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Chapter 21
Cutting-Tool Materials and Cutting
Fluids
Introduction
• Cutting Tool Characteristics:
1. Maintaining hardness, strength, and wear resistance at elevated
temperatures
2. Toughness
3. Thermal Shock resistance
4. Wear resistance
5. Chemical stability
Introduction
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1.
2.
3.
4.
5.
6.
7.
8.
9.
Tool Materials Categories:
High-speed steels
Cast-cobalt alloys
Carbides
Coated tools
Alumina-based ceramics
Cubic boron nitride
Silicon-nitride-base ceramics
Diamond
Whisker-reinforced materials
Hardness of Cutting
Tool Materials as a
Function of
Temperature
Figure 22.1 The hardness of
various cutting-tool materials as a
function of temperature (hot
hardness). The wide range in
each group of materials is due to
the variety of tool compositions
and treatments available for that
group.
General Properties of Tool Materials
General Characteristics of Cutting-Tool Materials
Operating Characteristics of Cutting-Tool Materials
HIGH SPEED STEELS
• Good wear resistance, relatively inexpensive
• Because of their toughness and high resistance to fracture, HSS
are especially suitable for:
1. high +ve rake-angle tools
2. interrupted cuts
3. machine tools with low stiffness that are subjected to vibration
and chatter.
• HSS tools are available in wrought, cast, and sintered forms
• They can be coated for improved performance
• HSS tools may also be subjected to:
a. surface treatments for improved hardness and wear resistance
b. steam treatment at elevated temperatures to develop a black oxide
layer for improved performance
CAST-COBALT ALLOYS
• 38%-53% Co, 30%-33% Cr, and 10%-20%W
• High hardness, good wear resistance, can maintain their
hardness at elevated temperatures
• They are not as tough as HSS and are sensitive to impact
forces
Stellite Tools
• These alloys are cast and ground into relatively simple tool
shapes.
• used only for special applications that involve deep,
continuous roughing cuts at relatively high feeds and speeds,
as much as twice the rates possible with HSS
CARBIDES
• Hardness over a wide range of temperatures.
• high elastic modulus and thermal conductivity.
• low thermal expansion.
Tungsten carbide (WC):
• Composite material consisting of WC particles bonded
together in a cobalt matrix
• Manufactured with powder-metallurgy techniques
• WC particles, 1-5 μm in size
• As Co content increases, the strength, hardness, and wear
resistance of WC decrease, while its toughness increases
because of the higher toughness of cobalt
CARBIDES
Titanium Carbide (TiC):
• Higher wear resistance than WC but is not as tough
• With a nickel-molybdenum alloy as the matrix, TiC is suitable
for machining hard materials, mainly steels and cast irons, and
for cutting at speeds higher than those for WC.
Inserts and Toolholders
• Individual cutting tools with several
cutting points
• A square insert has 8 cutting points
• The holes in the inserts are
standardized for interchangeability
Figure 22.2 Typical carbide inserts with various shapes and
chip-breaker features: Round inserts are also available, as
can be seen in Figs. 22.3c and 22.4. The holes in the inserts
are standardized for interchangeability in toolholders.
Source: Courtesy of Kyocera Engineered Ceramics, Inc.
CARBIDES - Insert Attachment
• Figure 22.3 Methods of
attaching inserts to
toolholders:
a. Clamping
b. Wing lockpins
c. Examples of inserts
attached to toolholders
with threadless lockpins,
which are secured with
side screws
d. Insert brazed on a tool
shank
Figure 22.3 Methods of mounting
inserts on toolholders: (a) clamping
and (b) wing lockpins. (c) Examples of
inserts mounted with threadless
lockpins, which are secured with side
screws. Source: Courtesy of Valenite.
CARBIDES - Insert Attachment
a. Wing lockpins
a. Clamping
Insert Edge Properties
Figure 22.4 Relative edge strength and
tendency for chipping of inserts with various
shapes. Strength refers to the cutting edge
indicated by the included angles. Source:
Courtesy of Kennametal, Inc.
Figure 22.5 Edge preparation for
inserts to improve edge strength.
Source: Courtesy of Kennametal, Inc.
• Insert shape affects strength of
cutting edge
• To further improve edge
strength and prevent chipping,
all insert edges are usually
honed, chamfered, or
produced with a negative land.
CARBIDES – General notes
• Stiffness of the machine tool is of major importance when
using carbide tools
• Light feeds, low speeds, and chatter are detrimental because
they tend to damage the tool's cutting edge.
 Light feeds, for example, concentrate the forces and
temperature closer to the edges of the tool, increasing the
tendency for the edges to chip off
 Low cutting speeds tend to encourage cold welding of the
chip to the tool
• Cutting fluids, if used to minimize heating and cooling of the
tool in interrupted cutting operations, should be applied
continuously and in large quantities.
ISO Classification of Carbide Cutting Tools
Classification of Tungsten Carbides According
to Machining Applications
COATED TOOLS
• Because of their unique properties, such as lower friction and
higher resistance to cracks and wear, coated tools can be used
at high cutting speeds, reducing both the time required for
machining operations and costs.
• Coated tools can have tool lives 10 times longer than those of
uncoated tools.
Relative Time Required to Machine with Various
Cutting-Tool Materials
Figure 22.6 Relative time required to machine with various cutting-tool materials, indicating the
year the tool materials were first introduced. Note that machining time has been reduced by two
orders of magnitude with a hundred years. Source: Courtesy of Sandvik.
COATED TOOLS - Coating Materials
• Coatings thickness of 2-15 μm, are applied on cutting tools and
inserts by the following techniques:
1. Chemical-vapor deposition (CVD), including plasma-assisted
2. Physical-vapor deposition (PVD)
• Coatings for cutting tools, as well as dies, should have the
following general characteristics:
1. High hardness at elevated temperatures
2. Chemical stability to the workpiece material
3. Low bonding to the substrate to prevent flaking or spalling
4. Little or no porosity
• Honing of the cutting edges is an important procedure for the
maintenance of coating strength; otherwise, the coating may peel or
chip off at sharp edges
COATED TOOLS - Coating Materials
Titanium Nitride coating (gold in color):
• low friction coefficient, high hardness, resistance to high temp,
and good adhesion to the substrate.
• perform well at higher cutting speeds and feeds
• Flank wear is significantly lower than that of uncoated tools
• do not perform as well at low cutting speeds because the
coating can be worn off by chip adhesion
Titanium Carbide coatings:
• on tungsten-carbide inserts have high flank-wear resistance in
machining abrasive materials
Typical Wear Patterns on High-Speed-Steel
Uncoated and Titanium-Nitride Coated Tools
Figure 22.7 Schematic illustration of typical wear patterns of
high-speed-steel uncoated and titanium-nitride coated tools.
Note that flank wear is significantly lower for the coated tool.
COATED TOOLS - Coating Materials
Ceramics Coatings:
• Chemical inertness
• Low thermal conductivity
• Resistance to high temperature
• Resistance to flank and crater wear
• Most commonly used ceramic coating aluminum oxide
(Al2O3). However oxide coating generally bond weakly to
the substrate.
COATED TOOLS - Coating Materials
Multiphase Coatings:
• Carbide tools with 2 or 3 layers of such coatings.
• Particularly effective in machining cast irons and steels.
• Typical applications of multiple-coated tools:
 High-speed, continuous cutting: TiC/Al2O3.
 Heavy-duty, continuous cutting: TiC/Al2O3/TiN.
 Light, interrupted cutting: TiC/TiC + TiN/TiN.
Multiphase Coatings on a Tungsten-Carbide
Substrate
Figure 22.8 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 thicknesses are typically in the range of 2 to 10 μm.
Source: Courtesy of Kennametal, Inc.
COATED TOOLS - Coating Materials
Multiphase Coatings:
•Functions of coatings:
1.TiN: low friction
2.Al2O3: high thermal stability
3.TiCN: fiber reinforced with a good balance of resistance to
flank and crater wear for interrupted cutting
4.A thin carbide substrate: high fracture toughness
5.A thick carbide substrate: hard and resistant to plastic
deformation at high temperatures.
COATED TOOLS - Coating Materials
Diamond-Coated Tools:
• Thin films are deposited on substrates with PVD and CVD
techniques.
• Thick films are obtained by growing a large sheet of pure
diamond, which is then laser cut to shape and brazed to a
carbide shank.
• Diamond-coated tools are particularly effective in
machining nonferrous and abrasive materials, such as Al
alloys containing Si, fiber-reinforced and metal-matrix
composite materials, and graphite.
