EDM Machining

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Introduction

This article will take an in-depth look at EDM Machining.

The article will bring more understanding on topics such as:

• Operating Principle of EDM Machining
• Components of EDM Machining and their Functions
• Types of EDM Machines
• Applications of EDM Machining
• Advantages and Disadvantages of EDM Machining
• And Much More…

Chapter 1: EDM Machining Operating Principle

This chapter will discuss EDM machining together with its working principle.

What is EDM Machining?

EDM machining is a contemporary machining method based on the removal of material from a part using thermal energy. The material is removed by local melting or vaporizing small areas on the surface of the part being machined.

This is done by a series of repeated electrical sparks between tools that are called electrodes and the work piece in the presence of a dielectric fluid.

Electrical discharge machining is also known by several names, which are: spark eroding, spark machining, burning, wire burning, die sinking, or wire erosion.

Working Principle of EDM Machining

A potential difference is applied, in EDM machining, between the tool and work material. Both the tool and the workpiece must be conductors of electricity. The tool and the workpiece are sunken dip in a dielectric medium. Generally, kerosene or deionized water is employed as the dielectric fluid. A gap is maintained between the tool and therefore the workpiece. An electric field is established depending upon the applied electric potential difference and the gap between the tool and work material.

Usually, the tool takes the negative terminal and the work material takes the positive terminal of the power generator. The free electrons on the tool are subjected to electrostatic forces, the moment when the establishment of the electric field starts. If there is a smaller amount of the work function or smaller bonding energy of the electrons, the emission of electrons would be from the tool (assuming that it is connected to the negative terminal). This type of emission of electrons is called cold emission.

Through the dielectric medium, the cold emitted electrons are then accelerated towards the work material. As they gain velocity and energy and begin moving towards the work, collisions occur between the electrons and dielectric molecules. Such collisions may end in ionization of the dielectric molecule depending upon the work function or ionization energy of the dielectric molecule and therefore the energy of the electron. Because electrons are accelerated, more positive ions and electrons would be generated because of the collisions.

This cyclic process increases the electron and ion concentration within the dielectric fluid. This occurs between the tool and the work material at the site of the spark gap. The concentration would be so high that the matter existing in the channel might be characterized as “plasma.” The electric resistance of such a plasma channel would be very less. Thus an outsized number of electrons will flux from the tool to the work and ions from the work to the tool suddenly. This motion of electrons is known as an avalanche.

Such movement of electrons and ions are often visually seen as a spark. Thus the electricity is dissipated like it is the thermal energy of the spark. The high speed electrons then hit the work material and ions on the tool. The K.E. of the electrons and ions on impact with the surface of the work and gear respectively would be converted into thermal energy or heat flux. This extreme localized heat flux results in an instantaneous confined increase in temperature which might go beyond 10,000° C.

Such localized extreme rise in temperature results in material removal. Material removal occurs because of the instant vaporization of the fabric and also because of melting. The molten metal isn’t removed completely but only partially. As the electric potential is withdrawn, the plasma channel is no longer sustained. The plasma channel generates pressure or shock waves as it collapses. This will evacuate the molten material forming a crater of material removed around the spark site.

Thus, the EDM fabric removal is facilitated by shock wave formation as the plasma channel collapses owing to applied electric potential discontinuation. Generally the work material is made positive and the tool negative. The electrons strike the workpiece resulting in a crater formation due to heat and melting and removal of material. Similarly, the positive ions hit the tool resulting in tool wear.

In EDM, the generator uses voltage pulses between the tool and the workpiece. A constant voltage is not applied. Only sparking is desired in EDM instead of arcing. Arcing leads to the removal of localized material at a particular point and causes the distribution of sparks all over the tool’s surface. This, in turn, will lead to a uniform distribution of material removal under the tool.

Chapter 2: Components of EDM Machining and Their Functions

The following are components of EDM Machining, with their functions.

