UNIT-3
INTRODUCTION OF NON CONVENTIONAL MACHINING PROCESS
unconventional machining process (or
non-traditional machining process) is a special type of machining process in
which there is no direct contact between the tool and the work piece. In unconventional
method form of energy is used to remove unwanted material from a given work piece.
In several industries, hard
brittle materials like tungsten carbide, high speed steels, stainless steels,
ceramics etc find a variety of applications.
For example, tungsten carbide is used for making cutting tools while high speed steel is used for making gear cutters, drills, taps, milling cutters etc.
For example, tungsten carbide is used for making cutting tools while high speed steel is used for making gear cutters, drills, taps, milling cutters etc.
If such materials are machined with
the help of conventional machining processes, either the tool undergoes extreme
wear (while machining hard work piece) or the work piece material is damaged
(while machining brittle work piece).
This is because, in conventional machining, there is a direct
contact between the tool and the work piece. Large cutting forces are involved
and material is removed in the form of chips. Huge amounts of heat are produced
in the work piece. This induces residual stresses, which degrades the life and
quality of the work piece material.
Hence, conventional machining produces poor quality work piece
with poor surface finish (if the work piece is made of hard and brittle
material).
To overcome all these drawbacks, we use unconventional machining
processes to machine hard and brittle materials.
We also use unconventional machining processes to machine soft
materials, in order to get better dimensional accuracy.
Classification of unconventional machining processes:
Unconventional machining processes can be broadly classified into
several types based on four main criteria. The classification of unconventional
machining processes is given below:
1. Mechanical Energy based
Unconventional Machining Processes
2. Electrical Energy based
Unconventional Machining Processes (e.g. Electrical Discharge Machining)
3. Electrochemical Energy
based Unconventional Machining Processes
4. Chemical Energy based
Unconventional Machining Processes (e.g. Chemical Machining)
5. Energy based Unconventional Machining Processes (e.g. Plasma
Arc Machining)
2. Based on the source of
energy
1. Current
2. Voltage
3. Hydraulic Pressure
4. Pneumatic Pressure
5. Ionised Particles
6. Light
3. Based on the medium of
energy transfer
1. Electrons
2. Atmosphere
3. Ions
4. Electrolyte
5. Pressurized gas
6. Water
7. Ultrasonic waves
8. Plasma
9. Laser
10. Chemical reagent
11. Radiation
4. Based on the mechanism
of material removal
1. Erosion
2. Electric Discharge
3. Shear
4. Chemical Etching
5. Vapourisation
6. Melting
7. Ion Displacement
Based on energy used, unconventional
machining processes can be broadly classified into five main
types. They are:
In these processes, unwanted material in the
work piece is removed by mechanical erosion. The mechanical erosion can be
facilitated by using any medium. For example, in abrasive
jet machining, high velocity abrasive jet
is used for eroding material from the work piece. In water jet
machining, high velocity water jet is used
for cutting the work piece material.
The four main mechanical energy based unconventional machining
processes are:
1. Abrasive Jet Machining
2. Water Jet Machining or
Water Jet Cutting
3. Abrasive Water Jet
Machining
4. Ultrasonic Machining
Here, electric spark discharge is used to cut and machine the
workpiece.
In electrical energy based processes, no arc is
produced Instead, thousands of sparks are produced every second. These sparks
increase the temperature of the work piece, melt the unwanted portions and
vapourise those portions.
A dielectric fluid is
used for cleaning the work piece and facilitating a smooth spark discharge.
Processes that come under this category are:
1. Electrical Discharge
Machining
2. Wire Cut Electrical
Discharge Machining
In these processes, unwanted portions of the work piece are
removed by electrochemical effect. The work piece (in contact with an
electrolyte) is machined by ion dissolution. Processes that come under this
category are:
1. Electrochemical
Machining
2. Electrochemical Grinding
3. Electrochemical Honing
Here, chemical energy is used to remove material from the
workpiece.
We know that metal can be easily converted to metallic salt, if
suitable reagent is used. Chemical energy based processes exploit this
principle.
Material is removed by controlled etching of the workpiece in the
presence of a reagent known as enchant.
Chemical machining, chemical milling and photochemical milling
(PCM) are the processes that comes under this category.
