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Ch. 26

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Title: Ch. 26


1
Ch. 26 Abrasive Machining and Finishing
Operations
  • Brenton Elisberg, Jacob Hunner, Michael Snider,
    Michael Anderson

2
Abrasive Machining and Finishing Operations
  • There are many situations where the processes of
    manufacturing weve learned about cannot produce
    the required dimensional accuracy and/or surface
    finish.
  • Fine finishes on ball/roller bearings, pistons,
    valves, gears, cams, etc.
  • The best methods for producing such accuracy and
    finishes involve abrasive machining.

3
Abrasives and Bonded Abrasives
  • An abrasive is a small, hard particle having
    sharp edges and an irregular shape.
  • Abrasives are capable of removing small amounts
    of material through a cutting process that
    produces tiny chips.

4
Abrasives and Bonded Abrasives
  • Commonly used abrasives in abrasive machining
    are
  • Conventional Abrasives
  • Aluminum Oxide
  • Silicon Carbide
  • Superabrasives
  • Cubic boron nitride
  • Diamond

5
Friability
  • Characteristic of abrasives.
  • Defined as the ability of abrasive grains to
    fracture into smaller pieces, essential to
    maintaining sharpness of abrasive during use.
  • High friable abrasive grains fragment more under
    grinding forces, low friable abrasive grains
    fragment less.

6
Abrasive Types
  • Abrasives commonly found in nature include
  • Emery
  • Corundum
  • Quartz
  • Garnet
  • Diamond

7
Abrasive Types
  • Synthetically created abrasives include
  • Aluminum oxide (1893)
  • Seeded gel (1987)
  • Silicon carbide (1891)
  • Cubic-boron nitride (1970s)
  • Synthetic diamond (1955)

8
Abrasive Grain Size
  • Abrasives are usually much smaller than the
    cutting tools in manufacturing processes.
  • Size of abrasive grain measured by grit number.
  • Smaller grain size, the larger the grit number.
  • Ex with sandpaper 10 is very coarse, 100 is
    fine, and 500 is very fine grain.

9
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10
Grinding Wheels
  • Large amounts can be removed when many grains act
    together. This is done by using bonded abrasives.
  • This is typically in the form of a grinding
    wheel.
  • The abrasive grains in a grinding wheel are held
    together by a bonding material.

11
Bonding Abrasives
  • Bonding materials act as supporting posts or
    braces between grains.
  • Bonding abrasives are marked with letters and
    numbers indicating
  • Type of abrasive
  • Grain size
  • Grade
  • Structure
  • Bond type

12
Bond Types
  • Vitrified a glass bond, most commonly used
    bonding material.
  • However, it is a brittle bond.
  • Resinoid bond consiting of thermosetting resins,
    bond is an organic compound.
  • More flexible bond than vitrified, also more
    resistant to higher temps.

13
Bond Types
  • Reinforced Wheels bond consisting of one or more
    layers of fiberglass.
  • Prevents breakage rather than improving strength.
  • Rubber flexible bond type, inexpensive.
  • Metal different metals can be used for strength,
    ductility, etc.
  • Most inexpensive bond type.

14
The Grinding Process
  • Grinding is a chip removal process that uses an
    individual abrasive grain as the cutting tool.
  • The differences between grinding and a single
    point cutting tool is
  • The abrasive grains have irregular shapes and are
    spaced randomly along the periphery of the wheel.
  • The average rake angle of the grain is typically
    -60 degrees. Consequently, grinding chips undergo
    much larger plastic deformation than they do in
    other machining processes.
  • Not all grains are active on the wheel.
  • Surface speeds involving grinding are very fast.

15
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16
Grinding Forces
  • A knowledge of grinding forces is essential for
  • Estimating power requirements.
  • Designing grinding machines and work-holding
    fixtures and devices.
  • Determining the deflections that the work-piece
    as well as the grinding machine may undergo.
    Deflections adversely affect dimensioning.

17
Grinding Forces
  • Forces in grinding are usually smaller than those
    in machining operations because of the smaller
    dimensions involved.
  • Low grinding forces are recommended for
    dimensional accuracy.

18
Problems with Grinding
  • Wear Flat
  • After some use, grains along the periphery of the
    wheel develop a wear flat.
  • Wear flats rub along the ground surface, creating
    friction, and making grinding very inefficient.

