Abrasive materials are hard, rigid materials used to grind, sand, or polish other softer materials. They are an essential tool in many manufacturing processes and are used in various industries, including metalworking, construction, and automotive. There are many different abrasive materials, each with unique properties and characteristics. This article will explore the different abrasive materials types, properties, features, and typical uses.
Table of Contents
What are abrasives?
Abrasives are natural and synthetic substances used to grind, polish, scrape, scrub, clean, or otherwise remove solid material, usually by rubbing action (as in a grinding wheel) but also by impact (shot blasting) under pressure.
Data on the discovery, early use, adaptation, and development of abrasives before the 20th century are scant. From the most premature and scant information available, the science of abrasives began when, to create tools, humans selected certain rocks over others because of their valuable properties. Evidence shows that some grinding machines sawed stones as early as 4000 BC in Egypt.
The actual expansion of the use of abrasives coincided with the beginning of metallurgy in the Middle East around 2000 BC. The first confirmed use of abrasives for grinding metal comes from a steel dagger found along with a whetstone dating to 1500 BC. c. Natural and manufactured abrasive substances play an essential role in shaping and finishing numerous products with various applications.
Properties and characteristics of abrasives
The most important physical properties that qualify as abrasives are hardness, toughness (or stiffness), grain size and shape, fracture (or cleavage), and purity (or uniformity).
Additional considerations for manufacturing bonded abrasive products, such as grinding wheels, including high-temperature stability and bonding characteristics of the grain surfaces.
The economic factors of cost and availability are always important.
No particular property is paramount to any use. For example, some applications require extreme hardness and toughness, such as diamonds for drill bits.
For other uses, the essential factors are hardness, the ability to break down slowly, and, in the process, develop fresh cutting edges when the grains wear away.
For example, highly cleavable, friable, or tough grains are not desirable in garnet sandpaper.
Extreme hardness may be objectionable for other uses, such as abrasives for toothpaste and window soaps.
The different abrasives can rarely be entirely interchangeable for efficiency uses in critical applications.
Although both crushed quartz and garnet are used in sandpaper, the two abrasives are not interchangeable in their applications.
In the final analysis, the choice of a high-grade abrasive depends on the quality and amount of work the abrasive does per unit cost.
The Initial cost of an artificial abrasive can be much higher than that of a natural abrasive. Still, synthetic minerals can work better and faster, so the final price is lower.
For this reason, artificial abrasives have replaced mainly natural abrasives.
Classification of abrasives
Abrasives can be divided into two general classes, natural and artificial industrial.
Natural abrasives
Natural abrasives are all rocks and minerals used for abrasive purposes without chemical or physical changes other than crushing, shaping, or adhering into suitable shapes.
The most common examples include diamonds, garnets, diatomite, tripoli, pumice, silica sand, and quartz.
Artificial or industrial abrasives
Metals or mineral raw materials’ thermal or chemical action obtains artificial or manufactured abrasives.
Manufactured products can be substituted for most natural products, generally at a higher initial cost but with greater efficiency, but this is only sometimes the case. For example, there is no satisfactorily manufactured garnet substitute for making paper and coated abrasive cloths.
For some abrasives, such as hunting stones, and others whose use is declining, manufacturing substitutes has not been economically attractive.
For a low-priced product, such as steel shot blasting, molten aluminum oxide and silicon carbide can be substituted.
The use of natural abrasives such as sand and quartz has seriously declined due to government regulation of free crystalline silica (a carcinogen).
The transition has not been a net loss to the mining industry because virtually all manufactured abrasives are made from raw mineral materials.
Types of abrasive products
Loose abrasive grains
Abrasive grains are produced from various materials for use as grains and incorporation into other products. Abrasive grains are essential products and are the starting point for making bonded shapes, coated abrasives, grinding tools, polishes, cleaners, grinding pastes, and other compounds. For example, garnet, flint, and chert are used to polish silica sand and other natural mineral grains such as corundum.
Manufactured grains such as fused alumina, silicon carbide, and steel shot are also used. Each use has its special requirements, and all grains except steel shot have a Mohs hardness of 7 or more.
