1. NAME: MOHAMMAD UMAIR ANSARI
TOPIC: ELECTRO CHEMICAL MACHINING
BRANCH: MECHANICAL
SUBJECT: UCMP
YEAR : 4-1
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2. ELECTRO CHEMICAL MACHINING
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3. It works on the faraday’s law of electrolysis. Need for ECM
As the name indicates it is the combination of electrical and chemical energy used for machining.
It works on the faraday’s law of electrolysis.
Need for ECM
hard & difficult to shape metals cannot be easily machined by conventional methods.
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4. It is characterized as reverse electroplating process
It removes material instead of depositing it.
ECM process makes use of an electrolyte & high electric current to ionize & remove metal atoms
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5. Elements or parts of ECM Process
The equipment mainly consist of following parts.
Cathode tool (-ve terminal)
Anode work piece (+ve terminal)
DC power & Control system
Electrolyte
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6. A standard overcut includes 0.76mm over all active surfaces
Cathode tool(-ve): the shape of the tool should be similar to the shape desired in the work piece. A round shaped tool will produce a round hole in the work piece, a square shaped tool produces a square hole & so on.
Since a small gap has to be left between the work piece & the tool for the electrolyte to flow
A standard overcut includes 0.76mm over all active surfaces
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7. The material selected for the tool should be easily machinable,exhibit good stiffness to resist high electrolyte pressures, resist chemical action, & good electrical and thermal conductor
The material that find wide applications in the manufacture of tools are Aluminium,brass, bronze,copper,carbon,stainless steel, titanium etc.
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9. Anode workpiece(+ve): there is no restriction in the workpiece material except that it must be good conductor of electricity.
The chemical characteristics of the electrolytes effects the metal removal rate.
The fixtures for holding the work are made of some insulating materials, like epoxy resins or glass fibers.
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10. Normal current requirements are as high as 800 amp/cm2
DC power & Control system: the process needs low voltage of the order of 2 to 20V,or 30V in rare cases
Normal current requirements are as high as 800 amp/cm2
Three phase ,440V,A.C power is converted to low voltage D.C by a step down transformer & a rectifier
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11. Electrolyte: An electrolyte is an substance containing free ions that make the substance elctrically conductive.
Electrolysis : Electrolysis is an electrochemical process in which direct current is passed through an electrolyte resulting an chemical reactions at the electrodes & electrolyte.
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12. Electrolyte performs following functions
Completes the electric circuit between the tool & the workpiece.
Acts as a conductor to carry current
Remove heat generated during the chemical reactions
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13. Properties of electrolyte and selection High electrical conductivity
Low viscosity & high specific heat
Chemical stability
Non-corrosive & non-toxic
Inexpensive &readily available
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14. Sodium chloride(Nacl),potassium chloride(Kcl), sodium nitrate(NaNO3),potassium nitrate(‎KNO3),sodium sulphate(Na2SO4),sodium chromate(Na2CrO4),sodium hydroxide(‎NaOH),sodium flouride(‎NaF‎), are use as electroytes In most application Nacl in water has been found to be satisfactory,but as with most electrolytes,its corrosiveness presents a problem.
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15. Sodium nitrate solution is also extensively used
Sodium nitrate solution is also extensively used. it has all the desirable characteristics & is less corrosive in nature then sodium chloride. sometimes sulphuric acid (Hcl) is used in some cases to produce good surface finish
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16. Theory of ECM
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19. The basic chemical reactions taking place for say Nacl as electrolyte & iron as workpiece Nacl Na+ + cl- H2O H+ + OH- Hydrogen ions(H+) takes away electrons from the cathode(tool) & forms hydrogen gas 2H+ + 2e- H2 at cathode
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20. The iron atoms come out of anode(w/p) as Fe Fe ++ + 2e-
iron ions combine with the chloride ions to form ion chloride & sodium ions combine with hydroxyl ions to form sodium hydroxide
Na+ + OH NaoH
Fe OH Fe(OH)2
Fe cl Fecl2
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21. ECM Operation:
In operation, the tool having a shape, similar to that desired in the work piece is fed towards the work piece maintaining a small gap of approximately 0.25mm between them. This is accomplished by utilizing a servo-drive on the tool feed axis.
A high-current, low voltage DC power supply is connected between the tool and the workpiece. The tool is connected to the negative terminal(cathode) & the workpiece to the positive terminal(anode).
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22. The electrolyte is pumped at a high-pressure through the small gap between the tool & the workpiece,thus providing the necessary path for electrolysis.
When the current is passed, dissolution of the work piece(anode) occurs. Meanwhile, the flowing electrolyte washes the metal ions away from the work piece before they have a chance to plate on the tool.
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23. The downward movement of the tool causes the work piece to take the same shape ass that of the tool.
It is important to maintain a uniform gap between the tool & the work piece.
