Electro discharge machining (EDM)

Types of non Convectional Machining:

1) MECHANICALl

         (a) Abrasive Jet Machining (AJM)

         (b) Ultrasonic Machining (USG)

2)CHEMICAL

         (a) Chemical Machining (CHM)

3)ELECTRO-CHEMICAL

         (a) Electro-Chemical Matching (ECM)

         (b) Electro-Chemical Grinding (ECG)

4)THERMO-ELECTRIC

         (a) Ion-Beam Machining (IBM)

         (b) Plasma Arc Machining (PAM)

         (c) Electrical Discharge Machining (EDM)

(a) Ram / Sink EDM

(b) Wire EDM

         (d) Electron-Beam Machining (EBM)

         (e) Laser-Beam Machining (LBM)

Electro discharge machining (EDM): Electrical discharge machine started with the observation of Josepth preistly in 1770. EDM employs high frequency sparks for machining electrically conductive materials. In the Electrical Discharge Machining process (EDM), metal is removed by generating high frequency sparks through a small gap filled with a dielectric fluid. This technique allows machining complicated shapes in hard metals, including refractory alloys. A necessary condition for achieving a good surface finish is a well controlled gap between the electrode (tool) and the workpiece. The sparking gap ranges from about 10 to 100 microns, respectively for finish and roughing. The control problem is therefore the regulation of the gap that is measured only indirectly by processing some secondary signals like ignition delays, average gap voltage, etc. The tool and work piece form a pair of electrodes, separated by about 20 to 200μm in a liquid dielectric through which the spark discharges occur. It is universally acknowledged that EDM is a highly complex and stochastic process. EDM  is a nontraditional  method of  removing  material by a series of rapidly recurring  electric arcing  discharges  between  an  electrode  (the cutting tool)  and  the  work piece, in  the  presence  of  an energetic electric field. The   EDM   cutting  tool is  guided  along  the  desired path   very   close  to  the  work   but   it   does   not   touch  the  piece. Consecutive   sparks   produce   a   series   of   micro-craters   on   the work piece and remove material along the cutting path by melting and vaporization. The particles are washed away by the continuously flushing fluid. In this process electrode is given particular shape & size according to component profile. The current density in the discharge of channel is of the order of 10.000 A/cm2 the power density, of the order of 500MW/cm2.   

Capabilities: EDM is a machining method primarily used for hard metals or those that would be impossible to machine with traditional techniques. One critical limitation, however, is that EDM only works with materials that are electrically conductive. EDM or Electrical Discharge Machining is especially well-suited for cutting intricate contours or delicate cavities that would be difficult to produce with a grinder, an end mill or other cutting tools. Metals that can be machined with EDM include hastalloy, hardened tool-steel, titanium, carbide, inconel and kovar. 

Theory: The square pulses generated and supplied by the power generators and the ideal spark discharge pulses are shown. When the inter electrode distance is high, open circuit pulses occur. As the servo control system feeds the electrode towards work piece, the spark discharges occur when the requisite spark gap is reached. The onset of spark discharge occurs after a short delay called ignition delay (td), required for ionization and spark channel formation which depends on spark gap. Thus the real discharge time is lower than the pulse on time (ti). If electrode feed rate is lower than erosion rate, spark gap goes on increasing leading to open circuit condition. If electrode feed rate is higher than the erosion rate, spark gap goes on reducing leading to short circuits. If spark gap is highly contaminated with erosion debris, successive sparks occur at the same location leading to arcing. For ideal machining conditions the spark gap should be uniform with stable sparking. This requires parity between electrode feed rate and work piece erosion rate, which does not occur. Short circuits and arcing pulses also occur when the flushing of spark gap is not efficient and the eroded debris cause progressive stagnation of the spark gap. When short circuit occurs, the tool is withdrawn to open circuit condition by the servo control whence the tool is given the nominal feed. The transition from open circuit and short circuit to normal sparking condition results in considerable ineffective pulses and loss of productivity. The servo control response characteristics should be such that minimum loss of effective pulses occurs in the transition zone.

