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					Evolution of the EDM process
EDM is among the earliest non traditional manufacturing process, having an
inception 50 years ago in a simple die-sinking application. Anyone who has ever
seen what happens when a bolt of lightening strikes the ground will have a fair idea
of the process of EDM




THE history of the EDM process dates back to the days of World Wars I and II.
Earlier very few saw the benefits of this process and the popularity of the primitive
technology wasd scarce. As much electrode material was removed as that of the
work piece and the manual feed mechanism lead to more arcing than sparking.
During this time the dither or the vibrator came into the picture and this represented
the first attempt towards controlling the spark gap. Vibrating the electrode allowed
material removal to be effective. Two Soviet scientists were convinced that many
more improvements could be made. Doctors B.R. and N.I. Lazarenko were the
people who invented the relaxation circuit, they invented a simple servo controller
too that helped maintain the gap width between the tool and the work piece. This
reduced arcing and made EDM machining more profitable.

The two principle types of EDM processes are the die sinking and the wire EDM
process. The die sinking process was refined as early as the 1940 with the advent of
the pulse generators, planetary and orbital motion techniques, CNC and the
adaptive control mechanism. From the vacuum tubes, to the transistors to the
present day solid state circuits, not only was it possible to control the Pulse on time,
but the pause time or the Off time could also be controlled. This made the EDM
circuit better, accurate, dependable and EDM industry began to grow.
During the 1960 the CIRP and ISEM conferences were held for the first time in
Czechoslovakia which proved to be a driving force in the progress of the EDM
process. EDM research was inspired by the old paradigm of technology that each
problem in a field must be solved. The founders of this paradigm were Newton and
Maxwell.




There were a number of problems faced when mathematical modeling of the ED
process was done. The gap pollution, the hydrodynamic and thermodynamic
behavior of the working fluid are hard to model. Getting a model in all with practical
technological results was difficult. This inability along with the high demand from the
market lead to a more pragmatic, application oriented research into the EDM
process.

The research going on today aims at a more application oriented field rather than
searching for a unified EDM model. Today the EDM market is growing owing to
increasing popularity of EDM in the manufacturing market and secondly due to the
indirect influence of fundamental and applied EDM R&D, carried out at various labs,
industrial ones and at universities.


The evolution of the wire EDM in the 70’s was due to powerful generators, new wire
tool electrodes, better mechanical concepts, improved machine intelligence, better
flushing. Over the years the speed of wire EDM has gone up 20 times when it was
first introduced, machining costs have decreased by atleast 30% over the years.
Surface finish has improved by a factor of 15, while discharge current has gone up
more than 10 time higher .
THEORIES OF MATERIAL REMOVAL :
The removal of material in electrical discharge machining is based upon the erosion
effect of electric sparks occurring between two electrodes. Several theories have
been forwarded in attempts to explain the complex phenomenon of "erosive spark".
The following are the theories,

1. Electro-mechanical theory
2. Thermo-mechanical theory
3. Thermo-electric theory


Electro-mechanical theory :

This theory suggests that abrasion of material particles takes place as a result of the
concentrated electric field. The theory proposes that the electric field separates the
material particles of the workpiece as it exceeds the forces of cohesion in the lattice
of the material. This theory neglects any thermal effects. Experimental evidence
lacks supports for this theory.

Thermo-mechanical theory :

This theory suggests that material removal in EDM operations is attributed to the
melting of material caused by "flame jets". These so - called flame jets are formed
as a result of various electrical effects of the discharge. However, this theory does
not agree with experimental data and fails to give a reasonable explanation of the
effect of spark erosion.

Thermo-electric theory :

This theory, best-supported by experimental evidence, suggests that metal removal
in EDM operations takes place as a result of the generation of extremely high
temperature generated by the high intensity of the discharge current. Although well
supported, this theory cannot be considered as definite and complete because of
difficulties in interpretation.

PROCESS MECHANISM:
It is not absolutely necessary to understand the operating principles of EDM to be a
successful machinist. However, an understanding of what is taking place between
the electrode and the workpiece can aid the EDMer in several important areas. A
basic knowledge of EDM theory can help with troubleshooting, in selecting the
proper workmetal/electrode combinations, and in understanding why what is good
for one job is not always good for the next.

The following description represents a combination of what is known plus what is
theorized about the process.

