UNIVERSITI TEKNIKAL MALAYSIA MELAKA

Study the Effect of EDM (Die Sinking) Parameters on Surface Roughness for Machining

Tool-Steel

Thesis submitted in accordance with the requirements of the Universiti Teknikal Malaysia Melaka for the Bachelor Degree of Manufacturing Engineering in

Manufacturing Process

By

Mohd Isnaini Bin Masran

Faculty of Manufacturing Engineering March 2008

APPROVAL

This thesis submitted to the senate of UTeM and has been accepted as partial

fulfilment of the requirements for the degree of Bachelor of Manufacturing

(Manufacturing Process).

The members of the supervisory committee are as follow:

………………………………………

Supervisor

(Official Stamp & Date)

DECLARATION

I hereby, declare this thesis entitled “Study the Effect Of EDM (Die-Sinking) Parameters on Surface Roughness for Machining Tool-Steel” is the result of my

own research except as cited in the references.

Signature : …………………………………………………….

Author’s Name : MOHD ISNAINI BIN MASRAN

Date : 27 March 2008

UTeM Library (Pind.1/2007)

BORANG PENGESAHAN STATUS TESIS*

UNIVERSITI TEKNIKAL MALAYSIA MELAKA

JUDUL:

SESI PENGAJIAN: 2007/2008 Saya _____________________________________________________________________ mengaku membenarkan tesis (PSM/Sarjana/Doktor Falsafah) ini disimpan di Perpustakaan Universiti Teknikal Malaysia Melaka (UTeM) dengan syarat-syarat kegunaan seperti berikut:

1. Tesis adalah hak milik Universiti Teknikal Malaysia Melaka. 2. Perpustakaan Universiti Teknikal Malaysia Melaka dibenarkan membuat salinan

untuk tujuan pengajian sahaja. 3. Perpustakaan dibenarkan membuat salinan tesis ini sebagai bahan pertukaran

antara institusi pengajian tinggi. 4. **Sila tandakan (√)

STUDY THE EFFECT OF EDM (DIE-SINKING) PARAMETERS ON SURFACES ROUGHNESS FOR MACHINING TOOL-STEEL

MOHD ISNAINI BIN MASRAN

Disahkan oleh:

(TANDATANGAN PENULIS)

Alamat Tetap: KG.LUBUK PUSING ,45300 Sungai Besar,Selangor Darul Ehsan Tarikh: 27 March 2008

(TANDATANGAN PENYELIA)

Cop Rasmi:

Tarikh: 27March 2008

* Tesis dimaksudkan sebagai tesis bagi Ijazah Doktor Falsafah dan Sarjana secara penyelidikan, atau disertasi bagi pengajian secara kerja kursus dan penyelidikan, atau Laporan Projek Sarjana Muda (PSM). ** Jika tesis ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasa/organisasi berkenaan dengan menyatakan sekali sebab dan tempoh tesis ini perlu dikelaskan sebagai SULIT atau TERHAD.

(Mengandungi maklumat TERHAD yang telah ditentukan oleh organisasi/badan di mana penyelidikan dijalankan)

(Mengandungi maklumat yang berdarjah keselamatan atau kepentingan Malaysia yang termaktub di dalam AKTA RAHSIA RASMI 1972)

TIDAK TERHAD

TERHAD √

SULIT √

UNIVERSITI TEKNIKAL MALAYSIA MELAKA Karung Berkunci 1200, Ayer Keroh, 75450 Melaka Tel : 06-233 2421, Faks : 06 233 2414 Email : fkp@kutkm.edu.my

FAKULTI KEJURUTERAAN PEMBUATAN

Rujukan Kami (Our Ref) : 27 March 2008 Rujukan Tuan (Your Ref): Pustakawan Perpustakawan Kolej Universiti Teknikal Kebangsaan Malaysia KUTKM, Ayer Keroh MELAKA. Saudara, PENGKELASAN TESIS SEBAGAI SULIT/TERHAD - TESIS SARJANA MUDA KEJURUTERAAN PEMBUATAN (PROSES PEMBUATAN): TAJUK: Sukacita dimaklumkan bahawa tesis yang tersebut di atas bertajuk “STUDY THE EFFECT OF EDM (DIE-SINKING) PARAMETERS ON SURFACES ROUGHNESS FOR MACHINING TOOL-STEEL” mohon dikelaskan sebagai terhad untuk tempoh lima (5) tahun dari tarikh surat ini memandangkan ia mempunyai nilai dan potensi untuk dikomersialkan di masa hadapan. Sekian dimaklumkan. Terima kasih. “BERKHIDMAT UNTUK NEGARA KERANA ALLAH” Yang benar, WAN HASRULNIZZAM WAN MAHMOOD Pensyarah, Fakulti Kejuruteraan Pembuatan (Penyelia Bersama)

