Non Traditional Manufacturing – Introduction

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Module9Non-conventionalmachiningVersion 2 ME, IIT Kharagpur

Lesson35Introductionand Abrasive JetMachiningVersion 2 ME, IIT Kharagpur

Instructional ii.xiv.xv.xvi.Identify the characteristics of conventional machiningIdentify the characteristics of non traditional machiningDifferentiate between conventional and non traditional machiningClassify different non traditional machining processesIdentify the need for non traditional machining processesDescribe the basic mechanism of material removal in AJMIdentify major components of AJM equipmentState the working principle of AJM equipmentDraw schematically the AJM equipmentIdentify the process parameters of AJMIdentify the machining characteristics of AJMAnalyse the effect of process parameters on material removal rate (MRR)Draw variation in MRR with different process parametersDevelop mathematical model relating MRR with abrasive jet machining parametersList three applications of AJMList three limitations of AJM(i)IntroductionManufacturing processes can be broadly divided into two groups and they are primarymanufacturing processes and secondary manufacturing processes. The former ones providebasic shape and size to the material as per designer’s requirement. Casting, forming,powder metallurgy are such processes to name a few. Secondary manufacturing processesprovide the final shape and size with tighter control on dimension, surface characteristicsetc. Material removal processes are mainly the secondary manufacturing processes.Material removal processes once again can be divided into mainly two groups and they are“Conventional Machining Processes” and “Non-Traditional Manufacturing Processes”.Examples of conventional machining processes are turning, boring, milling, shaping,broaching, slotting, grinding etc. Similarly, Abrasive Jet Machining (AJM), UltrasonicMachining (USM), Water Jet and Abrasive Water Jet Machining (WJM and AWJM), Electrodischarge Machining (EDM) are some of the Non Traditional Machining (NTM) Processes.(ii)Classification of Non Traditional Machining ProcessesTo classify Non Traditional Machining Processes (NTM), one needs to understand andanalyse the differences and similar characteristics between conventional machiningprocesses and NTM processes.Conventional Machining Processes mostly remove material in the form of chips by applyingforces on the work material with a wedge shaped cutting tool that is harder than the workmaterial under machining condition. Such forces induce plastic deformation within the workpiece leading to shear deformation along the shear plane and chip formation. Fig. 9.1.1depicts such chip formation by shear deformation in conventional machining.Version 2 ME, IIT Kharagpur

Shear planeCHIPChipToolTOOLVCWorkpieceWORKPIECEFig.9.1.1 Shear deformation in conventional machining leading to chip formation.Thus the major characteristics of conventional machining are: Generally macroscopic chip formation by shear deformationMaterial removal takes place due to application of cutting forces – energy domaincan be classified as mechanicalCutting tool is harder than work piece at room temperature as well as undermachining conditionsNon Traditional Machining (NTM) Processes on the other hand are characterised as follows: Material removal may occur with chip formation or even no chip formation maytake place. For example in AJM, chips are of microscopic size and in case ofElectrochemical machining material removal occurs due to electrochemicaldissolution at atomic levelIn NTM, there may not be a physical tool present. For example in laser jetmachining, machining is carried out by laser beam. However in ElectrochemicalMachining there is a physical tool that is very much required for machiningIn NTM, the tool need not be harder than the work piece material. For example, inEDM, copper is used as the tool material to machine hardened steels.Mostly NTM processes do not necessarily use mechanical energy to providematerial removal. They use different energy domains to provide machining. Forexample, in USM, AJM, WJM mechanical energy is used to machine material,whereas in ECM electrochemical dissolution constitutes material removal.Thus classification of NTM processes is carried out depending on the nature of energy usedfor material removal. The broad classification is given as follows: Mechanical Processes Abrasive Jet Machining (AJM) Ultrasonic Machining (USM) Water Jet Machining (WJM) Abrasive Water Jet Machining (AWJM)Electrochemical Processes Electrochemical Machining (ECM) Electro Chemical Grinding (ECG) Electro Jet Drilling (EJD)Electro-Thermal Processes Electro-discharge machining (EDM)Version 2 ME, IIT Kharagpur

