Electrical Discharge Machining (EDM)
1
Introduction
Sometimess it is referred to as spark machining , spark eroding , burning, die Sometime sinking or wire erosion
Its a manufacturing process whereby a desired shape is obtained using electrical discharges (sparks).
Material is remoed from the workpiece by a series of rapidly recurring current discharges between two electrodes, separated by a dielectric li!uid and sub"ect to an electric oltage.
#ne of the electrodes $ %tool&electrode' or %tool' or %electrode'.
#ther electrode & workpiece&electrode or %workpiece'.
s distance between the two electrodes is reduced, the current intensity becomes greater greater than the strength of the dielectric (at (at least in some some points) causing it to break. 2
Introduction
Sometimess it is referred to as spark machining , spark eroding , burning, die Sometime sinking or wire erosion
Its a manufacturing process whereby a desired shape is obtained using electrical discharges (sparks).
Material is remoed from the workpiece by a series of rapidly recurring current discharges between two electrodes, separated by a dielectric li!uid and sub"ect to an electric oltage.
#ne of the electrodes $ %tool&electrode' or %tool' or %electrode'.
#ther electrode & workpiece&electrode or %workpiece'.
s distance between the two electrodes is reduced, the current intensity becomes greater greater than the strength of the dielectric (at (at least in some some points) causing it to break. 2
History
*his allows current to flow between the two electrodes.
*his phenomenon is the same as the breakdown of a capacitor capacitor..
s a result, material is remoed from both the electrodes.
#nce the current flow stops, new li!uid dielectric is usually coneyed into the electrode +one enabling the solid particles (debris) to be carried away.
dding new li!uid dielectric in the electrode olume is commonly referred to as flushing.
lso, 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 li!uid dielectric breakdown can occur. occur.
History
In 1--, /nglish 0hysicist oseph 0riestley studied the erosie effect of electrical discharges.
urthering 0riestley3s research, the /4M process was inented by two 5ussian scientists, 4r. 6.5. 7a+arenko and 4r. 8.I. 7a+arenko in 19.
In their efforts to e:ploit the destructie effects of an electrical discharge, they deeloped a controlled process for machining of metals.
*heir 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.
*he discharge generator effect used by this machine, known as the 7a+arenko ;ircuit, was used for many years in the construction of generators for electrical discharge.
History
8ew researchers entered the field and contributed many fundamental characteristics of the machining method we know today.
In 19<2, the manufacturer ;harmilles created the first machine using the spark machining process and was presented for the first time at the /uropean Machine *ool /:hibition in 19<<.
In 19=9, gie launched the world3s first numerically controlled wire&cut /4M machine.
Seibu deeloped the first ;8; wire /4M machine in 19-2 and the first system was manufactured in apan.
5ecently, the machining speed has gone up by 2 times.
*his has decreased machining costs by at least percent and improed the surface finish by a factor of 1.< <
General Aspects of EDM
/4M is a machining method primarily used for hard metals or those that would be ery difficult to machine with traditional techni!ues.
/4M typically works with materials that are electrically conductie, although methods for machining insulating ceramics with /4M hae been proposed.
/4M can cut intricate contours or caities in hardened steel without the need for heat treatment to soften and re&harden them.
*his method can be used with any other metal or metal alloy such as titanium, hastelloy, koar, and inconel.
lso, applications of this process to shape polycrystalline diamond tools hae been reported.
=
EDM - System
-
EDM - Components
>
EDM - Components
*he main components in /4M?
/lectric power supply
4ielectric medium
@ork piece A tool
Sero control unit.
*he work piece and tool are electrically connected to a 4; power supply.
*he current density in the discharge of the channel is of the order of 1 Bcm2 and power density is nearly < M@Bcm 2.
gap, known as S05C D0 in the range, from .< mm to .< mm is maintained between the work piece and the tool.
4ielectric slurry is forced through this gap at a pressure of 2 kgfBcm 2 or lesser.
9
EDM – Working rinciple
It is a process of metal remoal based on the principle of material remoal by an interrupted electric spark discharge between the electrode tool and the work piece.
In /4M, a potential difference is applied between the tool and workpiece.
/ssential & 6oth tool and work material are to be conductors.
*he tool and work material are immersed in a dielectric medium.
Denerally kerosene or deionised water is used as the dielectric medium.
gap is maintained between the tool and the workpiece.
