9. El-Hofy Fundamentals of Machining Processes Tema 10

7/28/2019 9. El-Hofy Fundamentals of Machining Processes Tema 10

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10M odern Abrasive Processes

10.1 Ultrasonic MachiningUltrasonic machining (USM) is the removal of hard and brittle materialsusing an axially oscillating tool at ultrasonic frequency (18-20 kHz). Duringthat oscillation, the abrasive slurry of B4C or SiC is continuously fed into themachining zone, between a soft tool (brass or steel) and the workpiece. Theabrasive particles are, therefore, hammered into the workpiece surface andcause chipping of fine particles from it. The oscillating tool, at amplituderanging from 10 to 40 um, imposes a static pressure on the .abrasive grainsand feeds down as the material is removed to form the required tool shape(Figure 10.1).The machining system, shown in Figure 10.2, is composed mainly fromthe magnetostricter, concentrator, tool, and slurry feeding arrangement. Themagnetostricter 1S energized at the ultrasonic frequency and produces smallamplitude of vibration which is arnplified using the constrictor (mechanicalamplifier) that holds the tool. The abrasive slurry is pumped between theoscillating tool and the brittle workpiece.Tlic magnetostricer: The magnetostricter, shown in Figure 10.4, has a highfrequency winding on a magnetostricter core and a special polarizingwinding around an armature. The magnetostriction effect was firstdiscovered by Joule in Manchester in 1874. Accordingly, a magnetic fieldundergoing ultrasonic frequencies causes corresponding changes in aferromagnetic object placed within its region of influence. This effect isused to oscillate the USM tool, mounted at the end of a magnetostricter, atultrasonic frequencies of 18-20 kHz.

M C c ! l I 7n iC l 7 / l 7mp l i fi c r : The elongation obtained at the resonance frequency iston smaIl for practica! machining applications. The vibration amplitude is,therefore, increased by fitting an amplifier (acoustic horn) into the outputend of the magnetostricter. Larger amplitudes of typically 40-50 um are

2 9 9


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3 0 0 Fundamcntals of Mncllinil1g Procesees

Leadstotransducerwinding il~-::::-----.;1-;rr.r.:': ~----:... L------------ - - ------ .::: : .:. .:::= T I ~ t . : : - - ~ - - Magnetostrictionl l i : . ; g r 0 < l l l : . H - - - - - - . I ~ ; ; ; ; , " ,Coolingwater

Concentrator

ToolWorkpiece

FIGURE 10.1Main elernents of ultrasonic machining systcm.

suitable for practical USM applications. Depending on the amplitud erequired, theamplification process canbeachieved by oneormore acoustichorns. Tohave themaximum amplitude of vibration (resonance) the lengthof the concentrator ismade multiples of onehalf thewavelengths of soundin the concentrator (horn) material. Thechoice of the shape of the acoustichorn controls the final amplitude of vibration. Five acoustic horns, whichinclude cylindrical, stepped, exponential, hyperbolic cosine, and conical, arecommonly used inUSM.Aluminum bronze and marine bronze are cheap with high fatigue

strength of, respectively, 185and 150MN/m2, which makes them suitablefor acoustic horns. Themain drawbacks of themagnetostrictive transducer

Static pressure -vibrationsAbrasives +water

. -Workpiece

Localized hammering Cavitation erosionree impactFIGURE 10.2Material removal mechanism in USM.


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tviodcrn Abmsive Procl'SSCS 3 0 1are the high losses encountered, low efficiency (55/,,), and consequent heatup and need for cooling. Higher efficiencies (90-95%) are possible by usingpiezoclectric transformers to modern USM machines,Tools: Tool tips must have high wear resistance and high fatiguestrength. For machining glass and tungsten carbide, copper andchromium silver steel tools are recommended. Silver and chromiumnickel steel are used for machining sintered carbides. During USM, toolsare fed towards. and held against, the workpiece by rneans of a staticpressure that has to overcome the cutting resistance at the interface of thetool and workpiece.Abrasive slurri]: The abrasive slurry is usually composed of 50% (byvolume) fineabrasive grains (100-800 grit) ofboron carbide (B,.C),aluminurnoxide (Al203), or silicon carbide (SiC) in 50% water. The abrasive slurry iscirculated between the oscillating too! and workpiece through a nozzle closeto the tool-workpiece interface at an approximate rate of 25 L/min.

Materia! remooal proccss: Under the effect of the static feed force and theultrasonic vibration, the abrasive particles are hammered into the workpiecesurfacc causing mechanical chipping of minute particles. Figure 10.2 showsthe complete material removal mechanism of USM, which involves threedistinct actions: Mcchanical abrasion by localized direct hammering of the abrasivograins stuck between the vibrating tool and adjacent work surface The micro chipping by free impacts of particles which fly across themachining gClpand strike the workpiece at random locations Thc work surfnce erosin by cavitation in the slurry strearn

10.1.1 Mechanism of Material RemovalUsing the theory of Shaw (1956), material removal by USM due tocavitations under the tool and chemical corrosion due to slurry media areconsidered insignificant. Therefore, the material removal due to these twofactors has been ignored. The material removal by abrasive particles due tothrowing and hammering only has been considered.Abrasive particles are considered spherical in shape having diarneter d.Abrasive particles, suspended in a carrier, move under the high frequencyvibrating too\. There are two possibilities when the too! hits an abrasiveparticle. If the size of the particle is small and the gClpbetween the bottom ofthe tool and the work surface is large enough, then the particle will bethrown by the tool to hit the work surface (throwing model). Under thereverse condition, the particle will behammered ovcr the workpiece surface.In both cases, the particle creates a crater of depth I z p and radius rp' lt isassurned that the volume nf material removed isapproximately proportionalto the indentation diameter (2rp)'


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302 F undamentals 01Machining P rocesses

Tool

FIGURE 10.3Development of fracture in the workpieee due to hitting by a grain; (a) by throwing; (b) byhammering; and (e)erater shape.

