Unit ii usm

UNIT II
ULTRASONIC MACHINING
BY
Dr. A. Asha, Professor/Mechanical Engineering
Kamaraj College of Engineering & Technology, Madurai 625701

• PRINCIPLE OF USM:
It is a machining method, a slurry of small
abrasive particles are forced against the
workpiece by means of a vibrating tool and it
causes the removal of metal from the workpiece
in the form of extremely small chips

• CONSTRUCTION

• It consists of abrasive slurry, work piece, fixture, table cutting tool,
circulating pump, reservoir, ultrasonic oscillator, leads, excitation coil, feed
mechanism, ultrasonic transducer, transducer cone, connecting body, tool
holder
• The ultrasonic oscillator and amplifier also known as generator is used to
convert the applied electrical energy at low frequency to high frequency.
• The transducer is made up of magnetostrictive material and it consists of
stack of nickel laminations that are wound with a coil.
• The function of the transducer is to convert electrical energy to mechanical
energy.
• Tough and ductile tool materials is used in this process. Low carbon steels
and stainless steels are commonly used as tool materials

• The tool is generally soldered or fastened mechanically to the
transducer through a metal holder. Generally tool holder is of
cylindrical or conical in shape.
• The material used for tool holders are titanium alloys, monel
aluminium, stainless steel.
• Abrasive slurry usually a mixture of abrasive grains and water of
definite propotion ( 20 – 30%) is made to flow under pressure
through the gap between the tool and workpiece of the order of 0.02
to 0.1mm.
• The most commonly used abrasives are boron carbide, silicon carbide,
aluminum oxide and diamond .Boron carbide is most commonly used
abrasive slurry since it has the fastest cutting abrasive property

Working :
• Electric power is given to ultrasonic oscillator and this oscillator converts
the electrical energy of low frequency to high frequency(20kHz)
• This high frequency from oscillator is supplied to transducer.
• The transducer converts the electrical energy to mechanical vibrations. The
transducer is made up of magnetostrictive material, which is excited by
flowing high frequency electric current and this results in the generation of
mechanical vibrations.
• The vibrations generated in the transducer is of the order of 20kHz to
30kHz and hence ultrasonic waves are produced.
• The vibrations are then transmitted to the cutting tool through transducer
cone, connecting body and tool holder. This makes the tool to vibrate in a
longitudinal direction.

• Abrasive slurry is pumped from the reservoir and it is made to flow
under pressure through the gap between tool and workpiece.
• In an abrasive slurry when the cutting tool vibrates at high frequency
it leads to the removal of metal from the workpiece.
• The impact force arises from the vibration of the tool end and the
flow of slurry through the workpiece & tool gap causes thousands of
microscopic grains to remove the workpiece material by abrasion.
• A refrigerated cooling system is used to cool the abrasive slurry to a
temperature of 5 to 60C.
• The ultrasonic machining process is a copying process in which the
shape of the cutting tool is same as that of the cavity produc.ed

TRANSDUCERS
•Transducer : It is a device which converts one
form of energy into another form of energy. In
ultrasonic machining process it converts the
electrical energy into mechanical energy.
•Types of transducers:
(a) Magnetostriction transducer
(b) Piezoelectric transducer

• Principle :
• When a rod of ferromagnetic material such as iron or nickel is kept in
a magnetic field parallel to its length the rod suffers a change in its
length.
• The change in length is independent of the direction of the magnetic
field and depends only on the magnitude of the field and nature of
the material. This phenomenon is known as magnetostriction effect
MAGNETOSTRICTION TRANSDUCER

• Construction :

• A rod of (AB) ferromagnetic material iron or nickel is clamped
at the middle.
• The two ends A and B of the rod is wound by the coil L1 and
L2.
• The coil L1 wound on the right hand portion of the rod along
with a variable capacitor C1. The coil L2 wound on the left
hand portion of the rod is connected to the grid circuit.
• The frequency of the oscillatory circuit is adjusted by the
variable capacitor C1 and the current is noted by the
milliammeter.
• The LT battery and HT battery are provided to produce
current in the grid circuit

Working :
• The filament in the grid circuit is heated by the low tension battery. This
causes the production of electrons and these electrons are accelerated by
using high tension battery.
• So current is produced in the circuit.
• The Alternating current passes through the coil L1 which produces an
alternating magnetic field along the length of the rod.
• The result is the rod vibrated due to the magnetostriction effect. The
vibrations of the rod create ultrasonic waves which are sent out.
• The longitudinal expansion and contraction of the rod (AB) produces an
emf in the coil L2. This induced emf is fed due to the grid circuit
continuously as a feed back.
• In this way the current is built up and the vibrations of the rod is
maintained.

