Study of EDM and Optimization of its machining parameters Department of Mechanical Engineering M.M.M. Engineering College Gorakhpur 1 Historical Perspective In 1770, English physicist Joseph Priestley studied the erosive effect of electrical discharges. Furthering Priestley's research, the EDM process was invented by two Russian scientists, Dr. B. R. Lazarenko and Dr. N. I. Lazarenko, in 1943. In their efforts to exploit the destructive effects of an electrical discharge, they developed a controlled process for machining of metals. Their initial process used a spark machining process, named after the succession of sparks (electrical discharges) that took place between two electrical conductors immersed in a dielectric fluid. The discharge generator effect used by this machine, known as the Lazarenko circuit, was used for many years in the construction of generators for electrical discharge. Additional researchers entered the field and contributed many fundamental characteristics of the machining method we know today. In 1952, the manufacturer Charmilles created the first machine using the spark machining process and was presented for the first time at the European Machine Tool Exhibition in 1955. In 1969 Agie launched the world's first numerically controlled wire-cut EDM machine. Seibu developed the first CNC wire EDM machine 1972 and the first system manufactured in Japan. There is clearly a need to understand the process closely. When Japan began its reconstruction efforts after world war2, it faced an acute shortage of good quality of raw materials, high quality manufacturing equipment and skilled engineers. The challenge was to produce high quality products and continue to improve the quality under those circumstances. The task of developing a methodology to meet the challenge was assigned to Dr. Genichi Taguchi, who at that time was a manager in Nippon Telephone & Telegraph Company. Thorough his research in the 1950s and early 1960s Dr. Taguchi developed the foundations of robust design and validated its basic philosophies by applying them in the development of many products. In recognition of this contribution, he received the individual Deming Award in 1962, which is one of the highest recognition in quality field. The robust design method can be applied a wide variety of problems. The application of the method in electronics, automotive products, photography, and many others industries have been an important factor in the rapid industrial growth and the subsequent domination of international market in these industries by Japan.
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Study of EDM and Optimization of its machining parameters
Department of Mechanical Engineering M.M.M. Engineering College Gorakhpur
1
Historical Perspective
In 1770, English physicist Joseph Priestley studied the erosive effect of electrical discharges. Furthering Priestley's research, the EDM process was invented by two Russian scientists, Dr. B. R. Lazarenko and Dr. N. I. Lazarenko, in 1943. In their efforts to exploit the destructive effects of an electrical discharge, they developed a controlled process for machining of metals. Their initial process used a spark machining process, named after the succession of sparks (electrical discharges) that took place between two electrical conductors immersed in a dielectric fluid. The discharge generator effect used by this machine, known as the Lazarenko circuit, was used for many years in the construction of generators for electrical discharge.
Additional researchers entered the field and contributed many fundamental characteristics of the machining method we know today. In 1952, the manufacturer Charmilles created the first machine using the spark machining process and was presented for the first time at the European Machine Tool Exhibition in 1955.
In 1969 Agie launched the world's first numerically controlled wire-cut EDM machine. Seibu developed the first CNC wire EDM machine 1972 and the first system manufactured in Japan.
There is clearly a need to understand the process closely.
When Japan began its reconstruction efforts after world war2, it faced an acute shortage of
good quality of raw materials, high quality manufacturing equipment and skilled engineers.
The challenge was to produce high quality products and continue to improve the quality
under those circumstances. The task of developing a methodology to meet the challenge
was assigned to Dr. Genichi Taguchi, who at that time was a manager in Nippon Telephone
& Telegraph Company. Thorough his research in the 1950s and early 1960s Dr. Taguchi
developed the foundations of robust design and validated its basic philosophies by applying
them in the development of many products. In recognition of this contribution, he received
the individual Deming Award in 1962, which is one of the highest recognition in quality field.
The robust design method can be applied a wide variety of problems. The application of the
method in electronics, automotive products, photography, and many others industries have
been an important factor in the rapid industrial growth and the subsequent domination of
international market in these industries by Japan.