Laser Jet Machining (LJM) Electron Beam Machining (EBM)Chemical Processes Chemical Milling (CHM) Photochemical Milling (PCM) etc.Fig. 9.1.2 schematically depicts some of the NTM processes:toolf 20 – 25 kHza 10 25 μmtoolwork piecework piecetooltoolstand-off-distancework piecework pieceFig. 9.1.2 Schematic representation of various metal cutting operations.(iii) Need for Non Traditional MachiningConventional machining sufficed the requirement of the industries over the decades. Butnew exotic work materials as well as innovative geometric design of products andcomponents were putting lot of pressure on capabilities of conventional machiningprocesses to manufacture the components with desired tolerances economically. This led tothe development and establishment of NTM processes in the industry as efficient andeconomic alternatives to conventional ones. With development in the NTM processes,currently there are often the first choice and not an alternative to conventional processes forcertain technical requirements. The following examples are provided where NTM processesare preferred over the conventional machining process:Version 2 ME, IIT Kharagpur

Intricate shaped blind hole – e.g. square hole of 15 mmx15 mm with a depth of30 mmDifficult to machine material – e.g. same example as above in Inconel, Ti-alloysor carbides.Low Stress Grinding – Electrochemical Grinding is preferred as compared toconventional grindingDeep hole with small hole diameter – e.g. φ 1.5 mm hole with l/d 20Machining of composites.(iv) Abrasive Jet MachiningIn Abrasive Jet Machining (AJM), abrasive particles are made to impinge on the workmaterial at a high velocity. The jet of abrasive particles is carried by carrier gas or air. Thehigh velocity stream of abrasive is generated by converting the pressure energy of thecarrier gas or air to its kinetic energy and hence high velocity jet. The nozzle directs theabrasive jet in a controlled manner onto the work material, so that the distance between thenozzle and the work piece and the impingement angle can be set desirably. The highvelocity abrasive particles remove the material by micro-cutting action as well as brittlefracture of the work material. Fig. 9.1.3 schematically shows the material removal process.High velocity abrasive gas jet (150 300 m/s)nozzledi (0.2 0.8 mm)Stand off distance(0.5 15 mm)workpieceFig. 9.1.3 Schematic representation of AJMAJM is different from standard shot or sand blasting, as in AJM, finer abrasive grits are usedand the parameters can be controlled more effectively providing better control over productquality.In AJM, generally, the abrasive particles of around 50 μm grit size would impinge on thework material at velocity of 200 m/s from a nozzle of I.D. of 0.5 mm with a stand off distanceof around 2 mm. The kinetic energy of the abrasive particles would be sufficient to providematerial removal due to brittle fracture of the work piece or even micro cutting by theabrasives.Version 2 ME, IIT Kharagpur

(v)EquipmentIn AJM, air is compressed in an air compressor and compressed air at a pressure of around5 bar is used as the carrier gas as shown in Fig. 9.1.4. Fig. 9.1.4 also shows the other majorparts of the AJM system. Gases like CO2, N2 can also be used as carrier gas which maydirectly be issued from a gas cylinder. Generally oxygen is not used as a carrier gas. Thecarrier gas isPressure controlvalveAbrasivefeederfilterAbrasive mixed with carrier ¼ turn valveAir compressorNozzleworkpieceElectro-magnetic on-off valvetableFig. 9.1.4AJM set-upfirst passed through a pressure regulator to obtain the desired working pressure. The gas isthen passed through an air dryer to remove any residual water vapour. To remove any oilvapour or particulate contaminant the same is passed through a series of filters. Then thecarrier gas enters a closed chamber known as the mixing chamber. The abrasive particlesenter the chamber from a hopper through a metallic sieve. The sieve is constantly vibratedby an electromagnetic shaker. The mass flow rate of abrasive (15 gm/min) entering thechamber depends on the amplitude of vibration of the sieve and its frequency. The abrasiveparticles are then carried by the carrier gas to the machining chamber via an electromagnetic on-off valve. The machining enclosure is essential to contain the abrasive andmachined particles in a safe and eco-friendly manner. The machining is carried out as highvelocity (200 m/s) abrasive particles are issued from the nozzle onto a work piece traversingunder the jet.(vi) Process Parameters and Machining Characteristics.The process parameters are listed below: Abrasive Material – Al2O3 / SiC / glass beads Shape – irregular / spherical Size – 10 50 μm Mass flow rate – 2 20 gm/minCarrier gas Composition – Air, CO2, N2 Density – Air 1.3 kg/m3Version 2 ME, IIT Kharagpur

