GRINDING

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GRINDING

• Definition Another material removal process, in
  which abrasive particles are contained in bonded
  grinding wheel, that operates at very high
  surface speed.
• The grinding wheel is usually in disk shaped and
  is precisely balanced for high rotational speeds.

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Chip formation

• In grinding, the chips are small but are formed
  by the same basic mechanism of compression and
  shear. Burning chips are the sparks observed
  during grinding with no cutting fluid, because
  the chips have heat energy to burn or melt in the
  atmosphere. The feeds and depths of cut in
  grinding are small, while the cutting speed is
  high.
• Grinding may be classified as non-precision or
  precision, according to purpose and procedure.

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Non-precision grinding

• The common forms are called, snagging and
  off-hand grinding. Both are done primarily to
  remove stock that can not be taken off as
  conveniently by other methods. The work is
  pressed hard against the wheel or vice versa. The
  accuracy and surface finish are of secondary
  importance.

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Precision grinding

• Precision grinding is concerned with producing
  good surface finishes and accurate dimensions.
• 3 types of precision grinding exists
• External cylindrical grinding
• Internal cylindrical grinding
• Surface grinding

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Grinding wheel

• A grinding wheel is made of abrasive grains
  held together by a bond. These grains cut like
  teeth when the wheel is revolved at high speed
  and is brought to bear against a work piece. The
  properties of a wheel that determine how it acts
  are the kind and size of abrasive, how closely
  the grains are packed together and amount of the
  bonding material.

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Abrasive materials

• Different abrassive materials are appropriate
  for grinding different work material. Abrasives
  are hard substances used in various forms as
  tools for grinding and other surface finishing
  operations. They are also able to cut materials
  which are too hard for other tools and give
  better finishes and hold closer tolerances.

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Common abrasive materials

• Aluminum Oxide (Al2O3) known as Alundum or
  Aloxide. Various substances may be added to
  enhance hardness, toughness, etc. Plain Al2O3 is
  white, and used to grind steel, ferrous, high
  strength alloys.
• Silicon Carbide (SiC) known in trade as
  Carborundum and Crystalon. Harder than Al2O3 but
  not as tough. Used to grind aluminum, brass,
  stainless steel, cast irons, certain brittle
  ceramics.

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Common abrasive materials (continue)

• Boron Nitride in the forms of single-crystal
  cubic boron nitride (CBN) and microcrystalline
  cubic boron nitride (MCBN) under trade names such
  as Borazon or Borpax. Used for hard materials
  such as hardened tool steels and aerospace
  alloys.
• Diamond, a pure form of carbon, both natural and
  artificial. Used on hard materials such as
  ceramics, cemented carbides and glass.

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Grain size

• Important parameter in determining surface
  finish and material removal rate. Small grit
  sizes produce better finishes, larger grit sizes
  permit larger material removal rates. Also,
  harder materials need smaller grain sizes to cut
  effectively, while softer materials require
  larger grit size.
• Grain sizes used in grinding changes between
  8-250, whilw 8 is very coarse, but 250 is very
  fine.

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Bonding materials

• To get wide range of properties needed in
  grinding wheels, abrasive materials bonded by
  using organic or ingorganic materials.
• Inorganic bonds
• Vitrified bond Clay bond melted to a porcelain
  or glass like consistency. It can be made strong
  and rigid for heavy grinding and not effected by
  water, oil, acids. Most grindig wheels have
  vitrified bonds.

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Inorganic bonds (continue)

• Slicate bond is essentially water glass hardened
  by baking. It holds grains more loosely than a
  vitrified bond and give closer cut. Large wheels
  can be made more easily with slicate bond.
  Usually used in situation where heat generation
  must be minimized.
• Metallic bond Cubic boron nitride and diamond
  abrassives are usually (but not always) embeded
  in metallic bonds, for utmost in strength and
  tendency to hold the costly long-wearing grains.

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Organic bonds

• Rubber bond is a flexible bond, used in cutoff
  wheels.
• Resinoid bond is a high strengt bond, used for
  rough grinding and cutoff operations.
• Shellac bond is relatively strong but not rigid,
  used in applications that requires good finishing.

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Wheel grade and wheel structure

• The grade of a grinding wheel is a measure of
  how strongly the grains are held by the bond.
  Typical structure of a grinding wheel contains
  abrasive grains, bond material and pores (air
  gaps) Volumetric proportions can be expressed as
• Vg Vb Vp 1
• If Vp relatively large and Vg relatively small,
  it is called open structure.
• If Vg relatively large and Vp relatively small,
  it is called dense structure.

