Electro chemical machining - Thanuj Kumar M

Introduction to Electro-Chemical NTM Processes
Prof. Thanuj Kumar M. Associate Professor,
Dept. of Mech. Engg. RRCE, B’luru, Karnataka, India

1) Introduction
2) Principle of Electro Chemical Machining: ECM Equipment,
3) Elements of ECM Operation, Chemistry of ECM.
4) ECM Process Characteristics: Material Removal Rate, Accuracy, Surface Finish.
5) Process Parameters: Current Density, Tool Feed Rate, Gap between Tool & Work
Piece, Velocity of Electrolyte Flow, Type of Electrolyte, its Concentration
Temperature and Choice of Electrolytes.
6) ECM Tooling: ECM Tooling Technique & examples, Tool & Insulation Materials.
7) Applications ECM: Electrochemical Grinding and Electrochemical Honing Process.
8) Advantages, Disadvantages and Application of ECG, ECH.
Thanuj Kumar M., Assoc. Prof. Dept. of Mech. Engg., RRCE, Bengaluru. 02
Content:
ELECTROCHEMICAL MACHINING (ECM): 10 hours

 The thermal energy is applied to a very small portion of the
work surface, causing that portion to be removed by fusion
and/or vaporization of the material.
 The thermal energy is generated by conversion of electrical
energy.
 Examples:
a) ECM – Electro Chemical Machining
b) EDM – Electro Discharge Machining
c) LBM – Laser Beam Machining
d) PBM – Plasma Beam Machining
e) IBM – Ion Beam Machining
Thermal Non-Traditional Machining:

 Electrochemical: Mechanism is reverse of electroplating.
 Chemical: Most materials (metals particularly) are susceptible to chemical
attack by certain acids or other etchants. In chemical machining, chemicals
selectively remove material from portions of the work part, while other
portions of the surface are protected by amask.
 Chemical milling and photochemical machining or photochemical blanking all
use a chemical dissolution action to remove the machining allowance
through ions in an etchant.
 Electrochemical machining uses the electrochemical dissolution phase to
remove the machining allowance using ion transfer in an electrolytic cell.
Chemical & Electro-Chemical Machining in NTM:

Non-Traditional Machining
Electrochemical &
Chemical
Mechanical Thermal
CHM
USM EDM
PCM
WJM ECM
ECM
AJM LBM
IJM IBM
PBM
Classification:

Chemical and Electrochemical Machining
CHM
CD
Work Piece
Etchant
Ions
ECM
ECD
Work Piece
Electrolyte
Ions
PCM
CD
Work Piece
Etchant
Ions

 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.
Introduction:

 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.
 In a metal, electricity is conducted by free electrons but in a
solution the conduction of electricity is achieved through the
movement of ions.
 Thus the flow of current through an electrolyte is always
accompanied by the movement of matter.
 In the ECM process the work-piece is connected to a positive (+ve)
electrode and the tool to the negative (-ve) terminal for metal
removal.

 The first introduction of ECM was in 1929 by Gusseff,
 Its industrial applications have been extended to electro-
chemical drilling, electro-chemical deburring, electro-
chemical grinding and electro-chemical polishing .
 Its machining technique was started in late 60’s & 70’s.
History:

Schematic Diagram of Electro-Chemical Machining (ECM)

Machining processes
Complex parts machined
Electro Chemical Machine and Machined Parts

Gif file Showing How Electro Chemical Machining Works

Steps for Electro Chemical Machining Works
Step-6
Step-5 Step-7
Step-4
Step-2
Step-1 Step-3

Important Elements of ECM are:
1. Electrolyte
2. Tool (Cathode)
3. Work-Piece (Anode)
4. D.C Power Supply
Elements of ECM Machine:

 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.
 The main functions of an electrolyte in ECM are:
1. It carries the current between tool and work-piece.
2. It removes the products ofmachining and other insoluble products from
the cutting region.
3. It dissipates heat produced in the operation.
Electrolyte:

 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:
2H++ 2e- H2 (Hydrogen evolution)
M++ e-
M (M denotes metal)
Electrolysis Process:

 At anode, the following reactions takes place with a halogen electrolyte
M M++ e-(Metal dissolution)
2H2O
2Cl-
O2+ 4H++ 4e- (Oxygen evolution)
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 15g/min and
generate hydrogen at a rate of about 300cm3/min.

 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 used for tool shaping.
Tool (Cathode):

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.
1) Work-piece should be conductor of electricity. So it is almost
limited to metals only.
General requirements of tool material:
Work-Piece (Anode):

Current type : DC
Voltage : 2 to 35 V
Current : 50 to 40,000A
Current density : 0.1 A/mm2 to 5A/mm2
Material : NaCl and NaNO3
Temperature : 20oC – 50oC
Flow rate : 20 lpm per 100 Acurrent
Pressure : 0.5 to 20bar
Dilution : 100 g/l to 500 g/l
Working gap : 1mm to 2mm
Overcut : 2mm to 3mm
Feed rate : 0.5mm/min to 15mm/min
Electrode material : Copper, Brass, Bronze
Surface roughness : Ra 0.2 to 1.5 μm
Power Supply:
Electrolyte:

 In this process of ECM, the tool (mirror image of work-piece) is the
cathode and work-piece is the anode.
 A gap (0.05 to 0.7 mm) is provided between the tool & work-piece so
that electrolyte flows through the gap at a velocity of 30 to 60 m/s,
thus completes the electrical circuit.
 Electrolyte is pumped at high pressure of 20kgf/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.
Working of ECM:

 Metal is removed from the work-piece by dissolution.
 The electric current is of the order of 50 to 40,000A at 5 to 35V (DC) for
current density of 20 to 300A/cm2.
 Power of 3KWh is needed to remove 16cm3 of metal which is almost
30times the energy required in the conventional process (when the
material is readily machinable).

