INTRODUCTION TO ULTRASONIC MACHINING

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Chapter 7
ULTRA SONIC MACHINING (USM)
INTRODUCTION
Ultrasonic machining (USM), sometimes called ultrasonic impact grinding, employs
ultrasonically vibrating tool to impel the abrasives in a slurry at high velocity against
work piece. The tool is fed into the part as it vibrates along an axis parallel to the tool
feed at an amplitude on the order of several thousandths of an inch and a frequency of 20
kHz. As the tool is fed into the work piece, a negative of the tool is machined into the
work piece. The cutting action is performed by the abrasives in the slurry which is
continuously flooded under the tool. The slurry is loaded up to 60% by weight with
abrasive particles. Lighter abrasive loadings are used to facilitate the flow of the slurry
for deep drilling (to 5mm deep). Boron carbide, aluminum oxide, and silicon carbide are
the most common used abrasives in grit sizes ranging from 400 to 2000. The amplitude of
the vibration should be set approximately to the size of the grit. The process can use
shaped tools cut virtually any material but is most effective on materials with hardness
greater than Rc 40 including brittle and non conductive materials such as glass. Figure
28-15 shows a simple schematic of this process

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A comparative study on ultrasonic machining of hard and brittle
materials.
Precision abrasive processes are commonly employed to machine glasses, single crystals
and ceramic materials for various industrial applications. Until now, precision machining
of hard and brittle solids are poorly investigated in Brazil from the fundamental and
points of views. Taking into account major technological importance of this subject to the
production of functional and structural components used in high performance system, the
present study investigated the ultrasonic abrasion of different work piece materials-
alumina, Zirconia, quartez, glass, ferrite LiF—by using a stationary ultrasonic machine.
Experiments were conducted using a rectangular shaped cutting tool and sic particles
with mean grain size of 15m. the machined surfaces were characterized by surface
profilometry and scanning electron microcopy. In the case of alumina, zirconium and
quartz, the rates of material removal decreases with the depth of machining. The rate of
material removal remained constant for the other materials. The micrographs showed that
brittle micro cracking was the primary mechanism involved with material removal. The
rates of material removal and the machined surface topographies were discussed as a
function of intrinsic stiffness, hardness and fracture toughness of the work piece
materials.
· First built 1950s.
· Originally used for finishing EDM surfaces.
· Best suited to poorly conducting materials, and brittle materials.
· The basic process is a ductile and tough tool is pushed against the work with a constant
force. A constant stream of abrasive slurry passes between the tool and the work to
provide abrasives and carry away fractured particles. The majority of the cutting
action comes from an ultrasonic (cyclic) force applied in addition to the other force.

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· The basic components to the cutting action are believed to be,
1. 1. The direct hammering of the abrasive into the work by the tool
(major factor)
2. 2. The impact of the abrasive on the work
3. 3. Cavitation induced erosion
4. 4. Chemical erosion caused by slurry
· M.C. Shaw generated a model to estimate the cutting action.
· Consider the impact pictured below.

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· Material is also removed by grains moving quickly and building up kinetic energy.
When they strike the work surface, they transfer their energy quickly causing surface
work. This effect is smaller than hammering.
· The grains are not actually perfectly spherical, and as a result smaller rounds actually
lead to faster machining.

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· mmr decreases when static force F gets high enough to crush abrasive grains.

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· f = 16.3 KHz, A = 12.5 micro m, grain = 100 mesh.
· If d approaches A the grains start to crush.
· Example - Find the machining time for a hole 5mm in diameter in a tungsten carbide
plate 1cm thick. The grains are .01mm in diameter, the feed force is 3N, and the

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amplitude of oscillation is 20 micro m at a frequency of 25KHz. The fracture hardness
is approximately 6900N/mm2. The slurry is mixed in equal parts water and abrasive.
· Basic machine layout,

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· The acoustic head is the most complicated part of the machine. It must provide a static
force, as well as the high frequency vibration
· The magnetostrictive head is quite popular,
· Magnetostrictive materials should have a good coupling of magnetic and mechanical
energy,
· The vibrating head is supplied with a constant force using,
1. - counter weights
2. - springs
3. - pneumatics and hydraulics
4. - motors

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· If a tool is designed to increase flow, better cutting speeds will occur.
· Tools
1. - hard but ductile metal
2. - stainless steel and low carbon
3. - aluminum and brass tools wear near 5 to 10 times faster
· Abrasive Slurry
1. - common types of abrasive
1. - boron carbide (B4C) good in general,
but expensive

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2. - silicon carbide (SiC) glass, germanium,
ceramics
3. - corundum (Al2O3)
4. - diamond (used for rubies, etc)
5. - boron silicarbide (10% more abrasive
than B4C)
2. - liquid
1. - water most common
2. - benzene
3. - glycerol
4. - oils
3. - high viscosity decreases mrr
4. - typical grit size is 100 to 800
· Little production of heat and stress, but may chip at exit side of hole. Sometimes glass is
used on the back side for brittle materials.
· Summary of USM characteristics
1. - mechanics of material removal - brittle fracture caused by impact of
abrasive grains due to vibrating at high frequency
2. - medium - slurry
3. - abrasives: B4C; SiC; Al2O3; diamond; 100-800 grit size
4. - vibration freq. 15-30 KHz, amplitude 25-100 micro m
5. - tool material soft steel
6. - material/tool wear = 1.5 for WC work piece, 100 for glass
7. - gap 25-40 micro m
8. - critical parameters - frequency, amplitude, tool material, grit size,
abrasive material, feed force, slurry concentration, slurry viscosity
9. - material application - metals and alloys (particularly hard and brittle),
semiconductors, nonmetals, e.g., glass and ceramics
10. - shape application - round and irregular holes, impressions
11. - limitations - very low mrr, tool wear, depth of holes, and cavities
small.
· USM twist drilling has been done by attaching a magnetostrictive head to the spindle
shelf.