CBN in space: vitrified CBN superabrasives prove effective in aerospace applications. (Grinding).

Stephen G. Hess

The cost-performance ratio of the latest vitrified bonded cubic boron nitride (CBN) grinding wheels make them a technical and economical tooling choice for most aerospace applications. Their advantages in terms of quality, throughput and lower total grinding cost more than compensate for the higher grinding wheel purchase price. These latest vitrified CBN superabrasives should be considered whenever grinding needs include high volume and adherence to tight tolerances, and where the enduser has invested in advanced, small-footprint, stiff, grinding machines.

Careful analysis of the application will be necessary because the cost-per-piece of the vitrified CBN abrasive (i.e., the price of the abrasive itself needed to grind a single workpiece), can be higher than for conventional vitrified aluminum oxide wheels. The latest vitrified CBN wheels also are more expensive than single-layer CBN wheels, the other traditional superabrasive alternative. However, the total process cost-per-piece often will be 25to 35-percent less with vitrified CBN superabrasive.

Because the total cycle time using vitrified CBN wheels is shorter than when using aluminum oxide or single-layer CBN wheels, users will require fewer grinding machine hours to accomplish a given amount of work. Also, because vitrified CBN wheels are longer lasting than the alternatives, there will be fewer wheel changes, which translates into fewer setups and greater up-time.

The current trend toward smaller, stiffer grinding machines is another reason to select vitrified CBN wheels. These machines provide the higher accuracy and improved repeatability needed to take advantage of the superior durability of today's vitrified CBN wheels. These grinders also provide the flexibility needed for one-piece flow manufacturing processes. Conventional abrasives are not appropriate for these applications because they wear out too quickly and require frequent wheel changes.

Comparing numbers

For a given application, a superabrasive wheel may cost $5,000 and a conventional abrasive wheel only $150. However, field tests have shown that as many as 40 conventional wheels will be needed to generate the same number of parts that one superabrasive wheel can produce. Plus, we have to add in the additional grinding time, greater consumption of filters and coolants, the cost to dispose of filtered waste, disposal of wheel stubs, storage space for 40 wheels, manhours to change 40 wheels at prevailing wage rates, and additional shopfloor space.

In combination, all of these factors make vitrified CBN less expensive. Vitrified CBN also holds very tight tolerances, grinds cooler, and provides the safety, environmental and worker-comfort benefits of water-soluble coolants.

Vitrified CBN draws less power and is very durable. It will not break down or require re-dressing--even on large production runs. Internal tests documented that we could grind 40 parts, each 8" long, while incurring less than 0.00015" wear, at high metal removal rates. We also were able to achieve a surface finish below 20 [micro]-in. Ra, versus a typical finish requirement of 63 [micro]-in. Ra.

In field trials, we ground the root forms of more than 10 high-pressure turbine blades without dressing, while holding tolerances of better than 0.0005". We compared the overall performance of aluminum oxide abrasives with vitrified CBN and found that the latter registered a G-ratio of 1,700-- more than 400 times the G-ratio of aluminum oxide. In addition, while a vitrified aluminum oxide wheel may last one hour or one day (depending upon the material being removed), a vitrified CBN wheel may last three to nine months in a similar application.

For these reasons, the most advanced grinding machines, with their small footprints and superior stiffness, are being designed specifically for vitrified CBN abrasives. This relegates conventional abrasives to the larger, and sometimes older, workstations--of which there is a large installed base.

The price for the entire package of smaller machines, vitrified CBN, filtration system, and tool changer, usually is in the $300,000 to $400,000 range. This contrasts with a typical price for a large, traditional grinding system of $500,000 to $1.5 million.

The new, smaller systems offer better repeatability with their glass scale encoders and their ability to use vitrified CBN wheels. Because aluminum oxide abrasives often require continuous dressing as the wheel is in use, repeatability is diminished to 0.002" to 0.003". Vitrified CBN wheels do not require continuous dressing so they can produce parts with repeatability tighter than 0.001".

Pluses and minuses

Until now, the two abrasive choices for aerospace applications have been vitrified aluminum oxide and singlelayer CBN. Vitrified aluminum oxide has been the traditional choice for the last 20 years. It is available either in a fused aluminum oxide abrasive grit or seeded gel (SG) abrasive. Most are held together by a vitrified glass bond. Until the current generation of vitrified CBN wheels, vitrified aluminum oxide offered superior porosity along with all of its attendant benefits over its superabrasive competition.

In some applications, vitrified aluminum oxide wheels remain the best choice. They 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 redress the wheels.

The shortcomings of this type of wheel include low CpK tolerance limits, with only adequate consistency grind-to-grind. In addition, 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 large amounts of waste. This reduces machine life and drives up maintenance costs of machines and filtration systems.

The second traditional choice in aerospace abrasives has been single-layer CBN. 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 abrasive follows the shape of the pre-form.

This grinding tool is easy to use with little or no dressing required. It can be used on the most modern, high-technology grinders as well as on older, low-technology machines. Although they are more expensive than vitrified aluminum oxide wheels, single-layer CBN is the least expensive form of CBN wheel.

With a 100-piece order of a small, complex part, job shops often can use one single-layer CBN wheel to finish the job, and then discard it.

The most critical shortcomings of single-layer CBN is its inconsistency in the depth of crystal plating from wheel to wheel and the ability to hold part tolerances of 0.004" or better. 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 result, single-layer CBN is not the best choice for large batches where frequent wheel changes degrade productivity.

In aerospace and virtually every other manufacturing industry, workpiece tolerances and surface finish requirements have tightened with part demand continuing at a very high level. 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, rapidly are becoming the technical and economical technology of choice.

Norton Co., Worcester, MA, http://www.RSLeads.com/109tp-374 or circle 374