ultasonic machining process
DESCRIPTION
USMTRANSCRIPT
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Chapter Ⅰ USMChapter Ⅰ USM
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USM is able to efectively machine all hard materialwhether they are electrically conductive or not.
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The process and cutting toolThe process and cutting tool
• The process is performed by a cutting tool, which oscillates athigh frequency, typically 20-40 kH, in abrasi!e slurry"
• The shape of the tool corresponds to the shape to be
produced in the workpiece"• The high-speed reciprocations of the tool dri!e the abrasi!e
grains across a small gap against the workpiece .
• The tool is gradually fed with a uniform force"
• The impact of the abrasi!e is the energy principallyresponsible for material remo!al in the form of small wearparticles that are carried away by the abrasi!e slurry"
• The tool material, being tough and ductile, wears out at amuch slower rate"
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Ultrasonic MachiningUltrasonic Machining
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Elements of ultrasonic machiningElements of ultrasonic machining
• The tool is oscillated by alongitudinal magnetostriction
• # magnetic field !ariation at
ultrasonic frequencies
• The length of a ferromagnetic
ob$ect changes
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Material removalMaterial removal
• %ccurs when the abrasi!e particles, suspended in theslurry between the tool and workpiece, are struck by thedownstroke of the !ibration tool"
• The impact propels the particles across the cutting gap,hammering them into the surface of both tool and
workpiece" &ollapse of the ca!itation bubbles in the
abrasi!e suspension results in !ery high local pressures"
• 'nder the action of the associated shock wa!es on the
abrasi!e particles, microcracks are generated at the
interface of the workpiece"
• The effects of successi!e shock wa!es lead to chipping of
particles from the workpiece"
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Material removalMaterial removal
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The basic components to the cuttingThe basic components to the cutting
action are believed to beaction are believed to be
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• (mall, tabletop-sied units to large-capacity machinetools,
• )ench units, and as self-contained machine tools"
• *ower range from about 40 + to 2" k+"
• The power rating strongly influences the materialremo!al rate"
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Subsystems of USM SystemSubsystems of USM System
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EE
CC
DD
AA
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• The power supply is a sine-wa!e generator
• The user can control o!er both the frequency and power of thegenerated signal"
• t con!erts low-frequency .0/0 H1 power to high-frequency.0- kH1 power
• (upply to the transducer for con!ersion into mechanical
motion"
AA
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• Two types of transducers are used in '(3 to con!ert the
supplied energy to mechanical motion"• They are based on two different principles of operation
- 3agnetostriction
- *ieoelectricity
BB
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• 3agnetostricti!e transducers are usually constructed from alaminated stack of nickel or nickel alloy sheets"
• 3agnetostriction is eplained in terms of domain theory "
BB
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• 5omains are !ery small regions, of the order of l0-6 7 l0-8 cm9,
• n which there are forces that cause the magnetic moments of the
atoms to be oriented in a single direction"
• n each domain the atomic magnetic moments are oriented in one
of the directions of easy magnetiation
BB
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• n the cubic-lattice crystals of iron and nickel there are
si directions of easy magnetiation"
• n unmagnetied material all these directions are
present in equal numbers, the magnetic moments of the
orderless, unorientated domains compensate oneanother
BB
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• +hen the material is placed in a sufficiently strongmagnetic field, the magnetic moments of the domainsrotate into the direction of the applied magnetic fieldand become parallel to it"
• 5uring this process the material epands or contracts,until all the domains ha!e become parallel to oneanother"
BB
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• #s the temperature is raised, the amount of
magnetostricti!e strain diminishes "
• 3agnetostricti!e transducers require cooling by fans or
