Electrical Discharge Machining (EDM) - Principle, Equipment, Types and Working

Electrical Discharge Machining (EDM) is a non traditional machining and electro thermal process in which material from the workpiece is removed by using electrical discharges (sparks).
  • It was first observed in 1770 by Joseph Priestley. He was an English physicist.
  • In EDM machine the material is removed by rapidly recurring (repeating) discharges of current in between the electrodes. The electrodes are separated by dielectric liquid and a high voltage is applied across it.
  • It is used to machine those materials which are difficult to machine and have high strength temperature resistance.
  • EDM can be used to machine only electrically conductive materials. Otherwise it cannot be used.
  • One of the electrodes is called as tool and other is called as workpiece. Here the tool is connected with the negative terminal of the power supply and the workpiece is connected with the positive terminal.

Working Principle


In Electrical discharge machining; a potential difference is applied across the tool and w/p in pulse form. The tool and workpiece must be electrically conductive and a small gap is maintained in between them. The tool and workpiece is immersed in a dielectric medium (kerosene or deionized water). As the potential difference is applied, electrons from the tool start to move towards the workpiece. Here the tool is negative and w/p is positive. The electrons moving from the tool to the w/p collide with the molecules of dielectric medium. Due to the collision of electrons with the molecule, it gets converted into ions. This increases the concentration of electrons and ions in the gap between the tool and w/p. The electron moves towards the w/p and ions towards the tool. An electric current is set up in between the tool and w/p and called as plasma. As the electrons and ions strikes the w/p and tool, its kinetic energy changes to heat energy. The temperature of the heat produced is about 10000 degree Celsius. This heat vaporizes and melts the material from the workpiece. As voltage is break down, the current stops to flow between the tool and w/p. And the molten material in the w/p is flushed by circulating dielectric medium leaving behind a crater.

The spark generation is not continuous because constant voltage is not applied across the electrodes. The voltage is applied in pulse form.

Also Read: Electron Beam Machining Process

Types of EDM

There are two types of EDM machine

(i) Ram/Sinker EDM : This EDM machine consists of tool and workpiece immersed in a dielectric medium. It consists of ram type tool and it may be created according to the shape or form required to produce on the workpiece. It is also called as cavity type or volume EDM.

(ii) Wire EDM: In wire EDM, thin single-strand wire is used to cut the material from the workpiece. The wire is usually made of brass. A constant gap is always maintain between the wire and w/p. The wire is continuously fed through the workpiece submerged in a tank with dielectric medium. Here spark is generated in the gap between the wire and workpiece. It is used to cut metal as thick as 300 mm and to make punches, dies, and tools from hard metals that are difficult to cut from other methods.

Equipment

The various equipments used in EDM are

1. Dielectric Reservoir, Pump and Circulating system

Pump is used to circulate the dielectric medium between the two electrodes ( tool and workpiece). Kerosene or deionized water is used as dielectric medium.

2. Power Generator and Control Unit

Generator is used to apply potential difference. The voltage used in this machining process is not constant but it is applied in pulse form. A control unit is used to control the different operation during machining process.

3. Working Tank with Work Holding Devices

It has working tank with a work holding device. The workpiece is hold in the work holding devices. The tank contains dielectric medium.

4. Tool Holder

It is used to hold the tool.

5. Servo System to Move the Tool

 A servo system is used to control the tool. It maintains the necessary gap between the electrodes ( tool and workpiece).

Working of Electrical Discharge Machining (EDM)

  • In EDM, first the tool and w/p is clamped to the machine. After that with the help of a servo mechanism a small gap (of human hair) is maintain in between the tool and workpiece. 
  • The tool and workpiece is immersed in dielectric medium (kerosene of deionised water). 
  • A potential difference is applied across the Electrode. An electric spark is generated in between the tool and workpiece. This spark generates a heat of about 10000 degree Celsius. And due to this heat the material from the workpiece starts to vaporize and melts.
  • The spark generation in electrical discharge machining is not continuous. As the voltage breaks, the dielectric fluid flushes away the molten materials leaving behind a crater.
  • This process keep continue and machined the workpiece.