19
Problems with Grinding
  • Sparks
  • Sparks produced from grinding are actually
    glowing hot chips.
  • Tempering
  • Excessive heat, often times from friction, can
    soften the work-piece.
  • Burning
  • Excessive heat may burn the surface being ground.
    Characterized as a bluish color on ground steel
    surfaces.

20
Problems with Grinding
  • Heat Checking
  • High temps in grinding may cause cracks in the
    work-piece, usually perpendicular to the grinding
    surface.

21
Grain Fracture
  • Abrasive grains are brittle, and their fracture
    characteristics are important.
  • Wear flat creates unwanted high temps.
  • Ideally, the grain should fracture at a moderate
    rate so as to create new sharp cutting edges
    continuously.

22
Bond Fracture
  • The strength of the abrasive bond is very
    important!
  • If the bond is too strong, dull grains cannot
    dislodge to make way for new sharp grains.
  • Hard grade bonds are meant for soft materials.
  • If too weak, grains dislodge too easily and the
    wear of the wheel increases greatly.
  • Soft grade bonds are meant for hard materials.

23
Grinding Ratio
  • G (Volume of material removed)/ Volume
  • of wheel wear)
  • The higher the ratio, the longer the wheel will
    last.
  • During grinding, the wheel may act soft or
    hard regardless of wheel grade.
  • Ex pencil acting hard on soft paper and soft on
    rough paper.

24
Dressing, Truing, Shaping
  • Dressing a wheel is the process of
  • Conditioning worn grains by producing sharp new
    edges.
  • Truing, which is producing a true circle on the
    wheel that has become out of round.
  • Grinding wheels can also be shaped to the form of
    the piece you are grinding.
  • These are important because they affect the
    grinding forces and surface finish.

25
Grinding Operations and Machines
  • Surface Grinding
  • Cylindrical Grinding
  • Internal Grinding
  • Centerless Grinding
  • Creep-feed Grinding
  • Heavy Stock Removal by Grinding
  • Grinding fluids

26
Grinding Operations and Machines
  • Surface Grinding - grinding of flat surfaces
  • Cylindrical Grinding axially ground

27
Grinding Operations and Machines
  • Internal Grinding - grinding the inside diameter
    of a part
  • Creep-feed Grinding large rates of grinding for
    a close to finished piece

28
Grinding Operations and Machines
  • Centerless Grinding continuously ground
    cylindrical surfaces

29
Grinding Operations and Machines
  • Heavy Stock Removal - economical process to
    remove large amount of material
  • Grinding Fluids
  • Prevent workpiece temperature rise
  • Improves surface finish and dimensional accuracy
  • Reduces wheel wear, loading, and power consumption

30
Design Consideration for Grinding
  • Part design should include secure mounting into
    workholding devices.
  • Holes and keyways may cause vibration and
    chatter, reducing dimensional accuracy.
  • Cylindrically ground pieces should be balanced.
    Fillets and radii made as large as possible, or
    relieved by prior machining.

31
Design Considerations for Grinding
  • Long pieces are given better support in
    centerless grinding, and only the largest
    diameter may be ground in through-feed grinding.
  • Avoid frequent wheel dressing by keeping the
    piece simple.
  • A relief should be include in small and blind
    holes needing internal grinding.

32
Finishing Operations
  • Coated abrasives
  • Belt Grinding
  • Wire Brushing
  • Honing
  • Superfinishing
  • Lapping
  • Chemical-Mechanical Polishing
  • Electroplating

33
Finishing Operations
  • Coated Abrasives have a more pointed and open
    structure than grinding wheels
  • Belt Grinding high rate of material removal
    with good surface finish

34
Finishing Operations
  • Wire Brushing - produces a fine or controlled
    texture
  • Honing improves surface after boring, drilling,
    or internal grinding

35
Finishing Operations
  • Superfinishing very light pressure in a
    different path to the piece
  • Lapping abrasive or slurry wears the pieces
    ridges down softly

36
Finishing Operations
  • Chemical-mechanical Polishing slurry of
    abrasive particles and a controlled chemical
    corrosive
  • Electropolishing an unidirectional pattern by
    removing metal from the surface

37
Deburring Operations
  • Manual Deburring
  • Mechanical Deburring
  • Vibratory and Barrel Finishing
  • Shot Blasting
  • Abrasive-Flow Machining
  • Thermal Energy Deburring
  • Robotic Deburring

