38

CHAPTER 6 - RESEARCH METHODOLOGY ......................................................................... 40

CHAPTER 7 - RESEARCH ANALYSIS .................................................................................... 41

Figure7.5 ....................................................................................................................................... 51

7.4 Surface Roughness .................................................................................................................. 51

CHAPTER 8 - conclusion ............................................................................................................. 58

6

CHAPTER 9- REFRENCES ...................................................................................................... 59

7

List of Figure

Figure 1 Production process ...................................................................................... 9

Figure 2 Classification of Manufacturing ................................................................10

Figure 3 EDM machine set up .................................................................................12

Figure 4 EDM machine set up .................................................................................12

Figure 5 Geometry of workpiece and tool ..............................................................12

Figure 6 Formation of Ions during machining .........................................................13

Figure 7 working mechanism of EDM ....................................................................14

Figure 8 Varition of capacitor voltage with time in RC circuit ..............................15

Figure 9 Voltage and Current waveform during EDM ............................................15

Figure 10 EDM components ....................................................................................16

Figure 11 Die sinking EDM .....................................................................................17

Figure 12 wire cut EDM ..........................................................................................17

Figure 13 Principle of power mixed EDM (Kensal et.al 2007). ..............................19

Figure 14 Actual profile of a single EDM pulse (Fuller 1996) ...............................20

Figure 15 Typical EDM pulse current .....................................................................23

Figure 16 A typical cryogenic treatment cycle ( yildiz)Error! Bookmark not

defined.

Figure 17 Dielectric Reservoir ................................ Error! Bookmark not defined.

Figure 18 Electric pump ...........................................................................................36

Figure 19 Power generator and control unit ............................................................36

Figure 20 CRO .........................................................................................................37

Figure 21 x- y table coordinates ...............................................................................37

Figure 22 Tool holder, workpiece and tool ..............................................................38

Figure 23 Servo system for tool feed .......................................................................38

Figure 24 work plan ................................................ Error! Bookmark not defined.

8

9

CHAPTER 1 INTRODUCTION

1.1 PRODUCTION PROCESS

Production process is the process of converting raw material into the form of a final product (Fig.

1.1). Mainly it is divided in three parts:

1) Raw material extraction

2) Transformation Processes

3) Final output

1.1.1 Raw material extraction: Mainly it deals with collecting input materials required for

producing desired output. Generally these inputs are man, machine, material and money

etc.

1.1.2 Transformation processes: Transformation processes are the main preliminary operations

techniques required for conversion of raw material into final product or some part of the final

assembly. These operations are called manufacturing processes. Manufacturing operations are

further divided into two main categories (Fig. 1.2):

i) Primary Processes: The primary processes are those processes which provide basic shape

and size to the material as per designer’s requirement. Casting, forming, powder

metallurgy are such processes to name a few.

ii) Secondary processes: Secondary processes are the processes that provide the final shape

and size with tighter control on dimension, surface characteristics etc. Material removal

processes are mainly the secondary manufacturing processes.

Figure 1 Production process

10

1.2 MATERIAL REMOVAL PROCESS

It is basically a shaping operation in which material is removed in a way to get the finished

surface. Depending on the requirements, different material removal techniques are used for

different operations. Material removal processes are mainly divided in two types:

i) Conventional machining: In conventional machining processes, tool is always in

contact with work piece and material is removed in the form of chips. Chips may be

continuous or dis-continuous depending on work piece material, like may be ductile or

brittle material. Stresses on the work piece are very high in this case and hence it can’t be

used for machining fragile, delicate materials. As well in this case tool is required to be

harder than that of the work piece and hence it can also be not used for very hard

materials. Examples are Turning, Drilling, Boring, Shaping and Milling etc.

ii) Non-conventional machining: Keeping in view the drawbacks of conventional

machining, further advanced techniques were discovered for machining very hard, tough,

intricate shape, delicate and fragile materials. In case mechanical energy is not used for

machining but different other forms of energies are used for material removal. In this

case neither tool is in contact with work piece nor chips are produced but only debris are

produced which are very small in size. Depending in different types of energies used for

machining, non-conventional machining is divided into different types and classification

is shown in flow diagram (Fig. 1.2).

Figure 2 Classification of Manufacturing

11

Newer machining methods can be classified on the basis of the type of energy they employ for

purpose of metal removal. Broadly speaking they can be classified as below:

1.2.1 Mechanical Energy (Mechanical Processes): In mechanical processes metal removal

takes place either by a mechanism of simple shear or by erosion mechanism where high velocity

particles are used as transfer or by erosion mechanism where high velocity particles are used as

transfer media and pneumatic/hydraulic pressure acts as source of energy. It includes ultrasonic

machining, water jet machining and abrasive jet machining etc.

1.2.2 Thermal Energy (Thermal Process): Thermal processes involve the application of the

application of very thin intense local heat. Here melting or vaporization from the small area sat

the surface of the workpiece removes material. The source of energy used is amplified light,

ionized material and high voltage. Examples are laser beam machining, ion beam machining,

plasma arc machining and electric discharge machining.

1.2.3 Electrical Energy (Electro chemical Processes): Electrochemical processes involve

removal of metal by mechanism of ion displacement. High current is required as the source of

energy, and electrolyte acts as transfer media. It includes electro-chemical machining, electro

chemical grinding etc.

1.2.4 Chemical Energy (Chemical processes): Chemical processes involve the application of

resistant material (acidic or alkaline in nature) to certain portion of the workpiece. The desired

amount of material is removed from the remaining area of the workpiece by subsequent

application of an etching that converts the work piece material into a dissolve metallic salt. It

includes chemical machining and photochemical machining.

1.3 ELECTRICAL DISCHARGE MACHINING

It is an advanced machining process primarily used for hard and difficult metals which are

difficult to machine with the traditional techniques. Only electrically conducting materials are

machined by this process. The EDM process is best suited for making intricate cavities and

contours which would be difficult to produce with normal machines like grinders, end-mills or

12

other cutting tools. Metals such as hardened tool-steels, carbides, titanium, inconel and kovar are

easily machined through EDM.

EDM is a thermal process which makes use of spark discharges to erode the material from

workpiece surface. The cavity formed in EDM is a replica of the tool shape used as the erosions

occur in the confined area.

