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Unit 3. ELECTRICAL ENERGYTECHNIQUES

ME0028 NON TRADITIONAL MACHININGTECHNIQUES

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Unit 3 - ELECTRICAL ENERGY TECHNIQUES

Electro Chemical Machining (ECM): Operatingprinciples Equipment and sub systems Parametersinfluencing metal removal Benefits and Applications

Advantages and Limitations current developments inECM.

Electro Chemical Grinding (ECG): Operating principles

Equipment and sub systems Parameters influencingmetal removal Benefits Applications Advantagesand Limitations

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Electro Chemical Machining (ECM):

Electrochemical Machining (ECM) is a non-traditionalmachining (NTM) process belonging to Electrochemical

category.

ECM is opposite of electrochemical or galvanic coating

or deposition process.

Thus ECM can be thought of a controlled anodicdissolution at atomic level of the work piece that is

electrically conductive by a shaped tool due to flowof high current at relatively low potential differencethrough an electrolyte which is quite often water basedneutral salt solution.

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Principle of operation of ECM:

During ECM, there will be reactions occurring at theelectrodes i.e. at the anode or workpiece and at thecathode or the tool along with within the electrolyte.

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Let us take an example of machining of low carbon steelwhich is primarily a ferrous alloy mainly containing iron.

For electrochemical machining of steel, generally aneutral salt solution of sodium chloride (NaCl) is takenas the electrolyte.

The electrolyte and water undergoes ionic dissociationas shown below as potential difference is applied

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As the potential difference is applied between the workpiece (anode) and the tool (cathode), the positive ionsmove towards the tool and negative ions move towards

the workpiece.

Thus the hydrogen ions will take away electrons fromthe cathode (tool) and from hydrogen gas as:

Similarly, the iron atoms will come out of the anode

(work piece) as:

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Within the electrolyte iron ions would combine withchloride ions to form iron chloride and

similarly sodium ions would combine with hydroxyl ions

to form sodium hydroxide

In practice FeCl2 and Fe(OH)2 would form and get

precipitated in the form of sludge. In this manner it can be noted that the work piece gets

gradually machined and gets precipitated as the sludge.

Moreover there is not coating on the tool, only hydrogen

gas evolves at the tool or cathode. . Fig. 2 depicts the electro-chemical reactions

schematically.

As the material removal takes place due to atomic level

dissociation, the machined surface is of excellent

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Equipment

The electrochemical machining system has the followingmodules:

1. Power supply

2. Electrolyte filtration and delivery system

3. Tool feed system

4. Working tank

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2-30V dc of the order of 50 to 40,000 amps is suppliedacross the anode and the cathode.

The electrolyte flows in the gap between the tool and

the work piece at a velocity of 30 to 60 m/s. The temperature of the electrolyte is maintained at

about 40C.

In operation, the tool (cathode) is moved towards the

workpiece (anode) and the electrolyte by electrolysisprocess dissociates into ions carrying positive andnegative' electrical charges.

The cathode attracts the positively charged ions

(cations) from the electrolyte, while the negativelychanged ions (anions) move towards the anode, therebycompleting the electric circuit.

The anions cause the anode to dissociate and dissolveinto the electrolyte.

These dissolved metal ions are continuously carried

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The commonly used electrolytes are sodium chloride(NaCl), sodium nitrate (NaN03) potassium chloride(KCl), sodium hydroxide (NaOH), sodium fluoride

(Na2F) and sulfuric acid (H2S04).

Functions of Electrolytes:

The electrolyte is the working medium and part of toolingin an ECM process. It performs three importantfunctions:

a) It carries the current between the tool and the workpiece .

b) It carries away the products of machining from thework tool gap.

c) It removes the heat produced in operation from the

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Characteristics of ECM:

Tool and work material electrically conductive.

Atomic level dissolution.

Surface finish excellent.

Almost stress free machined surface.

No thermal damage.

