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UNIVERSITI TEKNIKAL MALAYSIA MELAKA
OPTIMIZATION OF MACHINING PARAMETERS IN WIRE
EDM
This report submitted in accordance with requirement of the Universiti Teknikal
Malaysia Melaka (UTeM) for the Bachelor Degree of Manufacturing Engineering
(Manufacturing Process) with Honours.
by
KW AN CHIN SENG
FACULTY OF MANUFACTURING ENGINEERING
2010
UNIVERSITI TEKNIKAL MALAYSIA MELAKA
RORANG PENGESAHAN S T:... -r JS LAPORAN PROJEK SARJANA .If UDA
T AJUK: Optimization of Machining Parameters in Wire EDM
SESI PENGAJIAN: 2009/2010
Saya KWAN CHIN SENG (8050610050)
mengaku membenarkan Laporan PSM ini disimpan di Perpustakaan Universiti Teknikal Malaysia Melaka (UTeM) dengan syarat-syarat kegunaan seperti berikut:
1. Laporan PSM adalah hak milik Universiti Teknikal Malaysia Melaka dan penulis. 2. Perpustakaan Universiti Teknikal Malaysia Melaka dibenarkan membuat salinan
untuk tujuan pengajian sahaja dengan izin penulis. 3. Perpustakaan dibenarkan membuat salinan laporan PSM ini sebagai bahan
pertukaran antara institusi pengajian tinggi. 4. **Sila tandakan (..J)
D SULIT
D TERHAD
E1 TIDAK TERHAD
(Mengandungi maklumat yang berdarjah keselamatan atau kepentingan Malaysia yang termaktub di dalam AKT A RAHSIA RASMI 19n) (Mengandungi maklumat TERHAD yang telah ditentukan oleh organisasi/badan di mana penyelidikan dijalankan)
Di~t;ran oleh:
;~~ (TANDATANGAN PENULIS) (TANDAT~ GAN PENYELIA)
UEWPAYJUN C R ·• ,..,.~h
op asml. Falwlti t<ejuruteraan ~emil ta U
. .. ua n neveraiti Teknikal Malaysia Melalca
Alamat Tetap: 13, LORONG GALING 34,
JALAN HAJI AHMAD, 25300 KUANTAN, PAHANG
Tarikh: 09t11 APRIL 2010. Tarikh: q { Jt { ~\ t>
- Jika Laporan PSM ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasa/organisasi berkenaan dengan menyatakan sekali sebab dan tempoh PSM ini perlu dikelaskan sebagai SULIT atau TERHAD.
DECLARATION
I hereby, declared this thesis entitled ''Optimization of Machining Parameters in
Wire EDM'' is the results of my own research except as cited in references.
Signature
Author's Name
Date
·'i.~· ······~· · ·· ···· · ····· ·· ·············· KWAN CHIN SENG
09th APRIL 2010
APPROVAL
This report is submitted to the Faculty of Manufacturing Engineering of UTeM as
a partial fulfillment of the requirements for the degree of Bachelor of
Manufacturing Engineering (Manufacturing Process) with Honours. The member
of the supervisory committee is as follow:
.. . (Sl .. =.~~)· · · (Official Stamp of Supervisor)
UEWPA.YJUN Pensyaralt
Fakulti Kf'juruteraan ll'embuatan Universiti Teknikal Malaysia Melaka,.
ABSTRACT
The objective of this project is to optimize the machining parameters in wire
electrical discharge machining (WEDM) in order to machine an electrode with the
desirable quality and accuracy. The intended purpose of the electrode is for the
application of miniature machining in EDM die-sinking. The machining of small and
precision part is problematic due to the presence of high mechanical force in most of
the manufacturing processes that might damage or deform the part. The application
of electrical discharge to remove material in WEDM is a potential solution to the
problem due to the non-contact nature between the electrode and the workpiece
during the machining process. Throughout the study, the variable machining
parameters such as open circuit voltage, discharge current and voltage gap were
investigated. Response surface methodology (RSM) has been used to design the
experiment and analyze the results. The quality and accuracy of the electrode were
measured on the surface roughness and kerf width, respectively. The significant
machining parameters on the surface roughness are open circuit voltage and voltage
gap while the significant machining parameter on the kerf width is discharge current.
