Nontraditional Machining and

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					      Nontraditional Machining and
       Thermal Cutting Processes
• Nontraditional machining refers to a group a processes
  which removes excess material by various techniques
  involving mechanical, thermal, electrical or chemical
  energy
• These processes do not use a sharp cutting tool in the
  conventional sense
• Nontraditional processes have been developed in
  response to new and unusual machining requirements,
  including
   – The need to machine newly developed materials with special
     properties (high strength, high hardness, high toughness)
   – The need for unusual and/or complex geometries
   – The need to avoid surface damage
• Classification of nontraditional manufacturing
  processes by principle form of energy
   – Mechanical - mechanical energy in some form different from
     the action of a conventional cutting tool; erosion of the
     workpiece material is typical
   – Electrical - electrochemical energy to remove material
– Thermal - thermal energy generally applied to a small portion
  of the work surface, causing removal by fusion and/or
  vaporization; thermal energy is generated by conversion of
  electrical energy
– Chemical - most materials are susceptible to chemical attack
  by certain acids or other etchants; chemicals selectively remove
  material from portions of the workpiece, while other portions
  of the surface are protected
• Available nontraditional material removal processes
   – Mechanical
      • AFM - abrasive flow machining
      • AJM - abrasive jet machining
      • HDM - hydrodynamic machining
      • LSG - low stress grinding
      • RUM - rotary ultrasonic machining
      • TAM - thermally assisted machining
      • TFM - total form machining
      • USM - ultrasonic machining
      • WJM - water jet machining
– Electrical
   • ECD - electrochemical deburring
   • ECDG - electrochemical discharge grinding
   • ECG - electrochemical grinding
   • ECH - electrochemical honing
   • ECM - electrochemical machining
   • ECP - electrochemical polishing
   • ECS - electrochemical sharpening
   • ECT - electrochemical turning
   • ES - electro-stream
   • STEM - shaped tube electrolytic machining
– Thermal
   • EBM - electron beam machining
   • EDG - electrical discharge grinding
   • EDM - electrical discharge machining
   • EDS - electrical discharge sawing
   • EDWC - electrical discharge wire cutting
   • LBM - laser beam machining
   • LBT - laser beam torch
   • PBM - plasma beam machining
   – Chemical
      • CHM - chemical machining
      • ELP - electropolish
      • PCM - photochemical machining
      • TCM - thermochemical machining
      • TEM - thermal energy machining
• While many processes are available, only the most
  commercially important processes are discussed here
       Mechanical Energy Processes