Study of EDM and Optimization of its machining parameters
Department of Mechanical Engineering M.M.M. Engineering College Gorakhpur
2
Introduction
Electric discharge machining (EDM), sometimes colloquially also referred to as spark machining, spark eroding, burning, die sinking or wire erosion, is a manufacturing process whereby a desired shape is obtained using electrical discharges (sparks). Material is removed from the work piece by a series of rapidly recurring current discharges between two electrodes, separated by a dielectric liquid and subject to an electric voltage. One of the electrodes is called the tool-electrode, or simply the ‘tool’ or ‘electrode’, while the other is called the work piece-electrode, or ‘work piece’.
When the distance between the two electrodes is reduced, the intensity of the electric field in the volume between the electrodes becomes greater than the strength of the dielectric (at least in some point(s)), which breaks, allowing current to flow between the two electrodes. This phenomenon is the same as the breakdown of a capacitor (condenser) (see also breakdown voltage). As a result, material is removed from both the electrodes. Once the current flow stops (or it is stopped – depending on the type of generator), new liquid dielectric is usually conveyed into the inter-electrode volume enabling the solid particles (debris) to be carried away and the insulating proprieties of the dielectric to be restored. Adding new liquid dielectric in the inter-electrode volume is commonly referred to as flushing. Also, after a current flow, a difference of potential between the two electrodes is restored to what it was before the breakdown, so that a new liquid dielectric breakdown can occur.
Study of EDM and Optimization of its machining parameters
Department of Mechanical Engineering M.M.M. Engineering College Gorakhpur
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Study of Electro Discharge Machining:
Process :
In EDM, a potential difference is applied between the tool and work piece. 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 deionised water is used as the dielectric medium. A gap is maintained between the tool and the work piece. Depending upon the applied potential difference and the gap between the tool and work piece, an electric field would be established. Generally the tool is connected to the negative terminal of the generator and the work piece 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 ionisation of the dielectric molecule depending upon the work function or ionisation 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 characterised as “plasma”. The electrical resistance of such plasma channel would be very less. 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.
Study of EDM and Optimization of its machining parameters
Department of Mechanical Engineering M.M.M. Engineering College Gorakhpur
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Process Parameters :
The process parameters in EDM are mainly related to the waveform characteristics.
Fig. 2 shows a general waveform used in EDM.
The waveform is characterised by the
• The open circuit voltage - Vo • The working voltage - Vw • The maximum current - Io • The pulse on time – the duration for which the voltage pulse is applied - ton • The pulse off time - toff • The gap between the work piece and the tool – spark gap - δ • The polarity – straight polarity – tool (-ve) • The dielectric medium • External flushing through the spark gap.
Study of EDM and Optimization of its machining parameters
Department of Mechanical Engineering M.M.M. Engineering College Gorakhpur
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Characteristics of EDM
(a) The process can be used to machine any work material if it is electrically conductive
(b) Material removal depends on mainly thermal properties of the work material rather
than its strength, hardness etc.
(c) In EDM there is a physical tool and geometry of the tool is the positive impression of
hole or geometric feature machined.
(d) The tool has to be electrically conductive as well. The tool wear once again depends on
the thermal properties of the tool material
(e) Though the local temperature rise is rather high, still due to very small pulse on time,
there is not enough time for the heat to diffuse and thus almost no increase in bulk
temperature takes place. Thus the heat affected zone is limited to 2 – 4 μm of the spark
crater
(f) However rapid heating and cooling and local high temperature leads to urface hardening
which may be desirable in some applications
(g) Though there is a possibility of taper cut and overcut in EDM, they can be controlled and
compensated.
Dielectric:
In EDM, as has been discussed earlier, material removal mainly occurs due to thermal
evaporation and melting. As thermal processing is required to be carried out in absence of
oxygen so that the process can be controlled and oxidation avoided. Oxidation often leads
to poor surface conductivity (electrical) of the workpiece hindering further machining.