water"
BB
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• (uch as quart or lead,irconate,titanate, generate a small
electric current when compressed"
• &on!ersely, when an electric current is applied, the
material increases minutely in sie"
• +hen the current is remo!ed, the material instantlyreturns to its original shape"
BB
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• *ieoelectric materials are composed of small particles boundtogether by sintering"
• The material undergoes polariation by heating it abo!e the
&urie point"
• (uch transducers ehibit a high electromechanical con!ersionefficiency that eliminates the need for cooling"
BB
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• The magnitude of the length change is limited by the
strength of the particular transducer material"
• The limit is approimately 0"02 mm"
BB
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• ts function is to increase the tool !ibration amplitudeand to match the !ibrator to the acoustic load"
• t must be constructed of a material with good acousticproperties and be highly resistant to fatigue cracking"
CC
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• 3onel and titanium ha!e good acoustic properties and are oftenused together with stainless steel, which is cheaper"
• Howe!er, stainless steel has acoustical and fatigue properties thatare inferior to those of 3onel and titanium, limiting it to lowamplitude applications"
• :onamplifying holders are cylindrical and result in the samestroke amplitude at the output end as at the input end"
• #mplifying toolholders ha!e a cross section that diminishestoward the tool, often following an eponential function"
• #n amplifying toolholder is also called a concentrator"
CC
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• #mplifying holders remo!e material up to 0 times fasterthan the nonamplifying type"
• The disad!antages of amplifying toolholders includeincreased cost to fabricate, a reduction in surface finishquality, and the requirement of much more frequent runningto maintain resonance"
CC
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• Tools should be constructed from relati!ely ductile
materials"
• The harder the tool material, the faster its wear rate will
be"
• t is important to realie that finishing or polishing
operations on the tools are sometimes necessary because
their surface finish will be reproduced in the workpiece"
DD
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• The geometry of the tool generally corresponds to thegeometry of the cut to be made,
• )ecause of the o!ercut, tools are slightly smaller thanthe desired hole or ca!ity
• Tool and toolholder are often attached by sil!er braing"
DD
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• The criteria for selection of an abrasi!e for a particular
application include hardness, usable life, cost, and particlesie"
• 5iamond is the fastest abrasi!e, but is not practical because of
its cost"
• )oron carbide is economical and yields good machining rates"
• (ilicon carbide and aluminum oide are also widely used"
EE
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• &oarse grits ehibit the highest remo!al rates,when the grainsie becomes comparable with the tool amplitude, cut moreslowly"
• The larger the grit sie, the rougher the machined surface"
EE
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• +ith an abrasi!e concentration of about 0; by weight
in water , but thinner mitures are used to promote
efficient flow when drilling deep holes or when forming
comple ca!ities"
EE
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EE
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ExampleExample
• <ind the machining time for a hole mm in diameter in a
tungsten carbide plate cm thick"• The grains are 0"0mm in diameter, the feed force is 9:, and
the amplitude of oscillation is 20 micro m at a frequency of
2=H"
• The fracture hardness is approimately 800:/mm2"
• The slurry is mied in equal parts water and abrasi!e"
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ExampleExample
The acoustic head is the
most complicated part of
the machine"
t must pro!ide a static
force, as well as the highfrequency !ibration
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- Basic machine layout- Basic machine layout
ExampleExample
3agnetostricti!e materials should ha!e a good coupling of
magnetic and mechanical energy
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Basic machine layoutBasic machine layoutExampleExample
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• f a tool is designed to increase flow, better cutting speeds willoccur"
• Tools
