Unit 2 ucm
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Energy based process

Energy based process

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Unit 2 ucm Unit 2 ucm Presentation Transcript

  • MECHANICAL ENERGY BASED PROCESSES UNIT II
  • Syllabus  Abrasive Jet Machining  Working Principles  Water Jet Machining  Equipment used  Abrasive Water Jet Machining &  Process parameters  Ultrasonic Machining  MRR  Variation in techniques used  Applications
  • Abrasive Jet Machining Working Principles – equipment used – Process parameters – MRR-Variation in techniques used – Applications
  • Working Principle     A stream of abrasive grains (Al2O3 or SiC) is carried by high pressure gas or air (compressed). Impinges on the work surface at very high velocity through a nozzle of 0.3 to 0.5 mm diameter Sand Blasting (SB) - a similar process The major differences between are SB and AJM    smaller diameter abrasives a more finely controlled delivery system Material removal – by mechanical abrasion action of the high velocity abrasive particles
  • Working Principle
  • Equipment Used  Abrasive delivery system  Control system  Pump  Nozzle  Motion system
  • Working  A gas (Nitrogen, CO2 or air) is supplied at 2 – 8 kg/cm2  Oxygen should never be used. (because, it causes violent chemical action with the work piece chips or abrasive particles).  Gas passes through a mixing chamber after filtration and regulation.  In the mixing chamber, abrasive particles (10 – 40 µm) are present and vibrated at 50 Hz.  Amplitude of vibration – to control the feed rate of abrasives.  (Gas + abrasives) - passed through a 0.45 mm diameter tungsten carbide nozzle at a speed of 150 – 300 m/s.  The nozzle is directed over the area to be machined
  • Working  Aluminum oxide (Al2O3) and silicon carbide (SiC) powders are used for heavy cleaning, cutting and deburring.  Magnesium carbonate is recommended for use in light cleaning and etching.  Sodium bicarbonate – fine cleaning and cutting of soft materials.  Commercial grade powders are not suitable – b’cos their sizes are not well classified. Also, they may contain silica which can cause a health hazard.  Abrasive powders are not reused. B’cos, contaminations and worn grits will reduce the machining rate (MRR).  The nozzle stand off distance is 0.81 mm
  • Process parameters VS Metal Removal Rate  Mass flow rate  Abrasive grain size  Gas pressure  Velocity of abrasive particles  Mixing Ratio  Nozzle tip clearance
  • Process Characteristics
  • Application Drilling holes Cutting slots Cleaning hard surfaces Deburring and polishing Machining intricate shapes or holes in sensitive, brittle, thin, or difficultto-machine materials  Frosting glass and trimming of circuit boards, hybrid circuit resistors, capacitors, silicon, and gallium     
  • Advantage The process is used for machining super alloys and refractory materials. It is not reactive with any work piece material. No tool changes are required. Intricate parts of sharp corners can be machined No initial hole is required for starting the operation as required by wire EDM.  Material utilization is high.  It can machine thin materials     
  • Disadvantage       The removal rate is slow. The tapering effect may occur especially when drilling in metals. The abrasive may get impeded in the work surface. Suitable dust-collecting systems should be provided. Soft materials can’t be machined by the process. Silica dust may be a health hazard
  • Water Jet Machining Working Principles – equipment used – Process parameters – MRR-Variation in techniques used – Applications
  • Working Principle  Water is pumped at a sufficiently high pressure, 200-400 MPa (2000-4000 bar) using intensifier technology.  An intensifier works on the simple principle of pressure amplification using hydraulic cylinders of different cross-sections as used in “Jute Bell Presses”.  When water at such pressure is issued through a suitable orifice (generally of 0.2- 0.4 mm dia), the potential energy of water is converted into kinetic energy, yielding a high velocity jet (1000 m/s).
  • Working
  • Water jet Equipments  It is consists of three main units (i) A pump along with intensifier. (ii)Cutting head comprising of nozzle and work table movement. (iii) filter unit for debris & impurities
  • Process parameters  Material Removal Rate  Geometry and surface Finish of work  Wear rate of the nozzle
  • Advantage  There are no bits or tools touching the material surface, thus there is no tool replacement costs.  Ultrahigh-pressure Water-jets cut to accuracy's of +/-0.010".  Low level of mechanical stress (less than a pound) placed on the work piece preventing damage and deformations.
  • Application  Used for cutting      Composites Plastics Fabrics Rubber wood products etc.  Also used in food processing industry
  • Abrasive Water Jet Machining Working Principles – equipment used – Process parameters – MRR-Variation in techniques used – Applications
  • Introduction  WJM - suitable for cutting plastics, foods, rubber insulation, automotive carpeting and headliners, and most textiles.  Harder materials such as glass, ceramics, concrete, and tough composites can be cut by adding abrasives to the water jet.  Abrasive water jet machining (AWJM) – Developed in 1974 to clean metal prior to surface treatment of the metal.  The addition of abrasives to the water jet enhanced MRR and produced cutting speeds between 51 and 460 mm/min.  Generally, AWJM cuts 10 times faster than the conventional machining methods of composite materials.  Zheng et al. (2002) claimed that the abrasive water jet is hundreds of times more powerful than the pure water jet.
  • WJM & AWJM  Different approaches and methodologies in WJM & AWJM   WJM – with stabilizer  AWJM – entrained – three phase (water + air + abrasives)  AWJM – suspended – two phase (water + abrasives)  Direct pumping  Indirect pumping   WJM – pure Bypass pumping In all the above variants, the basic methodology remains the same.