The volume of material removed, Qw shown by the dotted line inFigure 10.3, assuming a hemispherical crater, due to fracture per grit percycle is given by

Q =! ( ~ 7 T T 3 )2 3 pAccording to Figure 10.3,it can be shown that

r = ( d a) 2 _ ( d a_h ) 2 ss d hp 2 2 p apTherefore, Qy becomes

where kl isaconstant and thenumber of impacts Ni on theworkpiece by thegrits in each cycle depends on the number of grits beneath the tool at anytime. This is inversely proportional to the diameter of the grits (assumedspherical) as

1N =k2-1 d2awhere k2is aconstant ofproportionality. All theabrasive particles under thetool need not necessarily beeffective. Letk2betheprobability of an abrasiveparticle under thetool being effective. Then thevolume ofmaterial removed


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Modern Abrasive Processes 303per second VRR equals the frequency i r times the amount of materialremoved per cycle Qv:

f h rVRR =Q v!r =klk2k3yti rToevaluate thedepth ofpenetration hp of anabrasive particle, Shaw (1956)

proposed the following.For the grain-throwing model :

where hth is thedepth of penetration due togrit throwing inmm, a/2 is theamplitude of tool oscillation, i r is the frequency of tool oscillation, da is thegrit diameter, P '1 is the density of abrasive grits, and a.; is themean stressacting upon theworkpiece surface.The volumetric removal rate due to the throwing mechanism VRRthbecomes

[1 f2a2p ]VRR = klk2k3 t a d {S/2th 6 zt! r(Jw

F or thc grain-hammering model: When the gap between the tool and theworkpiece is smaller than thediameter of thegrit d


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304 F undamcntals 01Machinng ProccssesThe volumetric removal rate from the workpiece due to the hammeringmechanism VRRh can be evaluated as

The computational results of [ain (2004) showed that

where hh is the depth of penetration due to grit hammering in mm, ;. is thefrequency of tool oscillation in l/s, (}is the mean stress acting upon the toolin Nz'mm''. p~ is the density of abrasive grits in g/cm3, (}wis the mean stressacting upon workpiece surface in Nz'mm'', and Fav is the mean force on thegrit in N.10.1.2 Sol ved ExampleCalculate the ultrasonic machining time required for a hole of diameter6mm in tungsten carbide plate (fracture hardness =6900 N/mnl) if thethickness of the plate is 1.5 hole diameter. The mean abrasive grain size is1511mdiameter. The feed force is equal to 3.5N. The amplitude of tooloscillation is 25 um and the frequency is equal to 25kHz. The toa] materialused is copper having fracture hardness equal to 1.5X 103N/ mm/. Theslurry contains one part abrasives to one part water. Take the values ofdifferent constants as kl =0.3, k2=1.8mm/, k3=0.6, and abrasive density =3.8g/ cm'. Calcula te the ratio of the volume removed by throwing to thatremoved by hammering (Jain, 2004).Solution Given:Hole diameter, da=6X10-3 mPlate thickness=1.5Xhole diarneter =Rx IT mMean abrasi ve grain size=15X 10- hmFeed force=3.5 NAmplitude of tool vibration, at/2=25X10-6 mFrequency of oscillation, ; .=25,000cpsFracture hardness of workpiece material, (}w=6.9X109 N/m2Abrasive grain density, p" =3.8 X 103kg/m3kl =0.3k2=1.8 mm2=1.8X10-6 m2k3=0.6Throwing model:


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Modern Abrasive P rocesses 305

n =1.78 X 10-5 mm

f E 3VRR =kJ k2k3 ~{th d )ra(1.78 X 10-5)32.5 X 104VRRth =0.3 X 1.8 X 0.6 1.5 X 10-2

Hammering model:4Favatda

(T w7rk2(j +1)j =(Tw =6900 =4.6

(Tt 1500

4 X 3.5 X (2 X 25 X 1O---{)X (1.5 X 10-5)hw = 7rX (1.8 X 10---{)X (6.9 X 10':1)X (4.6+1)

hw =2.182 X 10-4 mm

(2.192 X 10-4)3 4VRRh =0.3 X 1.8 X 0.6 2 2.5 X 101.5X 10-

The total removal rate VRR =VRRth +VRRh. The machining time t-.becomesVolume of holet-: = VRR (7r/4)62 X 90.21987


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306 F undamenials 01M achining P rocesses

tm =19.289mnVRRth =0.023VRR

It isclear that thematerial removed byhammering ismuch more than thatremoved by throwing (43 times); therefore, for approximate calculations,VRRth can be ignored.InUSM, the linear (theoretical) material removal rate VRRL inmm/s cangenerally bedescribed using the following imperial formula (Jain, 1993)

(at) ( d a ) 0.5 ( a t)o.5

VRR =5.9fr Hr 2 2where fr is the frequency of oscillation inHz, at is the static stress on tool inNz'rnm', H, is the surface hardness of the workpiece (rrXcompressivefracture) strength (Nz'mrrr'), da/2 is themean radius of grit inmm, and at/2is the amplitude of vibration inmm.Incaseofhard andbrittle materials suchasglass, themachining rateishigh

and the role played by the free impact is noticed. When machining porousmaterials such asgraphite, themechanism of erosion is introduced. Therateof material removal in USM depends, first of all, on the frequency of toolvibration, staticpressure, thesizeof themachined area, and theabrasive andworkpiece material. The material removal depends on the brittlenesscriterion, which is the ratio of shearing tobreaking strength of amaterial.According toTable10.1,glasshas ahigher removal rate than that of ametal ofsimlar hardness. Moreover, due tothelowbrittleness criterion of steel, whichis softer, it is used as atool material. Figure 10.5summarizes the importantparameters that affecttheperformance of USM,which aremainly related toT AB L E 10.1Typical Process Characteristics of USM (Tool: Low Carbon Steel; Slurry: 30-40% of180-240 grit B4C; Amplitude: 0.025-0.035 mm; Frequency: 25 kHz)

Material Removal RatePenetration MaximumWork Volume Rate Practical ToolMaterial (mm3/min) (mm/rnin) Area (mrrr') Wear Ratioa

Glass 425 3.8 2580 100:1Ceramic 185 1.5 1935 75:1Ferrite 390 3.2 2260 100:1Quartz 200 1.7 1935 50:1Tungsten 40 0.4 775 1.5:1carbideToa] steel 30 0.3 775 1:1Source: FromRao, PN., Manujacturing Technology: Metal Cutting and Iviacnine Tools, 8thEd.,NewDelhi, TataMcGraw-Hill Publishing Company Limted, 2000.a Ratio of material removed fromthework to that removed fromthe too!.


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Modern Abrasive Processes 307.~I

Machining conditions Frequency Amplitude Pressure Depth Area Machine condition+ WorkpieceAbrasive slurry Ductility Type Hardness Size ~ Removal rate, Compression~ Carrier liquid Surface quality, strength Feeding method Accuracy Tensile strength Concentration tTool Hardness Wearability Accuracy Fatigue strength Mounting

FIGURE 10.5Factors affecting USM performance.

thetool, workpiece material, the abrasives, machining conditions, and themachine tool.