Hence, dielectric fluid should provide an oxygen free machining environment. Further it
should have enough strong dielectric resistance so that it does not breakdown electrically
too easily but at the same time ionise when electrons collide with its molecule. Moreover,
during sparking it should be thermally resistant as well.
Generally kerosene and deionised water is used as dielectric fluid in EDM. Tap water cannot
be used as it ionises too early and thus breakdown due to presence of salts as impurities
occur. Dielectric medium is generally flushed around the spark zone. It is also applied
through the tool to achieve efficient removal of molten material.
Study of EDM and Optimization of its machining parameters
Department of Mechanical Engineering M.M.M. Engineering College Gorakhpur
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Equipment:
• Dielectric reservoir, pump and circulation system
• Power generator and control unit
• Working tank with work holding device
• X-y table accommodating the working table
• The tool holder
• The servo system to feed the tool
Modelling of Material Removal and Product Quality
Material removal in EDM mainly occurs due to intense localised heating almost by point
heat source for a rather small time frame. Such heating leads to melting and crater
formation as shown in Fig.
The molten crater can be assumed to be hemispherical in nature with a radius r which forms
due to a single pulse or spark. Hence material removal in a single spark can be expressed as
the energy content of a single spark is given as
Study of EDM and Optimization of its machining parameters
Department of Mechanical Engineering M.M.M. Engineering College Gorakhpur
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A part of this spark energy gets lost in heating the dielectric, and rest is distributed between
the impinging electrons and ions. Thus the energy available as heat at the workpiece is given
by
Now it can be logically assumed that material removal in a single spark would be roportional
to the spark energy. Thus
Now material removal rate is the ratio of material removed in a single spark to cycle time.
Thus
The model presented above is a very simplified one and linear relationship is not observed
in practice. But even then such simplified model captures the complexity of EDM in a very
efficient manner. MRR in practice does increase with increase in working voltage, current,
pulse on time and decreases with increase in pulse off time.
Product quality is a very important characteristic of a manufacturing process along with
MRR. The followings are the product quality issues in EDM
• Surface finish
• Overcut
• Tapercut
No two sparks take place side by side. They occur completely randomly so that over time
one gets uniform average material removal over the whole tool cross section. But for the
sake of simplicity, it is assumed that sparks occur side by side as shown in Fig.
Study of EDM and Optimization of its machining parameters
Department of Mechanical Engineering M.M.M. Engineering College Gorakhpur
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Thus it may be noted that surface roughness in EDM would increase with increase in spark
energy and surface finish can be improved by decreasing working voltage, working current
and pulse on time.
In EDM, the spark occurs between the two nearest point on the tool and workpiece. Thus
machining may occur on the side surface as well leading to overcut and tapercut as depicted
in Fig. 5. Taper cut can be prevented by suitable insulation of the tool. Overcut cannot be
prevented as it is inherent to the EDM process. But the tool design can be done in such a
way so that same gets compensated.
Study of EDM and Optimization of its machining parameters
Department of Mechanical Engineering M.M.M. Engineering College Gorakhpur
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RC type Relaxation Circuit of EDM:
In RC type generator, the capacitor is charged from a DC source. As long as the voltage in
the capacitor is not reaching the breakdown voltage of the dielectric medium under the
prevailing machining condition, capacitor would continue to charge. Once the breakdown
voltage is reached the capacitor would start discharging and a spark would be established
between the tool and work piece leading to machining. Such discharging would continue as
long as the spark can be sustained. Once the voltage becomes too low to sustain the spark,
the charging of the capacitor would continue. Fig. 8 shows the working of RC type EDM
relaxation.
