Water Jet And Abrasive Water Jet Machining

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Module9Non conventionalMachiningVersion 2 ME, IIT Kharagpur

Lesson37Water Jet and AbrasiveWater Jet MachiningVersion 2 ME, IIT Kharagpur

Instructional ObjectivesoooooooooooooList four different non conventional machining processesDifferentiate between water and abrasive water jet machiningList different WJM and AWJM systemsList ten different modules of AWJM systemsList four applications of AWJMList three advantages of AWJMList materials that can be processed by AWJMMention functions of different elements of AWJMIdentify mechanism of material removalDevelop models for mechanism of material removalIdentify parameters related to product qualityIdentify five limitations of AWJMIdentify environmental issues in the area of AWJMIntroductionWater Jet Machining (WJM) and Abrasive Water Jet Machining (AWJM) aretwo non-traditional or non-conventional machining processes. They belong tomechanical group of non-conventional processes like Ultrasonic Machining(USM) and Abrasive Jet Machining (AJM). In these processes (WJM andAJWM), the mechanical energy of water and abrasive phases are used toachieve material removal or machining. The general grouping of some of thetypical non-traditional processes are shown below:o Mechanical Processes USM AJM WJM and AWJMo Thermal Processes EBM LBM PAM EDM and WEDMo Electrical Processes ECM EDG EJDo Chemical Processes Chemical milling Photo chemical machiningWJM and AWJM can be achieved using different approaches andmethodologies as enumerated below: WJM - Pure WJM - with stabilizer AWJM – entrained – three phase – abrasive, water and air AWJM – suspended – two phase – abrasive and waterVersion 2 ME, IIT Kharagpur

o Direct pumpingo Indirect pumpingo Bypass pumpingHowever in all variants of the processes, the basic methodology remains thesame. Water is pumped at a sufficiently high pressure, 200-400 MPa (20004000 bar) using intensifier technology. An intensifier works on the simpleprinciple of pressure amplification using hydraulic cylinders of different crosssections as used in “Jute Bell Presses”. When water at such pressure isissued through a suitable orifice (generally of 0.2- 0.4 mm dia), the potentialenergy of water is converted into kinetic energy, yielding a high velocity jet(1000 m/s). Such high velocity water jet can machine thin sheets/foils ofaluminium, leather, textile, frozen food etc.In pure WJM, commercially pure water (tap water) is used for machiningpurpose. However as the high velocity water jet is discharged from the orifice,the jet tends to entrain atmospheric air and flares out decreasing its cuttingability.Hence, quite often stabilisers (long chain polymers) that hinder thefragmentation of water jet are added to the water.In AWJM, abrasive particles like sand (SiO2), glass beads are added to thewater jet to enhance its cutting ability by many folds. AWJ are mainly of twotypes – entrained and suspended type as mentioned earlier. In entrained typeAWJM, the abrasive particles are allowed to entrain in water jet to formabrasive water jet with significant velocity of 800 m/s. Such high velocityabrasive jet can machine almost any material. Fig. 1 shows the photographicview of a commercial CNC water jet machining system along with close-upview of the cutting head.Fig. 1 Commercial CNC water jet machining system and cutting heads(Photograph Courtesy – Omax Corporation, USA)Version 2 ME, IIT Kharagpur

ApplicationThe applications and materials, which are generally machined using WJ andAWJ, are given below:Application Paint removal Cleaning Cutting soft materials Cutting frozen meat Textile, Leather industry Mass Immunization Surgery Peening Cutting Pocket Milling Drilling Turning Nuclear Plant DismantlingMaterials SteelsNon-ferrous alloysTi alloys, Ni- alloysPolymersHoneycombsMetal Matrix CompositeCeramic Matrix CompositeConcreteStone – GraniteWoodReinforced plasticsMetal Polymer LaminatesGlass Fibre Metal LaminatesThe cutting ability of water jet machining can be improved drastically byadding hard and sharp abrasive particles into the water jet. Thus, WJM istypically used to cut so called “softer” and “easy-to-machine” materials likethin sheets and foils, non-ferrous metallic alloys, wood, textiles, honeycomb,polymers, frozen meat, leather etc, but the domain of “harder and “difficult-tomachine” materials like thick plates of steels, aluminium and other commercialmaterials, metal matrix and ceramic matrix composites, reinforced plastics,layered composites etc are reserved for AWJM.Other than cutting (machining) high pressure water jet also finds application inpaint removal, cleaning, surgery, peening to remove residual stress etc.AWJM can as well be used besides cutting for pocket milling, turning, drillingVersion 2 ME, IIT Kharagpur