COATED TOOLS - Coating Materials
• Miscellaneous Coating Materials
• Titanium carbonitride (TiCN) and titanium-aluminum nitride (TiAlN)
are effective in cutting stainless steels.
• TiCN (which is deposited through physical-vapor deposition) is harder
and tougher than TiN and can be used on carbides and high-speed steel
tools.
• TiAlN is effective in machining aerospace alloys.
• Chromium based coatings, such as chromium carbide (CrC), have been
found to be effective in machining softer metals that tend to adhere to
the cutting tool, such as aluminum, copper, and titanium.
• Other new materials include zirconium nitride (ZrN) and hafnium
nitride (HfN)
• More recent developments are:
• nanolayer coatings, including carbide, boride, nitride, oxide &
combination
• Composite coatings, using a variety of materials.
ALUMINA-BASED CERAMICS
• Consist primarily of fine-grained, high-purity Al2O3. They are
cold-pressed into insert shapes under high pressure and sintered at
high temp; the end product is referred to as white, or cold-pressed,
ceramics.
• Additions of TiC and ZrO help improve toughness and thermalshock resistance.
• Alumina-based ceramic tools have very high abrasion resistance
and hot hardness.
• More stable than HSS and carbides, so they have less tendency to
adhere to metals during cutting leading to lower tendency to form a
BUE.
ALUMINA-BASED CERAMICS
• Consequently, in cutting cast irons and steels, good surface
finish is obtained with ceramic tools.
• Ceramics lack toughness, and their use may result in
premature tool failure by chipping or catastrophic failure.
• Effective in high-speed, uninterrupted cutting operations.
• -ve rake angles are preferred in order to avoid chipping.
• Tool failure can be reduced by increasing stiffness & damping
capacity of machine tools, mountings, & workholding devices,
thus reducing vibration and chatter.
Ranges of Mechanical Properties for Groups of
Tool Materials
Figure 22.9 Ranges of mechanical properties for various groups of
tool materials. See also Tables 22.1 through 22.5.
CUBIC BORON NITRIDE (CBN)
• made by bonding 0.5-1-mm layer of polycrystalline CBN to a
carbide substrate by sintering under pressure
• CBN tools are also made in small sizes without a substrate
• Figure 22.10 Construction of a polycrystalline CBNor a
diamond layer on a TiC insert
• Because CBN tools are brittle, stiffness of machine tool and
fixturing is important in order to avoid vibration and chatter
• to avoid cracking due to thermal shock, machining should
generally be performed dry, particularly in interrupted cutting
operations such as milling.
• Figure 22.11 Inserts with polycrystalline CBN tips (top row)
and solid polycrystalline CBN inserts (bottom row)
Cubic Boron Nitride Inserts
Figure 22.10 An insert of
polycrystalline cubic boron nitride or a
diamond layer on tungsten carbide.
Figure 22.11 Inserts with
polycrystalline cubic boron nitride tips
(top row), and solid-polycrystalline
cBN inserts (bottom row). Source:
Courtesy of Valenite.
SILICON-NITRIDE BASED CERAMICS
• Consist of SiN with various additions of Al2O3, yttrium oxide,
and TiC
• Toughness, hot hardness, and good thermal-shock resistance.
• An example of a SiN-base material is sialon, composed of : Si,
Al, On, and N
• It has higher thermal-shock resistance than silicon nitride
• recommended for machining cast irons and nickel-based
super-alloys at intermediate cutting speeds
• Because of chemical affinity to iron, SiN-based tools are not
suitable for machining steels
DIAMOND
•
•
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•
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•
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Low friction
High wear resistance
Ability to maintain sharp edge
Used when good surface finish and dimensional accuracy are
req. (soft non-ferrous & abrasive non-metallic materials)
Low rack angles are generally used > strong cutting edge
Used at high speed
Most reasonable for light uninterrupted finishing cut
Diamond is not recomm for mach plain carbon steels or
titanium, because of its strong chem. Affinity
Manufacturing, Engineering & Technology, Fifth Edition, by Serope Kalpakjian and Steven R. Schmid.
ISBN 0-13-148965-8. © 2006 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Proper Methods of Applying Cutting Fluids
Figure 22.12 Schematic illustration of the proper methods of applying
cutting fluids (flooding) in various machining operations: (a) turning, (b)
milling, (c) thread grinding, and (d) drilling.