• Ultrasonic machining (USM)
   – Abrasives contained in a slurry are driven at high velocity
     against the work by a tool vibrating at low amplitude (.003in)
     and high frequency (20-100khz)
   – The tool oscillates in a direction perpendicular to the
     workpiece surface and is fed slowly into the workpiece so that
     the shape of the tool is formed in the part
– The action of the abrasives impinging against the work surface
  performs the cutting
– Tool materials - soft steel, stainless steel
– Abrasive materials - boron nitride, boron carbide, aluminum
  oxide, silicon carbide and diamond
– The vibration amplitude should be set approximately equal to
  the grit size, and the gap size should be maintained at about
  two times the grit size
– The ratio of work material to tool material removed during the
  cutting process ranges from ~100:1 for cutting glass down to
  ~1:1 for cutting tool steel
– Workpiece materials: hard and brittle such as ceramics, glass
  and carbides; successfully used on certain metals such as
  stainless steel and titanium
– Shapes obtained by USM include nonround holes, holes along
  a curved axis and coining operation, in which an image pattern
  on the tool is imparted to a flat work surface
• Water jet cutting (WJC)
   –   Nozzle diameter: 0.004-0.016 in
   –   Pressure: up to 60,000psi
   –   Jet velocity: up to 3000 ft.Sec
   –   Nozzle made of sapphire, ruby or diamond
   –   Cutting fluids: polymer solutions; preferred because of their
       tendency to produce a coherent stream
– Important process parameters: standoff distance, nozzle
  operating diameter, water pressure and cutting feed rate
– Typical feed rates: 12 in/min to well over 1200 in/min
– The water jet cutting process is usually automated using CNC
  robots to manipulate the nozzle unit along the desired
  trajectory
– Materials cut by water jet: plastic, textile, composites, tiles,
  carpet, leather and cardboard
– Advantages: no crushing or burning of the work surface,
  minimum material loss because of the narrow cut slit, no
  environmental pollution, and easy automating the process
– Limitation: not suitable to cut brittle material because of their
  tendency to crack during cutting
• Abrasive water jet cutting (AWJC)
   – Introduction of abrasive particles into the stream adds to the
     number of parameters that must be controlled; among these
     are: abrasive type, grit size and flow rate
   – Type of abrasive materials: aluminum, oxide, silicon dioxide
     and garnet (a silicate mineral)
   – Grit size: ranges between 60 and 120
   – Flow rate: approximately 0.5 lb/min
   – Nozzle orifice diameter: 0.010 - 0.025in; somewhat larger that
     in water jet cutting to permit higher flow rates and more
     energy to be contained in the stream prior to the infection of
     abrasives
• Abrasive jet machining (AJM)
   – A high velocity stream of gas containing small abrasive
     particles
   – Pressure: 25 - 200 psi
   – Nozzle orifice diameter: 0.003 - 0.040 in
   – Velocities: 500 - 1000 ft/min
   – Gases: dry air, nitrogen, carbon dioxide and helium
   – The process is usually carried out manually by an operator
   – AJM is normally used as a finishing process
   – Applications: deburring, trimming and deflashing, cleaning
     and polishing
   – Applied on hard, brittle materials (glass, silicon, mica and
     ceramics) that are in the form of thin flat stock
   – Typical abrasives: aluminum oxide (for aluminum and brass),
     silicon carbide (for stainless steel and ceramics), and glass
     beads (for polishing)
– Grit sizes are small, 15-40m in diameter and must be very
  uniform in size for a given application
– No recycling of abrasives; abrasive grains are fractured, worn
  and contaminated
Electrochemical Machining Processes

• Electrochemical machining (ECM)
   – It removes metal from an electrically conductive workpiece by
     anodic dissolution, in which the shape of the workpiece is
     obtained by a formed electrode tool in close proximity to, but
     separated from the work by a rapidly flowing electrolyte
   – Underlying principle: material is deplated from the anode and
     deposited onto the cathode in the presence of an electrolyte
     bath
– The difference in ECM is that the electrolyte bath flows
  rapidly between the two poles to carry off the deplated
  material
– The electrode tool, usually made of copper, brass or stainless
  steel, is designed to posses approximately the inverse of the
  desired final shape of the part
– Gap distance: usually from 0.003 - 0.030 in
– A water solution of sodium chloride is commonly used as the
  electrolyte
– Electrolyte serves for:
    • Carrying off the material that has been removed from the
      workpiece
    • Removing hear and hydrogen bubbles created in the
      chemical reactions of the process
– Removed material in the form of microscopic particles must be
  separated from the electrolyte through centrifuge,
  sedimentation or other means
– Large amount of electrical power is required to perform ECM
– Voltage is kept relatively low to minimize arcing across the gap
– Use when:
   • The material is very hard or difficult to machine or
   • Where the workpiece geometry is difficult or impossible
     to accomplish by conventional machining methods
– Typical ECM applications
   • Die sinking
   • Multiple hole drilling
   • Holes that are not round
   • Deburring
– Advantages:
   • Little surface damage to the work part
   • No burrs as in conventional machining
   • Low tool wear
   • Relatively high metal removal rates for hard and
     difficult to machine metals
– Disadvantages
   • Significant cost of electrical power to drive the
     operation
   • Problems of disposing of the electrolyte sludge
• Electrochemical deburring (ECD)
   – An adaptation of ECM designed to remove burrs or round
     sharp corners
   – The same ECM principles of operation apply to ECD
   – Much less material is removed in ECD, thus cycle times are
     much shorter
• Electrochemical grinding (ECG)
   – Special form of ECM
   – A rotating grinding wheel with a conductive bond material is
     used to augment the anodic dissolution of the metal workpart
     surface
   – Bond material: metallic (diamond abrasives) or resin bond
     impregnated with metal particles (aluminum oxide)
   – Most of the machining is accomplished by electrochemical
     action, therefore the grinding wheel lasts much longer
– Applications:
   • Sharpening of cemented carbide tools
   • Grinding of surgical needles, other thin wall tubes and
     fragile parts
         Thermal Energy Processes

