the influence of abrasive water jet … pressure, traverse speed, abrasive mass flow rate, standoff...

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Int. J. Mech. Eng. & Rob. Res. 2015 Abdulhafiz A Shaikh and Jignesh K Patel, 2015

THE INFLUENCE OF ABRASIVE WATER JETMACHINING PARAMETERS ON VARIOUS

RESPONSES—A REVIEWJignesh K Patel1 and Abdulhafiz A Shaikh1*

*Corresponding Author: Abdulhafiz A Shaikh,[email protected]

Composites materials are getting difficult to machine owing to its constituents properties, fiberorientation and relative volume fraction of matrix. Abrasive water jet machining is a recent non-traditional machining process, and widely used in many industrial applications. Abrasive waterjet cutting of material involves the effect of a high pressure velocity jet of water with inducedabrasive particle on to materials to be cut. The present paper collects the findings on influencesof distinct parameters of AWJ machining of composite materials. The parameters such ashydraulic pressure, traverse speed, abrasive mass flow rate, standoff distance, types of abrasivematerials, grit size, jet nozzle oscillation and cutting orientation are focused for kerf tapper angle,surface roughness and depth of cut. From the literature survey it was found that by increase ofwater pressure, kerf tapper angle and surface roughness gets decreased. However, traversespeed and standoff distance shows reverse effects.

Keywords: Abrasive water jet machining, Kerf tapper ratio, Surface roughness, Fiberreinforced composite materials, Abrasive materials

INTRODUCTIONNature has taught that even the hardest rockscan be eroded by a stream of water andmoved away from the area. Because thiseffect could be seen, it could also be adaptedto man’s use. In the late 1960s Franz foundthat very high pressure jets could be used tocut through wood products with little damageto the material on the outside of the cutsurface and at relatively high cutting speed(Byran, 1963). The first equipment was

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Int. J. Mech. Eng. & Rob. Res. 2015

1 Department of Mechanical Engineering, SV National Institute of Technology, Ichchhanath, Surat 395007, Gujarat, India.

installed at Alton Boxboard in 1972 and ledto the development of a new tool formanufacturing industry (Walstad andNoecker, 1972). In the AWJ machiningprocess, high pressure water is supplied bya pump at the orifice inside the cutting headfrom where it is converted into a high velocityjet. While passing through a mixing chamber,water creates a vacuum which draws theabrasive particles into a focusing tube wherethe Abrasive Water Jet (AWJ) mixture is

Review Article

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formed (Momber, 1998). The jet plume(mixture of abrasives and water droplets)impacts the target surface it results in thegeneration of a unique footprint (kerf). As aresult of this, the work-piece material removalis mainly caused by the impact of amultitudeof high velocities abrasive particles asdiscussed by Momber and Kovacevic (1998).It has advantage over other machiningprocess; such as (a) it can cut any materiallike titanium, diamond, glass, plastics andcomposite, etc. (b) Any 2-D profile can cutwith high tolerance. (c) There is no directcontact between cutting tool and workmaterial, so no heat generation, no wear oftool and no structural change in work material.(d) No special fixture or tooling is requiredfor cutting. (e) It has minimum cutting force injet direction so no special clamping isrequired.

A typical abrasive water jet system includesmain four components: (1) the water purifyingand storage system. (2) High pressure

generating system. (3) Cutting head and (4)Abrasive delivery system and catcher asshown in Figure 1.

The Water Purifying and StorageSystemThe water purifying and storage system is usedfor supplying pressure to ultrahigh pressurepump continuously. Typical AWJ systemincludes two different storage tanks (i) cuttingtank, and (ii) cooling water tank as shown inFigure 1. Generally, particle having size greaterthan 1 µm needs to be removed from the wateras it creates wear of the critical part of pump,which leads to failure of pump. Cooling wateris used to reduce the temperature of oil pump.

High Pressure Generating SystemThis system is equipped with intensifier andaccumulator to generate high pressure andstorage of high pressure water respectively.Intensifier includes double acting reciprocatingpump which is operated by oil pressure. Ultrahigh pressure pump includes two different

Figure 1: Schematic Diagram of an Abrasive Water Jet Cutting System

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circuits to generate high pressure up to 600MPa. Accumulator stores the high pressurewater energy to reduce the pressure loss innext stage.

Cutting HeadThe cutting head equipped with focusing tube,orifice, nozzle and mixing chamber. Generally,focusing tube is made up of stainless steelhaving length of 76.2 mm and diameter 0.76mm. Sapphire, ruby and diamond material canbe used for orifice having diameter rangingfrom 0.08 to 0.8 mm. High pressure tubecarries the pressured water from accumulatorto cutting head through focusing tube. Highpressure water passes through orifice whichconverts pressure energy of water into kineticenergy of water particle due to convergentshape of orifice. The high speed water jet thenpasses through a mixing chamber, which isdirectly connected to orifice. Water loses itspressure energy as it passes through mixingchamber due to venturi effect which createsvacuum in mixing chamber. Due to vacuumabrasive enters in mixing chamber mixes withwater. In mixing chamber high energy of waterparticle transfers to abrasive particle then mixerof water and abrasive pass through nozzle withact as saw to cut the material.