38
Deburring Operations
  • Vibratory and Barrel Finishing abrasive pellets
    are tumbled or vibrated to deburr
  • Abrasive-flow Machining a putty of abrasive
    grains is forced through a piece

39
Deburring Operations
  • Thermal Energy Deburring natural gas and oxygen
    are ignited to melt the burr
  • Robotic Deburring uses a force-feedback program
    to control the rate and path of deburring

40
Economics of Abrasive Machining and Finishing
Operations
  • Creep-feed grinding is an economical alternative
    to other machining operations.
  • The use of abrasives and finishing operations
    achieve a higher dimensional accuracy than the
    solitary machining process.
  • Automation has reduced labor cost and production
    times.
  • The greater the surface-finish, the more
    operations involved, increases the product cost.
  • Abrasive processes and finishing processes are
    important to include in the design analysis for
    pieces requiring a surface finish and dimensional
    accuracy.

41
Chapter 27 Advanced Machining Processes
42
Chapter 27 Advanced Mechanical Processes
  • Advanced Machining Processes can be used when
    mechanical methods are not satisfactory,
    economical or possible due to
  • High strength or hardness
  • Too brittle or too flexible
  • Complex shapes
  • Special finish and dimensional tolerance
    requirements
  • Temperature rise and residual stresses

43
Advanced Mechanical Processes
  • These advanced methods began to be introduced in
    the 1940's.
  • Removes material by chemical dissolution,
    etching, melting, evaporation, and hydrodynamic
    action.
  • These requirements led to chemical, electrical,
    laser, and high-energy beams as energy sources
    for removing material from metallic or
    non-metallic workpieces.

44
Chemical Machining
  • Chemical machining
  • Uses chemical dissolution to dissolve material
    from the workpiece.
  • Can be used on stones, most metals and some
    ceramics.
  • Oldest of the advanced machining processes.

45
Chemical Machining
  • Chemical milling - shallow cavities are produced
    on plates, sheets, forgings, and extrusions,
    generally for the overall reduction of weight.
  • Can be used with depths of metal removal as large
    as 12 mm.
  • Masking is used to protect areas that are not
    meant to be attacked by the chemical.

46
Chemical Machining
  • Chemical Blanking similar to the blanking of
    sheet metals with the exception that the material
    is removed by chemical dissolution rather than by
    shearing.
  • Printed circuit boards.
  • Decorative panels.
  • Thin sheet-metal stampings.
  • Complex or small shapes.

47
Chemical Machining
  • Surface Roughness and Tolerance table

48
Chemical Machining
  • Photochemical blanking/machining
  • Modification of chemical milling.
  • Can be used on metals as thin as .0025 mm.
  • Applications
  • Fine screens.
  • Printed circuit boards.
  • Electric-motor laminations.
  • Flat springs.
  • Masks for color televisions.

49
Chemical Machining
  • Chemical machining design considerations
  • No sharp corners, deep or narrow cavities, severe
    tapers, folded seam, or porous workpiece
    materials.
  • Undercuts may develop.
  • The bulk of the workpiece should be shaped by
    other processes prior to chemical machining.

50
Electrochemical Machining
  • Electrochemical machining (ECM)
  • An electrolyte acts as a current carrier which
    washes metal ions away from the workpiece (anode)
    before they have a chance to plate on the tool
    (cathode).
  • The shaped tool is either solid or tubular.
  • Generally made of brass, copper, bronze or
    stainless steel.
  • The electrolyte is a highly conductive inorganic
    fluid.

51
Electrochemical Machining
  • Electrochemical machining cont.
  • The cavity produced is the female mating image of
    the tool shape.
  • Process capabilities
  • Generally used to machine complex cavities and
    shapes in high strength materials.
  • Design considerations
  • Not suited for producing sharp square corners or
    flat bottoms.
  • No irregular cavities.

52
Electrochemical Machining
53
Electrochemical Machining
  • Pulsed electrochemical machining (PECM)
  • Refinement of ECM.
  • The current is pulsed instead of a direct
    current.
  • Lower electrolyte flow rate.
  • Improves fatigue life.
  • Tolerance obtained 20 to 100 micro-meters.

54
Electrochemical Grinding
  • Electrochemical grinding (ECG)
  • Combines ECM with conventional grinding.
  • Similar to a conventional grinder, except that
    the wheel is a rotating cathode with abrasive
    particles.
  • The abrasive particles serve as insulators and
    they remove electrolytic products from the
    working area.
  • Less then 5 of the metal is removed by the
    abrasive action of the wheel.

