1.7.1 A Short List of Tool Materials

 

• Carbon Steels

- Limited tool life. Therefore, not suited to mass production

- Can be formed into complex shapes for small production runs

- low cost

- suited to hand tools, and wood working

- Carbon content about 0.9 to 1.35% with a hardness ABOUT 62°C Rockwell

- Maximum cutting speeds about 26 ft/min. dry

- The hot hardness value is low. This is the major factor in tool life.

 

• High Speed Steel

- an alloyed steel with 14-22% tungsten, as well as cobalt, molybdenum and chromium, vanadium.

- Appropriate heat treating will improve the tool properties significantly (makers of these steels often provide instructions)

- can cut materials with tensile strengths up to 75 tons/sq.in. at speeds of 50-60 fpm

- Hardness is in the range of 63-65°C Rockwell

- The cobalt component give the material a hot hardness value much greater than Carbon Steels

- Used in all type of cutters, single/multiple point tools, and rotary tools

 

• Stellite

- a family of alloys made of cobalt, chromium, tungsten and carbon

- The material is formed using electric furnaces, and casting technique, and it cannot be rolled, or worked.

- The material has a hardness of 60-62°C Rockwell without heat treating, and the material has good hot hardness properties

- Cutting speed of up to 80-100 fpm can be used on mild steels

- The tools that use this method either use inserts in special holders, or tips brazed to carbon steel shanks

 

• Tungsten Carbide

- Produced by sintering grains of tungsten carbide in a cobalt matrix (it provides toughness).

- Other materials are often included to increase hardness, such as titanium, chrome, molybdenum, etc.

- Compressive strength is high compared to tensile strength, therefore the bits are often brazed to steel shanks, or used as inserts in holders

- These inserts may often have negative rake angles

- Speeds up to 300 fpm are common on mild steels

- Hot hardness properties are very good

- coolants and lubricants can be used to increase tool life, but are not required.

- special alloys are needed to cut steel

 

• Ceramics

- sintered or cemented ceramic oxides, such as aluminum oxides sintered at 1800°F

- Can be used for turning and facing most metals, except for nimonic alloys and titanium. Mild steels can be cut at speeds up to 1500 fpm.

- These tools are best used in continuous cutting operations

- There is no occurrence of welding, or built up edges

- coolants are not needed to cool the workpiece

- Very high hot hardness properties

- often used as inserts in special holders

 

• Diamonds

- a very hard material with high resistance to abrasion

- very good for turing and boring, producing very good surface finish

- operations must minimize vibration to prolong diamond life

- also used as diamond dust in a metal matrix for grinding and lapping. For example, this is used to finish tungsten carbide tools

 

• Cemented Oxides

- produced using powder metallurgy techniques

- suited to high speed finishing

- cutting speeds from 300 to 7500 fpm

- coolants are not required

- high resistance to abrasive wear and cratering