While several theories of how EDM works have been advanced over the years, most
of the evidence supports the thermoelectric model. The following nine illustrations
show step-by-step what is believed to happen during an EDM cycle. The graphs
below the illustrations show the relative values of voltage and current at the point
depicted

Illustration 1:

A charged electrode is brought near the workplace. Between them is insulating oil,
known in EDM as dielectric fluid. Even though a dielectric fluid is a good insulator, a
large enough electrical potential can cause the fluid to break down into ionic
(charged) fragments, allowing an electrical current to pass from electrode to
workpiece. The presence of graphite and metallic particles suspended in the fluid
can aid this electrical transfer in two ways : the particles(electrical conductors) aid in
ionizing the dielectric oil and can carry the charge directly; and the particles can
catalyze the electrical breakdown of the fluid.

The electrical field is strongest at the point where the distance between the
electrode and workpiece is least, such as the high point shown. The graph in the
illustration shows that the potential(voltage) is increasing, but current is zero.
Illustration 2:

As the number of ionic(charged) particles increases, the insulating properties of the
dielectric fluid begin to decrease along a narrow channel centered in the strongest
part of the field. Voltage has reached its peak, but current is still zero.




Illustration 3:

A current is established as the fluid becomes less of an insulator. Voltage begins to
decrease.
Illustration 4:

Heat builds up rapidly as current increases, and the voltage continues to drop. The
heat vaporizes some of the fluid, workpiece, and electrode, and a discharge channel
begins to form between the electrode and workpiece.




Illustration 5:

A vapour bubble tries to expand outward, but its expansion is limited by a rush of
ions towards the discharge channel. These ions are attracted by the extremely
intense electro-magnetic field that has built up. Current continues to rise, voltage
drops.
Illustration 6:

Near the end of the on-time, current and voltage have stabilized, heat and pressure
within the vapour bubble have reached their maximum, and some metal is being
removed. The layer of metal directly under the discharge column is in molten state,
but is held in place by the pressure of the vapour bubble. The discharge channel
consists now of a superheated plasma made up of vaporized metal, dielectric oil,
and carbon with an intense current passing through it.




Illustration 7:

At the beginning of the off-time, current and voltage drop to zero. The temperature
decreases rapidly, collapsing the vapor bubble and causing the molten metal to be
expelled from the workpiece.
Illustration 8:

Fresh dielectric fluid rushes in, flushing the debris away and quenching the surface
of the workpiece. Unexpelled molten metal solidifies to form what is known as the
recast layer.




Illustration 9:

The expelled metal solidifies into tiny spheres dispersed in the dielectric oil along
with bits of carbon from the electrode. The remaining vapor rises to the surface.
Without a sufficient off-time, debris would collect making the spark unstable. This
situation could create a DC arc which can damage the electrode and the workpiece.
This on/off sequence represents one EDM cycle that can repeat up to 250,000 times
per second. There can be only one cycle occuring at any given time. Once this cycle
is understood we can start to control the duration and intensity of the on/off pulses to
make EDM work for us.

Analysis of the Pulses used in the EDM Process

Performance measures such as MRR, tool wear, and surface finish for the same
energy depend on the shape of the current pulses. Depending upon the situation in
the gap which separates both electrodes, principally four different electrical pulses
may be distinguished:

a) Open circuit or open voltage

b) Effective discharges or real Sparks

c) Arcs and

d) Short circuits

They are usually defined on the basis of time evolution of discharge voltage and (or)
discharge current (Fig). Their effect upon material removal and tool wear may differ
quite significantly. Open voltages, occurring when the distance between both
electrodes is too large, obviously do not contribute to any material removal or
electrode wear.
When contact between tool and workpiece takes place, a short circuit occurs which
also does not contribute to material removal.The range of the electrode distances in
between these two extreme cases can be considered to be a practical working gap
yielding actual discharges, i.e., sparks and arcs. Both pulse types do show a
characteristic voltage drop across the gap during a pulse. The difference between
sparks and arcs is quite difficult to establish. It is believed that arcs occur in the
same spot, or on the electrode surface and may therefore severely damage tool and
workpiece.It is assumed that arcs occur when the plasma channel of the previous
pulse is not fully deionized; the current during the following pulse will flow by
preference along the same current path. Therefore, in such a case, no time is
required to form a new gaseous current path. The formation of the gaseous channel
is normally considered to be necessary to initiate a new spark breakdown. This
peculiarity of EDM arcs is often proposed as a discrimination characteristic with
respect to effective discharges or real sparks.It is believed that only "sparks" really
contribute to material removal in a desired mode. Until now it remains an open
question how much arcs contribute in terms of material removal and tool wear.

