UNIT- 3

Chemical and Electro Chemical

Techniques
 Chemical Machining
The chemical machining process is also knowns as the Etching process. This process sounds like magic due to
its easy outputs.

In this process, we are just dipping the workpiece into a tank of chemical solution and in just a few seconds,
we will be obtaining the desired structure on the workpiece. This machining process is not magic, but
scientifically practical. This process uses a strongly acidic or alkaline chemical reagent to remove material from
the workpiece

Chemical machining is a process of material removal to obtain a desired shape on the workpiece by dipping
the workpiece into a strong chemical reagent.
Chemical Machining Construction or Parts:
It consists of the following parts:
•Tank
•Heating coil
•Stirrer and
•Workpiece
Tank:
This process has a tank with its face open. The tank is built of strong metal coated with materials that are non-
reactive to etchant depending on the applications and concentration of chemical reagent.
Heating coil:
A heating coil is mounted at the lowest section of the tank to maintain the temperature of the tank at a constant
level. It is practical that in any metal removal process the heat generation is natural. Also, the coil does cooling in
necessary conditions.
Stirrer:
A stirrer is placed in the etchant whose main purpose is to mix the etchant consistently to maintain a uniform
concentration and heat along the volume of the etchant. It is well known to us that the hot particles always
accumulate at the top, leaving the cold below.
So, to spread the heat uniformly along the etchant, the stirrer is used. The stirrer also helps in the flushing of
dissolved metal from the workpiece simultaneously breaking the bubbles formed during machining due to
oxidation.
Workpiece: The workpiece is held in the etchant by the use of a hanger in the case of small applications. The
length of the hanger is fastened over a masked area so that the fixing of the workpiece does not disturb the
machining zone.In the case of a larger workpiece, fixtures coated with rubbers and polymers are used to hold the
workpiece.

Working Principle:

The working principle of chemical machining is based on chemical etchant. An etchant is a mixture of strong
chemical acids which are reactive to metal. When the workpiece is dipped in the etchant, the etchant reacts with
the workpiece causing a uniform rate of dissolution of metal from the workpiece.

To obtain a desired shape or structure, an elemental coating that is non-reactive to a chemical reagent called
‘Maskant’ is applied on the workpiece before machining.

Localized machining is achieved by applying a suitable mask on all the areas where we do not want the etchant
to react. Thus, exposing the machining zone for the necessary removal of metal.
Chemical Machining Operation Types:

1. Chemical milling (CHM): This operation is performed to obtain pockets, contours on the workpiece or to
remove bulk material from the workpiece.

2. Chemical engraving (CHE): This operation is performed to reproduce a special design on a workpiece with a
lot of precision. E.g., titles, brand names, serial numbers, etc.

3. Chemical polishing (CHP): This operation is performed to make up a fine finishing or deburring of the
workpiece. This is possible by using of usage of lighter diluted chemical reagent.

4. Photochemical machining (PCM): Photochemical machining is a process of manufacturing stress-free and

crack-free components. PCM is used where micro details are to be machined on a workpiece.
Advantages:
This machining process removes metal uniformly.
Good surface finish with close tolerances.
Complex contours can be easily machined.
Material removal along all the axes at the same time.
Less skilled operator is required.
No mechanical stress is produced on the workpiece.
Low initial cost.
Low machining cost.
Disadvantages:
Less operator safety, a minor spill of etchant on the skin can cause harm to the operator.
Chances of corrosion after the days of machining.
Machining of alloys can lead to poor surface finish.
Process is not eco-friendly.
Disposal of byproducts can cause harm to the surrounding.
Material Removal Rate (MRR) is less in comparison with other machining processes.
Chances of bubble formation, which may result in improper machining.
Applications:
Weight reduction of complex contours, impossible by conventional methods.
Machining of thin and delicate components.
Used to machine the contours present inside a hole.
Used in automobile and aviation industries.
Making of fine screens and meshes.
Types of maskant
• Cut and peel
• Screen printing
• Photoresist masks

Factors to consider to select the type of maskant:

It should be inert to the chemical reagent used.
Tough to withstand handling.
It should not change its characteristics during the machining process.
Also withstand heat.
It should allow itself for cutting and scribing.
It should adhere well to the workpiece.
Availability and low cost.
Etchants
Types ofetchants:FeCl3, Chromic acid, FeNO3, HF, HNO3
Etchants are the most influential factor in the chemical machining of any material. Various etchant are
available due to workpiece material. The best possible etchant should have properties as follow
a. High etch rate
b. Good surface finish
c. Minimum undercut
d. Compatibility with commonly used maskants,
e. High dissolved-material capacity
f. Economic regeneration
g. Etched material recovery
h. Easy control of process.
i. Personal safety maintenance
 Electrochemical Machining (ECM)
Electrochemical Machining process, the combination of electrical energy and chemical energy makes the removal of
material from the surface of a work-piece. It works on the principle of Faraday's law of electrolysis.
Principle-Faraday's law of electrolysis:
The mass of a substance deposited or liberated at any electrode on passing a certain amount of charge is directly
proportional to its chemical equivalent weight
Parts of Electrochemical Machining:
The electrochemical machining setup consists of
Fixture
Table
Workpiece
Tool
Electrolyte
Pump
Filter
Pressure gauge
Flowmeter
Feed unit
Power supply
Diagram of Electrochemical Machine:

The Diagram of Electrochemical Machining Process was shown below.

