ABRASIVE JET MACHINING - Nitkkrncmp.files.wordpress

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Non Traditional MachiningABRASIVE JET MACHININGDefinition:In abrasive jet machining, a focused stream of abrasive particles, carried by highpressure air or gas is made to impinge on the work surface through a nozzle and thework material is made to impinge on the work surface through a nozzle and workmaterial is removed by erosion by high velocity abrasive particles.Process:In Abrasive jet machining abrasive particles are made to impinge on work material athigh velocity. Jet of abrasive particles is carried by carrier gas or air. The highvelocity stream of abrasives is generated by converting pressure energy of carriergas or air to its Kinetic energy and hence high velocity jet. Nozzles directs abrasivejet in a controlled manner onto work material. The high velocity abrasive particlesremove the material by micro-cutting action as well as brittle fracture of the workmaterial.This is a process of removal of material by impact erosion through the action ofconcentrated high velocity stream of grit abrasives entrained in high velocity gasstream. AJM is different from shot or sand blasting, as in AJM, finer abrasive grits areused and parameters can be controlled more effectively providing better control overproduct quality.In AJM, generally, the abrasive particles of around 50 microns grit size wouldimpinge on the work material at velocity of 200 m/s from a nozzle of ID 0.5mm witha stand off distance of around 2mm. The kinetic energy of the abrasive particleswould sufficient to provide material removal due to brittle fracture of the work pieceor even micro cutting by the abrasives.Physics of the Process: Fine particles (0.025mm) are accelerated in a gas stream The particle are directed towards the focus of machining As the particles impact the surface, it causes a micro fracture, and gas carriesfractured particles away Brittle and fragile work betterJagadeesha T, Assistant Professor. MED, National Institute of Technology, Calicut

Non Traditional MachiningEquipment:A schematic layout of AJM is shown above. The gas stream is then passed to thenozzle through a connecting hose. The velocity of the abrasive stream ejectedthrough the nozzle is generally of the order of 330 m/sec.Abrasive jet Machining consists of1.2.3.4.5.Gas propulsion systemAbrasive feederMachining ChamberAJM NozzleAbrasivesGas Propulsion SystemSupplies clean and dry air. Air, Nitrogen and carbon dioxide to propel the abrasiveparticles. Gas may be supplied either from a compressor or a cylinder. In case of acompressor, air filter cum drier should be used to avoid water or oil contamination ofabrasive powder. Gas should be non-toxic, cheap, easily available. It should notexcessively spread when discharged from nozzle into atmosphere. The propellantconsumption is of order of 0.008 m3/min at a nozzle pressure of 5 bar and abrasiveflow rate varies from 2 to 4 gm/min for fine machining and 10 to 20 gm/min forcutting operation.Abrasive Feeder.Required quantity of abrasive particles is supplied by abrasive feeder. The filletedpropellant is fed into the mixing chamber where in abrasive particles are fed througha sieve. The sieve is made to vibrate at 50-60 Hz and mixing ratio is controlled bythe amplitude of vibration of sieve. The particles are propelled by carrier gas to amixing chamber. Air abrasive mixture moves further to nozzle. The nozzle impartshigh velocity to mixture which is directed at work piece surface.Jagadeesha T, Assistant Professor. MED, National Institute of Technology, Calicut

Non Traditional MachiningMachining chamberIt is well closed so that concentration of abrasive particles around the workingchamber does not reach to the harmful limits. Machining chamber is equipped withvacuum dust collector. Special consideration should be given to dust collectionsystem if the toxic material (like beryllium) are being machined.AJM nozzleAJM nozzle is usually made of tungsten carbide or sapphire ( usually life – 300 hoursfor sapphire , 20 to 30 hours for WC) which has resistance to wear. The nozzle ismade of either circular or rectangular cross section and head can be head can bestraight, or at a right angle. It is so designed that loss of pressure due to the bends,friction etc is minimum possible. With increase in wear of a nozzle, the divergence ofjet stream increases resulting in more stray cutting and high inaccuracy.Jagadeesha T, Assistant Professor. MED, National Institute of Technology, Calicut