TOOL WEAR AND RECTANGULAR AND
NONRECTANGULAR CURRENT PULSES:
The advent of the rectangular pulse generator in EDM has resulted in improved
metal removal rates and reduced electrode wear compared to the performance of
the former relaxation-type generator. Nevertheless, it is a fact that the problem of
electrode wear still persists and that it is particularly important during finishing
operations. During finishing operations higher MRR, though preferable, is not very
important. Accuracy is the key factor. As a solution to the tool wear problem,
besides parameter selection, non-rectangular current pulses, comb current, and
other types of current pulses have been applied successfully. The state-of-art
electrical generators are designed to produce both rectangular and non-rectangular
current pulses; Therefore, it has become possible to use different types of pulses
and observe their effect on machining performance. Previous investigations indicate
that considerable tool wear reduction is possible when sloped current pulses are
used. However, the full effects of pulse on-time, which for a given average current
level has a considerable influence in determining both tool wear and material
removal rate, were not taken into account. In the study reported in an attempt made
was to find the region in which non-rectangular pulses have a clear advantage over
rectangular pulses. Due to machine limitations, a maximum peak current of 16
amperes was tried, so the results of this study cannot be considered as valid for all
settings as they are from a small experimental range.One other study reported in
made use of trapezoidal pulses in rough machining. A maximum average pulse
current of 22 amps was used and only pure trapezoidal current pulses were
employed. Also, a lift cycle for the tool was used to get rectangular current pulses
was not used. Using the lift cycle for the tool normally does not give precise results
but this probably could have been a machine constraint. The removal of material in
electro-discharge machining is based primarily on the conversion of electric energy
into thermal energy. Accordingly, the distribution of the thermal energy on the
electrode surface is of considerable importance. The plasma channel is believed to
expand throughout the pulse duration approximately with the relation
                                       r = sqrt(t')

                               where 0<=t'<=t(on time)

r is the radius of the plasma in microm, t(on time) is pulse on-time in microSec and t' is
time in pSec
The heat density on the electrode surfaces decreases by the same degree when the
instantaneous discharge energy is constant. Therefore, at the beginning of a
discharge with a small diameter of the discharge channel, maximum heat density
exists. As the channel diameter increases the heat admitting surface expands and
the proportion of energy dissipated by radiation, convection, and conduction rises
markedly. If these physical mechanisms are true, the supplied discharge energy is
not optimally utilized with the rectangular current pulse, because at the beginning of
the discharge the current density is so high that the material to be removed is
heated far beyond the required melting temperature. The current density varying
during a discharge must be regarded as the criterion for the electrode erosion
because it is argued that at the beginning of the discharge, only the anode is
thermally affected . In this phase the electrons in the plasma channel are
accelerated towards the anode and transfer their energy of motion to the anode by
impact. At that time the plasma channel rapidly expands but its diameter is still
small. The energy imposed on the anode decreases after a few microseconds
because of the decrease in energy density on the anode, along with further
expansion of the plasma channel. Thermal modelling of the process also shows that
the input power and tool wear are related by thermal conductivity of the tool and
pulse on-time. Therefore, an adaptation of the supplied power input will result in a
reduction of the tool.

Another factor that influences tool wear is the pulse interval time. It is known that
with the diminishing of the pulse interval to the limit at which the discharges
deteriorate into stationary arcs leads to the generation of socket discharges by
favoring deposition of graphite protective film. This increases the resistance to
electrical erosion of the copper tool electrode.Inversely, an increase of pause
interval determines both an increase of tool wear and the diminishing of graphite film
deposition. Thus, the adequate conditions for the deposition of the graphite film and
the diminishing of current at the beginning of discharge, causes the decrease of tool
wear.
DIELECTRIC FLUID :
The EDM setup consists of a power supply whose one lead is connected to the
workpiece immersed in a tank having dielectric coil. The tank is connected to a
pump, oil reservoir, and a filter system. The pump provides pressure for flushing the
work area and moving the oil while the filter system removes and traps the debris in
the oil. The oil reservoir restores the surplus oil and provides a container for
draining the oil between the operations.




