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Note:Norton Company's latest case study compares vitrified the CBN superabrasives of today with vitrified aluminum oxide and single-layer CBN of yesterday.

Abstract
A new, high-performance superabrasive technology, vitrified CBN (cubic boron nitride), is meeting the rigorous grinding requirements of the aerospace industry. Today's new vitrified CBN superabrasives keep the tight tolerances while grinding cooler and removing more material with less wheel abrasion than previous technologies. CBN superabrasives are manufactured, thermally stable abrasive grit that is second only to diamond in hardness. Its hardness makes vitrified CBN grinding wheels especially applicable for grinding ferrous alloys and other aerospace materials. The principal difference between today's vitrified CBN versus similar products manufactured 10 years ago is increased porosity. Until now, the two traditional choices have been vitrified aluminum oxide and single-layer CBN.

Summary and Conclusion Highlights
Field tests involving creepfeed grinding of inconel Z-notch forms produced abrasive cost/piece that were 38 percent lower for vit CBN versus single layer (or plated) CBN.

align=right border=0>A new, high-performance superabrasive technology is meeting the rigorous grinding requirements of the aerospace industry. Today's new vitrified CBN (cubic boron nitride) superabrasives keep the tight tolerances demanded by the industry while grinding cooler and removing more material with less wheel abrasion than previous technologies.

Using the appropriate grinding machine, current-day vitrified CBN is easier and more effective to use than conventional grinding wheels in many of today's high-precision applications in the aerospace industry. For example, vitrified CBN wheels are being used with increasing frequency to grind Inconel and Rene alloys with Z-notch, root (or fir tree), and other forms.

CBN superabrasives are manufactured, thermally stable abrasive grit, which is second only to diamond in hardness. Its hardness makes vitrified CBN grinding wheels especially applicable for grinding ferrous alloys and other aerospace materials. CBN will not react with these materials, but diamond does. The vitrified bond is a glass material — similar to that used in conventional aluminum oxide abrasives — that is strong, durable, and brittle. It is very effective in high-precision, tight-tolerance grinding because of its ability to hold the abrasive, as well as its ability to be easily dressed.

Available from several manufacturers, these vitrified superabrasive wheels come with different amounts of CBN content. There is no industry standard for product coding, but, as a general rule, higher concentration numbers indicate higher amounts of CBN.

The principal difference between today's vitrified CBN versus similar products manufactured 10 years ago is increased porosity. Vitrified CBN manufactured today can match the porosity of aluminum oxide abrasives, while offering substantially greater hardness and therefore greater durability and longer life. Pore inducers enable manufacturers to offer 20 to 55 percent porosity. As in all abrasives, porosity facilitates coolant flow to the grinding surface for improved temperature control reduced sliding friction and removal of swarf.

Those familiar with only the older vitrified CBN of years past will automatically associate this technology with oil lubricants. Present-day vitrified CBN wheels are made to be used with water-soluble coolants, with the attendant benefits to the environment and to workers' safety and comfort.

Today's CBN superabrasives are available in segmented and continuous rim designs. With the segmented wheel, sections of the matrix (CBN, bond, porosity) are cemented to steel, aluminum, or ceramic cores. In the continuous design, the matrix is pressed and sintered onto a ceramic core in a continuous rim. The segmented design allows the properties of the wheel to be measured with far greater accuracy than continuous rim design to provide greater product consistency.

Measurability and their shape-holding capabilities are leading to increased application of vitrified CBN superabrasive wheels to creepfeed grinding, which is reserved for the grinding and finishing of complex parts made of hard-to-grind materials. Turbine blades and other components characterized by complex shapes and geometries that require ultra-tight tolerances are typical aerospace uses for this superabrasive.

Two Traditional Alternatives: Pluses and Minuses
Until now, the two abrasives of choice for aerospace applications have been vitrified aluminum oxide and single-layer CBN.

Vitrified aluminum oxide has been the traditional choice for the last 20 years, available in either a fused aluminum oxide abrasive grit or seeded gel (SG) abrasive, both held together by a vitrified glass bond. Until the current generation of vitrified CBN, vitrified aluminum oxide offered superior porosity, with all its attendant benefits over its superabrasive competition.

In some applications, it still remains the best choice. Vitrified aluminum oxide wheels are inexpensive and relatively easy to use. It is simple and inexpensive to dress complex forms into vitrified aluminum oxide wheels, as well as to re-dress the wheels. The shortcomings of this type of wheel include low CPK tolerance limits, with poor consistency grind-to-grind. And, the wheels are short-lived, often requiring many changes, sometimes per-part, when grinding large turbines. Because the user is dressing away large volumes of abrasive grit and bond, vitrified aluminum oxide wheels accumulate huge amounts of waste. This reduces the machine life and drives up maintenance costs of machines and filtration systems.