06-2332122 s.k. - Penyelia Utama: Abdul Rahim Samsudin

ABSTRACT

The Electrical Discharge Machining (EDM) removes workpiece by an electrical

spark erosion process. The variations in the machining parameters, such as polarity,

pulse-on time, open discharge voltage, discharge current and dielectric fluid greatly

affect the measures of the machining performance, for example surface roughness,

electrode wear ratio and material removal rate. Therefore, proper selection of the

machining parameters can result in better machining performance. This study is

devoted to a study of the influences of EDM parameters on surface roughness for

machining tool steel (AISI D2 Cold-work) which is widely used for various dies and

cutters for its high strength and wear resistance due to formation of chrome carbide

in heat treatment. The selected EDM parameters were Current (16-64A), Voltage

(90-200 V) and Pulse OFF Time (5-10 µs). From the experimental, evaluation and

analyzing result obtained, the higher discharge energy apparently producing the

worst surface texture and fastest machining time. In the other hand, when applying

lower discharge energy, machining time will be much longer but the surface texture

will be better.

i

ABSTRAK

Permesinan Nyahcas Elektrik (EDM) membuang benda kerja melalui proses hakisan

percikkan bunga api. Variasi parameter dalam permesinan adalah, kekutuban

laksana, nadi masa, voltan pemberhentian terbuka, arus elektrik dan dielektrik

bendalir dimana menjejaskan prestasi permesinan, seperti kekasaran permukaan

untuk nisbah kehausan elektrod dan kadar buangan material. Oleh itu, pemilihan

parameter yang sesuai dapat menghasilkan prestasi permesinan yang baik. Laporan

tertumpu pada kajian tentang parameter- parameter EDM terhadap kekasaran

permukaan untuk pemesinan bahan kerja “tool steel” unsur gred AISI D2 kerja-

sejuk yang mana biasa digunakan untuk acuan dan mata alat pemotong disebabkan

tinggi tahap kekerasan dan tahan terhadap kehausan yang mana akibat dari

pembentukan krom karbida dalam proses pemanasan. Parameter EDM yang dipilih

adalah arus elektrik (16-64A), voltan (90-200 V) dan nadi mase berhenti (5-10 µs).

Hasil percubaan,penilaian dan penganalisaan, didapati lebih tinggi tenaga luahan

dibekalkan lebih teruk bentuk permukaan yang terhasil dan kerja pemesinan menjadi

semakin cepat. Berbeza apabila tenaga luahan dibekal adalah rendah, bentuk

permukaan akan menjadi lebih baik namun masa pemesinan akan bertambah lama.

ii

DEDICATION

For my beloved mother, father and Wan Aida

iii

ACKNOWLEDGEMENTS

Assalamualaikum,

Alhamdulillah,a very grateful to ALLAH S.W.T in every way giving me the will and

strength to completed this research.

I would like to dedicate my thankfulness to my love one mother and father as my

main supportive through the completion of Projek Sarjana Muda.I would like to

express my highest appreciation to my supportive supervisor, Dr. Mohd.Rizal Bin

Salleh for Projek Sarjana Muda 2 and Pn.Rosidah Bt. Jaafar for Projek Sarjana Muda

1 for their supervision and support in completing this thesis.

