An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing[1] which leads to part of the workpiece being worn away by friction. While finishing a material often means polishing it to gain a smooth, reflective surface, the process can also involve roughening as in satin, matte or beaded finishes. In short, the ceramics which are used to cut, grind and polish other softer materials are known as abrasives.
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Abrasives are extremely commonplace and are used very extensively in a wide variety of industrial, domestic, and technological applications. This gives rise to a large variation in the physical and chemical composition of abrasives as well as the shape of the abrasive. Some common uses for abrasives include grinding, polishing, buffing, honing, cutting, drilling, sharpening, lapping, and sanding (see abrasive machining). (For simplicity, "mineral" in this article will be used loosely to refer to both minerals and mineral-like substances whether man-made or not.)
Files are not abrasives; they remove material not by scratching or rubbing, but by the cutting action of sharp teeth which have been cut into the surface of the file, very much like those of a saw. However, diamond files are a form of coated abrasive (as they are metal rods coated with diamond powder).
Main article: Abrasion (mechanical)Abrasives generally rely upon a difference in hardness between the abrasive and the material being worked upon, the abrasive being the harder of the two substances. However, it is not strictly necessary, as any two solid materials that repeatedly rub against each other will tend to wear each other away; examples include, softer shoe soles wearing away wooden or stone steps over decades or centuries or glaciers abrading stone valleys.
Typically, materials used as abrasives are either hard minerals (rated at 7 or above on Mohs scale of mineral hardness) or are synthetic stones, some of which may be chemically and physically identical to naturally occurring minerals but which cannot be called minerals as they did not arise naturally. (While useful for comparative purposes, the Mohs scale is of limited value to materials engineers as it is an arbitrary, ordinal, irregular scale.) Diamond, a common abrasive, for instance occurs both naturally and is industrially produced, as is corundum which occurs naturally but which is nowadays more commonly manufactured from bauxite.[2] However, even softer minerals like calcium carbonate are used as abrasives, such as "polishing agents" in toothpaste.
These minerals are either crushed or are already of a sufficiently small size (anywhere from macroscopic grains as large as about 2 mm to microscopic grains about 0.001 mm in diameter) to permit their use as an abrasive. These grains, commonly called grit, have rough edges, often terminating in points which will decrease the surface area in contact and increase the localised contact pressure. The abrasive and the material to be worked are brought into contact while in relative motion to each other. Force applied through the grains causes fragments of the worked material to break away, while simultaneously smoothing the abrasive grain and/or causing the grain to work loose from the rest of the abrasive.
Some factors which will affect how quickly a substance is abraded include:
Abrasives may be classified as either natural or synthetic. When discussing sharpening stones, natural stones have long been considered superior but advances in material technology are seeing this distinction become less distinct. Many synthetic abrasives are effectively identical to a natural mineral, differing only in that the synthetic mineral has been manufactured rather than mined. Impurities in the natural mineral may make it less effective.
Some naturally occurring abrasives are:
Some abrasive minerals (such as zirconia alumina) occur naturally but are sufficiently rare or sufficiently more difficult or costly to obtain such that a synthetic stone is used industrially. These and other artificial abrasives include:
Abrasives are shaped for various purposes. Natural abrasives are often sold as dressed stones, usually in the form of a rectangular block. Both natural and synthetic abrasives are commonly available in a wide variety of shapes, often coming as bonded or coated abrasives, including blocks, belts, discs, wheels, sheets, rods and loose grains.
A bonded abrasive is composed of an abrasive material contained within a matrix, although very fine aluminium oxide abrasive may comprise sintered material. This matrix is called a binder and is often a clay, a resin, a glass or a rubber. This mixture of binder and abrasive is typically shaped into blocks, sticks, or wheels. The most common abrasive used is aluminium oxide. Also common are silicon carbide, tungsten carbide and garnet. Artificial sharpening stones are often a bonded abrasive and are readily available as a two sided block, each side being a different grade of grit.