10.1.3 Factors Affecting Material Removal RateTool oscillation: The amplitude of tool oscillation has the greatest effectof alltheprocess variables. Theamplitud eof oscillation varies within thelimits of0.04-0.08 mm. Thematerial removal rate increases with riseintheamplitudeof tool vibration (Figure 10.6). The vibration amplitude determines thevelacity of the abrasive particles at the interface between the toal andworkpiece. Under such circumstances, the kinetic energy rises at largeramplitudes, which enhances the mechanical chipping actian and conse-quently increases theremoval rateoGreater vibratian amplitudes may lead tathe accurrence af splashing, which causes a reduction af the number afactive abrasive grains and results in thedecrease of themetal removal rateoThe increase af feed force induces greater chipping farces by each grain,which raises the averall remaval rate (Figure 10.6and Figure 10.7).Regarding theeffectaf vibratian frequency on theremaval rate, foragiven

amplitude, the increase in vibration frequency reduces the removal rate(Figure 10.8). This trend may be related ta thesmall chipping timeallawedforeach grain such that lower chipping action prevails causing adecrease inremaval rateoThesame figureshaws that, foragiven frequency, theincreaseaf removal rate at higher amplitudes.


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308 F undamentals of Machining P rocesses

ro>oEoE


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Modern Abrasive P rocesses 309

~oEQ)a:

Frequency

AmplitudeFIGURE 10.8Variation of removal ratewith vibration amplitude and frequency.

flow of slurry results in an enhanced machining rateo In practice, avolumetric concentration of about 30-35% of abrasives is recommended.The increase of abrasive concentration up to 40%enhances themachiningrateoMore cutting edges become available in the machining zone, whichraises the chipping rate and consequently the overall removal rate(Figure 10.12).Workpiece impact hardness: Themachining rate isaffectedby theratio of tooltoworkpiece hardness (Figure10.13).Inthis regard, thehigher theratio, thelower will be the material removal rateoFor this reason, soft and toughmaterials are recommended for USM tools.

ai>oEQ)a:

Theoretical////// Actual

Mean grain sizeFIGURE 10.9Variationof removal ratewith mean grain size.


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310 Fnndiuncntal o f MIlc/lil/il/g Pro(('~SCS

Q)~((l>oEQ)a:

------------~Particle velocity

FIGURE 10.10Variation of rCIl1l1vZlI ratc with particlc vclocity.

Ton! stuu: The machining rate is affcctcd by the tool shape and area. Theincreasc of tool


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v fodc l'I1 A brasioe PrO(I 's s ,S 31 1

m>oEQ)n ::

>

SiC

30 % Abrasive concentrationFIGURE 10.12Variation of rl'1ll0V,11 rute abr.isive conccntr.rtion and typc,

tool censes to break aWoEQ)o:

Tool/Work hardnessFIGURE 10.nVari.ition of n-moval I"


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eoc o-.Q )eQ Je,

lool diameter, d1

~d2>d1

d3>d2

Feed forceFIGURE 10.14Variation of pcnetration rate with fecd force at diffcrent tool diameters,

Inaccurate feed of the tool holder Form error of the tool Unsteady and uneven supply of abrasive slurry around theoscillating tool

Overcut: The overcut is considered to be about 2-4 times greater than thcmean grain size when machining glass and tungsten carbide. It is about 3times greater than the mean grain size of B4C (mesh number 280-600). Themagnitude of the overcut depends on many other process variables,including the type of workpiece material and the method of ultrasonictool feed. I n general, USM accuracy levels are limited to +0.05mm.Conicity: The conicity of holes is approximately 0.20when drilling of


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P rocessl~Ii1t!ern Abrasive P rocesses

e

he ,t3 I

he j1iic ,I,

a ,I. t1I

313

100C /lC /l Glass)e 75..cOl::JeQ) 50al't:: :J(j)

25 Tungsten carbide

50 100 150Mean grain size

FIGURE 10.15Effectof grain size on surface roughness for different workpiece materials.

A relationship canbe found between thecrater dimensions: crater diameteris one third of the abrasive grain diameter and the depth is one tenth(McGeough,2002).Astheamplitude israised theindividual grains arepressed further into the

workpiece surface, thus causing deeper craters and, therefore, a roughersurface finish. Smoother surfaces canalsobeobtained when theviscosity ofthe liquid carrier of theabrasive slurry isreduced. Thesurface irregularitiesof the sidewall surfaces of the cavities areconsiderably larger than those ofthebottom. This results fromthesidewalls beingscratched bygrains enteringand leaving themachining zone. Cavitations damage tothemachined surfaceoccurs when the tool particles penetrate deeper into the workpiece. Undersuch circumstances, it ismore difficult to replenish adequately the slurry inthese deeper regions and arougher surface isproduced.

l'

TABLE 10.2Grit Number, Grit Size, and Surface Roughness in USMGrit Number Grit Size (rnm) Surface Roughness (um)180240320400600800

0.0860.0500.0400.0300.0140.009

0.550.510.450.40.280.21

Source: Metals Handbook (1989). Reproduced by permission of ASM International.


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314 Fundanzentals of Machining Processes

ToolVibration +Static pressure +1000 RPM~c:Slurry

Finished \workpiece ~

Slurry/Finishedworkpiece

Finishedworkpieceut-off Feed

FIGURE 10.16Rotary USM configurations.

10.1.6 ApplicationsUSM should beapplied for shallow cavities cut inhard and brittle materialshaving asurface arealess than 1000mm'.Rotary ultrasonic machining: A modified version of USM is shown inFigure 10.16where a tool bit is rotated against the workpiece in a simlarfashion to conventional coring, drilling, and mlling. RUM ensures highremoval rates, lower tool pressures for delicate parts, improved deep holedrilling, less breakout or through holes, and no core seizing during coredrilling. The process allows the uninterrupted drilling of small diameterholes. Conventional drilling necessitates a tool retraction, which increasesthemachining time. Thepenetration rate depends on the size and depth ofcavity. Small holes require more timeastherateofmachining decreases withdepth ofpenetration due tothedifficulty inmaintaining acontinuous supplyof new slurry at the tool face. Generally, adepth-to-diameter ratio of 2.5 isachievable by RUM.Sinking: During USM sinking, the material removal is difficult when themachined depth exceeds 5-7mm or when the active section of the toolbecomes important. Under such conditions the removal of theabrasive gritsat theinterfacebecomes difficult and, therefore, thematerial removal processisimpossible. Moreover, themanufacture of such atool isgenerally complexand costly. Contouring USM (Figure 10.17), employs simple tools that aremoved in accordance to the contour required.Production 01EDM electrodes:USMhasbeen used toproducegraphite EDMelectrodes. Typical ultrasonic machining speed ingraphite ranges from0.4to1.4cm/mno Surfacefinishrange from0.2to1.511mandaccuracies of1011maretypical. Small machining forcespermt themanufacture offragile graphiteEDM electrodes.Polishing: Ultrasonic polishing occurs by vibrating abrittle tool materialsuch as graphite or glass into the workpiece at ultrasonic frequency and


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Vibration +Static tRed Vibration +Static feed +NC rnotiont ! t !Slurry

Tool path

'./Sinking Warkpiece Contouring

FIGURE 10.17lJSM dil' xink inj; illldc(lnt()uring.

rel


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316 Fundamentals of M achining P rocessesPressure gauge

Powdersupplyand mixer

J et

RegulatoGassupply Nozzle

WorkpieceibratorFIGURE 10.18Abrasive jet machining systems.

