Classification & Types of Cutting Tools

Classification & Types of Cutting Tools

What are Cutting Tools?

A Cutting Tool or Cutter is a tool used to create inserts or remove any remainder material of a workpiece by clearing out the excessive deformity. Cutting Tools are pointed and are mounted on various machines tools to be used in the process of cutting.

Cutting Tools are always harder than the material they are used to cutting. The Cutting Tools used for work should also withstand the heat released in the process.


Classification of Cutting Tools is done on the basis of various factors. The basic classification can be done measuring the contact distance between a tool and the metal.
Cutting Tools are of classified according to two types-

Single Point Tools

The tools which make use of a single sharp cutting edge to remove deformities are known as Single Point Tools. The act of turning, the exact opposite of boring, makes use of Single Point Tools as it works on the exterior diameter of a cast hole. The sharp tip of a Single Point Tool is usually a round, forming a nose radius.

Multiple Cutting Tools

Multiple Cutting Tools have more than one cutting edge to them and are used for various different purposes. A Multiple Cutting Tool is mounted on a machine and used by utilising the tool in a rotation motion. The activities of drilling and mining make use of Multiple Cutting Tools.

Cutting tools can be classified in the basis of following properties:

Wear Resistance-

Wear Resistance is the ability of a tool to resist wearing with continuous use. Wearing of a tool happens as friction is created when a cutting tool mounted on a machine gets in contact with the piece of metal it is being worked on. If a tool does not have the ability to resist this wear, then it is bound to break or not be usable because of the tool shape changing due to it.

To reduce the damage caused by friction, Cobalt is added in the tool composition. This increases the longevity of the tool 

Strength & Toughness-

It is the ability of a tool to withstand immense cutting pressure with working on complicated pieces or types of cuts required. A tool should withstand the vibrations created while cutting.

To improve a tool's toughness and strength, some manufactures add Nickel and/or Molybdenum to the composition. When a tool is tough, it won’t chip away or break down with prolonged use.

Hot Hardness-

Hot Hardness is the ability of a tool to withstand the heat and temperatures produced while trying to cut a piece of metal. The act of cutting produces significant heat on the tool. This heat can melt the cutting tool and break it off.

To increase a cutting tools hot hardness, metals like- Tungsten, Aluminium, Vanadium and Molybdenum are added. These metals are responsible for increasing the heating capacity of the tool. Generally, the tool’s hot hardness needs to be more than the material it is going to interact with.

( TO take reference of the table from http://www.e-codestar.com/rshuster/@http@zdaps@addr@rickspage/lib/mach/cuttoolclass.html for the above three properties)

Young’s Modulus-

It is the measure of how stiff a solid is. Stiffness isn’t to be confused with hardness, toughness, or even strength. Young's modulus E, can be calculated by dividing the tensile stress,  by the engineering extensional strain,  in the elastic (initial, linear) portion of the physical stress–strain curve:



where,

E is the Young's modulus (modulus of elasticity);
F is the force exerted on an object under tension;
A is the actual cross-sectional area, which equals the area of the cross-section perpendicular to the applied force;
ΔL is the amount by which the length of the object changes (ΔL is positive if the material is stretched, and negative when the material is compressed);
L0 is the original length of the object.

Tool Life-

The life of a tool is the time a tool will take before it wears out. The equation to calculate this is called the Taylor Tool Life equation. The Taylor tool life equation can be written as:

v(T)n = C

Where,

v is the cutting speed, m/min;
T is the tool life, in minutes;
C is the cutting speed for a tool life of 1 minute;
n is the Taylor exponent (Do not confuse this use of n with the cold working index n).

Cutting tools can be classified on the basis of their material composition:

Carbon Steel

A Carbon Steel is made up of plain carbon steel with additional composition mentioned below:

Silicon – 0.1 to 0.4 %
Carbon –   0.8 to 1.3 %
Manganese  – 0.1 to 0.4 %

High Speed Steel

These tools are made up of steel that is meant for speed cutting. There are different types of High Speed Steel to make cutting tools. The popular ones are:

Cobalt High Speed Steel

This type of High Speed Steel is composed of:

Cobalt= 12%
Tungsten= 20%
Chromium= 4%
Vanadium= 2 %

18 – 4 – 1 High Speed Steel

18 - 4 - 1 High Speed Steel is composed of:

Tungsten = 18%
Chromium= 4%
Vanadium= 1%
Carbon= 0.75%

Molybdenum High Speed Steel

This type of High Speed Steel is composed of:

Molybdenum = 6%
Tungsten= 5%
Chromium= 4%
Vanadium= 2%

Cast Alloys

These type of tools are now limited in use. The composition of Cast Alloys is:

Cobalt = 40% - 55%
Chromium = 30%
Tungsten = 10% - 20%

Carbides

These are made by mixing tungsten and carbon at a high temperature of 1500 Centigrade. Then Cobalt is added to this mixture and heated at a temperature of 1400 Centigrade. The composition of Carbides is:

Tungsten= 82%
Titanium carbide= 10%
Cobalt= 8%

Ceramics

The material composition of a ceramics tool is:

Aluminium oxide=  90%
Chromium oxide= 10%

Types of Cutting Tools

What is a Boring Tool?

Boring is the act of enlarging an already casted hole by a drill. A boring tool is a Multi Point Tool which works on the internal diameters of a cast hole. With the help of a boring tool, the diameter of the hole can be increased. A boring tool also gives the hole a clean and a smooth finish, getting the piece ready for the next step.

What is an Indexing Tool?

An Indexing tool is a specialised tool used on a workpiece to make it circularly indexed. Indexing is the process in which a particular piece moves or is being moved from one location to the other quickly and precisely. The piece of work is held in between an index tool in the same way as in a metalworking lathe. Indexing head tools are usually used on milling machines and many other drill tools

What is a Brazed Tip Tool?

Brazing is the process of joining two dissimilar materials with a third material. Brazing and Brazing Alloying are close concepts except Brazing is done on a higher temperature than Brazing Alloying.  Brazed tip tools are often a cheaper alternative available than other specialised tools tips. These tools are also tougher when it comes to resistance and wearing than other standard tool in use.

The types of tools are also based on their material properties. These are as follows

Carbon Steel

This material is amongst the lowest grade of tools and falls in the family of low grade alloys. A tool made from Carbon Steel has is hard, tough and has strength when it is hardened at a certain temperature.

Carbon Steel Tools are suitable to be cut with at a lower cutting speed as above a temperature of 180oC, the Carbon starts melting. This limits this type of tool to lower speed machines in turn, rendering them unable for metal cutting.

The materials it is composed of are cheap, easily available and comfortably forged. Carbon Steel tools have a hardness of about 62 Rc and usually opted for working with wood.

High Speed Steel

HSS has a higher resistance and hot hardness than Carbon Steel because of its material composition. Tools made from High Speed Steel can be used for Metal cutting at a speed of about 2 to 3 times faster than Carbon Steel Tools.

HSS tools can cut through a metal comfortably and can perform high speed cut with faster metal removal rate. The melting point of this steel is about 900oC. Cutting Tools made with High Speed Steel are suitable for interrupted cuts on metals using different machines and processes.

Cast Alloys

Cast Alloys were introduced in the early 1900’s and were in regular use then. They have a maximum hardness value of about 55 - 64 Rc. Cast Alloys have a better resistance than its toughness, lower comparatively to HSS.

Cast Alloys can be used at a slight higher speed than High Speed Steels. These tools maintain their hardness from up-to 760oC and they are highly alloyed, which makes them brittle and susceptible to damage. Tools made from Cast Alloys are now limited in use.

Carbides

Carbides or Cemented Carbides are materials that have a high Hot Hardness over various temperatures, high thermal conductivity and a high Young’s Modulus making them a suitable material for manufacturing carbide cutting tools. Usually Carbides are made of Tungsten powder and Carbon, mixed in a ratio of weight- 94 : 6. Then the next step involves sintering it with Cobalt at high temperatures.

There are a wide range of grade of Tungsten Carbide available in the Market. The addition of Cobalt gives increased hardness, depending on the amount present in the mixture. Tungsten Carbide has a higher wear resistance than Tungsten. Typical cutting speed is of- 30 - 150 mm; when coated it is about 100 - 250 mm.

There are three main types of Carbides- Straight Tungsten Carbide, Titanium Carbide or Tantalum Carbide & Composite Carbides. Straight Tungsten Carbide is strong and has a high resistance. Titanium Carbides help in reducing the chip rate of a tool and help in improving the hot hardness.

Carbide tools are also known as Tungsten Carbide Cutting Tools.

Ceramics

Developed in the 1960’s, Ceramics tools are made by sintering Boron Nitride in powder form and Aluminium Oxide at about a temperature of 1700oC. Because of Ceramics being usually refractory, they can withstand high temperatures of heat, up to 1200oC. Ceramics are brittle even more than Cast Alloys, which makes them sensitive to thermal shock.

Ceramics are costly to produce and are sensitive, this renders them usable for only high quality machinery with a high production rate thus lowering their use.

Which is the Hardest Cutting Tool Material?

The Hardest Cutting Tool is Diamond. Diamonds are the hardest substance known to man yet they are comparatively brittle. Traditionally, Single Diamond Tools used for the purpose of cutting. Nowadays, machines make use of Poly-crystalline Diamonds as a replacement. Though Diamonds are costly and are hence not used a lot to make tools, other than specific industries.

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