Velocity – 500 700 m/s Pressure – 2 10 bar Flow rate – 5 30 lpmAbrasive Jet Velocity – 100 300 m/s M abr M gas Mixing ratio – mass flow ratio of abrasive to gas – Stand-off distance – 0.5 5 mm Impingement Angle – 600 900 Nozzle The material removal rate (MRR) mm3/min or gm/minThe machining accuracyThe life of the nozzle Material – WC / sapphire Diameter – (Internal) 0.2 0.8 mm Life – 10 300 hoursThe important machining characteristics in AJM areFig. 9.1.5 depicts the effect of some process parameters on MRRMRRdgMRRMixing ratioabrasive flow rateConstantmixing ratioMRRgas flow rateMRRfor constantmixing ratioabrasive flow rateGas pressure(a)(b)MRR(c)(a)(b) SOD(d)(c) (d)Fig. 9.1.5 Effect of process parameters MRRVersion 2 ME, IIT Kharagpur

(vii) Modelling of material removalAs mentioned earlier, material removal in AJM takes place due to brittle fracture ofthe work material due to impact of high velocity abrasive particles.Modelling has been done with the following assumptions:(i)(ii)(iii)(iv)Abrasives are spherical in shape and rigid. The particles are characterised by themean grit diameterThe kinetic energy of the abrasives are fully utilised in removing materialBrittle materials are considered to fail due to brittle fracture and the fracturevolume is considered to be hemispherical with diameter equal to chordal length ofthe indentationFor ductile material, removal volume is assumed to be equal to the indentationvolume due to particulate impact.Fig. 9.1.6 schematically shows the interaction of the abrasive particle and the work materialin AJM.Abrasive gritEquivalent grit2r2rIndentation (δ)Ductile materialBrittle materialMaterial removalABCδD2rFig. 9.1.6 Interaction of abrasive particles with workpieceVersion 2 ME, IIT Kharagpur

From the geometry of the indentationAB 2 AC 2 BC 2BC 2 r 2 AB 2 AC 22 dg dg r δ 2 2 22r δ d gδ d gδ22r dgδ Volume of material removal in brittle material is the volume of the hemispherical impactcrater and is given by:ΓB 22ππ r3 (d g δ )3 / 233For ductile material, volume of material removal in single impact is equal to the volume of theindentation and is expressed as:2 d g δ πδ d gΓD πδ 2 2 3 2Kinetic energy of a single abrasive particle is given byK .E.g π11 π 3 3mg v 2 d g ρ g v 2 d g ρgv 222 612 where,v velocity of the abrasive particlemg mass of a single abrasive gritdg diameter of the gritρg density of the gritOn impact, the work material would be subjected to a maximum force F which would lead toan indentation of ‘δ’. Thus the work done during such indentation is given byW 1Fδ2Now considering H as the hardness or the flow strength of the work material, the impactforce (F) can be expressed as:F indentatio n area x hardnessF π r2 Hwhere, r the indentation radius W 11Fδ π r 2 H δ22Now, as it is assumed that the K.E. of the abrasive is fully used for material removal, thenthe work done is equated to the energyVersion 2 ME, IIT Kharagpur