Non Traditional MachiningABRASIVESAluminum oxide (Al2O3) Silicon carbide (SiC) Glass beads, crushed glass and sodiumbicarbonate are some of abrasives used in AJM. Selection of abrasives depends onMRR , type of work material , machining accuracy.AbrasivesAluminum oxide(Al2O3)Grain Sizes12, 20, 50 micronsSilicon carbide (SiC)25,40 micronGlass beads0.635 to 1.27mmApplicationGood for cleaning, cuttingand deburringUsed for similarapplication but for hardmaterialGives matte finishDolomite200 meshEtching and polishingSodium bi carbonate27 microsCleaning, deburring andcutting of soft materialLight finishing below 500CProcess parametersFor successful utilization of AJM process, it is necessary to analyze the followingprocess criteria.1. Material removal rate2. Geometry and surface finish of work piece3. wear rate of the nozzleHowever, Process criteria are generally influenced by the process parameters asenumerated below: Abrasivesa) material – Al2O3 SiC Glass beads Crushed glass Sodium bi carbonateb) shape – irregular/regularc) Size – 10 to 50 micronsd) Mass flow – 2-20 gm/min Carrier Gasa) Composition – Air, CO2, N2b) Density – 1.3 kg/m3c) Velocity - 500 to 700 m/sd) Pressure - 2 to 10 bare) Flow rate - 5 to 30 microns Abrasive Jetb) Velocity - 100 to 300 m/sc) Mixing ratio – Volume flow rate of abrasives/Volume flow rate of gasd) Stand off distance – SOD- 0.5 to 15mm.e) Impingement angle – 60 to 90 deg.Jagadeesha T, Assistant Professor. MED, National Institute of Technology, Calicut

Non Traditional Machining Nozzlea) Material – WC/Sapphireb) Diameter – 0.2 to 0.8 mmc) Life – 300 hours for sapphire, 20 to 30 hours for WCProcess capability1. Material removal rate – 0.015 cm3/min2. Narrow slots – 0.12 to 0.25mm 0.12mm3 Surface finish -0.25 micron to 1.25 micron4 Sharp radius up to 0.2mm is possible5. Steel up to 1.5mm ,Glass up to 6.3mm is possible to cut6. Machining of thin sectioned hard and brittle materials is possible.Applications1. This is used for abrading and frosting glass more economically as compared toetching or grinding2. Cleaning of metallic smears on ceramics, oxides on metals, resistive coating etc.3. AJM is useful in manufacture of electronic devices , drilling of glass wafers, deburring of plastics, making of nylon and Teflon parts permanent marking onrubber stencils, cutting titanium foils4. Deflashing small castings, engraving registration numbers on toughened glassused for car windows5. Used for cutting thin fragile components like germanium, silicon etc.6. Register treaming can be done very easily and micro module fabrication forelectrical contact , semiconductor processing can also be done effectively.7. Used for drilling , cutting , deburring etching and polishing of hard and brittlematerials.8. Most suitable for machining brittle and heat sensitive materials like glass, quartz,sapphire , mica , ceramics germanium , silicon and gallium.9. It is also good method for deburring small hole like in hypodermic needles andfor small milled slots in hard metallic components.Advantages1. High surface finish can be obtained depending upon the grain sizesParticle size ( in microns)Surface roughness ( in microns)100.152 to 0.20325 to 270.355 to 0.675500.965 to 1.272. Depth of damage is low ( around2.5 microns)3. It provides cool cutting action, so it can machine delicate and heat sensitivematerial4.Process is free from chatter and vibration as there is no contact between the tooland work piece5.Capital cost is low and it is easy to operate and maintain AJM.6.Thin sections of hard brittle materials like germanium, mica, silicon, glass andceramics can be machined.7.It has the capability of cutting holes of intricate shape in hard materials.Jagadeesha T, Assistant Professor. MED, National Institute of Technology, Calicut