Since spark discharges occur in EDM, it is also called as "spark machining". The material

removal takes place in EDM through a rapid series of electrical discharges. These discharges

pass between the electrode and the workpiece being machined. The fine chips of material

removed from the workpiece gets flushed away by the continuous flowing di-electric fluid. The

repetitive discharge creates a set of successively deeper craters in the work piece until the final

shape is produced.

Figure 4 EDM machine set up Figure 3 EDM machine set up

Figure 5 Geometry of workpiece and tool

13

1.4 PRINCIPLE OF ELECTRICAL DISCHARGE MACHINING

In EDM, a potential difference is applied between the tool and workpiece. Both the tool and the

work material are to be conductors of electricity. The tool and the work material are immersed in

a dielectric medium. Generally kerosene or de-ionized water is used as the dielectric medium. A

gap is maintained between the tool and the workpiece. Depending upon the applied potential

difference and the gap between the tool and workpiece, an electric field would be established.

Generally the tool is connected to the negative terminal of the generator and the workpiece is

connected to positive terminal. As the electric field is established between the tool and the job,

the free electrons on the tool are subjected to electrostatic forces. If the work function or the

bonding energy of the electrons is less, electrons would be emitted from the tool (assuming it to

be connected to the negative terminal). Such emission of electrons are called or termed as cold

emission. The “cold emitted” electrons are then accelerated towards the job through the dielectric

medium.

As they gain velocity and energy, and start moving towards the job, there would be collisions

between the electrons and dielectric molecules. Such collision may result in ionization of the

dielectric molecule depending upon the work function or ionization energy of the dielectric

molecule and the energy of the electron. Thus, as the electrons get accelerated, more positive

ions and electrons would get generated due to collisions. This cyclic process would increase the

concentration of electrons and ions in the dielectric medium between the tool and the job at the

spark gap. The concentration would be so high that the matter existing in that channel could be

characterized as “plasma”. The electrical resistance of such plasma channel would be very less.

Figure 6 Formation of Ions during machining

14

Thus all of a sudden, a large number of electrons will flow from the tool to the job and ions from

the job to the tool. This is called avalanche motion of electrons. Such movement of electrons and

ions can be visually seen as a spark. Thus the electrical energy is dissipated as the thermal energy

of the spark.

The high speed electrons then impinge on the job and ions on the tool. The kinetic energy of the

electrons and ions on impact with the surface of the job and tool respectively would be converted

into thermal energy or heat flux. Such intense localized heat flux leads to extreme instantaneous

confined rise in temperature which would be in excess of 10,000o

C.

Such localized extreme rise in temperature leads to material removal. Material removal occurs

due to instant vaporization of the material as well as due to melting. The molten metal is not

removed completely but only partially. As the potential difference is withdrawn the plasma

channel is no longer sustained. As the plasma channel collapse, it generates pressure or shock

waves, which evacuates the molten material forming a crater of removed material around the site

of the spark. Thus to summarize, the material removal in EDM mainly occurs due to formation

of shock waves as the plasma channel collapse owing to discontinuation of applied potential

difference. Generally the workpiece is made positive and the tool negative. Hence, the electrons

strike the job leading to crater formation due to high temperature and melting and material

removal. Similarly, the positive ions impinge on the tool leading to tool wear.

Charging and discharging of the capacitors take place in a particular manner (Fig. 1.6), which

decides the duration for machining to take place.

Figure 7 working mechanism of EDM

15

In EDM, the generator is used to apply voltage pulses between the tool and the job. A constant

voltage is not applied. Only sparking is desired in EDM rather than arcing. Arcing leads to

localized material removal at a particular point whereas sparks get distributed all over the tool

surface leading to uniformly distributed material removal under the tool.

1.5 HISTORY OF EDM

In dates back to 1770, English chemist Joseph Priestly discovered the erosive effect of electrical

discharges on metal. After a long time, in 1943 at the Moscow University where B.R. and N.I.

Lazarenko decided to exploit the destructive effect of electrical discharges for constructive use.

They developed a controlled process of machining to machine metals by vaporizing material

from the surface of workpiece. Since then, EDM technology has developed rapidly and become

Figure 8 Varition of capacitor voltage with time in RC circuit

Figure 9 Voltage and Current waveform during EDM

16

indispensable in manufacturing applications such as die and mould making, micro-machining,

prototyping, etc. In1950s The RC relaxation circuit was introduced, in which provided the first

consistent dependable control of pulse times and also a simple servo control circuit to

automatically find and hold a given gap between the electrode (tool) and the workpiece. In

the1980s, CNC EDM was introduced which improved the efficiency of the machining operation.

1.6 EDM COMPONENTS

1.7 CLASSIFICATION OF EDM PROCESS

Basically, there are two different types of EDM

(a) Die-sinking EDM

(b)Wire cut EDM

(c) Powder mixed EDM

1.7.1 DIE-SINKING EDM- It reproduces the shape of the tool used (electrode) in the part

where the tool shape complements the final desired shape of the workpiece. The wear has to be

very low, in order to keep the electrode original shape unmodified during the whole machining

process. The asymmetry in the material removal rate is thus crucial for die-sinking. The electrode

Figure 10 EDM components

17

is generally of copper or graphite, and the dielectric is oil. In case of die-sinking EDM the tool is

usually the anode and the workpiece is the cathode for coarse machining operations while the

polarities are reversed for fine machining operations. During any single discharge the anode

starts to melt first because it absorbs the fast moving electron sat the start of the pulse but re-

solidifies fast because of the expanding radius of the plasma channel, which tends to reduce the

heat intensity on the anode surface. The melting of the cathode is, however, delayed because it

absorbs slow moving ions. Moreover, the plasma radius at the cathode is smaller because item its

electrons. Hence the heating of the cathode is over a smaller area and thus more intense. This

difference of spark behaviour at the cathode and the anode results in more material being

removed from the cathode than from the anode and is the rationale for choosing the work piece

as the cathode in coarse machining operation. Fig 1.3 (a) shows a die-sinking type EDM.

1.7.2 WIRE CUT EDM- It uses a continuously circulating metallic wire (typical diameter

0.1mm, generally in steel, brass or copper), which cuts the workpiece along a programmed path.