4epending upon the applied potential difference (< to < E) and the gap between the tool and workpiece, an electric field would be established.
Denerally the tool is connected to the negatie terminal (cathode) of the generator and the workpiece is connected to positie terminal (anode). 1
EDM – Working rinciple
s the electric field is established between the tool and the "ob, the free electrons on the tool are sub"ected to electrostatic forces.
If the bonding energy of the electrons is less, electrons would be emitted from the tool.
Such emission of electrons are called or termed as % cold emission’ .
*he Fcold emittedG electrons are then accelerated towards the "ob through the dielectric medium.
s they gain elocity and energy, and start moing towards the "ob, there would be collisions between the electrons and dielectric molecules.
Such collision may result in ioni+ation of the dielectric molecule.
Ioni+ation depends on the ioni+ation energy of the dielectric molecule and the energy of the electron. 11
EDM – Working rinciple
s the electrons get accelerated, more positie ions and electrons would get generated due to collisions.
*his cyclic process would increase the concentration of electrons and ions in the dielectric medium between the tool and the "ob at the spark gap.
*he concentration would be so high that the matter e:isting in that channel could be characterised as FplasmaG.
*he electrical resistance of such plasma channel would be ery less.
*hus all of a sudden, a large number of electrons will flow from tool to "ob and ions from "ob to tool.
*his is called aalanche motion of electrons.
Such moement of electrons and ions can be isually seen as a spark .
*hus the electrical energy is dissipated as the thermal energy of the spark. 12
EDM – Working rinciple
*he high speed electrons then impinge on the "ob and ions on the tool.
*he kinetic energy of the electrons and ions on impact with the surface of the "ob and tool respectiely would be conerted into thermal energy or heat flu:.
Such intense locali+ed heat flu: leads to e:treme instantaneous confined rise in temperature which would be in e:cess of 1,o;.
Such locali+ed e:treme rise in temperature leads to material remoal.
Material remoal occurs due to instant apori+ation of the material as well as due to melting.
*he molten metal is not remoed completely but only partially.
1
EDM – Working rinciple
Hpon withdrawal of potential difference, plasma channel collapses.
*his ultimately creates compression shock waes on both the electrode surface.
0articularly at high spots on work piece surface, which are closest to the tool.
*his eacuates molten material and forms a crater around the site of the spark.
*he whole se!uence of operation occurs within a few microseconds. 1
EDM – Sc!ematic
1<
EDM – Working rinciple
*hus to summarise, the material remoal in /4M mainly occurs due to formation of shock waes as the plasma channel collapse owing to discontinuation of applied potential difference
Denerally the workpiece is made positie and the tool negatie.
ence, the electrons strike the "ob leading to crater formation due to high temperature and melting and material remoal.
Similarly, the positie ions impinge on the tool leading to tool wear .
In /4M, the generator is used to apply oltage pulses between the tool and "ob.
constant oltage is not applied. #nly sparking is desired rather than arcing.
rcing leads to locali+ed material remoal at a particular point whereas sparks get distributed all oer the tool surface leading to uniform material remoal. 1=
EDM – Working rinciple
1-
EDM – o"er # Control Circuits
*wo broad categories of generators (power supplies) are in use on /4M.
;ommercially aailable? 5; circuits based and transistor controlled pulses.
In the first category, the main parameters to choose from at setup time are the resistance(s) of the resistor(s) and the capacitance(s) of the capacitor(s).
In an ideal condition, these !uantities would affect the ma:imum current deliered in a discharge.
;urrent deliery in a discharge is associated with the charge accumulated on the capacitors at a certain moment.
7ittle control is e:pected oer the time of discharge, which is likely to depend on the actual spark&gap conditions.
dantage? 5; circuit generator can allow the use of short discharge time more easily than the pulse&controlled generator. 1>
EDM – o"er # Control Circuits
lso, the open circuit oltage (i.e. oltage between electrodes when dielectric is not broken) can be identified as steady state oltage of the 5; circuit.
In generators based on transistor control, the user is usually able to delier a train of oltage pulses to the electrodes.
/ach pulse can be controlled in shape, for instance, !uasi&rectangular.
In particular, the time between two consecutie pulses and the duration of each pulse can be set.
*he amplitude of each pulse constitutes the open circuit oltage.
*hus, ma:imum duration of discharge is e!ual to duration of a oltage pulse.