Power Generator

This is the power supply given to the EDM machining process. The negative terminal is connected to the tool and the positive terminal is connected to the part being machined (i.e. the workpiece). Different types of power generators are used in the process of electrical discharge machining with some listed below:

• Resistance-capacitance type (RC type) relaxation generator
• Rotary impulse type generator
• Electronic pulse generator
• Hybrid EDM generator

Workpiece Component

A workpiece is the part to be machined. It’s fixed in the dielectric container using a fixture and is connected to the positive terminal of the power supply.

Fixture Component

The fixture is used for holding the workpiece properly in the dielectric container.

Dielectric Fluid

Aspects of dielectric fluid are discussed below.

The Dielectric Medium

The dielectric medium plays a key role in the functioning of the EDM Machining process. Usually, the dielectric fluid is hydrocarbon oil with low viscosity. The dielectric serves to separate the workpiece and the electrode. During the EDM machining process, as the spark is produced, a surge of current takes place when the dielectric is ionized to form a column or path in the tool. After this happens, the dielectric medium is raptured, when the gap between the tool and work gap is about 0.03 mm and at a voltage of about 7V.

The spark discharge will be at around 10,000°C and thousands of atmospheric pressure in a micro small area. It takes less than a microsecond for this to take place for each spark. As the column of ionized dielectric vapor collapses, a small tiny part of the workpiece vaporizes due to the arc being forced out. The small tiny metal particles are then cooled down into small spheres and the flux of the dielectric fluid sweeps the particles from the area.

Functions of the Dielectric Fluid

The following are essential functions of a dielectric fluid used in the EDM machining process:

• The dielectric fluid must remain electrically non-conductive to the time when the breakdown voltage is reached (i.e. its dielectric strength must be high)
• Once the breakdown voltage is attained, instantly, it should breakdown electrically.
• The dielectric fluid must deionize the spark gap (i.e. after the discharge has occurred, it must quench the spark rapidly)
• It carries away the metal particles that have been removed from the arc gap.
• The dielectric fluid acts as a good cooling medium.

Desirable Properties of Dielectric Fluid

The dielectric fluid must possess the following properties to act as a good dielectric medium and meet various functional requirements:

• High level of electrical strength for proper insulation
• High fire and flashpoints in order to prevent fire hazards
• Good wetting properties and low viscosity
• Chemically neutral in order to prevent corrosion
• Non-toxic in nature
• Low decomposition rate for a high life expectancy
• Low cost
• Good quenching properties

Common Dielectric Fluids

The most commonly used dielectrics are hydrocarbon and mineral oil, both of low viscosity. The different types of dielectric fluids are kerosene, paraffin oil, lubricating oil, transformer oil, etc. Another recommendation is distilled water, when higher rates of material removal are desired. Kerosene is the most used dielectric fluid in EDM machining processes.

Different types of fluids perform differently in EDM machining processes. Kerosene possesses a low material removal rate and a medium wear ratio. Distilled water has a medium material removal rate and a least wear ratio. Tetraethylene glycol possesses a high material removal rate and a high wear ratio. The dielectric fluid must be filtered first before reuse so that removed metal from the work material and tool electrode is separated. This ensures efficient performance during the process.

Flushing of Dielectric

Flushing is the proper circulation of the dielectric fluid at the gap that is between the workpiece and the electrode tool in EDM machining. The efficiency of the cutting process depends on the flushing of the dielectric fluid to a greater extent. To achieve good machining conditions, good flushing is essential in EDM machining.

The dielectric fluid gets contaminated with eroded metal particles during the machining process. The dielectric fluid also gets contaminated with carbon particles resulting from the cracking of the dielectric fluid due to heat. This contamination, in turn, will reduce the insulation strength of the dielectric fluid, leading to early discharge of the spark. If the contamination exceeds the permissible level, then bridges are formed in the tool and the work gap. This will lead to short circuiting and damage to the tool and the work surface. Such bridges are eliminated by proper flushing in the gap.

