Abrasive Machining

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Abrasive Machining

• General Manufacturing Processes Engr.-20.2710
• Instructor - Sam Chiappone

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Abrasive Machining Processes

• Abrasive machining is one of the oldest forms of
  metal removal. It is also one of the most
  important. Abrasive machining can produce
  surface finishes ranging from rough to extremely
  fine.
• Abrasive machining is a process where chips are
  formed by small cutting edges on abrasive
  particles.

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Abrasive Machining Processes
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Applications

• Manufactured products which are candidates for
  abrasive machining include
• Hard to machine metals such as super alloys or
  heat treated tool steels
• Parts which have closes tolerances
• Size (/- .0005)
• Parallelism
• Flatness
• Sheet type products such as
• Plywood, thin stainless steels, and decorative
  plaque material

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Process Characteristics

• Abrasive machining has two unique
  characteristics
• Small cutting edges cut simultaneously.
• The abrasives used are very hard and capable of
  cutting materials in the range of Rc45 and above

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Abrasives

• Abrasives can be natural or manmade.
• Natural include
• Sand stone
• Emery
• Diamond
• Garnet
• Quartz

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Abrasives

• Manmade (1891 time frame most commonly used
  today)
• Silicon Carbide
• Aluminum Oxide
• Cubic Boron Nitride

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Abrasive Grain Size

• Grains are separated by mechanical sieving
  machines. The number of openings per linear inch
  in the sieve(or screen) through which the
  particles can pass determines the grain size.
• Typical classifications
• Course, medium, and fine
• Silicon Carbides range from 2-240 in size
• Aluminum Oxides range from 4-240 in size
• 600 range are generally used in honing or lapping
  operations

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Abrasive Grain Size
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Forms

• Abrasive particles can be
• A Free slurry
• Adhered to resin on a belt
• Close packed into wheels or stones held together
  by a bonding agent.

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Forms
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The Grinding Process

• One process, which utilizes abrasive machining,
  is the grinding process.
• A machining process where the abrasives used are
  bonded together into a wheel. The grinding wheel
  is the cutting tool in this process. In
  grinding, high precision and tight tolerances are
  possible. Typical tolerances associated with
  this process are /- .0001. Product examples
  include bearings, machining fixtures, shafts, and
  precision measurement gauges.

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Chip Formation

• Chips in this process are formed by the same
  mechanism of compression and shear as other
  machining processes.
• As the grains or abrasives become dull, the
  cutting forces increase. The increase in the
  cutting force causes the grains to plow and rub
  rather than cut. As the plowing and rubbing
  increases, the grains fracture at the cutting
  edge to revile a new cutting edge.

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Chip Formation
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The Tool Grinding Wheels

• The spacing of particles with respect to one
  another is called the structure of the wheel.
  Wheel structure can be open, medium, or dense.
• Dense structure is used for hard materials, for
  high speed grinding operations, and also for
  producing fine finishes and tight tolerances.
• Open structure is used for high contact area,
  grinding of tough materials, and high stock
  removal rates.
• Hardness - resistance to penetration
• Toughness - ability of a metal to absorb energy
  without failure.

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The Tool Grinding Wheels
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Wheel Bonding Agents

• Vitrified - composed of clay and other ceramic
  substances. Abrasive particles are mixed with
  wheel material then pressed together and fired in
  a kiln.
• Resiniod or phonolic - Plastic compound wheels
  designed for a wide variety of applications.
  This is a bit more flexible than other wheels.

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Wheel Bonding Agents

• Silicate - This bond uses silicate of soda as the
  bonding agent. Wheels are formed then baked at
  500F for a day or longer. They are not as strong
  as vitrified-bonded wheels. Because they are not
  as strong, abrasive grains fracture more easily.
  This results in lower operating temperatures
  which can be a positive factor depending on the
  application of the wheel.

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Wheel Bonding Agents

• Shellac - Abrasives are mixed with shellac,
  heated (330 F) and pressed or rolled into the
  desired shape. These wheels are thin, elastic,
  and strong.
• Rubber - High speed wheels with a considerable
  amount of flexibility. Typically these wheels
  will operate at speeds of up to 16,000 ft/min.

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Wheel Forms
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Wheel Classification
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Grinding Wheel Operating Procedures

• Truing - Restores the original geometry to the
  grinding wheel. It also can be used to ensure
  the wheel is running concentric with the spindle
  of the grinding machine.
• Dressing - Exposes a new cutting surface on the
  grinding wheel.
• Balancing - Typically performed on large and
  diamond wheels this procedure ensures the wheel
  is running true with the axis of rotation.

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Grinding Wheel Operating Procedures
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Cutting Fluid

• Cutting fluid is an important factor in the
  grinding process.
• It assists in
• Washing away chips,
• Keeping the wheel from becoming clogged,
• Reduces operating temperatures.

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Grinding Machines

• Machine tools in this process are classified by
  the type of surface they produce.
• Typical classifications include
• Surface
• Cylindrical
• OD and ID

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Grinding Machines
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2
3
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Grinding Machines
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Grinding Operations

• Operations include
• Cutting off - Slicing or slotting
• Cylindrical between cntrs. - OD of cylindrical
  parts
• Cylindrical centerless - OD of cylindrical parts
  with a regulating wheel
• Internal - Bores and large holes
• Snagging - Large amounts of material no surface
  finish requirements
• Surface - Flat workpieces
• Tool grinding - Grinding cutting edges on tools
• Off-hand grinding - tool or work held by hand

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Grinding Machines

• Grinding operation on all machines can be
  accomplished in three ways
• In-feed - moving the wheel into the work
• Cross feed - traversing the wheel across the work
• Plunge feed - wheel is forced in on the radius of
  the work. This is similar to form turning on a
  lathe.