ME 350 � Lecture 22 � Chapter 26 by L830s3dw

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									 ME 350 – Lecture 22 – Chapter 26
NONTRADITIONAL MACHINING PROCESSES
1. Mechanical Energy Processes (USM, WJC, AJM)
2. Electrochemical Processes (ECM)
3. Thermal Processes (EDM, Wire EDM, EBM, LBM, PAC)
4. Chemical Processes (CHM, Chemical Blanking, PCM)
Nontraditional machining is characterized by material
  removal that:
ME 350 – Final Exam Update

          Location:
          DCL 1320

             Date:
      Friday May 13th, 2011

             Time:
       1:30 pm – 4:30 pm
 Nontraditional Processes Used When:

1. Material is either very hard, brittle or both; or
   material is very ductile:

2. Part geometry is complex or geometric
   requirements impossible with conventional
   methods:

3. Need to avoid surface damage or contamination
   that often accompanies conventional machining:
1. Mechanical Energy Processes

• Ultrasonic machining (USM)

• Water jet cutting (WJC)

• Abrasive jet machining (AJM)
1a) Ultrasonic Machining (USM & UW)




Abrasives in a slurry are driven at high velocity against
work by a vibrating tool (low amplitude & high frequency)
• Tool oscillation is perpendicular to work surface
• Abrasives accomplish material removal
• Tool is fed slowly into work
• Shape of tool is formed into part
USM Applications
• Used only on hard and brittle work materials:
  ceramics, glass, carbides, and hard metals.
• Shapes include non-round holes, holes along a
  curved axis
• “Coining operations” - pattern on tool is
  imparted to a flat work surface
• Produces virtually stress free shapes
• Holes as small as 0.076 mm have been made
1b) Water Jet Cutting (WJC)

• Uses high pressure, high
  velocity stream of water
  directed at work surface
  for cutting
 WJC Applications
• Usually automated using CNC or industrial robots
• Best used to cut narrow slits in flat stock such as:
  plastic, textiles, composites, tile, and cardboard
• Not suitable for:
• When used on metals, you need to add to the
  water stream:
• Smallest kerf width about 0.4 mm for metals, and
  0.1mm for plastics and non-metals.
• More info: http://www.waterjets.org/index.html
WJC Advantages
• No crushing or burning of work surface
• Minimum material loss
• No environmental pollution
• Ease of automation
1c) Abrasive Jet Machining (AJM)
 High velocity gas stream containing abrasive
 particles (aka: sand blasting or bead blasting)




  – Normally used as a finishing process rather than
    cutting process (e.g. gas sandpaper)
  – Applications: deburring, cleaning, and polishing.
2. Electrochemical Machining Processes
• Electrical energy used
  in combination with
  chemical reactions to
  remove material
• Reverse of:


• Work material must be    Courtesy of AEG-Elotherm-Germany
  a:
• Feature dimensions
  down to about 10 μm
    Electrochemical Machining (ECM)

Material removal by anodic dissolution, using
electrode (tool) in close proximity to work but
separated by a rapidly flowing electrolyte
ECM Operation
Material is deplated from anode workpiece (
  pole) and transported to a cathode tool (
  pole) in an electrolyte bath
• Electrolyte flows rapidly between two poles to
  carry off deplated material, so it does not:


• Electrode materials: Cu, brass, or stainless steel
• Tool shape is the:
  – Tool size must allow for the gap
ECM Applications
• Die sinking - irregular shapes and contours for
  forging dies, plastic molds, and other tools
• Multiple hole drilling - many holes can be
  drilled simultaneously with ECM
• No burrs created – no residual stress




    Schuster et al, Science 2000   Trimmer et al, APL 2003
Material Removal Rate of ECM
• Based on Faraday's First Law: rate of metal dissolved is
  proportional to the current
             MRR = Aƒr = ηCI
   where I = current; A = frontal area of the electrode (mm2),
     ƒr = feed rate (mm/s), and η = efficiency coefficient
      M
  C=     = specific removal rate with work material;
     nrF

           M = atomic weight of metal (kg/mol)
           r = density of metal (kg/m3),
           F = Faraday constant (Coulomb)
           n = valency of the ion;
 Equations for ECM (Cont’)


                       Gap, g




• Resistance of Electrode:

             g
       R = r
                                  Area, A
             A
    ρ is the resistivity of the
      electrolyte fluid (Ohm∙m)
 Example: ECM through a plate
• Aluminum plate, thickness t = 12 mm;
• Rectangular hole to be cut:
  L = 30mm, W = 10mm
• Applied current: I = 1200 amps.               10mm
                                         30mm
• Efficiency of 95%,
• MRR = Aƒr = ηCI

Determine how long it will take to cut the hole?