Ma:imum current during a discharge that the generator deliers can also be controlled.
19
EDM – o"er # Control Circuits
4etails of generators and control systems on /4Ms are not always easily aailable to their user.
*his is a barrier to describing the technological parameters of /4M process.
Moreoer, the parameters affecting the phenomena occurring between tool and electrode are also related to the motion controller of the electrodes.
framework to define and measure the electrical parameters during an /4M operation directly on inter&electrode olume with an oscilloscope e:ternal to the machine has been recently proposed by erri
et al.
*his would enable the user to estimate directly the electrical parameter that affect their operations without relying upon machine manufacturer3s claims.
@hen machining different materials in the same setup conditions, the actual electrical parameters are significantly different.
2
EDM – o"er # Control Circuits
@hen using 5; generators, the oltage pulses, shown in ig. are responsible for material remoal.
series of oltage pulses (ig.) of magnitude about 2 to 12 E and fre!uency on the order of < k+ is applied between the two electrodes.
21
EDM – o"er # Control Circuits
22
EDM – o"er # Control Circuits
2
EDM – o"er # Control Circuits
2
EDM – Electrode Material
/lectrode material should be such that it would not undergo much tool wear when it is impinged by positie ions.
*hus the localised temperature rise has to be less by properly choosing its properties or een when temperature increases, there would be less melting.
urther, the tool should be easily workable as intricate shaped geometric features are machined in /4M.
*hus the basic characteristics of electrode materials are?
igh electrical conductiity $ electrons are cold emitted more easily and there is less bulk electrical heating
igh thermal conductiity $ for the same heat load, the local temperature rise would be less due to faster heat conducted to the bulk of the tool and thus less tool wear.
2<
EDM – Electrode Material
igher density $ for less tool wear and thus less dimensional loss or inaccuracy of tool
igh melting point $ high melting point leads to less tool wear due to less tool material melting for the same heat load
/asy manufacturability
;ost $ cheap
*he followings are the different electrode materials which are used commonly in the industry?
Draphite
/lectrolytic o:ygen free copper
*ellurium copper $ 99J ;u K .
6rass
2=
EDM – Electrode Material
Draphite (most common) & has fair wear characteristics, easily machinable.
Small flush holes can be drilled into graphite electrodes.
;opper has good /4M wear and better conductiity.
It is generally used for better finishes in the range of 5 a L .< m.
;opper tungsten and siler tungsten are used for making deep slots under poor flushing conditions especially in tungsten carbides.
It offers high machining rates as well as low electrode wear.
;opper graphite is good for cross§ional electrodes.
It has better electrical conductiity than graphite while the corner wear is higher.
6rass ensures stable sparking conditions and is normally used for speciali+ed applications such as drilling of small holes where the high electrode wear is acceptable. 2-
EDM – Electrode Mo$ement
In addition to the sero&controlled feed, the tool electrode may hae an additional rotary or orbiting motion.
/lectrode rotation helps to sole the flushing difficulty encountered when machining small holes with /4M.
In addition to the increase in cutting speed, the !uality of the hole produced is superior to that obtained using a stationary electrode.
/lectrode orbiting produces caities haing the shape of the electrode.
*he si+e of the electrode and the radius of the orbit (2.< mm ma:imum) determine the si+e of the caities.
/lectrode orbiting improes flushing by creating a pumping effect of the dielectric li!uid through the gap.
2>
EDM – Electrode Wear
29
EDM – Electrode Wear
*he melting point is the most important factor in determining the tool wear.
/lectrode wear ratios are e:pressed as end wear, side wear, corner wear, and olume wear.
F8o wear /4MG - when the electrode&to&workpiece wear ratio is 1 J or less.
/lectrode wear depends on a number of factors associated with the /4M, like oltage, current, electrode material, and polarity.
*he change in shape of the tool electrode due to the electrode wear causes defects in the workpiece shape.
/lectrode wear has een more pronounced effects when it comes to micromachining applications.
*he corner wear ratio depends on the type of electrode.
*he low melting point of aluminum is associated with the highest wear ratio.
EDM – Electrode Wear
1
EDM – Electrode Wear
Draphite has shown a low tendency to wear and has the possibility of being molded or machined into complicated electrode shapes. *he wear rate of the electrode tool material (@ t) and the wear ratio (5 w) are gien by Calpak"ian (199-).