- hard but ductile metal
- stainless steel and low carbon
- aluminum and brass tools wear near to 0 times faster
• #brasi!e (lurry
- common types of abrasi!e
- boron carbide .)4&1 good in general, but epensi!e- silicon carbide .(i&1 glass, germanium, ceramics
- corundum .#l2%91
- diamond .used for rubies , etc1
- boron silicon-carbide .0; more abrasi!e than )4&1
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• liquid- water most common
- benene
- glycerol
- oils• high !iscosity decreases mrr
• typical grit sie is 00 to 600
>ittle production of heat and stress, but may chip at eitside of hole"
(ometimes glass is used on the back side for brittle
materials"
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• 3echanics of material remo!al - brittle fracture caused by impact ofabrasi!e grains due to !ibrating at high frequency
• 3edium - slurry
• #brasi!es? )4&@ (i&@ #l2%9@ diamond@ 00-600 grit sie
• Aibration freq" -90 =H, amplitude 2-00 micro m
• Tool material soft steel
• 3aterial/tool wear B " for +& workpiece, 00 for glass
• Cap 2-40 micro m
• &ritical parameters - frequency, amplitude, tool material, grit sie,abrasi!e material, feed force, slurry concentration, slurry !iscosity
• 3aterial application - metals and alloys .particularly hard and brittle1,
semiconductors, nonmetals, e"g", glass and ceramics
• (hape application - round and irregular holes, impressions
• >imitations - !ery low mrr, tool wear, depth of holes, and ca!ities small"
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" # cylindrical impression with a diameter of 0mm and a depthof mm has to be made on a tungsten carbide surface" Thefeed force is constant and equal to :" The a!erage diameterof the grains in the abrasi!e slurry is 0"0mm" The tooloscillates with an amplitude of 90 micro m at 20 =H" Theslurry contains part of abrasi!e to about part of water" Thefracture hardness of tungsten carbide workpiece may betaken as D000 :/mm2" Estimate the machining time"
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2" # square through hole of mm by mm has to be drilled in amm thick tungsten carbide sheet" The slurry is made of part of 0 micro m radius boron carbide grains mied with "parts of water" The feed force is 4:" The tool oscillates with an
amplitude of 0"0mm at 2=H" #ssuming that only 20; ofthe pulses are effecti!e, calculate the time required tocomplete the $ob"
9" n an E&3 operation, a pure copper block is being machined"f a current of 000# is used, determine the !olume rate of
material remo!al from the copper block"
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4" The composition of a :imonic alloy turbine blade is 6;
cobalt, 2; :i, and 20; chromium" t is being machined
electrochemically with a current of 00#" <ind out the !olume
remo!al rate if the density of the alloy is 6"9g/cm9" The
dissolution !alency of chromium is , whereas that for both
nickel and cobalt is 2"
" The composition of a monel alloy workpiece undergoing
electrochemical machining is as gi!en here? 9; :i, 9"D; &u, 2"; <e, 2; 3n, 0"; (i, 0"9; &
if the machining current is 000#, estimate the !olume
remo!al rate"
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" The equilibrium gap when machining .electrochemically1 iron,
using :a&l solution in water as the electrolyte, is found to be
0"2mm" The current density is 200#/cm2, the operating
!oltage being 2A" ron dissol!es at a !alency 2, the density ofiron is D"6 g/cm9, and the specific resistance of the electrolyte
is 2"6 ohm cm" &alculate the metal remo!al rate/unit work
surface area" The o!er!oltage may be taken as "A"
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D" n an electrochemical trepanning operation on a flat ironsurface, an electrode in the form of a tube .with an outerdiameter of cm1" # laser beam with a power intensity of
2 F 0 +/mm2 is used to drill a 0"2mm diameter hole in atungsten sheet of 0"4mm thickness" f the efficiency of theoperation is only 0;, estimate the time required"
6" TG'E / <#>(E - +ater is the main cutting tool in 'ltra
(onic machining"
8" +hy are the !ibrations in '(3 so small
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0" '(3 will be used to add the following pattern to an ob$ect,
f the tool is Tungsten carbide, and the work is &u, with an
amplitude of oscillation of 0 Im, at 90=H, how long will the
operation take .:ote? the grain diameter is 20Im, and thehead has a static force of :1
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" +hen is the abrasi!e added into the flow for the !arious
abrasi!e $et machining processes
2" +hy is the depth of material remo!ed by abrasi!e $etmachining so !ariable
9" 5escribe the ability of the abrasi!e processes to
produce sharp corners"