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Singh, Chandan Deep: Electrical Discharge Machining. Optimization of chromium
powder mixed EDM parameters during machining of H13 tool steel, Hamburg, Anchor
Academic Publishing 2017
PDF-eBook-ISBN: 978-3-96067-710-9
Druck/Herstellung: Anchor Academic Publishing, Hamburg, 2017
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ABSTRACT
In the present study, optimization of chromium powder mixed EDM parameters is studied
during machining of H13 tool steel. Four input parameters of powder mixed EDM namely
peak current, pulse on time, duty cycle and powder concentration are varied, each at three
levels, to get the optimum responses. Material removal rate (MRR), Tool wear rate
(TWR) and Surface Roughness (Ra) are considered as performance measures. Copper
electrode of 16 mm is used as the tool. Response Surface Methodology is used to
correlate input and output parameters. The variation of responses due to variation in input
parameters has been studied and shown in the form of surface plots and contour plots.
Results reveals that pulse on time, powder concentration, duty cycle and peak current are
the significant factors affecting MRR while TWR is significantly affected by peak current
only. In case of Surface Roughness, pulse on time, peak current and powder concentration
are the significant factors affecting the Ra. Furthermore, it is found that maximum MRR
is there when both peak current and duty cycle are increased simultaneously. Minimum
TWR is obtained when current is at its low level (10 Amp). In case of surface roughness,
minimum value of Ra is obtained when pulse on time and peak current simultaneously are
at low levels (100µsec and10 amp) while powder concentration and duty cycle are at their
intermediate levels (10g/l and 6%). Desirability Method is used to optimize the input
parameters to get optimal values of responses. Optimal solution that has been found for
the present study is 16.1250 mm
3
/min MRR, 0.3161 mm
3
/min TWR and 7.6987 µm Ra
with peak current 20 ampere, duty cycle 5.0909%, pulse on time 200 µsec and powder
concentration of 11.667 g/l.
Keywords: MRR, TWR, Surface Roughness (Ra), RSM and Desirability Method.
TABLE OF CONTENTS
Chapter-1 Introduction 1-14
1.1 Introduction to Non Conventional Machining Processes 1
1.2 Electrical Discharge Machining 2
1.3 Working Principle of EDM 3
1.4 Mechanism of Material Removal in EDM 4
1.5 Powder Mixed EDM 6
1.6 Major Components of EDM or PMEDM 7
1.7 Important Process Parameters of PMEDM or EDM 9
1.8 General Requirements of Dielectric Medium in EDM 11
1.9 Flushing 11
1.10 Applications of EDM 13
1.11 Advantages of EDM 13
1.12 Disadvantages of EDM 14
Chapter-2 Literature Review & Problem Formulation 15-27
2.1 Literature Review 15
2.2 Problem Formulation 25
2.3 Objectives of the Present Study 27
Chapter-3 Optimization Technique Used 28-32
3.1 Response Surface Methodology 28
3.2 Surface Plot 29
3.3 Contour Plot 30
3.4 Desirability Approach 31
3.5 Optimization Plot by Desirability Method 32
Chapter-4 Experimental Setup
33-38
4.1 EDM Machine 33
4.2 Experimental Setup for Powder Mixed in the Dielectric 34
4.3
Preparing Workpiece for Experimentation 36
4.4 Weighing the Specimen 37
4.5 Specification of Balance 38
4.6 Surface Roughness Measurement 38
Chapter-5 Results & Discussion 39-68
5.1 Response Surface Methodology 39
5.2 Input Parameters and their Levels 40
5.3 Responses Variables 41
5.4 Design Matrix and Observation Table 43
5.5 Results for Material Removal Rate 46
5.6 Results for Tool Wear Rate 52
5.7 Results for Surface Roughness (Ra) 57
5.8 Optimization of the Powder Mixed EDM Parameters 63
Chapter-6 Conclusion & Future Scope 69-71
6.1 Conclusion 69
6.2 Future Scope 70
References 72-77
LIST OF FIGURES
Fig. No.
Title
Page No.
1.1
Setup of EDM
3
1.2
Working principle of EDM
3
1.3
Diagram of the EDM physical process
5
1.4
Working Principle of PMEDM
6
1.5
Positioning System
7
1.6
Tool Holder
8
1.7
Concept of pulse on time and pulse off time
10
1.8
Normal and Reverse Polarity
11
1.9
Injection flushing through electrode
12
1.10
Side flushing
12
1.11
Suction flushing through electrode
12
3.1
Surface plot generated by RSM
30
3.2
Contour plot generated by RSM
30
3.3
Optimization plot by Desirability Method
32
4.1
EDM used for experimentation
33
4.2
Experimental set up for powder mixed dielectric EDM
35
4.3
Specimens for experimentation
36
4.4
Electronic balance
37
5.1
Design of experiment by RSM using MINITAB software
39
5.2 (a)
Workpiece after machining
42
5.2 (b)
Workpiece before machining
42
5.3
Copper electrode used for machining
42
Fig. No. Title
Page No.