Advantages and Disadvantages

Advantages

  • It can be used to machine any material that is electrically conductive.
  • It can easily machine thin fragile sections such as webs or fins without deforming the part.
  • Complex dies sections and molds are produced accurately, faster and at lower price.
  • It is burr-free process.
  • It does not involve contact between the tool and workpiece. So delicate sections and work material can be machined easily without any distortion.
  • It can machined complex shapes which is not manufactured by the conventional machine tools.
  • It can produce tapered holes.

Disadvantages

  • It can machine only electrically conductive materials.
  • Low rate of metal removal.
  • More tool wear during machining.
  • Takes extra cost and time for the preparing electrodes for ram/sinker EDM.
  • High power consumption.
  • Overcut is formed in EDM.

Application

  1. It is mostly used by mold making and dies industries.
  2. It is used in prototype manufacturing in aerospace, automobile and electronic industries.
  3. It is used for coinage die making.
  4. It is used to create small holes in variety of application.
  5. It is used to disintegrate parts which cannot be disintegrate easily such as broken tools (studs, bolts drill bit and taps) form the workpiece.
This is all about the Electrical Discharge Machining - principle, equipment, types, working, advantages and disadvantages with application. If you have any query regarding this than comment us. If you find this article valuable than share it on Facebook and Google+.


What is Carburetor - Parts and Working?

Today we will learn about what is carburetor - parts and its working. As we know that spark ignition engines used volatile liquid fuels such as petrol for its working. Petrol is high volatile and its ignition temperature is quite below the temperature occurs in engine cylinder before full compression. Hence, mixing of air and fuel can’t possible inside the engine cylinder of SI engines. But we require properly mixed, homogenous mixture of air-fuel in correct ratio for proper combustion. This needs a device which can mix air-fuel in correct ratio and form a homogenous mixture outside the cylinder. This device is known as Carburetor.

What is Carburetor?


Carburetor is a device which is used in spark ignition engine to mix air-fuel in correct ratio outside the cylinder. This process is known as carburation.

Carburetor Parts


What is Carburetor - Parts and Working?

1. Float Chamber:    


The float chamber serves as a storage tank of fuel for continuous supply of fuel. It contains a float valve which maintains the level of fuel in float chamber. When the level of fuel decreases in float chamber the float moves downward, which open the fuel supply valve and allow flow of fuel into float chamber. As the fuel level increases, the float moves upward which close and stop the fuel supply. This fuel level is maintained below the discharge nozzle outlet hole to prevent overflow.

2. Strainer:


It is a device which is used to filter the fuel before entering into float chamber. It consist a fine wire mesh which filters the fuel and removes dust and other suspended particles from it. These particles if not removed can cause blockage of nozzle.

3. Metering System:


The metering system controls the flow of fuel into nozzle. It is responsible to form correct mixture of air fuel.  It consist two main parts, first one is known as metering orifice and other one is known as fuel discharge nozzle.
When the air passes through venturi, it generates a low pressure field across throat compare to pressure at float chamber. Due to this pressure difference, fuel is discharge into the air stream. The quantity of fuel is control by the metering orifice and discharge hole at the exit of fuel discharge nozzle.

4. Idling System:


It consist a passage directly from the float chamber to venturi tube. It provides rich mixture during idling and at low speed. It works during idling or when the throttle is open below 15%.

5. Throttle Valve:


It is a butterfly valve situated at the exit of the venturi tube. It controls the speed of the vehicle by providing control amount of mixture. It controls the quantity of air fuel mixture. If throttle is fully opened, than more mixture drawn into cylinder and thus gives high output. But if it is little opened, less mixture is drawn into the cylinder, which gives less power.

6. Choke Valve:


It is same as throttle valve in construction but situated at the entrance of venturi tube. It is used to provide very rich mixture during starting in cold season. It controls the quantity of air flow through the venturi tube. If the choke is fully open, normal amount of air flow through venturi, which forms a normal mixture. But if the choke is partially closed, it results low amount of air flow through venturi and large amount of fuel flow through discharge nozzle. It gives rich mixture.

Working of Carburetor:


Now we know about basic parts of carburetor and its function. These all parts work together to perform a common function of providing homogenous air-fuel mixture in proper ratio. Its working can be summarized into following points.