An Explanation for the Parts of Electrochemical Machining Process:

Fixture:
The fixture is used to hold the table firmly.
Table:
The table is used to hold the work piece properly.
Workpiece:
It is the material on which machining is carried out to remove the material from the surface of the workpiece.
Here, the workpiece acts as an anode.
Tool:
With the help of the tool, material removal takes place in the workpiece. Here, the tool acts as a cathode.
Electrolyte:
The electrolyte acts as a medium for the flow of ions and leads to the removal of material from the surface of
workpiece.
Pump:
It pumps the electrolyte from the sump to all the parts of the system.
Filter:
It removes the impurities present in the electrolyte which is being pumped to the system or work region.
Pressure gauge:
It is used to check the pressure of the electrolyte coming from the pump via a filter to the work region.
Flow meter:
It is used to measure the discharge or mass flow rate of the fluid(electrolyte).
Feed Unit:
To give the feed to the tool, servomotor is used such that whenever material removal takes place from the
workpiece, the servomotor gives the necessary amount of feed to the tool.
Power Supply:
The power supply is to be given to the machine to work properly. Here +ve supply is given to the
workpiece(acts as an anode) and the -ve supply is given to the tool(acts as a cathode).
Working of Electrochemical Machining (ECM) Process:
The electrolyte is pumped to the work region by the pump via a filter, pressure gauge, flow meter and
finally, it enters into the work region from the passage.
When the Power supply is given, an optimum gap is maintained between the tool and workpiece
because of Faraday's laws of electrolysis, the ions have started displacing from the workpiece and trying
to deposit over the tool.
Before the ions are depositing on the tool, the electrolyte present between tool and workpiece is pumped
out. Then, the ions also moving along with electrolyte without depositing on the tool.
From the above, the mechanism of material removal is Ion displacement and because there is no
disturbance taking place in the tool, the same tool can be used for producing an infinite number of
components.
Hence, we can say that the wear ratio of the tool is infinity(because of no tool wear)
This is the detailed explanation of the electrochemical machining process along with the basic terms and
working.
Advantages of Electrochemical Machining(ECM) Process:
Complex Concave curvature components can be easily produced by using Complex Concave curvature
tools.
Because of ion displacement, the surface finish produced is excellent.
Because no forces are acting, no residual stresses will be present. because of no heat generation, no
thermal effects are present on the workpiece.
Because of no tool wear, the same tool can be used for producing an infinite number of components.
Disadvantages of Electrochemical Machining(ECM) Process:
Workpiece material must be electrically conductive.
Out of all the Non-traditional machining methods, electrochemical machining requires the highest specific
cutting energy Therefore, the cost of machining will be high.
This is preferable for producing contours only but not for holes.
Applications of Electrochemical Machining Process:
It is mainly used for producing Complex concave curvature components such as Turbine blades etc.
The ECM process is used for profiling and contouring, die sinking operation, drilling, trepanning, and
micromachining.
 High feed rate produces high MRR
 It decreases equilibrium machining gap thus increases surface finish & tolerance

 High voltage increases equilibrium machining gap

- gives poor surface finish and tolerance

 Small gap results in more current density

- which in-turn results in more MRR

 higher concentration increases equilibrium machining gap

- results in poor surface finish and tolerance

 low velocity – causes heat and byproducts build up

- results in non uniform MRR
 High velocity – causes cavitation effect
- results in non uniform MRR
ELECTRO-CHEMICAL GRINDING (ECG) OR ELECTROLYTIC GRINDING
INTRODUCTION

It is a process that is quite similar to the electrochemical machining. As per the name, it is the metal
removal process which is nothing but the combination of the grinding process and electrochemical process.
It is also termed as the “electrolytic grinding”

The materials which cannot be easily shaped due to their extreme hardness or high tensile strength can be
ground by using Electro- chemical grinding process. Examples: Cemented carbides, hardened steel etc.

