I have to study some technical English :( It would help me. Thank you in advance!
Preformed tool tips made from metallic carbides are harder than Stellite, cheaper than Stellite, and capable of operating at the same temperatures. Carbides can only be machined by grinding using silicon carbide abrasive wheels.
Since carbides are very brittle compared with metallic cutting-tool materials they have to be securely supported by the tool shank, to which they may be brazed or clamped.
Carbide cutting tools fall into three categories:
1) TUNGSTEN CARBIDE:
Tungsten carbide is very hard and brittle and is used to machine such materials as grey cast iron and cast bronzes. These metals have a relatively low tensile strength and form a discontinuous chip. However, they have a hard and abrasive skin as a result of the casting process. The tungsten carbide particles are mixed with metallic cobalt particles to form a composite cutting material after sintering.
2) MIXED CARBIDES:
Mixed carbides are mixtures of tungsten and titanium carbides. They are less hard and abrasion resistant than straight tungsten carbide, but they are very much stronger and tougher and are used for cutting high-strength materials. They are also less porous than straight tungsten carbide and are therefore less prone to form a built-up edge. The comments concerning particle size and quantity apply equally to mixed carbides.
3) COATED CARBIDES:
Coated carbides are more expensive than tungsten or mixed carbides but can be run at cutting speeds up to 30 per cent greater than those recommended for tungsten or mixed carbides without any reduction in tool life. Tungsten or mixed-carbide tool tips are coated with a very hard and abrasion-resistant film of titanium nitride (TiN). They cannot be reground as this would destroy the coating, hence coated carbides are only available as disposable inserts.
The use of throwaway inserts has a number of advantages:
1) The inserts are mass-produced and, relatively speaking, they are very cheap, much cheaper than the cost of regrinding.
2) The tolerances on the inserts are such as to ensure consistent cutting performance from one insert to the next.
3) The tolerances on the inserts also ensure there is no need to reset the tooling after changing an insert. This reduces the downtime and lost production through tool replacement.
