2. Introduction
Electrochemical Machining (ECM) is one of the newest and most useful
non-traditional machining (NTM) process belonging to Electrochemical
category.
Electrochemical machining (ECM) is used to remove metal and alloys
which are difficult or impossible to machine by mechanical machining
process.
The reverse of electroplating.
This machining process is based Michael Faraday’s classical laws of
electrolysis, requiring basically two electrodes, an electrolyte, a gap and a
source of D.C power of sufficient capacity.
4. Working of ECM
In the actual process of ECM, the cathode is tool shaped(mirror image of
work-piece) and anode is the work-piece.
A gap(0.05 to 0.7 mm) is provided between the tool and work-piece and
electrolyte flows through the gap at a velocity of 30 to 60 m/s and it
completes the electrical circuit.
Electrolyte is pumped at high pressure of 20 kgf/cm2(1.96 MPa) through
the gap.
Electrolyte must be circulated at a rate sufficiently high to conduct
current between them and to carry heat.
Metal is removed from the work-piece by dissolution through electrical
action and cathode shape is reproduced on the work-piece.
5. The electric current is of the order of 50 to 40,000 A at 5 to 35 V D.C for current
density of 20 to 300 A/cm2.
Power of 3 KWh is needed to remove 16 cm3 of metal which is almost 30 times
the energy required in the conventional process(when the material is readily
machinable).
6. Electrolysis process
In the electrolytic circuit the electron is flow from the work-piece through
the power supply to the tool.
The electrolysis process that takes place at the cathode liberates
hydroxyl ions(OH-) and free hydrogen.
The hydroxyl ions combine with the metal ions of the anode to form
insoluble metal hydroxides and material is thus removed from the
anode.
At cathode, the following reactions takes place:
1. M++ e- M(M denotes metal)
2. 2H++ 2e- H2(Hydrogen evolution)
7. At anode, the following reactions takes place with a halogen
electrolyte:
1. M M++ e-(Metal dissolution)
2. 2H2O O2+ 4H++ 4e- (Oxygen evolution)
3. 2Cl- Cl2+ 2e- (Halogen gas evolution)
As an example, in machining of iron in NaCl electrolyte, at the cathode
the reaction products are FeCl2, Fe(OH)2, Fe(OH)3 which forms a layer
and this is how iron is removed by electrolytic action.
Current of 1000 A would dissolve iron at the rate of about 15 g/min and
generate hydrogen at a rate of about 300 cm3/min.
8. Elements of ECM
Important elements of ECM are:
1. Electrolyte
2. Tool (cathode)
3. Work-piece (anode)
4. D.C power supply
Electrolyte:
Common electrolytes used are sodium chloride, sodium nitrate, sodium
hydroxide, sodium fluoride, sodium chlorate, potassium chloride and sulphuric
acid.
These solution on reaction produce an insoluble compound in the form of
sludge.
9. The main functions of an electrolyte in ECM are:
1. It carries the current between tool and work-piece.
2. It removes the products of machining and other insoluble products from
the cutting region.
3. It dissipates heat produced in the operation.
The essential characteristics of electrolyte includes:
1. Good electrical conductivity.
2. Non-toxicity and chemical stability.
3. Non-corrosive property.
4. Low viscosity and high specific heat.
High velocity flow over the electrode surface is one of the key factors of
ECM. It is necessary to prevent crowding of hydrogen gas and debris of
machining.
10. Tool (cathode):
The most commonly used tool material are copper, brass, titanium, copper
tungsten and stainless steels when electrolyte is made of salts of sodium
and potassium.
Titanium has been found to be the most suitable tool where the electrolyte
has the tendency to anodize the tool as in case of sulphuric acid.
The other tool materials are aluminium, graphite, bronze, platinum and
tungsten carbide.
The accuracy of tool shape directly affects the work-piece accuracy.
Electro-forming and cold forging are two methods of tool shaping.
11. The general requirements of tool material in ECM are:
1. It should be conductor of electricity.
2. It should be rigid enough to take up the load due to fluid pressure.
3. It should be chemically inert to the electrolyte.
4. It should be easily machinable to make it in the desired shape.
Work-piece (anode):
Work-piece should be conductor of electricity. So it is almost limited to
metals only.
12. 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 = 96500 coulumbs
ρ = density of the material
m
tρ
IA
Fρν
13. Power Supply:
1. Type: direct current
2. Voltage: 2 to 35 V
3. Current: 50 to 40,000 A
4. Current density: 20 A/cm2 to 300 A/cm2
14. Accuracy of ECM:
There are number of factors which govern the accuracy of the parts
produced by ECM. The major ones are:
1. Machining voltage.
2. Feed rate of electrode (tool).
3. Temperature of electrolyte.
4. Concentration of electrolyte.
Under ideal conditions with properly designed tooling, ECM is capable of
holding tolerances of ±0.02 mm.
Surface finish in ECM is of the order of 0.2 to 0.8 micron.
No burrs and sharp edges are left on the work-piece.
15. Application
The most common application of ECM is high accuracy duplication.
Because there is no tool wear, it can be used repeatedly with a high degree
of accuracy.
It is commonly used on thin walled, easily deformable and brittle material
because they would probably develop cracks with conventional machining.
It is used in machining of hard-heat-resisting alloys.
It is used in cutting cavities and holes in various products, machining of
complex external shapes like that of turbine blades, aerospace components
and machining of tungsten carbide and nozzles of alloy steels.
Any conducting material can be machined by this method.
16. Advantages
There is no cutting forces therefore clamping is not required except for controlled
motion of the work piece.
It can machine configurations which is beyond the capability of conventional
machining processes.
Very accurate (tolerance of ±0.02 mm).
Relatively fast.
Can machine harder metals than the tool.
Extremely thin materials can be easily worked without distortion.
Tool wear is nearly absent.
Better surface finish (0.2 to 0.8 micron).
17. Disadvantages
High energy consumption.
Non conducting material cannot be machined.
Corrosion and rust of ECM machine can be hazardous but preventive
measures can help in this regard.