Unwanted portions of a metal can be easily
removed, if it is melted or vapourised. Thermo-electrical energy based unconventional
machining processes make use of this principle.
In these processes, electrical energy is converted to a huge
amount of heat by some means. This heat is applied on a small region of the
workpiece. That particular region is either melted or vapourised. By this way,
material is removed.
The following are some of the important thermo-electrical energy
based unconventional machining processes:
1. Plasma Arc Machining
2. Electron Beam Machining
3. LASER Beam Machining
4. Ion Beam Machining
An unconventional machining process (or non-traditional machining process) is a special type of machining process in which there is no direct contact between the tool and the workpiece. In unconventional machining, a form of energy is used to remove unwanted material from a given workpiece.
Why unconventional machining processes are used?
The answer is simple. In several industries, hard and brittle
materials like tungsten carbide, high speed steels, stainless steels, ceramics
etc., find a variety of applications.
For example, tungsten carbide is used for making cutting tools while high speed steel is used for making gear cutters, drills, taps, milling cutters etc.
For example, tungsten carbide is used for making cutting tools while high speed steel is used for making gear cutters, drills, taps, milling cutters etc.
If such materials are machined with the help of conventional
machining processes, either the tool undergoes extreme wear (while machining
hard workpiece) or the workpiece material is damaged (while machining brittle
workpiece).
This is because, in conventional machining, there is a direct
contact between the tool and the workpiece. Large cutting forces are involved
and material is removed in the form of chips. Huge amounts of heat is produced
in the workpiece. This induces residual stresses, which degrades the life and
quality of the workpiece material.
Hence, conventional machining produces poor quality workpiece with
poor surface finish (if the workpiece is made of hard and brittle material).
To overcome all these drawbacks, we use unconventional machining
processes to machine hard and brittle materials.
We also use unconventional machining processes to machine soft
materials, in order to get better dimensional accuracy.
Classification of unconventional machining processes:
Unconventional machining processes can be broadly classified into
several types based on four main criteria. The classification of unconventional
machining processes is given below:
1. Based on the type of energy used
1. Mechanical Energy based
Unconventional Machining Processes (e.g. Abrasive Jet
Machining, Water Jet
Machining)
2. Electrical Energy based
Unconventional Machining Processes (e.g. Electrical Discharge Machining)
3. Electrochemical Energy
based Unconventional Machining Processes (e.g. Electrochemical Grinding)
4. Chemical Energy based
Unconventional Machining Processes (e.g. Chemical Machining)
5. Thermo-electrical (or
Electro-thermal) Energy based Unconventional Machining Processes (e.g. Plasma
Arc Machining)
2. Based on the source of
energy
1. Current
2. Voltage
3. Hydraulic Pressure
4. Pneumatic Pressure
5. Ionised Particles
6. Light
3. Based on the medium of
energy transfer
1. Electrons
2. Atmosphere
3. Ions
4. Electrolyte
5. Pressurized gas
6. Water
7. Ultrasonic waves
8. Plasma
9. Laser
10. Chemical reagent
11. Radiation
4. Based on the mechanism
of material removal
1. Erosion
2. Electric Discharge
3. Shear
4. Chemical Etching
5. Vapourisation
6. Melting
7. Ion Displacement
8. Blasting
WATER JET MACHINE
Water Jet Machining (WJM) is a mechanical energy based non-traditional machining process used to cut and machine soft and non-metallic materials.
It involves the use of high velocity water jet to smoothly
cut a soft workpiece. It is similar to Abrasive
Jet Machining (AJM).
In water jet machining, high velocity water jet is allowed
to strike a given workpiece. During this process, its kinetic energy is
converted to pressure energy. This induces a stress
on the workpiece. When this induced stress is high enough, unwanted particles
of the workpiece are automatically removed.
This
article contains the following sections:
1. Schematic
diagram of Water Jet Machining
2. Construction
3. Working
4. Advantages
5. Disadvantages
6. Applications
The apparatus of water jet machining consists of the following
components:
1. Reservoir: It is used for
storing water that is to be used in the machining operation.
2. Pump: It pumps the water
from the reservoir.
4. Accumulator: It is used for
temporarily storing the pressurized water. It is connected to the flow
regulator through a control valve.
