Electrical Discharge Machining

Electrical discharge machining is an important ‘non-traditional manufacturing method. It is a standard method of creating tools to produce die castings, plastics mouldings, forging dies etc. In the material science new developments must control new engineering composite materials, metallic materials, and high tech ceramics, having thermal characteristics and good mechanical properties as well as suitable electrical conductivity.

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This is to keep them ready for machining by spark erosion. Electrical discharge machine (EDM) is a worldwide technique used in manufacturing process for high care machining of all types of conductive materials such as: graphite, metals, metallic alloys. Even EDM is possible in ceramic materials.

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Electrical discharge machining principle:

EDM is an electro-thermal non – traditional machining process, electrical energy is used to produce electrical spark, and thermal energy produced by the spark is used to remove the material from the work piece.

EDM is mainly used to machine the tough materials and high strength temperature alloys. EDM can be used to machine difficult geometries in minor batches and job-shop basis. Electrically conductive materials are to be machined with the help EDM. It is one of the most important conventional equipment in manufacturing industries. By using the EDM we can machine many complex 3D shapes with simple shaped electrode tool.

Process:

In the electrical discharge machining, before initializing the process the work piece is connected to the positive terminal and the tool is connected to the negative terminal. In between the tool material and the work piece material a potential difference is created. Electrically conductive materials are connected to the tool and the work piece material. Both tool and work piece material must be submerged in the di-electric fluid and subject to electric voltage. In between the tool and the work piece material a gap is maintained to develop the electric field.1

Due to the electro static forces developed between the tool and the work piece, results to free flow of electrons. In the process, series of regular current is released between the tool and the work piece at that instance there is a metal exclusion from the work piece. The metal removal must be taken place in the tool and the work piece materials.

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 Basic components of electrical discharge machining process:

Die sinking:

The Die sinking is also known as ram. From the figure, we can see the machine shaft arrangement and also the electrodes attached to the shaft.

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An individual can view the up and down movement of the shaft. Electro magnet is placed above the shaft. When it is energized it pulls the shaft toward the work piece material. Later when the electro magnet is de- energized, the shaft moves in the upward direction. During the movement of the shaft the tool is separated from the work piece material. Then we can see the accurate reverse shape of the tool on the work piece material as the final product in the process. Mainly the die shrink EDM is used to remove the taps and drills from the aluminum casting. The die electric fluid used in the process is hydrogen oils.

Wire EDM

In the wired EDM process a wire is used as electrode. We can see the spark between the tool and the work piece during the process. A continuous wire is used as an electrode. The die electric fluid used in the process is ionized water.

Types of EDM

  • Sinker EDM
  • Wire EDM
  • Powder Mixed EDM

Sinker EDM

Sinker EDM is also called as Volume EDM or cavity EDM. It consists of tool material and work piece material and they are submerged in the dielectric fluid. The tool must be connected to the negative terminal and the work piece to the positive terminal. Between the tool and the work piece material power is generated. When the tool is near to the work piece then there is a break down that occurs at the dielectric fluid, plasma channel is formed and sparks are seen. The sparks are occurred simultaneously in all the locations. Between the electrode and the work piece the sparks are seen incessantly in random locations. If the base metal is warned then spark gaps are increased. The electrode is depressed mechanically where the process continuous in a standard manner. With the help of the setup parameters the processes are controlled by the actual duty cycle, then several numbers of sparks are seen per second. Sometimes the controlling cycle is known as ON TIME and OFF TIME. The duration of the spark is determined with the help of the ONLINE controlling cycle. During the process the spark are seen for very long period of time, where it creates a rough surface finish on the work piece material. If the one spark is replaced with the other spark then it is known as the OFFTIME.

Wire EDM

The wired EDM is also known as the wire cut EDM. In this process a wire acts as electrode. In the wire EDM a series of sparks are generated between the electrode and the work piece to machine the conductive materials.  High frequency current is passed between the wire and the work piece material to maintain a spark gap in presence of the dielectric fluid. The discharged sparks ranges from \frac{1}{1,000,000} of seconds. Depending upon the surface finish and cutting speed the metal removal is experienced at the time of spark discharge. The spark must be discharge at a heat of 15,000 to 21,000; the material is to be melted and vaporized from the work piece. Mainly this is used in the low residual stress; it does not require any high cutting forces for the removal of metal. If any mechanical properties are changed, at that instance it leads to the residual stress. Depends upon the parameters used the work piece must undergone the thermal cycle. It causes formation of the residual tensile stress on the work piece material. The machining takes place after the completion of the heat treatment, where the dimensional accuracy does not affect the heat treatment distortion.

Equipment used in the process:

Circuits, tools, work piece material, dielectric fluid,

Circuit:

Different types of circuits are there they are

LC circuit, RLC circuit, RL circuit, RC circuit

RC circuit

In this process we use basic RC type circuit. They are also known as RC networks and RC filters. The RC circuit consists of resistor and capacitor driven by a current or voltage source. The simplest RC circuit consists of one resistor and one capacitor, which is a basic type of the RC circuit.

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The purpose of the RC circuit is to filter the signal by blocking the frequencies and by passing new once. The most common type of the filters is ‘low pass filter’ and ‘high pass filters’.

Tools:

Tools are also known as electrodes. Generally used tools are copper, brass, aluminum etc.

