Ultrasonic Machining (USM)

USM components
Introduction
• Ultrasonic machining (USM) is the removal of hard and brittle
materials using an axially oscillating tool at ultrasonic
frequencies [18–20 kilohertz (kHz)].

• During that oscillation, the abrasive slurry of B4C or SiC is

continuously fed into the machining zone between a soft tool
and the workpiece.

• The abrasive particles are, therefore, hammered into the

workpiece surface and cause chipping of fine particles from it.

• The oscillating tool, at amplitudes ranging from 10 to 40 µm,

imposes a static pressure on the abrasive grains and feeds
down as the material is removed to form the required tool
shape.
The machining system
• The machining system, is composed mainly from the
magnetostrictor, concentrator, tool, and slurry feeding
arrangement.

• The magnetostrictor is energized at the ultrasonic frequency

and produces small-amplitude vibrations.

• Such a small vibration is amplified using concentrator

(mechanical amplifier) that holds the tool. The abrasive slurry
is pumped between the oscillating tool and the brittle
workpiece.

• A static pressure is applied in the tool-workpiece interface

that maintains the abrasive slurry
Main elements of an ultrasonic machining system
USM system components
The magnetostrictor
• The magnetostrictor used in USM, has a high-frequency
winding wound on a magnetostrictor core and a special
polarizing winding around an armature.
• Magnetic field undergoes ultrasonic frequency changes in a
ferromagnetic object placed within its region of influence.

Magnetostriction transducer
• This effect is used to oscillate the USM tool, which is mounted
at the end of a magnetostrictor, at ultrasonic frequencies (18
to 20 kHz).
• The coefficient of magnetostriction elongation εm

• where ∆l is the incremental length of the magnetostrictor core and l is the

original length of the magnetostrictor core, both in millimeters.

• Materials having high magnetostrictive elongation are

recommended to be used for a magnetostrictor
Mechanical amplifier

• The elongation obtained after transducer is usually 0.001 to

0.1 µm, which is too small for practical machining applications.
• The vibration amplitude is increased by fitting an amplifier
(acoustic horn) into the output end of the magnetostrictor.
Larger amplitudes, typically 40 to 50 µm, are found to be
suitable for practical applications.
• Depending on the final amplitude required, the amplitude
amplification can be achieved by one or more acoustic horns.
 Types of Horns

• The choice of the shape of the acoustic horn controls the final
amplitude.
Tools

• Tool tips must have high wear resistance and fatigue

strength.

• For machining glass and tungsten carbide, copper

and chromium silver steel tools are recommended.
• During USM, tools are fed toward, and held against, the
workpiece by means of a static pressure that has to
overcome the cutting resistance at the interface of the tool
and workpiece.

• Different tool feed mechanisms are available that utilize

pneumatic, periodic switching of a stepping motor or
solenoid, compact spring-loaded system, and counterweight
techniques.
Abrasive slurry
• Abrasive slurry is usually composed of 50 percent (by
volume) fine abrasive grains (100–800 grit number) of boron
carbide (B4C), aluminum oxide (Al2O3), or silicon carbide
(SiC) in 50 percent water.

• The abrasive slurry is circulated between the oscillating tool

and workpiece.

• Under the effect of the static feed force and the ultrasonic
vibration, the abrasive particles are hammered into the
workpiece surface causing mechanical chipping of particles.

• The slurry is pumped through a nozzle close to the tool-

workpiece interface at a rate of 25 liters per minute (L/min).
Material removal process
1. Mechanical abrasion by localized direct hammering of the
abrasive grains stuck between the vibrating tool and
adjacent work surface.

2. The microchipping by free impacts of particles that fly across

the machining gap and strike the workpiece at random
locations.

3. The work surface erosion by cavitation in the slurry stream.

• The relative contribution of the cavitation effect is reported
to be less than 5 percent of the total material removed.

• The dominant mechanism involved in USM of all materials is

direct hammering.

• Soft and elastic materials like mild steel are often plastically
deformed first and are later removed at a lower rate.

• In case of hard and brittle materials such as glass, the

machining rate is high and the role played by free impact
can also be noticed.

• When machining porous materials such as graphite, the

mechanism of erosion is introduced.
• The rate of material removal, in USM, depends, first of all,
on the frequency of tool vibration, static pressure, the size
of the machined area, and the abrasive and workpiece
material.
Factors affecting USM performance
Factors affecting material removal rate
Tool oscillation
• The amplitude of the tool oscillation has the greatest effect
of all the process variables.

• The material removal rate increases with a rise in the

amplitude of the tool vibration.

• The vibration amplitude determines the velocity of the

abrasive particles at the interface between the tool and
workpiece.

• Under such circumstances the kinetic energy rises, at larger

amplitudes, which enhances the mechanical chipping
action and consequently increases the removal rate.
• A greater vibration amplitude may lead to the occurrence of
splashing, which causes a reduction of the number of active
abrasive grains and results in a decrease in the material
removal rate.
Abrasive grains
• Both the grain size and the vibration amplitude have a similar
effect on the removal rate.