Die electric fluid: Light hydrocarbon oils seem to satisfy these requirements best of all. The fluid that is circulating b/w the tool and the work piece during the edm process is known as die electric fluid. It is nothing is a coolant. It at in the electron bombardment for effective cooling. Water is the best dielectric used in wire EDM. It is used mostly because of its low viscous properties. It can enter through very small gaps. Since the process of removal of material (both from work and tool)  mainly depends on thermal evaporation and melting the presence of oxygen in the atmosphere surrounding the spark would lead to formation of metal oxides which adversely affect the continuation or generation of repetitive sparks (most of metal oxides are bad conductor). Hence, it is pertinent to use a dielectric fluid which contains no oxygen for liberation during the process, to help ionization, without disturbing the process.  But the performance (mainly the failure) of the dielectric to suit the process is extremely important. The failure of dielectric under electric stress termed as breakdown, is found to spread over a wide range of applied stresses, depending upon it environment and mode of use. In general, the main basic mechanisms of dielectric breakdown in the three states of matter are:

1. intrinsic, 

2. thermal and discharge or avalanche

Dielectric fluid is a very important part of the wire EDM process. The dielectric fluid is used to cool the wire and flush resolidified particles from the gap.  Experienced EDM operators say that the three most important factors of wire EDM are “flushing, flushing, flushing.” Normal tap water is added to the EDM system and is cycled through a two step process.  First, the water is sent through a paper filter system. Paper filters are rated in microns, which indicate the largest particle size allowed to pass through. The typical wire EDM filter is between 3 and 10 microns.  The fluid is pressurized into the filtration bank where it is stripped of all larger particles and then returned to the dielectric holding tank. A set of electronic probes monitor the fluid quality in the holding tank. When the conductivity level climbs above the determined value, it is then sent to the second process. The second process consists of a bottle called a resin cell. The resin cell contains small, round resin beads. These resin beads are electrically charged and attract the fine particles that were allowed to pass through the paper filters. Once the fluid is stripped of these fine particles, it is returned to the holding tank. The resin cell is shut off once the fluid conductivity level has been returned to the determined value.

Break down mechanism: The earlier theories of breakdown in liquids have assumed that it occurs by avalanche ionization of the atoms induced by conduction electrons accelerated in the applied field.  The cathode electrode is assumed to be source of these electrons which are emitted either by field-effect or by Scotty-effect. The electron liberated from the cathode gains from the applied field more energy than it loses in vibration collisions with the molecules of the liquid dielectric.  These electrons are accelerated until they gain sufficient energy to ionize the liquid molecules and initiate an electron available.

Electrode material: As already discussed, the tool electrode in EDM process is the means of providing electrical energy to the work-material, as well as the necessary form to the latter.  The work-surface sometimes being the inverse profile of the tool, its accuracy depends on the form stability of the tool under the severe electrical and flushing stress conditions. Moreover, the share of eat that the tool receives from the plasma channel, is to be dissipated away faster unlike work-material in order to reduce surface temperature to minimize evaporation and melting of the tool material responsible for its wear rate.  So it is necessary to me made of highly thermal conductive material over and above its first character of higher electrical conductivity.  Then it is evident that the tool material should have highly melting point to reduce its wear-rate. However, from the material science point of view, higher thermal conductivity and higher melting point are sometime two contradicting character for pure elements.  So a proper choice is necessary to be made, since the electrode cost can represent more than 50% of the total machining cost. Therefore, the basic desirable characters of the too-material are: High electric conductivity, High thermal conductivity, High melting temperature, Cheapness, Easier manufacturability. Theoretically, any material that is good electrical conductor can be used as a tool with more or less advantages; in general, tool or electrode material can be classified into four groups:

Metallic electrodes: 

Copper Electrodes:  It is one of the oldest and commonly preferred as tool material.  Its melting point at 1083˚C density = 8.9g/cc and electrical resistively of 0.0167 ohm mm²/m, coefficient of expansion of 4.318 x 10   -4 mm/ K and its abundant availability is the reason behind its use Copper being difficult to cast for tool material and since at it molten state it tends to absorb oxygen free, phosporized or arsenide. 