• Electric discharge machining (EDM)
   – One of the most widely used nontraditional processes
   – Shape of the finished work surface is produced by a formed
     electrode tool
   – EDM process must take place in the presence of a dielectric
     fluid
– Discharge region heated to extremely high temperature so that
  a small portion of the work surface is melted and removed
– Individual discharges occur hundreds or thousands of times
  per second to give a gradual erosion of the entire surface
– Process variables:
    • Discharge current
    • Frequency of discharges
– The high spark temperature causes the tool to melt, resulting
  in a small cavity opposite the cavity produced in the work
– Wear ratio:
    • Work material removed/tool material removed
    • Ranges from 1.0 - 100 depending on the combination of
      work and electrode materials
– Electrode materials: graphite, copper, brass, copper tungsten,
  silver tungsten, etc.
– Metal removal rate:
    • MRR = KI/Tm1.23
– Dielectric fluids used: hydrocarbon oils, kerosene and distilled
  or deionized water
– Applications:
   • tool fabrication and parts production
   • delicate parts
   • hole drilling with hole axis at an acute angle to the surface
   • production machining of hard and exotic metals
• Electric Discharge Wire Cutting (EDWC or wire EDM)
   – special form of EDM using a wire as the electrode
   – cutting action achieved by thermal energy from electric
     discharges between the electrode wire and the workpiece
– Workpiece fed continuously and slowly past the wire to achieve
  cutting path
– NC used to control workpart motions
– Wire EDM must be carried out in the presence of a dielectric
– Wire diameters: 0.003 - 0.012 in.
– Wire materials: brass, copper, tungsten and molybdenum
– Dielectric fluids: deionized water or oil
– Overcut ranges from 0.0008 - 0.002in. And remains fairly
  constant and predictable once cutting conditions are
  established
• Electron Beam Machining (EBM)
  – A high velocity stream of electrons is focused on the workpiece
    surface to remove material by melting and vaporization
  – Electron beam gun accelerates a stream of electrons to ~3/4 c
    and focused through an electromagnetic lens
  – Kinetic energy of beam converted to thermal energy of
    extremely high density, melting or vaporizing material in a
    very localized area
  – EBM must be carried out in a vacuum
– Can be used on any known material
– Applications:
   • drilling of extremely small diameter holes - down to 0.002
     in
   • drilling holes with high depth/diameter ratios, greater than
     100:1
– Limitations:
   • need of a vacuum
   • high energy required
   • expensive equipment
• Laser Beam Machining (LBM)
  – Uses light energy from a laser to remove materials by
    vaporization and ablation
  – Types of lasers:
     • CO2
     • solid-state
  – Energy is concentrated optically and in terms of time
  – Light beam pulsed so that the released energy results in an
    impulse against the work surface, producing evaporation and
    melting
– Used for:
    • drilling - down to 0.001 in
    • slitting
    • slotting
    • scribing
    • marking
– Not considered a mass production process; generally used on
  thin stock
– Range of work materials virtually unlimited
• Plasma Arc Cutting (PAC)
   – Plasma - a superheated, electrically ionized gas
   – PAC uses a plasma stream operating at temperatures in the
     range from 18,000o - 25,000o F to cut metal
   – The high-velocity plasma stream is directed at the workpiece,
     melting it and blowing the molten metal through the kerf
– Plasma arc generated between an electrode inside the torch
– Plasma flows through a water-cooled nozzle, which constricts
  and directs the stream
– Hot enough to cut through metal 6 in thick
– Gases used:
   • nitrogen, argon-hydrogen or a mixture (primary gases)
   • secondary gases or water directed to surround the plasma
     jet to confine the arc and clean the kerf
– Most applications consist of cutting flat metal sheets and plates
– Can be used to cut nearly any electrically conductive metal
– Feed rates:
   • as high as 430 in/min for 1/4 in. aluminum
   • 200 in/min for 1/4 in. steel
   • 20 in/min for 4 in. aluminum
– Advantage: high productivity
– Disadvantages: rough cut surface, metallurgical damage
• Air Carbon Arc Cutting
   – arc generated between a carbon electrode and the metallic
     work
   – High-velocity air jet used to blow away the melted portion of
     the metal
   – Used to form a kerf for severing the piece or to gouge a cavity
     in the pat
   – Used on a variety of metals, including cast iron, carbon steel,
     low alloy and stainless steels
   – Sputtering of molten metal is a hazard
• Other Arc Cutting Processes
   –   Gas metal arc cutting
   –   Shielded metal arc cutting
   –   Gas tungsten arc cutting
   –   Carbon arc cutting
               Chemical Machining

