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Metal is cut either by moving work piece like in lathe or by moving tool like in shaping machine, drilling or milling machine. Clearance angles must be provided to prevent tool surface below cutting edge from rubbing against work piece. Rake angles are often provided on cutting tools to cause a wedging action in formation of chips and to reduce friction and heat.
In order to remove chips from a work piece, a cutting tool must be harder than work piece and must maintain a cutting edge at temperature produced by friction of cutting action.
Carbon steel tools even though comparatively inexpensive tend to lose cutting ability at temperatures around 400 degree F (205 degree C).
High-speed steel, containing 18 percent tungsten, 4 percent chromium, 1 percent vanadium, and only 0.5 to 0.8 percent carbon, permits operation of tools twice or three times speeds allowable with carbon steel
Cast-alloy cutting-tool materials containing cobalt, chromium, and tungsten are effective in cutting cast iron and retaining their cutting ability even when red hot.
Cemented Tungsten Carbide
The hardness of Tungsten Carbide approaches that of a diamond. Tungsten carbide tools can be operated at cutting speeds many times higher than those used with high-speed steel.
Ceramic, or oxide, tool tips consist primarily of fine aluminum oxide grains, which are bonded together. These are very hard.
An overheated tool can become blunt and soft very fast. Therefore very often, cooling fluids cools cutting points of tool. This serves to lubricate and cool.
Water is an excellent cooling medium, but it corrodes ferrous materials. Sulfurized mineral oil is one of most popular coolants as it can both cool as well as lubricate. The sulfur prevents chips from work from melting on to tip of tool.
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