It is not practical to reuse the abrasive powder because contaminations andworn grit cause adecline in themachining rateoTheabrasive powder feedrate is controlled by the amplitude of vibrations of the mixing chamber.The nozzle stand-off distance is kept at 0.81mm. The relative motionbetween the workpiece and the nozzle is manually or automaticallycontrolled using camdrives, pantographs, tracer mechanisms, or computercontrol according to the cut geometry required. Masks of copper, glass, orrubber may be used to concentrate the jet stream of the abrasive particlesto a confined location on the workpiece. Intricate and precise shapes areproduced by using masks with corresponding contours. Dust removalequipment is incorporated to protect the environment.

Air and abrasivesstream

Sapphire nOZZle~ ~ ~~ ~ J et diameter (0.3-0.5 mm)et velocity (150-300 ms). Stand-off distance (0.8mm)rT777"7A rr:,.,.- ." ..r ,. ,

...;1>--WorkpieceFIGURE 10.19Abrasive jet machining terminology.


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tviodem Abrasioe P rocesses 31710.2.1 Material Removal RateIn AJM, the abrasive particles frorn the nozzle follow parallel paths for ashort distance and then the abrasive jet flares outward like a narrow cone.When the abrasive particles of Ah03 or SiC, having sharp edges, hit a brittleand fragile material at high speed, they dislodge a small particle fram it by atiny brittle fracture. The lodged out particle is carried away by the air or gas.The material rernoval rate, VRR, is given by

where K J is a constant, N" is the number of abrasive particles impacting/unitarca, d" is the rnean diarneter of abrasive particles, P" is the density ofabrasive particles, H, is the hardness of the work material, and vis the speedof abrasive particles.Material removal rate, workpiece accuracy, surface raughness, and nozzle

wear are influenced by the size and distance of the nozzle, composition,strength, size and shape of abrasives, flow rate, and composition, pressure,and veloci ty of the carrier gas. The material rernoval rate is rnainlydependen! on the flow rate and the size of abrasives. Larger grain sizeproduces greater rcmoval rates.The typical material rernoval rate is 16.4 mm3/min when cutting glassand for metals it vares from 1.6 to 4.1 mm 'ymin. For harder ceramics,

cutting rates are about 50'1,.higher than those for glass. The minimum widthof cut is O.IJ mm. Toleranccs are typically 0.13 rnrn with O.05 mrnpossible using good fixation and motion control. The produced surfacehas a randorny matte texture. Surface roughness of 0.2 to 1.5um using 10ami 50 um particlcs, respectively, can be attained. Taper is present indeep cuts. High nozzle pressure results in greater rernoval rate, but thenozzle life is dccreased. Table "10 .3 sumrnarizes the overall processcharacteristics./vbrasit: flmu I' I7 tc : !\t a particular prcssurc, thc volumetric rernoval ratcincreases with abrasive flow rate up to an optimum value then decreaseswith further increase in the flow rateo This is mainly due to the fact that masstlow rate of the gas dccreases with the mercase of the abrasivo flow rateo Themixing ratio increases, causing a decrease in rernoval rate beca use of thedecreasing flow vclocity ,1I1d thc kinetic energy available for materialremoval (Figure '10.20 and Figure 10.21).Nozzle ~ tl1J /d -o fld i~ tI7 llc L ': The cffect of nozzle stand-off distance is shown inFigure '10.22. Thc rcmoval rate attains


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318 F undnmcntal of M achining P rocesscs

TA BL E 10.3Abrasivo Jet Machining Proccss CharactcristicsAhrasiocTypeSizcFlow rateMcdill/ llTypeVelocityPressureFlow ratcNozzleMaterialShapc

AI2o.1or SiC (used once)Around 25 um3-20 g/minAir or CO2150-300 ms2-8 kg/cm228 Llmin

Tip distanccL ifeOperating angleAreaTolcranccSurfacc roughness

WC or sapphircCircular, 0.3-0.5 mrn diamcter. ' 2Rectangular (0.08X 0.51mm to 6.6] X 0.51mm)0.25.:...15mrn

WC (12-30 h), sapphirc (300 h)Vcrtica I tu 60 off vcrtica I0.05-0.2 mm20.05 mm0.15-0.2 urn (10 um particles)0.4-0.8 um (25 um particles)1.0-1.5 um (20 um particles)

Gas p resslIre : The increase of gas pressure increases the kinetic energy and,therefore, the rernoval rate by AIM process (Figure 10.23).Mixil1g ratio: The rnixing ratio Vx, is defined asVolurne flow rate of abrasive particles Q"VX = . =.. Volurne flow rate of carner gas Qg

C '>oE(j)a:

Abrasive flow rateFIGURE 10.20Variation of material removal rate with the abrasive flow rateo


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Modern Abrasive P rocesses 3 1 9

oEQ)((

Velocity of particlesFIGURE 10.21Variation of material removal rate with the velocity of particles.The increase of Vx increases the removal rate, but a large value of Vxdecreases the jet velocity and sometimes blocks the nozzle. Thus an

optimum value of mixing ratio has been observed that gives themaximumremoval rate (Figure 10.24).Themass ratio Mx isdetermined by

Mass flow rate of abrasive particles __ MaA 1 x =- - - - - - - - - - - - - - - - - - - - - - - - ~- - - - - - - - -Mass flow rate of (carrier gas+particles) Ma+g'10.2.2 Appl ications

Drilling holes, cutting slots, cleaning hard surfaces, deburring,polishing and radiusing

(1J>oEQ)((

Stand-off distanceFIGURE 10.22Effect of nozzle stand-off distance on removal rateo


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320 F U l ldl l l l /m /tl l s o f M tl c //i l l i l l g ProCt'SSL'S

C ii>oE(l)a:

~- - - - - - - - - - - - - - - - - - - - - - - 7Gas pressureFIGURE 10.23Efft-ct uf gas prl'sslIrc on n-rnoval r.itc.