The second traditional choice in aerospace abrasives has been single-layer CBN. (Fig. 1) As its name implies, one layer of CBN abrasive grit is plated or braised onto a steel body, or preform. This makes single-layer CBN applicable to intricate shapes, because the abrasive follows the shape of the preform. It is easy to use with little or no dressing required, and can be used on the most modern, high-technology grinders as well as on older, low-tech machines. Although more expensive than vitrified aluminum oxide wheels, single-layer is the least expensive CBN wheel. Its ability to hold intricate forms and its relative hardness make it ideal for small-batch orders. With a 100-piece order of a small complex part, job shops can often use one wheel to finish the job, then discard it.

The most critical shortcoming of single-layer CBN is its inconsistency in the depth of crystal plating wheel to wheel. The orientation of the crystal is random and held in the matrix longer than with any CBN abrasive, including vitrified CBN. This makes it difficult to estimate wheel life. As a general rule, the crystal should be 50 percent of the diameter that needs to be plated. As a result, single-layer CBN is not the best choice for large batches, where frequent wheel changes degrade productivity. With large batch runs common to the aerospace industry, single-layer CBN is not the product of choice.

Vitrified CBN vs. Vitrified Aluminum Oxide vs. Single-Layer CBN
We recommend vitrified CBN over both vitrified aluminum oxide and single-layer CBN for most aerospace applications. One of the most important trends in grinding today is toward smaller, stiffer grinding machines. These machines are required for the increasingly prevalent one-piece flow manufacturing processes. Conventional abrasives are not appropriate for these applications because they wear out so quickly that they require too-frequent wheel changes. The Japanese approach to manufacturing, Keisen, also demands compactness. It calls for the grinding wheel to be no more than 10 times the size of the part, and for the production line to be no more than 500 times the size of the part. This efficient use of space can be achieved only with the new, smaller grinding machines, which require vit CBN.

Vit CBN usually offers the highest performance at the lowest total per-piece or per-shift cost. For a given application, a superabrasive wheel may cost $5,000 and a conventional abrasive wheel only $150. But, you may need 40 of those conventional wheels where one superabrasive wheel suffices. Then, add filters and coolants, disposal of filtered-out waste, disposal of wheel stubs, storage space for 40 wheels, time to change 40 wheels at prevailing wage rates, and shop floor space. In combination, all these factors make vit CBN less expensive. Vit CBN also holds very tight tolerances, grinds cooler, and provides the safety, environmental and worker-comfort benefits of water-soluble coolants. Vitrified CBN draws low power (Fig. 2), and is very durable. It will not break down or require re-dressing on even large production runs. Internal tests documented that we could grind 40 parts, each eight inches long, incurring less that 0.00015 in. wear, at high metal removal rates (Fig. 3). We were also able to achieve a surface finish below 20 µin. Ra, versus a typical finish requirement of 63 µin. Ra (Fig. 4).

In field trials, we ground the root forms of more than 60 high-pressure turbine blades without dressing, holding tolerances of less than 0.0005 in. We compared the overall performance of aluminum oxide abrasives vs. vitrified CBN (Fig. 5), finding that vit CBN registered a G ratio of 1,700 — 400 to 500 times the G ratio of aluminum oxide.

Most important, while a vitrified aluminum oxide wheel may last one hour or one day (depending on the material to be removed) a vitrified CBN wheel may last six months to one year in a similar application.

For these reasons, the most advanced grinding machines, with their small footprints and superior stiffness, are being designed especially for vitrified CBN abrasives. This relegates conventional abrasives to the larger, and sometimes older, workstations (of which there is, to be sure, a large installed base). The price for the entire package of smaller machines, vitrified CBN, filtration system, and tool changer is usually in the $300,000 to $400,000 range. This contrasts with a typical price of a large, traditional grinding system of $500,000 to $1.5 million.

The new, smaller systems — with their glass scale encoders and their ability to use vitrified CBN wheels — offer better repeatability. Because the older abrasives require continuous dressing as the wheel is in use, repeatability is diminished to 0.0002 to 0.0003. But, vitrified CBN wheels do not require continuous dressing. Therefore, they can produce parts with repeatability tighter than 0.0001.

In aerospace and virtually every other manufacturing industry, workpart tolerances and surface finish requirements have tightened. At the same time there is greater attention than ever on achieving the lowest total cost per part. In some instances, current grinding systems can be modified to meet these stiffer requirements. However, as machine and abrasive wheel technologies improve, only the best performance at the lowest per-part cost will suffice. Today's porous, high-performance vitrified CBN abrasive wheels, with their ability to hold the tightest tolerances, grind cooler, and last longer are rapidly becoming the technical and economical technology of choice.

Authors: Stephen G. Hess and Patrick D. Redington, Norton Company (Worcester, MA); Rounan Li, Saint-Gobain (Worcester, MA); and Martin Cooper, GE Aircraft Engines (Bromont, Quebec)

Norton Company, 1 New Bond St., Box 15008, Worcester, MA 01615-0008. Tel: 508-795-5000.

Source: Norton Company