PRINCIPLE

In Electrochemical grinding method, the work is machined by the combined action of electrochemical
effect and conventional grinding operation. But the major portion of the metal (about 90%) is removed by
electrochemical effect.
CONSTRUCTION AND WORKING OF ECG PROCESS
Construction
It consists of workpiece, work table, grinding wheel, spindle, D.C power source, electrolyte, pump, motor for
pump, nozzle, filter for incoming electrolyte, and reservoir for electrolyte.
The grinding wheel is mounted on a spindle, which rotates inside suitable bearings.
 The workpiece is held on the machine table in suitable fixtures. The table can be moved forward and backward
to feed the work or to withdraw it.
The grinding wheel and spindle are separated from the machine by using an insulating sleeve.
Sodium nitrate, sodium chloride and potassium nitrate with a concentration of 0.150 to 0.300 kg/ litre of water
are usually used as electrolyte.
The electrolyte from the reservoir is pumped and passed through nozzle in the gap between the wheel and
workpiece.
A constant gap of 0.025 mm is maintained between the grinding wheel and workpiece.
The grinding wheel is made of fine diamond particles. These particles are slightly projecting out from the
surface and come in contact with work surface with very little pressure.
The grinding wheel runs at a speed of 900 to 1800 m/min .The workpiece is connected to positive terminal
(anode) of battery and grinding wheel is connected to negative terminal (anode)
Working
A mild D.C voltage of about 3 to 30 V is applied between the grinding wheel and workpiece.
Due to the applied voltage, the current flows through the electrolyte with positively charged ions and
negatively charged ions. The positive ions move towards the grinding wheel (cathode) while the negative ions
move towards the work piece (anode).
The electrochemical reaction takes place due to this flow of ions and it causes the removal of metal from the
workpiece.
It can be seen that the workpiece is fed against the rotation of grinding wheel and the metal is removed from
the workpiece surface by simultaneous abrasive action and electrolytic reaction. In fact 10% of the workpiece
metal removed by abrasive cutting and 90% by electrolytic reaction.
Grinding wheel wear is negligible because the major part of the cutting action is electrolytic, and little
dressing of grinding wheel is needed.
The short-circuit between the wheel and work is prevented due to point contact made by the fine diamond
points.
ADVANTAGES OF ECG
1. Since the tool wear is negligible, the life of the grinding wheel is increased. This factor is most valid in the
grinding of hard metals such as tungsten carbide, where, costly diamond grinding wheels are used. In ordinary
grinding there are high wear rates on these expensive diamond wheels.
2. Work is free of surface cracks and distortion because heat is not generated in the process.
3. As compared to conventional grinding, a very little cutting force is applied to the workpiece.
4. Good surface finish is obtained.
5. Work material is not subjected to any structural changes.
6. Intricate parts can be machined without any distortion.
7. The surface finish produced by this process is varied from 0.2 to 0.4µm.
8. Accuracy of the order of 0.01 mm can be achieved by proper selection of wheel grit size and abrasive
particles.
9. Burr free and stress free components are produced.
10. The wheel bond wears very slowly. So, the grinding wheel need not be dressed frequently.
DISADVANTAGES
1. Initial cost is high.
2. Power consumption is high.
3. Metal removal rate is lower than conventional grinding.
4. Non-conducting materials cannot be machined.
5. Preventive measures are needed against corrosion by the electrolyte.
6. Maintenance cost is high.
7. Since the tolerances achieved are slightly low, the workpiece need final abrasive machining.
Applications
1.ECG is used for grinding turbine blades.
2.It is used in aerospace industries for grinding honeycomb.
3.Also used for finishing hard surfaces.
4.It is also used for creating sharp objects.
5.It is also used for grinding fragile articles.
Process Parameters of ECG
Electrochemical Honing ( ECH )
Introduction to Electrochemical Honing :
•ECH is a process in which the metal removal capabilities of ECM are combined with the accuracy capabilities
of honing. The process consists of a rotating and reciprocating tool inside a cylindrical component.
•Material is removed through anodic dissolution and mechanical abrasion – 8% or more, of the material
removal, occurs through electrolytic action
•As with conventional ECM, the workpiece is the anode and a stainless steel tool is the cathode.
ECH tool construction
•Tool consists of a hollow stainless steel body that has expandable, nonconductive honing stones protruding
from at least three locations around the circumference
•The honing stones are identical with those used in conventional honing operations, except that they must
resist the corrosiveness of the electrolyte
•The honing stones are mounted on the tool body with a spring-loaded mechanism so that each of the stones
exerts equal pressure against the workpiece
•The length of the stones is selected to be approximately one-half of the length of the bore being processed.
Electrochemical Honing ( ECH )
Working
 A mild D.C voltage of about 25V is applied between the honing tool and workpiece.
Due to the applied voltage, the current flows through the electrolyte with positively charged ions and
negatively charged ions. The positive ions move towards the honing tool (cathode) while the negative ions
move towards the workpiece (anode).
The electrochemical reaction takes place due to this flow of the ions and it causes the removal of metal from
the work piece.
It can be seen that work piece (cylinder) is fed against the rotation of honing tool and the metal is removed
from the workpiece by the simultaneous abrasive action and electrolytic reaction.
Automatic gauging devices designed into the system which initiates a signal and when the cylinder is of the
desired diameter size, the cycle is automatically terminated.
It is mostly used for internal cylindrical grinding, with a size tolerance of 0.012 mm on diameter and 0.005
mm on roundness
Advantages of electrochemical honing :
Increased MRR particularly on hard materials
Since most of the material is removed electrochemically, honing stone life is greatly extended
Burr-free operation
Unlike conventional honing, no micro-scratches are left on the work surface
Less pressure required between stones and work
Reduced noise and distortion when honing thin-walled tubes
Limitations of electrochemical honing :
High capital cost
Corrosive environment
High preventive maintenance cost
Non-conductive materials cannot be machined
Requires disposal and filtering of electrolytes
Electrochemical Deburring