2
EDM – Dielectric
In /4M, material remoal mainly occurs due to thermal eaporation and melting.
s thermal processing is re!uired to be carried out in absence of o:ygen so that the process can be controlled and o:idation aoided.
#:idation often leads to poor surface conductiity (electrical) of the workpiece hindering further machining.
ence, dielectric fluid should proide an o:ygen free machining enironment.
urther it should hae enough strong dielectric resistance so that it does not breakdown electrically too easily.
6ut at the same time, it should ioni+e when electrons collide with its molecule.
Moreoer, during sparking it should be thermally resistant as well.
Denerally kerosene and deionised water is used as dielectric fluid in /4M.
EDM – Dielectric
*ap water cannot be used as it ionises too early and thus breakdown due to presence of salts as impurities occur.
4ielectric medium is generally flushed around the spark +one.
It is also applied through the tool to achiee efficient remoal of molten material.
*hree important functions of a dielectric medium in /4M? 1.
Insulates the gap between the tool and work , thus preenting a spark to form until the gap oltage are correct.
2.
;ools the electrode, workpiece and solidifies the molten metal particles.
.
lushes the metal particles out of the working gap to maintain ideal cutting conditions, increase metal remoal rate.
It must be filtered and circulated at constant pressure.
EDM – Dielectric
*he main re!uirements of the /4M dielectric fluids are ade!uate iscosity, high flash point, good o:idation stability, minimum odor, low cost, and good electrical discharge efficiency.
or most /4M operations kerosene is used with certain addities that preent gas bubbles and de&odoring.
Silicon fluids and a mi:ture of these fluids with petroleum oils hae gien e:cellent results.
#ther dielectric fluids with a arying degree of success include a!ueous solutions of ethylene glycol, water in emulsions, and distilled water.
<
EDM – %lus!ing
#ne of the important factors in a successful /4M operation is the remoal of debris (chips) from the working gap.
lushing these particles out of the working gap is ery important, to preent them from forming bridges that cause short circuits.
/4Ms hae a built&in power adaptie control system that increases the pulse spacing as soon as this happens and reduces or shuts off the power supply.
lushing $ process of introducing clean filtered dielectric fluid into spark gap.
If flushing is applied incorrectly, it can result in erratic cutting and poor machining conditions.
lushing of dielectric plays a ma"or role in the maintenance of stable machining and the achieement of close tolerance and high surface !uality.
Inade!uate flushing can result in arcing, decreased electrode life, and increased production time. =
EDM – %lus!ing
Four methods:
1. 8ormal flow
2. 5eerse flow
. et flushing
. Immersion flushing
-
EDM – %lus!ing
8ormal flow (Ma"ority)
4ielectric is introduced, under pressure, through one or more passages in the tool and is forced to flow through the gap between tool and work.
lushing holes are generally placed in areas where the cuts are deepest. 8ormal flow is sometimes undesirable because it produces a tapered opening in the workpiece.
5eerse flow
0articularly useful in machining deep caity dies, where the taper produced using the normal flow mode can be reduced.
*he gap is submerged in filtered dielectric, and instead of pressure being applied at the source a acuum is used.
@ith clean fluid flowing between the workpiece and the tool, there is no side sparking and, therefore, no taper is produced. >
EDM – %lus!ing
et flushing
In many instances, the desired machining can be achieed by using a spray or "et of fluid directed against the machining gap.
Machining time is always longer with "et flushing than with the normal and reerse flow modes.
Immersion flushing
or many shallow cuts or perforations of thin sections, simple immersion of the discharge gap is sufficient.
;ooling and debris remoal can be enhanced during immersion cutting by proiding relatie motion between the tool and workpiece.
Eibration or cycle interruption comprises periodic reciprocation of the tool relatie to the workpiece to effect a pumping action of the dielectric.
9
EDM – %lus!ing
Synchroni+ed, pulsed flushing is also aailable on some machines.
@ith this method, flushing occurs only during the non&machining time as the electrode is retracted slightly to enlarge the gap.
Increased electrode life has been reported with this system.
Innoatie techni!ues such as ultrasonic ibrations coupled with mechanical pulse /4M, "et flushing with sweeping no++les, and electrode pulsing are inestigated by Masu+awa (199).