  • First fuel is supplied into the float chamber through strainer. Strainer works as a filter. It does not allow dust and other suspended particles into the float chamber which can choke any fuel passage.
  • The float maintains a constant level of fuel into float chamber. If the amount of fuel in the float chamber goes down below designed limit, the float goes down which opens the fuel supply valve and allow fuel to flow into float chamber. If the fuel reaches designed limit, the float goes up, which closes the fuel supply valve and thus stop fuel supply into float chamber.
  • The fuel discharge nozzle connects float chamber to venturi tube. The one end of fuel supply nozzle connected to the bottom of the float chamber and other one is to the venturi tube slightly above the designed fuel level in the float chamber. This will avoid overflow when engine is not running.
  • During suction stroke air is drawn into cylinder through venturi tube. Venturi is a tube of decreasing cross section and has a minimum area at throat. The fuel supply nozzle connects at the throat of venturi tube.  This air has maximum velocity at throat. Due to this high velocity, the pressure at the throat goes down below float chamber pressure.
  • This will create a pressure difference between float chamber and venturi tube. This pressure difference is known as carburetor depression. It acts as driving force for fuel. It drives fuel from float chamber to venturi tube through fuel supply tube and the fuel is discharged into the air stream.
  •  The fuel-air ratio depends on the size of discharge jet and metering system. So they are chosen as such, they can give desired air-fuel ratio.
  • This air fuel mixture provided to the cylinder through throttle valve. The SI engine is a quantity governed engine. So the quantity of the mixture provided into the cylinder is controlled by the throttle valve and hence control output power.
  • For idling or when required rich mixture, extra fuel is supplied by the idling system into venturi tube.
This is all about what is carburetor main parts and working. If you have any query regarding this article, or you find anything missing or wrong, ask by commenting.   

Author Bio:


Mitesh Bairwa is Mechanical Engineer by profession and blogger by hobby. He is owner of www.mech4study.com.


Electron Beam Machining Process

Electron Beam Machining is process in which high velocity electrons are concentrated in a narrow beam and then directed towards the workpiece for machining. When this high velocity electron strikes the workpiece, it melts and vaporizes the material from the workpiece.

Working Principle

In an electron beam machining, the electrons strike the workpiece with a high velocity. As the electron strikes the workpiece, the kinetic energy of the electron changes into heat energy. The heat energy so produced is used to melt and vaporize the materials from the w/p. The whole process takes place in vacuum. Vacuum environment is used to prevent the contamination and avoid collision of electrons with air molecules. Ff the electrons collide with the air molecules, it will lost its Kinetic energy.

Diagram

Electron Beam Machining

Equipment

The various equipment used in EBM machine are

1. Cathode

The cathode is negatively charged and it is used to produce Electrons.

2. Annular Bias Grid

It is present next to the cathode. Annular bias grid is a circular shaped bias grid and prevents the diversion of electrons produced by the cathode. It works as a switch and makes the electron gun to operate in pulse mode.

3. Anode 

It is placed after the annular bias grid. It is positively charged. Annular anode attracts the beam of electron towards it and gradually the velocity of the electron increases.  As the electron beam leave the anode section, its velocity becomes half of the velocity of light.

4. Magnetic Lenses

The magnetic lenses reduce the divergence of electron beam and shape them. It allows only convergent electrons to pass and captures the low energy divergent electrons from fringes. It improves the quality of the beam.

5. Electromagnetic Lens

It helps the Electron beam to focus on the desired spot.

6. Deflector Coils

The deflector coil carefully guides the high velocity electron beam to a desired location on the  workpiece and improves the shape of the holes.

Working Process

  • In Electron Beam Machining, first the electron is generated by the cathode and an annular biased grid does not allows the electron to diverge.
  • From the annular bias grid, the electron produced by the cathode is attracted towards the anode and gradually its velocity increases. As the electron beam leaves the anode section, its velocity reaches to half of the velocity of the light.
  • After that, it passes to the series of magnetic lenses. The magnetic lenses allows only convergent beam to pass through it and captures the divergent beam from the fringes. And then a high quality electron beam is made to pass through the electromagnetic lens and deflector coils.
  • The electromagnetic lens focuses the electron beam to the desired spot on the workpiece. The deflector carefully guides the beam to the desired locations and improves the shape hole.