A fine surface roughness of 1.68 J.lm and a small kerf width of 0.271 mm were
achieved. The confirmation runs have shown the acceptable percentage of variation
between the predicted value and actual value on the surface roughness with 6.52%
and kerf width with 2.01%.
ABSTRAK
Objektif projek ini adalah untuk mengoptimumkan parameter pemesinan dalam
teknologi wire electrical discharge machining (WEDM) untuk menghasilkan
elektrod yang berkualiti dan berketepatan. Tujuan elektrod ini adalah untuk
digunakan dalam aplikasi pemesinan telus dalam teknologi EDM die-sinking.
Pemesinan bahagian yang kecil dan jitu adalah rumit kerana kewujudan daya
mekanikal yang tinggi yang dikenakan dalam kebanyakan process pemesinan yang
boleh merosakkan atau mengubah bentuk bahagian. Aplikasi pembuangan bahan
dengan penyahcasan elektrik di dalam teknologi WEDM adalah berpotensi untuk
mengatasi masalah tersebut disebabkan keadaan elektrod yang tidak bersentuh
dengan bahan kerja dalam process pemesinannya. Dalam projek ini, parameter
pemesinan yang telah digunakan dalam kajian ialah open circuit voltage, discharge
current dan voltage gap. Response surface methodology (RSM) telah digunakan
untuk mereka eksperimen dan menganalisa keputusan eksperimen. Kualiti dan
ketepatan elektrod telah dikaji ke atas kekasaran permukaan dan Iebar pemotongan.
Parameter pemesinan yang berpengaruh ke atas kekasaran permukaan ialah open
circuit voltage dan voltage gap, manakala parameter pemesinan yang berpengaruh ke
atas Iebar pemotongan ialah discharge cu"ent. Kekasaran permukaan yang rendah
sebanyak 1.68 !Jrn dan Iebar pemotongan yang kecil sebanyak 0.271 mm telah
tercapai. Ujian ketentuan telah menunjukkan sesaran keputusan yang boleh diterima
iaitu 6.52% pada kekasaran permukaan dan 2.01% pada kelebaran pemotongan.
DEDICATION
To my beloved family and friends
ACKNOWLEDGEMENT
First of all, I would like to express my gratitude to Universiti Teknikal Malaysia
Melaka (UTeM) for the opportunity to pursue study in Bachelor of Manufacturing
Engineering (Manufacturing Process) and to conduct and complete this "Projek
Sarjana Muda". UTeM has provided many facilities such as machines and
equipments to aid the success of this project.
A very special thanks to my supervisor Miss Liew Pay Jun for her guidance,
encouragement and contribution in this project. She has given me useful advices and
motivation when I encountered problems and willingly shares the knowledge
regarding this project.
Besides that, I would like to thank ADTECH for providing the WEDM machine to
conduct my study and experimental runs. Not forgetting, thanks to Mr. Zamree who
is a lecturer in ADTECH that has guided me throughout the entire handling and
running of the WEDM machine.
Last but no least, I would like to thank my family for their continuous support and
courage throughout the study and the project. Not forgettable, thanks to my friends
that have been working side by side in completing this project.