EDM – %lus!ing
or proper flushing conditions, Metals andbook (19>9) recommends? 1.
lushing through the tool is more preferred than side flushing.
2.
Many small flushing holes are better than a few large ones.
.
Steady dielectric flow on the entire workpiece&electrode interface is desirable.
.
4ead spots created by pressure flushing, from opposite sides of the workpiece, should be aoided.
<.
ent hole should be proided for any upwardly concae part of the tool&electrode to preent accumulation of e:plosie gases.
=.
flush bo: is useful if there is a hole in the caity.
1
EDM – rocess arameters *he waeform is characteri+ed by the?
*he open circuit oltage $ E o
*he working oltage $ E w
*he ma:imum current $ I o
*he pulse on time $ the duration for which the oltage pulse is applied & t on
*he pulse off time $ t off
*he gap between the workpiece and the tool $ spark gap & N
*he polarity $ straight polarity $ tool (&e)
*he dielectric medium
/:ternal flushing through the spark gap.
2
EDM – rocess arameters
*he process parameters & mainly related to the waeform characteristics.
EDM – &ypes – Sinker EDM
Sinker /4M, also called caity type /4M or olume /4M.
;onsists of an electrode and workpiece submerged in an insulating li!uid such as oil or other dielectric fluids.
*he electrode and workpiece are connected to a suitable power supply.
*he power supply generates an electrical potential between the two parts.
s the electrode approaches the workpiece, dielectric breakdown occurs in the fluid, forming a plasma channel, and a small spark "umps.
*hese sparks happen in huge numbers at seemingly random locations.
s the base metal is eroded, and the spark gap subse!uently increased, the electrode is lowered automatically so that the process can continue.
Seeral hundred thousand sparks occur per second, with the actual duty cycle carefully controlled by the setup parameters.
*hese controlling cycles are sometimes known as Oon timeO and Ooff timeF.
EDM – &ypes – Sinker EDM
*he on time setting determines the length or duration of the spark .
ence, a longer on time produces a deeper caity for that spark and all subse!uent sparks for that cycle.
*his creates rougher finish on the workpiece.
*he reerse is true for a shorter on time.
#ff time is the period of time that one spark is replaced by another .
longer off time, for e:ample, allows the flushing of dielectric fluid through a no++le to clean out the eroded debris, thereby aoiding a short circuit.
*hese settings can be maintained in micro seconds.
*he typical part geometry is a comple: 4 shape, often with small or odd shaped angles.
<
EDM – &ypes – Wire EDM 'WEDM(
lso known as wire&cut /4M and wire cutting.
thin single&strand metal wire (usually brass) is fed through the workpiece submerged in a tank of dielectric fluid (typically deioni+ed water).
Hsed to cut plates as thick as mm and to make punches, tools, and dies from hard metals that are difficult to machine with other methods.
Hses water as its dielectric fluidP its resistiity and other electrical properties are controlled with filters and de&ioni+er units.
*he water flushes the cut debris away from the cutting +one.
lushing is an important factor in determining the ma:imum feed rate for a gien material thickness.
;ommonly used when low residual stresses are desired, because it does not re!uire high cutting forces for material remoal.
=
EDM – Material )emo$al )ate
-
EDM – Material )emo$al )ate
In /4M, the metal is remoed from both workpiece and tool electrode.
M55 depends not only on the workpiece material but on the material of the tool electrode and the machining ariables such as pulse conditions, electrode polarity, and the machining medium.
In this regard a material of low melting point has a high metal remoal rate and hence a rougher surface.
*ypical remoal rates range from .1 to mm Bmin.
M55 or olumetric remoal rate (E55), in mmBmin, was described by Calpak"ian (199-)?
where
I
&
/4M current ()
*w
-
Melting point of the workpiece (Q;).
>
EDM – Material )emo$al )ate
/ffect of pulse current (energy) on M55 A surface roughness. 9
EDM – Material )emo$al )ate
/ffect of pulse on&time (energy) on M55 A surface roughness. <
EDM – Surface Integrity
Surface consists of a multitude of oerlapping craters that are formed by the action of microsecond&duration spark discharges.
;rater si+e depends on
physical and mechanical properties of the material
composition of the machining medium
discharge energy and duration.
Integral effect of thousands of discharges per second leads to machining with a specified accuracy and surface finish.
4epth of craters & the peak to alley (ma:imum) of surface roughness 5 t.