Abrasive Delivery System andCatcherThe abrasive delivery system includesabrasive hopper and pneumatically operatedvalve to control the abrasive mass flow rate.Generally, three different types of abrasivematerial used in industries such as garnet,aluminum oxide and silicon carbide withdifferent mesh size ranging from 60 to 120mesh. The target material is being cut by water,

which contains large amount of energy neededto absorb before it can damage any part.Catcher is used to collect the pressurizedwater after cutting.

LITERATURE OVERVIEWAbrasive Water Jet Machining (AWJM) is anew non-conventional material removaltechnology which is increasingly used inindustry (Ciglar et al., 2009). The AWJMprocess has a high-potential and is applicableto both metals and non-metals (Jain, 2008).Thus, AWJM offers a productive alternative toconventional techniques. In AWJM processmaterial removal occurs through erosion andresults from the interaction between anabrasive laden water jet and target (Arola andRamulu, 1997). In this section an extensivereview of the current state of research anddevelopment in AWJM conducted. AWJ cuttinginvolves a large number of variables, andvirtually all these variables affect the cuttingresults, only the major and easy-to-adjustvariables were considered. The performanceof AWJM depends upon number of processparameters and can be classified into twocategories: the input parameters and outputparameters. The abrasive water jet machiningprocess is characterized by large number ofprocess parameters that determine efficiency,economy and quality of the whole process.Figure 2 demonstrates the factors influencingAWJ machining process.

Shanmugam and Masood (2009) havemade an investigation on the kerf taper angle,generated by Abrasive Water Jet (AWJ)machining of two kinds of composite materials(i) epoxy pre-impregnated graphite wovenfabric and (ii) glass epoxy. The experiments

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have been conducted on a flow water jetsystem with sapphire orifice having diameterof 0.254 mm and abrasive garnets with a meshsize of #80. Taguchi experiments design wasused to construct the design of experimentsfor the process parameters. Effect of differentfour process parameters were studied namelywater pressure, traverse speed, standoffdistance and abrasive mass flow rate on kerftaper angle. They concluded that, increase ofwater pressure and traverse speed shows theopposite effect on kerf taper angles as shownin Figures 3a and 6a. With increasing standoffdistance the kerf taper angle increases asdemonstrated in Figure 9a. As increase inabrasive mass flow rate kerf taper angle isdecreases insignificantly as illustrated inFigure 11a.

Wang (1999) investigated the machinabilityand kerf characteristics of polymer matrixcomposite sheets under abrasive water jetmachining. Experiments were conducted ona Flow Systems International water jet cutterequipped with a model 20X dual intensifierhigh output pump (up to 380 MPa) and a fiveaxis robot positioning system to cut 300 x 300mm test specimens. The specimens wereprepared from Teflon and phenolic resin. Threemajor parameters have been studied mainly,water pressure, the nozzle traverse speed andthe standoff distance. It can be noted fromFigure 3b that both the top and bottom kerfwidths increase approximately linearly with thewater pressure. The kerf taper angle alsoincreases with the water pressure. The effectof traverse speed on the top kerf width, bottom

Figure 2: Process Parameters Influencing the AWJ Machining

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kerf width and kerf taper is shown in Figure6b. As shown in Figure 9b, that the top andbottom kerf widths increase with an increasein the standoff distance.

Azmir and Ahsan (2009) explained theinfluence of six machining parameters onsurface roughness (Ra) and kerf taper ratio(TR) characteristics during an abrasive waterjet machining of glass/epoxy laminatedcomposite. Taguchi’s design of experimentsand analysis of variance were used todetermine the effect of machining parameterson Ra and TR. In this case, six machiningparameters abrasive types, hydraulicpressure, standoff distance, abrasive massflow rate, traverse rate and cutting orientationwere selected as control factors. Theequipment used for machining the sampleswas Excel-CNC abrasive water jet cuttingmachine equipped with Ingersold Rand modelof water jet pump with the designed pressureof 345 MPa. The machine is equipped with agravity feed type of abrasive hopper, anabrasive feeder system. For the nozzleassembly, it has an orifice of 0.25 mmdiameter of sapphire jewel and a focusing tubeof 0.76 mm internal diameter of carbide with afocus length of 70 mm. The effect of hydraulicpressure, traverse speed, standoff distance,abrasive mass flow rate, types of abrasive andcutting orientation on mean kerf taper ratio andmean surface roughness as shown in Figures3c, 6c, 9c, 12b, 14a,15a and 4a, 7a, 10a, 13a,14a and 15a respectively.