5.4
Main effects plot for MRR
49
5.5
Surface plot and Contour plot for MRR
50
5.6
Residual Plots for MRR
51
5.7
Main effects plot for TWR
54
5.8
Surface plots for TWR
55
5.9
Contour plot for TWR
56
5.10
Residual plots for TWR
57
5.11
Main effects plot for Ra
59
5.12
Surface plot for Surface roughness (Ra)
60
5.13
Contour plots for Surface Roughness (Ra)
61
5.14
Residual plots for Surface roughness (Ra)
62
5.15
Optimal plot for MRR
64
5.16
Optimal plot for TWR
65
5.17
Optimal plot for Surface roughness (Ra)
66
5.18
Optimization plot for the experiment
68
LIST OF TABLES
Table No. Title
Page No.
4.1
Specification of EDM machine used for experimentation
34
4.2
Composition of H13 die steel
37
4.3
Specification of Electronic balance
38
5.1
Input Parameters with Levels
40
5.2
Constant Parameters
41
5.3
Design matrix for experimentation
43
5.4
Response values after EDM operation
45
5.5
ANOVA table for MRR
47
5.6
ANOVA table for TWR
52
5.7
ANOVA table for Ra
58
5.8
Single objective optimization table for MRR
63
5.9
Single objective optimization table for TWR
64
5.10
Single objective optimization table for Ra
65
5.11
Validation table for Desirability Method
66
5.12
Multi-objective optimization by Desirability Method
67
List of Abbreviations & Symbols
EDM = Electric Discharge Machining
PMEDM = Powder Mixed Electric Discharge Machining
MRR = Material removal Rate
TWR = Tool Wear Rate
Ra = Average Surface Roughness
RSM = Response Surface Methodology
ANOVA = Analysis of Variance
µsec = 10
-6
sec
µm = 10
-6
m
t = Time for Machining (8 min)
Wtb = Weight of tool before machining in grams
Wta = Weight of tool after machining in grams
= Density of copper = 8.96gm/cm
3
Wjb =Weight of the workpiece before machining in grams
Wja = Weight of the workpiece after machining in grams
, = Density of H13 steel= 7.80gm/cm
3
CHAPTER 1
INTRODUCTION
1.1 Introduction to Non Conventional Machining Processes
Traditional or Conventional machining processes work on the principle that there should
be the physical contact between the tool and the workpiece and the tool must be harder
than the workpiece for the removal of the material. But newly developed materials such
as carbides, nickel based alloys, Hastelloy, Inconel, hot die steels etc have very high
strength, hardness, corrosion resistance and other properties which make them almost
impossible to machine with conventional processes. Therefore, there is a need to develop
new tool materials and processes for the machining of such type of materials with high
accuracy and productivity. Non conventional processes provide solution for machining of
these types of advanced materials. These processes are non conventional in the sense that
these do not use the tool to remove the material from the workpiece but use the energy for
material removal hence material is removed without the formation of chips.
Based on the type of energy used by these processes for the machining of workpiece,
these are classified into following type of categories.
a) Mechanical processes: These are the processes in which the material is removed
from the workpiece by the mechanical action of abrasive particle or fluid or both. In
some cases mechanical action is achieved by vibrating the abrasive particles at high
frequency as in Ultrasonic machining, while in Abrasive jet machining and Water jet
machining, kinetic energy of abrasive jet and fluid respectively striking the workpiece
provides the mechanical action for the erosion of the material.
b) Electrochemical processes: These processes require electrochemical energy for the
removal of metal from the workpiece. Material is removed from the anode workpiece
and transported to the cathode tool in an electrolyte bath. Electrolyte flows rapidly
1
between the two poles to prevent the plating of the eroded material on to the tool.
Electrochemical machining, electrochemical grinding and electrochemical deburring
are the processes that utilize electrochemical energy for material removal.
c) Chemical processes: It involves the application of strong chemical etchant to remove
the material from desired portion of the workpiece. The remaining portion of the
workpiece is covered with the maskant to prevent the etching from this undesired
portion. Chemical milling, chemical blanking and photochemical machining are the
examples of this process.
d) Thermal processes: These processes utilize the energy in the form of localized heat,
light, electron bombardment for material removal by melting or vaporizing the area of
the workpiece from where material is to be removed. Examples are electron beam
machining, laser beam machining, plasma arc machining and electric discharge
machining.