5. Control Valve: It controls the
direction and pressure of pressurized water that is to be supplied to the
nozzle.
6. Flow regulator: It is used to
regulate the flow of water.
7. Nozzle: It renders the
pressurized water as a water jet at high velocity.
§
Water from the reservoir is pumped to the intensifier using a
hydraulic pump.
§
The intensifier increases the pressure of the water to the
required level. Usually, the water is pressurized to 200 to 400 MPa.
§
Pressurized water is then sent to the accumulator. The accumulator
temporarily stores the pressurized water.
§
Pressurized water then enters the nozzle by passing through the
control valve and flow regulator.
§
Control valve controls the direction of water and limits the
pressure of water under permissible limits.
§
Flow regulator regulates and controls the flow rate of water.
§
Pressurized water finally enters the nozzle. Here, it expands with
a tremendous increase in its kinetic energy. High velocity water jet is
produced by the nozzle.
§
When this water jet strikes the workpiece, stresses are induced.
These stresses are used to remove material from the workpiece.
§
The water used in water jet machining may or may not be used with
stabilizers. Stabilizers are substances that improve the quality of water jet
by preventing its fragmentation.
§
For a good understanding of water jet machining, refer the
schematic diagram above.
1. Water jet machining is a
relatively fast process.
2. It prevents the
formation of heat affected zones on the workpiece.
3. It automatically cleans
the surface of the workpiece.
4. WJM has excellent
precision. Tolerances of the order of ±0.005″ can be obtained.
5. It does not produce any
hazardous gas.
6. It is eco-friendly.
1. Only soft materials can
be machined.
2. Very thick materials
cannot be easily machined.
3. Initial investment is
high.
1. Water jet machining is
used to cut thin non-metallic sheets.
2. It is used to cut
rubber, wood, ceramics and many other soft materials.
3. It is used for machining
circuit boards.
4. It is used in food
industry.
AJM
AJM
Process
parameters of Abrasive
Jet Maching (AJM) are factors that
influence its Metal Removal Rate (MRR).
In a machining process,
Metal Removal Rate (MRR) is the volume of metal removed from a given workpiece
in unit time.
The following are some of the important process parameters of
abrasive jet machining:
1. Abrasive mass
flow rate
2. Nozzle tip distance
3. Gas Pressure
4. Velocity
of abrasive particles
5. Mixing ratio
Mass flow rate of the abrasive particles is a major process
parameter that influences the metal removal rate in abrasive jet machining.
In AJM, mass flow rate of the gas (or air)
in abrasive jet is inversely proportional to the mass flow rate of
the abrasive particles.
Due to this fact, when continuously increasing the abrasive mass
flow rate, Metal Removal Rate (MRR) first increases to an optimum value
(because of increase in number of abrasive particles hitting the work piece)
and then decreases.
However, if the mixing ratio is kept constant, Metal Removal Rate (MRR) uniformly
increases with increase in abrasive mass flow rate.
Nozzle Tip Distance (NTD) is the gap provided between
the nozzle tip
and the workpiece.
Upto a certain limit, Metal Removal Rate (MRR) increases with
increase in nozzle tip distance. After that limit, MRR remains constant
to some extent and then decreases.
In addition to metal removal rate, nozzle tip distance influences
the shape and diameter of cut.
For optimal performance, a nozzle tip distance of 0.25 to 0.75 mm
is provided.
Air or gas pressure has a direct impact on metal
removal rate.
In abrasive jet machining, metal removal rate is directly
proportional to air or gas pressure.
Whenever the velocity of abrasive particles is increased, the
speed at which the abrasive particles hit the workpiece is increased. Because
of this reason, in abrasive jet machining, metal removal rate increases with
increase in velocity of abrasive particles.
Mixing ratio is a ratio that determines the quality of the
air-abrasive mixture in Abrasive Jet Machining (AJM).
It is the ratio between the mass flow rate of abrasive particles
and the mass flow rate of air (or gas).
When mixing ratio is increased continuously, metal removal rate
first increases to some extent and then decreases.
Size of the abrasive particle determines the speed at which metal
is removed.
If smooth and fine surface finish is to be obtained, abrasive particle
with small grain size is used.If metal has to be removed rapidly, abrasive
particle with large grain size is used.
