Ntm kiran

Electrical parameters in R-C circuits
 supply voltage (Us) & breakdown voltage (Ub).
Charging resistance (R).
Capacitance(C).
Gap setting.

supply voltage (Us) & breakdown
voltage (Ub).
• Keeping all other factors constant, an increase
in brake down voltage will result in increased
energy per spark, thus MMR will increases.
• The D.C supply voltage used in EDM machine
ranges fro 30-200 V.

Charging resistance (R).
with constant gap setting, the cutting power
available varies inversely as R .
MMR
RESIATNCE
Critical Resistance

Capacitance(C).
An increase in capacitance also
increase the energy per spark &
at the same time reduces the
spark frquency for a given
setting.
the value of capacitance C range
between 10- 100 microfarads.

Gap setting.
In practice , it becomes increasing difficult to endure
optimum conditions of gap setting as power is increases
because
1) The dielectric gets contaminated with metal particles and
breakdown will occur at lower voltage.
2) With increasing power, R is reduced. This help in dielectric break
down at lower voltages.

9
EDM – Electrode Material
 Electrode material should be such that it would not undergo much tool wear
when it is impinged by positive ions.
 Thus the localised temperature rise has to be less by properly choosing its
properties or even when temperature increases, there would be less melting.
 Further, the tool should be easily workable as intricate shaped geometric
features are machined in EDM.
 Thus the basic characteristics of electrode materials are:
 High electrical conductivity – electrons are cold emitted more easily and
there is less bulk electrical heating
 High thermal conductivity – 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.

10
 Higher density – for less tool wear and thus less dimensional loss or
inaccuracy of tool
 High melting point – high melting point leads to less tool wear due to less
tool material melting for the same heat load
 Easy manufacturability
 Cost – cheap
 The followings are the different electrode materials which are used commonly
in the industry:
 Graphite
 Electrolytic oxygen free copper
 Tellurium copper – 99% Cu + 0.5% tellurium
 Brass

11
 Graphite (most common) - has fair wear characteristics, easily machinable.
 Small flush holes can be drilled into graphite electrodes.
 Copper has good EDM wear and better conductivity.
 It is generally used for better finishes in the range of Ra = 0.5 μm.
 Copper tungsten and silver 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.
 Copper graphite is good for cross-sectional electrodes.
 It has better electrical conductivity than graphite while the corner wear is
higher.
 Brass ensures stable sparking conditions and is normally used for specialized
applications such as drilling of small holes where the high electrode wear is
acceptable.

12
EDM – Electrode Movement
 In addition to the servo-controlled feed, the tool electrode may have an
additional rotary or orbiting motion.
 Electrode rotation helps to solve the flushing difficulty encountered when
machining small holes with EDM.
 In addition to the increase in cutting speed, the quality of the hole produced
is superior to that obtained using a stationary electrode.
 Electrode orbiting produces cavities having the shape of the electrode.
 The size of the electrode and the radius of the orbit (2.54 mm maximum)
determine the size of the cavities.
 Electrode orbiting improves flushing by creating a pumping effect of the
dielectric liquid through the gap.

14
EDM – Electrode Wear
 The melting point is the most important factor in determining the tool wear.
 Electrode wear ratios are expressed as end wear, side wear, corner wear, and
volume wear.
 “No wear EDM” - when the electrode-to-workpiece wear ratio is 1 % or less.
 Electrode wear depends on a number of factors associated with the EDM, like
voltage, current, electrode material, and polarity.
 The change in shape of the tool electrode due to the electrode wear causes
defects in the workpiece shape.
 Electrode wear has even more pronounced effects when it comes to
micromachining applications.
 The corner wear ratio depends on the type of electrode.
 The low melting point of aluminum is associated with the highest wear ratio.

16
EDM – Electrode Wear
 Graphite has shown a low tendency to wear and has the possibility of being
molded or machined into complicated electrode shapes.
 The wear rate of the electrode tool material (Wt) and the wear ratio (Rw) are
given by Kalpakjian (1997).

17
EDM – Dielectric
 In EDM, 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 work
piece 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, it should ionize when electrons collide with its molecule.
 Moreover, during sparking it should be thermally resistant as well.

 Tap water cannot be used as it ionizes 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.
 For most EDM operations kerosene is used with certain additives that prevent
gas bubbles and de-odoring.
 Silicon fluids and a mixture of these fluids with petroleum oils have given
excellent results.
 Other dielectric fluids with a varying degree of success include aqueous solutions
of ethylene glycol, water in emulsions, and distilled water.