Grinding wheels are cylinders that are rotated at high speed. While once worked with a foot pedal or hand crank, the introduction of electric motors has made it necessary to construct the wheel to withstand greater radial stress to prevent the wheel flying apart as it spins. Similar issues arise with cutting wheels, which are often structurally reinforced with impregnated fibres. High relative speed between abrasive and workpiece often makes necessary the use of a lubricant of some kind. Traditionally, they were called coolants as they were used to prevent frictional heat build up which could damage the workpiece (such as ruining the temper of a blade). Some research suggests that the heat transport property of a lubricant is less important when dealing with metals as the metal will quickly conduct heat from the work surface. More important are their effects upon lessening tensile stresses while increasing some compressive stresses and reducing "thermal and mechanical stresses during chip formation".[3]
Various shapes are also used as heads on rotary tools used in precision work, such as scale modelling.
Bonded abrasives need to be trued and dressed after they are used. Dressing is the cleaning of the waste material (swarf and loose abrasive) from the surface and exposing fresh grit. Depending upon the abrasive and how it was used, dressing may involve the abrasive being simply placed under running water and brushed with a stiff brush for a soft stone or the abrasive being ground against another abrasive, such as aluminium oxide used to dress a grinding wheel.
Truing is restoring the abrasive to its original surface shape. Wheels and stones tend to wear unevenly, leaving the cutting surface no longer flat (said to be "dished out" if it is meant to be a flat stone) or no longer the same diameter across the cutting face. This will lead to uneven abrasion and other difficulties.
Main article: Coated abrasiveA coated abrasive comprises an abrasive fixed to a backing material such as paper, cloth, rubber, resin, polyester or even metal, many of which are flexible. Sandpaper is a very common coated abrasive. Coated abrasives are most commonly the same minerals as are used for bonded abrasives. A bonding agent (often some sort of adhesive or resin) is applied to the backing to provide a flat surface to which the grit is then subsequently adhered. A woven backing may also use a filler agent (again, often a resin) to provide additional resilience.
Coated abrasives may be shaped for use in rotary and orbital sanders, for wrapping around sanding blocks, as handpads, as closed loops for use on belt grinders, as striking surfaces on matchboxes, on diamond plates and diamond steels. Diamond tools, though for cutting, are often abrasive in nature.
Sand, glass beads, metal pellets copper slag and dry ice may all be used for a process called sandblasting (or similar, such as the use of glass beads which is "bead blasting"). Dry ice will sublimate leaving behind no residual abrasive.
Cutting compound used on automotive paint is an example of an abrasive suspended in a liquid, paste or wax, as are some polishing liquids for silverware and optical media. The liquid, paste or wax acts as a binding agent that keeps the abrasive attached to the cloth which is used as a backing to move the abrasive across the work piece. On cars in particular, wax may serve as both a protective agent by preventing exposure of the paint of metal to air and also act as an optical filler to make scratches less noticeable. Toothpaste contains calcium carbonate or silica as a "polishing agent" to remove plaque and other matter from teeth as the hardness of calcium carbonate is less than that of tooth enamel but more than that of the contaminating agent.
Very fine rouge powder was commonly used for grinding glass, being somewhat replaced by modern ceramics, and is still used in jewellery making for a highly reflective finish.
Cleaning products may also contain abrasives suspended in a paste or cream. They are chosen to be reasonably safe on some linoleum, tile, metal or stone surfaces. However, many laminate surfaces and ceramic topped stoves are easily damaged by these abrasive compounds. Even ceramic/pottery tableware or cookware can damage these surfaces, particularly the bottom of the tableware, which is often unglazed in part or in whole and acts as simply another bonded abrasive.[4]
Metal pots and stoves are often scoured with abrasive cleaners, typically in the form of the aforementioned cream or paste or of steel wool and non woven scouring pads which holds fine grits abrasives.
Human skin is also subjected to abrasion in the form of exfoliation. Abrasives for this can be much softer and more exotic than for other purposes and may include things like almond and oatmeal.[5] Dermabrasion and microdermabrasion are now rather commonplace cosmetic procedures which use mineral abrasives.