IV © UrWeRIU Tel<nbl Mlllllylla ~
TABLE OF CONTENT
Abstract
Abstrak ll
Dedication lll
Acknowledgement IV
Table of Content v
List of Tables viii
List of Figures IX
List of Abbreviations XI
1.0 CHAPTER 1: INTRODUCTION 1
1.1 Introduction 1
1.2 Problem Statement 2
1.3 Objective 3
1.4 Scope 3
1.5 Importance of Study 4
1.6 Expected Result 4
2.0 CHAPTER 2: LITERATURE REVIEW s 2.1 Wire Electrical Discharge Machining (WEDM) 5
2.1.1 WEDM Process 6
2.2 Machining Parameters in WEDM 7
2.2.1 Open Circuit Voltage 8
2.2.2 Discharge Current 9
2.2.3 Voltage Gap 9
2.3 Response Variables 9
2.3.1 Surface Roughness 10
2.3.2 Kerf Width 11
2.4 Wire for WEDM 12
2.4.1 Brass Wire 12
2.5 Workpiece Material 13
2.5.1 Copper 13
2.6 Design of Experiment (DOE) 14
2.6.1 Response Surface Methodology (RSM) 15
2.7 Past Researches on WEDM 16
2.8 Summary of Literature Review 22
3.0 CHAPTER3:METHODOLOGY 23
3.1 Objective of Experiment 23
3.2 Design of Experiment - Response Surface Methodology (RSM) 23
3.2.1 Constant Machining Parameter 24
3.2.2 Variable Machining Parameter 24
3.2.3 Design of Experiment Matrix 25
3.2.4 Response Variables 26
3.3 Material Preparation 27
3.3.1 Workpiece 27
3.3.2 Wire 27
3.4 Machine and Procedures 28
3.4.1 WEDM Machine 28
3.4.2 WEDM Machining Procedures 29
3.5 Test and Measurement 29
3.5.1 Surface Roughness Measurement 29
3.5.2 Kerf Width Measurement 30
3.6 Data Analysis 31
3.7 Discussion on the Results 31
3.8 Conclusion and Recommendation 31
3.9 FlowChart 32
3.9.1 Project Flow Chart 32
3.9.2 WEDM Machining Flow Chart 34
4.0 CHAPTER 4: RESULTS AND DISCUSSION 35 4.1 Results 35
4.1.1 Surface Roughness Measurement 35
4.1.2 Kerf Width Measurement 37
4.2 Results Analysis by Using Design Expert Software 39
VI © UrWeRIU Tel<nbl Mlllllylla ~
4.2.1 Surface Roughness Analysis
4.2.1.1 Transformation
4.2.1.2 Fit Summary
4.2.1.3 ~()"}\
4.2.1.4 Diagnostics
4.2.1.5 Model Graphs
4.2.2 Kerf Width Analysis
4.2.2.1 Transformation
4.2.2.2 Fit Summary
4.2.2.3 ~()"}\
4.2.2.4 Diagnostics
4.2.2.5 Model Graphs
4.3 C>ptimization
4.3.1 Numerical C>ptimization
4.3.2 Confirmation Run
5.0 CONCLUSION AND RECOMMENDATIONS
5.1 Conclusion
5.2 Recommendations
REFERENCES
APPENDICES
i\ Gantt Chart for PSM 1
B Gantt Chart for PSM 2
C(i) FX-K Series Machining Data Sheet
C(ii) Names of Machining Conditions Setting Notches
C(iii) Notch Settings
D Workpiece and Electrode
E Table of Hole Measurement
F Table of Electrode Size Measurement
Vll
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39
39
40
42
44
47
60
60
61
63
65
68
80
80
82
84
84
85
86
LIST OF TABLES
2.1 Important properties of brass wire in WEDM 13
2.2 Important properties of copper in EDM die-sinking 14
2.3 Journals of past researches on WEDM 16
3.1 Constant machining parameters 24
3.2 Variable machining parameters 24
3.3 Design of experiment matrix 25
3.4 General properties of copper 27
3.5 General properties ofbrass 27
3.6 Specifications ofMitsubishi Wire-Cut EDM FX20K machine 28
4.1 Surface roughness measurement 35
4.2 Kerf width computation 37
4.3 Sequential model sum of square (Surface roughness) 40
4.4 Lack of fit test (Surface roughness) 41
4.5 Model summary statistics (Surface roughness) 41
4.6 Table of ANOVA for Response Surface 2FI Model (Surface roughness) 42
4.7 Table of summary statistics (Surface roughness) 43
4.8 Sequential model sum of square (Kerf width) 61
4.9 Lack of fit test (Kerf width) 62
4.10 Model summary statistics (Kerf width) 62
4.11 Table of ANOVA for Response Surface 2FI Model (Kerf width) 63
4.12 Table of summary statistics (Kerf width) 64
4.13 Solution generated by Design Expert software 80
4.14 Comparison of measurement between the predicted value and actual
value for surface roughness and kerf width 82
4.15 Deviation between the predicted value and actual value for the surface
roughness 83
4.16 Deviation between the predicted value and actual value for the kerf width 83
Vlll
