Choosing The Right Grinding Wheel

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The grinding wheel is an abrasive cutting tool, which functions similarly to the teeth of a saw. However, unlike a saw that has teeth only along its edge, the grinding wheel has abrasive grains distributed throughout its structure. Thousands of these hard grains work against the workpiece to cut away small chips of material.

Abrasive suppliers provide a wide array of products designed for various grinding applications in metalworking. Choosing the wrong product can be costly for your shop in terms of time and resources. This article outlines the key factors to consider when selecting the best grinding wheel for your specific needs.

Abrasives, Grits, and Grains

Grinding wheels consist of two primary elements: the abrasive grains responsible for cutting, and the bond that holds these grains together, providing support while cutting. The ratio of grain to bond and their arrangement in the wheel dictate its overall structure.

The choice of abrasive depends on its interaction with the work material. The optimal abrasive maintains sharpness with minimal dulling. As dulling occurs, the abrasive fractures, producing new cutting edges.

Different abrasive types possess unique properties, including hardness, strength, toughness, and impact resistance.

Aluminum oxide remains the most commonly used abrasive for grinding wheels, particularly for carbon steel, alloy steel, high-speed steel, annealed malleable iron, wrought iron, bronzes, and similar metals. Numerous types of aluminum oxide are tailored for specific grinding tasks, each identified by a unique designation consisting of letters and numbers varying by manufacturer.

Zirconia alumina represents another aggressive abrasive family, composed of differing ratios of aluminum oxide and zirconium oxide. This combination results in a tough, durable abrasive suitable for rough grinding applications, such as cut-off operations, across various steels and alloys. Several types of zirconia alumina are also available for matching specific needs.

Silicon carbide is employed for grinding gray iron, chilled iron, brass, soft bronze, aluminum, stone, rubber, and other non-ferrous materials.

Ceramic aluminum oxide is a significant innovation in abrasive materials. Manufactured through a gel sintering process, it yields an abrasive characterized by controlled fracturing at a submicron level, continuously generating numerous new cutting points. This highly durable abrasive is ideal for precision grinding on difficult-to-grind steels and alloys, often blended with other abrasives to enhance performance for specific applications.

Grain size is another crucial factor to consider. Each grinding wheel is marked with a number that signifies the grit size, indicating the size of individual grains. A higher number corresponds to a finer grit, while lower numbers (such as 10, 16, or 24) indicate coarser grains suitable for rapid stock removal without concern for finish quality. Conversely, fine grit wheels (70, 100, or 180) are employed for delicate finishes and hard, brittle materials.

Buying Bonds

For an abrasive wheel to cut efficiently, it must contain the appropriate bond material. The bond not only holds the abrasive grains but must also wear down as the grains are utilized, exposing fresh cutting points.

Conventional grinding wheels utilize three principal bond types, each imparting distinct characteristics to the wheel's grinding action. The bond selected depends on factors such as operating speed, grinding process, required precision, and the material being ground.

The majority of grinding wheels feature vitrified bonds, comprised of a blend of clays. These clays fuse with abrasive grains under the high temperatures of the grinding wheel manufacturing process, creating a glass-like structure that supports each abrasive grain securely.

Wheels constructed with vitrified bonds are rigid, strong, and porous, effectively removing material while adhering to precise specifications. They are impervious to water, acids, oils, and temperature variations. However, although hard, vitrified bonds are also brittle and may fracture under grinding pressure.

Organic bonds, such as resinoid bonds, soften with heat during grinding. They are ideal for applications that require rapid stock removal or improved finishes. Resinoid bond wheels are preferred in fabrication shops, foundries, and for saw sharpening tasks.

Another type, rubber bonds, offers a smooth grinding action, making them suitable for applications demanding high-quality finishes, such as bearings and cut-off wheels where burrs or burning must be minimized.

The bond strength is denoted by the wheel's grade. Wheels classified as hard grade provide extended life, effectively handling high-horsepower operations and tasks with narrow contact areas. Soft grade wheels, on the other hand, promote rapid stock removal on larger areas and harder materials.

Wheel Shapes

Grinding wheels come in various shapes, with the straight wheel being the most commonly recognized form. The part of the wheel that engages with the workpiece is located on the periphery. A recessed wheel allows it to fit onto a machine spindle flange assembly.

Some wheels feature cutting faces on their sides, including cylinder, cup, and dish wheels. Segmented wheels consist of bonded abrasive sections arranged for continuous or intermittent grinding, commonly used for hard-to-reach areas like cutting tool teeth.

Mounted wheels, typically small and featuring unique shapes like cones or plugs, are permanently mounted on steel mandrels for various internal grinding tasks.

Grinding wheels are labeled with maximum safe operating speeds. It's crucial not to exceed these limits, and the safest practice is to avoid mounting any wheel on a grinder capable of surpassing that speed.