Most copper marketed for commercial use contains traces of silicon or other hardeners (impurities as well) and small percentage for arsenic (0.4%) these affect to reduce the conductivity of copper seriously. Oxygen-free copper is about 99.9% pure and exhibits extremely high conductivity. Phosporized copper contains residual phosphorus and exhibits lower conductivity of copper seriously. Electrolytic copper can be machined as readily as coppers containing selenium lead or tellurium.  The recommended materials for better machining are:

Tellurium Copper - 99.5% copper and 0.6% tellurium

Leaded Copper - 99% copper and 0.1% lead

Selenium Copper - 99.4% copper and 0.6% selenium 

These three copper are utilized for most EDM applications.  The amount of temper applied to each determines the best performance of a given EDM operation.  Copper with half-hard temper exhibits the best overall performance.  Under high current, soft Copper will temper and begins to wrap.  So it is required to put chillers at the hot spot when softer electrodes are used.

Copper-tungsten electrodes: (Cu: W varies between 50: 50 and 20: 50) however, do not exhibit war page problems.  This is used when small holes are to be drilled a tubular tool.  Although, expensive Cu-W can be used for those applications where the use of alternate electrode material would be impractical. Because of its low flexural strength and high rigidity, it is a preferred material for slots. 

Small pieces of Cu-W can be Silvered soldered an steel shanks to form intricate tool shapes Copper-tungsten has high density  (15-18 gm/cm²) along with high strength  (BHN: 85 – 240  kg/mm²  Rb 94) good thermal and electrical conductivity  (resistively : 0.045 ~ 0.55 ohm mm²/m) high density enables it impart high surface finish. It is commonly recommended for EDM of dies with fine contoured details.

Brass electrodes: Free machined brass is often used as an electrode material.  Because of its high wear-rate it is not preferred for generation of 3-D surfaces, it has been seen to be one of the best tool materials for machining titanium-alloys at low material-removal-rate conditions.

The high wear rate is attributed by the presence of low melting alloy of element Zn.  However, the plasma channel stability (machining stability) is achieved nicely with brass tool since its high-rate or erosion allow zinc-vapors in the plasma channel might reduce arc resistance and helps quicker ionization also. 

Silver tungsten: This has very similar characteristics to those of the Cu-W.   It contains very high percentage of tungsten and hence extremely costly which restrict applications.

However, the lower corner wear helps to produce very sharp corners (small corners radius) if required and is better than Cu-W.

Tungsten: Because of its high melting point exhibit extremely low erosion rate but its high cost only restricts it for use in producing fine holes or wire cutting operations where high order accuracy is required. 

Wire electrodes of Tungsten of less 0.01mm are available for use owing to its high strength character.  To enable its use Tungsten coated electrodes are being developed for use.

Aluminum: In spite of high thermal and electrical conductivity and low density it has not found a suitable place in edging tool because of its low melting temperature and high tarnishing property. When machining large 3D cavities which do not require higher surface finish, one can use aluminum alloy known as SILUMIN as tool material. It has a composition of about (Al: 85%; Si:  11% Mg:  0.4% to 0.6%; Zn: 1 %;) (Ti: 1%; Mo, Fe and Cu: 1%)

This is easily cast able.  Shrinkage of about 1% can be off-set by cold-forming preceded by annealing at 540˚C followed by water cooling.  It is also easily machine able.

Steel: Steel sometimes is used as electrodes even if it has lower efficiency as compared to copper or granite. In case of irregular parting lines for plastic moulds and die casting dies are matched to prevent flash by using upper part of the mould or die as an electrode.