Scratched compact discs and DVDs may sometimes be repaired through buffing with a very fine compound, the principle being that a multitude of small scratches will be more optically transparent than a single large scratch. However, this does take some skill and will eventually cause the protective coating of the disc to be entirely eroded (especially if the original scratch is deep), at which time, the data surface will be destroyed if abrasion continues.
Silicon carbide powders are commonly used as abrasive materials in various machining processes, including grinding, water-jet cutting, and sandblasting.[6] These powders are effective for fine grinding or rough polishing of semiconductors, ceramics, and ferrous materials.[7]
The shape, size and nature of the workpiece and the desired finish will influence the choice of the abrasive used. A bonded abrasive grind wheel may be used to commercially sharpen a knife (producing a hollow grind), but an individual may then sharpen the same knife with a natural sharpening stone or an even flexible coated abrasive (like a sandpaper) stuck to a soft, non-slip surface to make achieving a convex grind easier. Similarly, a brass mirror may be cut with a bonded abrasive, have its surface flattened with a coated abrasive to achieve a basic shape, and then have finer grades of abrasive successively applied culminating in a wax paste impregnated with rouge to leave a sort of "grainless finish" called, in this case, a "mirror finish".
Also, different shapes of adhesive may make it harder to abrade certain areas of the workpiece. Health hazards can arise from any dust produced (which may be ameliorated through the use of a lubricant) which could lead to silicosis (when the abrasive or workpiece is a silicate) and the choice of any lubricant. Besides water, oils are the most common lubricants. These may present inhalation hazards, contact hazards and, as friction necessarily produces heat, flammable material hazards.[8]
An abrasive which is too hard or too coarse can remove too much material or leave undesired scratch marks. Besides being unsightly, scratching can have other, more serious effects. Excessive abrasion or the presence of scratches may:
A finer or softer abrasive will tend to leave much finer scratch marks which may even be invisible to the naked eye (a "grainless finish"); a softer abrasive may not even significantly abrade a certain object. A softer or finer abrasive will take longer to cut, as it tends to cut less deeply than a coarser, harder material. Also, the softer abrasive may become less effective more quickly as the abrasive is itself abraded. This allows fine abrasives to be used in the polishing of metal and lenses where the series of increasingly fine scratches tends to take on a much more shiny or reflective appearance or greater transparency. Very fine abrasives may be used to coat the strop for a cut-throat razors, however, the purpose of stropping is not to abrade material but to straighten the burr on an edge. The final stage of sharpening Japanese swords is called polishing and may be a form of superfinishing.
Different chemical or structural modifications may be made to alter the cutting properties of the abrasive.[9]
Other very important considerations are price and availability. Diamond, for a long time considered the hardest substance in existence, is actually softer than fullerite and even harder aggregated diamond nanorods, both of which have been synthesised in laboratories, but no commercial process has yet been developed. Diamond itself is expensive due to scarcity in nature and the cost of synthesising it. Bauxite is a very common ore which, along with corundum's reasonably high hardness, contributes to corundum's status as a common, inexpensive abrasive.
Thought must be given to the desired task about using an appropriately hard abrasive. At one end, using an excessively hard abrasive wastes money by wearing it down when a cheaper, less hard abrasive would suffice. At the other end, if the abrasive substance is too soft, abrasion does not take place in a timely fashion, effectively wasting the abrasive as well as any accruing costs associated with loss of time.
Aside from the aforementioned uses of shaping and finishing, abrasives may also be used to prepare surfaces for application of some sort of paint of adhesive. An excessively smooth surface may prevent paint and adhesives from adhering as strongly as an irregular surface could allow. Inflatable tyre repair kits (which, on bicycles particularly, are actually patches for the inner tube rather than the tyre) require use of an abrasive so that the self-vulcanising cement will stick strongly.
Inadvertently, people who use knives on glass or metal cutting boards are abrading their knife blades. The pressure at the knife edge can easily create microscopic (or even macroscopic) cuts in the board. This cut is a ready source of abrasive material as well as a channel full of this abrasive through which the edge slides. For this reason, and without regard for the health benefits, wooden boards are much more desirable. A similar occurrence arises with glass-cutters. Glass-cutters have circular blades that are designed to roll not slide. They should never retrace an already effected cut.
