6) Cutting Tool Materials

- lathe cutting tools are single-point cutting tools
- as known as toolbits.
- usually manufactured from one of four materials
- high-speed steel
- cast alloys
- cemented carbides
- aluminum oxide
- each of the materials having its unique advantages and have same characteristic
- the cutting tools material is harder than the material to be cut
- most effective cutting material may be selected for a particular machining operation,knowledge of the capabilities of cutting tools is a necessity.

6.1 High-Speed Steel
- as known as HSS
- probably the most common cutting tool material used on an engine lathe
- toolbits made of HSS are inexpensive (compared to other materials)
- capable of withstanding heavy shocks or interrupted cuts
- retain their sharp cutting edge at temperatures as high as 1100⁰F
- HSS toolbits are easier to grind than those made of other types of material and are available in a variety of sizes and shapes.
- molybdenum, vanadium, tungsten, chromium, and cobalt are principal elements used to make HSS tools
- classified as 18-4-1, that is, 18 percent tungsten, 4 percent chromium and about 1 percent vanadium, with the remainder being iron.
- carbon content of HSS is generally 0.75 to 0.9 percent
- when higher cutting speeds are needed, the cobalt or high vanadium types are used
- cobalt high-speed steels contain cobalt in amounts varying from 5 to 12 percent
- the presence of cobalt increases the toolbit's "red hardness", the ability to cut at elevated temperatures
- abrasion resistance is improbed somewhat with cobalt, but there is a decrease in the ability of the material to withstand shocks
- high vanadium HSS contain up to 5 percent vanadium and somewhat more carbon
- the increase of vanadium in HSS provides improved resistance to abrasion

6.2 Cast Alloys
- cast alloy toolbit materials are composed of varying amounts of tungsten, cobalt, and chromium
- combination of these materials allows cast alloy cutting tools to retain their hardness up to 1300⁰F
- above 1500⁰F the tools lose their hardness, but when cooled they reharden without heat treatment
- nonmachinable
- cast to shape and finished by grinding and polishing operations
- as a rule, cast alloy toolbits are more brittle than tools made from HSS, but they will withstand 20 to 50 percent higher cutting speeds and more sever abrasion
- this material is more difficult to grind than HSS
- should never be cooled in water during the grinding process
- recognized by such trade names as Stellite, Armide, and Tantung

6.3 Cemented Carbides
- are composed primarily of carbon mixed with tungsten, tantalum, or titanium powders and bonded by cobalt in a sintering process (powdered metallugy)
- a typical tungsten carbide toolbit is composed of 94 percent tungsten and 6 percent pure carbon by weight
- varying amounts of cobalt (the binder)n are used to regulate the toughness of the cutting tool
- higher amounts of cobalt result in greater toughness, with a decrease in the hardness of the cutting tool material
- titanium and tantalum are added as alloying elements in varying amounts to change the cutting characteristics of cemented carbides
- it necessary to be very careful when considering the use of carbides as a cutting tool
- many factors must be analyzed prior to using this material and the manufacturers' specifications shoudl be consulted prior to selection
- the disadvantage of having to select the proper type of carbide is far outweighed by the advantages carbides give in machining
- it have excellent red hardness capabilities, can remove large amounts of materials in a short period of time, have high hardness and are capable of cutting speeds three to four times greater than HSS cutting tools
- considerable care is required when sharpening cemented carbides
- special wheels as known as silicon carbide or diamond must be used
- used on a lathe in two ways which is as tips brazed on special shanks and as throwaway inserts
- brazed carbides are available in a variety of shapes and size
- the carbide tip is brazed to a steel shank, and the shank is held in the toolholder of the lathe
- throwaway inserts are generally used more often in industry than brazed tips
- they are held in special holders by some type of clamping device
- it is simply indexed to the next side, clamped, and ready for continued use when a cutting edge on a carbide insert become dull
- means a square-shaped insert that is held in a negative toolholder can be indexed four times, turned over, and indexed four more times, for a total of eight usable cutting edges
- carbide insert is then throw away for saves time in production operations by eliminating the necessity of resharpening a dull tool

6.4 Oxides
- one of the latest developments in cutting tool materials is the use of aluminum oxide
- toolbits made with this material are generally referred to as ceramic cutting tools
- the manufacturing process of ceramic tools is similar to that of carbide cutting tools
- used as the principal material, with nickel as the binder
- pressures exceeding 400lb and temperatures as high as 3000⁰F are used in the sintering process
- carbides, ceramics are generally used as throwaway inserts
- ceramic cutting tools have very low heat conductivity and high compressive strength but are quite brittle
- extreme care must be taken while using ceramics, and setups must be rigid to avoid breaking the cutting tool
- ceramic tools can outperform carbides in rigid and powerful machines
- they are not generally used where interrupted cuts are required
- they are very resistant to abrasive wear and can withstand cutting speeds two to three times faster than carbide tools