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Wheel grade and wheel structure (continue)

• Open structures are recommended in situations
  where the clearance for chips must be provided.
  Dense structures are used to obtain better
  surface finish and dimensional control.
• If Vb small relatively soft wheel, as Vb
  increase, hardness is supplied.
• Hard wheels used to achive high stock removal
  rates and for grinding of relatively soft work
  materials, while soft wheels generally used for
  applications requiring low material removal rates
  and grinding of hard work materials.

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Grinding wheel specifications

• All grinding wheel manufacturers use
  substantially the same standard wheel marking
  system. This system uses numbers and letters to
  specify abrasive type, grit size, grade
  structure, and bonding material. However,
  properties of the wheels are determined to a
  large extent by the ways the wheels are made. The
  processes vary from one plant to another, and
  wheels carrying the same symbols but made by
  different manufacturers are not necessarily
  identical.

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American National Standard Institudes marking
system for standard wheels

• Prefix-Abbrasive type-Abrasive grain
  size-Grade-Structure-Bond type-Manufacturer
  record
• Ex 51-A-36-L-5-V-23
• PrefixManufacturer symbol indicating exact kind
  of abrasive (use optional)
• Abrassive type AAluminum oxide
• CSilicon Carbide
• B Boron Nitride
• DDiamond
• Grain size coarse8-24, medium30-60,
  fine70-180, very fine gt220

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American National Standard Institudes marking
system for standard wheels (continue)

• GradeRanges from A-Z, where A represents soft, Z
  represents hard wheel grade.
• Structure Scale isnumerical. 1 very dense and
  15very open.
• Bond type B Resinoid
• E Shellac
• R Rubber
• S Silicate
• V Vitrified
• Manufacturer record Manufacturers private
  marking to idetify the wheel.

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Wheel shape and sizes

• There are different wheel shapes that are
  recognized as standard
• Straight cylinders with or without recesses in
  their sides
• Tapered two sides
• Straight cup
• Flaring cup
• Dish
• Saucer
• Other shapes may be obtain as specials.

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Wheel shape and sizes (continue)

• The principle dimensions that designate the size
  of a grinding wheel are the outside diameter,
  width, and hole diameter. Standard wheel shapes
  are made in certain sizes only, but the variety
  is large.

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Wheel wear

• The overall wear of bonded abrasive is caused by
  3 distinct mechanism
• Attritious wear Sharp edges of an abrasive grain
  become dull by attrition, developing flat areas.
• Fracture of the grainsPortion of the grain
  breaks off. The fractured area becomes new
  cutting area.
• Fracture of the bond Part of the grains fall
  out.

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Grain action

• There are 3 types of grain action in grinding
• CuttingGrit can penetrate to the surface and
  actually performs chip removing.
• PlowingGrit can penetrate to the surface but can
  not perform cutting. The work surface deformes.
• RubbingGrits rubs to the surface, energy
  consumed witout cutting.

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Grinding Chips
Relative grain force

• Actual forcerelative grain force x strength of
  metal

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Surface Grinding and Plowing
(ploughing)
C number of cutting points per unit area of
wheel surface

• Surface temperature rise

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Approximate Specific Energy Requirements for
Surface Grinding

• Specific energy u uchip uploughing usliding

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Surface Grinding Operations
(a) Traverse grinding with a horizontal-spindle
surface grinder. (b) Plunge grinding with a
horizontal-spindle surface grinder, producing a
groove in the workpiece. (c) A vertical-spindle
rotary-table grinder (also known as the Blanchard
type).
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Surface Grinding
horizontal-spindle surface grinder.
(a) Rough grinding of steel balls on a
vertical-spindle grinder the balls are guided by
a special rotary fixture. (b) Finish grinding of
balls in a multiple-groove fixture. The balls
are ground to within 0.013 mm (0.0005 in.) of
their final size.
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Cylindrical Grinding Operations
Examples of various cylindrical grinding
operations. (a) Traverse grinding, (b) plunge
grinding, and (c) profile grinding.
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Centerless Grinding
Schematic illustrations of centerless grinding
operations (a) through feed grinding. (b)
Plunge grinding. (c) A computer numerical
control cylindrical grinding machine.