Ramulu and Arola (1994) examined theinfluence of cutting parameters on the surfaceroughness and kerf taper of an abrasive waterjet machined graphite/epoxy laminate. All theexperiments were performed with a PowerJet

model water jet, driven by a Model 20-35 waterjet pump. The material used for theexperiments was graphite/epoxy (Gr/Ep)laminated sheets with 16 mm thickness. Threeseries of cutting tests were performed eachconstructed using a Taguchi’s experimentaldesign array. Five independent variablesassociated with AWJ cutting process werevaried including jet pressure, standoff distance,traverse speed, grit size and abrasive massflow rate. Surface roughness parametersincluding Ra, Rq, Ry, and Rz were obtained withSurfAnalyzer 4000 profilometer using a 5µdiameter probe. Measurements were reducedusing analysis of variance (ANOVA)techniques of Taguchi to designate the effectsand interaction effects of the processparameters on cut quality. Kerf taper (TR) wasdefined as the ratio of the jet entrance kerfwidth to th exit kerf width. Taper (TR)measurements were obtained by recordingthe entrance and exit kerf widths an opticalmicroscope and calculating the ratio of themeasurements.

Azmir et al. (2009) investigated the effectof Abrasive Water Jet Machining (AWJM)process parameters on kerf taper ratio (TR)and surface roughness (Ra) of Aramid FiberReinforced Plastics (AFRP) composite.Taguchi’s design of experiment was used asthe experimental approach. The equipmentused for machining the samples was Excel-CNC abrasive water jet cutting machineequipped with Ingersold Rand model of waterjet pump with a designed pressure of 345MPa. In that study, Kevlar 129 was used andhand laminated in the prepreg form ofmodified phenolic resin having its realweight of 410 g/m2. The aramid fibres which

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was readily available in a woven fabric andnamed for its manufacture’s style of 258 (2× 2 basket weave) were used for thepreparation of the laminates Throughanalysis of variance (ANOVA), it was foundthat the traverse rate was considered to bethe most significant factor in both TR and Ra

quality criteria. TR and Ra were reduced asincreasing the hydraulic pressure as shownin Figures 3e and 4b. TR and Ra wereincreases as the traverse rate and standoffdistance increases, as demonstrated inFigures 6e, 7b, 9d and 10b respectively.However, there was no clear pattern forabrasive mass flow rate on both Ra and TR

as shown in Figures 12c and 13b.

Wang and Guo (2002) developed semi-empirical model to predict the depth of jetpenetration in abrasive water jet cutting ofpolymer matrix composites. All theexperiments have been conducted on a FlowSystems International water jet cutter to cut 300X 300 mm2 test specimens of 16 mm thick.The water jet cutter was equipped with a model20X dual intensifier high output pump (up to380 MPa) and a five-axis robot manipulatorfor positioning and moving the nozzle. All thespecimens were Phenolic Fabric PolymerMatrix Composites which are non-metalliclaminated sheets made by impregnated layersof fiber (cotton) reinforcement with resin matrix.They have studied the effect of threeparameters likely, water pressure, nozzletraverse speed, and abrasive mass flow rateon depth penetration while kept all otherparameters as constant. As shown in Figure 5and Figure 11, depth of penetration isincreases as water pressure and abrasivemass flow rate increases. While jet traverse

rate shows the opposite effect on depth ofpenetration as given in Figure 8.

Xu and Wang have been presented anddiscussed an experimental investigation ofAbrasive Water Jet (AWJ) cutting of aluminaceramics with controlled nozzle oscillation. Inthat experiment, the specimens used were87% alumina ceramic plates with a thicknessof 12.7 mm, to represent brittle materials. Theabrasive water jet cutting system employedwas the Flow International Water jet Cutterdriven by a “Model 20X” dual intensifierpumping system, with an operating pressureof up to 380 MPa. A Taguchi experimentaldesign array was used to construct the cuttingtests. They found that, larger oscillation anglesincrease the overlap cutting action and thenumber of scanning actions on a given part ofsurface, so that the scanning action wasdominant and thus reduces the surfaceroughness. It has been found that oscillationangle have a similar effect on kerf taper andsurface roughness.

Hydraulic Pressure

Effect of Hydraulic Pressure onKerf GeometryShanmugam and Masood have been madean investigation on AWJ machining of graphitewoven fabric and glass epoxy and studied theinfluence of hydraulic pressure on kerfgeometry. Figure 3a shows the influence ofwater pressure on the kerf taper angles. Theyconcluded that, within the operating rangeselected, increase of water pressure resultsin decrease of kerf taper angles. When waterpressure is increased, the jet kinetic energyincreases that lead to a high momentumtransfer of the abrasive particles, generatinga wider-bottom kerf. Therefore, the difference

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in top and bottom kerf width is reduced,leading to a decrease in kerf taper angle.

