What is a grinding wheel? Grinding wheels contain abrasive grains and layers of fiberglass bonded into a wheel shape by another substance. The abrasive grains act as grinding tools, removing material from a workpiece to shape and refine it. Grinding wheels are useful in many grinding and machining operations.
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Several types of grinding wheels are available, so when a facility is choosing a wheel, it's essential to consider the specifications of contrasting styles and how well they can handle different environments and operational challenges. In this guide to grinding wheels, we discuss a few grinding wheel types, as well as their materials, design and benefits for specific applications.
Grinding wheels ' along with other more portable grinding products like cones and plugs ' come in various styles. Selecting the right type of wheel for a given application allows users to get demanding metal fabrication jobs done quickly and accurately.
There are three main types of grinding wheels, where various numbers differentiate between wheels with specific properties and uses ' type 1 snagging wheels, type 27 grinding wheels and type 28 grinding wheels.
A type 1 snagging wheel has a straight profile and a relatively small diameter of about 2 to 4 inches. Its size makes it ideal for use on high-speed die grinders for grinding off excess metal. Weiler Abrasives' type 1 snagging wheels incorporate aluminum oxide grains for a long life grinding and a consistent cut-rate.
Type 27 is by far the most common abrasive grinding wheel. Type 27 grinding wheels differ from other wheels in that they have a flat profile with a depressed center. A depressed center allows for clearance when the operator must work at a constrained angle.
Using a wheel with a depressed center allows for a range of grinding angles, typically from 0 to 45 degrees. However, the optimal angles for working with type 27 grinding wheels range from 25 to 30 degrees. The steeper the grinding angle, the more aggressive the cut will be.
Working at shallow angles with these wheels requires some consideration of potential ramifications. Grinding at shallow angles can prolong the wheel's lifespan, but it also often compromises the cut-rate. On harder materials, shallow grinding angles may also increase unwanted vibration and chatter.
Type 28 grinding wheels, also known as saucer wheels, have similarly depressed centers and are optimized for low grinding angles. They differ from type 27 wheels in that their concave or saucer-shaped design allows for better access to the workpiece ' especially in tighter areas, such as corners, fillets and overhangs ' and increased aggression at smaller working angles. They can work at angles between 0 and 30 degrees but typically work best for use with grinding angles from 0 to 15 degrees.
GRINDING WHEEL MATERIALS
The materials in each grinding wheel break down into a few main components ' the grains, the bond and the fiberglass that reinforces the wheels to give them strength and stability for use in demanding applications. The grit of the wheel is also an essential element that helps determine performance.
The abrasive grains provide the essential functionality of a grinding wheel because they remove material from the workpiece. A few commonly used grinding wheel abrasives are ceramic alumina, zirconia alumina, aluminum oxide, white aluminum oxide, aluminum oxide and silicon carbide. Grains can be blended together to achieve different performance characteristics as well.
Grains also come in various sizes ' the size of a grain refers to the size of the individual abrasive particles, similar to the grades for sandpaper particles.
The bond is the substance that causes the abrasive grains to adhere to the wheel. Bonds can consist of different materials. Common materials include shellac, resinoids, rubber and glass or glass-ceramic. At Weiler Abrasives, our portable grinding wheels contain resinoid bonds.
The bond on a grinding wheel may be either hard or soft. A harder bond extends the wheel's lifespan, provided the user operates and maintains the wheel correctly. A softer bond, on the other hand, allows for smoother grinding and exposes new grains more quickly. Choosing the correct bond for a given application can help balance performance and longevity. The type of metal can also influence the bond that's best for your application.
A grinding wheel's bond sometimes contains iron, sulfur and chlorine, which can pose challenges if they adhere to the workpiece during grinding. Weiler Abrasives offers several wheels that minimize these elements. Our Tiger® Ceramic, Aluminum, and INOX wheels are contaminant-free, containing less than 0.1% chlorine, sulfur and iron. They help prevent corrosion on stainless steel and aluminum workpieces.
