Page 631 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

612 Chapter 22 Cutting-Tool Materials and Cutting Fluids

provides only limited cooling, but is very effective at clearing chips from the cutting
interface.

Cryogenic Machining. More recent developments in machining include the use of
cryogenic gases such as nitrogen or carbon dioxide as a coolant. With small-diameter
nozzles and at a temperature of -200°C, liquid nitrogen is injected into the cutting
zone. Because of the reduction in temperature, tool hardness is maintained and tool
life is enhanced, thus allowing higher cutting speeds. Also, the chips are more brittle;
hence, machinability is increased. Furthermore, the nitrogen simply evaporates and
therefore has no adverse environmental impact.

SUMMARY

° A wide variety of cutting-tool materials have been developed over the past century,
the most commonly used ones being high-speed steels, carbides, ceramics, cubic
boron nitride, and diamond. Tool materials have a broad range of mechanical and
physical properties, such as hot hardness, toughness, chemical stability and inert-
ness, and resistance to chipping and wear.
° Various tool coatings have been developed with major improvements in tool life,
surface finish, and the economics of machining operations. Common coating ma-
terials are titanium nitride, titanium carbide, titanium carbonitride, and alu-
minum oxide; diamond coatings also are gaining acceptance. The trend is toward
multiphase coatings for even better performance.
° The selection of appropriate tool materials depends not only on the material to be
machined, but also on process parameters and the characteristics of the machine
tool.
° Cutting fluids are important in machining operations and reduce friction, wear,
cutting forces, and power requirements. Generally, slower cutting operations and
those with high cutting-tool pressures require a fluid with good lubricating char-
acteristics. In high-speed operations, where the temperature rise can be signifi-
cant, fluids with good cooling capacity and some lubricity are required. The
selection of cutting fluids must take into account their possible adverse effects on
the machined parts, on machine tools and their components, on personnel, and
on the environment.

KEY TERMS

Alumina-based ceramics Cutting fluids Multiphase coatings Stellite
Carbides Diamond coatings Nanocrystalline Titanium carbide
Cast-cobalt alloys Diamond tools Near-dry machining Titanium nitride
Ceramics Dry machining Polycrystalline cubic Tool costs
Cermets Finishing cuts boron nitride Tool reconditioning
Chemical stability Flooding Polycrystalline diamond Toughness
Chip breaker High-speed steels Reconditioning of tools Tungsten carbide
Coated tools Inserts Roughing cuts Uncoated carbides
Coolants Lubricants Sialon Wear resistance
Cryogenic machining Micrograin carbides Silicon-nitride-based Whisker-reinforced tools
Cubic boron nitride Mist ceramics