Analytical Results and Formulae:
During charging, at any instant, from circuit theory,
Differential equation:
Solution:
where, Ic = charging current Vo= open circuit voltage Rc= charging resistance C = capacitance Vc= instantaneous capacitor voltage during charging
Study of EDM and Optimization of its machining parameters
Department of Mechanical Engineering M.M.M. Engineering College Gorakhpur
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During discharging, the electrical load coming from the EDM may be assumed a totally
resistive and is characterised by a machine resistance of Rm. then the current passing
through the EDM machine is given by:
where, Id = discharge current or current flowing through the machine
Vc= instantaneous capacitor voltage during discharging
Rm= machine resistance
Study of EDM and Optimization of its machining parameters
Department of Mechanical Engineering M.M.M. Engineering College Gorakhpur
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Study of Taguchi’s method of Design of experiment using
orthogonal arrays:
Literature Survey: The model quality improvement focuses on the reduction on variability of the manufactured
product. It is more costly to costly the cause of variations than to make a process insensitive
to control the cause of variations than to make a process insensitive to those variations. The
manufacturing variability can never be controlled if checked at the manufacturing and the
inspection stages only. Considerable advantages can be obtained by achieving product
quality at manufacturing process stage (Design stage) instead of controlling quality at
manufacturing process stage or through the inspection of the finished product.
Taguchi method is a powerful tool for the design of high quality system. It provides a simple,
efficient and systemic approach to optimize design of performance, quality and cost. The
methodology is valuable when the design parameters and qualitative or discrete. Taguchi
parameter design can optimize the performance characteristics through the setting of
design parameters and reduce the sensitivity of the system performance to source of
variations. In recent years the rapid growth of interest in the Taguchi method has led to
numerous applications of the method in a world-wide range of industries and nations.
In the present research work, the above methodology is employed to optimize the
machining parameters and to find out the most important factors and their influence on the
quality characteristics, i.e. surface roughness. In order to obtain better surface roughness,
the proper setting of cutting parameters is crucial before the process takes place. As a
starting point for determining cutting parameters, technologists could use the hands on
data tables that are furnished in machining data handbooks. Lin (1994) suggested that a trial
and error approach could be followed in order to obtain the optimal machining conditions
for a particular operation. Consequently, it is a very time consuming process of identifying
the optimum cutting conditions for a particular operation. Recently, a Design of Experiment
(DOE) has been implemented to select manufacturing process parameters that could result
in a better quality product.
The DOE is an effective approach to optimize the throughput in various manufacturing
related processes. In their study, three independent variables, each with three levels, had
total of (33) = 27 experimental runs. Oftentimes, the optimum metal cutting process
required studying more than three factors for the cutting parameters. For example, if a DOE
setup considered 4 or 5 independent variables, each with at least three levels, then (34)=81
runs or (35)=243 runs were required in the experiments. Imagining the total cost of these
experimental runs, one could conclude that it was very costly for the industry. In addition,
Study of EDM and Optimization of its machining parameters
Department of Mechanical Engineering M.M.M. Engineering College Gorakhpur
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the time of these runs could delay any quality resolving action for the industry. More
industrial technology (IT) graduates are facing challenges to improve the quality of products
and processes with minimum cost and time constraints in their careers. The Taguchi
parameter design techniques has been proved to be successful in the meeting this challenge
over the past 15 years. Therefore, there is a need to not only introduce our IT graduates to
DOE but also Taguchi parameter design.
In the present work, the experiments have been designed using highly fractional factorial
experimental design (Taguchi’s orthogonal array) to determine the influence of various
factors on the response. The experimental findings are used to identify the best
combination of parameters. It has been shown that this method yield the same or even
better results (in term of precision) as a complete factorial experiment. Basically the Taguchi
methodology is valuable when the design parameters are qualitative and discrete. In recent
years, the rapid growth of interest in the Taguchi method has led to numerous applications
of the method in a worldwide range of industries and nations.