Non Traditional MachiningDisadvantages /Limitations1. Limited capacity due to low MRR. MRR for glass is 40 gm/minute2 Abrasives may get embedded in the work surface, especially while machining softmaterial like elastomers or soft plastics.3. The accuracy of cutting is hampered by tapering of hole due to unavoidable flaringof abrasive jet.4. Stray cutting is difficult to avoid5. A dust collection system is a basic requirement to prevent atmospheric pollutionand health hazards.6. Nozzle life is limited (300 hours)7. Abrasive powders cannot be reused as the sharp edges are worn and smallerparticles can clog the nozzle.8. Short stand off distances when used for cutting , damages the nozzle.Machining characteristicsFollowing are the AJM process criteria1. Material removal rate2. Geometry and surface finish of work piece3. wear rate of the nozzleProcess criteria are generally influenced by the process parametersThe characteristics of above process parameters on process criteria are as follows1.Effect of abrasive flow rate andgrain size on MRRIt is clear from the figure that at aparticular pressure MRR increasewith increase of abrasive flow rateand is influenced by size of abrasiveparticles.Butafterreachingoptimum value, MRR decreases withfurther increase of abrasive flowrate. This is owing to the fact thatMass flow rate of gas decreases withincrease of abrasive flow rate andhence mixing ratio increases causinga decrease in material removal ratebecause of decreasing energy available for erosion.2. Effect of exit gas velocity and abrasiveparticle densityThe velocity of carrier gas conveying the abrasiveparticles changes considerably with the change ofabrasive particle density as indicated in figure.The exit velocity of gas can be increased tocritical velocity when the internal gas pressure isnearly twice the pressure at exit of nozzle for theabrasive particle density is zero. If the density ofJagadeesha T, Assistant Professor. MED, National Institute of Technology, Calicut

Non Traditional Machiningabrasive particles is gradually increased exit velocity will go on decreasing for thesame pressure condition. It is due to fact that Kinetic energy of gas is utilized fortransporting the abrasive particle3.Efect of Mixing ratio on MRRIncreased mass flow rate of abrasive will result in adecreased velocity of fluid and will thereby decreasesthe available energy for erosion and ultimately theMRR. It is convenient to explain to this fact by termMIXING RATIO. Which is defined asMixing ratio Volume flow rate of carrier gasVolume flow rate of carrier gasThe effect of mixing ratio on the material removal rate is shown above.The material removal rate can be improved byincreasing the abrasive flow rate provided the mixingratio can be kept constant.The mixing ratio isunchanged only by simultaneous increase of both gasand abrasive flow rate.An optimum value of mixing ratio that gives maximumMRR is predicted by trial and error. In place of Mixingratio, the mass ratio ( α ) may be easier to determine.Which is defined as.α Mass flow rate of carrier gasMass flow rate of carrier gas and abrasive ma.ma g3.Effect of Nozzle pressure on MRRThe abrasive flow rate can be increased byincreasing the flow rate of the carrier gas.This is only possible by increasing the internalgas pressure as shown in the figure. As theinternal gas pressure increases abrasive massflow rate increase and thus MRR increases.As a matter of fact, the material removal ratewill increase with the increase in gas pressureKinetic energy of the abrasive particles isresponsible for the removal of material byerosion process. The abrasive must impinge on the work surface with minimumvelocity for machining glass by SIC particle is found to be around 150m/s.Jagadeesha T, Assistant Professor. MED, National Institute of Technology, Calicut

Non Traditional Machining5. Stand off distance.Stand off distance isdefined as the distancebetween the face of thenozzle and the worksurface of the work. SODhas been found to haveconsiderable effect on theworkmaterialandaccuracy. A large SODresults in flaring of jetwhichleadstopooraccuracy.It is clear from figure thatMRR increase with nozzletip distance or Stand offdistance up to certain distanceandthendecreases.Penetration rate also increaseswith SOD and then decreases.Decrease in SOD improvesaccuracy, decreases kerfwidth,and reduces taper in machinedgroove.Howeverlightoperation like cleaning, frostingetc are conducted with largeSOD.(say 12.5 to 75mm)Jagadeesha T, Assistant Professor. MED, National Institute of Technology, Calicut