Jun Wang studied the effect of hydraulicpressure on kerf taper angle on AWJ machiningof polymer matrix composite. As shown in Figure3b that both the top and bottom kerf widthsincrease approximately linearly with the waterpressure, as higher water pressure results ingreater jet kinetic energy impinging onto thematerial and opens a wider slot. The kerf taperangle also increases with the water pressure.This is because the bottom kerf width is notincreased in the same order as the top kerf width,as indicated in the figure. It follows that as the jetloses its kinetic energy, it cannot remove thematerial adequately at the lower section, resultingin a narrow bottom kerf. It is interesting to notethat the characteristics of the taper angle in termsof water pressure and traverse speed discussed

above are opposite to those reported in glass/epoxy cutting. This may stem from the differenttypes of materials processed, different pressureand speed ranges selected as well as differentratios of jet energy used to the energy requiredto cut the materials.

Azmir and Ahsan have been made aninvestigation on AWJ machining glass/epoxyand studied the influence of hydraulic pressureon kerf geometry. Higher hydraulic pressureresults in greater jet kinetic energy and opensa wider slot on the work-piece on both of thetop and bottom widths. Consequently, the kerftaper ratio calculated as the ratio of top to thebottom width is reduced with further increaseof supply hydraulic pressure due to the morerapidly increasing of top kerf width comparedto the bottom kerf width. This is clearlyillustrated in Figure 3c.

Figure 3: Effect of Hydraulic Pressure on Kerf Taper Ratio

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Ramulu and Arola studied the effect ofhydraulic pressure on kerf taper ratio of AWJmachining of graphite/epoxy laminatedcomposite. The influence of pressure on kerftaper is shown in Figure 3d. High supplypressures increase the kinetic energy of theabrasive particles and retain their capacity formaterial removal. As expected, higher supplypressures reduce the kerf taper over the cuttingdepth. Further increases in the supply pressurewould reduce the kerf taper.

Azmir et al. have been made aninvestigation on AWJ machining aramid fiberreinforced plastics and studied the influenceof hydraulic pressure on kerf geometry. Higherhydraulic pressure results in greater jet kineticenergy and opens a wider slot on the work-piece on both of the top and bottom widths. TR

is reduced with further increase of supplyhydraulic pressure due to the more rapidlyincreasing of top kerf width compared to thebottom kerf width. This is clearly illustrated inFigure 3e.

Effect of Hydraulic Pressure onSurface RoughnessAzmir and Ahsan studied the effect ofhydraulic pressure on surface roughness ofAWJ machining of glass/epoxy laminatedcomposite. While, in case of hydraulicpressure, a higher hydraulic pressureincreases the kinetic energy of the abrasiveparticles and enhances their capability formaterial removal. As a result, the surfaceroughness decreases as illustrated inFigure 4a.

Azmir et al. have been made aninvestigation on AWJ machining aramid fiberreinforced plastics and studied the influenceof hydraulic pressure on surface roughness.Looking to case of hydraulic pressure, a higherhydraulic pressure increases the kinetic energyof the abrasive particles and enhances theirability for material removal. Whenever thesupply pressure provides sufficiently highenergy to the abrasives, the cutting process isenabled to be carried out without severe jet

Figure 4: Effect of Hydraulic Pressure on Surface Roughness

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Traverse SpeedEffect of Traverse Speed on KerfGeometryShanmugam and Masood have been madean investigation on AWJ machining of graphitewoven fabric and glass epoxy and studied theinfluence of traverse speed on kerf geometry.Increase of traverse speed increases kerftaper angle as shown in Figure 6a. Theincrease in kerf taper angle is a direct resultof the exposure time because at highertraverse, less time is available for cutting,leading to less overlapping of the jet on thetarget material.

Jun Wang studied the effect of traversespeed on the top kerf width, bottom kerf widthand kerf taper is shown in Figure 6b. It canbe seen from the figure that the traversespeed has a negative effect on both the topand bottom kerf widths. Taper angledecreases slightly with an increase in thetraverse speed. The negative effect of thetraverse speed on both the top and bottomkerf widths is due to the fact that a fasterpassing of abrasive water jet allows fewerabrasives to strike on the jet target and hencegenerates a narrower slot. It is interesting tonote that the characteristics of the taper anglein terms of water pressure and traversespeed discussed above are opposite tothose reported in glass/epoxy cutting.

Azmir and Ahsan examined the effect oftraverse rate on the kerf taper as shown inFigure 6c. It could be concluded that thenegative effect of traverse rate on the kerf widthis due to the fact that a faster passing ofabrasive water jet allows fewer particles tostrike on the target material and, hence,generates a narrower slot. In other words, the

deflection which in turn minimizes the wavinesspattern. As a result, the Ra decreases asillustrated in Figure 4b. In the AWJM of AFRPlaminate, higher degrees of waviness weregenerally noted on the specimens that weremachined with low jet pressure.