Surface finish is one of the important criteria to be considered for proper functioning of
many machined part. It is found from result of metal cutting experiments that machining
variables like peak current, Pulse on Time and feed, and tool geometry have a marked
influence on the quality of surface produced. The roughness is directly dependent on the
peak current. In practice, the machining variables are often adjusted so as to give
dimensional accuracy only and further, if the finish is unsatisfactory, the setup is adjusted
until this difficulty is eliminated. The objective of this investigation is to experimentally
determine the effect of process variables on the surface finish obtainable during the process
and also to identify the parameters affecting the surface roughness
Subject overview:
Design of experiments (DOE) is a powerful statistical technique introduced by R.A Fisher in
England in 1920’s to study the effect of multiple variables simultaneously. In his early
applications, fisher wanted to find out how much rain, water, fertilizer, sunshine etc. are
needed to produce the best crop. Since that time, much development of the technique has
been taken place in the academic environment, but did help generate many applications in
the production floor.
As a researcher in Electronic Control Laboratory in Japan, Dr. Genechi Taguchi carried out
significant research with DOE techniques in the late 1940’s he spent considerable effort to
make his experimental technique more user friendly (easy to apply) and applied it to
improve the quality of manufactured products. Dr. Taguchi’s standardized version of DOE,
popularly known as the Taguchi approach, was introduced in the USA in the early 1980’s.
Study of EDM and Optimization of its machining parameters
Department of Mechanical Engineering M.M.M. Engineering College Gorakhpur
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Today it is one of the most effective quality building tools used by engineers in all types of
manufacturing activities.
The DOE using Taguchi approach can economically satisfy the needs of problem solving and
product/process design optimization projects. By learning and applying this technique,
engineers, scientists and researchers can significantly reduce the time required for
experimental investigations. DOE can be highly effective when we wish to:
Optimise product and process design, study the effects of multiple factors (i.e.-
variables, parameters, ingredients, etc.) on the performance, and solve production
problems by objectively laying out the investigate experiments. (Overall application
goals).
Study influence of individual factors on the performance and determine which factor
has more influence, which one have one less. We can also find out which factor
should have tighter tolerance and which tolerance should be relaxed. The
information from the experiments will tell us how to allocate quality assurance
resources based onobjective data. It will indicate whether a supplier’s part causes
problems or not(ANOVA data), and how to combine different factors in their proper
settings to get the best results(Specific Objectives).
Further the experimental data will allow us to determine:
How to substitute a less expensive part to get the same performance
How much money we can save the design improvement we propose
How we can determine which factor is causing most variations in the result
How we can set up our process such that it is insensitive to the uncontrollable
factors
Which factors have more influence on the mean performance
What we need to do to reduce performance variation around the target
How we can adjust the factors for a system whose response varies proportional
to signal factor(Dynamic response)
How to combine multiple criteria of evaluation into a single index
How we can adjust factor for overall satisfaction of criteria of evaluations
How the uncontrollable factors affect the performance
Study of EDM and Optimization of its machining parameters
Department of Mechanical Engineering M.M.M. Engineering College Gorakhpur
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Taguchi Parameter Design:
In the early 1950s, Dr. Genichi Taguchi, “The father of quality engineering”, introduced the
concept of offline quality control techniques known as Taguchi parameter design. Offline
quality control techniques are those activities performed during the product (or process)
design and development phases. Taguchi parameter design is based on the concept of
fractional factorial design. The two major goals of parameter design are:
1. To minimise the process or product variation
2. To design robust and flexible processes or products those are adaptable to
environmental conditions.
“Robust” means that the process or product performs consistently and is relatively
insensitive to factors that are difficult to control.
Two important tools used in parameter design are orthogonal arrays and signal-to-
noise(S/N) ratios. Orthogonal arrays have a balanced property in which every factor setting
occurs the same number of times for every setting of all other factors in the experiment.
Orthogonal arrays allow researchers or designers to study many design parameters which
can be obtained with minimum time and resources. The signal-to-noise ratio is simply a
quality indicator by which the experimenters and designers can evaluate the effect of
changing a particular design parameter on the performance of the process or product. The
following are the steps of Taguchi parameter design: