Dry, Near-Dry wet EDM- parameters, capabilities,
Application and process
DRY EDM
Dry electric discharge machining (Dry EDM) is one of the novel EDM technology in
which gases namely helium, argon, oxygen, nitrogen etc. are used as a dielectric
medium at high pressure instead of oil based liquid dielectric.
Principle of dry edm
The gas used Is normally oxygen gas when the workplece Is made of steel. The gas let
also nushes the removed materlal awy from the dlscharge gap and prevents them from
adharlng onto the surfaces of the tool electrode and workplece. Furthermore, durlng
pulse Interval, the gas 1st blow of the plasma formed by the prevlous dlscharge, thus
guaranteelng the recovery of the dieledrlc strength of the gap.
The greatest advantage of dry EDM Is that the tool electrode wear ratlo Is very low
compred wlth that of conventlonal EDM In Ilquld. The materlal removal rate Improves
as the concentratlon of oxygen gas In the gas 1st Is Increased and
even hlgher materlal removal rate than the conventlonal EDM can be obtalned when
pure oxygen gas Is used.
Parameter and optimization in dry edm
I. Discharge current - It points out the different levels of power that can be supplied by
the generator of the EDM machine and represents the mean value of the discharge
current intensity.
II.Pulse-on time - It is the duration of time (µs) the current is allowed to flow per cycle.
Material removal is directly proportional to the amount of energy applied during this
pulse-on time. This energy is controlled by the discharge current and the duration of
the pulse-on time.
III.Duty cycle - It is a percentage of the pulse-on time relative to the total cycle time.
This parameter is calculated by dividing the pulse-on time by the total cycle time (pulse
on time plus pulse-off time). The result is multiplied by 100 for the percentage of
efficiency, called duty cycle.
The discharge energy is directly proportional to the current, and hence, with increase in
the current, the discharge energy also increases. The higher values of current help in
the effective melting of the work piece, which leads to better results. The higher value
of pressure helps to flush the debris particles effectively. This reduces the arcing to a
greater extent and hence, better results can be achieved.
The SEM image corresponding to the first experiment exhibits long cracks and also
more debris particles are attached to the surface. Since the duty factor is fixed at a
constant level, less pulse off time is available for a lower value of pulse on time, and
hence, only less time is available for flushing. Also, the lower value of pressure
reduces the flushing efficiency. The poor flushing of the debris particles may lead to
arcing which also affects the surface finish.The SEM image of the surface machined
under the optimal values (12 A, 200 µs, 60 V and 2.5 KPa)
At a higher value of the current, more amount of energy is released, which helps in the
effective melting of the work piece. The higher value of the pressure helps in effective
flushing of the debris particles. Also, the increased pulse off time (due to constant duty
factor) provides more time for effectively flushing the debris particles. The probability of
arcing reduces due to the effective flushing of the debris particles, which results in a
better surface finish.
The voltage is also directly proportional to the discharge energy. When the gap voltage
is raised to a higher value, the distance between the electrode and the work piece
increases, leading to inefficient flushing in the dry EDM process.
A white layer (recast layer) is usually observed in the EDMed surface. The molten
material eroded from the work piece will solidify and harden due to the cooling effect of
the dielectric fluid, and get adhered to the machined surface. The rapid heating and
quenching cycles lead to the formation of the heat affected zone beneath the recast
layer
The microstructure of the surface machined with the initial parameters shows a thin
discontinuous white layer interrupted by some inter metallics at the surface. Also, the
patches of a non-grainy layer might also be due to excessive hot spots, which may be
due to insufficient cooling. The microstructure of the surface machined under the
optimal values also exhibits a thin white layer. Less heat affected zones are observed
in the sub-surface layer immediate to the recast layer.
Special Points
1.Gases Helium ,NItrogen,Argon gas and mixture
2.Filteration ,cleaning not required
3.High Cost
4.Operators health
5.Environment friendly
6.Fire safety
7.Oxidation layer protection
8.Voldage requirement
9.Therein they conclude MRR is lower with tool electrode as a positive polarity
compared with the negative polarity, during this polarity of the tool electrode is positive
MRR is higher, within the case of EDM during a liquid. In these machining
characteristics between EDM in air with a negative tool electrode and EDM in oil with a
positive tool electrode is compared.