Mechanics of EDM: Though the surfaces may appear smooth, asperities and irregularities are always present, as indicated (in an exaggerated manner, of course) As a result, the local gap varies, and at a given instant, it is minimum at one point (say, A) when a suitable voltage is built up across the tool and the work piece (the cathode and anode, respectively) an electrostatic field of sufficient strength is established, causing cold emission of electrons from the cathode at A.  These liberated electrons accelerate towards the anode. After gaining a sufficient velocity, the electrons collide with the molecules of the dielectric fluid, breaking them into electrons and positive ions.  The electrons so produced also accelerate and may ultimately dislodge the other electrons from the dielectric fluid molecules.  Ultimately, a narrow column of ionized dielectric fluid molecules is established at A connecting the two electrodes (causing an avalanche  of electrons, since the conductivity of the ionized column is very large, which is normally seen as a spark)  As a result of this spark, a compression shock wave is generated and a very high temperature is developed on the electrodes (10,000 – 12,000ºC ) This high temperature causes the melting and vaporization of the electrode materials, and the molten metal’s are evacuated by a mechanical blast, resulting in small craters on both the electrodes a. As soon as this happens, the gap between the electrodes at A increases and the next location of the shortest gap is somewhere else (say, B). Therefore, when the cycle is repeated, the next spark takes place at B.  In this way, the sparks wander all over the electrode surface and, ultimately, the process result in a uniform gap.  So, depending on the negative electrode shape, an impression is created on the other electrode. Generally, the rate of material removal from the cathode is comparatively less than that from the anode due to the following reasons: 

(i) The momentum with which the stream of electrons strikes the anode is much more than that due to the stream of the positive ions impinging on the cathode though the mass of an individual electron is less than that of the positive ions. The hydrolysis of the dielectric fluid (normally a hydrocarbon) creates a thin film of carbon on the cathode. A compressive force is developed on the cathode surface.  Therefore, normally, the tool is connected to the negative terminal of the dc source. If the tool is stationery relative to the work piece, the gap increases as the material removal progresses, necessitating an increased voltage to initiate the sparks.  To avoid this problem, the tool is fed with the help of a servo drive which senses the magnitude of the average gap and keeps it constant.

Principle behind Electrical Discharge Machining (EDM): The electric field causes electrons and positive free ions to be accelerated to high velocities between the wire and the high point on the work piece.  An ionized channel is formed across the gap between this point and the wire. At this stage, current can flow and the spark takes place between the electrodes. A bubble of gas forms due to vaporization of the electrodes and the dielectric.  The pressure caused by the bubble rises until it becomes very high. A plasma zone is formed, which very quickly reaches extremely high temperatures (~8000-12000 degrees C).  This causes instantaneous local melting of a certain amount of material at the surface of both the wire and the work piece. The voltage is now dropped.  The sudden reduction in temperature causes implosion of the bubble.  This implosion creates dynamic forces which pull the melted material away from the two conductors. This eroded material then resolidifies in the dielectric and is swept away.

Axis Wire EDM: In addition to the normal Wire EDM process. A true 5 Axis Wire EDM incorporates a rotary indexing system under the direct servo control of the Wire EDM machine. 5 Axis Wire EDM actually is on different axes as compared to the 5 Axis CNC Machining processes. Often, people would consider the tilting of the wire head as being 5 Axis work. In actuality, true 5 Axis EDM would have a rotary indexer that could actually rotate the part - even simultaneously during the Wire EDM process. More commonly, the rotary indexer is actually used to index on preset increments, for example to create axial slots. A 5 Axis Wire EDM with fine wire capability would be able to use a .001" diameter wire and can even offer sub micron tolerances. Wire EDM (Electrical Discharge Machining) uses a continuous wire electrode that travels through the workpiece. The wire is monitored by a computer numerically controlled (CNC) system. Wire EDM removes metal just like any other machining tool, but it uses electricity to remove the metal by spark erosion. During this process, the wire never comes in contact with the workpiece. The wire electrode leaves a path slightly larger than the wire. A commonly used wire of 0.010” usually creates a 0.013” to 0.014” gap. Thinner wires have smaller gaps. The wire electrode is only passed through the work piece one time, it does not get reused. Wire EDM generates rapid electrical pulses between the wire and the workpiece. With sufficient voltage, a controlled spark precisely causes a small part of the workpiece to melt and vaporize. These pulses are repeated thousands of times per second. Each spark produces a temperature of approximately 10,000°C. The size of the spark penetration into the material depends on the energy turned out by the power supply.

Terminology:

Gap – The narrow path that the wire makes as it travels through the work piece.  Also referred to as the “kerf”. The gap is always slightly larger than the wire diameter being used. 