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ELEMENT OF ECM PROCESS:

1. Cathode tool (of a shape which is almost the mirror image

of the cavity to be machined into the work piece).2. Anode work piece.

3. Source of D.C power.

4. Electrolyte, a conductive liquid.

Advantages of Nacl or KCl electrolyte are

1. Inexpensive.

2. Non toxic.

3. No fire hazard.4. Can be used for variety of materials.

5. Current efficiency is high.

6. At high concentration conductivity is easy to control.

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MODELLING OF MATERIAL REMOVAL RATE :

Material removal rate (MRR) is an important characteristic toevaluate efficiency of a non-traditional machining process.

In ECM, material removal takes place due to atomicdissolution of work material. Electrochemical dissolution isgoverned by Faradays laws.

The first law states that the amount of electrochemicaldissolution or deposition is proportional to amount ofcharge passed through the electrochemical cell, which

may be expressed as:where W = mass of material dissolved or deposited

The second law states that the amount of material

deposited or dissolved further depends on

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PROCESS PARAMETERS

Power Supply

Type direct current

Voltage 2 to 35 V Current 50 to 40,000 A

Current density 0.1 A/mm2 to 5 A/mm2

Electrolyte

Material NaCl and NaNO3 Temperature 20oC 50oC

Flow rate 20 lpm per 100 A current

Pressure 0.5 to 20 bar

Dilution 100 g/l to 500 g/l

Working gap 0.1 mm to 2 mm

Overcut 0.2 mm to 3 mm

Feed rate 0.5 mm/min to 15 mm/min

Electrode material Copper, brass, bronze

Surface roughness, Ra 0.2 to 1.5 m

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The important process variables that affect the ECMoperation are as follows:

1) Voltage: The voltage across the cutting gap between

the tool and work influences the current and hence thematerial removal rate. This is the primary controllingfactor in most ECM operations. However for a givenconstant voltage, current also depends on theelectrical resistance between the gap, which is furthera factor of 'the conductivity of the electrolyte and thegap size.

2) Feed Rate: This .is the rate of penetration in ECM

process. For a given voltage, both the frontal gap andside gap (see Fig. 2-8) are inversely proportional to thefeed rate. The distance across the frontal gap is afunction of the feed rate. As the gap reduces, the

resistance drops, increasing the amperage and thusincreasin the machinin rate. Similarl side a also

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3. Current density: The feed rate varies directly with thecurrent. Higher machining rates requires higher currentdensities .with higher voltages which in turn increases

the power consumption

4) Electrolyte flow rate: Electrolyte flow rate is importantin controlling the machining rate, and temperaturecontrol of the ECM process. Flow rate also has aninfluence on the level of turbulence and hence on thesurface finish and taper on the work material. The flowshould carry away the sludge formed along with it.

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Advantages

Very hard materials can be machined.

Complex shapes can be produced. Tool wear is negligible.

High surface finish can be obtained.

Very thin metals sheets can be machined.

Residual stresses induced are almost nil, and no distortion. There is no cutting forces therefore clamping is not required

except for

controlled motion of the work piece.

There is no heat affected zone. Very accurate.

Relatively fast and a time saving process.

During drilling, deep holes can be made or several holes at

once. ECM deburrin can debur difficult to access areas of arts.

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Limitations:

More expensive than conventional machining.

Need more area for installation.

Electrolytes may destroy the equipment.

Not environmentally friendly (sludge and other waste)

High energy consumption.

Material has to be electrically conductive.

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APPLICATIONS:

ECM is used for machining hard metals and alloys, formaking dies and tools, machining of complex contours,

machining of tungsten carbide and high strength heatresisting alloys. The important applications are-

Machining hard heat resistant alloys.

Machining of thorough holes of any cross section.

Machining of shaped cavities (like forging dies). Machining of complex external shapes (like turbine

blades).

Wire cutting of heavy slugs of metals.

Machining of blind holes (regular and irregular shapes).

Die sinking

Profiling and contouring

Trepanning

Grinding

Drilling

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