De-ionized water is used as dielectric, directly Injected around the wire. The wire is capable of

achieving very small cutting angles. The wire in wire-EDM applications acts almost like an

electrical saw. The quality of the machining, i.e. precision and surface roughness, is directly

related to the discharge parameters (current, voltage, discharge duration, polarity), and also on

the dielectric cleanliness. Sparks with strong current produce deep craters: a high removal rate is

obtained but with a high surface roughness. On the other hand, sparks with low current will

produce small craters: the surface roughness is low but the removal rate is also low. Fig.1.3 (b)

shows a wire-cut EDM

Figure 11 Die sinking EDM Figure 12 wire cut EDM

18

1.7.3 POWDER MIXED EDM

Powder mixed EDM is one of the recent innovations for the enhancement of capabilities of EDM

process. In powder mixed EDM, the electrically conductive powder is mixed in the dielectric of

EDM, which reduces the insulating strength of the dielectric fluid and increases the spark gap

between the tool and work piece. As a result, the process becomes more stable, thereby,

improving the MRR and reducing SR, i.e., improving surface finish. Moreover, the surface

develops high resistance to corrosion and abrasion. Powder mixed EDM also termed as ‘Additive

EDM’ was originally invented during late seventies as a revolutionary technique for achieving

mirror like finish relatively at high machining rates on already machined components. The

powder particles get energized and behave in a zigzag fashion (Fig. 1.12). These charged

particles are accelerated by the electric field and act as conductors. The conductive particles

promote breakdown in the gap and increase the spark gap between tool and the work piece.

Under the sparking area, the particles come closer to each other and arrange themselves in the

form of chain like structures between both the electrodes. The interlocking between the different

powder particles occurs in the direction of flow of current. The chain formation helps in bridging

the discharge gap between both the electrodes. Due to bridging effect, the insulating strength of

the dielectric fluid decreases. The easy short circuit takes place, which causes early explosion in

the gap. As a result, a series discharge starts under the electrode area. The faster sparking within

a discharge takes place causing faster erosion from the work piece surface and hence the MRR

increases. At the same time, the added powder modifies the plasma channel. The plasma channel

becomes enlarged and widened (Zhao 2002). The sparking is uniformly distributed among the

powder particles, hence

electric density of the spark

decreases. Due to uniform

distribution of sparking among

19

the powder particles, shallow craters are produced on the work piece surface. This results in

improvement in surface finish.

Figure 13 Principle of power mixed EDM (Kensal et.al 2007).

20

1.8 PROCESS PARAMETERS

Discharge voltage

Peak Current

Pulse configuration

Polarity

Electrode gap

Pulse on time

Pulse off time

Discharge current

1.8.1 Discharge voltage- Discharge voltage in EDM is related to the spark gap and

breakdown strength of the dielectric (Kansal et al. 2005b). Before current can flow, the open

gap voltage increases until it has created an ionization path through the dielectric. Once the

current starts to flow, voltage drops and stabilizes at the working gap level. The preset voltage

determines the width of the spark gap between the leading edge of the electrode and work piece.

Higher voltage settings increase the gap, which improves the flushing conditions and helps to

stabilize the cut. Actual profile of single EDM pulse is shown (Fig. 1.14). MRR, TWR and

surface roughness (SR) increases by increasing open circuit voltage, because electric field

strength increases.

Figure 14 Actual profile of a single EDM pulse (Fuller 1996)

21

1.8.2 Peak Current-This is the amount of power used in discharge machining, measured in

units of amperage, and is the most important machining parameter in EDM. In each on-time

pulse, the current increases until it reaches a present level, which is expressed as the peak

current. Higher value of peak current leads to rough surface finish operations and wider craters

on work materials. Its higher value improves MRR, but at the cost of surface finish and tool

wear. Hence it is more important in EDM because the machined cavity is a replica of tool

electrode and excessive wear will hamper the accuracy of machining.

1.8.3 Pulse configuration-Metal removal is directly proportional to the amount of energy

applied during the on-time (Kansal et al. 2005a). This energy is controlled by the peak

amperage and the length of the on-time. Pulse on-time is commonly referred to as pulse duration

and pulse off-time is called pulse interval. With longer pulse duration, more work piece material

will be melted away. The resulting crater will be broader and deeper than a crater produced by

shorter pulse duration. However, excessive pulse duration can be counter-productive. When the

optimum pulse duration for each electrode-work material combination is exceeded, MRR starts

to decrease. At the same time, pulse interval must be greater than the deionization time to

prevent continued sparking at one point (Fuller 1996).

The pulse shape is normally rectangular, but generators with other pulse shapes have also been

developed. Using a generator which can produce trapezoidal pulses, Bruyn (1968) succeeded in

reducing relative TWR to very low values.

1.8.4 Polarity- The Polarity normally used is normal polarity in which the tool is negative and

workpiece is positive. Sometimes positive polarity can be used depending upon the requirement,

where tool is positive and workpiece is negative. The negative polarity of the workpiece has an

inferior surface roughness than that under positive polarity in EDM. The current passing through

the gap creates high temperature causing material evaporation at both electrode spots. The

plasma channel is composed of ions and electrons flow. As the electron processes has smaller

mass than anions show quicker reaction, the anode material is worn out predominantly. This

effect causes minimum wear to the tool electrodes and becomes of importance under finishing

22

operations with shorter on-times. However, while running longer discharges, the early electron

process predominance changes to positron process (proportion of ion flow increases with pulse

duration), result in gin high tool wear. In general, polarity is determined by experiments and is a

matter of tool material, work material, current density and pulse length combinations.

1.8.5 Electrode gap - The servo feed system is used to control the working gap at a proper

width. Mostly electro-mechanical (DC or stepper motors) and electro-hydraulic systems are

used, and are normally designed to respond to average gap voltage. Larger gap widths cause

longer ignition delays, resulting in a higher average gap voltage. If the measured average gap

voltage is higher than the servo reference voltage pre-set by the operator, the feed speed

increases. On the contrary, the feed speed decreases or the electrode is retracted when the

average gap voltage is lower than the servo reference voltage, which is the case for smaller gap

widths resulting in a smaller ignition delay. Therefore short-circuits caused by debris particles

and humps of discharge craters can be avoided. Also quick changes in the working surface area,

when tool electrode shapes are complicated, does not result in hazardous machining. In some

cases, the average ignition delay time is used in place of the average gap voltage to monitor the

gap width.