The word deburring means removal of burr from the source of any workpiece to provide a smooth
finished surface. The term electrochemical means the mode of energy used for deburring. Together
electrochemical chemical deburring refers to a machining process in which burrs are removed using
electrochemical energy.

A special type of tool is used for this operation which is called a deburring tool. Like any other
electrochemical process, in electrochemical deburring also there is no contact between the deburring tool
and the workpiece.

It acts as a reverse electroplating process. In electrochemical deburring, the tool acts as the cathode and the
workpiece acts as the anode.

Both the tool and the workpiece are kept in a flowing electrolyte medium. ECD is also known as
electrolytic deburring due to the use of electrolytes. It is a fast and easy process.
 Need of Electrochemical Deburring:
Removal of burrs is considered a serious problem in Many industries which deal with high accuracy.
•A burr can sometimes have sharp edges which may harm the operator or a worker hence it is important to
remove it.
•It can cause cracks on the surface of the mating parts. As the pressure increases when the area of contact
decreases.
•It also lessens the beauty of the workpiece.
•Electrochemical deburring is also important for deburring hard metals.

Components of Electrochemical Deburring:

•Supply tank
•Pumps
•Collection tank
•Reaction tank
•DC power supply
•Base
•Electrolyte and
•Tool
Supply tank:
The tank carrying electrolyte for its supply is called a supply tank.

Pumps:
There are two pumps P1 and P2. P1 is used to supply electrolytes to the reaction tank and P2 is used to supply
the electrolyte from the collection tank to the supply tank.

Collection tank:
The tank which collects the filtered electrolyte is called a collection tank.

Reaction tank:
The area or the container carrying the tool-workpiece and the electrolyte are called a reaction tank. The
electrochemical reaction between the workpiece and the tool takes place in the reaction tank.
The tank is designed in such a way that there is a constant flow of electrolytes in the tank to carry the slag.

DC power supply:
The voltage values of the DC power supply used in electrochemical deburring are low. But the current value is
high, this promotes faster removal of metal from the surface of the workpiece.
Base:
The base here is made of a conducting material that is used to keep the workpiece stable. The base also
connects the two workpieces electrically. DC supply is given to the base which connects the two workpieces.

Electrolyte:
The electrolyte is a solution of simple salt with water. It is generally a conductive solution of Sodium chloride
and Sodium nitrate in water.

Working
The workpiece is kept on the base and the tool is positioned between the workpieces.
The workpiece is connected to the positive terminal of the DC power supply. And the tool is connected to the
negative terminal of the DC power supply. The pump is activated and the flow of electrolytes is started.
The electrolyte reaches the reaction tank by passing through a filter. Then the DC power supply is switched
on and the reaction starts.
Electron transfer takes place between the workpieces and the electrolyte. The electron transfer results in the
removal of burrs from the surface of the workpiece.
The remaining electrolyte is made to flow through the filter (F2) into the collection tank. From the collection
tank, the electrolyte is again supplied to the supply tank. And the process is repeated.
Once the burr is removed the power supply is switched off and the workpiece is removed.

Advantages of Electrochemical Deburring:

Electrochemical deburring is a highly accurate process which gives excellent surface finish.
Heat generation is negligible.
There are no thermal stresses developed in the workpiece.
No wear of tool takes place.
The efficiency is more.
Faster process increases productivity of the plant.
Quality products are manufactured.
Disadvantages
High initial cost of the equipment.
Different tool must be designed for different workpieces.
Complex process.
Highly skilled operator is required.
Only the workpieces that conducts electricity can be machined.

Applications
It is used for deburring of gears.
It is also used for removing sharp edges from highly precise equipment.
Also be used for surface finishing of hard materials.