19
EDM – Dielectric
 Three important functions of a dielectric medium in EDM:
1. Insulates the gap between the tool and work, thus preventing a
spark to form until the gap voltage are correct.
2. Cools the electrode, work piece and solidifies the molten metal
particles.
3. Flushes the metal particles out of the working gap to maintain
ideal cutting conditions, increase metal removal rate.
 It must be filtered and circulated at constant pressure.

Essential Requirements of EDM
 The dielectric fluid should have sufficient and stable dielectric
strength to serve insulation between the electrode and the tool.
 It should deionize rapidly after the spark discharge has taken place.
 It should have low viscosity and a good wetting capacity.
 The flash point must be high to avoid any fire hazards.
 It should not emit any toxic vapours or have unpleasant odours
 It should be easily available in the market at a reasonable price.

21
EDM – Flushing
 One of the important factors in a successful EDM operation is the removal of
debris (chips) from the working gap.
 Flushing – 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.
 Flushing of dielectric plays a major role in the maintenance of stable machining
and the achievement of close tolerance and high surface quality.
 Inadequate flushing can result in arcing, decreased electrode life, and increased
production time.

EDM – Flushing
 Four methods:
1. Normal flow 2. Reverse flow
3. Jet flushing 4. Immersion flushing

EDM – Flushing
 Normal flow (Majority)
 Dielectric is introduced, under pressure, through one or more passages
in the tool and is forced to flow through the gap between tool and work.
 Flushing holes are generally placed in areas where the cuts are deepest.
 Normal flow is sometimes undesirable because it produces a tapered
opening in the work piece.
 Reverse flow
 Particularly useful in machining deep cavity dies, where the taper
produced using the normal flow mode can be reduced.
 The gap is submerged in filtered dielectric, and instead of pressure being
applied at the source a vacuum is used.
 With clean fluid flowing between the work piece and the tool, there is
no side sparking and, therefore, no taper is produced.

24
EDM – Flushing
 Jet flushing
 In many instances, the desired machining can be achieved by using a
spray or jet of fluid directed against the machining gap.
 Machining time is always longer with jet flushing than with the normal
and reverse flow modes.
 Immersion flushing
 For many shallow cuts or perforations of thin sections, simple immersion
of the discharge gap is sufficient.
 Cooling and debris removal can be enhanced during immersion cutting
by providing relative motion between the tool and workpiece.
 Vibration or cycle interruption comprises periodic reciprocation of the
tool relative to the workpiece to effect a pumping action of the
dielectric.

25
EDM – Flushing
 Synchronized, pulsed flushing is also available on some machines.
 With 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.
 Innovative techniques such as ultrasonic vibrations coupled with
mechanical pulse EDM, jet flushing with sweeping nozzles, and electrode
pulsing are investigated by Masuzawa (1990).

26
EDM – Flushing
 For proper flushing conditions, Metals Handbook (1989) recommends:
1. Flushing through the tool is more preferred than side flushing.
2. Many small flushing holes are better than a few large ones.
3. Steady dielectric flow on the entire workpiece-electrode interface is
desirable.
4. Dead spots created by pressure flushing, from opposite sides of the
workpiece, should be avoided.
5. A vent hole should be provided for any upwardly concave part of the
tool-electrode to prevent accumulation of explosive gases.
6. A flush box is useful if there is a hole in the cavity.

27
EDM – Process Parameters
The waveform is characterized 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 workpiece and the tool – spark gap - δ
 The polarity – straight polarity – tool (-ve)
 The dielectric medium
 External flushing through the spark gap.

28
 The process parameters - mainly related to the waveform characteristics.
EDM – Process Parameters

29
EDM – Types – Wire EDM (WEDM)
 Also known as wire-cut EDM and wire cutting.
 A thin single-strand metal wire (usually brass) is fed through the workpiece
submerged in a tank of dielectric fluid (typically deionized water).
 Used to cut plates as thick as 300 mm and to make punches, tools, and dies
from hard metals that are difficult to machine with other methods.
 Uses water as its dielectric fluid; its resistivity and other electrical properties
are controlled with filters and de-ionizer units.
 The water flushes the cut debris away from the cutting zone.
 Flushing is an important factor in determining the maximum feed rate for a
given material thickness.
 Commonly used when low residual stresses are desired, because it does not
require high cutting forces for material removal.

30
EDM – Material Removal Rate

31
 In EDM, the metal is removed from both workpiece and tool electrode.
 MRR depends not only on the workpiece material but on the material of the
tool electrode and the machining variables such as pulse conditions,
electrode polarity, and the machining medium.
 In this regard a material of low melting point has a high metal removal rate
and hence a rougher surface.
 Typical removal rates range from 0.1 to 400 mm3 /min.
 MRR or volumetric removal rate (VRR), in mm3/min, was described by
Kalpakjian (1997):
where I - EDM current (A)
Tw - Melting point of the workpiece (°C).