1.8.6 Pulse on time - Pulse on-time is the time period during which machining takes place.

MRR is directly proportional to amount of energy applied during pulse on-time. The energy of

spark is controlled by the peak amperage and the length of the on-time. The longer the on-time

pulse Is sustained, the more workpiece material will be eroded. The resulting crater will be

broader and deeper than a crater produced by a shorter on-time. These large craters will create a

rougher surface finish. Extended on times gives more heat to workpiece, which means there cast

layer will be larger and the heat affected zone will be deeper. Hence, excessive on-times can be

counter-productive. When the optimum on-time for each electrode-work material combination is

exceeded, material removal rate starts to decrease.

1.8.7 Pulse off time - Pulse off-time is the time during which re-ionization of dielectric takes

place. The discharge between the electrodes leads to ionization of the spark gap. Before another

23

spark can take place, the medium must de-ionize and regain its dielectric strength. This takes

some finite time and power must be switched off during this time. Too low values of pulse off

time may lead to short-circuits and arcing. Large value on other hand increases the overall

machining time since no machining can take place during the off-time. Each cycle has an on-

time and off-time that is expressed in units of micro seconds.

1.8.8 Discharge current - The discharge current (Id) is a measure of the power supplied to the

discharge gap. A higher current leads to a higher pulse energy and formation of deeper discharge

craters. This increases the material removal rate (MRR) and the surface roughness (Ra) value.

Similar effect on MRR and Ra is produced when the gap voltage (Vg) is increased. Once the

current starts to flow, voltage drops and stabilizes at the working gap level. The present voltage

determines the width of the spark gap between the leading edge of the electrode and workpiece.

Higher voltage settings increase the gap, which improves the flushing conditions and helps to

stabilize the cut.

1.9 CHARACTERISTICS OF EDM

1.9.1 Advantage of EDM

One of the main advantages of EDM is a consequence of the thermal process. It is based on:

removing material by melting and evaporation, so the hardness of the work piece is no limitation

for machining. Even the hardest steel grades can be machined with almost same machining speed

as for softer steels.

Figure 15 Typical EDM pulse current

24

1.9.1a) Machining hard materials

The capability of machining hard materials is a major benefit as most tools and moulds are made

of hard materials to increase their lifetime. The recent developments in cutting tools for turning

and milling and the processes of high speed machining allow to machine harder materials than

before, but EDM still remains the only available process for machining many hard materials (e.g.

carbides).

1.9.1.b) Absence of forces

As the EDM-process is based on a thermal principle, almost no mechanical forces are applied to

the work piece. This allows to machine very thin and fragile structures. It should 15 be noticed

that some small mechanical, electrical and magnetic forces are produced by the EDM-process

and that, as already mentioned, flushing and hydraulic forces may become large for some work

piece geometry. The large cutting forces of the mechanical materials removal processes,

however, remain absent.

1.9.1.c) Machining of complex shapes

Complex cavities can often be machined without difficulties by die-sinking EDM, provided an

electrode is available, having the opposite shape of the cavity. In most cases, the soft electrode

(Cu, graphite or W-Cu) can be machined rather easily by conventional processes as milling and

turning or by wire-cutting EDM.I n this way, complex cavities can be eroded, even on simple

die-sinking machines which can only erode in the downward direction. Due to the modern NC

control systems on die sinking machines, even more complicated work pieces can be machined.

Modern, multi-axis NC controlled wire-cutting machines (where e.g. the wire inclination can be

constantly) also allow to achieve very intricate work pieces. Besides complex shapes,

conventional processes, can easily be machined by EDM. EDM is also one of the only processes

capable of machining three dimensional micro work pieces. A large growth of applications for so

called micro electro mechanical systems (MEMS) is predicted for the near future.

1.9.1.d) High degree of automation

The high degree of automation and the use of tool and workpiece changers allow the machines to

work unattended for overnight or during the weekends.

25

1.9.1.e) Accuracy of the process

EDM is a process where very accurate structures can be machined (typically 1 to 5μm). In the

case of workpiece with a higher thickness, the accuracy and the fine surface equality remains the

same over the whole thickness of the workpiece, due to the fact that EDM is machining with the

same process conditions over the total workpiece height. Process like laser beam or water jet

machining can also achieve a high surface finish, but they can be used only for work pieces with

a limited thickness. When the thickness increases, focusing problems induce a loss of quality.

One of the main application fields of EDM is the mould and dies making industry. To achieve a

high life time of the workpiece, very hard materials should be used. The high hardness is often

obtained by a thermal treatment. After the treatment however, most workpiece can no longer be

machined by conventional processes, so EDM is the appropriate way to manufacture these work

pieces.

1.9.2 Disadvantage of EDM

1.9.2.a) The need for electrical conductivity

To be able to create discharges, the workpiece has to be electrically conductive. Isolators, like

plastics, glass and most ceramics, cannot be machined by EDM, although some exception like

for example diamond is known. Machining of partial conductors like Si semi-conductors,

partially conductive ceramics and even glass is also possible.

1.9.2.b) Predictability of the gap

The dimensions of the gap are not always easily predictable, especially with intricate work piece

geometry. In these cases, the flushing conditions and the contamination state of differ from the

specified one. In the case of die-sinking EDM, the tool wear also contributes to a deviation of the

desired work piece geometry and it could reduce the achievable accuracy. Intermediate

measuring of the work piece or some preliminary tests can often solve the problems.

1.9.2.c) Optimization of electrical parameters

The choice of the electrical parameters of the EDM-process depends largely on the material

combination of electrode and workpiece and EDM manufactures only supply these parameters

26

for a limited amount of material combinations. When machining special alloys, the user has to

develop his own technology.

1.9.2.d) Low material removal rate

The material removal of the EDM-process is rather low, especially in the case of die-sinking

EDM where the total volume of a cavity has to be removed by melting and evaporating the

metal. With wire-EDM only the outline of the desired work piece shape has to be machined.

Due to the low material removal rate, EDM is principally limited to the production of small

series although some specific mass production applications are known.

1.9.3 APPLICATION OF EDM:

(a)In the machining of very hard metals and alloys used in aerospace, automotive and nuclear

industries.

