Many types of tool materials, ranging from high carbon steel to ceramics and diamonds, are used
as cutting tools in today’s metalworking industry. It is important to be aware that differences do
exist among tool materials, what these differences are, and the correct application for each type
of material.
The various tool manufacturers assign many names and numbers to their products. While many
of these names and numbers may appear to be similar, the applications of these tool materials
may be entirely different. In most cases, the tool manufacturers will provide tools made of the
proper material for each given application. In some particular applications, a premium or higher
priced material will be justified.
This does not mean that the most expensive tool is always the best tool. Cutting tool users can’t
afford to ignore the constant changes and advancements that are being made in the field of tool
material technology. When a tool change is needed or anticipated, a performance comparison
should be made before selecting the tool for the job. The optimum tool is not necessarily the least
expensive or the most expensive, and it is not always the same tool that was used for the job last
time. The best tool is the one that has been carefully chosen to get the job done quickly,
efficiently, and economically.
A cutting tool must have the following characteristics in order to produce good quality and
economical parts:
Hardness — harness and strength of the cutting tool must be maintained at elevated
temperatures, also called hot hardness (Figure 1.1)
Toughness — toughness of cutting tools is needed so that tools don’t chip or fracture, especially
during interrupted cutting operations.
Wear Resistance — wear resistance means the attainment of acceptable tool life before tools
need to be replaced.
The materials from which cutting tools are made are all characteristically hard and strong. There
is a wide range of tool materials available for machining operations, and the general
classification and use of these materials are of interest here.
Tool Steels and Cast Alloys
Plain carbon tool steel is the oldest of the tool materials dating back hundreds of years. In simple
terms, it is a high-carbon steel, which contains about 1.05% carbon. This high carbon content
allows the steel to be hardened, offering greater resistance to abrasive wear. Plain high carbon
steel served its purpose well for many years. However, because it is quickly over tempered
(softened) at relatively low cutting temperatures (300 to 500°F), it is now rarely used as cutting
tool material except in files, saw blades, chisels, etc. The use of plain high carbon steel is limited
to low heat applications.
High Speed Tool Steel: The need for tool materials that could withstand increased cutting speeds
and temperatures led to the development of high-speed tool steels (HSS). The major difference
between HSS and plain high carbon steel is the addition of alloying elements to harden and
strengthen the steel and make it more resistant to heat (hot hardness).
Some of the most commonly used alloying elements are manganese, chromium, tungsten,
vanadium, molybdenum, cobalt, and niobium. While each of these elements will add certain
specific desirable characteristics, it can be generally state that they add deep hardening
capability, high hot hardness, resistance to abrasive wear, and strength, to HSS. These
characteristics allow relatively higher machining speeds and improved performance over plain
high carbon steel.
The most common HSS used primarily as cutting tools are divided into the M and T series. The
M series represents tool steels of molybdenum type and the T series represents Tungsten.
Although there seems to be a great deal of similarly among these HSS, each one serves a specific
purpose and offers significant benefits in its special application.
An important point to remember is that none of the alloying elements for either series of HSS is
in abundant supply and the cost of these elements is skyrocketing. In addition, U.S.
manufacturers must rely on foreign countries for supply of these very important elements.
Some of the HSS are now available in powdered metal (PM) form. The difference between
powdered and conventional metals is in the method by which they are made. The majority of
conventional HSS is poured into an ingot, and then, either hot or cold, worked to the desired
shape. Powdered metal is exactly as its name indicates. Basically the same elements that are used
in conventional high-speed steel are prepared in a very fine powdered form. These powdered
elements are carefully blended together, pressed into a die under extremely high pressure, and
then sintered in an atmospherically controlled furnace. (The PM method of manufacturing
cutting tools is explained later in this chapter.)
HSS Surface Treatment: Many surface treatments have been developed in an attempt to extend
tool life, reduce power consumption, and to control other factors that affect operating conditions
and costs. Some of these treatments have been used for many years and have proven to have
some value. For example, the black oxide coatings that commonly appear on drills and taps are
of value as a deterrent to build-up on the tool. The black oxide is basically ‘dirty’ surface that
discourages the build-up of work material.
One of the more recent developments in coatings for HHS is titanium nitride by the physical
vapor deposition (PVD) method. Titanium nitride is deposited on the tool surface in one of
several different types of furnace at relatively low temperature, which does not significantly
affect the heat treatment (hardness) of the tool being coated. This coating is known to extend the
life of a cutting tool significantly or to allow the tool to be used at higher operating speeds. Tool
life can be extended by as much as three times, or operating speeds can be increased up to 50%.
Cast Alloys: The alloying elements in HSS - principally cobalt, chromium, and tungsten -
improve the cutting properties sufficiently, that metallurgical researchers developed the cast
alloys, a family of materials without iron.
A typical composition for this class was 45% cobalt, 32% chromium, 21% tungsten, and 2%
carbon. The purpose was to obtain a cutting tool with hot hardness superior to HSS.
When applying cast alloy tools, their brittleness should be kept in mind and sufficient support
should be provided at all times. Cast alloys provide high abrasion resistance and are thus useful
for cutting scaly materials or those with hard inclusions.
Cemented Tungsten Carbide
Henri Moissan discovered tungsten carbide in 1893 during a search for method of making
artificial diamonds. Charging sugar and tungsten oxide, he melted tungsten sub-carbide in an arc
furnace. The carbonized sugar reduced the oxide and carburized the tungsten Moissan recorded
that the tungsten carbide was extremely hard, approaching the hardness of diamond and
exceeding that of sapphire. It was more than 16x as heavy as water. The material proved to be
extremely brittle and seriously limited its industrial use.
Commercial tungsten carbide with 6% cobalt binder was first produced and marketed in
Germany in 1926. Production of the same carbide began in the U.S. in 1928 and in Canada in
1930.
At this time, hard carbides consisted of the basic tungsten carbide system with cobalt binders.
These carbides exhibited superior performance in the machining of cast iron, nonferrous, and
non-metallic materials, but were disappointed when used for the machining of steel.
Most of the subsequent developments in the hard carbides have been modifications of the
original patents, principally involving replacement of part or all of the tungsten carbide with
other carbides, especially titanium carbide and/or tantalum carbide. This led to the development
of the modern multi-carbide cutting tool materials permitting the high speed machining of steel.
A new phenomenon was introduced with the development of the cemented carbides, again
making higher speeds possible. Previous cutting tool materials, products of molten metallurgy,
depended largely upon heat treatment for their properties and these properties could, in turn, be
destroyed by further heat temperatures, these products of molten metallurgy failed.
A different set of conditions exist with the cemented carbides. The hardness of the carbide is
greater than the of most other tool materials at room temperature and it has the ability to retain its
hardness at elevated temperatures to a greater degree, so that greater speeds can be adequately
supported.
Manufacture of Carbide Products
The term “tungsten carbide” describes a comprehensive family of hard carbide compositions
used for metal cutting tools, dies of various types, and wear parts. In general, these materials are
composed of the carbides of tungsten, titanium, tantalum, or some combination of these, sintered
or cemented in a matrix binder, usually cobalt.
Blending: The first operation after reduction of the tungsten metal powder is the milling of
tungsten and carbon prior to the carburizing operation. Here, 94 parts by weight of tungsten and
six parts by weight of carbon - usually added in the form of lampblack - are blended together in a
rotating mixer or ball mill. This operation must be performed under carefully controlled
conditions in order to insure optimum dispersion of the carbon in the tungsten. Carbide blending
equipment - better known as a ball mill - is shown in figure 1.2.
In order to provide the necessary strength, a binding agent, usually cobalt is added to the
tungsten in powder form and these two are ball milled together for a period of several days, to
form a very intimate mixture. Careful control of conditions, including time, must be exercised to
obtain a uniform, homogeneous product. (See figure 1.3)
Compacting: The most common compacting method for grade powders involves the use of a die,
made to the shape of the eventual product desired. The size of the die must be greater than the
final product size to allow for dimensional shrinkage that takes place in the final sintering
operation. These dies are expensive, and usually made with tungsten carbide liners. Therefore,
sufficient number of the final product (compacts) are required, to justify the expense involved in
manufacturing a specific die. Carbide compacting equipment - better known as a pill press - is
shown in figure 1.4, and various pill pressed carbide parts are shown in figure 1.5.
If the quantities are not high, a larger briquette, or billet, may be pressed. This billet may then be
cut up (usually after pre-sintering) into smaller units and shaped or preformed to the required
configuration, and again, allowance must be made to provide for shrinkage. Ordinarily pressures
used in these cold compacting operations are in the neighborhood of 30,000 PSI. Various
carbide-preformed parts are shown in figure 1.6.
A second compacting method is the hot pressing of grade powders in graphite dies at the
sintering temperature. After cooling, the part has attained full hardness. Because the graphite dies
are expendable, this system is generally used only when the part to be produced it too large for
cold pressing and sintering.
A third compacting method, usually used for large pieces, is isostatic pressing. Powders are
placed in a closed, flexible container that is then suspended in a liquid in a closed pressure
vessel. Pressure in the liquid is built up to the point where the powders become properly
compacted. This system is advantageous for pressing large pieces, because the pressure acting on
the powders operates equally from all directions, resulting in a compact of uniform pressed
density.