Effect of Hydraulic Pressure onDepth PenetrationWang and Guo have been made aninvestigation on AWJ machining of Phenolicfabric polymer matrix composites and studiedthe influence of hydraulic pressure on depthpenetration. The trend of depth of penetrationwith respect to the water pressure is shown inFigure 5. In general, the depth of penetrationincreases with water pressure, as more energywill be able to remove more material. This isdue to the fact that a higher water pressuretends to open a wider kerf which will have anegative effect on the depth of penetration. Inaddition, particle fragmentation increases withwater pressure, which reduces the cuttingeffectiveness of the particles.

Figure 5: Effect of Hydraulic Pressureon Depth of Penetration

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decrease in the exposure time that was causedby increasing the traverse rate resulted to thereduction in both of the kerf top and bottomwidth. Whereas, the increasing trend of the kerftaper ratio is the result of the more rapidlydecreasing kerf width at the bottom than at thetop as the traverse rate increases.

Ramulu and Arola studied the influence oftraverse speed on the taper of the laminatedmaterial at the aforementioned cuttingconditions. This trend resulted from thereduction in kerf entrance width withincreasing traverse speed due to thedecrease in exposure time. Kerf exit widthwas nearly independent of the traverse speedused for cutting in this scenario and, therefore,did not influence the taper ratio. At lower jetpressures, kerf taper generally increases with

an increase in traverse speed, which isattributed to an increase in jet deflection asshown in Figure 6d.

Azmir et al. investigated the effect oftraverse rate on the kerf taper ratio as shownin Figure 6e. It is concluded that the negativeeffect of traverse rate on the kerf width is dueto the fact that a faster passing of abrasivewater jet allows fewer particles to strike on thetarget material and, hence, generates anarrower slot. In other words, the decrease inthe exposure time that was caused byincreasing the traverse rate resulted to thereduction in both of the kerf top and bottomwidth. Whereas, the increasing trend of the TR

is the result of the more rapidly decreasing kerfwidth at the bottom than at the top as thetraverse rate increases.

Figure 6: Effect of Traverse Speed on Kerf Taper Angle

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Effect of Traverse Speed onSurface RoughnessAzmir and Ahsan investigated the influence oftraverse speed on prediction of the kerf profileshape under different traverse speed in AWJMof glass/epoxy, it was found that the roughnessof the cut profiles changes with traverse rateand it is more obvious at the highest traverserate. In this case, a lower traverse rate isdesirable to produce a better surface finish asshown in Figure 7a.

Azmir et al. in case of traverse rate, it canbe anticipated as increasing the traverse rateallows less overlap machining action and fewerabrasive particles to impinge the surface,increasing the roughness of the surface. Also,a faster traverse rate increases the jetdeflection which results to a higher magnitudeof surface roughness. The roughness of thecut profiles changes with traverse rate and itis more obvious at the highest traverse rate. Inthis case, a lower traverse rate is desirable toproduce a better surface finish as shown inFigure 7b.

Effect of Traverse Speed on DepthPenetrationWang and Guo Figure 8 shows that, thedepth of penetration decreases with anincrease in jet traverse rate in an exponentialform. As the traverse speed increases, thenumber of particles impinging on a givenexposed target area decreases, which inturn reduces the material removal rate. Theyhave found that the damping and frictioneffect on the jet decreases as the jetexposure time decreases. Thus, an increasein the jet traverse speed will reduce theenergy loss of the particles and improve thematerial removal rate. It has been reportedthat with a faster travel of the jet, fewerparticles will be able to strike on the targetmaterial and open a narrower slot.Consequently, as a result of the reducedenergy loss and the narrowing kerf width ata high traverse speed, the rate of decreasein the depth of penetration is reducing andthe curves tend to flattening in the graphs asthe traverse speed increases.

Figure 7: Effect of Traverse Speed on Surface Roughness

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Standoff DistanceEffect of Standoff Distance on KerfGeometryShanmugam and Masood With increase in

standoff distance, the kerf taper increaseswithin the range 2-5 mm as shown in Figure9a. By increasing the standoff distance thematerial surface is exposed to the downstreamof the jet. At downstream, the jet starts todiverge losing its coherence thereby reducingthe effective cutting area that directly affectsthe kerf taper angle.

Jun Wang Figure 9b shows that the top andbottom kerf widths increase with an increasein the standoff distance although a smaller rateassociated with the bottom kerf width isobserved. This may be explained as the resultof jet divergence when high-velocity water jetsspread out (at different angles) as they exitfrom the mixing tube. Since the jet is losing itskinetic energy as it penetrates into the workmaterial, the outer rim of the diverged jet willnot take effect as it approaches the lower part

Figure 8: Effect of Traverse Speedon Depth Penetration

Figure 9: Effect of Standoff Distance on Kerf Taper Angle

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of the kerf. As such, the stand-off distance hasa lesser effect on the bottom kerf width thanthe top kerf width. As a consequence of thiseffect, the kerf taper angle is increasing withthe stand-off distance, as shown in Figure 9b.

