Grinding wheel

(design engineering) A wheel or disk having an abrasive material such as alumina or silicon carbide bonded to the surface.

Background

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 1890s, 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 1840s, when bonds containing rubber or clay were introduced, and by the 1870s 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.

Raw Materials

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 1812 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.

The Manufacturing
Process

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.

Mixing the ingredients

• Preparing the grinding wheel mixture begins with selecting precise quantities of abrasives, bond materials, and additives according to a specific formula. A binder, typically a water-based wetting agent in the case of vitrified wheels, is added to coat the abrasive grains; this coating improves the grains' adhesion to the binder. The binder also helps the grinding wheel retain its shape until the bond is solidified. Some manufacturers simply mix all materials in a single mixer. Others use separate steps to mix abrasive grains with binder.

  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.

Molding

• For the most common type of wheel, an annular disc, a predetermined amount of grinding wheel mixture is poured into a mold consisting of four pieces: a circular pin the size of the finished wheel's arbor hole (its center hole); a shell with a 1-inch (2.5-centimeter) wall, about twice as high as the desired grinding wheel is thick; and two flat, circular plates with diameter and arbor hole sizes equal to those of the wheel. A variety of methods are used to distribute the mixture evenly. Typically, a straight edge pivots about the center arbor pin to spread the mixture throughout the mold.
• Using pressures in the range of 100 to 5000 pounds per square inch (psi) for 10 to 30 seconds, a hydraulic press then compacts the mixture into the grinding wheel's final shape. Some manufacturers use gage blocks between the two face plates to limit their movement and establish uniform thickness. Others control wheel thickness by closely monitoring the consistency of the mix and the force of the press.
• After the mold has been removed from the press and the wheel stripped from the mold, the wheel is placed on a flat, heatproof carrier. Final shaping of the wheel may take place at this time. All work at this stage has to be done very carefully because the wheel is held together by only the temporary binder. Lighter wheels can be lifted by hand at this stage; heavier ones may be lifted with a hoist or carefully slid on a carrier to be transported to the kiln.

Firing

• Generally, the purposes of the firing are to melt the binder around the abrasives and to convert it to a form that will resist the heat and solvents encountered during grinding. A wide range of furnaces and kilns are used to fire grinding wheels, and the temperatures vary widely depending upon the type of bond. Wheels with a resin bond are typically fired at a temperature of 300 to 400 degrees Fahrenheit (149 to 204 degrees Celsius), and wheels with vitrified bonds are fired to temperatures between 1700 and 2300 degrees Fahrenheit (927 to 1260 degrees Celsius).

Finishing

• After firing, wheels are moved to a finishing area, where arbor holes are reamed or cast to the specified size and the wheel circumference is made concentric with the center. Steps may be necessary to correct thickness or parallelism of wheel sides, or to create special contours on the side or circumference of the wheel. Manufacturers also balance large wheels to reduce the vibration that will be generated when the wheel is spun on a grinding machine. Once wheels have received labels and other markings, they are ready for shipment to the consumer.

Quality Control

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.

The Future

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.

Where To Learn More

Books

Borkowski, J. Uses of Abrasives & Abrasive Tools. Prentice Hall, 1992.

Burkar, W. Grinding & Polishing. State Mutual Book & Periodical Service, 1989.

Hahn, Robert S. Handbook of Modern Grinding Technology. Chapman & Hall, 1986.

Salmon, Stuart C. Modern Grinding Process Technology. McGraw-Hill, 1992.

Periodicals

Murray, Charles J. "Retainer System Eases Wheel and Blade Replacement." Design News. January 18, 1988, p. 104.

[Article by: Theodore L. Giese]

The noun has one meaning:

Meaning #1: a wheel composed of abrasive material; used for grinding
  Synonym: emery wheel

Grinding wheel

This article does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (June 2008)

A grinding wheel is an expendable wheel that is composed of an abrasive compound. These wheels are used in grinding machines.

Contents 1 Composition and manufacture 2 Functions 3 Safety 4 Types 4.1 Straight wheel 4.2 Cylinder or wheel ring 4.3 Tapered wheel 4.4 Straight cup 4.5 Dish cup 4.6 Saucer wheel 4.7 Diamond wheel 4.8 Diamond mandrels 5 Cut off wheels 6 See also

Composition and manufacture

The wheel is generally made from a matrix of coarse particles pressed and bonded together to form a solid, circular shape, various profiles and cross sections are available depending on the intended usage for the wheel. They may also be made from a solid steel or aluminium disc with particles bonded to the surface.