32
Effect of pulse current (energy) on MRR & surface roughness.

33
Effect of pulse on-time (energy) on MRR & surface roughness.

34
EDM – Surface Integrity
 Surface consists of a multitude of overlapping craters that are formed by the
action of microsecond-duration spark discharges.
 Crater size 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.
 Depth of craters - the peak to valley (maximum) of surface roughness Rt.
 Maximum depth of damaged layer can be taken as 2.5 times of roughness Ra.
 According to Delpreti (1977) and Motoki and Lee (1968), the maximum peak
to valley height, Rt, was considered to be 10 times Ra.

35
 Average roughness can be expressed in terms of pulse current ip (A) and pulse
duration tp (μs) by
 Surface roughness increases linearly with an increase in MRR.
 Jeswani (1978) - Graphite electrodes produce rougher surfaces than metal
ones.
 Kuneida and Furuoya (1991) claimed that the introduction of oxygen into
discharge gap provides extra power by the reaction of oxygen.
 This in turn increased workpiece melting and created greater expulsive forces
that increased MRR and surface roughness.
 Choice of correct dielectric flow has a significant effect in reducing surface
roughness by 50 %, increasing the machining rate, and lowering the thermal
effects in the workpiece surface.
 Dielectrics having low viscosity are recommended for smooth surfaces.

36
 Metallurgical changes occur in the surface – Temperature 8000 to 12,000°C.
 Additionally, a thin recast layer of 1 μm to 25 μm – depending on power used.
 Delpretti (1977) and Levy and Maggi (1990) claimed that the heat-affected
zone (HAZ) adjacent to the resolidified layer reaches 25 μm.
 Some annealing can be expected in a zone just below the machined surface.
 Not all the workpiece melted by discharge is expelled into the dielectric.
 Remaining melted material is quickly chilled, primarily by heat conduction into
the bulk of the workpiece, resulting in an exceedingly hard surface.
 Depth of annealed layer is proportional to power used.
 It ranges from 50 μm for finish cutting to ~ 200 μm for high MRR.
 Annealing is usually about two points of hardness below the parent metal for
finish cutting.

37
 In roughing cuts, the annealing effect is ~ five points of hardness below the
parent metal.
 Electrodes that produce more stable machining can reduce the annealing
effect.
 A finish cut removes the annealed material left by the previous rough cut.
 The altered surface layer significantly lowers the fatigue strength of alloys.
 It consists of a recast layer with or without microcracks, some of which may
extend into the base metal, plus metallurgical alterations such as rehardened
and tempered layers, heat-affected zones, and inter-granular precipitates.
 During EDM roughing, the layer showing microstructural changes, including a
melted and resolidified layer, is less than 0.127 mm deep.
 During EDM finishing, it is less than 0.075 mm.
 Post-treatment to restore the fatigue strength is recommended to follow EDM
of critical or highly stressed surfaces.

38
 There are several effective processes that accomplish restoration or even
enhancement of the fatigue properties.
 These methods include
 Removal of the altered layers by low-stress grinding or chemical
machining
 Addition of a metallurgical-type coating
 Re heat-treatment
 Application of shot peening.

39
EDM – Characteristics
 Can be used to machine any work material if it is electrically conductive.
 MRR depends on thermal properties (job) rather than its strength, hardness
etc.
 The volume of the material removed per spark discharge is typically in the
range of (1/1,000,000) to (1/10,000) mm3.
 In EDM, geometry of tool - positive impression of hole or geometric feature.
 Tool wear once again depends on the thermal properties of tool material.
 Local temperature rise is rather high, but there is not enough heat diffusion
(very small pulse on time) and thus HAZ is limited to 2 – 4 μm.
 Rapid heating and cooling leads to surface hardening which may be desirable
in some applications.
 Tolerance value of + 0.05 mm could be easily achieved by EDM.
 Best surface finish that can be economically achieved on steel is 0.40 m.