  • Basic Methodology  Water is pumped at a sufficiently high pressure, 200-400 MPa (2000 – 4000 bar).  “Intensifier” works on the principle of pressure amplification using hydraulic cylinders of two different cross-sections.  When water at such a pressure is passed through a suitable orifice (nozzle having φ = 0.2 – 0.4 mm), the potential energy of water is converted into kinetic energy.  This yields high velocity (~ 1000 m/s) jet of water.  Such a high velocity water jet can machine thin sheets/foils of aluminium, leather, textile, frozen foods, etc.  WJM – commercially pure water (tap water) is used for machining.
  • Variants in WJM & AWJM  Problem in WJM – as the high velocity water jet is discharged from the orifice, the jet tends to entrain atmospheric air and flares out – decreasing the machining ability.  Hence, stabilizers (long chain polymers) are added to water (WJM with stabilizer).  Stabilizers hinders the fragmentation of water jet.  In AWJM, abrasive particles are added to the water jet to enhance its cutting ability by many folds.  In entrained type AWJM, the abrasive particles are allowed to entrain in water jet to form abrasive water jet wit sufficient velocity of as high as 800 m/s.  Such high velocity abrasive jet can machine almost any material.
  • Commercial CNC WJM
  • WJM & AWJM - Applications  Paint removal  Cleaning  Cutting soft materials  Cutting frozen meat  Textile, Leather industry  Mass Immunization  Surgery  Peening  Pocket Milling  Drilling & Turning  Nuclear Plant Dismantling
  • WJM & AWJM - Materials  Steels & Non-ferrous alloys  Ti alloys, Ni- alloys  Polymers  Honeycombs  Metal Matrix Composite & Ceramic Matrix Composite  Concrete  Stone – Granite  Wood  Reinforced plastics  Metal Polymer Laminates  Glass Fibre Metal Laminates
  • Applications of WJM & AWJM  The cutting ability of WJM can be improved drastically by adding hard and sharp abrasive particles into the water jet.  Thus, WJM is typically used to cut so called “softer” and “easy-to-machine” materials like thin sheets and foils, non-ferrous metallic alloys, wood, textiles, honeycomb, polymers, frozen meat, leather etc.  But, the domain of “harder” and “difficult-to-machine” materials like thick plates of steels, aluminium and other commercial materials, 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 in paint removal, cleaning, surgery, peening to remove residual stress etc.  AWJM can as well be used besides cutting for pocket milling, turning, drilling, etc.  One of the strategic areas where “robotic AWJM” is finding critical application is dismantling of nuclear plants.
  • WJM & AWJM - Advantages  Extremely fast set-up and programming  Very little fixturing for most parts  Machine virtually any 2D shape on any material  Very low side forces during the machining  Almost no heat generated on the part  Can machine thick plates 30
  • AWJM - Elements  AWJM accelerates a jet of water (70 percent) and abrasive (30 percent) from 4.2 bar up to a velocity of 30 m/s.  Silicon carbides, sand (SiO2), corundum, and glass beads of grain size 10 to 150 μm are often used as abrasive materials.  Using such a method, burrs of 0.35 mm height and 0.02 mm width left in steel component after grinding are removed by the erosive effect of the abrasives while water acts as an abrasive carrier.  The introduction of compressed air to the water jet enhances the deburring action. 31
  • Some Machining Components
  • Ultrasonic Machining Working Principles – equipment used – Process parameters – MRR-Variation in techniques used – Applications
  • ULTRASONIC MACHINING (UM)  In UM the tip of the tool vibrates at low amplitude and at high frequency. This vibration transmits a high velocity to fine abrasive grains between tool and the surface of the work piece.  Material removed by erosion with abrasive particles.  The abrasive grains are usually boron carbides.  This technique is used to cut hard and brittle materials like ceramics, carbides, glass, precious stones and hardened steel
  •  Metal cutting as in this process abrasives contained in a slurry are driven at high velocity against the work piece by a tool vibrating at low amplitude and high frequency  Amplitude is kept of the order of 0.07 mm and frequency is maintained at approximately 20,000 Hz.  The work piece material is removed in the form of extremely small chips  Abrasive slurry acts as a multipoint cutting tool and does the similar action as done by a cutting edge
  • Process details  Abrasive Slurry  Work piece  Ultrasonic Oscillator or Transducer  Feed Mechanism
  • Process Parameters  MRR  Tool Material  Tool Wear Rate  Abrasive Material and Abrasive Slurry  Surface Finish  Work Material
  • Metal Removal Rate  Grain Size of Abrasive  Abrasive Material  Concentration of slurry  Amplitude of Vibration  Frequency of Ultrasonic Waves
  • Ultrasonic Oscillator or Transducer  Magnetostriction Transducer  Piezoelectric Transducer
  • Feed Mechanism  Gravity feed Mechanism  Spring loaded Feed Mechanism  Pneumatic or Hydraulic Feed Mechanism
  • Application  The machining of hard and brittle materials like carbides glass, ceramics, precious stones, titanium  It is used for tool making and punch and die making  It is widely used for several machining operations like turning, grinding, trepanning and milling
  • Advantage  Its main advantage is the work piece after machining is free from any residual stress as to concentrated force or heat is subject to it during the machining process.  Extremely hard and brittle materials can be machined, their machining is very difficult by conventional methods.  Very good dimensional accuracy and surface finish can be obtained.  Operational cost is low.  The process is environmental friendly as it is noiseless and no chemical and heating is used
  • Limitation  Its metal removal rate (MRR) is very low and it can not be used for large machining cavities.  Its initial setup cost and cost of tool is very high, frequency tool replacement is required as tool wear takes place in this operation.  Not recommended for soft and ductile material due to their ductility.  Power consumption is quite high.  Slurry may have to be replaced frequently
  • Compiled by D.Vasanth kumar, Assistant Professor, Jansons Institute of technology