The following are different methods of flushing in EDM machining:

Pressure Dielectric Flow

Pressure flow types are a broadly used method for circulating dielectric fluid in EDM. The fluid is forced to pass through the gap between the tool and the workpiece, by forcing it through holes in the electrode. Under pressure, the dielectric fluid flushes out the solid metal particles and cools the workpiece and the tool electrode. A needle-like work material is left off in the electrode hole. This will then be removed afterward to get a clean machined surface.

Reverse Dielectric Flow

In this method of flushing, the dielectric fluid flows from the outside of the electrode tool from the bottom. In both reverse and pressure dielectric flow, a taper is formed at the mouth of the cavity.

Vacuum Dielectric

This method produces straight holes in the workpiece. A vacuum pump is used to create a vacuum, to draw the dielectric fluid around the tool electrode, which passes through a hole at the center. This type of method leaves a central needle like work material which will be removed afterwards in order to have a clean machined surface.

Vibration Flush Cut

In this method, the flushing action to the fluid is produced by vibrating the tool. This method is suitable for small tools that can not accommodate a fluid passage for the flux of the fluid. This method is the most suitable for deep hole drilling of small diameter.

Pump Component

A pump is used as a passage for the dielectric fluid to flow from the base of the container to the tool and the part to be machined such that more MRR takes place.

Filter in a Machine

A filter removes any irregularities or dust particles that may be present in the dielectric medium. The filter is situated just above the pump.

Tool Holder

The tool holder serves to hold the tool properly.

Spark Generation

Between the tool and the part to be machined, a spark is generated, in the presence of a dielectric medium. Therefore, the removal of material occurs from the surface of the workpiece.

Electrode Tool

Any electrical conducting material can work as an electrode tool in EDM machining. The shape of the tool is transferred in the cavity cut during machining in EDM machining processes. Thus the shape and accuracy of the machined surface are fully dependent on the electrode’s shape and accuracy. Erosion takes place on both the tool and the workpiece during cutting. The wear ratio range is usually between 5: 1 and 100: 1.

The wear of the tool compared to the wear of the workpiece i.e. the material removed is called the wear ratio. The physical and chemical properties of the tool and work material, the fluid used as the dielectric and the operating conditions of the machining process determine the wear ratio. The wear ratio of the workpiece and tool are affected by the melting point of the tool and work.

Wear ratio is given by: Wr=2.25MT-2.3
Where, Wr is the work/tool wear ratio; and MT is the Work/tool melting point ratio

Wear ratio is given by: Wr=2.25MT-2.3 Where, Wr is the work/tool wear ratio; and MT is the Work/tool melting point ratio

The wear ratio of the work and tool reduces as the cross sectional area of the work and tool increases. But the wear ratio is increased by higher cutting rates, when machining sintered hard metals like vanadium and molybdenum steels. Higher wear ratio results in narrow deep sections with sharp corners. To minimize tool wear, reverse the polarity and use copper tools. EDM electrode tools are made by casting, machining or by powder metallurgy. Tools with a diameter as small as 0.1mm have been successfully used in EDM processes.

EDM Tool Material

EDM tools must exhibit high melting and vaporization temperature or high thermal conductivity like graphite or copper respectively. The tool materials must also exhibit other properties like the ease of fabrication or shaping and wear resistance. Cost is another consideration when choosing a material to be used as a tool material in EDM machining.

The following are important factors that determine the suitability of a material for application as an electrode tool in EDM machining:

• Higher metal removal rate
• Lower tool wear
• High degree of electrical efficiency

Although any electrically good conductive material can serve as an electrode material, considering the essential factors and properties for better application in EDM, graphite has been the most used as a tool material. Graphite exhibits a low wear rate with a higher degree of electrical efficiency and it is relatively cheap and easy to fabricate.

Good structured isotropic grain graphite that has fine grains, possessing the same current carrying characteristics, together with wear characteristics in any direction, make an excellent material for the tool electrode in EDM Machining. Copper, copper-tungsten, and brass are also utilized as EDM tool materials. To obtain minimum tool wear in EDM, graphite is used as an electrode with a frequency of around 3000 kHz and a power supply of 30A.