When the polarity of the tool electrode is negative the tool electrode wear ratio is
additionally much
lower compared with the polarity of the tool electrode is positive.
10.investigated on performance of dry EDM using slotted electrodes during this copper
used as tool electrode, work piece is SS304 and dielectric medium is oxygen. By using
different number of slots on electrode and eventually concluded that four slots as
optimum number of slots for max material removal rate (MRR). In this 1.497 mm3/min
is that the highest average material removal rate is recorded. Using the electrodes with
peripheral slots improve the flush debris particles thanks to this increasing the fabri c
removal rate (MRR)
Similarities between Conventional EDM and Dry EDM
Both the processes can be applied for conductive workpiece material only
(because the workpiece is made one electrode in EDM).
In both the cases, shape of the tool electrode must be in accordance with the
intended feature.
Mechanism of material removal (i.e. melting and vaporization) is same for both
the cases.
Spark generation occurs in both the cases. In fact, this spark is the main source
of heat required for material removal.
Both die sinking EDM and wire EDM can be carried out either in conventional
mode or in dry mode.
If process parameters are controlled properly then dry-EDM can offer similar
material removal rate (MRR) as it is observed in conventional EDM.
Differences between Conventional EDM and Dry EDM
In conventional EDM, suitable liquid In dry-EDM, no liquid is used.
(such as kerosene, deionized Instead pressurized gas (like
water, etc.) is used as dielectric helium, nitrogen, argon or gas
fluid. mixture) is used as dielectric fluid.
Here spark is generated in a liquid Here spark is generated in a
medium. gaseous medium.
The dielectric liquid is usually The dielectric gas is usually not
reused for a longer duration. Thus recycled. Thus cooling and
the used dielectric is required to cleaning accessories are not
cool down and clean before required.
delivering the same into EDM zone.
Liquid dielectric usually has high Gaseous dielectric has low thermal
heat capacity. So it can cool down capacity. Thus cooling capability is
the tool electrode and workpiece very poor in dry EDM.
electrode quickly and efficiently.
Cost of dielectric medium is Dielectric medium is cheaper.
comparatively high.
Liquid dielectric is associated with It eliminates the risk of
the risk of environment pollution, environment pollution and fire, and
operator health hazard and fire at the same time it promotes
(mainly for hydrocarbon based oil). operator’s health.
When ferrous material is machined Chance of oxide layer formation on
by conventional EDM, the liquid the hot machined surface is high,
dielectric prevents the atmospheric especially during machining
oxygen to come in contact with hot ferrous metals.
sparking zone and thus can protect
the same from oxidation.
Liquid dielectric requires Breakdown voltage for gaseous
significantly higher voltage for dielectric is comparatively low.
breakdown. This helps in increasing
spark intensity.
Near Dry EDM
Comparison of Dry, Near-Dry and Conventional EDM
This section highlights the advantages of dry and near-dry EDM over conventional
EDM in terms of various machinability aspects as given below.
Productivity: The MRR of near-dry EDM was nearly 50–60% higher than conventional
EDM. In conventional EDM, carbon particles and other debris particles
are generated during spark erosion . Further, due to inefficient flushing they do not
flush away from the IEG effectively. Subsequently, they disturb the erosion process
resulting in ineffective sparks which results in low MRR. In near-dry EDM high pressure
dielectric medium provides better flushing than conventional
EDM. This reduces debris accumulation problem at IEG resulting in higher MRR.
Several investigations have been conducted where it was found thatnear-dry EDM
achieves higher MRR as compared to conventional EDM .
The experimental investigation conducted by Dhakar reveals that the MRR of near-dry
EDM was marginally higher than dry EDM
Consequently, lower down chances of possible short circuiting. This phenomenon
improves MRR in near-dry EDM. It was also interestingly observed that, conventional
EDM produced very high TWR at higher values of current than near-dry and dry EDM
process.