The selection of processes depends on various factors like process parameters, process
capabilities, shapes to be machined, properties of the material and economics of the
process.
1.2 Electrical Discharge Machining (EDM)
EDM is one of the extensively used non conventional machining processes for material
removal. In this, material is removed by the initiation of electrical discharge between the
tool and the workpiece. There is no direct contact between tool and workpiece. It is
capable of machining electrical conductive materials regardless their hardness and widely
used in aerospace industry, automobiles industry, die and mould making industry to
machine hard materials and their alloys. General set up of electrical discharge machine is
shown in fig 1.1
2
Fig 1.1 Setup of EDM [35]
1.3 Working Principle of EDM
EDM is a thermal process that uses spark discharges to machine the material. A shaped
electrode acts as a tool which makes cavities or holes in the workpiece. Electrically
conductive workpiece is connected to one pole of a pulsed power supply and electrode or
tool is connected to another pole of power supply.
Fig 1.2 Working principle of EDM [35]
A small gap is maintained between the electrode and the workpiece to provide electrical
resistance in gap. An intensive electric field is created between the tool and the workpiece
when a pulse of D.C electricity is delivered. This electric field is created at a point where
surface irregularities provide the narrowest gap. As a result of this field, naturally
occurring microscopic contaminants suspended in the dielectric fluid and the negatively
charged particles emitted from the workpiece form high conductive bridge across the gap.
3
As the voltage increases in the beginning, the temperature of this bridge increases and
formation of spark comes into play between two surfaces. At the mid- point of electrical
pulse, voltage is decreased by power supply and current is increased. Due to this, increase
in temperature and pressure in the spark channel takes place. Because of this increase in
temperature and pressure, small amount of material melts and vaporizes from both
electrode and workpiece at the point of spark contact Fed by gaseous by products of
vaporization, a bubble rapidly expand outward from the spark channel. The spark and
heating are stopped when the electrical pulse is terminated. This causes both the spark
channel and the vapour bubble to collapse. The injection of relatively cool dielectric fluid
results in an explosive expulsion of molten metal from both surfaces, resulting in the
formation of small crater in both surfaces. The entire sequence takes place in only micro-
seconds to mini-seconds. As there is no contact between the tool and the workpiece, so
there is no force generated during machining.
1.4 Mechanism of Material Removal in EDM
In EDM, material is removed from the workpiece and the electrode by the series of sparks
at the closest point which decrease the distance between the electrode and the workpiece.
The next spark occurs at the next closest point. Material at the closest point is heated.
This results in vaporization of material due to origination and termination of spark. Whole
sequence of material removal mechanism is discussed below with the help of fig 1.3.
In fig 1.3(a) when the voltage is applied between the tool and the workpiece, a strong
electric field is developed at the point of least distance between the tool and the
workpiece. Increasing the voltage ionizes the dielectric fluid. Fig 1.3(b) indicates that
when the voltage reached its peak value insulating properties of the dielectric decreases
along the narrow channel centered in the strongest part of the field. A current is
established as the number of ionic particles increase and discharge channel is formed.
4
5
Fig 1.3 Diagram of the EDM physical process [36]
In fig 1.3(c), due to the buildup of heat with increase in current in the discharge channel, a
plasma channel is formed consists of vaporized material from the electrode and the
workpiece. At the end of the pulse on time, the pressure inside the plasma is nearly at its
peak point which causes the growth of vapour bubbles as shown in fig 1.3(d). When the
pulse voltage ceases, the plasma channel collapses as there is sharp decrease in the plasma
channel pressure and the molten cavities explode violently into the dielectric liquid as
represented in fig 1.3(e). Finally the surface cools down instantaneously, where all
vaporized and fraction of melted material in the form of irregularly shaped or hollow
spherical particles are flushed away by the dielectric liquid.
As there is no direct contact between the tool and the workpiece, therefore problems like
vibration, chattering and mechanical stresses does not occur in EDM during machining of
components. In spite of these advantages, problems like poor surface finish, low material
removal rate etc prevent its uses in some industries to some extent. In order to remove
6
these problems, a new technology known as powder mixed electric discharge machining
(PMEDM) has been emerged to improve the machining performances of conventional
EDM
1.5 Powder Mixed EDM
In PMEDM, a suitable powder like silicon, vanadium, titanium etc is mixed with the
dielectric of EDM. Due to the applied electric field applied, the powder particles get
energized and accelerated and become conductors and promote breakdown in the gap and
also enhance the spark gap between the tool and the workpiece.