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Dielectric fluid:

During the Electrical Discharge Machining process, the work piece and the tool material are submerged in the dielectric fluid, which is an electrical insulator that helps to regulator the arc discharge. The dielectric fluid that provides a colouring that is pumped through the arc gap. This eliminates suspended particles of work piece and tool material (electrode) from the work cavity.

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Different types of dielectric fluids are present they are

  • Distilled water
  • White sprit
  • Kerosene

Function of dielectric fluid in EDM process:

  • In the spark gap, between the work piece material and tool material functions as spark conductor.
  • It acts as a coolant to cool the work piece and the tool material.
  • It works to remove the chips formed during the process and to maintain a perfect gap to get smooth operations.

Requirements to select the Dielectric fluid:

  • Dielectric acts as insulation between the work piece material and tool material to have steady and appropriate dielectric strength.
  • For high wettability and smooth flow it should have low viscosity
  • Dielectric fluid maintain high flash point
  • It should not emit poisonous vapours and unpleasant odour

Material selection:

Depends upon the type of the material the parameters are to be changed. The tool (electrode) material must be selected from the brass, copper and aluminum where the work pieces are the tool steels.

Tool steels:

In the manufacturing process the tool steels are used as the work piece materials and also the machining tools, plastics and woods. The tool materials are suitable to withstand at high loads and constant at the elevated temperatures. High speeds tools are used as the tool steels in the process, as they can freeze at the 62 to 67 HRC and at the high temperature at 540. These tools are applicable for the saws, taps, punches, end mills, lathe tools, drills, routers, chasers and planar tools.

Electrodes:

Brass:

It is an alloy, by which the proportions of copper and the zinc are maintained and the brass is created. Bronze is an alloy of tin and cooper. Some types of brass are known as bronze which is a substitution alloy. These are used in the application of valves, bearings, locks and gears etc.

Copper:

Copper is a ductile material having good electrical conductivity. We can find the copper in the pure state as well which looks like silver in color. It is used as an electrical and heat conductor. Copper is the basic of various alloys. It is a good conductor of electricity; it is ductile, malleable and good conductor of heat.

Aluminium

Aluminium has many advantages like high strength ratio to weight ratio, electrical conductivity, high thermal conductivity, corrosive resistance, reflectivity, nontoxicity and easy for machinability. Aluminium is non-magnetic. Aluminium is used in the packing, constructions, transportation, electrical applications and portable tools.

Material properties:

S.NO Material Thermal conductivity Boiling point Melting point
1 Brass 109 2624 K 900 K
2 Copper 401 2835.15 K 1357.77 K
3 Aluminum 237 2792.15 K 933.47 K

Metal removal rate in Electrical Discharge Machining:

Different types of machining conditions are involved in the metal removal from electric discharge machining they are radiation, heat condition, electrical forces, phase changes, bubble formation, rapid solidification and collapse. In the Electrical discharge machining the metal removal is associated with the erosive properties produced; this is seen when there is occurrence of separation and longitudinal discharges between the work piece and the tool materials. This is seen at the time of short spark occurrence in between the electrodes. The electrical energy is responsible for the removal of the material from both, i.e., the tool material and the work piece materials. To remove the melted pools, certain forces are applied like the hydrodynamic, thermodynamic and electrical forces.

MRR(in^3/hr)=\frac{Electrode \: Area(in^2)\times Depth \: of Cut \: (in)}{Time \: of Cut\:(min)}\times 60

Steps for the Metal removal process:

  • A gap is maintained between the tool and the work piece material. In that gap a strong electric filed is developed.
  • The dielectric fluid is used to build a high conductive bridge between the work piece and the tool
  • The di-electric liquid present between tool and the work piece is heated up when the voltage is increase. To produce a spark, dielectric must be ionized.
  • By increasing the pressure and temperature, gradually the spark is generated.
  • At the spark contact point there is a metal removal in the tool and work piece
  • The dielectric fluid present in the tank enters into the gap to remove the molten metal particles.

Factors for the metal removal rate:

  • Spark intensity
  • On time length
  • Seep and the stability influence the off time
  • The gap between the tool and the work piece material is smaller, at that instance we observe that there is high accuracy and the metal removal is slower

Advantages

  • Complex shapes are produced
  • Good surface finish is obtained
  • No contact between the tool material and the work piece material
  • Holes are also drilled
  • Small work pieces are also machined very effectively
  • Material of any hardness can be cut
  • High accuracy is attained
  • Good surface finish are possible
  • No cutting forces involved
  • Holes completed in one “pass”

Disadvantages of electrical discharge machining:

  • Electrically conductive materials are used
  • It is a slow process, high accuracy and good surface finish required
  • vapour released from the dielectric fluid can be dangerous
  • Heat Affected Zone near cutting edges
  • Life of the die-sinking tool is limited.
  • High power consumption
  • Metal removal rate must be slow
  • During the process we see the tool wear-out
  • Separate process are required for the electrically nonconductive materials

Applications of Electrical Discharge Machining

  • By using the Electrical Discharge Machining process we come across prepare parts like stamping tools, extrusion dies, hardened steel dies, header dies, intricate mould cavities and forging dies.
  • In automobile industries and aerospace industries, for the machining of the exotic materials this process is widely used.
  • Honey comb shaped parts are also manufactured with the help of the electrical discharge machining.
  • Micro pins, micro cavities and micro nozzles are produced with the help of micro EDM
  • EDM must be used as cutting force to make the slots, ribs and deep cavities.
  • To make slots, collects and mould cooling slots less and longer electrodes are used for the process.

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