Cutting Tool Materials

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Cutting Tool Materials
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Questions p.333

• List the properties of the ideal cutting
  tool material.
• What is the main advantage of carbon steel
  cutting tools ?
• Explain the term red hardness .
• What makes up high speed steel ?
• What are tungsten carbides ?
• Explain how tungsten carbides are produced .
• List the advantages of tungsten carbides as
  cutting tools.
• How are they graded ?
• List 4 uses for tungsten carbides.
• Give the uses and properties for the
  following cutting tool materials (i) cast
  alloys
• (ii) ceramics
• (iii) cubic boron
  nitride
• (iv) diamond .

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Cutting tool materials

• Selection of cutting tool materials is very
  important
• What properties should cutting tools have
• Hardness at elevated temperatures
• Toughness so that impact forces on the tool can
  be taken
• Wear resistance
• Chemical stability

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Cutting Tool Construction
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Types of tool materials

• Carbon steel
• High speed steel (HSS)
• Cemented Carbides
• Cast alloys
• Ceramics
• Cubic boron nitride (CBN)
• Diamond

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Carbon Steel

• Oldest of tool materials
• Used for drills taps,broaches ,reamers
• Inexpensive ,easily shaped ,sharpened
• No sufficient hardness and wear resistance
• Limited to low cutting speed operation

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Temperature effects
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High speed steel

• Retains its hardness at high temperature
• Red hardness.
• Relatively good wear resistance

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Tungsten Carbide

• Composite material consisting of tungsten-carbide
  particles bonded together
•  
• Alternate name is cemented carbides
• Manufactured with powder metallurgy techniques
  p335 Fig. 2
• Small particles are pressed sintered to
  desired shape
•  
• Amount of cobalt present affects properties of
  carbide tools
•  
• As cobalt content increases the tougher the
  tool

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Making tungsten carbides
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Step 1
Tungsten carbon mixed then heated to
give tungsten carbide
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Step 2

• Mix tungsten carbide powder with binder
• Usually cobolt

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Step 3
Pressing to shape
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Step 4
Sintered
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Cast alloys

• Commonly known as stellite tools
• Composition ranges 38 - 53 cobalt
• 30- 33 chromium
• 10-20tungsten
• Good wear resistance ( higher hardness)
• Less tough than high-speed steels and sensitive
  to impact forces
• Less suitable than high-speed steels for
  interrupted cutting operations
• Continuous roughing cuts relatively high
  gfeeds speeds
• Finishing cuts are at lower feed and depth of cut

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Inserts
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Inserts

• Individual cutting tool with severed cutting
  points
• Clamped on tool shanks with locking mechanisms
• Inserts also brazed to the tools
• Clamping is preferred method for securing an
  insert
• Carbide Inserts available in various
  shapes-Square, Triangle, Diamond and round
• Strength depends on the shape
• Inserts honed, chamfered or produced with
  negative land to improve edge strength

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Insert Attachment

• Fig Methods of attaching inserts to toolholders
  (a) Clamping and (b) Wing lockpins. (c)
  Examples of inserts attached to toolholders with
  threadless lockpins, which are secured with side
  screws.

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Ceramics

• Used as grinding wheels.
• as cutting tool inserts. These are used in a
  similar way to cemented carbide inserts.
• they can withstand extremely high machining
  temperatures.
• They also have a high resistance to abrasion.

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Ceramics

• Ceramic cutting tools can he used to machine
  difficult materials at really high cutting
  speeds sometimes over 2000 m/min. Compare this
  with the cutting speed for carbon steel cutting
  tools 6 m/min.
• Ceramic cutting tools are very brittle.
• They can be used only on machines which are
  extremely rigid and free of vibration.

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Cubic boron Nitride ( CBN )

• Made by bonding ( 0.5-1.0 mm ( 0.02-0.04-in)
• Layer of poly crystalline cubic boron nitride to
  a carbide substrate by sintering under pressure
• While carbide provides shock resistance CBN layer
  provides high resistance and cutting edge
  strength
• Cubic boron nitride tools are made in small sizes
  without substrate
• Fig (a) Construction of a polycrystalline cubic
  boron nitride or a diamond layer on a
  tungsten-carbide insert. (b) Inserts with
  polycrystalline cubic boron nitride tips (top
  row) and solid polycrystalline CBN inserts
  (bottom row).

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Diamond

• Hardest known substance
• Low friction, high wear resistance
• Ability to maintain sharp cutting edge
• Single crystal diamond of various carats used for
  special applications
• Machining copperfront precision optical mirrors
  for ( SDI)
• Diamond is brittle , tool shape sharpened is
  important
• Low rake angle used for string cutting edge