• When the frequency of the oscillatory circuit is equal to the frequency
of the vibrating rod the resonance occurs.
• At resonance the rod vibrates vigorously and ultrasonic waves are
produced with high frequencies.

• Advantages :
1. Production cost is low
2. Very simple design
3. At low ultrasonic frequencies large power output is possible
without any damage to the oscillatory circuit.
• Limitations :
1. It cannot produce ultrasonic waves of frequency above 3000 kHz.
2. The frequency of oscillations depends on temperature.
3. Losses of energy due to hysteresis and eddy current
4. As the frequency is inversely proportional to length of the rod the
length of the rod should be decreased to increase the frequency.

PIEZOELECTRIC TRANSDUCER
•Piezoelectric effect :
When mechanical force is applied to one pair of
opposite faces of certain crystals like quartz, tourmaline
equal and opposite electrical charges appear across its
other faces. But ultrasonic waves are generated based
on Inverse piezoelectric effect
•Inverse piezoelectric effect :
When an a.c. voltage is applied across the piezoelectric
crystal it starts vibrating at the frequency of the applied
voltage

• Construction :

•It consists of a primary and secondary circuit. The
primary circuit is arranged with coils L1 & L2.
•Coil L1 is connected to the grid circuit and the coil L2
is connected to the plate circuit.
•The frequency of the oscillatory circuit is varied by
using the capacitor C1.
•The quartz crystal is placed between two metal plates
A and B. The plates are connected to the secondary
coil L3 of the transformer

Working :
• The filament in the grid circuit is heated by the low tension battery (LT). This
causes the production of electrons and these electrons are accelerated with a
very high velocity by high tension battery (HT)
• So alternating current is produced in the circuit.
• This alternating current is passed through the coil L1 and L2 of the primary circuit
and it is transferred to the secondary circuit L3.
• This current is passed to the plates A and B and it leads the crystal to vibrate due
to the principle of inverse piezo electric effect. The vibrations of the crystal will
create ultrasonic waves.
• When the frequency of the oscillatory circuit is equal to the frequency of the
vibrating crystal resonance occurs.
• At resonance the crystal vibrates vigorously and ultrasonic waves are produced
with very high frequencies.

Advantages :
It is more efficient than magnenostriction transducer
It can produce frequency upto 500 MHz.
It is not affected by temperature and humidity.
Disadvantages:
The cost of piezoelectric quartz is high
Cutting and shaping of quartz crystal is very complex

FEED MECHANISM
• Feed system is used to apply the static load between the tool
and workpiece during ultrasonic machining process
• The basic requirements to tool mechanism are as follows
1. The tool should be moved slowly to prevent breaking
2. The tool has to come back to its initial position after
finishing its machining operation.
• There are three types of feed mechanism. They are
1. Gravity feed mechanism
2. Spring loaded feed mechanism
3. Pneumatic (or) Hydraulic feed mechanism

FEED MECHANISM
• Gravity feed mechanism

GRAVITY FEED MECHANISM
• In this mechanism counter weights are used to apply
the load to the head through the pulley.
• The overall impact force is the difference between the
weight of the acoustic head and that of the counter
weight used.
• The force here can be adjusted by varying the counter
weights
• In order to reduce the friction the ball bearings are
used.
• Gravity feed mechanism is generally preferred
because of simple construction

SPRING LOADED FEED MECHANISM
•In this mechanism the spring pressure is used to
feed the tool during the machining operation.
•This mechanism is also preferred because of its
sensitivity and compactness.

PNEUMATIC OR HYDRAULIC FEED MECHANISM

PNEUMATIC OR HYDRAULIC FEED
MECHANISM
• In order to get high feed rate, pneumatic feed
mechanism is used

•In usm the metal removal rate depends on the
following
(a)Grain size of the abrasive
(b)Abrasive materials
(c)Concentration of slurry
(d)Amplitude of vibration
(e)Frequency of Ultrasonic waves
METAL REMOVAL RATE

METAL REMOVAL RATE
(a) Grain size of the abrasive
MRR and surface finish are greatly influenced by the grain size of the
abrasive
Maximum rate in machining is attained when the grain size of the
abrasive is bigger.
For rough work operation grit size of 200-400 is used and for finishing
operation the grit size of 800-1000 is used

