Tech Talk
What is "Cubic Boron Nitride (CBN)"?
There are two major suppliers of CBN abrasive: GE Superabrasives and DeBeers. Originally each company had its own trade name: Borazon, CBN, and Amborite, ABN, respectively. Both companies have since agreed to call what is essentially the same mineral, "CBN".
CBN is manufactured by a process which requires very high temperatures and pressures. It is synthesized in crystal form from hexagonal boron nitride whichis commonly called "white graphite", derived from the pryolysis of boron chloride-ammonia (BCI3-NH3). The hexagonal borono nitride, composed of atoms of boron and nitrogen, is combined with a catalyst like metallic lithium in an envrionment in the range of 1650 degrees C, and pressures of up to 68,000 bar (1,000,000 psi). A black, opaque grain results. De Beers used lithium nitride as their catalyst and found that a yellow, translucent grain results, hence, their designation Amber Boron Nitride (ABN). The intense heat and pressure in the presence of the catalyst causes the nitrogen atom to donate an electron to a boron atom which then forms a chemical bond to the nitrogen atom and forms a very strong crystalline structure similar to that of diamond. The crystals are blocky in shape with noticeably sharp edges and corners. Until the mid-19070s CBN was manufactured in only two forms: Type I (GE) or ABN 300 (De Beers), consisting of 40 percent by weight uncoated and 60 percent nickel coated. The nickel coating provides increased bond streangth for use in resin bonds.
The CBN referred to above has a grain structure which fractures along cleavage planes. The grain therefore wears until the force on the grain is sufficient to cause shearing of the cleavage plane to expose a sharper, keener cutting edge. This shearing force is very high due to the high hardness and toughtness of the abrasive. Diamond truing is therefore the best method for forming or truing very fine detail in the grain on the periphery of a grinding wheel. Diamond has the hardness, and therefore the ability, to fashion the shape of the surface grains. The advent of reliable vitrified bond systems for superabrasives makes crush dressing practical, yet for the finest detail, diamond dressing systems are superior.
A new type of CBN grain structure has been developed by both GE Superabrasives and De Beers. The new grains are types 500, 510, 550, 560, and 570 CBN by GE and ABN 600, 610, and 660 by De Beers. This new structure is microcrystalline. The grains tend to be quite large, as large as FEPA B851 (20/30 U.S. mesh), and appear to sharpen themselves in-process by very fine crystallographic breakdown. This attritious wear mechanism therefore exposes an aggressive surface as opposed to a worn flat surface typical in monocrystal grain. CBN grinding wheels manufactured with the microcrystalline grains should be expected to require less truing and dressing.
Compared with aluminum oxide and silicon carbide, CBN has minimal wear and therefore stays sharper for a longer period of time between dressing. An individual CBN grain will exhibit a life 100 times that of an aluminum oxide grain. Because of its inherent sharpness CBN tends to machine cooler, providing high surface integrity and superior surface finish. The cost of the abrasive is high; however, the benefits dervied from the dramatically reduced amount of wheel dressing and the quality of the workpiece surface may be advantageous. It is improtant to appreciate that CBN requires a very rigid machine tool with the correct truing and dressing method employed. Machine tools designed along the more traditional lines of a grinding machine do not have the vibrational stability nor the high speed capability required for the economical use of CBN. CBN has been shown to perform more successfully with increased wheel speed, providing high stock removal rates and minimal wheel wear. High-speed grinding tests, using plated CBN wheels, have been carried out in excess of 300 ms-1 (60,000 sfm) and have shown longer wheel life and better quality workpiece surfaces. There are machines working in production in the United States using plated CBN wheels, running in excess of 150 ms-1 (30,000 sfm). At normal speeds there is one disadvantage with CBN in that it does not machine very soft or gummy materials easily. The plated grinding wheels clog and load up with the soft, sticky material. At ultrahigh speed however, it would appear that the chip formation changes and brittle fracture occurs in the softer materials, such that at high speed it would appear that both hard and soft materials will machine alike. The demands on the machine tool for dynamic stability under high-speed conditions are great. CBN, along with diamond, has been termed a "Superabrasive", so similarly it requires "super" machine tools which have been designed with the specific requirements of CBN and diamond abrasives in mind. The truing and dressing methods for these abrasives require high dynamic rigidity of the machine tool as well as the truing and dressing mechanism in order to cope with the inherent vibration experienced with surperabrasives. Dynamic wheel balancing is essential.