Rough Cut – The initial cut of the wire through solid material. High voltage is required for this type of cutting. 

Skim Cut – Secondary cutting that is performed after the rough cut. Skim cuts are optional and run at lower voltage than the rough cut. The reason for performing skim cuts would be to increase accuracy and improve surface finish.

Dielectric Fluid – Water that has been sent through the filtration and resin cell. This fluid acts as an insulator and a flushing agent during the cutting process.

Flushing – Dielectric fluid that is emitted from the upper and lower machine heads. The pressurized fluid helps in the production of the spark and in the removal of the eroded material. Insufficient flushing can result in reduced cutting speeds and wire breakage.

Terminology (Cont.):

Secondary Discharge – This is caused when particles exploded off of the work piece get too close to the wire again. If too close, the wire will discharge at them a second time. These secondary discharges will sometimes cause the particles to be re-melted onto the work piece. Optimized flushing greatly reduces this effect. 

Recast – Sometimes referred to as the white layer, this is a thin depth of material that has been re-welded back on the EDM surface.  It is softer than the parent material and can be of concern in close tolerance mating applications.  With older power supplies, it was not uncommon to see a recast depth of up to 0.005”. Today’s advanced power supplies have reduced this depth to approximately 0.0001”.   

Power Contacts – Typically made from carbide, every wire EDM has one in the upper and lower machine heads.  The wire holds a positive pressure against the contact, and the contact transfers electricity to the wire. Over time, the contact wears and should be replaced or indexed to a new location.

Wire – This is the electrode (cutting tool) that is used to deliver the electrical pulses to the work piece. Wire comes in many different diameters (0.0008” – 0.013”) as well as many different compositions, depending on the application. The wire diameter is usually dictated by the smallest internal radius to be made.

Wire Guides – They hold the wire precisely in the upper and lower machine heads.

Threading – Passing the wire through the upper head, work piece, and lower head of the machine. Most advanced wire EDM systems have an automatic threading system.

Slug – The scrap piece of material that is removed from an opening.

No Core – The process of spiraling the wire out to the finished opening size. This avoids having to remove a slug. 

Advantages of 5 axis Wire EDM technology: 5 axes Wire EDM can be extremely useful when close tolerances are involved. The use of fine wires with very small diameters in the range of .001-.004" can guarantee a very high precision work on wire EDM machines. Because EDM is a no-contact and no-force process, it is well suited for making frail or fragile parts that cannot take the stress of standard machining; it can cut parts 16 inches tall with a straightness of ±0.0005 inch per side! Parts requiring small inside radii are now easily achieved using this technology. The use of a .001" wire would be that the corner radii can be as sharp as .0015 (there is usually an over burn of .001" during the EDM process). But the most important advantage came with the increased sophistication of EDM controls in rams and the new EDM processes that use simple-shaped electrodes to 3D mill complex shapes with extremes accuracy! Now users demand and need maximum productivity and throughput, increased accuracy, higher angles of taper, thicker work pieces, automatic wire threading, and long periods of unattended operation, make the Wire EDM machining a breakthrough precision technology. 5 axis Wire EDM is used to manufacture complex parts for the aerospace, Medical, and telecommunication Industries.

Over cut: Over is the distance, the spark will penetrate the work piece from the tool and remove metal from work piece. Theoretically, it is slightly larger than the gap between the end of tool and the work piece. It is generally 0.025 to 0.2mm. This overcut is a function of the voltage of the spark. The overcut increase with higher current and decrease with higher frequency.

This break down voltage depends: On the distance between the electrode and the part, On the insulating capability of the dielectric, On the condition of pollution of the gap (erosion waste) A discharge builds up at the point of the strongest electrical field. This is, in fact, the result of a complicated process. Due to the action of this field, free positive ions and electron are accelerated reach very high speeds and rapidly form an ionized and therefore conductive channel. A plasma zone is formed. This rapidly reaches very high temperatures, in the order of 8000 to 12000c and develops to the increases number of shocks causing local and instantaneous melting of a certain quantity of material at the surface of both conductors. Simultaneously, a gas bubble, due to vaporization of the electrodes and its pressure increases regularly until it becomes very high. When the current is interrupted, the sudden drop in temperature causes the bubbles to implode causing dynamic forces which have the effect of projecting this melted material out of the crater. The eroded material then resolidifies in the dielectric in the form of small spheres, and is evacuated by the dielectric 