The bond on a grinding wheel helps provide a consistent cut rate by exposing new grains over time. As older grains become worn, the grain particles fracture as they are designed to ' thereby exposing new abrasive surfaces, leaving fresh abrasive particles exposed in their places. Ideally, the composition of the binding is such that under normal working conditions, wear and tear will remove worn abrasive particles and leave fresh ones in place, maintaining the wheel's superior cut-rate and performance.
The abrasive particles bound to the wheel also have a characteristic known as grade. Grade refers to hardness, but not the hardness of the particles themselves ' it refers instead to the strength of the bond holding the particles to the wheel. A wheel with a stronger bond typically has a longer life. A softer bond is designed to break down faster to maintain a consistent cut rate as new sharp grains take the place of worn ones. The letters N, R, S and T specify the hardness of a bond, with the letters that come later in the alphabet referring to harder bonds. As a general rule, a wheel with a softer bond will perform better on a hard metal, while a hard bond will perform better on a softer metal.
FIBERGLASS
The fiberglass structure and design on a grinding wheel provides reinforcement, rigidity and superior grinding ability. All Weiler grinding wheels come with triple-reinforced fiberglass that gives additional support and strength for aggressive stock removal. Our Tiger brand of performance grinding wheels has the outer layout of fiberglass cut back to allow for aggressive grinding from the outset with no break-in period.
The grit of a wheel is critical for supplying the right abrasion. Grit measurements generally range from coarse to fine. On Weiler Abrasives' grinding wheels, the coarsest grit has a rating of about 24 and the finest grit ' the grit on snagging wheels ' has a rating of about 36. Selecting the right grit level for a particular application helps ensure sufficient grinding power. A course grit has a better removal rate, while a finer grit requires less pressure during application and allows for a better final finish on the workpiece.
When selecting a grinding wheel, users should consider two primary factors ' diameter and thickness. Both metrics affect the wheel's usability and performance.
The choice of diameter for a grinding wheel generally depends on the available tool. The primary reason for fitting the grinding wheel diameter to the tool is safety ' the operation of the tool should never exceed the RPM rating on the abrasive. Smaller power tools tend to operate at higher RPM than larger power tools, and the design of abrasives and brushes enables them to meet the same standards. Running a wheel with only an 8,500 RPM rating on a grinder that operates at 13,000 RPM could cause the abrasive to fail and injure the operator.
Choosing the correct diameter also enhances safety because it allows for the use of proper guards. Trying to fit a 6-inch abrasive on a 4.5-inch grinder necessitates removing the protective guards, and running the grinder without guards would increase the operator's chance of injury if the abrasive failed.
Product life is an additional factor in the choice of grinding wheel diameter. Larger-diameter wheels last longer. Especially in applications where the operator must run the grinding wheel for a sustained period, using a larger-diameter wheel can improve productivity by reducing the number of times the operator must stop and replace the abrasive.
The configuration of the workspace and workpiece also influence the choice of diameter. For instance, an operator working in a cramped space or on a tricky area of the workpiece may choose a die grinder with a small 3-inch wheel for better access.
The thickness of a grinding wheel impacts its performance and wheel life. Our grinding wheels typically come with a quarter-inch thickness. This measurement gives our wheels a superior balance of precision, wheel life, and cut-rate when grinding.
Combination grinding and cutting wheels with 1/8-inch thickness are also available. These wheels allow for grinding and for making cuts that require a thinner wheel. The benefit of these thinner combo wheels is that they enable the operator to perform both 90-degree cuts and shallow-angled grinding without having to change the abrasive used on the wheel.
Thus far, we've discussed how different wheel types and compositions can influence the performance of a grinding wheel. And we've explored how selecting particular wheel diameters and thicknesses can optimize grinding wheels for specialized applications.
Now let's examine a few particular applications that require the use of grinding wheels and consider the optimal wheel specifications for each.
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Grinding wheels are made of natural or synthetic abrasive minerals bonded together in a matrix to form a wheel. While such tools may be familiar to those with home workshops, the general public may not be aware of them because most have been developed and used by the manufacturing industry. In this sector, grinding wheels have been important for more than 150 years.