W K.E.π1 2d 3 ρg v 2πr δH 12 g2d 3 ρg v 2gnow r d g δδ 6r 2H r 2 d gδd 2 ρg v 2gδ2 6H1/2 ρg δ d v g 6H Now MRR in AJM of brittle materials can be expressed as:MRRB ΓB x Number of impacts by abrasive grits per second ΓB N.6Γ mmama B3 aMRRB ΓBmass of a grit π 3d g ρ g πd g ρ g6.2π(d g δ )3 / 2 ma 4 m. a δ 3 / 26x3 3ρ g d g πd g ρ gas ΓB 2π(d g δ ) 3 / 233 . 2 4mδa MRRB ρg d g 1 ρg 2as δ dg v 6H .4 ma d g v MRRB . ρg dg 3/2 ρg 6H .MRRB as ΓD πδ 2d g2.4 ma v 3 / 263 / 4 ρg1/ 43/4 H 3/4ma v 3 / 21/ 4ρg H 3 / 4MRR for ductile material can be simplified as:.MRRD ΓD N ΓD6 ma3πd g ρ g πδ 2d g 6 ma32πd g ρ gVersion 2 ME, IIT Kharagpur

.MRRD as6πδ 2 ma2π dg 2 ρg1/2 ρg δ dg v 6H .6 ma d g 2v 2 ρg MRRD 2d g 2 ρg 6H .1 ma v 2MRRD 2 H(viii) Applications For drilling holes of intricate shapes in hard and brittle materialsFor machining fragile, brittle and heat sensitive materialsAJM can be used for drilling, cutting, deburring, cleaning and etching.Micro-machining of brittle materials(ix) Limitations MRR is rather low (around 15 mm3/min for machining glass)Abrasive particles tend to get embedded particularly if the work material is ductileTapering occurs due to flaring of the jetEnvironmental load is rather high.Quiz Test.1. AJM nozzles are made of(a) low carbon steel(b) HSS(c) WC(d) Stainless steel2. Material removal in AJM of glass is around(a) 0.1 mm3/min(b) 15 mm3/min(c) 15 mm3/s(d) 1500 mm3/min3. Material removal takes place in AJM due to(a) electrochemical action(b) mechanical impact(c) fatigue failure of the material(d) sparking on impactVersion 2 ME, IIT Kharagpur

4. As the stand off distance increases, the depth of penetration in AJM(a) increases(b) decreases(c) does not change(d) initially increases and then remains steadyProblem1. Estimate the material removal rate in AJM of a brittle material with flow strength of 4GPa. The abrasive flow rate is 2 gm/min, velocity is 200 m/s and density of the abrasiveis 3 gm/cc.2. Material removal rate in AJM is 0.5 mm3/s. Calculate material removal per impact if massflow rate of abrasive is 3 gm/min, density is 3 gm/cc and grit size is 60 μm as well asindentation radius.Solutions to the Quiz problems1 – (c)2 – (b)3 – (b)4 – (b)Solutions to the ProblemsSolution of Prob. 12 x10 33/2x (200 )ma v 3/ 260MRRB 1/ 4 3 / 4 ρg H(3000 )1/ 4 x 4 x10 9 3 / 4MRRB 8 x10 10 m 3 / s 8 x10 1 x 60 mm 3 / s 48.()mm 3 / minSolution of Prob. 2Mass of grit π6d g .ρ g33 x10 36xma60 No. of impact / time 6 3π 3x 3000d g ρ g πx 50 x106.()N 254648MRR 0.5mm3 / sΓB 1.96 x10 6 mm3 1960 μm 3N2546648 / s2Indentatio n volume π r 3 1960 μm 33Indentation radius, r 9.78 10 μmVersion 2 ME, IIT Kharagpur

In Abrasive Jet Machining (AJM), abrasive particles are made to impinge on the work material at a high velocity. The jet of abrasive particles is carried by carrier gas or air. The high velocity stream of abrasive is generated by converting the pressure energy of the carrier gas or air to its kinetic energy and hence high velocity jet. The nozzle directs the abrasive jet in a controlled manner .

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