(b)It is a promising technique to meet increasing demands for smaller components usually highly

complicated, multi-functional parts used in the field of micro-electronics.

(c) Application potential of EDM can be further enhanced if its machining rates can be increased

and resulting surface damage to the workpiece is accurately estimated and reduced.

CHAPTER 2 - SCOPE OF THE STUDY

The high capital cost of machines and satisfy the need of customer demand is great important in

the manufacturing process. This work presented an experimentation approach to studying the

27

impact of machining parameters on metal removal rate in EDM processes . Strong interactions

were observed among the machining parameters. Most significant interactions were high

precision is required. The efficient utilization of machine tools has been a problem for

manufacturing firms for a long time. In order to meet the demands for higher precision and

productivity in present industry, optimization of EDM processes is of a great concern for the

manufacturers. The surface finish is an important objective which is to due given due importance

in the process of optimization and the same has been incorporated in this work. The following

sections bring out the contribution of this research work.found between current parameters. A

systematic approach was provided to design and analyze the experiments, which is able to reduce

the cost and time of experiments and to utilize the data obtained to the maximum extend. The

Electrical Discharge Machining and Wire Electrical Discharge Machining processes are

important machining process in an industry. The EDM is extensively employed for the removal

material from the very hard materials. In these processes, the metal removal rate is low.but it can

machine most hard material. As high carbon high chromium die steel is too hart and used for

making die, punch hand tool etc.

28

CHAPTER 3 OBJECTIVE OF THE STUDY

In this paper EDM is performed on the high carbon high chromium die steel using the copper

electrode. The main problem in EDM process that is there is undesired wear of electrode which

in turn changes the geometry of tool which affects the impression on the workpiece. So for the

prevention of tool ear we are using the cryogenic treated tool which makes the tool hard and

prevention its ear.

The main objectives are as follows-:

To optimise the material removal rate and surface roughness on die sink EDM

Optimizing the parameter using RSM technique.

Effects of process parameteron machining of high carbon high chromium die steel.

29

CHAPTER 4- LITERATURE REVIEW

Che Haron et al. [1] investigated the machining characteristics when machining XW42

tool steel at two current settings (3A and 6 A), three diameter sizes (10, 15 and 20mm)and

kerosene as the dielectric. The results showed that the material removal rate is higher and the

relative electrode wear ratio is lower with copper electrode than graphite electrode. The increase

in the current and electrode diameter reduced tool wear rate as well as the material removal rate.

Tsai et al. [2] proposed a new method of blending the copper powders contained resin with

chromium powders to form tool electrodes. Such electrodes are made at low pressure (20 MPa)

and temperature (200°C) in a hot mounting machine. It was showed that using such electrodes

facilitated the formation of a modified surface layer on the work piece after EDM, with

remarkable corrosion resistant properties. The optimal mixing ratio, appropriate pressure, and

proper machining parameters (such as polarity, peak current, and pulse duration) were used to

investigate the effect of the material removal rate (MRR), electrode wear rate (EWR), surface

roughness, and thickness of the recast layer on the usability of these electrodes. Their work also

reveals that the composite electrodes obtained a higher MRR than Cu metal electrodes, the recast

layer was thinner and fewer cracks were present on the machined surface. A newly developed

lowpressure and low-temperature technique was used to fabricate composite electrodes to

conduct EDM on medium carbon steel. The conclusions based on the experimental results show

that by using pure copper powders contained resin as electrodes, and adopting a positive polarity

machining process can obtain a higher MRR than the Cu–Cr composite electrodes, but relatively

the EWR is also higher. The MRR is higher when a sintering pressure of 20 or 30 MPa is used to

fabricate composite electrodes than when a sintering pressure of 10 MPa is used. In the 10 MPa

case, Cu and Cr particles easily drop out of the electrode due to their weak bonding, resulting in

an unstable discharge state during the EDM process. The surface finish is poor when composite

electrodes are used in negative polarity machining, because of very many Cu

and Cr particles inside electrodes drop out accumulate or adhere to machined surfaces With use

the newly developed composite electrodes for EDM, causes both Cu and Cr

particles to drop easily, such that the elements (Cu and Cr) in the electrode can also

migrate to the machined surfaces during EDM, and the corrosion resistance increases

30

with the percentage of added Cr particles. This result suggests that using such composite

electrodes as tool electrode may improve the resistance of work piece surfaces to corrosion.

Simo et al. [3] studied surface alloying of various work piece materials using EDM. It can be

achieved by using powder metallurgy (PM) tool electrodes and the use of powders suspended in

the dielectric fluid, typically aluminium, nickel, titanium, etc. They presented experimental

results on the surface alloying of AISI H13 hot work tool steel during a die sink operation using

partially sintered WC/Co electrodes operating in a hydrocarbon oil dielectric. An L8 fractional

factorial Taguchi experiment was used to identify the effect of key operating factors on output

measures (electrode wear, work piece surface hardness, etc.). With respect to micro hardness, the

percentage contribution ratios (PCR) for peak current, electrode polarity and pulse on time were

~24, 20 and 19%, respectively.

Mohri et al. [4] proposed a new method of surface modification by EDM using

composite electrodes on workpieces of carbon steel or aluminum were carried out in

hydrocarbon oil. Copper, aluminum, tungsten carbide and titanium were used for the

materials of electrodes, it was revealed that there existed the electrode material in the work

surface layer and the characteristics of the surface of material are changed. Surfaces have lesser

cracks, high corrosion resistance and wear resistance.

Koshy et al. [5] used a rotating disk electrode which is more productive and accurate technique

than use conventional electrode. Material removal rate, tool wear rate, relative electrode wear,

corner reproduction accuracy and surface finish aspects of rotary electrode were compared with

those of a stationary one. The effective flushing of the working gap improves material removal

rate and machines surface with better finish. Despite the prevalent high tool wear rate, the

reproduction accuracy is least affected as the wear gets uniformly distributed over the entire

circumference of the disk. Machining of the sharp corner is possible even with aluminum

electrode, whose relative electrode wear is greater than unity.