In addition, I would like to convey thanks to Mr.Sivarao as my examiner for his

kindness in evaluating this project. Thank you so much to. Ms. Lee Pay Jun for her

kindness teaching me Design of Experiment (DOE). Not to forget Mr.Jaafar who

guide me in metrology research. Finally to all the staff of Faculty of Manufacturing

Engineering (UTeM) and colleagues, thank you very much for your help. Thank you.

iv

TABLE OF CONTENTS

Abstact ................................................................................................................................. i

Abstrak................................................................................................................................ ii

Dedication.......................................................................................................................... iii

Acknowledgements............................................................................................................ iv

Table of Contents................................................................................................................ v

List of Figure ................................................................................................................... viii

List of Table ..........................................................................................................xi

Sign and Symbols ..................................................................................................xii

1. INTRODUCTION ........................................................................................................ 1

1.1 Background............................................................................................................... 1

1.2 Problem Statement.................................................................................................... 2

1.3 Objectives ................................................................................................................. 3

1.4 Scopes ...................................................................................................................... 3

2. LITERATURE REVIEW ............................................................................................ 5

2.1 Electrical Discharge Machining (EDM)................................................................... 5

2.1.1 Typical (EDM) machine .................................................................................... 7

2.1.2 Die-Sinking........................................................................................................ 9

2.1.3 Principle of EDM (Die Sinking)...................................................................... 10

2.1.4 Process mechanism (stages 1-9) ...................................................................... 12

2.1.5 Dielectric fluid ................................................................................................. 18

2.1.6 Types of flushing ............................................................................................. 20

2.1.7 Application....................................................................................................... 22

2.2 Electrode EDM (Die-Sinking)................................................................................ 23

2.2.1 Type of electrode ............................................................................................. 23

v

2.2.2 Copper electrode .............................................................................................. 24

2.3 Workpiece material EDM (Die-Sinking)................................................................ 25

2.3.1 Tool steel.......................................................................................................... 26

2.3.2 Type of tool steel ............................................................................................. 27

2.3.3 Tool steel (AISI D2-Cold Work Steel). ........................................................... 27

2.4 Parameter EDM (Die-Sinking) ............................................................................... 29

2.4.1 Voltage (V). ..................................................................................................... 30

2.4.2 Pulse OFF Time (µs). ...................................................................................... 30

2.4.3 Peak Current (A).............................................................................................. 31

2.5 Machine performance ............................................................................................. 32

2.5.1 Surfaces Texture. ............................................................................................. 32

2.5.2 Surfaces Roughness (Ra). ................................................................................ 34

2.5.3 Surfaces effect in EDM.................................................................................... 37

2.5.4 Design of Experiment ...................................................................................... 39

2.5.5 Orthogonal Array (OA) Experiment................................................................ 40

3. METHODOLOGY ..................................................................................................... 43

3.1 Introduction............................................................................................................. 43

3.2 Method and experiment stages ............................................................................... 45

3.2.1 Identification of the control factors and their levels ........................................ 45

3.2.2 Design the experiment ..................................................................................... 46

3.2.3 Profile design and drawing .............................................................................. 48

3.2.4 Preparation of the experiment.......................................................................... 48

3.2.4.1 Parameter Setting.................................................................................. 49

3.2.4.2 Specimen Preparation. .......................................................................... 49

3.2.4.3 Machine preparation. ............................................................................ 51

3.2.4.4 Testing equipment. ............................................................................... 53

3.2.5 Conduct of Experiment.................................................................................... 54

3.2.5.1 Procedure EDM. ................................................................................... 56

3.2.6. Data collection and analysis ........................................................................... 57

3.2.6.1 Procedure Tester and Analyzer............................................................. 58

vi

4. RESULTS .................................................................................................................... 61

4.1 Finding of Surface Roughness................................................................................ 62

4.1.1 Factorial Design Analysis................................................................................ 63

4.1.2 Reduced Model ................................................................................................ 66

4.1.3 ANOVA Balanced ........................................................................................... 67

4.1.4 Application of Mathematical Model................................................................ 68

4.1.5 Factor Effect EDM........................................................................................... 70

4.1.6 EDM Surface Roughness................................................................................. 72

4.1.7 Surface Texture EDM Metallurgy Microscope ............................................... 75

5. DISCUSSION .............................................................................................................. 77

6.CONCLUSSION .......................................................................................................... 80

7. RECOMMENDATION FOR FUTURE WORK..................................................... 81

REFERENCES ............................................................................................................... 82

APPENDICES

A. Gant chart for PSM1

A Gant chart for PSM2

B Profile drawing

C Surface roughness results

vii

LIST OF FIGURES

2.1 A diagram of the components that form a typical EDM machine. 7

2.2 General feature of Die sinking EDM machine. (1.Servo control, 2.

Electrode, 3.Workpiece, 4.Dielectric fluid, 5.Pulse generator,

6.Oscilloscope, 7.DC motor)

9

2.3 Schematic diagram of ARC formation in EDM process. 11

2.4 (a)Typical surface generation in EDM process (a) Initial shape of

electrode and workpiece.