The main functions of the dielectric fluid are:

      To flush the eroded particles produced during machining, from the discharge
       gap and remove the particles from the oil to pass through a filter system.
      To provide insulation in the gap between the electrode and the workpiece.
      To cool the section that was heated by the discharge machining.

The two most commonly used fluids are petroleum based hydrocarbon mineral oils
and de-ionized water. The oils should have a high density and a high viscosity.
These oils have the proper effects of concentrating the discharge channel and
discharge energy but they might have a difficulty in flushing the discharge products.
For most EDM operations kerosene is the common die electric used with certain
additives, that prevent gas bubbles and de-odoring. Silicon fluids and mixture of
these fluids with petroleum oils have excellent results.
High removal rates, less tool wear, better surface finish have been obtained in the
machining of titanium alloys. Other dielectric fluids with a varying degree of success
include polar compounds such as aqueous solutions of ethylene glycol, water in
emulsions and distilled water.

The main things that the EDM user should be concerned with are:
Flash point: This is the temperature at which the vapors of the fluid will ignite. This
explanation is a little simplistic as conditions for testing are more involved but for the
sake of discussion and safety’s sake, the higher this number, the better. Unless you
are doing extremely small, low power cavity work or drilling the tinist of holes, be
especially concerned with anything on a spec sheet or MSDS rated lower than 180
degrees Fahrenheit.

Dielectric strength: This is the ability of the fluid to maintain high resistivity before
spark discharge and in turn the ability to recover rapidly with a minima amount of
OFF time. An oil with a high dielectric strength will offer a finer degree of control
throughout the range of frequencies used, especially those used when machining
with high duty cyles or poor flushing conditions. This will provide for better cutting
efficiency coupled with a reduced potential arcing.

Viscosity: The lower the viscosity of the fluid the better is the accuracy and finishes
that can be obtained . In mirror finishing or close tolerance operations, spark gaps
can be as small as 0.005 or less. With such tight, physical restrictions such as this, it
is much easier to flush small spark gaps with lighter and thinner oil. Good finishing
EDM oils are on the thin side. In the EDMing operations requiring moderate finishes
like in the forging dies, high MRR, high current values, heavier oils can be used.
Viscosity in such conditions can be high because of larger spark gaps and this will
also prevent the excess loss os fluid through vaporization.

Specific gravity: Often confused with viscosity, this is the “weight” of a substance
measure by a hydrometer. The “lighter” the oil or lower its specific gravity, faster the
heavier particles (chips) settle down. This reduces the gap contamination and
possibilities of secondary discharge and/or arcing.

Color: All dielectric oils will eventually darken with use, but it seems only logical to
start with a liquid that is as clear as possible to allow viewing of the submerged part.
Clear or “water-white” should be your choice, because any fluid that is not clear
when brand new certainly contains undesirable or dangerous contaminants.

Odor: Besides for the obvious reasons for aesthetic of choosing a fluid with no
discernable odor, the oils that have a strong odor give an indication for the presence
of sulfur which is undesirable in the EDM process.

Preventive Maintenance:
Depending on the use of the oil and maintenance the oils can last several years.
Regularly filtered oil prevention of water contamination will extend its useful life
considerably.
Water contamination cannot be eliminated completely as condensation will occur on
the electrode surface when the surface heats up. Graphite electrodes will contribute
more to the condensation than the metallic electrodes as they have a porous
structure and absorb moisture from the air. That is the reason why the graphite
electrodes should be stored in dry areas. Some shops will keep the electrodes in dry
ovens the night before they are used. A less obtrusive method to keep humidity and
moisture absorption to a minimum, would be to allow a 60 watt bulb remain to
remain lit within the strong cabinet.
The color of the oil is not necessarily an indication for oil replacement. All oils, no
matter how clear they are when new, will darken in shades from amber to brown
with use and age, because these products will break down when exposed to high
heat. Obviously, sustained high amperage machining will breakdown the oil more
rapidly. Tars, resins and hydrocarbons are generated when this occurs and this is
what “stains” the oil. No amount of filtration will remove this discoloration, so don’t
mistake “colored oil” for “dirty oil”.