Dcburring of cross holcs. slots, and thrcads in small prccision poE(l)a:

-_._._--._--- - ,-------> /Mixill~ratio

FIGURE 10.24Eff(-~ct01 ' rnixing rario un n~111(lv,11 rate.


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Modern Abrasive Processes Removal of films and delicate cleaning of irregular surfacesbecause the abrasive stream is able to follow contours

Advantages: Best suited for machining brittle and heat-sensitive materials likeglass, quartz, sapphire, ceramics, etc.

Machining super alloys, ceramics, glass, and refractory materials. Not reactive with any workpiece material. No tool changes arerequired. Intricate parts of sharp corners can bemachined. Workpiecematerial does not experience hardening. No initial hole is required for starting of operation as that requiredby wire EDM.

Material utilization is high. It canmachine thin materials.

Limitations: Slow removal rateo Stray cutting cannot beavoided (low accuracy 0.1 mm). Tapering effectmay occur, especially when drilling inmetals. Abrasive may get impeded in thework surface. Suitable dust collecting systems should beprovided. Softmaterials cannot bemachined by the process. Silicadust may beahealth hazard. Ordinary shop air should be filtered to remove moisture and oil.

10.3 Abrasive Water J et MachiningWater jetmachining is suitable for cutting plastics, foods, rubber insulation,automotive carpeting and headliners, and most textiles. Harder materialssuchasglass, ceramics, concrete, and tough composites canbecutby addingabrasives to the water jet during abrasive water jet machining (AWJM),which was first developed in 1974to clean metals prior to their surfacetreatment. Theaddition of abrasives to thewater jet enhanced thematerial-removal rate and produced cutting speeds between 51and 460mm/min.Generally, AWJM cuts 10 times faster than the conventional machiningmethods used for composite materials.

3 2 1


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322 F uudtuncntnls o / M ac/ il /i l /g P r oC C SS L 'SAWJM uses low pressure of 4.2 bar to accelerate a large volume of water

(70'1..)and abrasives (30%) mixture up to avelocity of 30 m/s. Silicon carbides.corundum, and glass beads of grain size 10-150 um are often used as abrasivematerials (Figure 10.25). Using such a method rernoves burrs left in steelcomponents after grinding that are 0.35 mm in hcight and 0.02 mm in width.The burrs are removed by the erosivc effect of the abrasives: water acts as anabrasive carrier that darnpens the impact effect on the machined surface. Theintroduction of compressed air to the water jet enhances thc deburring action.In AWJM, the water jet stream accelerates abrasive particles, not the water,

to cause the material removal. After the pure water jet is created, abrasivesare added using either the injection or suspension methods. Thc importantparameters of the abrasives are the material structure and hardness, themechanical behavior, grain shape, grain size and distribution, and theaverage grain sizc.Process capobilitcs: Typical process variables include pressure, nozzlediameter, stand-off distance. abrasive type, grit number, and workpiece feedrateo Abrasive water jet cuts through 356.6 mrn slabs of concrete or 76.6 mmthick tool steel plate at 38 mm/min in a single pass. The produced surfaceroughness ranges betwecn 3.8 and 6.4 um, although tolerances of 0.13 mrnare obtainable. Repeatability of 0.04 mrn, sguareness of 0.043 mm/m, andstraightness of 0.05 mm per axis are expected.During machining of glass the cutting rate of 16.4 mm1/min is achieved,

which is 4-6 times higher than that for metals. Surface roughness dependson workpiece material, grit size, and the type of abrasives. A material withhigh rernoval rate produces large surface roughness. For this reason, finegrains are used for machining soft mctals to obtain thc same roughness ofhard ones. The decrease of surface roughness at smaller grain size is related

Water~

Pressure generationL ---- ,-,--- 'Abrasive reservoir

Focusing tubeW!!p;;g Workpiece

FIGURE 10.25Abrasivo water jet machiniru; elcmcnts. (Frorn El-Hofy. H, Adonnccd Mllcliil/il/g Processcs, NOI/-Tradinonat C l l 1 d H y b r i d Processes,McGraw-Hill, New York, 2005. Reproduced by permission ofMcGraw-Hill eo.)


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Modern Abrasive Processes 323lo the reduced depth of cut and the undeformed chip cross section. Inaddition, the larger thenumber of grains per unit slurry volume, themoreofthem falls on aunit surface area.A carrier liquid consisting ofwater with anticorrosive additives has much

greater density than air. This contributes tohigher acceleration of the grainswith consequent larger grain speed and increased metal removal rateoMoreover, thecarrier liquid when spreading over thesurface fillsitscavitiesand forms a film that impedes the striking action of the abrasive grains.Bulgesand tops of thesurface irregularities arethefirst tobeaffected and thesurface quality improves. Awater air jetpermts onetoobtain, asanaverage,aroughness number higher by one ascompared with the effectof an air jet.In high-speed water jet machining of Inconel, the roughness increases at ahigher feed rate as well as at lower slurry flow rates.Advanced AWJ machines are now available where the computer loads a

CAD drawing from another system. The computer determnes the startingand end points and thesequence of operations. Theoperator then enters thematerial type and tool offset data. The computer determnes the feed rateand performs themachining operation.

10.3.1 ProcessCharacterist icsThe parameters that affect AWJM are water (flow rate and pressure),abrasives (type, size, and flow rate), water nozzle and abrasive jet nozzledesign, machining parameters (feed rate and stand-off distance), and workmaterial. Other machining parameters include angle of cutting, traversespeed (slotting), and the number of passes.Water jet pressure: Figure 10.26 shows the relationship between waterpressure on the depth of cut for low and high nozzle diameter. There is amnimum pressure below which no machining occurs. That mnimum

:: o'O..cC : . .(j)o

Nozzle diameter

Water pressureFIGURE 10.26Effect of water pressure and nozzle diarneter on the depth of cut.


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324 Fnndnntrnt at o( M n c l t i J l i J lg Proccssce

"5u'O.c1i(j)o

Water pressureFIGURE 10.27F:rkct of water prl'ssurl' and abrasivo flow r,lll' 011tlw d(Tlh ()f cut.

prcssure depends on the typc of workpicce material. As shown inFigure ]0.27, thc machining depth tends to stabilize beyond


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\'1odern /sbrasioe Processes 3 2 5

"5o'O.c.Q)o

Abrasive flow rateFIGURE 10.29Effect of abrasivo flow rate and nozzle diarneter on the depth of cut.