For manufacturers, grinding wheels provide an efficient way to shape and finish metals and other materials. Abrasives are often the only way to create parts with precision dimensions and high-quality surface finishes. Today, grinding wheels appear in nearly every manufacturing company in the United States, where they are used to cut steel and masonry block; to sharpen knives, drill bits, and many other tools; or to clean and prepare surfaces for painting or plating. More specifically, the precision of automobile camshafts and jet engine rotors rests upon the use of grinding wheels. Quality bearings could not be produced without them, and new materials such as ceramic or material composites would be impossible without grinding wheels to shape and finish parts.
Sandstone, an organic abrasive made of quartz grains held together in a natural cement, was probably the earliest abrasive; it was used to smooth and sharpen the flint on axes. By the early nineteenth century, emery (a natural mineral containing iron and corundum) was used to cut and shape metals. However, emery's variable quality and problems with importing it from India prior to its discovery in the United States prompted efforts to find a more reliable abrasive mineral.
By the s, the search had yielded silicon carbide, a synthetic mineral harder than corundum. Eventually, manufacturers figured out how to produce an even better alternative, synthetic corundum or aluminum oxide. In creating this bauxite derivative, they developed an abrasive material more reliable than both natural minerals and silicon carbide. Research into synthetic minerals also led to production of the so-called superabrasives. Foremost in this category are synthetic diamonds and a mineral known as cubic boron nitride (CBN), second in hardness only to the synthetic diamond. Today, development continues, and a seeded-gel aluminum oxide has just been introduced.
Throughout the grinding wheel's history, the bond that holds the abrasive grains together has proven as important as the grains themselves. The success of grinding wheels began in the early s, when bonds containing rubber or clay were introduced, and by the s a bond with a vitrified or glass-like structure was patented. Since then, bonds used in grinding wheels have been continually refined.
Grinding wheels are available in a wide variety of sizes, ranging from less than .25 inch (.63 centimeter) to several feet in diameter. They are also available in numerous shapes: flat disks, cylinders, cups, cones, and wheels with a profile cut into the periphery are just a few. Although many techniques, such as bonding a layer of abrasives to the surface of a metal wheel, are used to make grinding wheels, this discussion is limited to wheels composed of vitrified materials contained in a bonding matrix.
Two important components, abrasive grains and bonding materials, make up any grinding wheel. Often, additives are blended to create a wheel with the properties necessary to shape a particular material in the manner desired.
Abrasive grains constitute the central component of any grinding wheel, and the hardness and friability of the grinding materials will significantly affect the behavior of a given wheel. Hardness is measured in terms of a relative scale developed in by a German mineralogist named Friedrich Mohs. On this scale, extremely soft talc and gypsum represent hardnesses of one and two, and corundum and diamond represent hardness of nine and ten.
Friability refers to how easily a substance can be fractured or pulverized. People who design grinding wheels consider the friability of their abrasives'which can differ with the nature of the materials being ground'very carefully. For example, while diamond is the hardest known material, it is an undesirable steel abrasive because it undergoes a destructive chemical reaction during the cutting process; the same is true of silicon carbide. On the other hand, aluminum oxide cuts irons and steels better than diamond and silicon carbide, but it is less effective for cutting nonmetallic substances.
If selected correctly, an abrasive chosen to shape a particular substance will retain its friability when ground against that substance: because the grinding will cause the abrasive to continue fracturing along clean, sharp lines, it will maintain a sharp edge throughout the grinding process. This gives the grinding wheel the unique characteristic of being a tool that sharpens itself during use.
Although bonded abrasives began as tools made from natural minerals, modern products are made almost exclusively with synthetic materials. A bonding material holds the abrasive grits in place and allows open space between them. Manufacturers of grinding wheels assign a hardness to the wheel, which should not be confused with the hardness of the abrasive grain. Bonds that allow abrasives grains to fracture easily are classified as soil bonds. Bonds that restrict the fracturing of the grains and allow a wheel to withstand large forces are classified as hard bonds. Generally, soil wheels cut easily, produce poor surface finishes, and have a short useful life. On the other hand, harder wheels last longer and produce finer surface finishes, but cut less well and produce more heat during grinding.
