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

• Material is either very hard, brittle or both or
  material is very ductile
• Part geometry is complex or geometric
  requirements impossible with conventional
  methods
• 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