cutting tool materials

CUTTING TOOL MATERIALS

The various materials are used to remove the metal from the workpiece. The tool must be harder than the material which is to be cut. The selection of cutting tool material depends on the following factors:

• Volume of production

• Tool design

• Type of machining process

• Physical and chemical properties of work material

• Rigidity and condition of machine.

1. Properties/Characteristics of Cutting Tool Material

The cutting tool material should possess the following properties.

1. Hot hardness:

It is the ability of the cutting tool to withstand high temperature without losing its cutting edge. The tool must maintain its hardness at high temperature. This hardness is higher than the hardness of workpiece. The addition of following materials will improve the hot hardness such as Chromium, Molybdenum, Tungsten and Vanadium.

2. Wear resistance:

It is the ability to resist wear. During machining, the friction between workpiece and tool causes wear in the tool. If the tool is not having sufficient wear resistance, it will fail quickly. It leads to the poor surface finish. The addition of cobalt increases the wear resistance property of the tool.

3. Toughness:

It is the combined property of strength and ductility. The tool material should have sufficient toughness to withstand shock and vibrations. If the tool materials have sufficient toughness, the fine cutting edge of the tool does not break or chip when the tool is suddenly loaded. This property limits the hardness of the tool. The tool having high hardness is brittle and weak in tension. The addition of molybdenum and nickel increases toughness.

4. Low friction:

The coefficient of friction between tool and workpiece must be low. It reduces friction, heat developed and tool wear.

5. Cost of tool:

Tool material should be economical in production. It should be easy to manufacture the tool from the material. In addition to the above properties, the tool material should possess the following properties.

• High thermal conductivity

• Resistance to thermal shock

• Easy to grind and sharp

• Low mechanical and chemical affinity for the work material

2. Classification of Tool Materials

The following metals are suitably heat treated wherever required in manufacturing cutting tools.

(a) Carbon tool steel

(b) High speed steel

(c) Cemented carbides

(d) Ceramics

(e) Diamonds.

1. Carbon tool steels:

The composition of typical plain carbon steel used for cutting is as follows.

Carbon - 0.8 to 1.3%

Silicon - 0.1 to 0.4%

Manganese - 0.1 to 0.4%

These are suitable for low cutting speed and used in those applications where the cutting temperature is below 200°C. Such steels have good hardness, strength and toughness when hardened and tempered. It is done to provide a keen cutting edge. It is cheap, easy to forge and simple to harden. Cutting tools such as taps, dies, reamers and hacksaw blades are made by using these materials.

Medium alloy tool steels are similar to carbon tool steel. These are alloy steels alloyed with small quantities of tungsten, molybdenum, chromium and vanadium. It has carbon up to - 5%. Chromium and molybdenum are added to increase hardness of steel whereas the tungsten is added to improve wear resistance. The hardness is lost at 350°C. These metals are used to make taps, reamers, punches, dies, knives etc.

2. High speed steels (HSS):

This tool steel effectively cuts the metal even at high speeds. It has superior hot hardness and high wear resistance. The cutting speed can be 2 to 3 times higher than carbon steels. These tools give the improved cutting performance and higher metal removal rates. This tool steel maintains its hardness even up to 900°C. The various alloying elements to improve hot hardness and wear resistance are tungsten, chromium, vanadium, cobalt and molybdenum. HSS is widely used for drills, milling cutters, broaches, taps, turning tools and dies.

The various types of high-speed steels are:

(a) 18-4-1 High speed steel

(b) Molybdenum high speed steel

(c) Cobalt high speed steel.

(a) 18-4-1 High-speed steel:

It contains the following composition:

Tungsten - 18%

Chromium - 4%

Vanadium - 1%

It has about 0.75% carbon. It is commonly used for all purposes. This type of material gives excellent performance over a great range of materials and cutting speeds and it retains its hardness up to 600°C. Most of the cutting tools are made of this steel. The various tools such as drill bits, single point cutting tools, milling cutters etc. are made from this tool steel.

(b) Molybdenum high-speed steel:

This steel has the following composition:

Molybdenum - 6%

Tungsten - 5%

Chromium - 4%

Vanadium - 2%

It has high toughness and cutting ability.

(c) Cobalt high-speed steel:

Cobalt high-speed steel has the following composition:

Cobalt - 12%

Tungsten - 20%

Chromium - 4%

Vanadium - 2%

It is also known as super high-speed steel. This steel is used for heavy duty and rough cutting tools such as planer tools, milling cutters, lathe tools etc.