Characteristics of Electron Beam Machining

(i) The EBM machine is operated in pulse mode and this is achieved by the biasing annular biased grid.
(ii) The beam current can be as low as 200 μamp to 1 amp.
(iii) The pulse duration achieved in the EBM machine is 50 μs to 15 ms.
(iv) The energy possessed by the pulse is 100 j/pulse.
(v) It utilizes voltage in the range of 150 kV to 200 kV. And this voltage is used to accelerate Electrons to about 200,000km/s.

Advantages and Disadvantages

Advantages:

  • It can produce bolts of small sizes.
  • High accuracy and better surface finish.
  • Almost all types of materials can be machined.
  • Highly reactive metals such as Al and Mg can be machined easily.
  • As it does not apply any mechanical cutting forces on the workpiece, so cost of work holding and fixtures is reduced.

Disadvantages

  • High equipment cost.
  • Low metal removal rate.
  • High skilled operator is required.
  • High power consumption.
  • Not applicable to produce perfectly cylindrical deep holes.

Application

It is used to produce smaller size holes in various industries like automobile, aerospace, marine etc.

What is Stress Concentration - Definition, Causes, effects and Prevention?

Stress concentration is the accumulation of stress in a body due to sudden change in its geometry. When there is a sudden change in the geometry of the body due to cracks sharp corners, holes and decrease in the cross section area, then there is an increase in the localised stress near these cracks, sharp corners, holes, and decreased cross section area. The body tends to fail from these places where the stress concentration is more. So to prevent a body from getting failed, the stress concentration should be avoided or reduced.

It is also called as stress raisers or stress risers.

What is Stress Concentration

Cause:

The stress concentration in a body happens because of sudden change in the geometry of the body due to cracks, sharp corners, holes, decrease in the cross section area. Due to these irregularities, there is an increase in the intensity of stress in the body.

Effect

When a body has stress concentration in it, the chances of its failure increases. The body tends to fail from the place where it is has more concentration of stress. A body has less life that has more irregularities within it. In order to increase the life of the body, the intensity of stress should be reduced.

Stress concentration factor

The stress concentration factor is defined as the ratio of highest stress in the body to the reference stress. It is denoted by Kt.
Mathematically,


Where
σmax = Highest stress or maximum stress
σref = Reference stress

The value of stress concentration factor for
(i) A body free from irregularities is 1.
(ii) A body that has maximum irregularities or discontinuity is greater than 1.

Methods to Reduce Stress Concentration

1. There are no of ways to reduce stress concentration in a body and some of these are
Avoiding sharp corners by providing a fillet radius at the sharp corners. By providing the fillet radius at sharp corners, the cross section area decreases gradually instead of suddenly. And this distributes the stress in the body more uniformly. This is shown in the figure given below.

Methods to Reduce Stress Concentration


2. By providing small holes near big hole. If we have an object, that has an internal hole within it. Then the intensity of stress near that hole is more. To avoid this, some smaller holes are created near that hole. This distributes the stress more uniformly than it was before. This is shown in the figure given below.

Methods to Reduce Stress Concentration


3. By decreasing the nominal diameter of a threaded object and make it equal to the core diameter. Suppose we have a threaded object. And the intensity of stress at threaded part is more. The chances of object may fail is more at the threaded part. This can be avoided by decreasing the nominal diameter of the shank and make it equal to the core diameter. This will distribute the stress more uniformly in the object with threads.

Methods to Reduce Stress Concentration


4. By providing notches or undercut at the sharp corners.

Methods to Reduce Stress Concentration


Here we have discussed about what is stress concentration - definition, causes, effect, stress concentration factor and methods to reduce it. If you have any query about that than feel free to ask.
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Difference Between SI Engine and CI Engine

In this article we will discuss about the difference between SI engine and CI engine. The spark Ignition (SI) engine, as its name indicates uses spark to ignite the fuel. And in Compression Ignition (CI) engine, the air is compressed within the cylinder and the heat of this compression air is used to ignite the fuel. Since spark and compressed air is used to ignite the fuel in these engines, so they are called as spark ignition (SI) engine and compression ignition (CI) engine.
Difference Between SI engine and CI engine

SI Engine (Spark Ignition Engine)

Spark Ignition (SI) Engine is a type of engine in which the combustion takes place by the spark generated by the spark plug. It uses petrol as fuel and works on Otto cycle. In the spark ignition engine the air fuel mixture is inserted into the cylinder with help of carburetor. The compression of the fuel takes place but it has low compression ratio. The fuel is ignited by the spark generated by the spark plug. SI engine produces less noise and vibration and their starting is very easy. They are light in weight and have less maintenance cost. They are mostly used in light commercial vehicles such as scooters, motorcycles cars etc.

