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Chemical Machining
Fig : (a) Schematic illustration of the chemical machining process. Note
that no forces or machine tools are involved in this process. (b) Stages
in producing a profiled cavity by machining; not the undercut.
Steps in Chemical Machining
1. Cleaning - to insure uniform etching
2. Masking - a maskant (resist, chemically resistant
to etchant) is applied to portions of work surface
not to be etched
3. Etching - part is immersed in etchant which
chemically attacks those portions of work surface
that are not masked
4. Demasking - maskant is removed
Chemical Machining
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In this process, metal is removed from the work piece through a controlled
chemical attack or etching. Metal can be removed from selected portions or
from entire surface of the work piece, according to requirement.
Chemical machining involves the use of acids or alkali solution ( chemical )
known as enchant to etch away unwanted material, leaving final desired
pattern or part.
Principle of operation : As this is a controlled dissolution of work material in a
Chemical ( etchant ), the portion that need no machining is to be covered (
protected ) by applying a mask on work surface known as resist ( maskant ).
The work material is then submerged in a hot chemical solution and erosion of
unprotected area takes place. After machining , masking material is removed
from the surface of work piece and part is cleaned and inspected .
The basic function of etchant is to convert a material ( metal ) into a metallic
salt that can be dissolved in the etchant ( chemical ) and thus metal is
removed from the work surface.
Common etchants are HNO3 FeCl3 . Different etchants are used for different
material .
Common maskants ( resists ) are vinyl , neoprin rubber , butyl base material,
which are applied to work piece by flow , dip or spray coating.
Chemical machining - Applications
Chemical attacks metals and etch them by removing small amounts of material from
the surface using reagents or etchants
Fig : (a) Missile skin-panel section contoured by chemical milling to improve the
stiffness-to weight ratio of the part. (b) Weight reduction of space launch vehicles by
chemical milling aluminum-alloy plates. These panels are chemically milled after the
plates have first been formed into shape by processes such as roll forming or
stretch forming. The design of the chemically machined rib patterns can be modified
readily at minimal cost.
Chemical Machining
Advantages :
1. Tooling cost is very low
2. It is flexible process from the design point of view,
3. Extremely thin materials are machinable without the help of work
holding device .
4. Production of stress free and crack free surfaces.
5. Both faces of the work piece can be machined simultaneously
Disadvantages :
1. low metal removal rate , hence slower process.
2. Sharp corners can not be produced.
3. High manufacturing cost
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Chemical milling:
• Shallow cavities produced on plates, sheets, forgings, and extrusions. It
is used for metal removal from thicker work piece using screen resist
type maskants.
• Chemical blanking ( photochemical machining / blanking)
• Modification of chemical milling
• Material removed from flat thin sheet by photographic techniques ( using
photo resist type maskants )
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It is used for metal removal of thin electronic and computer metallic
components. Burr free etching of printed-circuit boards, decorative
panels, thin sheet metal stampings as well as production of small and
complex shapes
Chemical Engraving :
It is used for special precision contoured reproduction
Electro Chemical Machining ECM
Reverse of electro-plating
(workpiece is anode)
Cathode
hollow
Anode
NaCl
Shaped tool made of brass , copper , bronze , or stainless steel
Tool face has the reverse shape to be made on the work piece.
Fig : Schematic illustration of the
electrochemical-machining process.
Electrochemical Machining (ECM)
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Electrical energy used in combination with chemical reactions to remove
material
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Reverse of: electroplating
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Work material must be a: conductor
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Feature dimensions down to about 10 μm
Material removal by anodic dissolution, using electrode (tool) in close
proximity to work but separated by a rapidly flowing electrolyte
Electrochemistry of ECM
(Cu )
anode
Cathode work
piece ( Fe )
Instead of CuSO4 , if we use NaCl as
a electrolyte , then salt is not
consumed along with the process , it
only act as a carrier of current.
Electroplating
Electrochemistry of ECM
cathode
anod
e
W/P
W/P
7.5 V
0V
W/P
15 V
W/P
30 V
Electrochemistry of ECM
Let us consider aqueous solution of NaCl + H2O as a electrolyte.
When voltage difference is applied , across the electrode , reactions
At cathode and anode are
Fe  Fe ++ + 2 e
( At anode )
2H2O + 2 e  H2 + 2 ( OH ) - ( At cathode )
The positive metal ions tend to move towards cathode and negative
hydroxyl ions are attracted towards the anode . Thus positive metal
ions combine with negatively
charged ions to form ferrous oxide as
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Fe ++ + 2 ( OH ) - -  Fe ( OH )2
This ferrous oxide forms an insoluble precipitate , so in this anode
( work piece ) dissolves and only H2 gas is formed at the cathode ,
leaving shape of cathode ( tool ) unchanged , i.e. there is no
Deposition on tool . Therefore electrolyte should be chosen in such
a way that no deposition at either electrode takes place.
Electrochemical machining
• It is based on Faraday’s law of electricity.
• Electrolyte acts as current carrier i.e. conduction of electricity is
achieved through movement of ions existing in the electrolyte
• As the power supply switched on , current starts flowing through
the circuit , electrons are removed from the surface atoms of the
work piece ( Anode ) and becomes ions. Therefore work loses
material due to migration of ions towards the tool (cathode) and
hence material removal takes place
• But before these ions can get deposited on the cutting tool face ,
these are swept out away by rapidly flowing electrolyte out of
the gap between tool & work piece.
• Tool is fed towards the work piece automatically at constant
velocity to maintain desired gap between tool and work piece.