To control the actual wear of the graphite tool, it is coated with steel. But the no-wear machining condition results in a machined surface that is very rough for most applications and a finishing cut is constantly needed. The power supply to the system determines the major operating factors of material removal rate, wear ratio and machining stability. The selection of material for tool electrodes is also governed by the type of machine used, not only on the work material that is to be cut.

A certain amount of clearance must be provided on the tool side to obtain the correct size of the work produced afterwards, although the electrode tool in EDM machining is designed to act as a mirror image of the size and shape of the work to be produced. In designing the tool electrode, this is an important consideration.

The amount of clearance provided primarily depends on the tool material, material removal rate and the workpiece’s material. It is also dependent on the nature of the cut like roughing or finishing operation. The side clearance to be provided in machining hardened steel with brass as a tool material is 0.25 mm; 0.3 mm side clearance with duralumin and for graphite electrodes the side clearance will be 0.35 mm. The clearance to be provided with a high removal rate (rough cut) is 0.5 mm, 0.3 mm for medium rate, and 0.05 mm for the slow rate of cutting (fine surface).

Tool Feed Control

During the course of the EDM machining process, the material is removed from the part to be machined and the electrode tool. This increases the gap that is between the work and the tool. The tool must be fed properly using a device that controls the feed in order to maintain a constant gap and arcing voltage to prevent the occurrence of a short circuit.

It is essential for the system to respond rapidly, with a low inertia drive. If there is any overshooting, it will close the arc gap causing short circuiting. Hence, a rapid reversing feed motion is required. A signal obtained from an electrical sensor that responds to gap voltage or working current, provides actuation of the control.

Voltmeter Device

The voltmeter is a device for measuring voltage. Voltmeters are used to measure voltage in the EDM system.

Ammeter Measuring Device

An ammeter is used to measure or check the flow of current. The ammeter must be connected in order for us to check whether the current is flowing or not.

Servo Controlled Feed

A servo control mechanism automatically maintains a constant gap that is about the thickness of human hair between the work material and the tool. This mechanism is found in both wire and vertical EDM machines. It is important to ensure that there is no physical contact of the electrode and the work, otherwise arcing would occur. This in turn would result in the damaging of the workpiece and also breaking of the wire. As the operation progresses, the servomechanism advances the electrode into the work material and controls the work-wire to maintain the proper arc gap, by sensing the work-wire. The proper arc gap maintained is essential to a successful operation of the EDM.

Table Component

The table serves to hold the material to be machined (i.e. the workpiece).

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Chapter 3: Types of EDM Machines

This chapter will discuss two main types of EDM machines.

Conventional EDM Machines

This type of EDM machine is also known as sinker EDM, die sinking, volume EDM, ram EDM, and cavity-type EDM. This type of EDM is popular because of being suitable for creating complex shapes.

Conventional EDM makes use of machining to create a distinctive shape of an electrode. The electrode is then sunk deep into the material to be machined. A negative impression of its shape is created by the electrode, thereby forming a mold.

Capabilities and Applications of Conventional EDM Machines

Conventional EDM uses shaped electrodes, therefore it is useful for making dies and molds in particular. It is also suitable for small-batch production (production of prototypes). It has many applications in industries such as automotive and aerospace industries because it can accurately produce complex engine parts. It is also mostly used in a great diversity of industries for injection molding processes

Wire EDM Machines

These machines are also known as wire burning, spark EDM, and wire erosion. They use a thin heated wire as an electrode. Hard diamond is used as a guide to hold the wire steady. The wire electrode is moved through the workpiece to craft a particular shape, but only the electrical discharges of the wire do touch the workpiece; the wire itself does not touch the workpiece. In this type of EDM, the wire is put in slow motion.

In wire EDM, the wire is always available for cutting a smooth, uninterrupted form because the wire constantly unspools from an automated feeder. Sometimes, a cut through the middle rather than along the outside is required by a shape. In cases like these, wire EDM is coupled by hole-drilling EDM by machinists. As the name of this technique indicates, hole-drilling EDM involves drilling a small hole through the center of the work material. The wire can then be threaded through the hole to continue its shaping accurately. In cases like these, the electrodes’ shapes are tubes, and a dielectric fluid flows through the electrodes to the hole.