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LIST OF FIGURES
2.1 Classification of major WEDM research areas 6
2.2 Schematic ofWEDM 6
2.3 Definition of the arithmetic average height (R3) 10
2.4 Top view of the kerf machined by WEDM 11
3.1 Mitsubishi Wire-Cut EDM FX20K machine 28
3.2 Illustration of cutting mechanism 29
3.3 Mitutoyo Portable Surface Roughness Tester, SJ-301 30
3.4 Mitutoyo Toolmaker's Microscope, TM 30
3.5 Flow chart of project 33
3.6 Flow chart of WEDM machining 34
4.1 Box -cox plot for power transformation (Surface roughness) 39
4.2 Normal plot of residuals (Surface roughness) 44
4.3 Graph of studentized residuals versus predicted values (Surface
roughness) 45
4.4 Graph of externally studentized residuals versus run number (Surface
roughness) 46
4.5 Graph of one factor for surface roughness versus open circuit voltage 47
4.6 Graph of one factor for surface roughness versus discharge current 49
4.7 Graph of one factor for surface roughness versus voltage gap 51
4.8 Perturbation graphs of open circuit voltage (A), discharge current (B) and
voltage gap (C) on the surface roughness 52
4.9 Contour plot of discharge current and open circuit voltage with respond
to the surface roughness 53
4.10 3D surface of discharge current and open circuit voltage with respond
to the surface roughness 53
4.11 Contour plot of voltage gap and open circuit voltage with respond
to the surface roughness 55
lX
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4.12 3D surface of voltage gap and open circuit voltage with respond
to the surface roughness 55
4.13 Contour plot of voltage gap and discharge current with respond to the
surface roughness 57
4.14 3D surface of voltage gap and discharge current with respond to the
surface roughness 57
4.15 Cube plot of open circuit voltage (A), discharge current (B) and voltage
gap (C) on the surface roughness 59
4.16 Box-Cox plot for power transformation (Kerf width) 60
4.17 Normal plot of residuals (Kerfwidth) 65
4.18 Graph of studentized residuals versus predicted values (Kerf width) 66
4.19 Graph of externally studentized residuals versus run number (Kerf width) 67
4.20 Graph of one factor for kerf width versus open circuit voltage 68
4.21 Graph of one factor for kerf width versus discharge current 70
4.22 Graph of one factor for kerf width versus voltage gap 71
4.23 Perturbation graphs of factor voltage open (A), power setting (B) and
voltage gap (C) on the kerf width 72
4.24 Contour plot of factor discharge current and open circuit voltage with
respond to the kerf width 73
4.25 3D surface of factor discharge current and open circuit voltage with
respond to the kerf width 73
4.26 Contour plot of factor voltage gap and open circuit voltage with
respond to the kerf width 75
4.27 Contour plot of factor voltage gap and open circuit voltage with
respond to the kerf width 75
4.28 Contour plot of factor voltage gap and discharge current with respond to
the kerf width 77
4.29 3D surface of factor voltage gap and discharge current with respond to
the kerf width 77
4.30 Cube plot of factors open circuit voltage (A), discharge current (B)
and voltage gap (C) on the kerf width 79
4.31 Ramp function graph 81
2FI
AISI
ANN
ANOVA
c.v. Cr
DC53
DOE
EDM
FEA
lACS
MRR
NSGA
PRESS
RSM
SA
SKDll
SUS-304
WEDM
LIST OF ABBREVIATIONS
2-Factor Interactions
American Iron and Steel Institute
Artificial Neural Network
Analysis of Variance
Coefficient of Variation
Chromium
Tool steel
Design of Experiment
Electrical Discharge Machining
Finite Element Analysis
International Association Cryospheric Sciences
Material Removal Rate
Non-dominated Sorting Genetic Algorithm
Predicted Residual Sum of Squares
Arithmetic Average Height
Response Surface Methodology
Simulated Algorithm
Hardened steel
Stainless steel
Wire Electrical Discharge Machining
CHAPTER I
INTRODUCTION
This chapter includes the overview of wire electrical discharge machining (WEDM).
