ULTRASONIC MACHINING

1. Mechanical Machining
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Mechanical Machining

2. History:
Ultrasonic technology can be traced back to
research on the piezoelectric effect conducted by
Pierre Curie around 1880.
Ultra Sonic Machining; USM
Asymmetrical crystals such as quartz and
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Asymmetrical crystals such as quartz and
Rochelle salt (potassium sodium titrate) generate
an electric charge when mechanical pressure is
applied. Conversely, mechanical vibrations are
obtained by applying electrical oscillations to the
same crystals.

3. One of the first applications for Ultrasonic was
sonar (an acronym for sound navigation ranging).
It was employed on a large scale by the U.S. Navy
during World War II to detect enemy submarines.
Frequency values of up to 1Ghz (1 billion cycles
per second) have been used in the ultrasonic
industry.
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4. •The Ultrasonic waves are sound waves of frequency
higher than 20,000 Hz.
•Ultrasonic waves can be generated using
mechanical, electromagnetic and thermal energy
sources.
ULTRASONIC WAVES
sources.
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5. PRINCIPLE OF ULTRASONIC MACHINING
In the process of Ultrasonic Machining, material is
removed by micro-chipping or erosion with abrasive
particles.
In USM process, the
tool, is oscillated by the
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tool, is oscillated by the
Booster and Sonotrode
at a frequency of about
20kHz with an
amplitude of about 25.4
um (0.001 in).

6. PRINCIPLE OF ULTRASONIC MACHINING
The tool forces the abrasive grits, in the gap between the
tool and the workpiece, to impact normally and
successively on the work surface, thereby machining the
work surface.
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7. Piezoelectric effect & Piezoelectricity:
Is the method of obtaining piezoelectricity due to
the charge which accumulates in certain solid
materials such as crystals, certain ceramics in
response to applied mechanical stress.
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8. Process
 The basic USM process involves a tool vibrating with a
low amplitude and very high frequency and a continuous
flow of an abrasive slurry in the small gap between the
tool and the work piece.
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9.  The tool is gradually fed with a uniform force.
 The impact of the hard abrasive grains
fractures the hard and brittle work surface,
resulting in the removal of the work material in
the form of small wear particles.
 The tool material being tough and ductile
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 The tool material being tough and ductile
wears out at a much slower rate.

10. Mechanism of USM
The reasons for material removal in an USM process are
believed to be:
1. The hammering of the abrasive particles on the work
surface by the tool.
2. The impact of free abrasive particles on the work
surface.
3. The erosion due to cavitation.
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3. The erosion due to cavitation.
4. The chemical action associated with the fluid used.
The model proposed by M.C. Shaw is generally well
accepted and explains the material removal process well.

11. M.C. Shaw’s Model of USM Mechanics
In this model the direct impact of the tool on the grains in
contact with the work piece is taken into consideration.
Also, the assumptions made are:
1. The rate of work material removal is proportional to
the volume of the work material per impact.
2. The rate of work material removal is proportional to the
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2. The rate of work material removal is proportional to the
no. of particles making impact per cycle.
3. The rate of work material removal is proportional to the
frequency (no. of cycles per unit time).
4. All impacts are identical.
5. All abrasive grains are identical and spherical in shape.

12. Thus, volume of work material removal rate (Q)
Q α V Z ν
where, V = volume of the work material
removal per impact
Z = number of particles making impact
per cycle
ν = frequency
The position ‘A’
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• The period of movement from ‘A’ to ‘B’ represents the impact.
• The indentations, caused by the grain on the tool and the work
surface at the extreme bottom position of the tool from the position
‘A’ to position ‘B’ is ‘h’ (the total indentation).
The position ‘A’
indicates the
instant the tool
face touches the
abrasive grain.

13. MRR Vs Feed Force
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MRR increases with increasing feed force but after a
certain critical feed force it decreases because the
abrasive grains get crushed under heavy load.

14. The important parameters which affect the process are
the:
1. Frequency, f
2. Amplitude, A
3. Static loading (feed force),
4. Hardness ratio of the tool and the workpiece,
5. Grain size,
Process Parameters
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5. Grain size,
6. Concentration of the abrasive in the slurry.

15. With an increase in frequency of the tool head the
MRR should increase proportionally. However, there is a
slight variation in the MRR with frequency.
 When the amplitude of the vibration increases the
MRR is expected to increase. The actual nature of the
variation is shown in Fig. (b). There is some discrepancy
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variation is shown in Fig. (b). There is some discrepancy
in the actual values again. This arises from the fact that
we calculate the duration of penetration Δt by
considering average velocity.

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17.  We already said that with an increase in static
loading, the MRR tends to increase. However, at higher
force values of the tool head due to grain crushing the
MRR decreases.
 The ratio of workpiece hardness and tool hardness
affects the MRR quite significantly, and the
characteristics is shown below.
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characteristics is shown below.
 Apart from the hardness the brittleness of the work
material plays a very dominant role. The table below
shows the relative MRR for different work materials. As
can be seen the more brittle material is machined more
rapidly.

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MRR should also rise proportionately with the mean
grain diameter ‘d’. When ‘d’ becomes too large, the
crushing tendency increases.
 Concentration of the abrasives directly controls the
number of grains producing impact per cycle. MRR is
proportional to C1/4 so after C rises to 30% MRR increase
is not very fast.

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 Apart from the process parameters some physical
properties (e.g. viscosity) of the fluid used for the slurry
also affects the MRR. Experiments show that MRR drops
as viscosity increases.
 Although the MRR is a very important consideration for
judging the USM but so is the surface finish.

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The figure shows that the surface finish is more sensitive
to grain size in case of glass which is softer than tungsten
carbide.
This is because in case of a harder material the size of the
fragments dislodged through a brittle fracture does not
depend much on the size of the impacting particles.

21. Ultrasonic Machining Unit
The main units of an
Ultrasonic Machining
unit are shown in the
figure below. It
consists of the
following machine
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following machine
components:
(1)The acoustic head.
(2)The feeding unit.
(3)The tool.
(4)The abrasive
slurry and
pump unit.
(5)The body with
work table.

22. Abrasive Slurry
 The most common abrasives are Boron Carbide (B4C),
Silicon Carbide (SiC), Corrundum (Al2O3), Diamond and
Boron silicarbide.
 B4C is the best and most efficient among the rest but it
is expensive.
 SiC is used on glass, germanium and most ceramics.
 Cutting time with SiC is about 20-40% more than that
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 Cutting time with SiC is about 20-40% more than that
with B4C.
 Diamond dust is used only for cutting diamond and
rubies.
 Water is the most commonly used fluid although other
liquids such as benzene, glycerol and oils are also used.

23. Summery
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