Dirty oil can be judged by the following factors:

   1. Pressure gauge readings as described in the machines maintenance manual.
   2. Increased occurrence of DC arcing or pitting with settings that were
      previously successful. (Assuming of course that no other changes have been
      made such as the grade of the electrode material, flushing pressures etc.)
   3. Longer cutting cycles and / or degradation of finishes.
   4. A visual inspection of the oil. To do this, fill the work tank but do not machine.
      Collect a sample of oil in a clear container. Visually check the sample
      immediately for any cloudiness and again after several hours, to check for
      sediments on the bottom or color striations in the oil itself. Dirty oil is usually
      tinted gray or black and this coloration will dissipate if the oil is allowed to
      circulate through the filtration system. Replacing the filters is the least
      expensive ans most likely remedy to the above symptoms.
Some of EDM usage

Micro EDM :
Micro EDM, similar to conventional macro EDM, is an erosion process where the
material is removed by electrical discharges generated at the gap between two
electrically conductive electrodes. Micro EDM is used to machine micro holes ,
channels and 3D micro cavities in electrically conductive materials including super
alloy such as tungsten carbide and stainless steel. Micro EDM has been used to drill
not only circular holes but also holes with irregular cross sections. The shape and
size of the the micro hole made by micro EDM is determined by the electrode
prepared by the WEDG.

The micro EDM machine used is panasonic MG-ED72W , This machine includes
MG-ED71 ( Standard NC Boring machine ) + WEDG Unit ( Micro Electrode Tooling
unit )

Recently conducted projects include Integration of Uniform Wear Method with
CAD/CAM and Machining of Microhole with high aspect ratio and non circular blind
micro hole.
EDM Drilling :

Once relegated to a last resort method of drilling holes, fast hole EDM drilling is now
used for production work. Drilling speeds have been achieved of up to 2 ipm. Holes
can be drilled in any electrically conductive material, whether hard or soft, including
carbide. Fast hole EDM drilling is used for putting holes in turbine blades, fuel
injectors, cutting tool coolant holes, hardened punch ejector holes, plastic mold vent
holes, wire-EDM starter holes, and other operations. The term fast hole EDM drilling
is used because conventional ram EDM can also be used for drilling. However, ram
EDM hole drilling is much slower than machines specifically designed for EDM
drilling. Fast hole EDM drilling uses the same principles as ram EDM. A spark jumps
across a gap and erodes the workpiece material. A servodrive maintains a gap
between the electrode and the workpiece. If the electrode touches the workpiece, a
short occurs. In such situations, the servodrive retracts the electrode. At that point
the servomotor retraces its path and resumes the EDM process.

Recent research investigates the influence of process prameters on the surface
integrity of the electrodischarge drilling process.
Wire EDM
Wire EDM is an electrical discharge machining process with a continuously moving
conductive wire as tool electrode. The mechanism of metal removal in wire electrical
discharge machining (WEDM) involves the complex erosion effect of electric sparks
generated by a pulsating direct current power supply between two closely spaced
electrodes in dielectric liquid. The high energy density erodes material from both the
wire and workpiece by local melting and vaporizing. Because the new wire keeps
feeding to the machining area, the material is removed from the workpiece with the
moving of wire electrode. Eventually, a cutting shape is formed on the workpiece by
the programmed moving trajectory of wire electrode. The equipment is extensively
used in making dies and molds.
The related research projects include:
Avoidance of wire breakage, development of monitoring and control system,
database, machining of advanced materials,comparison of different wire
performance and thermal as well as vibration modeling.




Abrasive Electro Discharge Grinding ( AEDG )
AEDG is a hybrid process, which combines EDM and grinding. In AEDG mechanical
abrasion of a metal bonded diamond wheel is combined with the electro-erosion of
electrodischarge machining (EDM). The removal of conductive or partially
conductive material is by a combination of rapid, repetitive spark discharges
between workpiece and rotating tool, separated by a flowing dielectric fluid and also
by a mechanical action of irregularly shaped abrasive particles on the periphery of
the wheel.

The recent research includes monitoring and control, new power generator, 2-axis
NC wheel dressing unit , environmental performance of different dielectric fluids.
The current research involves strategy for optimizing neural network modeling, the
study of self dressing characteristics and sequence of operations and using neural
networks for controls in AEDG.

				
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posted:11/11/2011
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