The problem statement and objective of the study are also included. Then, it is
followed by the scope, importance of study and expected result of the project.
1.1 Introduction
According to Ho eta/. (2004), WEDM is a specialized thermal machining process
capable of accurately machining parts with varying hardness or complex shapes,
which have sharp edges that are very difficult to be machined by the main stream
machining process. Since the introduction of the process, WEDM has evolved from a
simple means of making tools and dies to the best alternative of producing micro
scale parts with the highest degree of accuracy and surface finish quality.
Sanchez and Ortega (2009) stated that WEDM is unique that the machining process
does not involve mechanical force exerted on the workpiece whereby material is
removed by a series of discrete electrical discharge (spark) generated between a
workpiece and wire electrode under the localized stream of dielectric fluid.
Throughout the cutting process, the wire electrode is continuously fed and collected
by spools or nozzles at certain speed and the movement of the wire electrode is
controlled numerically to cut the desired path and dimension of the workpiece.
Prasad and Krishna (2008) explained that generally, the selection of optimal
parameters in WEDM is difficult as it is a complex process and involves a large
number of variables. The selection of optimal parameters in WEDM is primarily
dependent on the operator' s experience and machining parameter tables provided by
the machine-tool manufacturers. However, researches have been carried out widely
by various researchers in optimizing the machining parameters on various materials
and conditions to obtain the desired machining result in WEDM.
Kohkonen (2009) intorduced that WEDM process found extensive use in producing
parts, especially small parts with low wall thickness. The wire electrode and
workpiece never make contact where there is virtually no cutting force on the part.
This allows for small parts with thin wall sections to be cut without deformation.
Furthermore, Hewidy et al. clarified that the high degree of the obtainable accuracy
and the fme surface quality make WEDM valuable in miniature application. This has
made WEDM process to surpass most of the conventional machining process.
1.2 Problem Statement
Besides than WEDM, electrical discharge machining is also applied in the means of
die-sinking where the same electrical discharge concept is used but having a shaped
electrode tool being directed towards the surface of the workpiece. The electrode in
EDM die-sinking needs to be machined to the desire shape and size, where some
would have intricate shape and low cross-section. The electrode is hardly producible
by the conventional machining method. It is more crucial when the electrode is used
for miniature machining in EDM die-sinking where the electrode is of high length to
cross-section ratio. It is easily subjected to deformation by only little forces.
According to Kohkonen (2009), the manufacturing of small precision parts has
always been a challenge because most manufacturing processes require mechanical
cutting force. In conventional machining methods such as turning and milling, high
cutting force is used for material removal. The large mechanical force is not
desirable in small parts as it will damage or deform the parts. The WEDM process
has excellent capability for manufacturing delicate small parts because there are no
mechanical shearing forces to resist. Therefore, by using WEDM, the delicate and
weak section of the EDM die-sinking electrode can be machined without any
distortion.
Besides that, in order to get a good quality of miniature machining in EDM die
sinking, the electrode must be of good quality and accuracy. According to Mahapatra
and Amar (2007), the quality of the electrode is determined by the low surface
roughness of the machined surface while the accuracy of the electrode is determined
by the small kerf width of the WEDM process. Ho et al. (2004) stated that the
settings for various process parameters required in the WEDM process play a crucial
role in producing an optimal machining performance.
1.3 Objective
1. To optimize the machining parameters in WEDM in fabricating a quality
electrode with good surface finish and accuracy for the application of
miniature machining in EDM die sinking.
2. To determine the significant machining parameters and study the interaction
between the machining parameters and responses involved.
