Transcript Text Book: - National Institute of Technology Calicut

CUTTING TOOL MATERIALS
& CUTTING FLUIDS
TOPICS :
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Introduction
Carbon and medium
alloy steels
High speed steels
Cast-cobalt alloys
Carbides
Coated tools
Alumna-based
ceramics
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Cubic Boron Nitride
Silicon Nitride
based ceramics
Diamond
Whisker-reinforced
tool materials
Cutting-Tool
Reconditioning
Cutting fluids
Introduction:
Characteristics of cutting tool :
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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)
Cutting tool materials
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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
Carbon and Medium alloy steels :
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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)
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Hardened to various depths
Good wear resistance
Relatively
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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 &
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Coated for better performance
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sintered (Powder metallurgy)
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
Cast-Cobalt alloys
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Commonly known as stellite tools
Composition ranges – 38% - 53 % cobalt
30%- 33% chromium
10%-20%tungsten
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 g=feeds &
speeds
Finishing cuts are at lower feed and depth of cut
Carbides :
3-groups of materials
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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
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tungsten carbide
titanium carbide
Tungsten Carbide
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Composite material consisting of tungsten-carbide particles
bonded together
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Alternate name is cemented carbides
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Manufactured with powder metallurgy techniques
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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
Titanium carbide
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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
Inserts
Inserts
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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
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.
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.
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
Coated tools :
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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
Coating materials
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Titanium nitride (TiN)
Titanium carbide (Tic)
Titanium Carbonitride (TicN)
Aluminum oxide (Al2O3)thickness range – 2-15 µm (80600Mu.in)
Techniques used :
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Chemical –vapor deposition (CVD)
Plasma assisted CVD
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Physical-vapor deposition(PVD)
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Medium –temperature chemical- vapor
deposition(MTCVD)
Properties for Group of Materials
Fig : Ranges of
properties for
various groups
of tool
materials.
Cutting tool Characteristics for coating :
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High hardness
Chemical stability
Low thermal conductivity
Good bonding
Little or no Porosity
Titanium nitride (TiN) coating :
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Low friction coefficients
High hardness
Resistance to high temperatures
Good adhesion to substrate
High life of high speed-steel tools
Titanium carbide (TiC) coating:
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Titanium carbide coatings on tungsten-carbide inserts have high flank
wear resistance.
Ceramics :
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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 :
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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.
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.
Diamond Coated tools :
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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
New Coating materials :
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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 :
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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
Alumina-Based ceramics:
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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 :
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70% aluminum oxide & 30 % titanium carbide
cermets(ceramics & metal)
Cermets contain molybdenum carbide, niobium carbide and
tantalum carbide.
Cubic boron Nitride ( CBN ) :
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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).
Silicon-Nitride based ceramics (SiN)
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They consists various addition of Aluminum Oxide ythrium oxide,
titanium carbide
SiN have toughness, hot hardened & good thermal – shock
resistance
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SiN base material is Silicon
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High thermal & shock resistance
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Recommended for machining cast iron and nickel based super
alloys at intermediate cutting speeds
Diamond :
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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 copper—front precision optical
mirrors for ( SDI)
Diamond is brittle , tool shape & sharpened is
important
Low rake angle used for string cutting edge
Polycrystalline-Diamond ( PCD ) Tools:
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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
Whisker –reinforced & Nanocrystalline
tool materials
New tool materials with enhanced properties :
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High fracture toughness
Resistance to thermal shock
Cutting –edge strength
Hot hardness
Whiskers used as reinforcing fibers :
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Examples: Silicon-nitride base tools reinforced
with silicon-carbide( Sic)
Aluminum oxide based tools reinforced with
silicon-carbide with ferrous metals makes Sicreinforced tools
Progress in nanomaterial has lead to the
development of cutting tools
Made of fine grained structures as (micro grain)
carbides
Cutting-Tool Reconditioning
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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
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