Non Traditional MachiningMaterial removal models in AJMFollowing assumptions are made in deriving the Material removal models for AJM.1. Abrasive are spherical in shape and rigid2. Kinetic energy of particle is completely used to cut the material3. Brittle material are considered to fail due to brittle fracture and fracture of volumeis considered to be hemispherical with diameter equal to chordal length ofindentation4. For Ductile material volume of material removal is assumed to be equal toindentation volume due to particulate impact.Abrasive particles are assumed to be spherical in shape having diameter dg.From the geometryAB2 AC2 BC22 dg 2 dg δ r 2 2 dg 2 dg δ r 2 22r2 - δ2 d g δ Neglecting δ 2 term wecan writer dg δFor Brittle materialVolume of the material removed is the volume of the hemispherical crater due thefracture is given byVolume of the material removed31 4 1 4 ΓB π r 3 π (r ) 2 2 3 2 3 2 3 π (d g δ32) --------------------------------(1)Jagadeesha T, Assistant Professor. MED, National Institute of Technology, Calicut

Non Traditional MachiningLet us assume that grits also move with velocity (V) then we can writeKinetic Energy KE 12 2 M (V ) 1 π 3 2 2 6 d g ρ g (V ) --------------------------(2) On impact , work material would be subjected to maximum force F, Which wouldleads to indentation of δ .Work done during such indentation is1W D by the grit Fδ -----------(3)2F2or F σ w π r --------(4)Also we know the Flow strength of material σ w 2πr2F σ w π (d g δ )Using equation (4) in (3) we get.W D by the grit σ π (d g δ )δ1Fδ w22It is assumed that Kinetic energy of the abrasives is fully used for material removalKinetic energy of the particle W D by the particleσ w π (d g δ )δ 1 π 3 2 2 6 d g ρ g (V ) 2 Simplifying we getδ V dgρg6σ -----------------------(5)MRR in AJM material can be expressed as Volume of the material Number of MRR removed X impacts made by abrasives per second per grit per cycle. MRR Mass flow rate of abrasives ΓB x Mass of the abrasive grit Jagadeesha T, Assistant Professor. MED, National Institute of Technology, Calicut

Non Traditional Machining3Ma 2 π (d g δ )2 x 3π(d g )3 ρ g6Upon simplifying we get Ma MRR 13 (ρ g )4 (σ w ) 4 For ductile materialΓD 1π (d g ) (δ )22MRR [] Mass flow rate of abrasives ΓD x Mass of the abrasive grit Substituting and simplifying we get.MRR M a V2 2σW Problem 1 : Estimate the MRR in AJM of a brittle material with flow strength of 4GPA. The abrasive flow rate is 2 gm/min, velocity is 200m/s, density of abrasive is 3gm/sec.Data Given:Flow strength of work material σ w 4 x10 9 N/m 2 4000 N / mm 2Abrasive grain density 3gm/CC 3x10-3x10-3 3x10-6 kg/mm3Mass flow rate of abrasives ρ g 2gm/min 2x 10-3/60 kg/secVelocity V 200x1000mm/secSolutionSince the material is brittle. We need to use the MRR formula corresponding toThe brittle material.MRRBritlle M av 13 (ρ g )4 (σ w ) 4 3 3 2 x10 x 200000 2 13 69 (3x10 )4 (4 x10 )4 48 mm3 /minutesJagadeesha T, Assistant Professor. MED, National Institute of Technology, Calicut