1.4 Scope
This project included the study on the few variable machining parameters in WEDM
which are open circuit voltage, discharge current and voltage gap. The other
machining parameters such as pulse-off time, wire speed, wire tension, dielectric
flow rate and dielectric flow rate are set constant. The performance measures of the
study are evaluated on the surface roughness of the electrode and the kerf width of
the WEDM process. Response surface methodology (RSM) method is used to study,
analyze and relate the variables and responses involved. The significant machining
parameters in WEDM are obtained. Finally, mathematical model is developed to
represent the study and verification is conducted to determine the consistency of the
mathematical model.
1.5 Importance of study
The primary concerns in this study are the surface finish of the electrode and the kerf
width of the WEDM process, where both are of high importance in small precision
parts. The surface roughness determines the surface quality of the electrode, while
the kerf width determines the accuracy of the electrode. The quality of the electrode
machined for the application of miniature machining in EDM die-sinking is
important because it reflects the resulting machining surface and accuracy. The
optimized machining parameters of the study can also be used as a reference to
fabricate other intricate shapes such as tools and dies, fixtures and gauges, prototypes
and medical parts with the identical workpiece feature and machining condition.
1.6 Expected result
At the end of this project, the study and experiment is expected to successfully
fabricate an electrode with the desire surface roughness and kerf width by using the
optimized machining parameters in WEDM. The mathematical model developed to
represent the resulting surface roughness and kerf width under certain predetermined
factors value are also expected to meet the result of the actual electrode fabricated
with minimum error.
CHAPTER2
LITERATURE REVIEW
This chapter includes the theory of the fundamentals involved in the study. Besides
that, the related findings by past researchers are also included. At the end of this
chapter, a summary of the study is included.
2.1 Wire Electrical Discharge Machining (WEDM)
Spedding and Wang (1997) stated WEDM has grown tremendously in conductive
material machining recently because of its advantages of being unaffected by
material hardness, no cutting force, high accuracy, and the ability to achieve complex
workpiece shape as well as unmanned machining. Performance of the WEDM
process, however, is affected by many factors (workpiece material, wire, dielectric
medium, adjustable parameters, etc.) and a single parameter change will influence
the process in a complex way.
Tosun et al. (2004) conducted the survey of literature review and indicates that there
are published works on the effect of machining parameters on material removal rate
(MRR), surface roughness, cutting speed, wire rupture and wire craters.
Unfortunately, little research related to the kerf width variation has been done. Di et
a/. (2009) presented that it is significant to investigate the kerf variations in detail to
improve the machining accuracy in WEDM.
Optimizing the Process Variables
WEDM Research Areas
Monitoring & Control the Process
Process Parameters Design
Fuzzy Control System
WEDM Developments
WEDM Applications
Process Modeling Wire Inaccuracy Adaptive Control
Systems t_.___
Hybrid Machining Processes
Self-Tuning Adaptive Control
Systems
L
Figure 2.1: Classification of major WEDM research areas (Ho eta/., 2004).
2.1.1 WEDM Process
In WEDM process, a thin wire is used as an electrode tool to cut the workpiece. The
workpiece is set at a fixed position while the wire travels along the desire cutting
path. The cutting result is a narrow line when directly cut into the workpiece or can
be layer by layer material removal from the surface of the workpiece.
cl:J Wtrc ll a ;: tlOil rolk•
~~-= ' fd n
Figure 2.2: Schematic of WEDM (Spedding and Wang, 1997).
Sanchez and Ortega (2009) demonstrated that during cutting, electrical discharges
occur within the continuous flushing of dielectric fluid. The dielectric fluid used in
most WEDM machines is deionized water. During the application of each discharge,
local temperature rises by several thousand degrees (ranging between I 0,000°C-
20,0000C). Consequently, part of the material melts and vaporizes generating craters
on the surface of the workpiece, which is removed in the form of debris by dielectric
flushing. As the gap between the workpiece and wire is very small (ranges from
0.025 to 0.05 mm), it is difficult to sufficiently flush the cutting zone especially when
the part thickness is high. For that reason, most machining work is done in a
completely filled tank and the workpiece is submerged fully in the dielectric fluid.
