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ABRASIVES :

An abrasive is a small, nonmetallic hard particle with sharp edges and an irregular shape. Unlike cutting tools, abrasives are capable of removing small amounts of material from a surface through a cutting process that produces tiny chips. They are commonly used in various applications such as grinding, sanding, honing, lapping, buffing, and polishing. Abrasives can be found in forms like grinding wheels, sandpaper, abrasive discs, belts, and stones. They are essential in achieving fine surface finishes, precise dimensional tolerances, and shaping hard materials in manufacturing and other industries.

Abrasive wheel grade and structure

  1. Grade: The grade of an abrasive wheel refers to the hardness or strength of the abrasive material used in the wheel. It indicates the ability of the wheel to retain its shape and withstand the forces generated during grinding or cutting operations. The grade is typically represented by a letter (such as A, B, C, etc.) or a number (such as 1, 2, 3, etc.), with higher values indicating harder grades.
  • Softer grade wheels (lower values) are more suitable for grinding or cutting softer materials as they provide better self-sharpening characteristics and allow for cooler cutting action. They are less likely to cause excessive heat build-up or damage the workpiece.
  • Harder grade wheels (higher values) are used for grinding or cutting harder materials as they offer better resistance to wear and longer wheel life. They are capable of removing more material but may generate higher heat during the process.
  1. Structure: The structure of an abrasive wheel refers to the density or spacing of the abrasive particles and the bond material that holds them together. It determines the porosity or openness of the wheel and affects factors like chip clearance, heat dissipation, and stock removal rate. The structure is commonly described as open, dense, or standard.
  • Open structure wheels have more spacing between the abrasive particles, allowing for better chip clearance and cooler cutting. They are suitable for grinding softer materials and for applications where heat build-up is a concern.
  • Dense structure wheels have closely packed abrasive particles, providing better support and stability. They are used for grinding harder materials and for precision applications where dimensional accuracy is important.
  • Standard structure wheels strike a balance between open and dense structures and are suitable for general-purpose grinding and cutting tasks.

Both grade and structure should be carefully selected based on the material being worked on, the desired finish, and the specific requirements of the grinding or cutting operation to ensure optimal performance and safety.

VARIETIES OF ABRASIVES

Abrasives are materials, often minerals, used for shaping or finishing workpieces through the process of rubbing, which leads to the gradual removal of material by friction. Finishing typically involves achieving a smooth and reflective surface, but it can also involve creating rougher finishes like satin, matte, or beaded textures. In essence, ceramics used to cut, grind, and polish softer materials are referred to as abrasives.

Abrasive materials consist of hard crystals that can occur naturally or be manufactured. The most commonly utilized materials include aluminum oxide, silicon carbide, cubic boron nitride, and diamond. There are also other materials such as garnet, zirconium, glass, and even walnut shells used for specialized applications.

There are two main types of abrasives: NATURAL and MANUFACTURED.

Important natural abrasives like emery, corundum, and diamond are primarily used in specific types of grinding wheels and honing stones. Manufactured or synthetic abrasives are now considered to be superior and just as effective as natural abrasives. Examples of manufactured abrasives include aluminum oxide, silicon carbide, boron carbide, and titanium carbide. Aluminum oxide is mainly used for grinding carbide tools and non-ferrous metals. Approximately 40 percent of a grinding wheel consists of abrasive material.

Abrasives can be classified as bonded or coated. Bonded abrasives are attached to a matrix known as a binder, which is typically made of clay, rubber, resin, or glass. On the other hand, coated abrasives are affixed to a backing material such as cloth, paper, resin, rubber, polyester, or even metal. An example of a coated abrasive is sandpaper, where the abrasive particles are applied to paper that can be held by hand. The pressure and speed of hand movement can be adjusted to control the smoothing strength of the sandpaper. Abrasives play a vital role in various industries, as they enable the production of finely crafted furniture and the construction of complex buildings.

APPLICATIONS OF ABRASIVES:

The applications of abrasives generally fall into the following categories:

  1. Surface cleaning and removal of excess material, such as rough off-hand grinding in foundries.

  2. Shaping, including grinding and tool sharpening.

  3. Sizing, primarily in precision grinding.

  4. Separating, as in cut-off or slicing operations.

Over the past century, manufactured abrasives like silicon carbide and aluminum oxide have largely replaced natural abrasives. Synthetic diamonds have also largely replaced natural diamonds. The success of manufactured abrasives stems from their superior and controllable properties, as well as their consistent uniformity.

Both silicon carbide and aluminum oxide abrasives are extremely hard and brittle, which enables them to form sharp edges. These sharp edges facilitate the penetration of the work material and reduce the heat generated during abrasion. Such abrasives are suitable for precision grinding and finishing. Tougher abrasives, which resist fracture and have greater longevity, are used for rough grinding.

USES OF ABRASIVES:

Industries employ abrasives in three primary forms:

  1. Bonded forms, where abrasives are combined to create solid tools like grinding wheels, cylinders, rings, cups, segments, or sticks.

  2. Coated forms, where abrasives are applied to backings made of paper or cloth, resulting in sheets (such as sandpaper), strips, or belts.

  3. Loose forms, where abrasives are held in a liquid or solid carrier for tasks like polishing or tumbling, or propelled by air or water pressure against a work surface (such as sandblasting for buildings).

Abrasion typically occurs when a surface is scratched. As a general rule, a substance can only be seriously scratched by a material that is harder than itself. This principle forms the basis for the Mohs scale of hardness, which ranks materials based on their ability to scratch materials of lower hardness. Abrasives are typically refractory materials with hardness values ranging from 6 to 10 on the Mohs scale. They are used to reduce, smooth, clean, or polish surfaces of less hard substances like metal, glass, plastic, stone, or wood. During abrasion, abrasive particles initially penetrate the abraded material and then cause the removal of particles from the surface. The ease with which abrasive particles dig into the surface depends on the hardness of the abraded material, while the ease of tearing off the deformed surface depends on the strength and, in some cases, the toughness of the material. Hardness is usually the most important factor determining a material's resistance to abrasion, as when two surfaces move against each other, the peaks of microscopic irregularities must shift, increase in hardness, or break. If local stresses are high enough, a tiny volume of the abraded material will fail, resulting in the detachment of a small particle. This type of abrasion occurs regardless of whether the contact between the surfaces is due to sliding, rolling, or impact. Some forms of abrasion involve little or no impact, while in others, the energy of the impact plays a crucial role in determining the abrasive's effectiveness. Brittle materials, for example, tend to shatter upon impact, and their abrasion may resemble erosion more than fracture.



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