Material Removal Rate:
 MRR depends mainly on feed rates.
 The feed rate determines the current passed between the work & the tool.
 As the tool approaches the work, the length of the conductive current path
decreases & the magnitude of current increases.
 This continues until the current is just sufficient to remove the metal at a rate
corresponding to the rate of tool advance.
 A stable cut is then made with a fixed spacing between the work and the tool,
termed as the equilibrium-machining gap.
 If the tool feed rate is reduced, the tool advance will momentarily lag behind,
increasing the gap and thus resulting in a reduction of current. This happens until a
stable gap is once again established.
ECM Process Characteristics:

 Under ideal conditions & with properly designed tooling, ECM is capable of holding
tolerance of the order of 0.02 mm & less.
 Repeatability of the ECM process is also very good.
 This is largely due to the fact that the tool wear is virtually non-existent on a good
machine; tolerance can be maintained on a production basis in the region of 0.02 -
0.04 mm.
 As a general rule, the more complex the shape of the work, the more difficult is to
hold tight tolerances and the greater is the attention required for developing a
proper tooling and electrode shape.
Accuracy:

 ECM under certain conditions can produce surface finishes of the order
of 0.4µm.
 This can be obtained by the frontal cut or the rotation of the tool or
the work.
 The important variables affecting the surface finish are feed rate, gap
dimension, electrolyte composition, viscosity, temperature & flow.
 Any defect on the tool will cause machining defects on the surface of
the work.
Surface Finish:

Feed Rate:
A high feed rate results in higher metal removal rate.
It decreases the equilibrium machining gap resulting in improvement of surface finish
and tolerance control.
Voltage:
Low voltage decreases the equilibrium-machining gap and results in better surface
finish and tolerance control.
Nature of Power Supply and Machining Pulse:
The nature of applied power supply may be of two types, such as DC (full wave
rectified) and pulse DC. A full wave rectified DC supplies continuous voltage where the
current efficiency depends much more on the current density.
The efficiency decreases gradually when the current density is reduced, whereas in
pulse voltage (duration of 1 ms and interval of 10 ms) the decrease is much more rapid.
With decreasing current density the accuracy of the form of the work-piece improves.
Operating Parameters:

a) Passivity Electrolyte: containing oxidizing anions e.g. sodium nitrate and sodium
chlorate, etc. Passivity electrolytes are known to give better machining precision
due to their ability to form oxide films and evolve oxygen in the stray current
region.
b) Non-Passivity Electrolyte: containing relatively aggressive anions such as sodium
chloride.
Note:
NaClO3, NaNO3, and NaCl solution with different concentration are used for
electrochemical machining (ECM). The pH value of the electrolyte solution is chosen to
15 ensure good dissolution of the work-piece material during the ECM process without
the tool being attacked.
Electrolyte Type, Concentration and Flow:

 The tool must match the required shape of the workpiece depending
on the material and the profile to be produced.
 Tool materials used in ECM must have good thermal and electrical
conductivity, corrosion resistance must be highly machinable and
should be stiff enough to withstand the electrolytic pressure without
vibrating.
Size, Shape and Material of the Tool:

MRR vs. Tool Feed Rate
 From the graph it shows that the MRR is influenced by feed
rate. The material removal rate increases with feed rate
because the machining time decreases.
MRR vs. Tool Feed Rate
 From the graph it shows that the MRR is also influenced
by voltage change but moderately.
Conclusion:
MRR increases with the Tool Feed Rate as well as the
Voltage, but it depends mainly on the tool feed rate.
Process Parameters:

Advantages:
1. There is no cutting forces therefore clamping is not required except for
controlled motion of the work piece.
2. It can machine configurations which is beyond the capability of
conventional machining processes.
3. Very accurate (tolerance of ±0.02 mm).
4. Relatively fast.
5. Can machine harder metals than the tool.
6. Extremely thin material scan be easily worked without
7. Distortion
8. Tool wear is nearly absent.
9. Better surface finish (0.2 to 0.8 micron).

 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.
Disadvantages:

 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.
 Used for copying of internal and external surfaces, cutting of curvilinear slots,
machining of intricate patterns, production of long curved profiles, machining of
gears & chain sprockets, production of integrally bladed nozzle for use in diesel
locomotives, production of satellite rings and connecting rods, machining of thin
large diameter diaphragms. Also used for slotting very thin walled collets.
Applications:

Electro chemical machining - Thanuj Kumar M

Electro chemical machining - Thanuj Kumar M

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