Undesired abrasion may result from the presence of carbon in internal combustion engines. While smaller particles are readily transported by the lubrication system, larger carbon particles may abrade components with close tolerances. The carbon arises from the excessive heating of engine oil or from incomplete combustion. This soot may contain fullerenes which are noted for their extreme hardness—and small size and limited quantity which would tend to limit their effect.
Courtesy of Rex-Cut Abrasives
Whether the surface-treatment task involves deburring, blending, grinding, polishing, finishing, dimensioning, patterning or shaping a metal workpiece, there’s a coated-abrasive product available to get the job done. The products include belts, rolls, sheets, pads and flap discs and wheels.
The coated-abrasive name derives from a single layer of abrasive grains being coated, or deposited, onto a flexible or semirigid backing material, using an adhesive, such as resin, to bond the grains to the backing material. This article examines the coating process, various types of grains, or man-made minerals, and backing materials for coated-abrasive products used for several metalworking applications.
Similar to other metal-removal operations, the choice of product depends on the application. “Our salesmen go in, look at the application and decide what the best product to go with is,” said Caitlin Murak at National Abrasives Inc., Lewisberry, Pa. However, operator preference often plays a major role in product selection, she added. “Some people just like a certain brand over another brand and it really doesn’t matter how well it works.” In addition to bonded abrasives and various metalworking products, National distributes an array of coated abrasives, including those from Mirka USA, Norton, Radiac Abrasives and VSM Abrasives.
Applying Abrasives
Two layers of resins create the bonding system for coated abrasives, according to a technical paper from VSM Abrasives Corp., O’Fallon, Mo. The first layer is the make, or base, coat, which anchors grains to the backing. The second layer is the size coat, which is applied over the grains to further anchor and stabilize them.
VSM also stated that grains can be applied via the gravity coating process or the electrostatic coating process. In the gravity method, grains drop from an overhead hopper onto the adhesive-coated backing. In the electrostatic method, the adhesive-coated backing and grains pass through an electrically charged field, which propels the grains upward toward the backing that travels upside down above the grains. The grains are then embedded in the adhesive with the sharpest edge of the minerals exposed to ensure uniform cutting.
With these coating processes, grain coverage can be modified to produce open- or closed-coat products. According to information from Norton/Saint-Gobain, Worcester, Mass., an open coat typically has 75 percent of the backing covered with evenly spaced grains, which is ideal for operations where the grinding debris loads or clogs the surface, reducing cutting efficiency and shortening tool life. When making a closed-coat product, the backing is almost completely covered with grain, which is suitable when loading is not an issue and a fine surface finish is required.
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Norton/Saint-Gobain’s Director of Marketing and Strategy David J. Long said the abrasive products manufacturer produces large rolls of coated abrasives, called “jumbo rolls,” then converts them to make the required shapes, such as discs, belts and sheets, in a secondary operation.
Grain Types
The primary abrasive grains for metal-working are aluminum oxide, zirconia alumina and ceramic alumina. Less-frequently applied ones include crocus, a natural abrasive of iron-oxide particles used mainly for cleaning and polishing soft metals, and silicon carbide, a hard and brittle grain for nonferrous metals and hard materials.
“The nice thing about silicon carbide is it breaks down very quickly and easily, producing a sharp edge based on its crystalline properties,” said Jim Schnorr, general manager of Wendt USA LLC, Buffalo, N.Y., and president of the Coated Abrasives and Fabricators Association. “The downside is silicon carbide breaks down very quickly and easily, so, in terms of life, it wears out quickly. However, you can essentially grind or cut anything with it, including difficult materials like titanium and carbide.”
Courtesy of Coated Abrasives and Fabricators Association
Electrostatic coating is the most widely used process for applying abrasive to the backing of coated-abrasive products. The process leaves the abrasive grains standing upright, perpendicular to the backing, with the sharper ends of the grains pointing up and away from the backing.