Das et. al. [6] objective of the study was to find out the optimum combination of process

parameter in EDM process so that surface roughness reaches a minimum value and the metal

removing rate reaches a maximum value. In this study, five roughness parameter Center line

31

average roughness, root mean square roughness, mean line peak spacing, skewness and kurtosis

along with MRR is considered. To optimize the multi response problems, Taguchi method alone

is unable to solve the problem. Thus the multi response characteristics must be converted to a

single performance index. In this study Principal components analysis (WPCA) method is used

for conversion. The result of machining on EN31 shows that discharge current is the most

influencing parameter that significantly affects the roughness and MRR characteristics at a

confidence level of 95%

Khanra et. al. [7] developed a ZrB2–Cu composite as an EDM tool material to overcome wear

resistance of tool. The composites was tested as tool material at different EDM process

parameters during machining of mild steel. The ZrB2–40 wt% Cu composite shows highest

metal removal rate (MRR) with significant tool removal rate (TRR) than other composites. The

performance of ZrB2–40 wt% Cu composite was compared to conventional Cu tool. The

composite shows higher MRR with less TRR than Cu tool but it shows more average surface

roughness and diameteral overcut than Cu tool.

Dhar and Purohit [8] evaluates the effect of current (c), pulse-on time (p) and air gap voltage (v) on MRR, TWR,

ROC of EDM with Al–4Cu–6Si alloy–10 wt. % SiCP composites. This experiment can be using the PS LEADER

ZNC EDM machine and a cylindrical brass electrode of 30 mm diame-ter. And three factors, three levels full

factorial design was using and analyzing the results. A second order, non-linear mathematical model has been

developed for establishing the relationship among machining parameters. The significant of the models were

checked using technique ANOVA and find-ing the MRR, TWR and ROC increase significant in a non-linear fashion

with increase in current

B.Mohan and Satyanarayana[9] evolution the of effect of the EDM Current, electrode marital polarity, pulse

duration and rotation of electrode on metal removal rate, TWR, and SR, and the EDM of Al-Sic with 20-25 vol. %

SiC, Polarity of the electrode and volume present of SiC, the MRR increased with increased in discharge current and

specific current it decreased with increasing in pulse duration. Increasing thespeed of the rotation electrode resulted

in a positive effect with MRR, TWR and better SR than stationary. The electric motor can be used to rotate the

electrode(tool) AV belt was used to transmit the power from the motor to the electrode Optimization parameters for

EDM drilling were also devel-oped to summarize the effect of machining characteristic such as MRR, TWR and SR.

Singh et al. [10] carried out experimental investigation to study the effects of machining parameters

such as pulsed current on material removal rate, diameteral overcut, electrode wear, and surface

roughness in electric discharge machining of En-31 tool steel (IS designation: T105 Cr 1 Mn 60)

32

hardened and tempered to 55 HRc. The work material was ED machined with copper, copper

tungsten, brass and aluminium electrodes by varying the pulsed current at reverse polarity. The

investigations indicate that the output parameters of EDM increase with the increase in pulsed

current and the best machining rates are achieved with copper and aluminium electrodes.

B.S. Reddy et al. [11] carried out a study on the effect EDM parameters over MRR, TWR, SR

and hardness. Mixed factorial design of experiments and multiple regression analysis techniques

had been employed to achieve the desired results. The parameters in the decreasing order of

importance for; MRR: servo, duty cycle, current and voltage; TWR: current, servo and duty

cycle; SR: current; HRB: servo only. M.M. Rahman et al. [2] investigated the effect of the peak

current and pulse duration on the performance characteristics of the EDM. The conclusions

drawn were: the current and pulse on time greatly affected the MRR, TWR and SR, the MRR

increases almost linearly with the increasing current, the SR increases linearly with current for

different pulse on time, TWR increased with increasing peak current while decreased when the

pulse on time was increased.

Singh and Maheshwari [12] found that the input parameters such as current, pulse on time,

voltage applied and the workpiece material greatly influences overcut. It increases with the

increase of current but only up to a certain limit. It also depends on the gap voltage. Kiyak

and Cakir [8] found that SR of workpiece and electrode were influenced by current and pulse

on time, higher values of these parameters increased the surface roughness. Lower current

and pulse time and higher pulse off time produced a better surface finish

B. Bhattacharyya et al. [13] observed that peak current and pulse on time significantly

influenced different criteria of surface integrity such as surface crack density, surface

roughness and white layer thickness. S Dhar et al. [10] came to the following conclusions:

with increase in peak current MRR, TWR and ROC increased significantly in a nonlinear

fashion; MRR and ROC increased with the increase in pulse on time and gap voltage was found

to have some effect on the three responses.

33

4.1 GAPS IN LITERATURE REVIEW

Some gaps are found on the previous Research on the basis of which the topic for my further

study has been decided.

The gaps are as follows-:

1. In EDM there is removal of material from tool along with workpiece but very few

amount of work is done to stop the removal of material from tool.

2. EDM is generally used for metals, ceramics and for composite materials but very few

has performed EDM in Superalloy.

34

35

CHAPTER-5 EQUIPMENT, MATERIALS AND EXPERIMENTAL SETUP

5.1 Description of machine / set up

For this experiment the whole work can be down by Electric Discharge Machine, model

SPARKONIX (die-sinking type) with servo-head (constant gap) and positive polarity for

electrode was used to conduct the experiments. Commercial grade EDM oil (specific gravity=

0.763, freezing point= 94°C) was used as dielectric fluid. With external flushing of cylindrical

shaped cutting tool with a pressure of 0.2 Kgf/cm2 Experiments were conducted with positive

polarity of electrode. The pulsed discharge current was applied in various steps in positive mode.

The EDM consists of following major parts:-

5.1.1 Dielectric reservoir, pump and circulation system.

5.1.2 Power generator and control unit.

5.1.3 Working tank with work holding device.

5.1.4 X-y table accommodating the working table.

5.1.5 The tool holder.

5.1.6 The servo system to feed the tool.

5.1.1 Dielectric reservoirs pump and circulation system

Dielectric reservoirs and pump are used to circulate the EDM oil for every run of the experiment

and also used the filter the EDM oil. Dielectric reservoir is shown in Fig.17

Fig.17

36

5.1.2 Power generator and control unit

The power supply control the amount of energy consumed. First, it has a time control function

which controls the length of time that current flows during each pulse; this is called “on time.”