In addition to hardness, the physical properties of toughness, grain shape, grain size, uniformity, and specific gravity are essential. The grains must be rigid, so they do not quickly disintegrate under impact. For some uses, rounded grains are considered desirable. For others, sharp cutting edges are preferred.
Uniformity of grain size and other physical properties is always desirable. The higher the specific gravity, the greater the impact force for grains of equal measure.
Bonded abrasives
The reduced-size abrasive grains bond and press or shape into a wide variety of bonded abrasives such as grinding wheels, sharpening stones, and tumbling media. The abrasives in such products were originally corundum or emery.
The use of natural abrasives has decreased considerably with the introduction of electric furnace abrasives (aluminum oxide and silicon carbide).
The five main types of bonded abrasives, depending on the type of bond and method of manufacture, are as follows:
- Vitrified grinding wheels with a clay-feldspar bond are manufactured in ceramic kilns. Due to their high rigidity and dimensional stability, vitrified bonded wheels are preferred for precision grinding operations. They are unaffected by water, acids, oils, and ordinary temperature variations.
- Resin wheels with a hard synthetic resin binding. These are high-speed wheels used in foundries and welding shops and for billet conditioning. They are also used in thread-cutting and sanding operations.
- Rubber wheels bonded with natural or synthetic rubber. These are elastic and are used for grinding ball races, most centerless feed wheels, and for portable hookup operations where the finish is an important consideration. Rubber-bonded cut-off wheels can be made very thin.
- Shellac wheels produce high finishes on items such as camshafts and paper rolls.
- Silicate bonds for applications where heat generated during grinding must be kept to a minimum. Silicate bonded wheels are smooth acting and are used on abrasive-edged tools of all types.
Coated abrasives
Coated abrasives consist of sized abrasive grains cemented to a paper, woven cloth, polyester film, or vulcanized fiber backing. Sandpaper was initially coated with silica sand, but today crushed quartz is used in silica-like paper because its grains have much sharper cutting edges.
The leading abrasives used for this purpose are aluminum oxide, silicon carbide, alumina-zirconia, alumina Sol-Gel, crushed quartz, and garnet. Most emery cloths are now coated with silicon carbide or aluminum oxide.
Flint paper in the United States is coated with crushed quartz, but true flint is used in Europe.
Granules and powders for soap, cleaners, and polishes
Many different materials, mostly natural, but some manufactured, are used to make soaps, cleaners, and polishes. Feldspar, pumice, sand, ground quartz, tripoli, diatomaceous earth, clay, and wood flour are ingredients in hand soaps and degreasers. Low price is often the main factor in abrasive selection. Occasionally, the abrasive is a crucial factor in the formulation and use of a product.
A household cleaner should not contain quartz or any mineral with a hardness equal to or greater than that of glass or enamel, most of which have a Mohs hardness of 6 or less. Otherwise, the glass or enamel surface would be easily scratched.
Such cleaners should not contain calcium carbonate, calcium sulfate, or other readily reactive lime compounds because lime reacts with most soaps to form insoluble substances that are difficult to remove from glass or enamel surfaces.
Uses and applications of abrasives
Abrasives are used to cut, grind, smooth, and drill all types of material, including steel, wood, fiberglass, plastic, and more.
There is a wide range of applications for abrasives, both industrial and home shop tools.
In summary, abrasives are used for:
- Cutting wheels are used to cut steel on two machines: hand-held angle grinders and bench-mounted chop saws or drop saws.
- Sanded: smooth surfaces are vital for woodworkers who want to avoid splinters and rough surfaces in home woodworking. Sanding discs will often work if you have a grinder.
- Smoothing is a more intensive metal smoothing process involving steel or metal discs.
- The sharp: sharpens objects, such as knives.
- The polish: makes the surface look nice and shiny. You may want to polish your car to make it look shiny and new from time to time!
- Drilling: Drill bits made with diamonds are used to make oil wells and in mining exploration.
In the home shop, abrasives are most often used for metalworking and woodworking. Most things you make or work on will require cutting, grinding, and finishing to a smooth, strong end product, and the abrasives are the solution.
While abrasive tools can last for years, implements such as sanding discs used to perform abrasions have limited use and need to be replaced, depending on the amount used.