Azmir and Ahsan Figure 9c, the kerf taperratio increases with the increase in standoffdistance. It was found that higher standoffdistance allows the jet to expand beforeimpingement and lowers the densities ofabrasive particles in the outer perimeter of theexpanding jet. This generally results in lowerpenetration depth as well as a higher surfaceroughness. Thus, increasing the standoffdistance between the nozzle and work-pieceis expected to result in higher differencebetween top and bottom kerf widths whicheventually gives higher kerf taper ratio.

Ramulu and Arola the influence of standoffdistance on kerf taper is shown in Figure 9e.At this combination of parametric levels, thekerf taper increases with an increase instandoff distance to 2.75 mm. Beyond thisstandoff distance the kerf taper begins todecrease, most likely due to the reducedeffects of jet expansion at the jet entrance withlittle change of kerf width at the jet exit. Althoughjet expansion increases with higher standoffdistance, as the standoff distance surpasses2.75 mm the energy of the exterior of the jetdecreases to levels which are below thosenecessary to create macro-damage. For thisreason, the kerf taper decreases with anincrease in standoff distance. Generally, whencutting with higher jet pressures or larger gritsizes, kerf taper increases with an increase instandoff distance.

Azmir et al. Referring to Figure 9d, the TR

increases with the increase in standoff

distance. Higher standoff distance allows thejet to expand before impingement and lowersthe densities of abrasive particles in the outerperimeter of the expanding jet. Thus,increasing the standoff distance between thenozzle and work-piece is expected to result inhigher difference between top and bottom kerfwidths which eventually gives higher TR.

Effect of Standoff Distance onSurface RoughnessAzmir and Ahsan in case of standoff distance,generally, higher standoff distance allows thejet to expand before impingement which mayincrease vulnerability to external drag from thesurrounding environment. Therefore, increasein the standoff distance results an increasedjet diameter as cutting is initiated and in turn,reduces the kinetic energy density of the jet atimpingement. It is desirable to have a lowerstandoff distance which may produce asmoother surface due to increased kineticenergy as shown in Figure 10a.

Azmir et al. in case of standoff distanceincreasing the standoff distance results anincrease in jet diameter as cutting is initiatedand in turn, reduces the kinetic energy densityof the jet due to impingement. It is desirable tohave a lower standoff distance which mayproduce a smoother surface due to increasedkinetic energy. As shown in Figure 10b,decreasing the standoff distance reduces thesurface roughness slightly. However, it isbelieved that the surface of the machinedlaminate may not be optimized with minimumstandoff distance. If the standoff distance istoo small, the abrasive water flow is dampedor decelerated by the target surface thatgenerates shallower depths of cut.

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Abrasive Mass Flow RateEffect of Abrasive Mass Flow Rateon Kerf GeometryShanmugam and Masood with an increase inthe abrasive mass flow rate the kerf taper

angle seems to decrease insignificantly, asshown in Figure 11a. It is implicit that a criticalenergy transfer from the jet to the particles isneeded to fracture the material, below whichany increase in abrasive mass flow rate doesnot have an effect on the kerf taper angle. In

Figure 10: Effect of Standoff Distance on Surface Roughness

Figure 11: Effect of Abrasive Mass Flow Rate on Kerf Taper Angle

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general, the effect of traverse speed and waterpressure is pronounced higher compared tostandoff distance with the abrasive mass flowrate having minimal effect. It is recommendedthat a combination of high water pressure, lowtraverse speed and short standoff distance areused to produce more vertical kerf wall.

Azmir and Ahsan the higher the abrasivemass flow rate, the higher the number ofparticles involved in the mixing and cuttingprocesses. Every increase in the abrasivemass flow rate leads to a proportional increasein the depth of cut. Therefore, the jet will havehigher kinetic energy and consequently will gainhigher capability to penetrate the workpiece.As a result, there will be a relatively wider widthfor both top and bottom kerf widths. With theincrease in penetration capability, the bottomwidth tends to be equal to the top widthresulting in kerf taper ratio approximating to1. As shown in Figure 11b, with the increasein abrasive mass flow rate consequently thekerf taper ratio is approaching to 1 as thepenetration capability increases.

