Superabrasive Safety

Imagine a glass disc spinning at a very high speed. The grinding-wheel manufacturer’s task is to prevent this disc from breaking. As superabrasive wheels are designed to operate at ever-increasing peripheral speeds, safety becomes an even greater concern. When a wheel breaks at high speed, it can cause considerable damage and injury. To prevent accidents caused by grinding-wheel failure, the manufacturer must ensure that the development of new diamond and cubic-boron-nitride (CBN) superabrasive products does not outpace the applicability of existing safety standards.

In a rotating disc, stresses generated by centrifugal force tend to pull the disc apart. These stresses, which increase as rotational speed increases, are highest at the wheel bore and decrease toward the periphery. If the disc material cannot withstand these stresses, the wheel will fail.

Wheels can be divided into four categories: vitrified-bond, resin-bond, metal-bond, and electroplated wheels. Each type can use conventional, diamond, or CBN abrasives. Except for electroplated wheels, all types are considered bonded-abrasive wheels.

Because vitrified- and resin-bond grinding wheels are inherently brittle, they’re more susceptible to fracture than are metal-bond and electroplated wheels. In a vitrified-bond wheel, the stresses generated by centrifugal force may cause the glass bond at the wheel bore to fail. To address this problem, the abrasives industry adopted a safety standard specifically for vitrified- and resin-bond grinding wheels. However, grinding-wheel manufacturers have raised questions about the applicability of the standard to some of the new superabrasive-wheel designs being developed.

Wheel Standards

The primary safety standard for vitrified- and resin-bond, conventional-abrasive grinding wheels is ANSI B7.1, "Safety Requirements for the Use, Care and Protection of Abrasive Wheels." This broad standard was adopted more than 70 years ago, when most grinding was done with low-speed vitrified- and resin-bond wheels. Today, there are many new types of abrasive bonds, grits, and grinding-wheel designs that operate at speeds considered impossible 20 years ago.

To the grinding-wheel industry’s credit, ANSI B7.1 has been updated regularly to address these technological advances, including superabrasives. The standard has evolved with each revision to incorporate changes in the industry such as wheel design and construction, grinding techniques, and end-user awareness. However, the standard’s primary goal - to prevent accident and injury during use of grinding wheels - remains unchanged.

To assure wheel safety, ANSI B7.1 calls on manufacturers to speed test the grinding wheels most susceptible to breakage. Before they are sold, all solid (coreless) vitrified- and resin-bond wheels 6" in diameter or larger must be tested at a speed greater than the maximum speed for which they are designed. By subjecting the wheels to stresses greater than they will ever see in service, particularly in the highly stressed bore region, the manufacturer can be assured that every wheel sold is safe to operate at its intended maximum speed.

When it comes to safety, superabrasive designs should be no exception to grinding-wheel standards.

Research conducted by the Grinding Wheel Institute, Cleveland, has further assured wheel manufacturers that the speed test provides adequate safety of their products. This research demonstrated that bonded-abrasive materials behave just like glass and ceramics; their strength is controlled by the number and size of preexisting microscopic flaws inherent in their manufacture. These flaws, which are areas of porosity that link to form defects, concentrate the stress and serve as fracture origins. The larger the flaw, the weaker the bonded-abrasive material, since less stress is required to cause failure. This is the basic principle of linear elastic-fracture mechanics, which is used to evaluate the likelihood of fracture when cracks are present.

The research showed that vitrified wheels are susceptible to moisture-assisted stress corrosion. This resulted in the adoption of ANSI B74.21, "Fatigue Proof Test Procedure for Vitrified Grinding Wheels," in 1986. This standard defines a test procedure to characterize the susceptibility of vitrified-bond materials to fatigue, or suffer a reduction in strength over time. The standard outlines a technique to calculate a specific test speed that will assure against premature wheel failure due to material fatigue.

The Grinding Wheel Institute’s research showed that 100% speed testing is the only way to assure the safety of vitrified- and resin-bond wheels. Most superabrasive wheels, however, do not have to meet this testing requirement, because they are designed differently. ANSI B7.1 refers only briefly to superabrasives, usually as exceptions to the requirements given for other wheels. Whether or not superabrasives should be considered exceptions depends on the design and construction of the wheel. To determine which wheels must be tested, manufacturers must understand how different types behave and why certain materials need additional testing.

Wheel Designs

All bonded abrasives can be formed into two basic wheel designs. One design is fabricated entirely out of the bonded abrasive. All wheels of this design must be speed tested if they are 6" in diameter or larger. In addition, they must be discarded after they are worn down to a minimum stub size. Because it does not efficiently use expensive superabrasive material, this solid-bond design is not common for superabrasive wheels.

A more efficient and, hence, more common wheel design for superabrasives is a bonded superabrasive attached in a continuous ring or in segments to the periphery of a core. This structure is used almost exclusively for superabrasive wheels. The core can be made of metal, resin, or other materials. When it comes to safety, superabrasive wheels with metal cores have two advantages over wheels without cores. First, the stresses applied to the bonded superabrasive at the periphery are much lower than the stresses at the wheel bore. Since the highest stresses are in the nonbrittle metal core, the abrasive is less likely to fail. The second advantage is that only small pieces of the periphery abrasive break during wheel failure. Because these pieces are much smaller than the fragments that break during failure of a solid-bond wheel, they are less likely to cause damage or injury.

However, even the smallest fragment can cause injury, and this is unacceptable to wheel manufacturers. But it is unclear whether or not 100% speed testing is necessary for superabrasive wheels that have metal cores, even if they have vitrified- or resin-bond abrasive segments adhered to the periphery. ANSI B7.1 does not require speed testing for these wheels, even though the bonds are fabricated from the same brittle material as conventional-abrasive wheels. Should they be treated the same? Yes.

Bond Interface

Figure 1: Debonding of the interface between grinding-wheel core and a bonded abrasive. (SEM photograph courtesy of Harry Cai.)

Because it is usually a weak link in the wheel structure, the interface between the bonded abrasive and the core makes safety concerns with superabrasive wheels different from those with solid-bond, conventional-abrasive wheels. Figure 1 shows a scanning-electron photomicrograph of a crack in the bond interface between a bronze core and a diamond-impregnated bronze abrasive. This debonding of the abrasive from the core typically occurs when operational and grinding stresses exceed the strength of the bond.

Figure 2: Stress in a segmented steel-core wheel determined by finite-element analysis.

It is more likely that failure will occur when a segment detaches or debonds from the wheel. Since the bonded abrasive is a brittle material that is subjected to stresses, speed testing is the only way to ensure that small pieces will not break off and that the wheel can sustain the rotational stresses generated during grinding. Therefore, 100% speed testing is necessary for wheels 6" in diameter or larger with vitrified- or resin-bond material bonded to a metallic or nonmetallic core.

However, speed testing may not adequately ensure the reliability of the bond interface. Can the bond sustain the localized stresses that occur in different wheel designs? Is the design adequate to sustain the operating stresses? Most importantly, does the strength of the interface degrade with time? Individual grinding-wheel manufacturers may be able to answer these questions for their specific products. But as an industry, they have not addressed the issues of wheel safety and reliability of attached segments.

To determine the reliability of the bond interface, wheel manufacturers must know how it behaves and where the stresses are applied. This can be difficult for wheels with complicated geometries. For example, Figure 2 shows a computer simulation of the stress in a segmented steel-core wheel subjected to rotational and grinding loads. Finite-element analysis determined that the highly stressed region is in the gullets near the interface.

Additional tests may be conducted to ensure the integrity of the interface. Although the United States has no such industry-wide standard, tests done in Europe can determine the interface strength of all wheel types. In these tests, a wheel is clamped in a fixture, and a small piece of the peripheral abrasive is bent off. The force required to remove the abrasive must exceed a minimum value for the wheel to be considered a safe design. The details of this test are stipulated in the German grinding-wheel standard DSA 304. Such design-testing standards, when combined with existing speed-testing requirements, will assure the safe and reliable operation of superabrasive grinding wheels.

About the Author

Thomas Service, Ph.D., P.E., is a senior research engineer for Thielsch Engineering Inc., Cranston, RI, and a member of the ANSI B7.1 Committee.