Capabilities and Applications of Wire EDM Machines

This type of EDM offers a few distinct capabilities and advantages. They provide performance that is robust, reliable, cutting edge, while remaining user-friendly. The following are advantages of wire EDM over conventional EDM:

Ease of production
Speed
Low maintenance

Ease of production Speed Low maintenance

With traditional EDM, the electrodes are prone to erosion and must be regularly replaced when they become too worn to function. Traditional EDM also needs the machining of electrodes of particular shapes, and this additional pre-machining consumes much time. On the other hand, wire EDM is ready to start as soon as the wire is put in place and does not need the time and expenditure on material, which is associated with pre-machining. It is suitable for applications that are time sensitive and shapes in which the machining of matching electrodes would present challenges. It is also mostly used in extrusion dies.

Chapter 4: Applications, Advantages, and Disadvantages of EDM Machining

This chapter will discuss the applications, advantages, and disadvantages of EDM machining. Aspects to consider when choosing the right EDM machining will be discussed.

Applications of EDM Machining

• It is used to make holes with sizes that are less than 0.1 mm.
• It is utilized in die sinking or die manufacturing, fixtures, and gauging.
• It is used to make holes in air brakes or pneumatic brakes.
• It is used in press tools and extrusion dies.
• It is utilized in die molds for plastics.
• It is applied in die-casting dies and mold inserts.
• EDM machining is utilized in the repair of worn dies for hot and cold forging and re-machining.
• It is used to make forging dies like connecting rod forging dies etc.
• EDM machining is used in Sintering dies.
• It is used in calibrating tools.
• It is utilized in the shaping of carbide tools and templates.
• It is used in the following applications: Aerospace; Medical; Electronics; Semiconductor.
• It is used in the manufacture of hard Electrodes.
• It is used in the manufacture of micro-tooling for Micro-EDM, Micro-USM and such other applications for micromachining.

Advantages of EDM Machining

• It can be used even in the heat-treated condition, and for any hard material.
• Complicated shapes made on the tool are able to be reproduced.
• A high accuracy can be achieved; about 0.005 mm.
• Economically, a good surface finish can be achieved; up to 0.2 microns.
• Machining time is not more than the conventional machining process.
• Mechanical stresses are not developed in this process (the tool and work do not have contact).
• The tool life is longer due to proper lubrication and cooling.
• Hard surfaces that are also resistant to erosion can be developed easily on the dies.
• It can be applied to any material that is electrically conductive.

Disadvantages of EDM Machining

• Excessive wear of the tool
• The power consumption is too high
• Reproduction of the sharp corner is not possible
• Change in metallurgical properties of materials due to the development of high heat.
• The material to be machined must be an electrical conductor
• Requirement for redressing of a tool for deep holes
• Over-cut is formed
• Expert machinists are hard to find

However, it must be noted that the disadvantages of EDM machining are far outweighed by the advantages.

Choosing the Right EDM Machine

• When opting for a wire EDM, it is important to pay attention to the latest cheaper features. For example, choose an equipment with an operational mode that assists in reducing wire consumption by lowering the infeed rate of the wire.
• Pay attention to the hole tapering capabilities
• Use the right dielectric fluid
• The ease of maintenance must be taken into consideration.

Conclusion

Electrical discharge machining is a process that consumes high power; hence the generators used in EDM machining must be able to supply a lot of power for the process to run efficiently. Careful considerations must be made when opting for a generator to use in EDM Machining. The type of fluid used as a dielectric fluid is another factor that must be taken into consideration, as it has been seen that different dielectric fluids possess different wear ratios and different material removal rates. A dielectric fluid of high material removal rate is required but also factoring in the wear ratio of the dielectric fluid. EDM machining offers a great diversity of benefits such as high accuracy, good surface finish, though EDM machining has disadvantages that must be noted when applying EDM machining.

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