Long described Al2O3 as an entry-level abrasive grain that provides a 20 to 25 percent utilization rate. The blocky grain is tough, meaning it resists fracturing, and is suited to grinding materials that are not considered difficult to machine, such as carbon steels. The next level is zirconia alumina, which he noted Norton invented in and continues to refine. The self-sharpening grain is well-suited for heavy grinding because the controlled fracturing continually produces sharp, new abrading points.
The top tier, according to Long, is ceramic alumina because it cuts at a higher rate compared to the other abrasives. It is a long-lasting, dense abrasive that produces new, sharp cutting edges as micron-sized particles break off during use. “You get about 80 to 85 percent utilization rate of the grain,” he said.
Ceramic alumina is produced via the seeded-gel process, which grows the grains to specific grit sizes, Long explained, whereas zirconia alumina and Al2O3 are “fired,” crushed and screened to achieve the desired grain size. Ceramic alumina and zirconia alumina are sometimes blended, depending on the application. “They work well together for high stock removal,” he said.
Instead of wearing and becoming dull like “old tried-and-true” Al2O3, zirconia and ceramic grains break apart to expose sharp, new cutting edges, Schnorr pointed out. “If they’re used properly, a physical reaction takes place that causes microfractures within the abrasive grains. The microfractures are generated by applying pressure to the abrasive media as it is used.”
Backing Options
Used to carry and support the abrasive grains, backing materials come in four types: paper, film, cloth and fiber. Some manufacturers consider a film, or latex, backing to be almost like plastic paper. The order of that list indicates the relative cost of each type, from low to high, Long said. “In many cases, you want to make sure you need the finishing capabilities that film provides. Otherwise, if you use film on a 60-grit disc, for example, you’re not getting a very good finish anyway so why spend the money on a more expensive film backing?”
Long explained that the grit range for film is usually 220 to 3,000, whereas the range for paper is 40 to 2,000, cloth is 24 to 600, and fiber, being the workhorse, is 16 to 80.
Being the lowest-cost option, paper tends to be the least durable, Schnorr noted, adding that a paper backing is often found in hand-use applications, such as sanding.
However, not all paper backings are equal, as they are divided into six weights designated with the letters A to F. A has a weight of 70 g/m2, B is 100 g/m2, C is 120 g/m2, D is 160 g/m2, E is 250 g/m2 and F is 300 g/m2, with E and F generally considered heavy enough for use as belt materials.
Schnorr said a cotton backing is stronger and more durable than paper, but tends break down fairly easily. “The benefit of cotton is it operates very cool and allows heat to dissipate.”
The stronger cloth materials include polyester and polyester-cotton blends. However, Schnorr added that more durable cloth materials tend to hold heat and can cause other problems, such as leaving residue on the work surface or causing heat discoloration.
VSM Abrasives stated that cloth backings are designated by their flexibility, identified as E (extremely flexible), F (very flexible), J (flexible), T (moderate), X (sturdy) and Y (very sturdy). Mechanical flexing of the coated abrasives creates this range of flexibility, and the types of flexes the company employs include (from stiffest to most flexible) single-, double- and full-flex.
Courtesy of VSM Abrasives
Four types of grains for coated abrasives are (left to right) aluminum oxide, silicon carbide, zirconia alumina and ceramic alumina.
The flexing procedure creates a controlled cracking in the bond, which ensures consistent flex characteristics and enhances stock removal through improved grain regeneration, according to VSM. The flex of a coated-abrasive product is inversely related to product life, and, as a rule of thumb, the company recommends using the stiffest product possible for an application.
When producing fiber backings, several sheets of paper stock are combined via chemicals, heat and pressure in a process called “vulcanizing.” Therefore, some refer to that type of backing as vulcanized fiber.
Courtesy of VSM Abrasives
A typical coated abrasive has a backing, make (base) coat, size coat and mineral.
Courtesy of Superior Abrasives LLC
The spring-like cutting action of a nonwoven abrasive (left) is easier on the workpiece surface than a coated abrasive (right) and is ideal for manual operations that require finesse, according to Superior Abrasives.