Abrasive particle size and material : Common abrasive particle sizes rangefrom 100to 150grit. For a particular workpiece material and machiningsystem, there is an optimum particle size that achieves the largest depth ofcut (Figure10.30).Hashish (1986)recommended theuseofdifferent abrasivesizes forachieving deeper cuts. Generally, theharder theworkpiecematerial,the harder the abrasives that should beused.Traverse rate: As shown in Figure 10.31, the decrease of traverse speedincreases the depth of machining. An optimum traverse rate formaximumcut area (traverse speed X depth of cut) is clear,Number of passes: Figure 10.32shows therelationship between thenumberof passes and the commutative depth of cut. As the number of passes

"5o'O.c.Q)o

Particle sizeFIGURE 10.30Effect of the abrasivcs particle size on the depth of cut.


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326 Fu ndtuncntal ol MncITi lli llg Proccsscs

eo~QJeQJOlCl lQJ. l:

Traverse rateFIGURE 10.31Rc-lationship lx-twccn travc-r-: ratc and arca geller


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Modern Abrasiue Processcs 327increases, the rate of increase of depth/pass increases because the previousslot tends to focus the abrasive jet stream for more effectivemachining.Stnnd-off disumce: An increase in thestand-off distance decreases thedepthof cut. As shown in Figure 10.33, there is an upper limt for the stand-offdistance beyond which no machining occurs.

10.4 Abrasive Flow MachiningAbrasive flow machining (AFM) finishes surfaces and edges by extrudingviscous abrasive media through or across the workpiece. Abrasion occursonly where the flowof themedia isrestricted. AFM isused todeburr, polish,radius, remove recast layers, and produce compressive residual stresses orprovide uniform air or liquid flow.In typical two-way flow, theworkpiece ishydraulically clamped betweentwo vertically opposed media cylinders. Material isremoved by the flow of

a semsolid abrasive compound through a restrictive passage formedby awork part/ tooling combination (Figure 10.34).This causes the mediaviscosity to ternporarily rise. Theabrasive grains areheld tightly in place atthis point and the media acts as a grinding stone that conforms to thepassage geometry. Consequently, the media slug uniformy abradesthe walls of the extrusion passage. Media viscosity returns to normal afterthe slug passes through the restricted area. By repeatedly extruding themedia fromonecylinder totheother, theabrasion action occurs as themedia

Hydraulicallyoperated -4..?r--""~""pistons

Fixture

Lowermediachamber

Lowermediachamber

--~~~~~~#~~2~~--orkpieceViscousabrasivemedia

Flow

FIGURE 10.34Abrasive flow machining schcmatic,


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32 8 Fundamentals 01M achining Processes

enter a restricted passage and travel through or across the workpiece. Thematerial removal mechanism is similar to the grinding or the lappingprocesses.The total volume of material removal, Qv, in anumber of cycles, no has

been described by Kumar (1998)as

where K1 is thepercentage of grains participating in the finishing action, K2is the flow stress to BHN hardness number (1for brittle material, >1 forductile material s), He is the number of cycles, Pm is the density of media in106g/ cm", H ; is the length of stroke inmm, 1 is the length of workpiece inmm, Vf is the velocity of media around the workpiece having a constantradius inmm/min, a,is thenormal stress acting upon theabrasive grains inN/rnm", P a is the density of abrasives in 106g/ cm", vp is the velocity of thepiston in mm/min, H, is the hardness of workpiece, Cwr is the weight ofabrasives to theweight of abrasives and carrier medium in percent, and Vmis the volume of abrasive media between workpiece.

I n a further work, [ain et al. (1999)presented the material removal rateMRR inmg/min as

Thesurface roughness value, Ra, is given by

where thevelocity ofmedia Vf isincm/min and Am istheabrasive mesh size(abrasive grain diameter da=5.24/Am).AFM parameters that have the greatest influence on the process

performance include the number of cycles, extrusion pressure, gritcomposition and type, workpiece material, and fixture designo AFM isused for finishing, radiusing, and edge finishing of internal inaccessiblepassages. Typical surface finish is0.05um. Theviscosity and flow rate of themedia affecttheuniformity of theremoval rateand theedge radius size. Lowand steady flowrates arebest foruniformmaterial removal fromthewalls ofadie. For deburring applications, Iow-viscosity AFM media and high flowrates arerecommended (Jain and [ain, 2001).Themedia used consist of apliable polymer carrier and aconcentration of

abrasive grains. Higher viscosity media are nearly solid and are used foruniform abrasion of the walls of large passages. Lower viscosity is suitablefor radiusing edges and for finishing small passages. The carrier of theabrasives is a mixture of a rubber-like polymer and a lubricating fluido


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1

Modern Abrasive Processes 329By changing the ratio of the polymer and the lubricating oil, differentviscosities are obtained.Abrasive grains are mostly made from silicon carbide, although boroncarbide, aluminum oxide, and diamond canbeused. Particle sizes range from0.005 to 1.5mm. Larger abrasives cut at a feed rate, although fine abrasivesprovide fine surface finishes and accessibility to small holes. Due to theabrasive wear, the effective life of the media depends on the quality of themedia, abrasive size and type, the flow speed, and the part configuration.The extrusion pressure is controlled between 7bar and 200 bar(100-3000 psi), as well as the displacement per stroke, and the number ofrecipracating cycles. One-way AFM systems flow theabrasive media throughtheworkpiece in only one direction, allowing themedia toexit freely from thepart for fast processing, easy cleaning, and simple quick-exchange tooling.AFM can simultaneously finish multiple parts or many areas of a singleworkpiece. Inaccessible areas and complex internal passages can be finishedeconomically and effectively. Automatic AFM systems are capable ofhandling thousands of parts per day, greatly reducing labor costs byeliminating the tedious handwork.Applications of AFM range frorn precision dies and medical componentsto high-volume praduction of electronic and automotive parts. Recently,AFM has been applied to the improvement in air and fluid flow forautomotive engine components. The pracess can also be used to remove therecast layers frorn fragile components.Figure 10.35shows that the original diameter gets wider as the machiningtime and flow pressure increase due to the increase in the duration and theforces of the abrasion component. High extrusion pressure also raises therate of media flow rate (Figure 10.36), which allows for greater number ofabrasives to do more machining to the hole.

~11li~

alenC ll~Ue

Flow pressure

.. .alO EC llO

TimeFIGURE 1035Effect of time and flow pressure on thc diameter increase.


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330 Fundamenials o / Machining P roccsses

Extrusion pressureFIGURE 10.36Effect of extrusion pressure on media flow rateo

Q J(f)C 'dQ Jtie Hole length'-Q JO EC 'd(5

TimeFIGURE 10.37Effect of time and hole length on diameter increase.