Any physical contact of the tool & the work piece results in arcing & serious damage to both the members
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24. Gap between workpiece & tool Type of electrolyte
Process parameters :
The following are the critical parameters which affect the metal removal rate and surface finish in ECM
Current density
Gap between workpiece & tool
Type of electrolyte
Velocity of electrolyte flow
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25. Current density: current density is simple the current that can be passed into a square inch of work area. At low current densities,MRR is small. The ECM used maintain a current density of A/in2
MRR 12 8 4
Current density
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26. The gap size may vary from 0.25-0.76mm.
Gap between workpiece & tool: the tool & the workpiece are positioned as close together to encourage efficient electrical transmission.
Small gap results in high current densities & hence, more metal removal rate.
The gap size may vary from mm.
a gap size of 0.25mm is often used.
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27. Type of electrolyte: the type of electrolyte depends on the tool & the workpiece material.
Sl.no
Electrolyte
Workpiece material 1
Nacl,Kcl, NaNO3
Steels, iron base alloys & steel alloys with nickel & cobalt base 2
Nacl, Kcl, or NaoH
Aluminium and Al alloys, copper and copper base alloys 3
Nacl, NaNO3
Grey cast iron 4
Nacl, Kcl
Titanium alloys
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28. Velocity of electrolyte flow: electrolyte flow may be between 15-60m/sec. If electrolyte flow is too low, the heat & by-products of the electrolytic reaction(hydrogen gas bubbles & sludge etc) build in the gap causing non-uniform metal.
Too high velocity will cause cavitation,also promoting non-uniform metal removal.
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29. Feed rate of electrode (tool) Temperature of electrolyte
There are a number of factors which govern the accuracy of the parts produced by ECM.
Machining voltage
Feed rate of electrode (tool)
Temperature of electrolyte
Concentration of electrolyte
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30. Final size of the part is made by controlling the voltage.
Machining voltage: machining voltage variations severely affect the overcut.
A higher voltage brings about a larger overcut and a lower voltage a smaller cut
Final size of the part is made by controlling the voltage.
The ECM power supply system attempts to rigidly maintain a constant pre-set voltage for varying voltage and current.
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31. Feed rate of electrode (tool):the feed rate of tool must be maintained under varying conditions of load & machining voltage, otherwise accuracy will be lost.
An increase in fed rate, other parameters remaining constant, decreases the overcut and vice versa.
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32. Temperature of electrolyte: Electrolyte temperature seriously affects the overcut.
The power loss in the electrolyte reaction gives rise to an increase in the temperature of the electrolyte
In general there is no appreciable change in the mechanical properties, such as tensile strength, yield strength, hardness, ductility, etc of the material of the parts due to ECM.
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33. Material removal rate:
Material removal rate (MRR) is an important characteristic to evaluate efficiency of a non-traditional machining process.
In ECM, material removal takes place due to atomic dissolution of work material which is governed by Faraday’s laws of Electrolysis.
MRR = =
where
m = ItA/Fv = mass of material dissolve
I = current ; A = Atomic weight ; v = valency
F = Faraday’s constant = coulumbs
ρ = density of the material IA m tρ
Fρν
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34. Electrode or tooling cost is a fixed cost.
ECONOMIC ASPECTS
Fixed costs of ECM installations are quite high as compared to its operating cost
Overhead costs are the same as for other conventional machining methods
ECM needs power of high current capacity, in localities where power is sufficiently cheap, this factor can be overlooked.
Electrode or tooling cost is a fixed cost.
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35. Electrolyte is not as costly as one might think it to be.
The most widely used electrolyte is sodium chloride(salt) & it is quite cheap
Cost of work piece fixtures are not very high.
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36. Advantages
One of the main advantage of ECM its ability to machine complex three-dimensional curved surfaces without the striation marks left by milling cutters.
This process is capable of machining metals & alloys irrespective of their strength & hardness.
There is little or no tool wear of the tool in this process.
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37. Accurate shape with good surface finish can be obtained.
Because of no tool wear a large number of components can be machined without the tool having to be replaced.
Accurate shape with good surface finish can be obtained.
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38. Dis-advantages
The main limitations of this process is that non-conductive materials cannot be machined
Inability to machine sharp interior edges & corners (< 0.2mm radius)because of very high current densities at those points
Corrosion & rust of the ECM machine can be a hazard.
High electrical power consumption.
High floor area is required.
Limited to mass production.
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39. Applications
Fragile parts, which are otherwise not easily machinable,can be shaped by ECM
Gas & steam turbine blades are now machined by this method
Use in die making industries, automotive, aerospace, power generation, oil and gas industries
Specific application include facing and turning complex 3dimensional surfaces
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40. Applicable in multiple hole drilling, milling deburring,grinding,honing,oddshape contouring etc.
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