ME 350 – Lecture 21 – Chapter 26

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ME 350 Lecture 21 Chapter 26

• NONTRADITIONAL MACHINING PROCESSES
• Mechanical Energy Processes (USM, WJC, AJM)
• - high velocity stream of abrasives or fluid (or
  both)
• Electrochemical Processes (ECM)
• - reverse of electroplating
• Thermal Processes (EDM, Wire EDM, EBM, LBM, PAC)
• - vaporizing of a small area of work surface
• Chemical Processes (CHM, Chemical Blanking, PCM)
• - chemical etching of areas unprotected by
  maskant
• Nontraditional machining is characterized by
  material removal that

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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

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1. Mechanical Energy Processes

• Ultrasonic machining (USM)
• Water jet cutting (WJC)
• Abrasive jet machining (AJM)

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1a) Ultrasonic Machining (USM)

• 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

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USM Applications

• Used only on hard and brittle work materials
  
• 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

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1b) Water Jet Cutting (WJC)

• Uses high pressure, high velocity stream of water
  directed at work surface for cutting

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WJC Applications

• Usually automated using CNC or industrial robots
• Best used to cut narrow slits in flat stock such
  as
• 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

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WJC Advantages

• No crushing or burning of work surface
• Minimum material loss
• No environmental pollution
• Ease of automation

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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.

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2. Electrochemical Machining Processes

• Electrical energy used in combination with
  chemical reactions to remove material
• Reverse of
• Work material must be a
• Feature dimensions down to about 10 µm

Courtesy of AEG-Elotherm-Germany
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Electrochemical Machining (ECM)

• Material removal by anodic dissolution, using
  electrode (tool) in close proximity to work but
  separated by a rapidly flowing electrolyte

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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

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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
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Material Removal Rate of ECM

• Based on Faraday's First Law rate of metal
  dissolved is proportional to the current
• MRR Aƒr ?CI
• where I current A frontal area of the
  electrode (mm2), ƒr feed rate (mm/s), and ?
  efficiency coefficient

specific removal rate with work material
M atomic weight of metal (kg/mol) r density
of metal (kg/m3), F Faraday constant
(Coulomb) n valency of the ion
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Equations for ECM (Cont)
Gap, g

• Resistance of Electrode

Area, A
r is the resistivity of the electrolyte fluid
(Ohmm)
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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.
• Efficiency of 95,
• Determine how long it will take to cut the hole?

10mm
30mm
Ideal CAl 3.4410-2 mm3/amps - other C
values in Table 26.1
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Solution
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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

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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)

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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

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Work Materials in EDM

• Work materials must be
• Hardness and strength of work material are
  
• Material removal rate depends primarily on
• Applications
• Molds and dies for injection molding and forging
• Machining of hard or exotic metals
• Sheetmetal stamping dies.

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3b) Wire EDM

• EDM uses small diameter wire as electrode to cut
  a narrow kerf in work similar to a

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Material Removal Rate of EDM

• Weller Equation (Empirical) Maximum rate RMR
  
• where K 664 (C1.23mm3/amps) I discharge
  current Tm melt temp of work material
• Actual material removal rate
• MRR vf hwkerf
• where vf feed rate h workpiece thickness
  wkerf kerf width

While cutting, wire is continuously advanced
between supply spool and take-up spool to
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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

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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

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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 1001
• Disadvantage slow and expensive

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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

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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

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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

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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

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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

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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
• In general, , Etch Factor Fe
• (see Table 26.2 pg 637)

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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 the thin sheetmetal, or
  tooling cost is prohibitive

Parts made by chemical blanking (photo courtesy
of Buckbee-Mears St. Paul).
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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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Quotes

• We are what we repeatedly do. Excellence, then,
  is not an act, but a habit. - Aristotle
• If you want others to be happy, practice
  compassion. If you want to be happy, practice
  compassion. - Dalai Lama
• When the heart grieves over what it has lost, the
  spirit rejoices over what it has left. - Sufi
  Epigram
• A great pleasure in life is doing what people say
  you cannot do. - Walter Bagehot