3. Cemented carbides:

Cemented carbides are made by mixing tungsten powder and carbon at high temperature (1500°C) in the ratio of 94 and 6 respectively by weight. This new compound is tungsten carbide. It is combined with cobalt, compacted and sintered in a furnace about 1400°C and it can be used at much higher cutting speed. The composition is 82% tungsten carbide, 10% titanium carbide and 8% Cobalt. They usually take the form of inserts (either braced or clamped form). The clamped inserts can be thrown, after wearing out of all cutting edges take place. The tool can withstand higher temperature up to 1000°C. Its cutting speed is 6 times higher than the high-speed steel. But, it is brittle and it has low resistance to shock. It must be supported strongly to prevent cracking.

These tool materials are classified into two main types, namely

(a) Straight tungsten carbide

(b) Alloyed tungsten carbide.

Straight tungsten carbides are very strong and more wear resistance. But, the rapid cracking takes place while machining steels. To improve resistance to cratering, alloyed tungsten carbides are used with the addition of carbides of titanium and molybdenum etc.

Titanium carbide improves the hot hardness and it reduces the tendency of chips to weld to the tool. The addition of titanium carbides helps to improve resistance to crater wear and it makes the structure fine grained. For optimum results, both titanium and tantalum carbides are often made. Carbides are used for machining hard steels and for machining brittle materials such as cast iron and bronze.

4. Ceramics:

Aluminium oxide and boron nitride powders are mixed together and sintered at 1700°C to form the ingredients of ceramic tools. These materials are very hard with good compressive strength. Ceramic tools are made in tips and clamped on the metal shanks of tools. It can be employed at cutting speeds as high as two to three times those employed with tungsten carbides. But, they are extremely brittle and cannot be used where more shocks and vibrations occur.

A well-known variety contains 90% Aluminium Oxide and remaining 10% shared by Chromium Oxide, Magnesium Oxide and Nitrogen Oxide. These ceramics have high compressive strength, longer tool life, greater machining flexibility, and superior surface finish. It can withstand the temperature up to 1700°C. This material is used for making single point cutting tools to machine cast iron and plastics. No coolant is needed but the tool must be strongly supported.

The various conditions for the effective use of carbide or ceramic tools are:

• high cutting speeds

• rigidity of tool and workpiece

• highly surface finish on cutting tool

• use of effective chip removal and chip guards

• elimination of any unbalanced forces.

5. Diamonds:

Diamond is the hardest cutting material. Polycrystalline diamond is manufactured by sintering under high pressure and temperature. It has a low coefficient of friction, high compressive strength and it is extremely wear resistant. It is used for machining very hard materials such as glass, plastics, ceramics etc. Due to high hardness, high compressive and bending strength, the deformation during the process is less. Diamond tools produce very good surface finish at high speeds with good dimensional accuracy. It is very small and best suited for light cuts and finishing operations. It can resist temperature up to 1250°C. The general properties of diamond are as follows:

• It is the hardest substance..

• It has low coefficient of thermal expansion.

• It has high heat conductivity.

• It has poor electrical conductivity.

• It has very low coefficient of friction.

It offers the highest tool life, 50-100 times more than cemented carbides. The main disadvantages of diamonds are their brittleness and high cost.

6. Cubic Boron Nitride:

Boron nitride is a chemical compound which is denoted by BN having equal numbers of boron and nitrogen atoms. Its hardness is less than diamond, but its thermal and chemical stabilities are more.

Boron nitride is not available in nature but it can artificially be produced from boric acid or boron trioxide. The first product of BN is amorphous BN powder and it could be converted to crystalline h-BN through heating in the nitrogen flow at 1500°C temperature. Opposite to diamond, larger c-BN pellets can be produced by sintering. Therefore, c-BN is mainly used in mechanical applications.

Cubic boron nitride (CBN or c-BN) is also used as an abrasive. Polycrystalline c-BN (PCBN) abrasives are used in machining steel. Materials with cubic boron nitride crystals are used in cutting tools in the form of cutting tips.

3. Technical and Economic Factors involved in Cutting Tool Material Selection 

Both technical and economic factors are equally considered. The technical factors are as follows:

(i) A tool material with sufficiently high hot hardness for strength and wear resistance.

(ii) Chemical stability and inertness for adhesive wear resistance.

(iii) Toughness for fracture prevention.

(iv) High thermal conductivity to minimize severe temperature gradients.

The economic factors are:

(i) Tool cost should be minimized.

(ii) The material should be readily available.