12

(b)Typical surface generation in EDM process (b) Final complimentary

shapes of electrode and workpiece after machining.

2.5 Building up of an electric field 13

2.6 Formation of a bridge by conductive particles 14

2.7 Beginning of discharge due to an emission of negative particles. 14

2.8 Flow of current by means of negatively and positively charged 15

2.9 Development of a discharge channel dies to a rise in temperature and

pressure.

15

2.10 Formation of a vapor bubble. 16

2.11 Reduction of the heat input after a drop in the current 17

2.12 Collapse of the vapor bubble. 17

2.13 Residues: metal particles, carbon and gas. 18

2.14 (a) Down through the electrode (b) Up through the workpiece 20

2.15 (c) By vacuum flow down through the workpiece (d) By vacuum flow

up through the electrode

21

2.16 By vibration 21

2.17 Standard terminology and symbol to describe surface finish (Quantity is

given in µin)

33

2.18 The Arimethic mean value. 35

viii

2.19

2.20

2.21

3.1

3.2

3.3

3.4

3.5

3.6

3.7

3.8

3.9

3.10

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

Coordinate used for surface roughness measurement

Heat-affected zones in EDM.

Typical SEM micrographs of the recast layer on the cross-section

Flow chart from starting the method of experiment until produced the

result

(a) Electrode copper profile (115mm x 19mm x 19mm)

(b) Workpiece D2-AISI Tool Steel (diameter 40 mm ,height 35mm)

Copper electrode

Tool steel AISI D2 workpiece material

EDM die sinking Sodick LN2/LQ Series

Mitutoyo Portable surface roughness (SJ 301)

(a) Vertical position for electrode. (b) The tool and the workpiece were

submerged in a dielectric fluid. (c) The electrical discharge starts to

erode the workpiece. (d) Dielectric fluid flush away. (e) Electrode gives

the cavity shape with depth requires. (f) Tool steel AISI D2 finish

machining.

Calibration using Mitutoyo Portable surface roughness (SJ 301)

Testing using Mitutoyo Portable surface roughness (SJ 301)

Metallurgy Microscope

Normal Probability Plot of the Effects

Main Effects Plot (data means) for (Ra)

Interaction Plot for (Ra)

Pareto Chart of the Effects standardized (First model)

Data of an ANOVA: Ra versus IP

Estimated Coefficient for Ra

Machining Time (s) versus Surface Roughness (Ra)

Peak Current (IP) versus Surface Roughness (Ra)

Voltage (V) versus Surfaces Roughness (Ra)

36

37

39

44

48

49

50

51

54

55

58

59

60

63

64

65

66

67

68

71

72

73

ix

4.10

4.11

4.12

4.13

Pulse OFF Time (µs) versus Surfaces Roughness (Ra)