Materials used are generally silicon carbide and diamond with a vitrified bonding agent. In production grinding, a wide array of materials is used. Wheels with different abrasives, structure, bond, grade, and grain sizes are available. The abrasive is the actual cutting material, such as cubic boron nitride, zirconia aluminum oxide, manufactured diamonds, ceramic aluminum oxide, aluminum oxide, and others. The abrasive is selected based on the hardness of the material being cut. The structure of the wheel refers to the density of the wheel (bond and abrasive versus airspace). A less-dense wheel will cut freely, and has a large effect on surface finish. A less dense wheel is able to take a deeper or wider cut with less coolant, as the chip clearance on the wheel is greater. The grade of the wheel determines how tightly the bond holds the abrasive. Grade affects almost all considerations of grinding, such as wheel speed, coolant flow, maximum and minimum feed rates, and grinding depth. Grain size determines the physical abrasive size in the wheel. A larger grain will cut freely, allowing fast cutting but poor surface finish. Ultra-fine grain sizes are for precision finish work, where a fine surface finish is required. The wheel bonding agent determines how the wheel holds the abrasives. This affects finish, coolant, and minimum/maximum wheel speed.

The manufacture of these wheels is a precise and tightly controlled process, due not only to the inherent safety risks of a spinning disc, but also the composition and uniformity required to prevent that disc from exploding due to the high stresses produced on rotation.

Functions

Grinding wheels are self sharpening to a small degree, for optimal use they may be dressed and trued by the use of grinding dressers. Dressing the wheel refers to removing the current layer of abrasive, so that a fresh and sharp surface is exposed to the work surface. Truing the wheel makes the grinding surface parallel to the grinding table or other reference plane, so the entire grinding wheel is even and produces an accurate surface.

The wheel type (eg:- cup or plain wheel below) fit freely on their supporting arbors, the necessary clamping force to transfer the rotary motion being applied to the wheels side by identically sized flanges (metal discs). The paper blotter shown in the images is intended to distribute this clamping force evenly across the wheels surface.

Safety

Before mounting and balancing a grinding wheel, the wheel must be sounded. Sounding, also known as a ring check, is loosely suspending the wheel by a bit of twine or other material so that it hangs free, and giving the wheel a very light tap with a non metallic object, such as a wooden stick. Care must be taken not to damage the wheel when sounding. A wheel that is safe to use will ring clearly and solidly, like a bell or tuning fork. A damaged wheel will not make any resonating sound, it will sound more like a dull thud.^{[citation needed]}

The clamping force of the grinding wheel flanges is an important safety parameter of a grinding operation:

• it must be high enough to drive the wheel without slippage under the most severe operating conditions of the machine.
• it must not apply to the wheel an excessive compression stress which could weaken the wheel.
• it must not distort the flanges.

This clamping force can be calculated by the formula detailed in the EN13218 standard, Annex C.

When flanges are clamped by screws it is essential to know the torque to use when tightening these screws to make sure that the needed clamping force is obtained and that each screw is loaded enough to avoid loosening. For multiple screw flanges this torque calculation is also detailed in the EN13218 standard Annex C.

The design of the flanges and their tightening parameters are the manufacturer's responsibility.

Types

Straight wheel

Straight wheel

To the left is an image of a straight wheel. These are by far the most common style of wheel and can be found on bench or pedestal grinders. They are used on the periphery only and therefore produce a slightly concave surface (hollow ground) on the part. This can be used to advantage on many tools such as chisels.

Straight Wheels are the kind of generally used for cylindrical, centreless, and surface grinding operations. Wheels of this form vary greatly in size, the diameter and width of face naturally depending upon the class of work for which is used and the size and power of the grinding machine.

Cylinder or wheel ring

Cylinder wheels provide a long, wide surface with no center mounting support (hollow). They can be very large, up to 12" in width. They are used only in vertical or horizontal spindle grinders. Cylinder or wheel ring is used for producing flat surfaces, the grinding being done with the end face of the wheel.

Tapered wheel

A straight wheel that tapers outward towards the center of the wheel. This arrangement is stronger than straight wheels and can accept higher lateral loads. Tapered face straight wheel is primarily used for grinding thread, gear teeth etc.

Straight cup

Straight cup wheels are an alternative to cup wheels in tool and cutter grinders, where having an additional radial grinding surface is beneficial.

Dish cup

A very shallow cup-style grinding wheel. The thinness allows grinding in slots and crevices. It is used primarily in cutter grinding and jig grinding.

Saucer wheel

A special grinding profile that is used to grind milling cutters and twist drills. It is most common in non-machining areas, as sawfilers use saucer wheels in the maintenance of saw blades.

Diamond wheel

Diamond wheel

Diamond wheels are grinding wheels with industrial diamonds bonded to the periphery.

They are used for grinding extremely hard materials such as carbide tips, gemstones or concrete. The saw pictured to the right is a slitting saw and is designed for slicing hard materials, typically gemstones.

Diamond mandrels

Diamond mandrels are very similar to their counterpart, a diamond wheel. They are tiny diamond rasps for use in a jig grinder doing profiling work in hard material.

Cut off wheels

Cut off or parting wheels are self-sharpening wheels that are thin in width and often have radial fibres reinforcing them. They are often used in the construction industry for cutting reinforcement steel (rebar), protruding bolts or anything that needs quick removal or trimming. Most handymen would recognise an angle grinder and the discs they use.

See also

• Diamond Blade
• Diamond tools

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