Fig 1.4 Working Principle of PMEDM [30]
Powder particles formed the chain type structure and arrange themselves in the direction
of current. This causes the bridging gap between the electrode and workpiece, hence
insulating strength of dielectric gets reduced which lead to easy short circuiting and hence
early explosion in the gap takes place. It results in the series of discharges under the
electrode area. Due to this, faster sparking causes the faster erosion from the workpiece
and hence MRR get increased. Addition of powder in the dielectric enlarged the plasma
which causes the electric density to decrease and hence uniform erosion occurs on the
workpiece leading to better surface finish along with high MRR.
1.6 Major Components of EDM or PMEDM
a) Positioning System
b) Servo Feed System
c) Power Supply System
d) Tool Holder
e) Dielectric System
f) Machining Tank
These systems are discussed in brief as following:
a) Positioning System: It consists of CNC two axes table. This system is used to
provide the movement to the workpiece in X and Y direction.
Fig 1.5 Positioning System
b) Servo Feed System: This system maintains the constant gap between the tool and
the workpiece throughout the machining operation by sensing and comparing the
gap voltage with the present value. If any difference exists, then this difference is
used to control the movement of servomotor to adjust the gap.
7
c) Power Supply System: Power supply system converts AC from main supply into
pulse DC required to produce spark discharges. Firstly, the input power is
transformed into continuous DC power by solid state rectifiers. A square wave
signal via digital multi vibrator oscillator circuit is generated by using small
percentage of DC power supply. This signal is used to start power transistors.
These transistors act as high speed switches to control the flow of remaining DC
power. This power is used to create sparks responsible for material removal [37].
d) Tool Holder: It is a tool holding device which holds the electrode for carrying out
the machining operations.
Fig 1.6 Tool Holder
e) Dielectric System: It supplies the required amount of dielectric fluid to the
cutting zone during machining. Dielectric fluid serves following functions:
1 Acts as insulator between both surfaces.
2 It performs the role of coolant and carries away the heat produce during
machining operation
8
3 It removes the material from the cutting zone hence acts as the flushing
medium. It is the one of the major and critical function of the dielectric
system. Poor flushing causes the dielectric fluid to stagnate and tiny
particles are build up in the gap. This will reduce the surface finish and
material removal rate
f) Machining Tank: In EDM machining tank is the actual tank in which machining
operation takes place but in PMEDM machining tank is another tank which is
smaller than the machining tank of EDM and is placed inside it. Powder is
allowed to mix with the oil present in this small tank rather than mixing with the
whole of the dielectric oil.
1.7 Important Process Parameters of PMEDM or EDM
Some of the important process parameters of PMEDM are discussed below:
a) Discharge Voltage: It is related to the spark gap and breakdown strength of
dielectric. Increase in the discharge voltage increase the gap which improves the
flushing conditions and result in higher material removal rate, tool wear rate and
surface roughness.
b) Pulse on Time: The duration of time (µsec) the current is allow to flow per cycle.
Material removal is directly proportional to the amount of energy applied during
this on time. This energy is really controlled by the peak current and the length of
the pulse on time.
c) Arc Gap: It is the distance between the electrode and workpiece during EDM
process. It may also be called spark gap. Spark gap can be maintained by servo
system.
9
d) Pulse off Time: It is the duration of time between the sparks. This time allows the
molten material to solidify and to be wash out of the arc gap. This parameter
affects the stability of the arc. If the pulse off time is too short, it will cause sparks
to be unstable.
Pulse Pulse
on Time off Time Total Pulse time
Fig 1.7 Concept of pulse on time and pulse off time [38]
e) Discharge Current: It is the amount of power used in electric discharge
machining. It is measured in ampere and one of the most important parameter in
EDM. High current improves the MRR but at the cost of tool wear and surface
finish.
f) Duty Cycle: It is a percentage of the on-time relative to the total cycle time. This
parameter is calculated by dividing the on-time by the total cycle time (on-time
pulse off-time).
g) Polarity: Polarity can be straight or reverse. In straight polarity tool electrode is
connected to the negative terminal of power supply and workpiece is connected to
positive terminal of power supply and in reverse polarity connection of terminal to
workpiece and tool electrode is opposite to that of straight polarity. Straight
polarity is normally used polarity in EDM. The negative polarity has high material
removal rate and low surface roughness as compared to positive polarity of the
10
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