Cost concerns: The near-dry and dry EDM are economically efficient processes
as compared to conventional EDM. Because, the cost of conventional EDM
dielectric is $17.68 per gallon (SHELL MACRON EDM 135) while the cost of tap
water and air/gas (used in dry and near-dry EDM) is almost negligible .
Environmental aspects: These processes are green because there is no generation
of hazardous gases/fumes; and no waste is produced from the dielectric liquid.
There is no risk of fire hazards because no flammable dielectric is used.
Efficiency: Highly efficient due to high MRR and fine surface finish. Further,
TWR in dry and near-dry EDM processes is negligible.
Space requirements: These processes do not require large floor space because
they do not need a huge dielectric circulation unit.
Process Parameters of Near-DRY EDM
Electrical Parameters
The current, gap voltage, duty factor, pulse on time, pulse off time and polarity are the
electrical parameters which dominantly affect the output characteristics in near-dry and
dry EDM processes. The current is a flow of electric charge which is carried by ions
and electrons in plasma. Gap voltage is the potential difference generated between
two electrodes (tool and workpiece) for breaking down the dielectric strength of fluid at
inter electrode gap (IEG). Finally, this results in spark generation. Pulse on time is
defined as the time during which the discharge occurs and machining is done. Pulse off
time is the time period of no discharge and reionization of the dielectric takes place.
Duty factor is the ratio of on time and the addition of on and off time.
Non-electrical Parameters
Non-electrical parameters such as flushing, lift and sensitivity also affect the response
characteristics to a great extent. Flushing plays an important role in EDM process. It is
the method of removing waste particles (debris) from the spark gap. The flushing when
ineffective, results in lower MRR and poor surface finish.
An effective flushing can greatly increase the MRR. Literature study revealed several
techniques for flushing in EDM namely internal flushing through tool electrodes, jet
flushing and suction,Internal flushing through tubular tool electrode is used in near-dry
EDM.
The lift parameter is the degree of “Z” axis (Z axis is the moving direction of tool
electrode) retraction after erosion time. The servo power on the cutting axis is
controlled by the sensitivity of the servo. This power resists the pressure increase
Tool Electrode Based Parameters
According to the working principle of EDM, any electrically conductive material
can be used as tool electrode. Literature study reveals that copper, brass,
graphite and steel are mostly used as tool electrodes in dry and near-dry EDM
processes.The selection of tool electrode is mainly dependent on tool-workpiece
combination, where one tool material may provide better results in comparison
with others.
A proper selection of tool and workpiece combination reduces the machining cost
considerably. Literature suggests that copper as tool electrode may eliminate
problems such as ignition delay and short circuiting etc. In addition, copper
receives much attention as a tool electrode material due to its properties to attain
stock removal, low wear and stable sparking. The size and shape of the tool
electrode greatly affects the responses in dry and near-dry EDM. Generally
tubular tool electrodes are used in these processes so that internal diameter of
tool can alter the velocity of dielectric fluid.
Near-Dry Based (Dielectric) Parameters
The near-dry parameters that influence the response characteristics are gaseous
pressure, gaseous medium, liquid medium, viscosity and flow rate. Gaseous pressure
is an important parameter in near-dry EDM, high gaseous pressure helps to easily
atomize liquid dielectric into mist. Further, it improves the flushing efficiency of the
process and results in the increased MRR. Dielectric fluid in EDM plays role of an
insulator between the tool and workpiece electrodes; however, debris flushing is also
one of its important work. Thus, viscosity of liquid is an important factor for selecting
the dielectric fluid in EDM. In EDM process debris accumulation is a key issue; thus,
dielectric liquids with low viscosity are preferable. However, high gaseous pressure
assists in debris flushing consequently eliminates this problem in near-dry EDM. Thus,
higher viscous liquid can also be used in this process. Liquid flow rate is the flow of
liquid constituent which mixes with air in a controllable manner.