Then it controls the amount of current allowed to flow during each pulse. These pulses are of

very short duration and are measured in microseconds. There is a handy rule of thumb to

determine the amount of current a particular size of electrode should use: for an efficient removal

rate, each square inch of electrode calls for 50 A. Low current level for large electrode will

extend overall machine time unnecessarily. Conversely, too heavy a current load can damage the

workpiece of electrode. The control unit controls all function of the machining for example of

Ton, Ip, duty cycle, putting the values and maintain the workpiece the tool gap. The control unit

is shown in Fig 1.19

Figure 16 Electric pump

Figure 17 Power generator and control unit

37

5.1.3Working tank with work holding device–

All the EDM oil kept in the working tank working tank is used to the supply the fluid during the

process of machining.

5.1.4 X-y table accommodating the working table

They are used to the moment of the workpiece form X and Y direction. It is shown in Fig .

Figure 18 CRO

Figure 19 x- y table coordinates

38

5.1.5 The tool holder

The tool holder holds the tool with the process of machining. The tool holder with work piece

and tool as shown in Fig

5.1.6 The servo system to feed the tool

The servo control unit is provided to maintain the pre-determined gap. It senses the gap voltage

and compares it with the present value and the different in voltage is then used to control the

movement of servo motor to adjust the gap.

5.2 Factors

Figure 20 Tool holder, workpiece and tool

Figure 21 Servo system for tool feed

39

For machining purpose to carry out various factors have to be decided. These factors are the

preliminary requirements of the study. These are:

Various parameters of EDM

i) Peak current

ii) Pulse on time

iii) Pulse of time

Work piece material

High carbon high chromium die steel

Tool material:

i) Copper

Responses to be measured:

i) MRR

ii) TWR

iii) Surface Roughness

40

CHAPTER 6 - RESEARCH METHODOLOGY

Response surface methodology (RSM) explores the relationships between several explanatory

variables and one or more response variables. The method was introduced by G. E. P. Box and

K. B. Wilson in 1951. The main idea of RSM is to use a sequence ofdesigned experiments to

obtain an optimal response. They acknowledge that this model is only an approximation, but use

it because such a model is easy to estimate and apply, even when little is known about the

process. An easy way to estimate a first-degree polynomial model is to use a factorial

experiment or a fractional factorial design. This is sufficient to determine which explanatory

variables have an impact on the response variable(s) of interest. The application of RSM to

design optimization is aimed at reducing the cost of expensive analysis methods (e.g. finite

element method or CFD analysis) and their associated numerical noise.

Advantages of RSM over Taguchi method

In Taguchi method, the orthogonal array do not test all variable combination or

interaction effects between all process parameters. In this method, only those

interactions can be analyzed that we believe truly exist. However, in RSM, interaction

effects between all process parameters can be analyzed.

In this method, the orthogonal array does not give information about the effects of

higher order control factor interactions (I2) on manufacturing system. However, in RSM,

higher order control factor interactions can be analyzed.

Taguchi method does not able to provide true optimal value of a factor setting. It

merely tells us the best level for a factor setting from the level chosen for

experimentation. However, RSM can actually predict the best combination of factors to

meet your goals.

http://en.wikipedia.org/wiki/Factorial_experiment

http://en.wikipedia.org/wiki/Factorial_experiment

http://en.wikipedia.org/wiki/Fractional_factorial_design

41

CHAPTER 7 - RESEARCH ANALYSIS

7.1 Introduction

After experimentation, the data has been analysed to find out the desirable combination of levels

of the input process parameters, their significance and relative contribution. Regression models

have been developed for performance measures. The results of experimentation with different

workpiece and powder combinations have been presented (Table 7.1) and analysed in this

chapter.

7.2 Material removal rate (MRR)

MRR for the parametric combinations mentioned in experimental matrix has been calculated

and tabulated. Regression analysis was performed for the collected data. For data analysis,

analysis of variance (ANOVA) was performed.

7.2.1 Regression analysis

The fit summary table formed by ‘Design Expert’ software recommended quadratic model as

statistically significant for analysis. . The quadratic model was suggested and chosen for

regression modelling. The results of the quadratic model in form of ANOVA are given in Table

7.2

The Model F-value of 5.59 implies the model is significant. There is only a 0.64% chance that a

"Model F-Value" this large could occur due to noise. Values of "Prob > F" less than 0.0500

indicate model terms are significant. In this case A2, B

2 are significant model terms. Values

greater than 0.1000 indicate the model terms are not significant.

The "Pred R-Squared"implies that overall mean is a better predictor of response than the current

model. "Adeq Precision" measures the signal to noise ratio. A ratio greater than 4 is desirable

(Myers and Montgomery, 1995). Model ratio of 8.914 indicates adequate signal.

42

Table 7.1 Data collected for various responses

43

Table 7.2 ANOVA table for MRR before backward elimination

7.2.2 Results and discussion

The perturbation plot shown in Figure 7.1 helps to compare the effect of all the factors at a

particular point in the design space. The response has been plotted by changing only one factor

over its range while holding all the other factors constant. By default, “Design-Expert” software

44

sets the reference point at the midpoint of all the factors. . A steep slope for current (A) and pulse

on time (B) for the materials hchcr die steel shows that the response is highly sensitive to that

factor.

Figure 7.1

Effect of current:

As it is evident from perturbation plot, MRR is highly sensitive to peak current. The MRR tends

to increase considerably with increase in peak current for any value of pulse on time as shown in.

Hence, maximum MRR is obtained at high peak current and high pulse on time. This is due to

dominant control of current on input energy . Increase in pulse current generates strong spark

45

which create the higher temperature causes the more material to melt and erode from the work

piece. Figure 7.2 shows 3D graph.

figure 7.2

Effect of pulse on time:

With increase in pulse on time, the MRR tends to increase during pulse on time. This energy is

controlled by the peak amperage and the length of the on-time. With longer pulse duration,

more work piece material will be melted away. The resulting crater will be broader and deeper

than a crater produced by shorter on-time. These larger craters will create a rougher

surface finish. Extended on times also allows more heat to work piece, which means the

recast layer will be larger and the heat affected zone will be deeper.