CI Engine (Compression Ignition Engine)

Compression Ignition (CI) Engine is an engine in which the combustion of fuel takes place by the heat of the compressed air. It uses diesel as fuel and works on Diesel cycle. In the compressed ignition engine, only air enters into the cylinder during suction stroke. It has high compression ratio because of the high ignition temperature of the diesel fuel. The fuel is ignited by the heat of the compressed air. Due to high compression ratio it produces more power. Due to incomplete combustion of the fuel, it produces more hydrocarbons which lead to air pollution. The noise and vibration problem is there in the CI engines. The maintenance cost of the CI engine is more as compared with the SI engines. They are mostly used in heavy duty vehicles such as buses, trucks, railways, ships etc.

Difference Between SI Engine and CI Engine in Tabular Form

S.no
Parameter
SI Engine
CI Engine
1.
Definition
It is an engine in which the spark is used to burn the fuel.
It is and engine in which heat of compressed air is used to burn the fuel.
2.
Fuel used
Petrol is used as fuel.
Diesel is used as fuel.
3.
Operating cycle
It operates on Otto cycle.
It operates on Diesel cycle.
4.
Compression ratio
Low compression ratio.
High compression ratio.
5.
Thermal efficiency
High thermal efficiency.
Less thermal efficiency.
6.
Method of ignition
Spark plug is used to produce spark for the ignition.
Heat of compressed air is used for the ignition.
7.
Engine Speed
High speed engines.
Low speed engines.
8.
Pressure generated
Low pressure is generated after combustion.
 High pressure is generated after combustion.
9.
Constant parameter during cycle
Constant volume cycle.
Constant pressure cycle.
10.
Intake
Air + fuel.
Only air.

Weight of engine
Si engine has less weight.
CI engine are heavier.
12.
Noise production
It produces less noise.
It produces more noise.
13.
Production of hydrocarbon
Less Hydrocarbon is produced.
More hydrocarbon is produced.
14.
Starting
Starting of SI engine is easy.
Starting of CI engine is difficult.
15.
Maintenance cost
Low
High
16.
Vibration problem
Less
Very High
17.
Cost of engine
Less cost
High cost
18.
Volume to power ratio
Less
High
19.
Fuel supply
Carburetor
Injector
20.
application
It is used in light commercial vehicles like motorcycle, cars etc.
It is used in heavy duty vehicles likes bus, trucks, ships etc.

So now, I hope you have clearly understood about the difference between SI engine and CI engine. If you have any confusion related to that than feel free to ask through your comments. If you find it informative or useful than share it for others.

Types of Springs

Since there are many types of springs but here I will discuss their types only on the basis of their shapes and how the load force is applied on them. The springs are defined as an elastic body which stores mechanical energy and gets distorted when loaded and regains its original shape when load is removed.  Springs main function is to get distorted when it is loaded and recover its original shape when load is removed.

The various applications of springs are as follows


  • To cushion, absorb or control energy due to shock and vibration as in bike or car springs, railway buffers, shock absorbers, aircraft landing gears and vibration dampers.
  • To apply forces, as in brakes, clutches and spring loaded valves.
  • To controls the motion by maintaining contact between two elements as in cams and followers.
  • To measure forces, as in spring balances and engine indicators.
  • To store energy, as in toys and watches.

Types of springs



On the basis of shape the types of springs are

1. Helical Springs or Coil Springs

It is a spring which is made up of a wire coiled in the form of helix. It is made to handle tensile and compressive loads.

2. Conical and Volute Springs

These are the compression spring have conical shapes. The conical springs are wounded with a uniform pitch whereas the volute springs are wounded in the form paraboloid with constant pitch and lead angles. Under compression, coils of these springs slide past each other and make the spring to compress to a very shorter length.