Temperature variation D.I. water chiller of work piece cause inaccuracy in job and it might possible that the wire guide will spoil due to the excessive generation of heat. Make sure the work piece fixed directly on the mounting surface of the clamping support with a plane-ground surface, pressure flushing is impossible. In the main cut this means a performance loss of up to 30%. It may cause wire marks on the job and brakes. It is always recommended that while cutting of punches or core and cavity inserts, take wire entry from inside the block, outer side entry leads inaccuracy in shape and size. It is always recommended to make program in close contour.

Wear ratio = volume of work material removed / volume of electrode consumed

Ultrasonic vibration: The higher efficiency gained by employment of ultrasonic vibration is mainly attributed to improvement in dielectric circulation which facilitates the debris removal and creation of a large pressure change between the electrode and the work piece, as an enhancement of molten ejection from the surface of the work piece. The work piece was vibrated during machining and the success was machining of micro holes as small as 5μ in diameter in quartz, glass and silicon. When the vibration was introduced on the work piece the flushing effect increased.

Dry EDM: In dry EDM, tool electrode is formed to be thin walled pipe. High –pressure gas or air is supplied through the pipe. The role of the gas is to remove the debris from the gap and to cool the inter electrode gap. The technique was developed to decrease the pollution caused by the use of liquid dielectric which leads to production of vapor during machining and the cost to manage the waste.

The characteristics of dry EDM: Tool electrode wear is negligible for any pulse duration, the processing reaction force is much smaller than in conventional, the residual stress is small since the melting resolidification layer is thin, Working gap is narrower than in conventional EDM.

EDM by powder additives: Fine abrasive powder is mixed into the dielectric fluid. Electrically conductive powder reduces the insulating strength of the dielectric fluid and increase in spark gap between the tool and the work piece. EDM process becomes more stable and improves machining efficiency, MRR and SQ. The characteristics of the powder such as the size, type and concentration influence the dielectric performance. 

The different powders are: Al, Si, Graphite, Cr, Sic etc.

EDM in water: Water as dielectric is an alternative to hydrocarbon oil. The approach is taken to promote a better health and safe environment while working with EDM. This is because hydrocarbon oil such as kerosene will decompose and release harmful vapor (CO & CH4). Machining in distilled water resulted in a higher MRR and lower wear ratio than in kerosene when a high pulse energy range was used.

Modeling: EDM process is influenced by many input factors. Various techniques viz. dimensional analysis, ANN and thermal modeling are employed to predict the output of the process mainly the surface finish, tool wear and MRR.

Wire Cut EDM: In wire electrical discharge machining (WEDM), or wire-cut EDM, a thin single-strand metal wire, usually brass is fed through the workpiece. The wire, which is constantly fed from a spool, is held between upper and lower guides. The guides move in the x–y plane, usually being CNC controlled and on almost all modern machines the upper guide can also move independently giving rise to the ability to cut tapered and transitioning shapes (circle on the bottom square at the top for example). This gives the wire-cut EDM the ability to be programmed to cut very intricate and delicate shapes. The wire-cut uses water as its dielectric with the water's resistivity and other electrical properties carefully controlled by filters and de-ionizer units.

There are types of wire EDM machining are presently available in the market:-

Submerge type: 

Advantage: In this machine we can achieve job accuracy better than flush type machine, especially in small cross-section. Where the rib size is very thin. Chances of thermal expansion are very less. Job height more than 100.00 mm. can also be machined in better accuracy.

Disadvantage: Life of submerge type machine is less than flush type because this machine is manufactured on box & nut basis. Some time it may possible that positional inaccuracy occur while working with heavy job.

Flushing: Flushing is the removal of erosion debris from the spark gap by effective circulation of dielectric. This facilitates deionization i.e., restoration of non conducting nature of the dielectric. Ineffective flushing leads to: Reduction of effective spark gap and eventual short circuits. Reduction of spark over voltage due to reduced resistance. Excessive debris contamination leads to arcing.