The increase in diameter decreases as the length of the hole increases(Figure 10.37).Additionally, the increase in the volume of themedia that isperforming machining causes thehole tobewider (Figure 10.38).The effectofmedia flow rate on temperature is shown in Figure 10.39.

10.5 Magnetic Abrasive MachiningAlthough magnetic abrasive finishing (MAF)originated intheUnited Statesduring the 1940s, itwas in the former USSRand Bulgaria that much of the


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Modern Abrasioe Processee 331

Media volumeFIGURE 10.38Effect of media volume on the diameter increase.

development tookplace in late1950sand 1960s.During the 1980s,[apanesefollowed the work and conducted research for various polishing appli-cations. Figure 10.40 shows the schematic diagram of MAF apparatus.A cylindrical workpiece is clamped into the chuck of the spindle thatprovides the rotating motion. Theworkpiece can beamagnetic (steel) or anon-magnetic (ceramic) material, themagnetic field linesgoaround throughthe workpiece. Axial vibratory motion is introduced in the magnetic fieldby the oscillating motion of the magnetic poles relative to the workpiece.A mixture of fine abrasives held in a ferromagnetic material (magneticabrasive conglomerate; (Figure10.41)is introduced between theworkpieceand the magnetic heads where the finishing process is exerted by the

Media flow rateFIGURE 10.39Effect of media flow rate on abrasive flow machining ternperature.


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332 F undamentals of M achining P rocesses

Vibratorymotion

==--+Magnetic abrasives

FIGURE 10.40Magnetic abrasive machining schematic.

magnetic field. Typically the size of themagnetic abrasive conglomerates is50-100 umand the abrasives are in the 1-10umrange. With non-magneticwork materials, themagnetic abrasives are linked toeachother magneticallybetween themagnetic poles N and Salong the lines of themagnetic forces,formng flexiblemagnetic abrasive brushes. Toachieve uniform circulationof the abrasives, themagnetic abrasives are stirred periodically.

10.5.1 ProcessDescription .MAF operates with magneto abrasive brushes where the abrasive grainsarrange themselves with their carrying iron particles to flexibly complywith the contour of thework surface. The abrasive particles are held firmyagainst the work surface, while short-stroke oscillatory motion is carriedout in the axial workpiece direction. MAF brushes contact and act upon thesurface-protruding elements that form the surface irregularities. Surfacedefects such asscratches, hard spots, lay lines, and tool marks areremoved;form errors like taper, looping, chatter marks can be corrected with alimted depth of 20um. Material removal rate and surface finish depend onthe workpiece circumferential speed, magnetic flux density, workingclearance, workpiece material, the size of the magnetic abrasive conglom-erates including the type of abrasives used, their grain size, and volume

Ferromagneticcomponent

FIGURE 10.41Magnetic abrasive conglomerate.

) .


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Modern Abrcsioe P rocesses 333fraction in the conglomerate (Fox et al., 1994). The magnetic pressurebetween the abrasives and the workpiece is expressed by Kim and Choi(1995) as

where Lois the magnetic permeability in vacuum, Pm is the magneticpressure, Lris the relative magnetic permeability of pure iron, Ha is themagnetic field strength in air gap, and W is the volume ratio of iron in amagnetic abrasive particle.The total volume removed by the magnetic abrasive brush, Qv in themachining time ti, is given by

The surface roughness value after amachining time ti is given by1[ NpN 1C tlfvtm] 21\(t ) = R (O)-- kl-----'----'--

,1 I ,1 I 7fHJ tan f)mwhere kl is the constant of proportionality, N is the number of magneticparticles acting in the machining regian simultaneously, Nac is the numberof abrasive grains in a single conglomera te, tlf is the force acting upon acutting edge of a single abrasive particle in N, Hr is the workpiece Brinellhardness in Ny mrn''. 1 is the length of work surface in mm, v is the velocityof magnetic abrasives in mm/ min, 20m is the mean angle of asperity ofabrasive cutting edge in degrees, R,1(0) is the initial surface roughness inum, and Ra(tm) is the surface roughness after time tm in um.

10.5.2 Process CharacteristicsFigure 10.42 shows the magnetic abrasive particle pressure Pm acting lIponthe work surface. which increases as the flux density on the magneticabrasive grains increases. Additionally, the pressure excreted by themagnetic abrasives decreases as the gap between the magnetic pole andthe workpiece is increased, provided that the filling density of the abrasivegrains in the gap remains constant (Figure 10.43).

10.5.3 Material Removal Rate and Surface FinishTypc and size of gmiJ /s: The surface roughness decreases rapidly in thebeginning then levels off to aconstant value. The increasein grain size raises- . . . . - - - - . - J1 :: . , < \I~.\ 1 ; ' "1 : ,;: "~_.~._ ._ .,. _ _ . -~.~ _ .. . . _ - -


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334 F undamcntals of Machillillg P roccsses

Magnetic pole

Rotation +vibrationMagnetic abrasive particles

N@ 0-!r 'ressure, P

IWorkpieceI

,'~Lines of magnetic force

FIGURE 10.42Magnetic field distribution and magnctic force acting on a magnctic abrasivc particle.

the surfacc roughness, as shown inFigure 10.44. The finishing process can beimproved by mixing small-sized diamond abrasives with irregular shapedlarge-sized ferromagnetic iron particles.M ixing weigl1t percentugc of i ron panictes: As shown in Figure 10.45, thereis an optimum value of mixing weight percentage of ferromagneticparticles for obtaining the best surface finish and the largestmachined depth.Magnetic flux density: As shown in Figure 10.46, an increase in themagnetic flux density and particle size increases the machined depth. Itdecreases with increasing the working clearance (Figure 10.47). Surface

Workingclearance

Magnetic flux densityFIGURE 10.43Effect of flux density and air gap 011 the magnetic abrasive pressure.


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fv lodern Abrasioe Processes

(f)(f)(1)e..cO): : : : J2(1)uco't:: ::: J(f)

335

Grain size

Finishing timeFIGURE 10.44Effcct of finishing time and grail1 size 011the final surface roughness.

roughness improves with magnetic flux density and finishing time(Figure W.48).10.5.4 ApplicationsPolsli/lg of balls and rol ler: Recently, MAF development involves the useof magnetic field to support abrasive slurries in polishing ceramic ballsand bearing rollers. A magnetic field, containing abrasive grains andcxtrernely fine ferromagnetic particles in a certain fluid such as water orkerosene, fills thechamber within a guide ring. The abrasive grains,

(f)( f ) . . caJ-eQ...c aJ0)-0::::J -o2 ~(1) .-u..cco u't: co::::J~(f)

Depth

Roughness

Mixing weight percentage of iron particlesFIGURE 10.45Effect of mixing wcight pcrccntugc 011the machined depth and surface.