• Mechanics and Chemistry of Chemical Machining
   – Differences in applications and the ways in which the steps are
     implemented account for the different forms of CHM; the
     steps are
       • Cleaning - to ensure that material will be removed
         uniformly from the surfaces to be etched
   • Masking - maskant, chemically resistant to the etchant,
     applied to portions of the work surface not to be etched
   • Etching - the material removal step; part immersed in an
     etchant that chemically attacks unmasked portions; part
     removed and washed when desired amount of material has
     been removed
   • Demasking - maskant removed from the part
– Masking and etching involve significant variations in methods,
  materials and process parameters
– Maskant materials: neoprene, polyvinyl chloride, polyethylene
  and other polymers
– Masking methods
   • Cut and peel - performed by hand, used for large
     workparts, low production quantities and where accuracy
     is not a critical factor
   • Photographic resist - normally applied where small parts
     are produced in high quantities and close tolerances are
     required
   • Screen resist - used in applications that are between the
     other two masking methods in terms of accuracy, part size
     and production quantity
– Etchant selection - depends on work material, desired depth
  and rate of etch, and surface finish requirements
• Chemical Milling
   – First CHM process to be commercialized
   – Used largely in the aircraft industry
   – Applicable to large parts where substantial amounts of metal
     are removed
   – Cut and peel maskant method employed
   – As depth increases, surface finish becomes worse
   – Metallurgical damage very small
• Chemical Blanking
   – Uses chemical erosion to cut very thin sheet-metal parts, down
     to 0.001 in. and/or for intricate cutting patterns
   – Produces burr free parts
   – Photoresist or screen resist method applied
   – Maximum stock thickness ~0.030 in.
   – Hardened or brittle materials can be processed
• Chemical Engraving
   – A chemical machining process used for making flat panels that
     have lettering and/or artwork on one side
   – Can be used to make raised or recessed lettering by reversing
     the portions of the panel to be etched
   – Masking done by either photoresist or screen resist methods
   – Filling operation to apply paint or other coating follows
     etching
• Photochemical Machining (PCM)
   – Chemical machining in which the photoresist masking method
     is used
   – Employed in metalworking when close tolerances and/or
     intricate patterns are required
   – Used extensively in electronics industry (makes VLSI possible)
   – Photoresist materials in current use are sensitive to UV light,
     but not other wavelengths
– No need to carry out process in a darkroom
– Anisotropy: depth of cut d divided by undercut u; reciprocal of
  the etch factor
    • A=1/Fe = d/u
    • A: degree of anisotropy
    • Fe: etch factor
         Application Considerations

• Workpart Geometry Features
   – Very small holes - (below 0.005 in. in diameter) use LBM
   – Holes with large depth/diameter ratios -     (d/D > 20) use
     ECM and EDM
   – Nonround holes - use EDM and ECM
   – Narrow slots that are not straight - use EBM, LBM, wire
     EDM, WJC and AWJC
– Micromachining - use PCM, LBM and EBM
– Shallow pockets and surface details in flat parts - use CHM
  and its variations
– Creation of special contoured shapes for mold and die
  applications - (die sinking) use EDM and ECM
• Work Materials
• Performance of Nontraditional Processes
   – Nontraditional processes are generally used when conventional
     methods are not practical or economical

				
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posted:11/25/2011
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