Silver Lining

Jewelers learned centuries ago that silver makes a beautiful setting for diamonds. The shiny metal enhances the sparkle of jewel-grade diamonds and complements the colors that flash across their facets when light strikes them.

As it turns out, silver is also a good setting for diamonds in the industrial world. The precious-metal coating offers material properties that help retain diamond crystals in a grinding wheel’s matrix while simultaneously providing higher levels of productivity when grinding tungsten carbide.

Researchers at GE Superabrasives began working with silver coatings for diamond grinding applications in the late 1970s. However, just as the product was about to be made available commercially, the Hunt brothers moved to corner the silver market, driving prices from a few dollars per ounce to close to $50. Reluctantly, the silver-coated-diamond product rollout was halted.

Recently, silver-coated diamond has returned to the spotlight. A number of trends have contributed to this. The first is the availability of silver at economically viable prices. Prices, which have remained relatively stable since the Hunt brothers’ silver empire collapsed, currently are hovering around $5 per ounce.

The second is the growing use of high-precision grinding equipment featuring advanced capabilities. Today’s multiaxis machines can move in myriad directions and perform more custom cuts. These machines also permit the gang-stacking of various types of wheels, allowing them to be applied to the workpiece more rapidly. A series of wheels can perform numerous tasks on a round blank, for example, producing a complete endmill, drill or other round tool in a single chucking. This eliminates multiple machines, each performing a single job.

Silver-coated-diamond wheels would have worked on the machines of a quarter-century ago, but they couldn’t have provided the payback that can be realized today. Then, a grinding machine’s operation was heavily dependent on the operator. Today’s machines are computer-controlled and incorporate features such as automatic loading/unloading that lend themselves to a systems approach. The main cost today is not the operator’s time, but equipping the machine so it produces parts quickly and at a continuous rate. Stopping the machine to change a wheel dramatically lowers production. Manufacturers are willing to invest in higher-priced wheels if they lower overall costs.

A third trend favoring the use of silver is the increased acceptance of oil as a coolant when grinding tungsten-carbide parts. This is largely due to newer grinding machine’s being designed with self-contained, high-volume, high-pressure delivery systems that contain oil mist. Straight oil’s benefits stem from its superior lubricity and chemical stability, resulting in an extremely long life. It is not uncommon for straight-oil coolants to last for years.

Combined, these trends led GE Superabrasives to continue its research into the silver-coated diamond. The product that has resulted from this effort is the RVG*-Ag50 diamond.

The RVG Diamond has been an industry standard since the 1950s. It has the unique ability to fracture in a controlled manner. The microchipping of crystals continuously presents fresh cutting points to the workpiece, allowing the wheel to self-sharpen. The crystal is coated with silver (about 50 percent by weight).

Why Silver Shines

Silver has three characteristics that improve superabrasive grinding: lubricity, the ability to form a “spiked” coating and excellent heat-dissipation characteristics. Let’s look at each of these in detail.

Lubricity reduces friction. In grinding applications involving straight-oil coolants, the majority of lubrication comes from the oil. However, microscopic amounts of silver coating melt or are sheared off the crystal surface. Researchers believe that this silver is deposited on the wheel surface, which reduces friction between the wheel and the workpiece. Consequently, the heat-generated damage to the wheel and workpiece surface is limited.

Spikes increase crystal retention. The diamond used in resin-bond grinding wheels is typically metal-coated, a process that enlarges the crystal’s surface area. This allows crystals to firmly anchor themselves in the bond, prolonging grinding wheel life.

Figure 1: Photomicrograph of RVG-Ag50 Diamond with silver coating. Because of the increased surface area, a spiked coating resists crystal pullout better than an unspiked coating.

A spiked coating, which is used on the RVG-Ag50 Diamond (Figure 1), extends wheel life even more than a conventional coating. Evidence of how much can be found in performance tests conducted on diamond crystals with a conventional nickel coating and those with a spiked nickel coating. (The RVG-Ag50 Diamond only comes with a spiked coating, so spiked vs. conventional tests could not be performed with it.) Tests showed that nickel-spiked-diamond wheels had a grinding ratio more than twice as high as the conventional nickel coating when grinding a tungsten-carbide workpiece.

Silver cools the grinding environment. The most important characteristic is silver’s ability to conduct heat. Silver has a coefficient of thermal conductivity (458 watts/Kelvin meter) that is well above other metals commonly used for coating diamond, such as nickel (58.5 W/K m) and copper (350-370 W/K m). Silver allows for the rapid transference of heat from the small area where the heat is generated—the crystal/workpiece interface—to the large area of contact between the silver coating and the bond.

A resin bond is used with the RVG-Ag50 Diamond because of its resiliency and ability to “give” somewhat during grinding, which maximizes the crystals’ longevity. (Vitreous and metal or electroplated wheels would be too stiff and rigid.)

The crystal’s extended life, in turn, means the grinding wheel will maintain its form longer, reducing the amount of downtime required for reprofiling. At the same time, the properties of the silver coating allow the grinding machine to be run at more aggressive feed rates, increasing the overall process productivity.

Among the early users of wheels made from the new crystal is a leading endmill manufacturer. During field tests, the toolmaker used wheels made with RVG-Ag50 to grind flutes on 5/8" and 3/4" tapered carbide endmills. CITCO Operations produced the wheel (1V1-7x1/2, 15°), which was mounted on a Huffman grinder. Straight oil was utilized. The toolmaker reportedly increased fluting speeds by 80 percent while extending the time between truing by a factor of four.

Besides fluting drills, endmills and other rotating tools, the crystal could be used for OD grinding, pointing, land grinding and, in certain circumstances, polishing.

Use Moderation

Grinding experiments performed on tungsten-carbide parts have revealed that, contrary to traditional thinking, slower wheel speeds improve process performance when straight oils are applied. The G-ratios for one series of tests conducted with RVG-Ag50 are charted in Figure 2. As can be seen, dramatically better wheel life was achieved at a speed of 20 m/sec. than at higher speeds.

Figure 2: Better wheel life was achieved with RVG-Ag50
at a speed of 20 m/sec. than at higher wheel speeds.

Another benefit of slowing down is a decreased power requirement. Figure 3 compares grinding power vs. time for two different wheel speeds. The plateau areas represent approximately 80 seconds of grinding time. Note the smooth power plateau at just over 2kW for the wheel speed of 20 m/sec. vs. the curve for 30 m/sec., which ranges from 3kW to about 3.8kW.

Figure 3: Moderate wheel speeds also positively impact the required power.

The extra energy required at 30 m/sec. directly translates into detrimental heat at the wheel/workpiece interface. Also note the rise in the power requirement toward the end of the grinding cycle as workpiece growth and the actual depth of cut increase. The higher wheel speed of 30 m/sec. requires significantly more power and can result in unwanted levels of heat, causing workpiece expansion and/or cracking, as well as loss of tolerance control. This, in turn, can halt production.

When it comes to silver-coated-diamond grinding wheels, a good piece of advice is to go slower and get there quicker.

About the Authors
Markus Jakobuss is product leader for GE Superabrasives’ RVG-Ag50 Diamond. James Dailey is an application development engineer at the Worthington, Ohio, company.