Rough surface texture

Moderate fine surface

Fine surface

74

75

75

76

x

LIST OF TABLES

2.1 Physical Properties of Copper Electrode 25

2.2 Basic types of tool and die steel 26

2.3a (a) Name and class alloy type for Cold work steel 27

2.4b (b) AISI D2 Tool Steel Composition 28

2.5 Mechanical Properties of AISI D2 28

2.6 Thermal Properties of AISI D2 28

2.7

2.8

2.9

2.10

2.11

IP electric discharge Peak Current Features

Features of OFF Time

Features of ON Time

ISO recommended roughness values and grade numbers for the

specification of surface roughness

Orthogonal Array Designation

29

29

30

34

41

3.1 The Level of Process Parameter for Copper Electrode 45

3.2 The Setting of EDM Parameters 46

3.3 Test table produce by MINITAB software 47

3.4 Array for test table Table 3.1 47

3.5

EDM die sinking Sodick LN2/LQ Series Specification 52

4.1 Data of surface Roughness (Ra) Test 61

4.3

Machining Time Data

69

xi

LIST OF ABBREVIATIONS, SYMBOLS, SPECIALIZED

NOMENCLATURE

% - percent

°C - Celsius

µ - micro

µm - micro meter

µs micro second

A - Ampere

cm - centimeter

CNC - computer numerical control

DC - direct current

DOE - design of experiment

EDM - electrical discharge machine

EWR - electrode wear rate

in - inch

kHz - kilohertz

kPa - kilopascal

m - meter

mm - millimeter

MPa - mega Pascal

MRR - material removal rate

OA - orthogonal Array

Ra - arithmetic mean value

Rq - root mean square average

V - Volt

xii

CHAPTER 1 INTRODUCTION

1.0 Background

Nowadays the important use of Electro Discharge Machine (EDM) has been known

worldwide. EDM has rapidly placed alongside the milling and the grinding

equipments. The most common use EDM is machining dies, tools and molds made

from hardened steel, high-speed steel, tungsten carbide and other work piece

materials. Beside of that, EDM also has replaced many of the traditional machining

process in some applications. EDM has many advantages. The most important thing

is no force or no contact involve in the machining process. Therefore no tool or work

piece force involve in the machining process like milling and grinding process in

traditional machining.

It as well suited for making complex or fragile parts that cannot be done by the

traditional machining process. Instead of that EDM process also can be used for high

ratio deep cavity machining for making deep holes or slots with only using long

electrodes. This process will keep the precision of making deep cavity machining

compared with the traditional machining. Since the EDM process do not involve with

forces, though material like titanium and nitralloy can be process easily. This make

the EDM process is very important in producing part using though material like in

the aerospace industries, military and car racing technologies.

The EDM process also not involve with the rotation of the tool. Therefore no

problem will occur when machining sharp internal corners, the dimension maintain

at high accuracy of precision. Compared with the traditional machining, problem will

1

occur for machining internal corner where it all depends to the radius of rotating tool.

EDM also has low cost for tooling. No die set needed to do EDM machining process.

Therefore complex parts and prototypes can be made easily.

At last, EDM is a system that can produce high quality and high precision products

in a very efficient way to meet the requirement and demand for precision machine

tools. The study of EDM process will make the evolution of the EDM grow and the

knowledge will play important role in the manufacturing process industry.

The typical parameter used in EDM such Electrode material, Electrode polarity (+/-),

pulse current Ip (A), pulse duration time (micro s), pulse off time to (micro s),

average voltage U (V), working current density Id (A/cm2), Average current I (A),

open gap voltage Vo (V), Dielectric and flushing mode.

The most important machining performance of EDM is the removal rate, the

electrode wear, accuracy and surface texture. In this paperwork “The effect of EDM

die sinking parameters on surface roughness for machining tool steel material” is

discussed. There are many parameters in EDM die sinking that can be considered.

Three level process parameter such as voltage (V), pulse OFF time (µs) and peak

current (A) with two level of factor (High) and (Low) considered for this study. By

controlling the chosen processes parameters the required surface roughness will be

derive and the final result will be evaluate in order to get the most significant

machining parameter that influences fine surfaces roughness.

1.1 Problem Statement

There have been many published studies considering surface finish of machined

materials by EDM. It was noticed that various machining parameters influenced

surface roughness and setting possible combination of these parameters were

difficult to produce optimum surface quality. The influences of some machining

parameters such as pulsed current, pulse time, pulse pause time, voltage, dielectric

2

liquid pressure and electrode material have been examined. One study examined P20

tool steel and provided useful information the effects of some machining parameters

on surface roughness, but the selected of pulsed current values was very low 1–8A

.The present study examines the effects of pulsed current, pulse time and pulse pause

time on surface roughness in the 40CrMnNiMo864 composite (AISI P20) tool steel.

Therefore the effect of EDM parameter for parameters voltage (V), pulse OFF time

(µs) and peak current (A) for machining tool steel with grade AISI D2 still being

studied to get the significant parameter and optimum value for machining

performance of tool steel material alloy steel (AISI D2 cold-work).

1.2 Objectives

The purposes of this project are:

i. To study the effect of EDM die sinking for three chosen parameters voltage

(V), pulse OFF time(µs) and peak current (A) on surfaces roughness for

machining tool steel.

ii. To evaluate the quality of the surface roughness that produced from the EDM

machining of tool steel.

iii. To find significant parameter and optimum value of surfaces roughness from

the surfaces quality evaluation.