Thus flushing away of particles generated during the process is vital to successful EDM operations.

Flushing type: 

Advantage of flush type machine is fixed table type than there is no chances of positional inaccuracy occur in heavy jobs. Disadvantage: It is difficult to cut job-having height more than 100.0m.m at high accuracy Deflection accuracy is more in this cross-section due to high flushing pressure.

Material selection for machining on wire edm: Electrical conductive should be more than 0.1 S/cm (0.08-1cm-1). Material must not undergo violent chemical reaction with water oxygen or hydrogen (non combustible).

Dielectric water: Dielectric liquid is a liquid that conducts electricity very poorly or not at all. In EDM it has 3main tasks: To ensure the deionization of the discharge channel between the electrode and work piece which is necessary for EDM; To remove the eroded particles from the work zone; To cool the work zone; In ED cutting, demonized water (demineralised water) is the most frequently used dielectric liquid because of the advantages it offers (lower risk of short circuit, good flushing conditions greater cleanliness). 

Water chiller: In wire EDM machine the water chiller is used to cool down the temperature of D.I. water. It is playing very important role in getting optimum performance of the machine .the temperature of D.I. water should be 20 Celsius 

Cut sequence: First cut (rough cut)     -     accuracy within 20micron and surface  

Cut sequence is totally depending upon the accuracy and surface finish required in the job. Finish will be 2.8 Ra value.

Second cut (semi finish) -     accuracy within 15 micron and surface. Finish will be 2.0 Ra value.

Third cut (finish)          -     accuracy with in 10 micron and Surface finish will be 1.5 Ra value.

Forth cut (finishing)      -    accuracy with in 5 micron and surface Finish will be 1.5 Ra value.

This accuracy and finish can vary according to the quality of wire and up keeping of machine. Superimposed cut sequences are carried out cutting height. While making of die plate of any press tool where land and taper is required to be cut, first taper cut than straight cut it will increases land and taper positional accuracy. This way we can reduce cutting time of die plate 30%. 

Spark gap: There is gap b/w the workpiece and the electrode is known as spark gap. When the m/c is on running cordlike on producing spark this is gap whose maintain this safety and gap is know or spark gap. The spark gap is maintained in the range of 0.005 to 0.05mm.

Types of cutting tool for wire edm: There are three types of wire available in market.             

Plane brass wire, Brace zinc coated, Tungsten wire, of the brass wire should be 500 N/ The hardness m2 and for the hard brass wire hardness is 900 N/m2. If the job height is more than 80 mm then the performance of hard brass wire is better than semi hard brass wire. To achieve high finish and accuracy, use of zinc coated brass wire is advisable. But the cutting speed will be less. Tungsten wire is basically is used where the diameter of wire is less than or equal to 0.1 mm. Generally, 0.25 mm brass wire diameter is being used in mind.

Concluding Remarks: Electro discharge sawing is essentially suitable for large bar stocks or ingots for fast, accurate and burr free cuts. The arc discharges in EDS result in higher roughness and kerf width compared to EDM. The efficient gap flushing of EDMD results in uniform and uninterrupted drilling unlike EDM with poor flushing and extensive short circuits. Effective gap flushing in EDMD combining with tool rotation results in highly uniform whole diameter along its length where as in EDM the hole has considerable taper. Several adaptations are possible for specialized applications of EDM. Dressing of diamond wheels, surface modification, generating accurate surfaces of rotation, machining of PCD etc.

Accuracy: Tolerance value of ±0.05mm could be easily achieved by EDM in normal production. However, by close control of the several variables a tolerance of ±0.003mm could be achieved. A typical taper value is about 0.005 to 0.05mm per 100mm depth. An overcut of 5 to 100 micron is produced, depending upon finishing or roughing. The best surface finish that can be economically achieved on steel is 0.4 micron.