Non Traditional MachiningProblem 2 : Material removal rate in AJM is 0.5 mm3/sec. calculate MRR/impact ifthe mass flow rate of abrasive is 3gm/min, density is 3 gm/CC and grit size is 60microns . Also calculate the indentation radius.Data Given:Material removal rate 0.5 mm3/sec.Abrasive grain size 60 microns 6x10-3 mmMass flow rate of abrasives ρ g 3gm/min 3x 10-3/60 kg/sec π 3 6 dg ρg Mass of grit MRR Volume of the material removed x230.5x10 π (r ) x3-6 3 1000 x 60 π 60 33 6 10000 1000 x 10 -6Mass flow rate of abrasivesMass of abrasive grit 10 microns Volume of the material Number of MRR removed X impacts made by abrasives per second per grit per cycle. 3x10 36x 60Number of impact/time 2546483 πx 50 x10 6 x3000 Problem 3: During AJM , Mixing ratio used is 0.2. calculate Mass ratio, if the ratio ofdensity of abrasives and density of carrier gas is equal to 20.()Solution :Mixing ratio(MR) Mass ratio( α ) Volume flow rate of abrasive particleVolume flow rate of carrier gasAbrasive mass flow ratecombined mass flow rate of abrasive and carier gas.MR Va.VgMaρa Va also α .M a g ρ V ρ V.agag.Ori.e1α1α .ρa Va ρ g Vg.ρa Va. ρ g V g 11 1 x . 1 20 0.2 ρ a V a 1.2 5 or α 0.80Jagadeesha T, Assistant Professor. MED, National Institute of Technology, Calicut

Non Traditional MachiningProblem 4: Diameter of nozzle is 1.0mm and jet velocity is 200m/s. Find thevolumetric flow rate ( cm3/sec ) of carrier gas and abrasive mixture.Cross sectional area of nozzle (πx0.52x10 2 ) πx 25 x10 4 cm 2Volumetric flow rate of carrier gas and abrasive mixture is area x velocity πx 25x 2 50 π cm3 /SecPractice Problem1.Find out the condition for which AJM will produce equal MRR both for ductile andbrittle materials. ( Hint : equate MRR equations and find the condition for criticalvelocity)2.Material removal rate in AJM is 0.3 mm3/sec. calculate MRR/impact if the massflow rate of abrasive is 4gm/min, density is 1.5 gm/CC and grit size is 23 micronsAlso calculate the indentation radius3During AJM , Mixing ratio used is 0.3. Calculate Mass ratio, if the ratio of densityof abrasives and density of carrier gas is equal to 10Give reasons for the following1. Abrasive machining is not suitable for soft materials2. Working temperature should be less than 50oc if we use Sodium bicarbonateabrasive particle3. Abrasive particles in AJM cannot be reused.4. Dust collection system is needed for Abrasive jet machining of beryllium5. In AJM, MRR for ductile material is lower than Brittle materials.Review Questions1. With the help of neat diagram explain Abrasive jet machining systemand label the parts.2. What are the advantages and limitations of AJM?3. Discuss the effect of following process parameters on MRR SOD and MRR Effect of abrasive grain size and flow on MRR Effect of Nozzle pressure for various MR on MRR Effect of Mixing ratio on MRR6. Derive an expression for MRR in AJM for brittle material7. Write at least five application of AJM in Industry8. State clearly the process capability of AJM9. Define AJM. What is the principle of Abrasive jet machining10. Write the applications of different types of abrasives used in AJM.11. Write five important variables of AJM process. Draw a sketch showing the effectof these variables on MRR12. Explain the working principle of AJM process13. With the help of sketches, show the effect of stand off distance on MRR.Jagadeesha T, Assistant Professor. MED, National Institute of Technology, Calicut

Abrasive jet Machining consists of 1. Gas propulsion system 2. Abrasive feeder 3. Machining Chamber 4. AJM Nozzle 5. Abrasives Gas Propulsion System Supplies clean and dry air. Air, Nitrogen and carbon dioxide to propel the abrasive particles. Gas may be supplied either from a compressor or a cylinder. In case of a compressor, air filter cum drier should be used to avoid water or oil .

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