• The removal rate rises at greater grain sizes until the size
reaches the vibration amplitude, at which stage, the material
removal rate decreases.

• When the grain size is large compared to the vibration

amplitude, there is a difficulty of abrasive renewal in the
machining gap.

• Because of its higher hardness, B4C achieves higher removal

rates than silicon carbide (SiC) when machining a soda glass
workpiece.
• Water is commonly used as the abrasive carrying liquid for
the abrasive slurry while benzene, glycerol, and oils are
alternatives.

• The increase of slurry viscosity reduces the removal rate.

• The improved flow of slurry results in an enhanced

machining rate.

• In practice a volumetric concentration of about 30 to 35

percent of abrasives is recommended.
• A change of concentration occurs during machining as a
result of the abrasive dust settling on the machine table.

• The actual concentration should, therefore, be checked at

certain time intervals.

• The increase of abrasive concentration up to 40 percent

enhances the machining rate.

• More cutting edges become available in the machining zone,

which raises the chipping rate and consequently the overall
removal rate.
Workpiece impact-hardness

• The machining rate is affected by the ratio of the tool

hardness to the workpiece hardness.

• In this regard, the higher the ratio, the lower will be the
material removal rate.

• For this reason soft and tough materials are

recommended for USM tools.
Dimensional accuracy
Generally the accuracy of machined parts suffers from the
following

• Side wear of the tool

• Abrasive wear
• Inaccurate feed of the tool holder
• Form error of the tool
• Unsteady and uneven supply of abrasive slurry around the
oscillating tool
Overcut

• The process accuracy is measured through the overcut

(oversize) produced during drilling of holes.

• The hole oversize measures the difference between the

hole diameter, measured at the top surface, and the tool
diameter.

• The side gap between the tool and the machined hole is
necessary to enable the abrasives to flow to the machining
zone under the oscillating tool.

• Hence the grain size of the abrasives represents the main

factor, which affects the overcut produced.
• The overcut is considered to be about two to four times
greater than the mean grain size when machining glass and
tungsten carbide.

• However, the magnitude of the overcut depends on many

other process variables including the type of workpiece
material and the method of tool feed.

• In general USM accuracy levels are limited to ±0.05 mm.

Conicity

• The overcut is usually greater at the entry side than at the exit
one due to the cumulative abrasion effect of the fresh and
sharp grain particles.

• As a result of such an effect, a hole conicity of approximately

0.2°arises when drilling a 20-mm-diameter hole to a depth of
10 mm in graphite.
The conicity can be reduced by

• The use of tools having negatively tapering walls

• The use of wear-resistant tool materials

• The use of an undersized tool in the first cut and a final tool
of the required size, which will cut faster and reduce the
conicity
Surface quality
The surface finish is closely related to the machining rate in
USM.

The larger the grit size, the faster the cutting but the coarser the
surface finish.

However, other factors such as tool surface, amplitude of tool

vibration, and material being machined also affect the surface
finish.

The larger the grit (smaller the grain size), the smoother
becomes the produced surface.

As mentioned earlier, the larger chipping marks formed on

brittle machined materials create rougher surfaces than that
obtained in the case of machined hard alloy steel.
• The amplitude of tool oscillation has a smaller effect on
the surface finish.

• As the amplitude is raised the individual grains are pressed

further into the workpiece surface thus causing deeper
craters and hence a rougher surface finish.

• Other process variables such as static pressure have a little

effect on the surface finish.

• Smoother surfaces can also be obtained when the viscosity

of the liquid carrier of the abrasive slurry is reduced.

• It is evident that the surface irregularities of the sidewall

surfaces of the cavities are considerably larger than those
of the bottom.
Applications
USM should be applied for shallow cavities cut in hard and
brittle materials having a surface area less than 1000 mm2.

Drilling
A modified version of USM , where a tool bit is rotated
against the workpiece in a similar fashion to conventional
drilling.

• The process is, therefore, called rotary ultrasonic

machining (RUM).

• RUM ensures high removal rates, lower tool pressures for

delicate parts, improved deep hole drilling, less breakout
or through holes.
Production of EDM electrodes

• Typical ultrasonic machining speeds in graphite, range from

0.4 to 1.4 centimeters per minute (cm/min).

• The surface roughness ranges from 0.2 to 1.5 µm and

accuracies of ±10 µm are typical. Small machining forces
permit the manufacture of fragile graphite EDM electrodes.
Other Applications
• Ultrasonic polishing

• Micro Ultrasonic machining

• Cutting off parts made from semiconductors at high

removal rates compared to conventional machining
methods

• Engraving on glass as well as hardened steel and sintered

carbide

• Parting and machining of precious stones including

diamond
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