Ma:imum depth of damaged layer can be taken as 2.< times of roughness 5 a.
ccording to 4elpreti (19--) and Motoki and 7ee (19=>), the ma:imum peak to alley height, 5 t, was considered to be 1 times 5 a. <1
EDM – Surface Integrity
erage roughness can be e:pressed in terms of pulse current i p () and pulse duration t p (s) by
Surface roughness increases linearly with an increase in M55 .
eswani (19->) & Draphite electrodes produce rougher surfaces than metal ones.
Cuneida and uruoya (1991) claimed that the introduction of o:ygen into discharge gap proides e:tra power by the reaction of o:ygen.
*his in turn increased workpiece melting and created greater e:pulsie forces that increased M55 and surface roughness.
;hoice of correct dielectric flow has a significant effect in reducing surface roughness by < J, increasing the machining rate, and lowering the thermal effects in the workpiece surface.
4ielectrics haing low iscosity are recommended for smooth surfaces
<2
EDM – Surface Integrity
Metallurgical changes occur in the surface $ *emperature > to 12,Q;.
dditionally, a thin recast layer of 1 m to 2< m $ depending on power used.
4elpretti (19--) and 7ey and Maggi (199) claimed that the heat&affected +one (R) ad"acent to the resolidified layer reaches 2< m.
Some annealing can be e:pected in a +one "ust below the machined surface. 8ot all the workpiece melted by discharge is e:pelled into the dielectric. 5emaining melted material is !uickly chilled, primarily by heat conduction into the bulk of the workpiece, resulting in an e:ceedingly hard surface.
4epth of annealed layer is proportional to power used.
It ranges from < m for finish cutting to 2 m for high M55 .
nnealing is usually about two points of hardness below the parent metal for finish cutting.
<
EDM – Surface Integrity
In roughing cuts, the annealing effect is fie points of hardness below the parent metal. metal.
/lectrodes that produce more stable machining can reduce the annealing effect effect..
finish cut remoes the annealed material left material left by the preious rough cut.
*he altered surface layer significantly significantly lowers the fatigue strength of alloys. alloys .
It consists of a recast layer with with or without microcracks, some of which may e:tend into the base metal, plus metallurg metallurgical ical alterations alterations such such as rehardened and tempered layers, heat&affec heat&affected ted +ones +ones,, and inter&granular precipitates. precipitates.
4uring /4M roughing, roughing, the layer showing microstructural changes, including a melted and resolidified layer, is less than .12- mm deep. deep.
4uring /4M finishing, finishing, it is less than .-< mm. mm.
0ost&treatment to 0ost&treatment to restore the fatigue strength is recommended to follow /4M of critical or highly stressed surfaces. <
EDM – Surface Integrity
*here are seeral effectie processes that processes that accomplish restoration or een enhancement of the fatigue properties.
*hese methods include
5emoal of the altered layers by layers by low&stress grinding or chemical machining
ddition of a metallurgical&type coating
5e heat&treatme h eat&treatment nt
pplication of shot peening.
<<
EDM – C!aracteristics
;an be used to machine any work material material if if it is electrically conductie.
M55 depends on depends on thermal properties ("ob) properties ("ob) rather than its strength, hardness etc.
*he olume of the material remoed per spark discharge is typically in the range of (1B1,,) (1B1,,) to to (1B1,) mm.
In /4M, geometry of tool & positie & positie impression impression of hole or geometric geometric feature.
*ool *o ol wear w ear once once again depends depends on on the thermal properties of tool material. tool material.
7ocal temperature rise is rather high, but there is not enough heat diffusion (ery small pulse on time) time ) and thus R is limited to 2 $ m.
5apid heating and cooling leads to surface hardening which hardening which may be desirable in some applications. applications.
*olerance alue *olerance alue of K .< mm could mm could be easily achieed by /4M.
6est surface finish that finish that can be economically achieed on steel is . µm. <=
Applications
4rilling of micro&holes, thread cutting, helical profile milling, rotary forming, and cured hole drilling.
4elicate work piece like copper parts can be produced by /4M.
;an be applied to all electrically conducting metals and alloys irrespectie of their melting points, hardness, toughness, or brittleness.
#ther applications? deep, small&dia holes using tungsten wire as tool, narrow slots, cooling holes in super alloy turbine blades, and arious intricate shapes.
