How To Choose Grinding Wheel - CRATEX Abrasives

Grinding wheels consist of two main components: the abrasive (cutting) grains and the bonding material that holds the grains together. The size of the grains and their spacing, the type of the bonding material and the abrasive used will determine the structure of the grinding wheel.

Types of Abrasives

Different abrasive types have different properties when it comes to impact resistance, strength, hardness and fracture toughness. Two types of abrasives can be used for making grinding wheels: natural and manufactured (almost entirely replaced natural materials). The abrasives consist of several chemical and mineral ingredients and the formula varies depending on the hardness of the material the wheel is intended to grind, polish or cut. There are two types of abrasive grains used to make grinding wheels. However, there are different formulas that are used even within these two categories. For example, a different formula will be used for wheels that are designed for hard steel and those designed for soft steel.

1. Conventional abrasives – aluminum oxide (A), silicon carbide (S), Ceramic (C)
2. Super-abrasives - diamond (D, MD, SD), cubic boron nitride (CBN)

Aluminum-oxide based abrasives used most often. They are used for metalwork–grinding most steels and other ferrous materials (alloy steel, high-speed steel, carbon-steel, annealed malleable and wrought iron, bronzes and other similar metals). The color and toughness of aluminum-oxide grinding wheels are influenced by the level of purity, and the abrasives used the most are 95% pure. There are various types of aluminum-oxide abrasives and their application varies depending on the grinding jobs. For example, nearly pure white wheels are the toughest, hard wearing and used for grinding heat-sensitive and high-strength steels.

Silicon-carbide based abrasives are used for cutting gray and chilled iron, cement, stone, and other non-ferrous materials (copper, brass, soft bronze, aluminum, magnesium) and nonmetallic (ceramics, gemstones, rubber), because they are harder and more brittle than aluminum-oxide wheels. However, although silicon-carbide abrasives are very sharp, they break down quickly and therefore not effective when it comes to high-pressure grinding. They work best under light pressure and are an excellent choice for putting on a smooth finish.

Ceramic grits are aluminum-oxide grits with microstructure much smaller than the conventional aluminum-oxide with a high level of purity. Its grains are sharp microcrystalline grains that cut aggressively when a light to moderate pressure is applied. Thanks to its fast cutting abilities and its strength and hardness, ceramic abrasives are used for precision grinding on titanium, stainless steel and high-nickel alloys. However, these abrasives are particularly heat-sensitive, so it is necessary to pay attention that the engineered process is executed in the correct manner.

Zirconia-alumina based abrasives are made from a different ratio of zirconium-oxide and aluminum-oxide and are in fact a type of ceramic abrasives. They are durable abrasives quite resistant to heat, used for rough grinding and cut-off operations on different steels and steel alloys.  

Natural diamonds can be used for grinding very hard materials (like granite, stone, cemented carbides, marble), but have almost completely been squeezed out of the market by synthetic diamond wheels, due to the low cost of the latter. Manufactured diamonds are used for grinding aluminum oxide cutting tools, very hard steels and cemented carbide.

Cubic boron nitride is sharp, cool and extremely hard abrasive (2.5 times harder than aluminum oxide and almost as hard as diamond). It is used for grinding super-hard, high-speed steels, hardened cast irons, stainless and die steels. Metal-coated abrasives are used for general-purpose grinding, and uncoated abrasives are used in vitrified bond and electroplated metal systems. 

Grit Size

Size of abrasive grains found in a grinding wheel can range from 6 to 10, which is the coarsest, to even 1000, which is the finest. It matches the number of openings in the screen the grains pass through, e.g. the higher the number, the smaller the opening. Grain size influences stock removal rate, surface finish and wheel chip clearance - the larger the grain, the faster the cutting, but the poorer the surface finish. The finer the grain, the more suitable for fine, precision finish work.

For example, the coarse size is used when surface finish is not that important, like snagging operations in foundries and steel mills; the medium size is used when there’s a need to get a certain control over the surface finish, like high-stock removal operations; fine sizes grains are used for fine-finish and for operations that require small-diameter grinding; and extra-fine grit sizes are used for lapping and polishing operations.  

Bond Strength & Wheel Structure

Wheels also differ based on the tightness of the bond that holds the abrasive and the grain spacing. The hardness or the grade of the bonding material ranges from A-H which would be classified as ‘soft’, I-P as ‘moderately hard’ to Q-Z which is ‘hard’. Wheel grade will determine the wheel speed, grinding depth, maximum and minimum feed rates, coolant flow, etc. Soft-bonded wheels are quick to release the grains, so the wheel wears out fast. When it comes to hard-bonded wheels, dull grains cause glazing, so the grains wear out fast, so the cutting edges are lost quickly.

As for the grain spacing, it ranges from 1, which would be the densest, to 17 which would be the least dense. Wheels with less grain density are open-structure wheels, they cut more freely, have a greater effect on the finish and can cut deeper and wider with less coolant. 