Abrasive Jet Machining (AJM), also known as micro-abrasive blasting, is
a mechanical
energy based unconventional
machining process used to remove
unwanted material from a given workpiece.
The process makes use of an abrasive jet with
high velocity, to remove material and provide smooth surface finish to hard
metallic workpieces. It is similar to Water Jet
Machining (WJM).
This article explains the construction and working of AJM listing
its advantages, disadvantages and applications. The contents of this article
are listed below:
1. Schematic
Diagram
2. Construction
3. Working
4. Operations
performed using AJM (Applications of AJM)
5. Advantages
6. Disadvantages
Schematic Diagram of
Abrasive Jet Machining:
A simple schematic diagram of Abrasive Jet Machining (AJM) is
shown below:
Construction of Abrasive
Jet Machining (AJM):
The constructional requirements of Abrasive Jet Machining (AJM)
are listed and described below:
1. Abrasive jet: It is a mixture of
a gas (or air) and abrasive particles. Gas used is carbon-di-oxide or nitrogen
or compressed air. The selection of abrasive particles depends on the hardness
and Metal Removal Rate
(MRR) of the workpiece. Most commonly, aluminium oxide or
silicon carbide particles are used.
2. Mixing chamber: It is used to mix
the gas and abrasive particles.
3. Filter: It filters the gas
before entering the compressor and mixing chamber.
4. Compressor: It pressurizes the
gas.
5. Hopper: Hopper is used for
feeding the abrasive powder.
6. Pressure gauges and flow
regulators: They are used to control the pressure and regulate the flow rate of
abrasive jet.
7. Vibrator: It is provided
below the mixing chamber. It controls the abrasive powder feed rate in the
mixing chamber.
8.
Nozzle: It forces the abrasive jet over the workpiece. Nozzle is
made of hard and resistant material like tungsten carbide.
Working:
Dry air or gas is filtered and compressed by passing it through the
filter and compressor.
A pressure gauge and a flow regulator are used to control the
pressure and regulate the flow rate of the compressed air.
Compressed air is then passed into the mixing chamber. In the
mixing chamber, abrasive powder is fed. A vibrator is used to control the feed
of the abrasive powder. The abrasive powder and the compressed air are
thoroughly mixed in the chamber. The pressure of this mixture is regulated and
sent to nozzle.
The nozzle increases the velocity of the mixture at the expense of
its pressure. A fine abrasive jet is rendered by the nozzle. This jet is used
to remove unwanted material from the workpiece.
For a good understanding of construction and
working of AJM, refer the schematic diagram above.
The following are some of the operations that can be performed
using Abrasive Jet Machining:
1. Drilling
2. Boring
3. Surface finishing
4. Cutting
5. Cleaning
6. Deburring
7. Etching
8. Trimming
9. Milling
Advantages of Abrasive
Jet Machining:
·
Surface of the workpiece is cleaned automatically.
·
Smooth surface finish can be obtained.
·
Equipment cost is low.
·
Hard materials and
materials of high strength can be
easily machined.
Disadvantages of
Abrasive Jet Machining:
·
Metal removal rate is low
·
In certain circumstances, abrasive particles might settle over the
workpiece.
·
Nozzle life is less. Nozzle should be maintained periodically.
·
Abrasive Jet Machining cannot be used to machine soft materials.
Ultrasonic machining schematic
Ultrasonic machining schematic
Ultrasonic machining is a subtraction manufacturing process that removes material from the surface of a part
through high frequency, low amplitude vibrations of a tool against the material
surface in the presence of fine abrasive particles. The tool travels vertically
or orthogonal to the surface of the part at amplitudes of 0.05 to 0.125 mm
(0.002 to 0.005 in.).
Ultrasonic machining physically operates by the mechanism of
micro-chipping or erosion on the work piece's surface. Since the abrasive
slurry is kept in motion by high frequency, low amplitude vibrations the impact
forces of the slurry are significant causing high contact stresses.
These high
contact stresses are achieved by the small contact area between the slurry's
particles and the work piece's surface. Brittle materials fail by cracking
mechanics and these high stresses are sufficient enough to cause micro-scale
chips to be removed from its surface. The material as a whole does not fail due
to the extremely localized stress regions