3. Torsion Springs

It is a spring that works on torsion or twisting. It stores the mechanical energy when twisted.

4. Laminated or Leaf Springs

It is a type of spring which is mostly used in automobile suspension, electrical switches and bows. It consists of a number of flat plates (known as leaves) of varying lengths held together by means of clamps and bolts

5. Disc or Belleville Springs

It is a disc shaped spring. It is commonly used to apply tension to a bolt. It is also called as Belleville washers and conical compression washers

On the basis of how the load force is applied springs are classified as

1. Tension or Extension Spring 

Tension or extension springs works on the application of tension loads. When tensile load is applied to this spring it extends to some length.

2. Compression Spring

The compression springs are designed to operate when compressive load is applied to it. It contracts under compression.

3. Torsion Spring

It is designed to work under twisting. It stores mechanical energy when twisted.

4. Constant Spring

It is a type of spring in which the supported load remains the same throughout the deflection cycle.

5. Variable Spring

Variable spring is a spring in which the resistance in the coil to load varies during compression.

This is all about different types of springs. If you find anything missing or incorrect than comment us. And if you find this article informative than don’t forget to like and share on Facebook and Google+


Difference Between Turbocharger and Supercharger

In this article we will learn about the difference between turbocharger and supercharger. They both are force induction system i.e. they compressed the air from the atmosphere and sends it to the engine cylinder. The compressed air allows double amount of fuel to enter into the cylinder for combustion. This doubles the power of engine. Turbocharger and supercharger perform the same operation, so most of us are unable to find difference between them. Here in this article I will try to clear all your doubts about turbocharger and supercharger.


What is Turbocharger?

It is a forced induction system that uses exhaust gases energy to compress the air form the atmosphere and sends it to the engine cylinder. The compressed air is rich in oxygen and so the quantity of the fuel entering into the cylinder is doubled. Now the fuel burnt into the cylinder produces double power as it was producing without the turbocharger.

The turbocharger is not directly connected to the engine. It works more efficiently on high speed and spins upto 150000 rpm. Its installation is not easy and has complex design as compared with the supercharger. It experiences lag problem due discontinuous energy supply from the exhaust.

The schematic diagram of turbocharger is shown below.          


    

 What is Supercharger?

It is also a forced induction system that compresses the air and sends it to the engine cylinder. it is generally placed on the top of the engine and directly connected to the engine crankshaft for its working. It doubles the power of engine. They are simple in design and installation. They can work on low rpm and its spin speed is upto 50,000 rpm. It more reliable and has negligible lag.

The schematic diagram of supercharger is shown below.


Difference between Turbocharger and Supercharger in Tabular Form

S.no
Turbocharger
Supercharger
1.
Turbocharger is a forced induction system that compresses the atmospheric gases and sends it to the engine cylinder.
Super charger is also a forced induction system. It compresses the atmospheric air and sends it to the engine cylinder.
2.
It uses exhaust gases for its energy.
It is connected to the crankshaft of the engine for its energy.
3.
It is not directly connected to the engine.
It is directly connected to the engine through belt.
4.
It has smog altering equipment which helps in lowering the carbon emission.
It doesn’t have wastegate, so the smog emits from the supercharger.
5.
It spins with a speed upto 150000 rpm.
It spins with a speed upto 50000 rpm.
6.
It is much quieter than supercharger.
It is not so quieter.
7.
It is less reliable.
It is more reliable.
8.
Maintenance is not easy.
Maintenance is easy.
9.
Turbocharger delivers their boost better at high rpm.
Supercharger can deliver their boost at lower rpm.
10.
It is more efficient.
It is less efficient.
11.
The compressed air in turbocharger has high temperature.
The compressed air in supercharger has less temperature.
12.
It requires intercooler for the compressed air to lower its temperature.
It may or may not require intercooler. But in some types, it requires intercooler.
13.
It is more complex.
It is less complex.
14.
It has lag problem due to discontinuous supply of energy.
It has negligible lag problem because of continuous supply of energy by crankshaft.
15.
The compressor is rotated by the turbine.
The compressor is rotated by the engine crankshaft through a belt.


For better explanation about difference between turbocharger and supercharger, must watch the video given below: