Various tool materials used in the machining process are listed below.
- Carbon tool steel
- High speed steel (HSS)
- Cast alloys
- Cemented or sintered carbides
- Ceramics or oxides
- Whisker reinforced alumina (or kyon)
- Cubic boron nitride (CBN)
Let’s now discuss these tool materials and their properties, uses and more.
#1) Carbon tool steel
Carbon tool steel has a carbon content from 0.6 to 1.5%.
Before 1900, most of the machining tools were made of carbon tool steel as it is easy to manufacture and ground.
Carbon tool steel has a major disadvantage that it cannot handle higher temperatures during the machining process.
As they reach the temperature above 200 °C, they lose their hardness.
Hence, carbon steel tools are generally used for low speed machining operations and low heat generating operations.
They are generally used for machining wood, magnesium, aluminum, brass, etc.
In carbon steel tools, the elements like silicon, manganese, tungsten, chromium, vanadium and molybdenum are added to it for improving its hot hardness, wear resistance and shock resistance.
#2) High speed steel (HSS)
High speed steels (HSS) contain alloying elements like tungsten, chromium, molybdenum, cobalt, vanadium, etc upto 25%.
By alloying these elements the properties like strength, wear resistance, hardness, etc improves at high temperatures (550 °C to 600 °C).
- Tungsten increases the tempering wear resistance of tool
- Molybdenum increases the toughness of the tool
- Vanadium increases the wear resistance of finishing tool
- Cobalt increases the hardening temperatures and improves the heat resistance of the tool.
High speed steel (HSS) has the highest toughness among all the machining tools.
HSS is used in various tools like drilling bits, reamers, braches, taps, milling cutters, gear cutters, etc.
The main advantage of using high speed steel tools is that they have high hardness, high toughness and they show good resistance to wear.
#3) Cast alloys
Cast alloys contain main four metals;
The cast alloys are manufactured by melting cobalt, chromium and tungsten together and then they are cast by molding process.
Cast alloys can also be manufactured by using powder metallurgy process in which a fine powder of metals are thoroughly mixed and compacted to a final shape.
The hardness of Cast alloys are less than that of the HSS at room temperature but they have the ability to maintain the higher hardness at higher temperatures.
They also show good shock resistance and wear resistance.
Cast alloy tools can operate at the speed twice that of the speed at which HSS can operate.
Cast alloy steels are made up of;
- 45% cobalt
- 33% chromium
- 17% tungsten and
- 3% iron
Cast alloys are generally preferred when deep roughing operations are to be done on a workpiece at relatively high speeds and feeds.
They are generally used for machining of metals like hard bronzes and cast iron.
#4) Cemented or sintered carbides
Today, more than half of all the machining tools manufactured worldwide are made up of cemented carbide.
Cemented carbide tools are made by powder metallurgy process by using different carbides and tough metals of the iron group of binders.
Various properties of carbides that makes the machining tool very effective as listed below.
- Carbides have high hot hardness, due to which they can retain their hardness even at high temperatures (around 1000 °C).
- They have high elastic modulus that is two to three times that of the steel.
- They do not show plastic deformation even at high stresses.
- They have good thermal conductivity.
- They have low thermal expansion.
Various types of carbides used in machining tools are mentioned below.
- Tungsten carbide
- Titanium carbide
- Tantalum carbide
- Niobium carbide
Cemented carbides are expensive and hence they are used to make the inserts or tips of the machining tool.
The inserts made of cemented carbide are available in different shapes like square, triangle, etc.
Cemented carbide tools are sometimes coated with a thin coating material in order to improve the tool life.
By applying coatings on the cemented carbide tools, its tool life can be increased up to 2-3 times than that of the conventional carbide tools.
#5) Ceramics or oxides
Ceramic materials are used as inserts in the machining tool.
Ceramic tools offers benefits like;
- It permits higher machining speed
- It reduces built-up edges
- Coefficient of friction will be less
- Better surface finish can be achieved
- Tool life is more
Generally the machining of cast iron is done with ceramic tools only which gives better results.
Ceramic tools are suitable for high speed uninterrupted turning operations.
#6) Whisker reinforced alumina (or kyon)
Whiskered reinforced alumina is a ceramic material that possesses a strengthening silicon carbide fibres added to the aluminium matrix.
The silicon carbide fibres provide reinforcement to the structure and they show good oxidation resistance, good mechanical strength as well as high hardness.
They are also very tough and resistant to wear and they are generally made in the form of inserts.
By using these tools, the machining can be done at higher speed (even 2-3 times higher than the machining done with carbide tools).
Sialon is Silicon-Aluminum-Oxygen-Nitrogen (Si-Al-O-Ni) complex compound of ceramic based materials.
They possess good toughness, thermal shock resistance and higher hot hardness.
Sialon is manufactured by mixing the powder of aluminum nitride, alumina, Si3N4 and yttria and then they are compacted into the final desired inserts.
Sialon inserted tools permit 2-3 times higher machining speeds as compared to carbide insert tools.
The hardest tool material that is available for machining is a Diamond.
Diamonds have extremely high hardness, low thermal expansion, low coefficient of friction and very high heat conductivity.
Polycrystalline diamond is used where good surface finish and dimensional accuracy is required. While single-crystal diamond is used to produce extremely fine surface finish on nonferrous alloys.
#9) Cubic boron nitride (CBN)
Cubic boron nitride is also a very hard material just next to the diamond.
Polycrystalline CBN material is used as a tool material which provides high wear resistance and edge strength.
This material is suitable for machining of high temperature alloys as well as various ferrous alloys because they are chemical inert to iron and nickel at high temperatures.
The powdered cubic boron nitride (CBN) is also used as abrasives in grinding wheels.
Cermets is a composite material that is made up of two different types of materials (i.e ceramics and metals).
Cermets are made of materials like;
- Aluminum oxide (ceramic)
- Titanium carbide (metal)
The main purpose of using two different materials is to gain the properties of both (i.e ceramics and metals).
Ceramics have properties like high temperature resistance and hardness, while metals have properties like toughness.
The Cermets have a self sharpening property which keeps the cutting forces less during the machining process and thus it enables longer tool life.
Coronite is a tool material that is made by combining high speed steels and tungsten carbide. The high speed steels provide strength and toughness while the tungsten carbide provides heat and wear resistance to the tool.
Coronite material is developed basically for manufacturing tools like small and medium sized drills, milling cutters, etc.