Sintering: A cobalt compact is heated in a hydrogen atmosphere or vacuum furnace in
temperatures ranging from 2,500 to 2,900°F, depending on the composition. Both time and
temperature are carefully adjusted in combination, to effect optimum control over properties and
geometry. The compact will shrink approximately 16% on linear dimensions, or 40% in volume.
The exact amount of shrinkage depends on several factors, including particle size of the powders,
and the composition of the grade. Control of the size and shape is most important and is least
predictable during the cooling cycle. This is particularly true with those grades of cemented
carbides with higher cobalt contents.
With cobalt having a lesser density than tungsten, it occupies a greater part of the volume than
would be indicated by the rated cobalt content of the grade. And, because cobalt contents are
generally a much higher percentage of the mass in liquid phase, extreme care is required to
control and predict with accuracy the magnitude and direction of shrinkage. Figure 1.7 shows
carbide parts being loaded into a sintering furnace, and a more detailed schematic diagram of the
cemented tungsten carbide manufacturing process is shown in figure 1.8.
Classification of Carbide Tools
Cemented carbide products are classified into three major categories:
• Wear Grades — used primarily in dies, machine and tool guides, and in everyday items such
as line guides on fishing rods and reels. Used anywhere good wear resistance is required.
• Impact Grades — also used for dies, particularly for stamping and forming, and in tools such
as mining drill heads.
• Cutting Tool Grades — the cutting tool grades of cemented carbides are divided into two
groups, depending on their primary application. If the carbide is intended for use on cast iron that
is a nonductile material, it is graded as a cast iron carbide. If it is to be used to cut steel, a ductile
material, it is graded as a steel grade carbide.
Cast iron carbides must be more resistant to abrasive wear. Steel carbides require more resistance
to cratering and heat. The tool wear characteristics of various metals are different, thereby
requiring different tool properties. The high abrasiveness of cast iron causes mainly edge wear to
the tool. The long chip of steel, which flows across the tool at normally higher cutting speeds,
causes.
In the context of machining, a cutting tool (or cutter) is any tool that is used to remove material
from the workpiece by means of shear deformation. Cutting may be accomplished by single-
point or multipoint tools. Single-point tools are used in turning, shaping, plaining and similar
operations, and remove material by means of one cutting edge. Milling and drilling tools are
often multipoint tools. Grinding tools are also multipoint tools. Each grain of abrasive functions
as a microscopic single-point cutting edge (although of high negative rake angle), and shears a
tiny chip.
Cutting tools must be made of a material harder than the material which is to be cut, and the tool
must be able to withstand the heat generated in the metal-cutting process. Also, the tool must
have a specific geometry, with clearance angles designed so that the cutting edge can contact the
workpiece without the rest of the tool dragging on the workpiece surface. The angle of the
cutting face is also important, as is the flute width, number of flutes or teeth, and margin size. In
order to have a long working life, all of the above must be optimized, plus the speeds and feeds
at which the tool is run.
Types[edit]
Linear cutting tools include tool bits (single-point cutting tools) and broaches. Rotary cutting
tools include drill bits, countersinks and counterbores, taps and dies, milling cutters, reamers, and
cold saw blades. Other cutting tools, such as bandsaw blades, hacksaw blades, and fly cutters,
combine aspects of linear and rotary motion
Cutting tools with inserts (indexable tools)[edit]
Cutting tools are often designed with inserts or replaceable tips (tipped tools). In these, the
cutting edge consists of a separate piece of material, either brazed, welded or clamped on to the
tool body. Common materials for tips include cemented carbide, polycrystalline diamond, and
cubic boron nitride.[1] Tools using inserts include milling cutters (endmills, fly cutters), tool bits,
and saw blades.
Solid Cutting Tools[edit]
The typical tool for milling and drilling has no changeable insert. The cutting edge and the shank
is one unit and built of the same material. Small tools cannot be designed with exchangeable
inserts.
Holder[edit]
To use a cutting tool within a CNC machine there is a basic holder required to mount it on the
machine's spindle or turret. For CNC milling machines, there are two (2) types of holder. There
are shank taper (SK) and hollow shank taper (HSK).
Tool setup[edit]
The detailed instruction how to combine the tool assembly out of basic holder, tool and insert
can be stored in a tool management solution.
Materials[edit]
To produce quality product, a cutting tool must have three characteristics:
Hardness: hardness and strength at high temperatures.
Toughness: so that tools do not chip or fracture.
Wear resistance: having acceptable tool life before needing to be replaced.[2]
Cutting tool materials can be divided into two main categories: stable and unstable.
Unstable materials (usually steels) are substances that start at a relatively low hardness point and
are then heat treated to promote the growth of hard particles (usually carbides) inside the original
matrix, which increases the overall hardness of the material at the expense of some its original
toughness. Since heat is the mechanism to alter the structure of the substance and at the same
time the cutting action produces a lot of heat, such substances are inherently unstable under
machining conditions.
Stable materials (usually tungsten carbide) are substances that remain relatively stable under the
heat produced by most machining conditions, as they don't attain their hardness through heat.
They wear down due to abrasion, but generally don't change their properties much during use.
Most stable materials are hard enough to break before flexing, which makes them very fragile.
To avoid chipping at the cutting edge, some tools made of such materials are finished with a
sightly blunt edge, which results in higher cutting forces due to an increased shear area, however,
tungsten carbide has the ability to attain a significantly sharper cutting edge than tooling steel for
uses such as ultrasonic machining of composites. Fragility combined with high cutting forces
results in most stable materials being unsuitable for use in anything but large, heavy and rigid
machinery and fixtures.
Unstable materials, being generally softer and thus tougher, generally can stand a bit of flexing
without breaking, which makes them much more suitable for unfavorable machining conditions,
such as those encountered in hand tools and light machinery.
Tool
Properties
material
Unstable. Very inexpensive. Extremely sensitive to heat. Mostly obsolete in
today's commercial machining, although it is still commonly found in non-
Carbon tool intensive applications such as hobbyist or MRO machining, where economy-grade
steels drill bits, taps and dies, hacksaw blades, and reamers are still usually made of it
(because of its affordability). Hardness up to about HRC 65. Sharp cutting edges
possible.
Unstable. Inexpensive. Retains hardness at moderate temperatures. The most
High speed
common cutting tool material used today. Used extensively on drill bits and taps.
steel (HSS)
Hardness up to about HRC 67. Sharp cutting edges possible.
Unstable. Moderately expensive. The high cobalt versions of high speed steel are
very resistant to heat and thus excellent for machining abrasive and/or work
HSS cobalt hardening materials such as titanium and stainless steel. Used extensively on
milling cutters and drill bits. Hardness up to about HRC 70. Sharp cutting edges
possible.
Stable. Expensive. Somewhat fragile. Despite its stability it doesn't allow for high
Cast cobalt
machining speed due to low hardness. Not used much. Hardness up to about HRC
alloys
65. Sharp cutting edges possible.
Stable. Moderately expensive. The most common material used in the industry
today. It is offered in several "grades" containing different proportions of tungsten
carbide and binder (usually cobalt). High resistance to abrasion. High solubility in
Cemented
iron requires the additions of tantalum carbide and niobium carbide for steel usage.
carbide
Its main use is in turning tool bits although it is very common in milling cutters
and saw blades. Hardness up to about HRC 90. Sharp edges generally not
recommended.
Stable. Moderately inexpensive. Chemically inert and extremely resistant to heat,
ceramics are usually desirable in high speed applications, the only drawback being
their high fragility. Ceramics are considered unpredictable under unfavorable
Ceramics conditions. The most common ceramic materials are based on alumina (aluminium
oxide), silicon nitride and silicon carbide. Used almost exclusively on turning tool
bits. Hardness up to about HRC 93. Sharp cutting edges and positive rake angles
are to be avoided.
Stable. Moderately expensive. Another cemented material based on titanium
carbide (TiC). Binder is usually nickel. It provides higher abrasion resistance
compared to tungsten carbide at the expense of some toughness. It is far more
Cermets
chemically inert than it too. Extremely high resistance to abrasion. Used primarily
on turning tool bits although research is being carried on producing other cutting
tools. Hardness up to about HRC 93. Sharp edges generally not recommended.
Stable. Expensive. Being the second hardest substance known, it is also the second
most fragile. It offers extremely high resistance to abrasion at the expense of much
Cubic boron toughness. It is generally used in a machining process called "hard machining",
nitride (CBN) which involves running the tool or the part fast enough to melt it before it touches
the edge, softening it considerably. Used almost exclusively on turning tool bits.
Hardness higher than HRC 95. Sharp edges generally not recommended.
Stable. Very Expensive. The hardest substance known to date. Superior resistance
to abrasion but also high chemical affinity to iron which results in being unsuitable
Diamond for steel machining. It is used where abrasive materials would wear anything else.
Extremely fragile. Used almost exclusively on turning tool bits although it can be
used as a coating on many kinds of tools. Sharp edges generally not recommended.