Azmir et al. the higher the abrasive massflow rate, the higher the number of particlesinvolved in the mixing and cutting processes.Every increase in the abrasive mass flow rateleads to a proportional increase in the depthof cut. Therefore, the jet will have higher kineticenergy and consequently will gain higher abilityto penetrate the work-piece. As a result, therewill be a relatively wider width for both top andbottom kerf widths. The increase in both topand bottom kerf widths does not give a higheror lower value in TR as it is calculated as theratio of top kerf width to the bottom kerf width.Figure 11c shows no clear trend on the effectof abrasive mass flow rate on the TR.

Effect of Abrasive Mass Flow Rateon Surface RoughnessAzmir and Ahsan have been made aninvestigation on AWJ machining glass/epoxyand studied the influence of abrasive massflow rate on surface roughness. In case ofabrasive mass flow rate, the higher the abrasivemass flow rate, the higher the number ofparticles involved in the mixing and cuttingprocesses. An increase in abrasive mass flowrate leads to a proportional increase in thedepth of cut. When the abrasive mass flow rateis increased, the jet can cut through thelaminate easily and as a result, the cut surfacebecomes smoother. However, the roughnessincreases with an increase in abrasive massflow rate up to a certain limit and beyond thatlimit it was found to decrease as illustrated inFigure 12a. This is due to the fact that anincrease in mass of abrasive particles resultsin inter-collision of particles among themselvesand hence causes a loss of kinetic energy.

Azmir et al. have reported that, in case ofabrasive mass flow rate, the higher the abrasivemass flow rate, the higher the number ofparticles involved in the mixing and cuttingprocesses. An increase in abrasive mass flowrate leads to a proportional increase in the depthof cut. When the abrasive mass flow rate isincreased, the jet can cut through the laminateeasily and as a result, the cut surface becomessmoother. However, the roughness increaseswith an increase in abrasive mass flow rate upto a certain limit and beyond that limit it wasfound to decrease as illustrated in Figure 12b.

Effect of Abrasive Mass flow rateon Depth PenetrationWang and Guo Figure 13 show that the depthof penetration increases with the abrasive

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mass flow rate This trend is in line with theearlier findings in many investigations, and thepredicted trend and values are in goodagreement with those from the experiments. Itis apparent that more particles tend to removemore materials and increase the depth ofpenetration. However, not all the abrasiveparticles in the jet will strike the target materialor at least not remove the material in the sameefficiency. This is due to the interferencebetween particles which reduces the particleenergy as well as the effectiveness of individualparticles in cutting the material. An increase inthe number of particles (or mass flow rate) inthe jet will increase the chance of particleinterference. Thus the overall cuttingperformance in terms of the depth ofpenetration does not increase linearly withabrasive mass flow rate. In addition, the kerfwidth also increases to some extent with theabrasive mass flow rate, which has a reducedeffect on the depth of penetration andcontributes to the reduced rate of increase inthe depth of penetration.

Types of AbrasiveEffect of Types of Abrasive on KerfGeometryAzmir and Ahsan have been made aninvestigation on AWJ machining glass/epoxyand studied the influence of types of abrasive

Figure 12: Effect of Abrasive Mass Flow Rate on Surface Roughness

Figure 13: Effect of Standoff Distanceon Depth Penetration

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on kerf geometry. It shows clearly that at higherhardness of abrasive particles tends toproduce lower taper ratio as shown in Figure14a. It is believed that with the higher hardnessof abrasive particles increases the kineticenergy of the water jet. Thus, results in highercapability of jet penetration into the targetmaterials. As a result the top kerf width will bebigger and the bottom also may have relativelybigger width giving a lower taper as it iscalculated based on the ratio of top and bottomkerf widths.

Effect of Types of Abrasive onSurface RoughnessAzmir and Ahsan have been made aninvestigation on AWJ machining glass/epoxyand studied the influence of types of abrasiveon surface roughness. It was found that higherhardness of abrasive material which wasaluminium oxide gave better surface finishcompared to lower hardness of abrasivematerial such as garnet. The abrasive materialhardness influences the fracture behavior of

the abrasive particles. The harder the material,the higher the probability of particle fractures.The use of the harder aluminium oxidesubstantially reduces the surface roughness.It was found that the use of harder abrasivematerial such as silicon carbide and aluminiumoxide resulted in retaining its cutting capability.Consequently, the surface of cuts becamesmoother as seen in Figure 14b.

Cutting OrientationAzmir and Ahsan have been made aninvestigation on AWJ machining glass/epoxyand studied the influence of cutting orientationon kerf geometry and surface roughness.Surface roughness may be influenced by thekinetic energy of the jet. It also depends on thearchitecture of the fibres. As pointed outearlier, cutting orientation is relativelysignificant in influencing the surface roughnessas illustrated in Figure 15b. However inpresent study, there is no clear trend on theeffect of fibre’s cutting orientation on surfaceroughness. It is believed that both constituents

Figure 14: Effect of Types of Abrasive on Surface Roughness

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of fibres and interstitial matrix experiencedindependent shear fracture during materialremoval process. Based on Figure 15b,surface roughness is lowest at cuttingorientation 22.50. However, the real effect ofcutting orientation is very much a subject fordiscussion where it may well depend on thenature of fibres, mechanics fracture of fibresand cohesiveness of matrix. Actually, the effectof traverse rate on the kerf taper was also foundto be similar to that observed on the surfaceroughness as shown in Figure 15a.