CASE STUDY
Compared to wet EDM, near dry EDM has higher material removal rate at low
discharge energy and generates a smaller gap distance. It also places a higher thermal
load on the electrode, which leads to wire breakage in wire EDM and electrode wear.
Dielectric fluid acts as an electrical insulation barrier in the gap between the work piece
and electrode. The dielectric strength, defined as the maximum electric field strength,
is an important parameter to determine the gap distance. Dry EDM uses gas; high
MRR can be achieved to cut steel work piece with the assistance of oxygen.
A fluid dispenser used in minimum quantity lubrication (MQL) or near dry machining is
adopted as the delivery system for near dry EDM. MRR envelopes, debris deposition,
and groove width are compared. A mathematical model and experimental validation of
gap distance are presented
EDM drilling tests were conducted on a Gromax MD20micro-hole EDM machine using
a brass tubular electrode with 1 mm outer diameter and 0.41 mm inner diameter. For
stable drilling under all EDM conditions, the gap voltage was set at 60 V and pulse
interval time was t0 70 ms.
The MRR in near dry EDM under 5.3 and 75 ml/min flow rates .A much higher flow rate
is required to increase the MRR because the nozzle is set near the discharge gap and
thus not all water droplets are delivered into the gap. EDM takes 428 s to drill a hole
through the 1.27-mm thick Al6061, which is very long compared to the 11 and 13
drilling time for the wet and near dry EDM, respectively. Dry EDM also has a severe
debris deposition problem , which subsequently creates a tapered hole.
A mathematical model is developed to predict and understand the effect of an air–
water mixture on the gap distance in near dry EDM. The critical distance at which the
applied gap voltage will cause the breakdown in the dielectric fluid is d1. Water–air
mixture properties are bounded between those of the air and water at s ¼ 0(100% air)
and 1 (100% water). The validity of these assumptions is evaluated in EDM
experiments. Values of s can be calculated based on the flow rate of water and air.
Near dry EDM improved the MRR and eliminated the problem of debris deposition.
Drilling EDM achieved better hole consistency with almost no taper.
The EDM process
The theory behind EDM is simple. An electric spark is made between an electrode and
the workpiece. As a spark creates intense heat, in the range of 8,000 to 12,000° C, it
melts virtually any surface it comes into contact with.
In the machining process, the spark is controlled very carefully and is highly focused on
a certain area making sure that only the surface of the desired material is affected.
EDM will not affect any heat treatment that has been applied to the material.
How Does Electrical Discharge Machining Work?
While the definition may seem simplistic, the physical process is a little bit more complex.
The removal of material from a workpiece using EDM occurs through a series of recurring
rapid current discharges between electrodes. These electrodes are separated using a
dielectric fluid. Then, a voltage is sent through the dielectric fluid. It is important to note
that EDM manufacturing only works for electrically conductive materials.
One of those electrodes serves to change shape to fit the exact purpose. This electrode is
the workpiece electrode or the “anode.” The other electrode is the tool-electrode or the
“cathode.” The basic principle behind this process is the erosion of the material with a
controlled electric spark. For this to occur, the two electrodes must not come in contact.
There is the application of potential difference across the workpiece and the electrode in
pulse form. As the electrode moves closer to the workpiece, the electric field present in
the small gap between them increases. This continues until it reaches breakdown volume.
The electrical discharge causes extreme heating of the material. The heating leads to the
melting away of some parts of the material. A steady flow of the dielectric fluid helps to
remove the excess material. The liquid also assists in cooling during the machining
process.
TYPES OF EDM
1. Wire EDM
2. Die sinker or ram EDM
3. Hole drilling EDM machines
Wire EDM machine
A wire EDM machine works in a similar way to a cheese cutter or a bandsaw cutting wood, although
the wire moves rather than the workpiece. A metallic wire (usually brass or copper) has high voltage
electrical discharges passed through it that allows it to cut through the entire thickness of the material.
Cutting will either take place from the edge, or a hole will be drilled in the piece to pass the wire
through if sections are to be cut out from the interior.