Manual EDM

1. Servo control only for Z-axis movement 

2. X,Y axes are manual movement

3. Tool profile to be made / Electrode pre-shaped

4. Kerosene is used as dielectric medium

5. Auto feed -screw rod, for spark gap

6. Frontal area spark & Arcing effect

7. One type surface finish

Applications

Wire EDM was primarily developed for the tool & die industry. With the advancements in cutting speed, reliability, unattended operation, and accuracy it has grown into many other industries. It is now used in medical, aerospace, automotive, defense, electronics, and extrusion applications to name a few. One rule to EDM is that the material must be electrically conductive.

In the case of wire EDM, the feature must be cut through the entire workpiece.

When an internal shape needs to be wire EDM’ed, you must first have a start hole to allow the wire to be threaded. This is typically done with a drill bit or “hole popper” EDM in a previous machining operation.

Many of today’s advanced wire EDM systems allow the upper wire guide to be programmed independently from the lower wire guide. An example of this would be a workpiece that is a square on the top and a circle at the bottom.

1. Plastic die moulds (Only moulds purpose)

2. Die casting dies & Press tool components

3. Injection molding tools ( 4 cavities die / 5 cavities die)

Die applications require two surfaces referred to as a land and a taper. Both of these surfaces can be cut using the wire EDM.  For example, the taper can be cut first by programming the desired angle in the opening. Once the taper is complete, the wire would be kept straight to cut the land.  

Wire EDM produces a sharp, burr-free edge. It is a highly desirable machining choice for workpiece such as medical implants and die openings.

Gears are another excellent application for specialty as well as production runs. This is a great way to produce precision gears without the expense of broaching, hobbing, or form grinding.

Stacking is another efficient method of cutting. When cutting multiples of the same thin workpiece, they can be stacked to reduce setup time and increase part output.

The applications for wire EDM continue to grow as more people realize its capabilities. It has offered many solutions to difficult or formerly impossible machining tasks.

CNC EDM

1. Servo control plus control feed back

2. X,Y,Z and C axes movement

3. Orbiting facility & Undercut machining is possible

4. Inclined holes can be possible

5. C – axis rotation, curved profiles can be machined

6. Spindle rotation / Electrode rotation

7. Commercially available hydrocarbons like ED30 / ED40 are  used as dielectric media

8. Pre shaping of electrode is not required

9. Merging is possible & Lateral sparking

10. Ball screws are used for axes movement

11. CAM can be done & 3D programs can be possible.

Limitation:

1. Work piece material should be electrically conductive.

2. Material must not undergo violent chemical reaction with water oxygen or hydrogen (non combustible).

Applications:

1. The EDM provides economic advantage for making stamping tools.

2. Cavities of intricate shapes (Automotive body components)

3. Extrusion press dies

4. Die casting moulds & Plastic moulds

5. Critical components (turbine blades)

6. Servo valves & Internal cavities

7. Drilling of micro holes & Thread cutting

Advantages:

1. EDM is applicable to all electrical conducting materials

2. No mechanical stress is present in the process, due to non contact of W/P and tool

3. The fragile and slender work pieces can be machined without distortion.

4. Hard and corrosion resistant surfaces needed for die making, can be developed.

5. The process can be applied to all electrically conducting materials and alloys irrespective of their melting points, hardness, toughness or brittleness.

6. Any complicated shape that can be made on the tool can be reproduces on the workpiece.

Time of machining is less than conventional machining processes.

7. Hard and corrosion resistant surface, essentially needed for die making, can be developed.

Because of its various advantages, wire EDM continues to grow in many manufacturing industries. The following are only a few of its many benefits.

Precision – Depending on the quality of the wire EDM system, profile accuracy of down to 0.00004” can be achieved.

Surface Finish – Many wire EDM systems can obtain surface finishes down to 8 micro inch Ra.

Unattended Operation – Once the machine is set up and running, there is time for the operator to carry out other job functions.  Multiple workpiece setups can extend that amount of time.

Cost of Operation – Many of today’s newest wire EDM systems can be operated for around $4 per hour in normal cutting applications. 

Disadvantages

1. Machining times are too long.

2. Machined surfaces undergo metallurgical changes due to large heat energy.

3. High power consumption & Tool wear. 

4. Machining heats the workpiece considerably and hence causes change in surface and metallurgical properties.