Grit SizeRamulu and Arola have been made aninvestigation on AWJ machining of graphite/epoxy and studied the influence of grit size onkerf taper ratio. The influence of grit size onkerf taper ratio at these cutting conditions isshown in Figure 16. As shown, generallysmaller grit sizes result in an increase in taperof the kerf when machining graphite/epoxy. Itis also illustrated that a greater kerf taperresults when using #80 garnet at thisparametric combination. This phenomenon ismost likely due to the rate of material removal

Figure 15: Effect of Cutting Orientation on Kerf Characteristics and Surface Roughness

Figure 16: Effect of Grit Size on Kerf TaperRatio

at the jet entrance when using large abrasives.Note that the kerf taper reaches a minimum atthese cutting conditions and is increased withlarger grit sizes. This phenomenon is attributedto the effect of larger abrasives on the entrancekerf width. Large abrasives increase the initialimpact zone which results in a wide, r entrancekerf and in turn, a larger taper ratio.

Controlled Nozzle OscillationXu and Wang have been made an

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investigation on AWJ machining of aluminaceramic and studied the influence of jetoscillation on kerf taper ratio and surfaceroughness. It has been found oscillation anglehave a similar effect on kerf taper and surfaceroughness. This is shown in Figures 17a and17b. This trend is consistent for cutting undervarious conditions. Figure 17b shows theeffect of oscillation angle on surfaceroughness. Here, it can be seen that initially,surface roughness increases slightly with anincrease in oscillation angle, and reaches amaximum turning point. As the oscillation anglefurther increases, surface roughness starts todecrease. This may be a result of the scanningaction of the jet on the cutting front. Thereappears to be an optimum scanning scopecorresponding to a set of cutting parameters,similar to the above discussion about the effectof standoff distance. As the oscillation angleincreases in the lower region, the jet scanningaction cannot effectively cut off the “peaks” lefton the cut surface, thus causing jet turbulenceor instability and system vibration that increasethe surface roughness. Larger oscillationangles increase the overlap cutting action and

the number of scanning actions on a given partof surface, so that the scanning action isdominant and thus reduces the surfaceroughness.

CONCLUSIONAbrasive water jet machining process includeslarge number of process parameters, whichaffects the quality of cutting surface. Processparameter which affect less or more on qualityof cutting in abrasive water jet machining arehydraulic pressure, traverse speed, stand-offdistance, abrasive mass flow rate, abrasivematerials, nozzle length and diameter, orificediameter, abrasive shape, size and hardness.Characteristics of cutting surface is measuredinform of surface roughness, surface waviness,material removal rate, kerf top width, kerfbottom width and kerf taper angle. It wasconcluded from literature (i) Hydraulic pressure(MPa) and type of abrasive materials wereconsidered as the most significant controlfactor in influencing surface roughness andtaper ratio respectively. (ii) High traversespeeds result in lower material removal ratesas material. (iii) Garnet abrasives produce a

Figure 17: Effect of Jet Oscillation Angle on (a) Kerf Taper Ratio and (b) SurfaceRoughness

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larger taper of cut followed by Al2O3. This isdue to higher hardness of Al2O3 compared togarnet. (iv) Decreasing the standoff distanceand traverse rate may improve both criteria ofmachining performance. Cutting orientationdoes not influence the machining performancein both cases. (v) Small oscillation angles ofcutting nozzle can improve the various cuttingperformance measures like surfaceroughness. Proper selection of orifice andnozzle length and diameter can improve thekerf quality.

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7. Jain V K (2008), “Machining Advanced(Non-Traditional) Machining Processes”,in J Paulo Davim (Ed.), Machining:Fundamentals and Recent Advances,pp. 299-327, Springer-Verlag LondonLimited, London.

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13. Walstad O M and Noecker P W (1972),“Development of High Pressure Pumpsand Associated Equipment for Fluid JetCutting”, Paper C3, in: Proceedings of the1st International Symposium on Jet CuttingTechnology, Coventry, UK.

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Abrasive Water Jet Cutting of PolymerMatrix Composites”, Journal of MaterialsProcessing Technology , Vol. 121,pp. 390-394.

15. Xu S and Wang J (2006), “A Study ofAbrasive Water Jet Cutting of AluminaCeramics with Controlled NozzleOscillation”, Int. J. Adv. Manuf. Technol.,Vol. 27, pp. 693-702.

the influence of abrasive water jet … pressure, traverse speed, abrasive mass flow rate, standoff...

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