Wire EDM creates a spark in deionised water, in which conductivity is highly controlled. The
deionised water cools the material and washes away the removed particles. Clean dielectric fluid is
continually pumped in to flush away debris.
The wire is adjustable and can be inclined to create a taper or to shape a different profile on the
edges. The electrical discharges make lots of little craters in the material, and the electrode and
workpiece never come into physical contact. Usually, a single cut will be passed right through a solid
section, and a scrap piece will drop off when complete.
If accuracy and smoothness are important factors, it may be necessary to skim the rough edges. A
skim cut involves passing the wire close to the roughed surface, this time with reduced power,
removing as much as 0.002” of surface imperfections on each pass, similar to sanding wood with a
very fine grade sandpaper.
Die sinker or ram EDM machine
This type of EDM machine is used to create cavities in a workpiece, which is useful in the
manufacture of tools and dies, metal stamping dies, and various plastic moulds, for example.
To bore the cavity, an electrode made from conductive graphite is shaped to form the required cavity
and is plunged or ‘rammed’ into the object. This creates complex, 3-D cavities, but is expensive to
produce and perform as the electrode has to be carefully machined, electrode wear is hard to control,
and there may be problems flushing out debris from the cut.
Hole drilling EDM machine
The simplest way to drill a hole with EDM is to traditionally drill a tiny pilot hole into the workpiece
before use. An EDM wire is then threaded through and used to widen the hole to the required
diameter.
If a pilot hole isn’t possible, a different type of EDM machine that ‘drills’ holes, sometimes known as a
‘hole popper’ can be used. This has a rotating electrode which cuts into the material while flushing it
continuously with dielectric.
The ‘hole popper’ machine is commonly used to make a small pilot hole allowing a wire EDM to be
threaded which is used to expand the hole. The advantage of this method is that very precise holes
can be made in tough materials. For instance, jet engine turbines have been drilled using this
process.
ABBREVIATIONS
Symbol Name
EDM Electro Discharge
Machining
MRR Material Removal Rate
SR Surface Roughness
Ip Discharge Current
Ton Pulse On Time
V Voltage
μs Micro second
μm Micro meter
Mm Millimeter
SEN Sensitivity
ASEN Anti-arc Sensitivity
A Ampere
Tw Tool work time
T↑ Tool lift time
DOE Design of experiment
Machining Parameters of EDM
1) Pulse On time (Ton): It is the duration of time expressed in micro seconds in which
the peak current is ready to flow in every cycle. This is the time in which energy
removes the metallic particles from the work piece.
2) Pulse Off time (Toff ): It is the period of time expressed in micro seconds between
the two pulse on time. This time permits the melted particle to coagulate on to the work
piece and to be wash away by flushing method of the arc gap.
3) Arc gap: It is gap between the electrode and work piece in which the spark
generate for eroding the metal from the work piece. It is very thin gap in the range of 10
– 125 μm.
4) Discharge current (Ip): Current is measured in ampere (A). Discharge current is
responsible directly for material removal. It contains energy for melting and
evaporation.
5) Duty cycle (τ): It is a ratio of the pulse on-time relative to the total cycle time
expressed in percentage. This factor is calculated by dividing the on-time by the total
cycle time (on plus off time).
6) Voltage (V): It is a potential difference that can be applied by the power supply in a
controlled manner. Voltage is also another main factor which affects the material
removal.
7) Diameter of electrode (D): It is the diameter of electrode or tool material. Diameter
of tool is one factor considered on machining. This experiment 10 mm tool diameter is
utilized.
8) Over cut – It is a measurement of clearance between tool and work piece after
completing each experiment by outline of the tool material.
Specification of EDM
Applications of Electrical Discharge Machining
1. The EDM process is extensively used because of its many advantages over
traditional machining. Its chief applications are in the manufacture of press tool and
forging dies as well as molds making for injection moulding.
2. EDM has also successfully employed for producing intricate and irregular shaped
profiles or outline common in tool rooms.
3. Small diameter holes in carbide or hardened steel can be machined by tube type
electrodes of copper tungsten, using a micro machining attachment.
