Chapter 21

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CUTTING TOOL MATERIALS CUTTING FLUIDS

• Chapter 21

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TOPICS

• Cubic Boron Nitride
• Silicon Nitride based ceramics
• Diamond
• Whisker-reinforced tool materials
• Cutting-Tool Reconditioning
• Cutting fluids

• Introduction
• Carbon and medium alloy steels
• High speed steels
• Cast-cobalt alloys
• Carbides
• Coated tools
• Alumna-based ceramics

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Introduction

• Characteristics of cutting tool
• Hardness (Elevated temperatures)
• Toughness (Impact forces on tool in interrupted
  operations)
• Wear resistance (tool life to be considered)
• Chemical stability or inertness (to avoid adverse
  reactions)

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Cutting tool materials

• Carbon medium alloy steels
• High speed steels
• Cast-cobalt alloys
• Carbides
• Coated tools
• Alumina-based ceramics
• Cubic boron nitride
• Silicon-nitride-base ceramics
• Diamond
• Whisker-reinforced materials

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Carbon and Medium alloy steels

• 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
• High speed steels (HSS)
• Hardened to various depths
• Good wear resistance
• Relatively
• Suitable for high positive rake angle tools

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• Two basic types of HSS
• Molybdenum ( M-series)
• Tungsten ( T-series)
• M-series - Contains 10 molybdenum, chromium,
  vanadium, tungsten, cobalt
• Higher, abrasion resistance
• H.S.S. are majorly made of M-series 
• T-series - 12 - 18 tungsten, chromium,
  vanadium cobalt
• undergoes less distortion during heat treating

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• H.S.S. available in wrought ,cast sintered
  (Powder metallurgy)
• Coated for better performance
• Subjected to surface treatments such as
  case-hardening for improved hardness and wear
  resistance or steam treatment at elevated
  temperatures
• High speed steels account for largest tonnage

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Cast-Cobalt 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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Carbides

• 3-groups of materials
• Alloy steels
• High speed steels
• Cast alloys
• These carbides are also known as cemented or
  sintered carbides
• High elastic modulus,thermal conductivity
• Low thermal expansion
• 2-groups of carbides used for machining
  operations
• tungsten carbide
• titanium carbide

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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
• Particles 1-5 Mum in size are pressed sintered
  to desired shape
•  
• Amount of cobalt present affects properties of
  carbide tools
•  
• As cobalt content increases strength hardness
  wear resistance increases

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Titanium carbide

• Titanium carbide has higher wear resistance than
  tungsten carbide
• Nickel-Molybdenum alloy as matrix Tic suitable
  for machining hard materials
• Steels cast irons
• Speeds higher than those for tungsten carbide

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Inserts
13
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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Edge Strength

• Fig Relative edge strength and tendency for
  chipping and breaking of inserts with various
  shapes. Strength refers to the cutting edge shown
  by the included angles.

Fig edge preparation of inserts to improve edge
strength.
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Chip breakers

• Purpose
• Eliminating long chips
• Controlling chip flow during machining
• Reducing vibration heat generated
• Selection depends on feed and depth of cut
• Work piece material,type of chip produced during
  cutting

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Coated tools

• High strength and toughness but generally
  abrasive and chemically reactive with tool
  materials
• Unique Properties
• Lower Friction
• High resistance to cracks and wear
• High Cutting speeds and low time costs
• Longer tool life

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Coating materials

• Titanium nitride (TiN)
• Titanium carbide (Tic)
• Titanium Carbonitride (TicN)
• Aluminum oxide (Al2O3)thickness range 2-15 µm
  (80-600Mu.in)
• Techniques used
• Chemical vapor deposition (CVD)
• Plasma assisted CVD
• Physical-vapor deposition(PVD)
• Medium temperature chemical- vapor
  deposition(MTCVD)

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Properties for Group of Materials

• Fig Ranges of properties for various groups of
  tool materials.

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Cutting tool Characteristics for coating

• High hardness
• Chemical stability
• Low thermal conductivity
• Good bonding
• Little or no Porosity
• Titanium nitride (TiN) coating
• Low friction coefficients
• High hardness
• Resistance to high temperatures
• Good adhesion to substrate
• High life of high speed-steel tools
• Titanium carbide (TiC) coating
• Titanium carbide coatings on tungsten-carbide
  inserts have high flank wear resistance.