55
Electrochemical Grinding
  • Electrochemical honing
  • Combines the fine abrasive action of honing with
    electrochemical action.
  • Costs more than conventional honing.
  • 5 times faster than conventional honing.
  • The tool lasts up to 10 times longer.
  • Design considerations for EGC
  • Avoid sharp inside radii.

56
Electrical Discharge Machining (EDM)
  • Principle of operation
  • Based on the erosion of metal by spark discharge
  • Components of operation
  • Shaped tool
  • Electrode
  • Workpiece
  • Connected to a DC power supply
  • Dielectric
  • Nonconductive fluid

57
Electrical Discharge Machining (EDM)
  • When the potential difference is sufficiently
    high, the dielectric breaks down and a transient
    spark discharges through the fluid, removing a
    very small amount of material from the workpiece
  • Capacitor discharge
  • 200-500 kHz
  • This process can be used on any electrically
    conductive material

58
Electrical Discharge Machining (EDM)
  • Volume of material removed per discharge
  • 10-10 to 10-8 in3
  • Material removal can be predicted
  • MRR 4104 ITw-1.23
  • MRR is mm3/min
  • I is current in amperes
  • Tw is melting point (C)
  • Mechanical energy is not a factor
  • The hardness, strength, and toughness do not
    necessarily influence the removal rate

59
Electrical Discharge Machining (EDM)
  • Movement in the XY axis is controlled by CNC
    systems
  • Overcut (in the Z axis) is the gap between the
    electrode and the workpiece
  • Controlled by servomechanisms
  • Critical to maintain a constant gap

60
Electrical Discharge Machining (EDM)
  • Dielectric fluids
  • Act as a dielectric
  • Provide a cooling medium
  • Provide a flushing medium
  • Common fluids
  • Mineral oils
  • Distilled/Deionized water
  • Kerosene
  • Other clear low viscosity fluids are available
    which are easier to clean but more expensive

61
Electrical Discharge Machining (EDM)
  • Electrodes
  • Graphite
  • Brass
  • Copper-tungsten alloys
  • Formed by casting, powder metallurgy, or CNC
    machining
  • On right, human hair with a 0.0012 inch hole
    drilled through

62
Electrical Discharge Machining (EDM)
  • Electrode wear
  • Important factor in maintaining the gap between
    the electrode and the workpiece
  • Wear ratio is defined as the amount of material
    removed to the volume of electrode wear
  • 31 to 1001 is typical
  • No-wear EDM is defined as the EDM process with
    reversed polarity using copper electrodes

63
Electrical Discharge Machining (EDM)
  • Process capabilities
  • Used in the forming of dies for forging,
    extrusion, die casting, and injection molding
  • Typically intricate shapes

64
Electrical Discharge Machining (EDM)
  • Material removal rates affect finish quality
  • High removal rates produce very rough surface
    finish with poor surface integrity
  • Finishing cuts are often made at low removal
    rates so surface finish can be improved
  • Design considerations
  • Design so that electrodes can be
    simple/economical to produce
  • Deep slots and narrow openings should be avoided
  • Conventional techniques should be used to remove
    the bulk of material

65
Wire EDM
  • Similar to contour cutting with a bandsaw
  • Typically used to cut thicker material
  • Up to 12 thick
  • Also used to make punches, tools and dies from
    hard materials

66
Wire EDM
  • Wire
  • Usually made of brass, copper, or tungsten
  • Range in diameter from 0.012 0.008 inches
  • Typically used at 60 of tensile strength
  • Used once since it is relatively inexpensive
  • Travels at a constant velocity ranging from 6-360
    in/min
  • Cutting speed is measured in cross sectional area
    per unit time (varies with material)
  • 18,000 mm2/hour
  • 28 in2/hour

67
Wire EDM
  • Multiaxis EDM
  • Computer controls for controlling the cutting
    path of the wire and its angle with respect to
    the workpiece plane
  • Multiheads for cutting multiple parts
  • Features to prevent and correct wire breakage
  • Programming to optimize the operation