etc. One of the strategic areas where robotic AWJM is finding criticalapplication is dismantling of nuclear plants.Fig. 2 Stainless steel plate(50 mm thick) machined withAWJ(Photograph Courtesy – OmaxCorporation, USA)Fig. 3 Different engineering componentsmachined with AWJ(Photograph Courtesy – OmaxCorporation, USA)Fig. 2 depicts a typical example of AWJM, where 50 mm thick stainless steelhas been machined. Fig. 3 shows the obtainable accuracy and precision withAWJM. Some of the job shop industries and manufacturers claim to havesuccessfully used AWJM in free form surface generation by milling as shownin the following web page:WJM and AWJM have certain advantageous characteristics, which helped toachieve significant penetration into manufacturing industries. Extremely fast set-up and programmingVery little fixturing for most partsMachine virtually any 2D shape on any materialVery low side forces during the machiningAlmost no heat generated on the partMachine thick platesMachineAny standard abrasive water jet machining (AWJM) system using entrainedAWJM methodology consists of following modules.Version 2 ME, IIT Kharagpur

LP booster pumpHydraulic unitAdditive MixerIntensifierAccumulatorFlexible high pressuretransmission lineOrificeMixing ChamberFocussing tube or insertsCatcherCNC tableAbrasive metering deviceCatcherOn-off valve61. LP Booster2. Hydraulic drive3. Additive mixer4. Direction control5. Intensifier5A.LP Intensifier5B.HP Intensifier6. Accumulator5B5 5A43Point A12Fig. 4 Schematic set-up of AWJMIntensifier, shown in Fig. 5 is driven by a hydraulic power pack. The heart ofthe hydraulic power pack is a positive displacement hydraulic pump. Thepower packs in modern commercial systems are often controlled bymicrocomputers to achieve programmed rise of pressure etc.Version 2 ME, IIT Kharagpur

pwpwphFig. 5 Intensifier – SchematicThe hydraulic power pack delivers the hydraulic oil to the intensifier at apressure of ph . The ratio of cross-section of the two cylinders in the intensifieris say A ratio (A A large / A small ). Thus, pressure amplification would takeplace at the small cylinder as follows.ph Al arg e pw Asmallpw ph Al arg eAsmallpw ph AratioThus, if the hydraulic pressure is set as 100 bar and area ratio is 40, pw 100x 40 4000 bar. By using direction control valve, the intensifier is driven bythe hydraulic unit. The water may be directly supplied to the small cylinder ofthe intensifier or it may be supplied through a booster pump, which typicallyraises the water pressure to 11 bar before supplying it to the intensifier.Sometimes water is softened or long chain polymers are added in “additiveunit”.Thus, as the intensifier works, it delivers high pressure water (refer Fig. 6). Asthe larger piston changes direction within the intensifier, there would be a dropin the delivery pressure. To counter such drops, a thick cylinder is added tothe delivery unit to accommodate water at high pressure. This is called an“accumulator” which acts like a “fly wheel” of an engine and minimisesfluctuation of water pressureHigh-pressure water is then fed through the flexible stainless steel pipes tothe cutting head. It is worth mentioning here that such pipes are to carry waterat 4000 bar (400 MPa) with flexibility incorporated in them with joints butwithout any leakage. Cutting head consists of orifice, mixing chamber andfocussing tube or insert where water jet is formed and mixed with abrasiveparticles to form abrasive water jet.Version 2 ME, IIT Kharagpur

Fig. 6 shows a cutting head or jet former both schematically andphotographically. Typical diameter of the flexible stainless steel pipes is of 6mm. Water carried through the pipes is brought to the jet former or singtubeCoverFig. 6 Schematic and photographic view of the cutting head(Photograph Courtesy – Omax Corporation, USA)The potential or pressure head of the water is converted into velocity head byallowing the high-pressure water to issue through an orifice of small diameter(0.2 – 0.4 mm). The velocity of the water jet thus formed can be estimated,assuming no losses as vwj (2pw / ρw)1/2 using Bernoulli’s equation where, pwis the water pressure and ρw is the density of water. The orifices are typicallymade of sapphire. In commercial machines, the life of the sapphire orifice istypically around 100 – 150 hours. In WJM this high velocity water jet is usedfor the required application where as in AWJM it is directed into the mixingchamber. The mixing chamber has a typical dimension of inner diameter 6mm and a length of 10 mm. As the high velocity water is issued from theorifice into the mixing chamber, low pressure (vacuum) is created within themixing chamber. Metered abrasive particles are introduced into the mixingchamber through a port.The abrasive particles are metered using different techniques like vibratoryfeeder or toothed belt feeder. The reader may consult standard literature ontransportation of powders.Version 2 ME, IIT Kharagpur