By referring to Prasad and Gopala (2008), the relative movement between the
workpiece and wire is controlled numerically to get the required shape and accuracy
of workpiece. The gap between the workpiece and the wire is maintained at an
effective distance by servo control system. As it cuts, the wire is continuously fed
and pulled by an automatic take-up mechanism. Sanchez and Ortega (2009) also
explained that as the wire is constantly renewed, it provides a constant-diameter wire
to the cutting and the wire wear is not a primary concern.
2.2 Machining Parameters in WEDM
Ho et a/. stated that WEDM process is a complex process involving a wide range of
machining parameters. Generally, the machining parameter settings can be obtained
from the manufacturer guide but the machining settings provided might not be
optimal due to the machining diversity. Levy and Maggi (1990) demonstrated that
the parameter settings given by the manufacturers are only applicable for common
steel grades.
Scot et a/. (1991) found that discharge current, pulse duration and pulse frequency
were the main significant control factors for both MRR and surface finish, while wire
speed, wire tension and dielectric flow rate were relatively significant. Tamg et a/.
(1995) found that the machining parameters such as the pulse on/off time duration,
discharge current, open circuit voltage, servo reference voltage, electrical
capacitance and table speed are the critical parameters for the estimation of the
cutting rate and surface roughness.
Mahapatra and Amar (2007) presented that based on the analysis of variance, it was
found that the discharge current, pulse duration and pulse frequency are significant
control factors for both the MRR and surface roughness. Tosun et aJ. investigated
that the most effective parameters with respect to kerf width are open circuit voltage
and pulse duration whereas the effect of wire speed and dielectric flushing pressure
on the kerf was insignificant.
From the literature review in machining parameters, the surface roughness is mostly
affected by discharge current, pulse duration, pulse-off time and open circuit voltage,
while the kerf width is mostly affected by open circuit voltage and pulse duration.
The variable machining parameters selected through the literature review and with
accordance to the restraint of the machine used were open circuit voltage, discharge
current and voltage gap.
2.2.1 Open Circuit Voltage
Open circuit voltage is the interpole voltage when no load is applied. According to
Elman (200 1 ), the open circuit voltage is the voltage between the electrodes when
the dielectric is not yet broken. Under these conditions there is no external electric
current between the terminals, even though there may be current internally. Voltage
must be larger for thicker plates. Di et al. stated that the open circuit voltage not only
determines the breakdown distance but also affects the wire vibration in the process,
and it is demonstrated to be a key factor to influence the kerf width. In addition, the
discharge energy increases with not only the peak current value but also the open
circuit voltage which is proven by Tamg eta/. (1994).
2.2.2 Discharge current
Discharge current refers to the peak current between poles. Hewidy et al. (2005)
presented that the increase in peak current causes an increase in discharge energy at
the point where the discharge takes place. Discharge current must be set to
commensurate with workpiece materials and wire electrode. Hewidy et al. (2005)
also stated that successive discharges that have random nature will result in the
formation of overlapped crater, pockmarks and chimneys. The diameter and the
depth of the crater depend on the discharge energy or in other words, on the peak
current value. Sanchez and Ortega (2009) found that accuracy can be greatly
improved by using low energy system.
2.2.3 Voltage Gap
Voltage gap is an average machining voltage used as a guideline value to machine
with optimum feed. Elman (200 1) stated that this voltage gap value fell from the
open circuit voltage when the machining starts to generate spark. Sanchez and Ortega
(2009) presented that the voltage gap, which is governed by the servo of gap, plays a
determining role in the evacuation of debris. Liao et al. (2004) stated that smaller
voltage gap ensure the occurrence of discharging spark but the electrostatic force
between the anode and cathode will increase. Therefore, wire deflection increase and
make it difficult to operate finishing process. This reflects on the surface finish and
the accuracy of the resulting machining.
2.3 Response Variables
According to Mahapatra and Amar (2007), the most important measures in WEDM
are ~ surface roughness and kerf width. Among other performance measures,
the kerf width, which determines the dimensional accuracy of the finishing part, is of
extreme importance. In WEDM operations, MRR determines the economics of
machining and rate of production. Besides that, the surface roughness is important
because it determines the quality of the WEDM operation.
