Superabrasives are cutting tools made from polycrystalline diamond (PCD) or cubic boron nitride (PCBN). PCD is used for non-ferrous materials, while PCBN is used for ferrous materials. These materials require diamond cutting wheels and new techniques to use properly. The grinding profession has developed new machinery and processes for working with these materials. Diamond wheels alone are capable of grinding superabrasives.
The term superabrasives was used to describe new cutting tool options based on a polycrystalline diamond (PCD) material developed by the General Electric (GE) Company in the United States in the 1970s. These very hard and brittle compounds represented an improvement in the grinding and cutting tools available, although they required new techniques to use properly. The materials are so hard that only diamond cutting wheels can be used to shape cutting tools made from these compounds.
In order for one material to polish another, the first material must be harder than the second. Hardness is a semi-quantitative value and refers to the overall ability of a material to avoid cracking upon the application of force. Hardness is usually associated with toughness in use and long life. The materials used in critical parts have become harder and more resistant to abrasion as improvements in metal alloys and bi-metal structures have been developed. These parts must be machined to specifications to meet the tightest tolerances, forming alone is not enough.
As metal parts became stronger, harder cutting tool materials were developed, including carbon tool steel, high speed steel, and cast alloys. Ceramic-based materials have also been employed, such as in cemented carbides and cermets. The introduction of superabrasives has immensely expanded the options available to toolmakers.
GE PCD superabrasives consist of a micron-sized layer of diamond — 3.3 × 10-6 feet or 1 × 10-6 meters — bonded to a carbide substrate. PCD is a synthetic diamond made up of multiple crystals. As you polish, these crystals break off and leave sharper edges for you to continue polishing. The sharp edges of the broken particle clear the grinding surface more easily, avoiding clogging of the surface. This self-cleaning attribute allows the wheel to operate at higher speeds, reduces electrical load, and produces less heat.
Polycrystalline cubic boron nitride (PCBN) is a similar class of superabrasives that substitute cubic boron crystals for diamond crystals. PCD is used for non-ferrous applications, such as aluminum and copper, and for organic materials, such as plastics, rubber compounds, and wood. PCBN materials are used for ferrous applications, such as gray cast iron. In both types of superabrasives, the medium by which the agent is bonded to the substrate is also important to performance. A vitrified bond appears to be superior to metallic or resinous bonds in many applications, showing the top edge finish of metal bonding agents and the aggressive shear of resinous agents.
To use superabrasive cutting tools, tool makers had to learn new techniques. Previously successful combinations of ground speed and wheel speed would not produce quality products. As a result, the grinding profession has developed new machinery and processes for working with the new materials.
PCD and PCBN cutting tools can only be ground by themselves with harder materials. Diamond wheels alone are capable of grinding superabrasives. Wheels must be accurately specified for diamond type, crystal size, and friability to efficiently grind these super-hard materials.
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