        Ideal CAl = 3.44×10-2 mm3/amp∙s
              - other ‘C’ values in Table 26.1
3. Thermal Energy Processes - Overview

• Very high temperatures, but only:
  – Material is removed by:

• Problems and concerns:
  – Redeposition of vaporized metal

  – Surface damage and metallurgical damage to the
    new work surface

  – In some cases, resulting finish is so poor that
    subsequent processing is required
3. Thermal Energy Processes

• Electric discharge machining (EDM)
• Electric discharge wire cutting (Wire EDM)
• Electron beam machining (EBM)
• Laser beam machining (LBM)
• Plasma arc cutting or machining (PAC)
    3a) Electric Discharge Machining (EDM)




•   One of the most widely used nontraditional processes
•   Shape of finished work is inverse of tool shape
•   Sparks occur across a small gap between tool and work
•   Holes as small as 0.3mm can be made with feature
    sizes (radius etc.) down to ~2μm
Work Materials in EDM

• Work materials must be:
• Hardness and strength of work material are:
  not factors
• Material removal rate depends primarily on:
  melting point of work material
• Applications:
  – Molds and dies for injection molding and forging
  – Machining of hard or exotic metals
  – Sheetmetal stamping dies.
3b) Wire EDM

• EDM uses small diameter wire as electrode to
  cut a narrow kerf in work – similar to a: bandsaw
  Material Removal Rate of EDM
• Weller Equation (Empirical);

  Maximum rate:         RMR = KI
                              1.23
                              Tm
     where K = 664 (°C1.23∙mm3/amp∙s);

     I = discharge current; Tm = melt

     temp of work material                 While cutting, wire is

• Actual material removal rate:            continuously advanced
                                           between supply spool
         MRR = vf ∙h∙wkerf
                                           and take-up spool to:

     where vf = feed rate; h = workpiece

     thickness; wkerf = kerf width
Wire EDM Applications
• Ideal for stamp and die
  components
   – Since kerf is so narrow, it is
     often possible to fabricate
     punch and die in a single cut
• Other tools and parts with
  intricate outline shapes,
  such as lathe form tools,
  extrusion dies, and flat
  templates
 3c) Electron Beam Machining (EBM)

• Part loaded inside a
  vacuum chamber

• Beam is focused through
  electromagnetic lens,
  reducing diameter to as
  small as 0.025 mm

• Material is vaporized in a
  very localized area
EBM Applications

• Ideal for micromachining
  – Drilling small diameter holes - down to 0.05 mm
    (0.002 in)
  – Cutting slots only about 0.025 mm (0.001 in.)
    wide
• Drilling holes with very high depth-to-diameter
  ratios
  – Ratios greater than 100:1
• Disadvantage: slow and expensive
3d) Laser Beam Machining (LBM)
• Generally used for:
  drilling, slitting,
  slotting, scribing,
  and marking
  operations
• Holes can be
  made down to
  0.025 mm
• Generally used on
  thin stock material
  3e) Plasma Arc Cutting (PAC)
• Uses plasma stream at
  very high temperatures to
  cut metal 10,000°C to
  14,000°C
• Plasma arc generated
  between electrode in torch
  and        workpiece
• The plasma flows through
  water-cooled nozzle that
  constricts and directs
  plasma stream to desired
  location
 Applications of PAC
• Most applications of PAC involve
  cutting of   metal sheets and plates
• Hole piercing and cutting along a
  defined path
• Can be operated by hand-held torch
  or automated by CNC
• Can cut any:
• Hole sizes generally larger than 2 mm
 4. Chemical Machining (CHM)




CHM Process:
• Cleaning - to insure uniform etching
• Masking - a maskant (resist, chemically resistant to etchant)
  is applied to portions of work surface not to be etched
• Patterning of maskant
• Etching - part is immersed in etchant which chemically
  attacks those portions of work surface that are not masked
• Demasking - maskant is removed
 Maskant - Photographic Resist Method
• Masking materials contain photosensitive chemicals
• Maskant is applied to work surface (dip coated, spin
  coated, or roller coated) and exposed to light through a
  negative image of areas to be etched
   – These areas are then removed using photographic
     developing techniques
   – Remaining areas are vulnerable to etching
• Applications:
   – Small parts on thin stock produced in high quantities
   – Integrated circuits and printed circuit cards
Material Removal Rate in CHM
• Generally indicated as penetration rates, i.e. mm/min.
• Penetration rate unaffected by exposed surface area
• Etching occurs downward and under the maskant
                                           d
• In general, d ≤ u ≤ 2d, Etch Factor: Fe=
                                           u
  (see Table 26.2 pg 637)
Chemical Blanking
• Uses CHM to cut very thin
  sheetmetal parts - down to
  0.025 mm thick and/or for
  intricate cutting patterns

• Conventional punch and
  die does not work because
  stamping forces damage        Parts made by chemical
  the thin sheetmetal, or       blanking (photo courtesy of
                                Buckbee-Mears St. Paul).
  tooling cost is prohibitive
CHM Possible Part Geometry Features
• Very small holes
• Holes that are not round
• Narrow slots in slabs and plates
• Micromachining
• Shallow pockets and surface details in flat
  parts
• Special contoured shapes for mold and die
  applications

								
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