40
 Drilling of micro-holes, thread cutting, helical profile milling, rotary forming,
and curved hole drilling.
 Delicate work piece like copper parts can be produced by EDM.
 Can be applied to all electrically conducting metals and alloys irrespective of
their melting points, hardness, toughness, or brittleness.
 Other applications: deep, small-dia holes using tungsten wire as tool, narrow
slots, cooling holes in super alloy turbine blades, and various intricate shapes.
 EDM can be economically employed for extremely hardened work piece.
 Since there is no mechanical stress present (no physical contact), fragile and
slender work places can be machined without distortion.
 Hard and corrosion resistant surfaces, essentially needed for die making, can
be developed.
Applications

41
 Uses a tubular tool electrode where the dielectric is flushed.
 When solid rods are used; dielectric is fed to the machining zone by either
suction or injection through pre-drilled holes.
 Irregular, tapered, curved, as well as inclined holes can be produced by EDM.
 Creating cooling channels in turbine blades made of hard alloys is a typical
application of EDM drilling.
 Use of NC system enabled large numbers of holes to be accurately located.
Applications – EDM Drilling

42
 An EDM variation - Employs either a special steel band or disc.
 Cuts at a rate that is twice that of the conventional abrasive sawing method.
 Cutting of billets and bars - has a smaller kerf & free from burrs.
 Fine finish of 6.3 to 10 μm with a recast layer of 0.025 to 0.130 mm
Applications – EDM Sawing

43
 Shichun and coworkers (1995) used simple tubular electrodes in EDM
machining of spheres, to a dimensional accuracy of ±1 μm and Ra < 0.1 μm.
 Rotary EDM is used for machining of spherical shapes in conducting ceramics
using the tool and workpiece arrangement as shown below.
Applications - Machining of spheres

44
 EDM milling uses standard cylindrical electrodes.
 Simple-shaped electrode (Fig. 1) is rotated at high speeds and follows
specified paths in the workpiece like the conventional end mills.
 Very useful and makes EDM very versatile like mechanical milling process.
 Solves the problem of manufacturing accurate and complex-shaped
electrodes for die sinking (Fig. 2) of three-dimensional cavities.
Applications - Machining of dies & molds
(Fig. 2)(Fig. 1)

45
 EDM milling enhances dielectric flushing due to high-speed electrode
rotation.
 Electrode wear can be optimized due to its rotational and contouring motions.
 Main limitation in EDM milling - Complex shapes with sharp corners cannot be
machined because of the rotating tool electrode.
 EDM milling replaces conventional die making that requires variety of
machines such as milling, wire cutting, and EDM die sinking machines.
Applications - Machining of dies & molds

46
Applications – Wire EDM
 Special form of EDM - uses a continuously moving conductive wire electrode.
 Material removal occurs as a result of spark erosion as the wire electrode is
fed, from a fresh wire spool, through the workpiece.
 Horizontal movement of the worktable (CNC) determines the path of the cut.
 Application - Machining of superhard materials like polycrystalline diamond
(PCD) and cubic boron nitride (CBN) blanks, and other composites.
 Carbon fiber composites are widely used in aerospace, nuclear, automobile,
and chemical industries, but their conventional machining is difficult.
 Kozak et al. (1995) used wire EDM for accurately shaping these materials,
without distortion or burrs.
 Recently used for machining insulating ceramics by Tani et al. (2004).

49
Applications – EDM of Insulators
 A sheet metal mesh is placed over the ceramic material.
 Spark discharges between the negative tool electrode and the metal mesh.
 These sparks are transmitted through the metal mesh to its interface with the
ceramic surface, which is then eroded.

50
Applications – Texturing
 Texturing is applied to steel sheets during the final stages of cold rolling.
 Shot blasting (SB) is an inexpensive method of texturing.
 Limitations of SB include its lack of control and consistency of texturing, and
the need for protection of other parts of the equipment holding the roll.
 EDT, is a variation of EDM and proved to be the most popular.
 Texturing is achieved by producing electrical sparks across the gap between
roll (workpiece) and a tool electrode, in the presence of dielectric (paraffin).
 Each spark creates a small crater by the discharge of its energy in a local
melting and vaporization of the roll material.
 By selecting the appropriate process variables such as pulse current, on and
off time, electrode polarity, dielectric type, and the roll rotational speed, a
surface texture with a high degree of accuracy and consistency can be
produced.

51
Some of the advantages of EDM include machining of:
 Complex shapes that would otherwise be difficult to produce with
conventional cutting tools.
 Extremely hard material to very close tolerances.
 Very small work pieces where conventional cutting tools may damage the part
from excess cutting tool pressure.
 There is no direct contact between tool and work piece. Therefore delicate
sections and weak materials can be machined without any distortion.
 A good surface finish can be obtained.
Advantages

52
Some of the disadvantages of EDM include:
 The slow rate of material removal.
 For economic production, the surface finish specified should not be too fine.
 The additional time and cost used for creating electrodes for ram/sinker EDM.
 Reproducing sharp corners on the workpiece is difficult due to electrode wear.
 Specific power consumption is very high.
 Power consumption is high.
 "Overcut" is formed.
 Excessive tool wear occurs during machining.
 Electrically non-conductive materials can be machined only with specific set-
up of the process
Disadvantages

Ntm kiran

Related slideshows

More Related Content

Ntm kiran