/4M can be economically employed for e:tremely hardened work piece.
Since there is no mechanical stress present (no physical contact), fragile and slender work places can be machined without distortion.
ard and corrosion resistant surfaces, essentially needed for die making, can be deeloped.
<-
Applications – EDM Drilling
Hses a tubular tool electrode where the dielectric is flushed.
@hen solid rods are usedP dielectric is fed to the machining +one by either suction or in"ection through pre&drilled holes.
Irregular, tapered, cured, as well as inclined holes can be produced by /4M.
;reating cooling channels in turbine blades made of hard alloys is a typical application of /4M drilling.
Hse of 8; system enabled large numbers of holes to be accurately located.
<>
Applications – EDM Sa"ing
n /4M ariation & /mploys either a special steel band or disc.
;uts at a rate that is twice that of the conentional abrasie sawing method.
;utting of billets and bars & has a smaller kerf A free from burrs.
ine finish of =. to 1 m with a recast layer of .2< to .1 mm
<9
Applications - Machining of spheres
Shichun and coworkers (199<) used simple tubular electrodes in /4M machining of spheres, to a dimensional accuracy of T1 m and 5a U .1 m.
5otary /4M is used for machining of spherical shapes in conducting ceramics using the tool and workpiece arrangement as shown below.
=
Applications - Machining of dies & molds
/4M milling uses standard cylindrical electrodes.
Simple&shaped electrode (ig. 1) is rotated at high speeds and follows specified paths in the workpiece like the conentional end mills.
Eery useful and makes /4M ery ersatile like mechanical milling process.
Soles the problem of manufacturing accurate and comple:&shaped electrodes for die sinking (ig. 2) of three&dimensional caities.
(ig. 1)
(ig. 2)
=1
Applications - Machining of dies & molds
/4M milling enhances dielectric flushing due to high&speed electrode rotation.
/lectrode wear can be optimi+ed due to its rotational and contouring motions.
Main limitation in /4M milling & ;omple: shapes with sharp corners cannot be machined because of the rotating tool electrode.
/4M milling replaces conentional die making that re!uires ariety of machines such as milling, wire cutting, and /4M die sinking machines.
=2
Applications – Wire EDM
Special form of /4M & uses a continuously moing conductie wire electrode.
Material remoal occurs as a result of spark erosion as the wire electrode is fed, from a fresh wire spool, through the workpiece.
ori+ontal moement of the worktable (;8;) determines the path of the cut.
pplication & Machining of superhard materials like polycrystalline diamond (0;4) and cubic boron nitride (;68) blanks, and other composites.
;arbon fiber composites are widely used in aerospace, nuclear, automobile, and chemical industries, but their conentional machining is difficult.
Co+ak et al. (199<) used wire /4M for accurately shaping these materials, without distortion or burrs.
5ecently used for machining insulating ceramics by *ani et al. (2).
=
Applications – Wire EDM
=
Applications – EDM of Insulators
sheet metal mesh is placed oer the ceramic material.
Spark discharges between the negatie tool electrode and the metal mesh.
*hese sparks are transmitted through the metal mesh to its interface with the ceramic surface, which is then eroded.
=<
Applications – e!turing
*e:turing is applied to steel sheets during the final stages of cold rolling.
Shot blasting (S6) is an ine:pensie method of te:turing.
7imitations of S6 include its lack of control and consistency of te:turing, and the need for protection of other parts of the e!uipment holding the roll.
/4*, is a ariation of /4M and proed to be the most popular .
*e:turing is achieed by producing electrical sparks across the gap between roll (workpiece) and a tool electrode, in the presence of dielectric (paraffin).
/ach spark creates a small crater by the discharge of its energy in a local melting and apori+ation of the roll material.
6y selecting the appropriate process ariables such as pulse current, on and off time, electrode polarity, dielectric type, and the roll rotational speed, a surface te:ture with a high degree of accuracy and consistency can be produced . ==
Ad$antages Some of the adantages of /4M include machining of?
;omple: shapes that would otherwise be difficult to produce with conentional cutting tools.
/:tremely hard material to ery close tolerances.
Eery small work pieces where conentional cutting tools may damage the part from e:cess cutting tool pressure.
*here is no direct contact between tool and work piece. *herefore delicate sections and weak materials can be machined without any distortion.
good surface finish can be obtained.
=-