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Tying It All Together

Several factors influence the selection of the best grinding wheel for your task. The type of material to be ground primarily determines the required abrasive. For steels and steel alloys, aluminum oxide or zirconia alumina is recommended, whereas silicon carbide is preferred for cast iron, nonferrous metals, and non-metallic materials.

Hard, brittle materials typically necessitate a fine grit size and a softer grade. As hard materials resist abrasive penetration, finer grits and softer grades allow dull grains to break away and expose new, sharp edges. Conversely, coarser grit and hard grade wheels are ideal for soft, ductile materials that are easy to penetrate.

The amount of stock removal required also plays a role. Coarse grits facilitate rapid stock removal due to their capacity for deep cuts, while slightly finer grits can be more effective for hard-to-penetrate materials with an abundance of cutting points.

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Wheels with vitrified bonds are apt for fast cutting, while resin, rubber, or shellac bonds suit applications requiring lesser stock removal or better finishing quality.

Another influencing factor for bond choice is the wheel's operational speed. Vitrified wheels typically operate below 6,500 surface feet per minute; faster speeds risk bond fractures. Organic bond wheels are preferred between 6,500 and 9,500 surface feet per minute, where specialized wheels may be needed for high-speed grinding.

Regardless of circumstances, never exceed the safe operating speeds indicated on the wheel or its label—be it in rpm or sfm.

The contact area between the wheel and workpiece is vital for grinding performance. For broader areas, select coarser grit and softer grade wheels for a cool cutting operation under heavier loads. Conversely, smaller contact areas require finer grits and harder grades to endure greater pressure.

The severity of the grinding action, defined by the pressure applied during grinding, also affects abrasive choice. Certain abrasives are engineered to endure tougher conditions while grinding steel or steel alloys.

Machine horsepower must be accounted for as well. Harder grade wheels generally perform best on higher horsepower machines, while softer grades are better suited for machines with lower horsepower.

Care and Feeding

Correct handling, mounting, and usage of grinding wheels are critical for safety and performance.

Wheels should be stored in environments free from deformation or damage, avoiding extreme temperatures and humidity that could degrade bond quality.

New wheels should be carefully inspected post-unpacking, and all used wheels returning to storage must also be checked for wear or damage.

Handle wheels with care to prevent drops or impacts that can cause cracks. Carry them rather than rolling; if they are too heavy, use a hand truck or similar equipment for safe transport.

Before mounting a vitrified wheel, perform a ring test according to the American National Standards Institute's Safety Code to check for cracks. Always discard cracked wheels.

Ensure that the machine's spindle rpm doesn't surpass the wheel's maximum safe speed. Always use a wheel with a snug fit on the spindle. Do not modify the wheel’s center hole, and utilize proper clean, recessed flanges for mounting.

Only tighten the spindle nut enough to secure the wheel; avoid overtightening. When using directional wheels, be sure the arrow indicating rotation aligns with spindle direction.

All guards and covers must be in position and secure before operating the machine. After mounting the wheel and ensuring safety, allow the machine to run at operating speed for a minute before commencing grinding.

Grind only on the face of a straight wheel and only on the side of cylinder, cup, or segment wheels. Make contact gently to prevent damage, and never force grinding to the point of noticeable motor slowdown or material heating, as indicated by the machine's ammeter.

In the event of a wheel breakage, inspect the machine to ensure that protective measures remain intact and that components such as flanges, spindles, and mounting nuts are undamaged.

System Analysis

The grinding wheel is a singular part of an engineered system composed of the wheel, machine tool, work material, and operational parameters, with each influencing the others. To enhance grinding performance, it is critical to select the most suitable grinding wheel for the specific process components.

About the author: Joe Sullivan was a senior product manager for Norton Company, Worcester, Massachusetts.

What Are Superabrasives?

Superabrasives form a unique category of bonded abrasives engineered for grinding some of the hardest and most challenging materials.

As carbides, high-speed steels, PCD, PCBN, ceramics, and similar materials approach the hardness of conventional abrasives, special superabrasives such as diamond and CBN are utilized to sharpen these materials.

Offering extreme hardness, superabrasives are generally more expensive than conventional options (silicon carbide and aluminum oxide). As such, their wheel construction differs from conventional products; superabrasive wheels have a core material shaping the wheel, with abrasive only on the cutting edge.

These wheels are available in standard grit ranges, typically from 46 to 2,000 grit. Similar to conventional options, they can be tailored in various grades and concentrations to suit the operation's requirements.

Four bond types are employed in superabrasive wheels: resinoid bond wheels provide fast, cool cutting; vitrified bond wheels offer a balance of rapid cutting and wear resistance; metal bond wheels are essential for grinding and cutting nonmetallic materials; and single-layer plated wheels excel in operations requiring both quick stock removal and complex form generation.