METAL REMOVAL RATE
(b) Abrasive Materials
The proper selection of the abrasive particles
depends on the type of material to be machined,
hardness of the material and metal removal rate and
the surface finish required.
The most commonly used abrasive materials are
boron carbide and silicon carbide used for machining
tungsten carbide and die steel.
Aluminum oxide is the softest abrasive used for
machining glass and ceramics

METAL REMOVAL RATE
(c) Concentration of slurry:
Abrasive slurry usually a mixture of abrasive grains and water of
definite proportion 20-30% is made to flow flow under pressure.
The fig shows how the metal removal rate varies with slurry
concentration

METAL REMOVAL RATE
(d) Amplitude of Vibration
 Metal removal rate in ultrasonic machining process increases with
increasing amplitude of vibration

METAL REMOVAL RATE
(e) Frequency
Ultrasonic wave frequency is directly proportional to the metal
removal rate as shown in the fig.

PROCESS PARAMETERS
• The various process parameters involved in USM
methods are as follows
(a)Metal removal rate
(b)Tool Material
(c)Tool Wear rate
(d)Abrasive materials and abrasive slurry
(e)Surface finish
(f) Work Material

PROCESS PARAMETERS
•Tool material
Low carbon steel and stainless steel are most
commonly used tool materials.
Since long length tools cause overstress the tool
should be short and rigid.
Hollow tool can be made with wall thickness greater
than 0.5 to 0.8 mm
Side Clearance is of the order of 0.06mm to 0.36 mm

PROCESS PARAMETERS
• Tool wear ratio
It is defined as the ratio of the volume of the material
removed from the work to the volume of material removed
from the tool
It is 1.5:1 for tungsten carbide work piece
100:1 for glass
50: 1 for quartz
75:1 for ceramics
1:1 for hardened tool steel

PROCESS PARAMETERS
Surface Finish
The maximum speed of penetration in soft and brittle
materials such as soft ceramics are of the order of
200mm/min.
Penetration rate is lower for hard and tough
materials.
Accuracy of this process is ± 0.006mm
Surface finish upto 0.02 to 08 micron value can be
achieved

PROCESS PARAMETERS
Work Materials
• Hard and Brittle materials, non metals like glass, ceramics etc and
semiconductors are used as work material in usm process.
• Wear ratio , average penetration rate, maximum machining area of
the different workpiece materials are shown in the table

ADVANTAGES OF USM
• Extremely hard and brittle materials can be easily machined
• Cost of metal removal is low
• Noiseless operation
• High accuracy and surface finish can be easily obtained
• No heat generation in this process. So the physical properties
of the workpiece material remain unchanged
• Equipment is safe to operate
• Non conducting materials such as glass, ceramics and semi
precision stones can be easily machined.
• The machined workpieces are free from stress

DISDAVANTAGES
•Wear rate of the tool is high
•The initial cost of equipment is high
•For effective machining the abrasive materials should
be replaced periodically since the dull abrasives stop
cutting action
•High power consumption
•Tool cost is high
•The size of the cavity that can be machined is limited

APPLICATIONS & LIMITATINS OF USM
Applications :
• Holes as small as 0.1mm can be drilled
• Precise and intricate shaped articles can be machined
• It has been efficiently applied to machine glass, ceramics, tungsten, precision
mineral stones.
• Used for making tungsten carbide and diamond wire drawing dies and dies for
forging and extrusion process
• Machining operations like drilling, turning, threading, cutting, grinding, profiling
on all materials both conducting and non conducting materials.
Limitations:
• Under Ideal conditions :
Penetration rate - 5mm3/min
Power - 500-1000 watts
Metal removal rate on brittle materials - 0.018 mm3/min
Hole tolerance - 25µm
Surface finish - 0.2 to 0.7µm

CHARACTERISTICS OF USM
• Metal removal mechanism : Slurry of small abrasive particles are forced
against the workpiece by means of vibrating tool and it causes the removal
of metal from the workpiece
• Abrasive : Silicon carbide, boron carbide, aluminum oxide and diamond
• Abrasive slurry : abrasive grains + 20-30% water
• Vibration frequency : 20 to 30 kHz
• Amplitude : 25 – 100m
• Wear ratio:1.5:1 for tungsten carbide work piece, 100:1 for glass,
50: 1 for quartz, 75:1 for ceramics ,1:1 for hardened tool
steel
• Work Material : Tungsten carbide, germanium, glass, ceramics, quartz, tool
steel
• Tool material : Low carbon steel, stainless steel
• Surface finish : 0.2 to 0.7 µ

ABRASIVE WATER JET MACHINING
(AWJM)

Unit ii usm

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