Ch. 26 – Abrasive Machining
and Finishing Operations
Brenton Elisberg, Jacob
Hunner, Michael Snider,
Michael Anderson
Abrasive Machining and
Finishing Operations
• There are many situations
where the processes of
manufacturing we’ve
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.
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.
Abrasives and Bonded
Abrasives
• Commonly used abrasives in abrasive
machining are:
– Conventional Abrasives
• Aluminum Oxide
• Silicon Carbide
– Superabrasives
• Cubic boron nitride
• Diamond
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.
Abrasive Types
• Abrasives commonly
found in nature
include:
–
–
–
–
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Emery
Corundum
Quartz
Garnet
Diamond
Abrasive Types
• Synthetically created
abrasives include:
–
–
–
–
Aluminum oxide (1893)
Seeded gel (1987)
Silicon carbide (1891)
Cubic-boron nitride
(1970’s)
– Synthetic diamond
(1955)
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.
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.
Bonding Abrasives
• Bonding materials act as supporting posts or
braces between grains.
• Bonding abrasives are marked with letters and
numbers indicating:
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–
–
–
–
Type of abrasive
Grain size
Grade
Structure
Bond type
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.
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.
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.
(a)
(b)
Grinding Forces
• A knowledge of grinding forces is essential
for:
– Estimating power requirements.
– Designing grinding machines and workholding fixtures and devices.
– Determining the deflections that the workpiece as well as the grinding machine may
undergo. Deflections adversely affect
dimensioning.
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.
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.
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.
Problems with Grinding
• Heat Checking
– High temps in grinding may cause cracks in
the work-piece, usually perpendicular to the
grinding surface.
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.
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.
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.
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.
Grinding Operations
and Machines
•
•
•
•
•
•
•
Surface Grinding
Cylindrical Grinding
Internal Grinding
Centerless Grinding
Creep-feed Grinding
Heavy Stock Removal by Grinding
Grinding fluids
Grinding Operations
and Machines
• Surface Grinding grinding of flat
surfaces
• Cylindrical Grinding
– axially ground
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
Grinding Operations
and Machines
• Centerless Grinding – continuously ground
cylindrical surfaces
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
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.
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.
Finishing Operations
•
•
•
•
•
•
•
•
Coated abrasives
Belt Grinding
Wire Brushing
Honing
Superfinishing
Lapping
Chemical-Mechanical Polishing
Electroplating
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
Finishing Operations
• Wire Brushing produces a fine or
controlled texture
• Honing – improves
surface after boring,
drilling, or internal
grinding
Finishing Operations
• Superfinishing –
very light pressure in
a different path to the
piece
• Lapping – abrasive or
slurry wears the
piece’s ridges down
softly
Finishing Operations
• Chemicalmechanical
Polishing – slurry of
abrasive particles and
a controlled chemical
corrosive
• Electropolishing –
an unidirectional
pattern by removing
metal from the surface
Deburring Operations
•
•
•
•
•
•
•
Manual Deburring
Mechanical Deburring
Vibratory and Barrel Finishing
Shot Blasting
Abrasive-Flow Machining
Thermal Energy Deburring
Robotic Deburring
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
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
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
Chapter 27 – Advanced
Machining Processes
Chapter 27 – Advanced
Mechanical Processes
• Advanced Machining
Processes can be used when
mechanical methods are not
satisfactory, economical or
possible due to:
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–
–
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High strength or hardness
Too brittle or too flexible
Complex shapes
Special finish and dimensional
tolerance requirements
– Temperature rise and residual
stresses
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.
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.
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.
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.
Chemical Machining
• Surface Roughness and Tolerance table
Chemical Machining
• Photochemical
blanking/machining
– Modification of chemical
milling.
– Can be used on metals as
thin as .0025 mm.
• Applications
–
–
–
–
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Fine screens.
Printed circuit boards.
Electric-motor laminations.
Flat springs.
Masks for color televisions.
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.
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.
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.
Electrochemical
Machining
Electrochemical
Machining
• Pulsed electrochemical machining (PECM)
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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.
(a)
(b)
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.
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.
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
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
Electrical Discharge
Machining (EDM)
• Volume of material removed per discharge
– 10^-10 to 10^-8 in^3
• Material removal can be predicted
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–
–
–
MRR = 4*10^4 I*Tw^-1.23
MRR is mm^3/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
Electrical Discharge
Machining (EDM)
• Movement in the X&Y
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
Electrical Discharge
Machining (EDM)
• Dielectric fluids
– Act as a dielectric
– Provide a cooling medium
– Provide a flushing medium
• Common fluids
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–
–
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Mineral oils
Distilled/Deionized water
Kerosene
Other clear low viscosity
fluids are available which
are easier to clean but
more expensive
Electrical Discharge
Machining (EDM)
• Electrodes
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–
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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
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
• 3:1 to 100:1 is typical
– No-wear EDM is defined as the EDM process with
reversed polarity using copper electrodes
Electrical Discharge
Machining (EDM)
• Process capabilities
– Used in the forming of
dies for forging,
extrusion, die casting,
and injection molding
– Typically intricate
shapes
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
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
Wire EDM
• Wire
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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 mm^2/hour
• 28 in^2/hour
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
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 mm^3/Amin
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
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)
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
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
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
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
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
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
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
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
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
References
http://www.electricaldischargemachining.com/
http://www.belmont4edm.com/
http://www.texasairsonics.com/Cabinet%20Style.html