MANUFACTURED DIAMOND

Diamond is used for truing and dressing grinding wheels and for the manufacture of diamond wheels. The need for a reliable source of diamond during World War II was realized when natural diamond was not readily available.

To produce diamond by a manufacturing process, the conditions of pressure and temperature found far below the earth’s surface had to be duplicated. This required a high-pressure, high-temperature belt apparatus capable of reproducing the conditions necessary for diamond growth. Graphite (a form of carbon) and a catalyst (such as iron, chromium, cobalt, and nickel) were subjected to high temperatures (2550° to 4260°F, or 1400° to 2350°C) and high pressures (800,000 to 1,900,000 lbs./sq. in. of 55,000 to 130,000 atmospheres) to form diamond crystals, Fig. 3-1-3. Under these conditions, the graphite is transformed into diamond and remains that way when it is cooled and the pressure is removed.

Fig. 3-1-3 The high-pressure, high-temperature belt apparatus used for manufacturing diamonds. (GE Superabrasives)

Types of Manufactured Diamond

There are many different types of manufactured diamond to suit various grinding applications. Manufactured diamond is available 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.

Fig. 3-1-4A Type RVG diamond is used to grind hard, abrasive nonferrous materials. (GE Superabrasives)

• 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.

Fig. 3-1-4B Type CSG-11 diamond is used to grind carbide and steel combinations. (GE Superabrasives)

• 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.

Fig. 3-1-4C Type MBG-11 diamond is used to grind carbides, glass, and ceramics. (GE Superabrasives)

• 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. 3-1-4D Type MBS diamond is used primarily for grinding stone, marble, concrete, and masonry products. (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.

• 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.

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.

Fig. 3-1-5 Diamond tools react chemically, under the proper temperature and pressure conditions, when cutting ferrous metals. (GE Superabrasives)

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.

Fig. 3-1-6 The main benefits of CBN grinding wheels. (GE Superabrasives)