MixingFig. 7 schematically shows the mixing process. Mixing means gradualentrainment of abrasive particles within the water jet and finally the abrasivewater jet comes out of the focussing tube or the nozzle.Water jetTrajectory of anabrasive particleInteraction withfocussing tubeMixing chamberFocussing tubeFig. 7 Schematic view of mixing processDuring mixing process, the abrasive particles are gradually accelerated due totransfer of momentum from the water phase to abrasive phase and when thejet finally leaves the focussing tube, both phases, water and abrasive, areassumed to be at same velocity.The mixing chamber, as shown in Fig. 7 and Fig. 8, is immediately followed bythe focussing tube or the inserts. The focussing tube is generally made oftungsten carbide (powder metallurgy product) having an inner diameter of 0.8to 1.6 mm and a length of 50 to 80 mm. Tungsten carbide is used for itsabrasive resistance. Abrasive particles during mixing try to enter the jet, butthey are reflected away due to interplay of buoyancy and drag force. They goon interacting with the jet and the inner walls of the mixing tube, until they areaccelerated using the momentum of the water jet.Mixing process may be mathematically modelled as follows. Taking intoaccount the energy loss during water jet formation at the orifice, the water jetvelocity may be given as,2 pwv wj Ψ (1)ρwVersion 2 ME, IIT Kharagpur

where,Ψ Velocity coefficient of the orificeThe volume flow rate of water may be expressed as q w φ v wj A orifice q w φ v wj qw φ Πdo24Πdo2 Ψ4q w cd Πdo2 42 pwρw2 pwρwwhere,φ Coefficient of “vena-contracta”cd Discharge coefficient of the orificeThus, the total power of the water jet can be given asPwj p w q wPwj p w c d Πdo2 4ΠPwj c d do2 42 pwρw2 pw3ρwDuring mixing process as has been discussed both momentum and energyare not conserved due to losses that occur during mixing. But initially it wouldbe assumed that no losses take place in momentum, i.e., momentum of thejet before and after mixing is conserved. m v before m v after m air v air m w v wj m abr v abr m air v air m w v wj m abr v abr before afterThe momentum of air before and after mixing will be neglected due to verylow density. Further, it is assumed that after mixing both water and abrasivephases attain the same velocity of vwj . Moreover, when the abrasive particlesare fed into the water jet through the port of the mixing chamber, their velocityis also very low and their momentum can be neglected.Version 2 ME, IIT Kharagpur

m w v wj m w m abr v awj v awj v awj mw m w m abr 1 1 R v wjv wjwhere, R loading factor mabr mwAs during mixing process momentum loss occurs as the abrasives collide withthe water jet and at the inner wall of the focussing tube multiple times beforebeing entrained, velocity of abrasive water jet is given as,vawj η1 1 R vwjwhere, η momentum loss factor.Suspension JetIn entrained AWJM, the abrasive water jet, which finally comes from thefocussing tube or nozzle, can be used to machine different materials.In suspension AWJM the abrasive water jet is formed quite differently. Thereare three different types of suspension AWJ formed by direct, indirect andBypass pumping method as already given in Table. 2. Fig. 8 shows theworking principle of indirect and Bypass pumping system of suspensionAWJM system.Version 2 ME, IIT Kharagpur

Indirect Pumpinghp-waterfrom pumpPressure vesselBypass PrincipleBypassPressure -waterfrom pumpFig. 8Schematic of AWJM (Suspension type)In suspension AWJM, preformed mixture of water and abrasive particles ispumped to a sufficiently high pressure and store in pressure vessel. Then thepremixed high-pressure water and abrasive is allowed to discharge from anozzle to form abrasive water jet.CatcherOnce the abrasive jet has been used for machining, they may have sufficientlyhigh level of energy depending on the type of app

AWJM, the abrasive particles are allowed to entrain in water jet to form abrasive water jet with significant velocity of 800 m/s. Such high velocity abrasive jet can machine almost any material. Fig. 1 shows the photographic view of a commercial CNC water jet machining system along with close-up view of the cutting head.

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