• Electrolyte is pumped at a high rate through the passages in the
tool
Electrochemical machining
Advantages :
1.
Tool doesn’t come
in contact with the
work piece or any
other friction So
wear and tear of
the tool is
negligible .
2.
Toughness and
brittleness of a
material has no
effect on the
machining process
3.
In this method ,
metal removal is an
atom by atom
resulting in higher
surface finish and
crack free surface
Disadvantages :
1.
Large power
consumption
2.
Sharp internal corner
cannot be achieved.
3.
Work material used
must be good
conductor of
electricity.
Parts made by Electrochemical Machining
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FIGURE : Typical
parts made by electrochemical machining. (a)
Turbine blade made of a nickel alloy. (b) Thin slots on a steel
roller-bearing cage. (c) Integral airfoils on a compressor disk.
Parts made by Electrochemical Machining
Applications :
- Dies and glass-making molds, turbine and compressor blades, Holes,
Due to low forces on tool,
ECM can be used to make
holes at very large angle to
a surface – an example is
shown in the turbine nozzle
holes in the figure here.
[source: www.barber-nichols.com]
Electrochemical Grinding
Combines electrochemical machining with conventional grinding
Fig : Schematic
illustration of the electrochemical – grinding
process. (b) Thin slot produced on a round nickel – alloy
tube by this process.
Typical structure of a grinding
wheel
Electrochemical Grinding
Electrochemical Grinding
1.Electrolyte grinding is a modification of both the grinding and electrochemical
Machining In this process metal is removed by electro-chemical decomposition
plus by some abrasive action.
2. On the periphery of the grinding wheel, abrasive particles are attached which act as
insulators, preventing a direct contact between wheel and work piece. The D.C. power
supply is connected to work piece( anode ) and conductive bond of grinding
wheel ( cathode ).
3. The insulating abrasive particles in the grinding wheel protrude evenly above
the wheel surface and the work piece is brought closer with wheel. The height of
abrasive particle above the wheel determines effective gap between anodic work piece
and cathodic wheel.
4. This gap prevents direct contact between wheel and work piece. Also the region
between gap is flooded with electrolyte where electrolysis actually takes place and
electrical circuit is completed through this electrolyte which also acts as a coolant.
5. The electric current flows through electrolyte and metal removal from the work piece
takes place due to electrolytic action . Simultaneously abrasive particle on the
surface of the wheel removes the decomposed material from work piece to provide
a fine surface finish and adequate dimensional control and thus high quality surface
finish is obtained
Electrical-Discharge Machining -- Electrode EDM
( graphite)
• FIGURE : Schematic illustration of the EDM process.
Role of Dielectric fluid ( transformer , hydrocarbon or mineral oil ) :
It can be used as a insulator or conductor to flow of current.
It acts as a flushing medium and carries away the debris.
It also acts as a cooling medium.
Electrical-Discharge Machining -- Electrode EDM
Electric Discharge Machining (EDM)
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One of the most widely used nontraditional processes
Shape of finished work is inverse of tool shape
Sparks occur across a small gap between tool and work
Holes as small as 0.3mm can be made with feature sizes (radius etc.)
down to ~2μm
EDM
1. It is process based on the principle of metal removal of
metals by the repetitive short lived sparks between tool
(cathode ) and work piece( anode ), both are immersed in a
dielectric fluid. Dielectric fluid initially is in deionised state
2. When the voltage is developed , emission of electrons from
cathode tool takes place. These liberated electrons accelerate
towards anode, during their path, the electrons collide with
molecules of dielectric fluid, breaking them into electrons and
positive ions and fluid gets ionized and it gives highly
conductive path for high current flow i.e. spark initiation takes
place to carry out metal removal operation .
3. The duration & magnitude of the spark discharge is closely
controlled and the tool is accurately fed into the work piece to
maintain a constant spark discharge gap.
4. The electric current varied from 0.5 to 400 A at 40 to 300
volts DC. Up to 10000 sparks per sec can be produced, having
temperature in the range of 100000C in spark zone.
Examples of EDM
FIGURE : (a) Examples of cavities produced by the
electrical-discharge-machining process, using shaped
electrodes. The two round parts (rear) are the set of dies
for extruding the aluminum piece shown in front.
(b) A spiral cavity produced by a rotating electrode.
(c) Holes in a fuel-injection nozzle made by electricaldischarge machining. Material: Heat-treated steel.
Examples of EDM
Stepped Cavities
FIGURE Stepped cavities produced with a
square electrode by EDM. The work piece
moves in the two principal horizontal
directions, and its motion is synchronized with
the downward movement of the electrode to
produce various cavities.
WIRE EDM
FIGURE : Schematic illustration of the wire EDM process. As much
as 50 hours of machining can be performed with one reel of wire,
which is then discarded.
WIRE EDM
• EDM uses small diameter
wire as electrode to cut a
narrow kerf in work –
similar to a: bandsaw
Gap is maintained to have spark
initiation
WIRE EDM / Wire cut EDM
WIRE EDM / Wire cut EDM
1.This process is similar to contour cutting with a band saw.
2. A slow moving wire travels along a prescribed path
,dielectric fluid is applied to the work area , cutting the work
piece with electric discharge sparks.
3. wire should have sufficient tensile strength and fracture
toughness.
wire is made of brass, copper or tungsten. (about 0.25mm in
diameter). The wire is generally used only once , it is
inexpensive .
Example of a wire EDM machine
Wire EDM Applications
Ideal for stamp and die
components
Other tools and parts with
intricate outline shapes, such as
lathe form tools, extrusion dies,
and flat templates
THE END