Although most applications for paper-backed coated abrasives are performed dry, paper can be chemically treated to make it waterproof. If an application, such as one using a fixed-base belt, requires coolant, the vast majority of Norton’s cloth-backed products are backed with polyester, which is waterproof, as is film. “There’s no treatment for fiber because nobody uses it wet,” Long said.
The backing may be protected when applying coolant, which is done to cool the process or dampen the dust created, but that doesn’t mean the abrasive is protected. Schnorr explained that coolants tend to dissolve the resin bonds that hold the grains in place. “Most coated abrasives do not react well to extended exposure to liquids, moisture or high humidity.”
Switching to Nonwoven
Although available for a couple of decades, nonwoven abrasives use is growing, according to Long. They differ from the traditional definition of a coated abrasive because a nonwoven backing can be impregnated with a layer of abrasives rather than just the top like coated abrasives. “For the most part, a nonwoven is coated,” he said. “It’s just coated on a synthetic nylon backing rather than on a cloth, film, paper or fiber backing.”
With nonwoven abrasives, end users can boost productivity when selecting a high-performance substrate and a premium abrasive, according to Long. “For example, I can offer a Blaze Rapid Strip product, which is a ceramic alumina grain coated onto a highly porous, highly aggressive nonwoven substrate,” he said. “With that particular product, an operator of a right-angle grinder can dramatically improve his wheel life and stock-removal rate over a comparable fiber disc.”
Courtesy of Norton/Saint-Gobain
Norton/Saint-Gobain offers an array of nonwoven abrasive discs.
National Abrasives’ Murak concurred that the nonwoven segment of the abrasives’ market is becoming more prevalent, noting “we mostly sell nonwoven.”
Compared to a bonded abrasive, such as a grinding wheel, which is comprised primarily of abrasive grain, a coated abrasive has only a single layer of abrasive. Therefore, Long noted, it’s not much more expensive to switch to a premium grain, say, going from Al2O3 to zirconia alumina, on a coated-abrasive product. “I can make a premium fiber disc with a lot less ‘premium’ than I can in a premium grinding wheel,” he said. “For example, we don’t even make aluminum-oxide flap discs anymore and hardly anyone else does in the industry. Why would you want to buy a disc for $4 when for $4.50 you could get something that lasts three times as long?”
Nonetheless, extending product life by switching to a longer-lasting grain is not always the best approach to enhancing efficiency, Schnorr pointed out, noting the abrasive is one of the lowest costs in any operation. Therefore, switching the grain to boost the metal-removal rate while achieving a comparable surface finish makes more sense. “What you should be looking for is time savings. Get the job done faster as labor costs tend to have a greater influence on the final cost of the product.” CTE
Coated abrasives can have coatings. Sometimes referred to as a grinding aid or sizing, a top coat is a chemical coating that can be deposited on a coated abrasive to help dissipate the heat of grinding. To effectively use zirconia-alumina and ceramic-alumina grains, an operator needs to exert a fair amount of pressure when processing stainless steel, for example, Wendt USA LLC’s Jim Schnorr noted. A grinding aid reduces the pressure-generated heat to prevent bluing or other discoloration of the workpiece surface.
Norton/Saint-Gobain’s David J. Long added that a high temperature at the abrasive/workpiece interface heats the bond, possibly causing premature grain release. “The more you can take the heat off of that grain by putting a lubricant on it,” he said, “the more the bond holds that grain in so it continually resharpens and does its work.”
In contrast to coating grains to increase lubricity around the grains—called “supersizing”—Long noted stearating applies a coating between the grains. He explained that supersizing coats the grain itself to lower the grinding temperature on individual grains when processing difficult-to-grind materials, while stearating fills the space between grains with a lubricant to prevent material from the workpiece loading the face of the abrasive tool and to keep the grains exposed. A stearate or other no-fill material, such as a wax, is particularly beneficial when processing soft metals, which tend to load the face.
—A. Richter
National Abrasives Inc.
(717) 697-
www.nainc.org
Norton/Saint-Gobain
(254) 918-
www.nortonabrasives.com
VSM Abrasives Corp.
(800) 737-
www.vsmabrasives.com
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