1.3 Scope and limitation

Scope of study including:

Run a machining process using EDM die sinking advanced machine type Sodick

LN2/LQ Series to examine methodically and in detail the machining performance

(surfaces roughness) in CNC laboratory Fasa B, UTeM. This project focuses on

study the effect of working surface and the parameter factor chosen was the Voltage

(V), Pulse OFF (µs) and Peak Current (A). Design of Experiment (DOE) will be

3

implementing to know how many times machining process should be run. Where the

chosen parameter was set in proportion with the DOE and the other parameters

remain constant. The material to be tested is tool steel D2-AISI Cold-Work and the

electrode were copper electrode. At the end of machining process, surface roughness

(Ra) value of the working surfaces will be evaluating using Portable Surface

Roughness Tester (SJ 301). Finally, DOE approach was apply to analyze the most

significant factor that influences the optimum value of the machining performance

using MINITAB software. Then from the result obtained, the comparison of the

surface roughness between the set of experiment using different level of parameter

setting will be discussed.

4

CHAPTER 2 LITERATURE REVIEW

2.1 Electrical Discharge Machining (EDM)

Electric discharge machining (EDM) is one of the most popular non-traditional

material removal processes and has became a basic machining method for the

manufacturing industries of aerospace, automotive, nuclear, medical and die-mold

production. The theory of the process was established by two Soviet scientists B.R.

and N.I. Lazarenko in the middle of 1940s. They invented the relaxation circuit and a

simple servo controller tool that helped to maintain the gap width between the tool

and the workpiece. This reduced arcing and made EDM machining more profitable

and produced first EDM machine in 1950s. Major development of EDM was

observed when computer numerical control systems were applied for the machine

tool industry. Thus, the EDM process became automatic and unattended machining

method (Kiyak and Cakir, 2007).

From the machining technique point of view, electro discharge machining (EDM) is

classified as a non-traditional machining technique. This technique has been widely

used in modern metal-working industry and its versatility and ability to cut fully

hardened steels has enabled it to be widely accepted, especially in the die making

industry in addition to high speed machining applications (Haron et al 2001).

The basic process in EDM is carried out by producing controlled electric sparks

between a tool (electrode) and the workpiece, both of which are immersed in a

dielectric fluid. The electric spark raises the surface temperature of both the electrode

and workpiece to a point where the surface temperatures are in excess of the melting

5

or even boiling points of the substances. Metal is thus primarily removed in the

liquid and vapour phases. By controlling the electrical parameters, removal of

material may be confined to some extent to the workpiece (Haron et al 2001).

Wear of the tool (electrode), however, cannot be ignored because when this occurs

the geometrical characteristics of the electrode will not be reproduced on the

workpiece. The surface generated by EDM consists of debris, which has been melted

or vapourised during machining, lying on or incorporated into the cratered spark-

eroded surface. This resulting product of the erosion process, commonly known as

debris, has an important relation to the various aspects of EDM. Debris formation is

analog to chip formation in traditional machining, and normally debris has a

spherical shape with slight ellipticity. Usually, the size as well as the formation of

debris depends on the current supplied during machining (Haron et al 2001).

Electrical discharge machining EDM, or spark machining, as it also called, removes

material with repetitive spark discharges from a pulsating DC power supply, with a

dielectric flowing between the workpiece and tool (Boothroyd and Knight, 2006).

The process uses thermal energy to generate heat that melts and vaporizes the

workpiece by ionization within the dielectric medium. The electrical discharges

generate impulsive pressure by dielectric explosion to remove the melted material.

Thus, the amount of removed material can be effectively controlled to produce

complex and precise machine components. However, the melted material is flushed

away incompletely and the remaining material resolidifies to form discharge craters.

As a result, machined surface has microcracks and pores caused by high temperature

gradient which reduces surface finish quality (Kiyak and Cakir, 2007).

6

2.1.1 Typical (EDM) machine

Figure 2.1: A diagram of the components that form a typical EDM machine. (Rao,

2000)

The basic EDM part component is spark servo system, generator, mechanical

structure and dielectric liquid (Figure 2.1).

a) Controlled Feed Servo System

A servo system can maintains a movement of the electrode towards the

workpiece at such a speed that the working gap, and hence, the sparking

voltage remains unaltered.This occurs when both electrode and workpiece are

eroded during the process, after a certain time dimensions of the electrodes

will be changed considerably. The result is increase in interelectrode gap.

This will increase the voltage required for sparking. This problem can be

7