4. Internal threads and internal helical gears can be cut in hardened materials by using
a rotary spindle along with thread cutting.
5. Another field of application of EDM is in grinding process is similar to EDM except
that the electrode is rotating wheel of graphite or brass.
6. EDG is advantageously adopted in grinding steel and carbide, thin and fragile
sections, brittle materials etc.
Injection Molding
Achieving the right dimension, depth, and shape of a mold is usually dependent on EDM.
It is the major injection molding process used by mold manufacturers. Wire EDM is the
main type used in this case.
Since injection molding requires various delicate and complex workpieces, this is usually
the best method to use. Moreover, it often produces high precision and fine EDM surface
finish.
Small Hole Drilling
Electrical discharge machining is a quick and unique way to create accurate deep small
holes drilling in materials, regardless of their hardness.
The hole drilling process involves using a brass electrode tube to channel the electrical
discharges onto the material. This helps to create holes of various small dimensions. The
exciting thing is that it can make holes on inclined faces and other challenging positions.
Die Casting
EDM is also very suitable for die-making applications. Manufacturing highly tailored dies
require extreme accuracy. These dies feature sharp internal corners, deep ribs, and other
intricate features.
Also, dies are often made from very hard steel alloys. These alloys are usually harder to
machine with traditional methods. The hard steel alloys may require finishing prior to heat
treatment, which may reduce the accuracy of details. Therefore, employing the EDM
process is more appropriate.
Advantages of Electrical Discharge Machining
There are several unique advantages associated with EDM manufacturing. Some of them
include:
1. Work on Any Type of Electrically Conductive Material
When you think of EDM manufacturing, the first thing that will come to your mind is its
ability to work on a wide range of materials. As long as your material is electrically
conductive, EDM is always the right process. This makes it possible to machine parts that
are difficult for traditional machining methods. These include parts made from titanium and
tungsten carbide.
2. No Mechanical Force is Involved
Another crucial benefit is that no mechanical force is put into the workpiece. Therefore,
you don’t have to worry about producing fragile outlines. This becomes easy because
there is no need for high cutting force before removing the material. Since no contact
occurs between the tool and the workpiece, there is no issue of mechanical stress.
3. Enables Various Shapes and Depths
With EDM, reaching shapes and depths seems impossible with a cutting tool. It is an
effective method for deep processing with very high tool lengths and diameter ratios. You
can easily cut sharp internal corners, narrow slots, and deep ribs with the EDM process.
4. Encourages Better Surface Finish
Manufacturers also argue that injection molding surface finish is often better with EDM
than traditional methods. This may be true because the EDM process gives surfaces high
precision and fine finishes.
5. Work on Hardened Material
Other conventional machining processes need to be done before hardening the
workpiece. On the other hand, EDM works perfectly on hardened material. Therefore, it is
easy to avoid any potential deformation from heat treatment.
EDM manufacturing, being a great part-production option, definitely has lots of
advantages in creating high-precision parts with desired shapes. If you have complex
parts to produce, consider EDM process or pick RapidDirect focuses on high precision
rapid prototyping service.
Limitation of EDM
1) Both the material the tool and work piece material has to be electrical
conductivity property. Because of this property creation of electric discharges is
possible.
2) Sometimes the wear rate on the electrode or tool is higher which requires use of
more than one tool to finish the machining on the work piece.
3) Sometimes the measurement of thin gap between the tool and work piece is not
easily predictable especially in case of complex geometries which demands the
flushing method to be differ from the simple one.
4) Optimum machining settings of the EDM process largly be influenced by on the
grouping of the tool and work piece. EDM manufacturers only fund these settings of the
required material combination. Therefore skill personnel required to develop his own
technology.
5) In case of die sinking EDM the cavity formed on the work piece with low metal
removal rate. In case of wire-cut EDM only outline of the required shape on the work
piece has to be machined. Therefore EDM is limited to small production applications.
References
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Electrical Discharge Machining of Stainless Steel 304, Journal of Applied Sciences, 11(3) (2011): pp. 549-554.
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