Exploring Advanced Manufacturing Technologies. Steve Krar
for grinding cemented carbides, carbide/steel combinations, nonferrous and nonmetallic materials, and many products such as natural stone, concrete, and masonry. The four main manufactured diamonds are:
▪TYPE RVG DIAMOND is an elongated, friable crystal with rough edges, Fig. 3-1-4A, and consists of thousands of tightly bonded small crystals that make up each abrasive grain. Type RVG (resin and vitrified) wheels are used to grind ultra-hard materials, such as tungsten carbide, and tough, abrasive nonmetallic and nonferrous materials.
▪TYPE CSG 11 DIAMOND, Fig. 3-1-4B, is designed to grind cemented carbide brazed tools where it may be necessary to grind both the carbide and some of the steel shank supporting the carbide insert.
▪TYPE MBG-11 DIAMOND, Fig. 3-1-4C, is used for grinding glass, ceramics, and carbides. The wheels with MBG (metal-bond grinding) abrasive have a metal bond to hold the tough diamond crystals in the wheel.
▪TYPE MBS DIAMOND - The Type MBS (metal-bond sawing) diamond, Fig. 3-1-4D, is used in metal-bond saws to cut granite, concrete, marble, and a variety of masonry and refractory materials.
Fig. 3-1-3 The high-pressure, high-temperature belt apparatus used for manufacturing diamonds. (GE Superabrasives)
Metal Coatings
The RVG diamond abrasive can be coated to prevent the diamond crystals from being pulled out from the resin bond. Coatings, such as nickel and copper provide better retention (holding power) for the RVG crystal in the wheel bond.
▪TYPE RVG-W (Resin, Vitrified, Grinding—Wet) is an RVG diamond with a special nickel coating that covers all surfaces of the crystal, providing a better holding or bonding surface for the resin bond, and results in much longer grinding wheel life.
Fig. 3-1-4A Type RVG diamond is used to grind hard, abrasive nonferrous materials. (GE Superabrasives)
Fig. 3-1-4B Type CSG-11 diamond is used to grind carbide and steel combinations. (GE Superabrasives)
Fig. 3-1-4C Type MBG-11 diamond is used to grind carbides, glass, and ceramics. (GE Superabrasives)
Fig. 3-1-4D Type MBS diamond is used primarily for grinding stone, marble, concrete, and masonry products. (GE Superabrasives)
▪TYPE RVG-D (Resin, Vitrified, Grinding—Dry) is an RVG diamond with a special copper coating that improves the bonding strength of the diamond in the wheel and controls its fracturing (tiny particles breaking away) under the stresses of grinding.
Under the pressure and temperatures created when grinding ferrous metals, diamond will react chemically and result in excessive diamond wear.
WORK MATERIALS
Diamond is used to machine and grind hard, abrasive nonferrous, nonmetallic, and composite materials. It is not recommended for grinding and machining ferrous materials because of the chemical characteristic known as carbon solubility potential, where steels will react with any source of carbon to absorb carbon into their surface. The reaction occurs under the temperature and pressure created during the grinding or machining process, thus causing excessive wear of the diamond-cutting tool, Fig. 3-1-5.
CUBIC BORON NITRIDE
A major breakthrough in the precision high-production grinding of hard, difficult-to-grind ferrous metals, was the discovery and manufacture of cubic boron nitride. CBN is twice as hard as aluminum oxide, and its performance on hardened steels is far superior. CBN is cool cutting, chemically resistant to inorganic salts and organic compounds, and can withstand grinding temperatures up to 1832°F (1000°C) before breaking down. Because of the cool-cutting action of CBN wheels, there is little or no thermal (heat) damage to the surface of the part being ground. The main benefits of grinding wheels made of CBN abrasive are shown in Fig. 3-1-6.
Manufacture
CBN is manufactured in crystal form from hexagonal boron nitride, sometimes referred to as white graphite. Hexagonal boron nitride, which is composed of boron and nitrogen atoms along with a solvent catalyst, is converted into cubic boron nitride through the application of heat (3000°F or 1650°C) and pressure (up to 1,000,000 lbs./sq. in., or 68,500 atmospheres). The combination of high temperature and high pressure causes each nitrogen atom to donate an electron to a boron atom, which uses it to form another chemical bond to the nitrogen atom. This produces a strong, hard, blocky, crystalline structure similar to diamond.
Fig. 3-1-5 Diamond tools react chemically, under the proper temperature and pressure conditions, when cutting ferrous metals. (GE Superabrasives)
Fig. 3-1-6 The main benefits of CBN grinding wheels. (GE Superabrasives)
CBN Types
There are various types of CBN available to suit a variety of steel grinding applications; CBN does not perform well on nonferrous or nonmetallic materials. Two main classes of CBN abrasive are monocrystalline and microcrystalline.
▪MONOCRYSTALLINE CBN - Monocrystalline CBN abrasive contains a large number of cleavage (break) planes along which a fracture can occur. This macrofracture (large break) is necessary for the abrasive grains to resharpen themselves when they become dull, Fig. 3-1-7.
▪MICROCRYSTALLINE CBN - Microcrystalline CBN abrasive consists of thousands of micron-size crystalline regions tightly bonded to each other to form a 100% dense particle. When the grains dull and the grinding pressure increases, they resharpen themselves by microfracturing (creating very small breaks), Fig. 3-1-8.
Table 3-1-1 lists the various types of abrasives and the workpiece materials for which each is best suited.
CHARACTERISTICS OF SUPERABRASIVES
The main physical properties of superabrasives that make them superior to conventional abrasives are shown in Fig. 3-1-9
▪Hardness - The harder the abrasive with respect to the workpiece, the more easily it can cut and remove material. The basic principle in material removal is that the cutting tool must be harder than the material being removed. Hardness of the cutting tool with respect to the material allows higher cutting speeds and greater feeds to decrease the amount of time required to complete the work cycle. Due to the higher hardness, superabrasive tools last longer.
Fig. 3-1-7 Monocrystalline CBN crystals macrofracture (large break)