68
Electrical Discharge Grinding
  • Similar to the standard grinder
  • Grinding wheel is made of graphite or brass and
    contains no abrasives
  • Material is removed by spark discharge between
    the workpiece and rotating wheel
  • Typically used to sharpen carbide tools and dies
  • Can also be used on fragile parts such as
    surgical needles, thin-wall tubes, and honeycomb
    structures
  • Process can be combined with electrochemical
    discharge grinding
  • Material removal rate is similar to that of EDM
  • MRR KI where K is the workpiece material factor
    in mm3/A-min

69
Laser Beam Machining
  • The source of the energy is the laser
  • Light Amplification by Stimulated Emission of
    Radiation
  • The focus of optical energy on the surface of the
    workpiece melts and evaporates portions of the
    workpiece in a controlled manner
  • Works on both metallic and non-metallic materials
  • Important considerations include the reflectivity
    and thermal conductivity of the material
  • The lower these quantities the more efficient the
    process

70
Laser Beam Machining
  • The cutting depth can be calculated using the
    formula t CP/vd where
  • t is the depth
  • C is a constant for the process
  • P is the power input
  • v is the cutting speed
  • d is the laser spot diameter
  • The surface produced is usually rough and has a
    heat affected zone (discussed in section 30.9)

71
Laser Beam Machining
  • Lasers may be used in conjunction with a gas such
    as oxygen, nitrogen, or argon to aid in energy
    absorption
  • Commonly referred to as laser beam torches
  • The gas helps blow away molten and vaporized
    material
  • Process capabilities also include welding,
    localized heat treating, and marking
  • Very flexible process
  • Fiber optic beam delivery
  • Simple fixtures
  • Low setup times

72
Laser Beam Machining
  • Design considerations
  • Sharp corners should be avoided
  • Deep cuts will produce tapered walls
  • Reflectivity is an important consideration
  • Dull and unpolished surfaces are preferable
  • Any adverse effects on the properties of the
    machined materials caused by the high local
    temperatures and heat affected zones should be
    investigated

73
Electron Beam Machining
  • Energy source is high velocity electrons which
    strike the workpiece
  • Voltages range from 50-200kV
  • Electron speeds range from 50-80 the speed of
    light
  • Require a vacuum

74
Electron Beam Machining
  • Plasma arc cutting
  • Ionized gas is used to rapidly cut ferrous and
    nonferrous sheets and plates
  • Temperatures range from 9400-17,000 F
  • The process is fast, the kerf width is small, and
    the surface finish is good
  • Parts as thick as 6 can be cut
  • Much faster than the EDM and LBM process
  • Design considerations
  • Parts must fit in vacuum chamber
  • Parts that only require EBM machining on a small
    portion should be manufactured as a number of
    smaller components

75
Water Jet Machining
  • Also known as hydrodynamic machining
  • The water jet acts as a saw and cuts a narrow
    groove in the material
  • Pressures range from 60ksi to 200ksi

76
Water Jet Machining
  • Process capabilities
  • Can be used on any material up to 1 thick
  • Cuts can be started at any location without
    predrilled holes
  • No heat produced
  • No flex to the material being cut
  • Suitable for flexible materials
  • Little wetting of the workpiece
  • Little to no burr produced
  • Environmentally safe

77
Water Jet Machining
  • Very similar to water jet machining
  • Water contains abrasive material
  • Silicon carbide
  • Aluminum oxide
  • Higher cutting speed than that of conventional
    water jet machining
  • Up to 25 ft/min for reinforce plastics
  • Minimum hole diameter thus far is approximately
    0.12 inches
  • Maximum hole depth is approximately 1 inch

78
Abrasive Jet Machining
  • Uses high velocity dry air, nitrogen, or carbon
    dioxide containing abrasive particles
  • Supply pressure is on the order of 125psi
  • The abrasive jet velocity can be as high as 100
    ft/sec
  • Abrasive size is approximately 400-2000
    micro-inches

79
Economics of Advanced Machining Processes
  • Advanced machining processes each have unique
    applications
  • The economic production run for a particular
    process depends on the costs of tooling,
    equipment, operating costs, material removal rate
    required, level of operator skill required, and
    necessary secondary and finishing operations
  • Chemical machining has the added cost of
    reagents, maskants, and disposal
  • Table 27.1 lists material removal rates for all
    advanced machining processes

80
References
http//www.electricaldischargemachining.com/ http
//www.belmont4edm.com/ http//www.texasairsonics.c
om/Cabinet20Style.html
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