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Ceramics

• Low thermal conductivity ,resistance ,high
  temperature
• Resistance to flank wear and crater wear
• Ceramics are suitable materials for tools
• Al2O3 (most commonly used)
• Multi Phase Coatings
• First layer Should bond well with substrate
• Outer layer Resist wear and have low thermal
  conductivity
• Intermediate layer Bond well compatible with
  both layers
• Coatings of alternating multipurpose layers are
  also formed.

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Multiphase Coatings

• Fig Multiphase coatings on a tungsten-carbide
  substrate. Three alternating layers of aluminum
  oxide are separated by very thin layers of
  titanium nitride. Inserts with as many as
  thirteen layers of coatings have been made.
  Coating thick nesses are typically in the range
  of 2 to 10 µm.

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Diamond Coated tools

• Use of Polycrystalline diamond as a coating
• Difficult to adhere diamond film to substrate
• Thin-film diamond coated inserts now commercially
  available
• Thin films deposited on substrate with PVD CVD
  techniques
• Thick films obtained by growing large sheet of
  pure diamond
• Diamond coated tools particularly effective in
  machining non-ferrous and abrasive materials

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New Coating materials

• Titanium carbo nitride (TiCN)
• Titanium Aluminum Nitride(TiAlN)
• Chromium Based coatings
• Chromium carbide
• Zirconium Nitride (ZrN)
• Hafnium nitride (HfN)
• Recent developments gives nano coating
  composite coating
• Ion Implementation
• Ions placed into the surface of cutting tool
• No change in the dimensions of tool
• Nitrogen-ion Implanted carbide tools used for
  alloy steels stainless steels
• Xeon ion implantation of tools as under
  development

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Alumina-Based ceramics

• Cold-Pressed Into insert shapes under high
  pressure and sintered at high temperature
• High Abrasion resistance and hot hardness
• Chemically stable than high speed steels
  carbides
• So less tendency to adhere to metals
• Good surface finish obtained in cutting cast iron
  and steels
• Negative rake-angle preferred to avoid chipping
  due to poor tensile strength
• Cermets, Black or Hot- Pressed
• 70 aluminum oxide 30 titanium carbide
• cermets(ceramics metal)
• Cermets contain molybdenum carbide, niobium
  carbide and tantalum carbide.

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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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Silicon-Nitride based ceramics (SiN)

• They consists various addition of Aluminum Oxide
  ythrium oxide, titanium carbide
• SiN have toughness, hot hardened good thermal
  shock resistance
• SiN base material is Silicon
• High thermal shock resistance
• Recommended for machining cast iron and nickel
  based super alloys at intermediate cutting speeds

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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

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Polycrystalline-Diamond ( PCD ) Tools

• Used for wire drawing of fine wires
• Small synthesis crystal fused by high pressure
  and temperature
• Bonded to a carbide substrate 
• Diamond tools can be used fir any speed
• Suitable for light un-interrupted finishing cuts
• To avoid tool fracture single crystal diamond is
  to be re-sharpened as it becomes dull
• Also used as an abrasive in grinding and
  polishing operations

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Whisker reinforced Nanocrystalline tool
materials

• New tool materials with enhanced properties
• High fracture toughness
• Resistance to thermal shock
• Cutting edge strength
• Hot hardness

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Whiskers used as reinforcing fibers

• Examples Silicon-nitride base tools reinforced
  with silicon-carbide( Sic)
• Aluminum oxide based tools reinforced with
  silicon-carbide with ferrous metals makes
  Sic-reinforced tools
• Progress in nanomaterial has lead to the
  development of cutting tools
• Made of fine grained structures as (micro grain)
  carbides

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Cutting-Tool Reconditioning

• When tools get worned, they are reconditioned for
  further use
• Reconditioning also involves recoating used
  tools with titanium nitride
• Cutting Fluids (Lubricants Coolants)
• Used in machining as well as abrasive machining
  processes
• Reduces friction wear
• Reduce forces and energy consumption
• Cools the cutting zone
• Wash away the chips
• Protect Machined surfaces from environmental
  corrosion

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Application of Cutting Fluids

• Fig Schematic illustration of proper methods of
  applying cutting fluids in various machining
  operations (a)turning, (b)milling, (c)thread
  grinding, and (d)drilling

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THE END