The bonding matrix in which the abrasive grains are fixed may include a variety of organic materials such as rubber, shellac or resin; inorganic materials such as clay are also used. Inorganic bonds with glass-like or vitreous structures are used on the tool-sharpening wheels for the home workshop grinder, while resin bonds are used in masonry or steel-cutting wheels. Generally, vitrified bonds are used with medium to fine grain sizes in wheels needed for precision work. Resin bonds are used generally with coarse grains and for heavy metal removal operations such as foundry work.
In addition to their abrasive and bonding materials, grinding wheels often contain additional ingredients that produce pores within the wheel or assist chemically when a particular abrasive is used to grind a special material. One important aspect of a grinding wheel that can be created or altered through additives is porosity, which also contributes to the cutting characteristics of the grinding wheel. Porosity refers to the open spaces within the bond that allow room for small chips of metal and abrasive generated during the grinding process. Porosity also provides pathways that carry fluids used to control heat and improve the cutting characteristics of the abrasive grains. Without adequate porosity and spacing between abrasive grains, the wheel can become loaded with chips and cease to cut properly.
A variety of products are used as additives to create proper porosity and spacing. In the past, sawdust, crushed nut shells, and coke were used, but today materials that vaporize during the firing step of manufacturing (for example, napthaline-wax) are preferred. Some grinding wheels receive additional materials that serve as aids to grinding. These include sulfur and chlorine compounds that inhibit microscopic welding of metal particles and generally improve metal-cutting properties.
Most grinding wheels are manufactured by the cold-press method, in which a mixture of components is pressed into shape at room temperature. The details of processes vary considerably depending upon the type of wheel and the practices of individual companies. For mass production of small wheels, many portions of the process are automated.
Wheel manufacturers often spend considerable effort to develop a satisfactory mixture. The blend must be free-flowing and distribute grain evenly throughout the structure of the grinding wheel to assure uniform cutting action and minimal vibration as the wheel rotates during use. This is particularly important for large wheels, which may be as big as several feet in diameter, or for wheels that have a shape other than the familiar flat disk.
There are no clear performance standards for grinding wheels. With the exception of those containing expensive abrasives such as diamonds, grinding wheels are consumable items, and the rates of consumption vary considerably depending on application. However, a number of domestic and global standards are accepted, voluntarily, by manufacturers.
Trade organizations, which represent some manufacturers in the highly competitive U.S. market, have developed standards covering such matters as sizing of abrasive grains, labeling of abrasive products, and the safe use of grinding wheels.
The extent to which grinding wheel quality is checked depends upon the size, cost, and eventual use of the wheels. Typically, wheel manufacturers monitor the quality of incoming raw materials and their production processes to assure product consistency. Special attention is given to wheels larger than six inches in diameter, because they have the potential to harm personnel and equipment if they break during use. Each large vitrified wheel is examined to determine the strength and integrity of the bonding system as well as the uniformity of grain through every wheel. Acoustical tests measure wheel stiffness; hardness tests assure correct hardness of bonds; and spin tests assure adequate strength.
Changes in manufacturing practices will determine the demand for various types of wheels in the future. For example, the trend in the steel industry towards continuous casting as a way to make steel has greatly reduced that industry's use of some types of grinding wheels. A push for greater productivity by manufacturers is responsible for market projections showing a shift from wheels made of traditional aluminum oxide abrasives to wheels made of newer forms of synthetic abrasives such as the seeded-gel aluminum oxide and cubic boron nitride. Also, the use of advanced materials such as ceramics and composites will increase demands for newer types of grinding wheels. The transition to new abrasive minerals, however, is being impeded by the fact that much manufacturing equipment and many industrial procedures are still unable to make effective use of the newer (and more expensive products). Notwithstanding trends, traditional abrasives are projected to continue serving many uses.
However, competition from several alternative technologies is likely to grow. Advances in cutting tools made of polycrystalline superabrasive materials'fine grain crystalline materials made of diamond or cubic boron nitride'will make such tools a viable option for shaping hard materials. Also, advances in the chemical vapor deposition of diamond films will affect the need for abrasives by lengthening the life of cutting tools and extending their capabilities.
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Murray, Charles J. "Retainer System Eases Wheel and Blade Replacement." Design News. January 18, , p. 104.
' Theodore L. Giese
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