46

Effect of pulse off time:

With increase in pulse off time, the MRR tends to decrease This is because increase in pulse off

time results in reduced pulse frequency there by reduction in MRR. In other words, the shorter

the interval, the faster will be the machining operation. But if the interval is too short, the

expelled work piece material will not be flushes away with the flow of the dielectric fluid and the

dielectric fluid will not be deionized. This will cause the next spark to be unstable. Unstable

conditions cause erratic cycling and retraction of the advancing servo, slowing down the

operation cycle and hence MRR .

7.3 Tool wear rate (TWR)

TWR for the parametric combinations mentioned in experimental matrix has been calculated

and tabulated. Regression analysis was performed for the collected data. For data analysis,

analysis of variance (ANOVA) was performed.



statistically significant for analysis. The recommended fit summary is shown in Table .

47

Table 7.3



48

indicate model terms are significant. In this case A, B, C,AB,BC are significant model terms.

Values greater than 0.1000 indicate the model terms are not significant.. "Adeq Precision"

measures the signal to noise ratio. A ratio greater than 4 is desirable. Model ratio of 15.274

indicates adequate signal. This model can be used to navigate the design space


The perturbation plot for hchcr material shown in figure 7.3 helps to compare the effect of all the

factors at a particular point in the design space. A steep slope for current (A) and pulse off time

(C) for the materials hchcr ) shows that the response is highly sensitive to that factor.figure 7.3

Figure 7.3

49

Effect of current:

The variation of TWR with peak current is shown in Figure 7.4,7.5. As shown in figures, TWR

increases linearly with increase in current for any value of pulse off time. This is due to the fact

that an increase in discharge current leads to increase of pulse energy. This results in increase in

heat energy rate, which is subjected to both of the electrodes. The increased heat energy rate

raises the rate of melting and evaporation of electrodes. Thus, the TWR increases with the

discharge current. The findings are closely agreed with the previous investigations

Effect of pulse on time:

With increase in pulse on time, the TWR tends to increase as shown in Figure 7.4,7.5 This is

because TWR is directly proportional to the amount of energy applied during pulse on time.

This energy is controlled by the peak amperage and the length of the on-time. With longer

pulse duration, more work piece and electrode material will be melted away. Extended on

times also allows more heat to work piece as well as electrode, thus the pulse energy is more,

so the tool will melt more and tool wear rate will be more.

Effect of pulse off time:

With increase in pulse off time, the TWR decreases significantly as shown in Figure 7.4,7.5.

This is because increase in pulse off time results in reduced pulse frequency there by reduction in

TWR.

50

Figure 7.4

51

Figure7.5

7.4 Surface Roughness

SR for the parametric combinations mentioned in experimental matrix has been calculated and

tabulated. Regression analysis was performed for the collected data. For data analysis, analysis

of variance (ANOVA) was performed.

52



statistically significant for analysis. The recommended fit summary is shown in Table .

Table 7.4

53



indicate model terms are significant. In this case A, AB, A2 are significant model terms. "Adeq

Precision" measures the signal to noise ratio. A ratio greater than 4 is desirable. Model ratio of

7.372 indicates adequate signal. This model can be used to navigate the design space.


The perturbation plot for hchcr material shown in figure 7.6 helps to compare the effect of all the

factors at a particular point in the design space. A steep slope for current (A) and pulse off time

(C) for the materials hchcr ) shows that the response is highly sensitive to that factor.figure 7.6

Figure 7.6

54

Effect of current:

The variation of SR with peak current is shown in Figure 7.6,7.7. As shown in figures, SR

increases linearly with increase in current for any value of pulse off time. This is due to the fact

that an increase in discharge current leads to increase of pulse energy.

Figure 7.6

55

Figure 7.7

As current is incresse SR of material also increases Increase in pulse current generates strong

spark which create the higher temperature causes the more material to melt and erode from the

work piece .

Effect of pulse on:

With increase in pulse on time, the SR tends to increase as showen in figure 7.6 and 7.7 during

pulse on time. This energy is controlled by the peak amperage and the length of the on-time.

With longer pulse duration, more work piece material will be melted away. The resulting crater

will be broader and deeper than a crater produced by shorter on-time

56

Effect of pulse off:

With increase in pulse off time, the SR tends to decrease as shown in Figure 7.6,7.7 This is

because increase in pulse off time results in reduced pulse frequency there by reduction in SR. In

other words, the shorter the interval, the faster will be the machining operation.and surface result

in rough.

57

58

CHAPTER 8 - conclusion

In this study the experiment was conducted by considering three variable parameters namely

current, pulse on time and pulse off time. The objective was to find the Material Removal Rate,

Surface Roughness and tool wear rate and to study the effects of the variable parameters on these

characteristics. The tool material was taken as copper and the workpiece was chosen as high

carbon high chromium die steel. Using response surface methodology experiments were

performed accordingly. The following conclusions were drawn:

1. For MRR the most significant factor was found to be peak current followed by pulse on time

and the least significant was duty cycle. The MRR increased nonlinearly with the increase in

current.

2. For SR the most significant factor was again current followed by pulse on time and lastly the

duty cycle. SR increased significantly with the increase in current in a nonlinear fashion. For

increase in pulse on time SR increased.

3.For less TWR most significant factor is less current and low pulse on time.

59

CHAPTER 9- REFRENCES

Dhar S., Purohit R., Saini N., Sharma A., Kumar G.H., Mathematical modelling of

electric discharge machining of cast Al-4Cu-6Si alloy-10 wt.% SiCp composites, Journal

of Materials Processing Technology, 194 (2007), 24-29

Tsai H.C., Yan B.H., Huang F.Y. (2003), “EDM performance of Cr/Cu-based composite

electrodes”, International Journal of Machine Tools & Manufacture, Vol. 43

Che Haron C.H., Ghani J.A., Burhanuddin Y. , Seong Y.K., Swee C.Y. (2008) “Copper

and graphite electrodes performance in electrical-discharge machining of XW42 tool

steel”,Journal of materials processing technology,

Mohri N., Satio N., Tsunekawa Y., Kinoshtia N. (1993), “ Metal surface modification

by electrical discharge machining with composite electrode”, CIRP Annals

Manufacturing Technology,

Koshy P., Jain V.K., Lal G.K. (1993), “Experimental investigation into electrical

discharge machining with a rotating disk electrode”, Precision Engineering.

Documents

Complete Research Paper