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33 FU l/dl1/ I lt'l/tn ls o / Mnc / il /il/g Pr o cesscs

s:.(J)-o-oQ)e.r:.uC1l

Iron particle diameter

Magnetic flux densityFIGURE 10.46Effect of 1ll.1gndic flux dl'nsity and iron partick- di.imotor on thc rnachincd dcpth.

cerarnic balls, and the float (made from non-rnagnetic material) aresuspended by the magnetic forces. The balls are preset against the rotatingdrive shaft and are polished by the rnechanical abrasion action. Bccausethe forces applied by thc abrasivo grains are extrcrnely small andcontrollable. the polishing action is very fine. The proeess is econornicaland the surfaces produced have little or no defeets.Fillis/il1g inner-i ubc surfaccs: A sehematie view for the interna] finishingof non-ferromagnetie tubes using MAF operation is shown in Figure 10.49.

s:.Q)-o-o(J)es:uC 1l

Iron particle diameter

7 -Working clearance

FIGURE 10.47Eff("d 01' working clearance and iron particle diameter 011 the machined depth.


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Mudan Abrnsii P rocessce

(/)(/)QJe.cOl:: : : l2QJro't:: :J(J)

337

Magnetic flux density

Finishing timeFIGURE 10.411Efcct of fini;-,hing time and grain si:: on the final surface roughness.

The magnetic abrasives inside the tubes are converged towards the finishingzone by the magnetic ficld, generating the magnetic force needed forfinishing. By rotating the tube at higher speed, the magnetic abrasives makethe inner surface srnoother. Figure 10.50 shows the finishing of aferrornagnetic tube where the magnetic fluxes flow into the tube (insteadof through the inside of the tube) due to their high magnetic permeability.Under such conditions, thc abrasives remain in the finishing zone when thetube is rotated.

Vibrations 0oMagnetic abrasives

Coil De source

Yoke

PaleLine of magnetic forceNon-ferromagnetic tube

RotationFIGURE 10.49M


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338 Fundamcnmts of Mnch ill il l~ Proccsscs,

DC source

Pole

Coil

s YokePole

Vibrations ~0Magnetic abrasives

RotationFIGURE 10.50M ilgnetic fin ishi ng of magnctic tubos. (F rorn Y . 1-1torn i and T Sh in iIIImil, M l I g uct ic abra-ne

. f i l / i ; ; / / i l / g of inucr surt / t" s of t ubr, lntcrnational symposium for electro machiniru; (lSEM-XI),Switzerland. pp. ';i{l-i)911, 1995.)

10.6 Problems1. A cylind rical imprcssion of diarncter 10mrn and depth 3 Illlll is to

be machined by USM in tungsten carbide. lf the feed force is 6N,the average diametcr of thc grains in thc abrasivo slurry is 10 ~J1I,the tool oscillation arnplitude is 30 um. and the frequency is20 kHz. The slurry contains one part of abrasives to about onepart of water. The fracture hardness of the tungsten carbideworkpiece is 7000 N /Illlll= and that of the copper tool is1500N /mm=. Calculate the rnachining time. Assurnc k1=.3,k2 =1.8mrrr'. and k3== 0.6 .

2. A sguare througb boje 5X 5mm is to be ultrasonically machinedin a tungsten carbide plate of 4mm thickness. The slurry is madeof one part of 10 ~LJ nB.tC abrasives in one part of water. If the feedforce is 5 N, thc tooJ oscillates at amplitudc of 15 um andfrequency of 25 kHz. Assuming that only 75'1" of pulses areeffective. calcula te the machining time. The fracture hardness oftungsten carbide workpiece is 7000 N/ mm'' and that of the coppertool is I5DON /mm=. Calcula te the machining time taking k1=0.3,k2 =1.8mrrr'. and k3 =0.6.

3. Estmate the machining time required to machine a hole in We,5mm thick. The grits are 20 um radi US, static stress is 15N/ mm''.the oscillation amplitude 35 um. and the machine operates alfrequency of 25,000 cps. The compressive fracture strength of WC


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Modern Abrnsive P rocesses

ti

is 2270N /jnrrr'. What would be the volumetric removal rate if theshape is a square 4X4 mm".4. During abrasive jet machining, the mixing ratio was 0.3. Calculatethe mass ratio if the ratio of the density of the abrasives anddensity of the carrier is 20.5. In AJM, if the nozzle diameter is 1.0mm and jet velocity is200mis, calculate the flow rate cm3/s of the carrier gas andabrasive mixture.

10.7 Review Questionsl. Explain how the material is removed in USM.2. What is the function of the abrasive slurry in USM?3. Show diagrammatically the main elements of aUSM machine.4. Explain the advantages and disadvantages of USM.5. A series of 5mm holes are to be drilled in aglass workpiece. Selectasuitable machining method. What are the variables that affect thefinal hole quality?6. Show diagrammatically RUM and USM contouring7. What are the rnain applications of USM?8. Explain the effect of USM parameters on the removal rateo9. What are the reasons behind errors in parts machined by USM?10. Sketch the machining arrangernent in AJM.11. Explain the rnain factors that affect the AJM removal rateo12. Show some applications for AJM.13. Show the main parts of the machining system in AWJM.14. Explain the effect of AWJM parameters on the removed depth framthe workpiece.15. Explain, using a simple diagram, how AFM is performed."1 6 . Explain the effect of AFM pararneters on diarnetral increase.17. Explain how the material is removed in MAF operation.18. Explain the effect of MAF parameters on the surface roughness andremoved depth.19. Describe some MAF applications.20. Compare AJM, AFM, and AWJM processes with respect toprincipIesof material rernoval, applications, advantages, and limitations.

339


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34 0 Fundamentals of Machining P rocesses21. Mark true (T) or false (F):

(a) The volurne of material removal in USM is directly related tothe frequency.(b) AFM canbeused to reduce thediameter of amild steel rod fram14to 12mm.(c) Stiffmedia are used for radiusing parts by AFM.(d) AWJM can be used to cut composite materials.(e) Material removal rate in AJM isgreater than that in AWJM.(f) A heat-affected layer of 0.5um is left after AFM.(g) In USM, for the sarne static load, the larger the tool diarneter, thegreater will be the penetration rateo

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9. El-Hofy Fundamentals of Machining Processes Tema 10