5.7.6.3 Cutting
Carbide tools have traditionally been used
for turning, cutting and drilling. These conventional cutting
materials, based on metals, can only withstand a limited thermal
stress, and do abrade. They are therefore, typically, supplied
with cooling lubricant.
Ceramic-based materials retain their strength
at significantly higher operating temperatures. These new
materials therefore permit higher machining speeds with higher
process reliability and with the omission of cooling lubricants.
This is not just helpful from the production point of view,
but is environmentally friendly due to the question of disposal.
A range of such cutting materials is now
available on the market based on aluminium oxide, silicon
nitride and mixed ceramics, supplemented by polycrystalline,
cubic boron nitride (PCBN). These are also outstandingly suitable
for cutting hard materials with high values of Vickers hardness.
In addition to simple hard turning, the use of hard turning
before grinding is increasingly found in practice. Mixed ceramics
and PCBN cutting materials are suitable for precision hard
turning of workpieces with hardnesses of up to 64 HRC. Production
times and production costs are significantly reduced through
the high-speed machining of cast-iron materials and in hard
cutting.
Silicon nitride ceramics have won a significant
market share in high-speed turning, drilling and cutting of
cast-iron in the manufacture of large series for the automobile
industry and its suppliers. Mixed ceramics including polycrystalline
cubic boron nitride are also being used more and more to replace
grinding with turning.
Cutting ceramics must have significantly
greater hardness than the material to be cut. This high hardness
does, however, finally have an effect on the toughness of
the cutting materials. Literature therefore often contains
advice to the effect that cutting ceramics cannot be used
for intermittent cutting. It must, however, be pointed out
that it is perfectly possible to use PCBN materials to machine
with intermittent cutting.
|