Types of Bond

The wheels are also differentiated by the bond or the substance that holds the wheel together with varying degrees of strength. Hard-grade bond is a bond that spans between each abrasive grain and provides an extremely strong hold, while soft-grade bonds span little, so a small force is required to cause the release of the grains. 

The bond influences the effectiveness of the cutting operation, and the type you will use depends on the wheel operating speed, the precision required, grinding material and the grinding operation. For example, the wheels with hard-grade bonds are used on high-power machines and jobs with small contact areas. Wheels with soft-grade bonds are used for fast stock removal, for hard materials and jobs with large contact areas.

Standard types of grinding wheel bonds are: 

• Vitrified (glass-based) – this type of bond is used most often - in 75% of all grinding wheels. The wheels with vitrified bond are rigid and strong even at elevated temperatures. Although they are unaffected by oils, acids and water, they have a rather poor shock resistance and can be broken down by the grinding pressure, which limits their application. They’re mostly used to 6,500 SFM;
• Resinoid (resin-based) – this type holds the second place as far as the popularity goes and it’s the kind of a bond that softens under the grinding heat. The resin-based wheels are primarily used in rough cut-off, rapid stock removal operations, as well as those where better finishes are required - all at speeds up to 16,500 SFM;
• Silicate (silicate-based) – silicate bonded abrasives are used when there must be minimal heat generated by grinding at a speed that is limited below 4,500 SFM.
• Shellac (shellac-based) – wheels with shellac bond are not used for heavy-duty grinding, but for producing smooth finishes on parts like crankpins, rolls, camshafts and cutlery;
• Rubber (natural or synthetic rubber-based) wheels are quite strong and therefore primarily used as thin cut-off wheels when burr and burn must be kept to a minimum, or driving wheels on centerless grinders, or to achieve extremely fine, smooth and high-quality finishes;
• Metal (various alloys) – these are primarily used for bonding diamond abrasives, in operations such as electrolytic grinding.

Sizes & Types of Grinding Wheels

The diameter of a grinding wheel can range from just 1" to 20", they can go from just 0.035" to up to 1/2" thick. The smallest wheels are used for different operations like auto body work while the largest ones can cut through railway tracks and thick metal constructions.

There are also different types of grinding wheels, such as:

• Straight wheels, which are the most common style of wheels used for cylindrical, centerless and surface grinding operations, and are found on bench or pedestal grinders; they’re used for creating slightly concave surface;
• Cylinder or wheel rings, which are used in vertical or horizontal spindle grinders and have no center mounting support; they’re used for creating flat surfaces by using the end face of the wheel;
• Tapered wheels, which typically used for operations, like grinding thread and gear teeth; it’s basically a straight wheel that tapers towards wheel center, but it can accept higher lateral loads;
• Straight cup wheels, which are used for producing additional radial grinding surface;
• Dish cup wheels, which are used primarily in cutter grinding and jig grinding;
• Saucer wheels, typically used for grinding milling cutters and twist drills;
• Diamond wheels, used for grinding extremely hard materials (like carbide cutting tips, concrete, gemstones, etc.);
• Small mounted wheels, which are mounted on a steel mandrel, typically of hand tool; they come at all kinds of variations (different grains, bonds, etc.) and are therefore used for various operations, typically different off-hand and precision grinding jobs;
• Cut-off wheels, which are normally used on angle grinders for quick removal or trimming; they’re thin and usually have radial fibers as reinforcement. 

Conclusion: The Perfect Wheel Is…?

We showed you different grinding wheel properties, all you need to do now is to put the correct ones together and get a perfect grinding wheel for your needs!

In case you are working on a hard material, you’ll probably want to use a softer grade and a fine-grit wheel, as hard materials resist the drilling of the abrasive grains into the surface, which dulls them quite quickly. The combination of a finer grit and a softer grade would be perfect, as the grains will dull, break away and leave new, sharp cutting points. In case you are working on softer materials, wheels with a coarse grit and harder grade would be the correct way to go.

Stock-removal speed is another factor to consider, so in case you need a wheel with rapid stock removal, you should choose coarser grits. The penetration level is greater, and they leave heavier cuts. Wheels with finer grits cut faster, so they are a perfect option for material that is hard to penetrate.

The abrasive material used in the making of the wheels, as well as the bond material, are one of the first things to consider. Different abrasive types are intended to be used on different materials – like we mentioned earlier in the article, aluminum oxide or zirconia-alumina should be used on steels and steel alloys, while silicon carbide is a suitable option for non-metallics, non-ferrous metals and cast iron. As for the different types of bonds, vitrified bonds have fast cutting abilities, rubber, resin or shellac bonds are used in case you wish to remove a smaller amount of stock or in case you want to achieve finer finishes.

Wheel speed is also an important factor to consider as different wheels can be used at different speeds. For example, vitrified wheels are used at speeds up to 6,500 SFPM, while organic-bond wheels can be used on speeds between 6,500 and 9,500 SFPM.

You should also think about the contact area between the grinding wheel and workpiece. In case you are dealing with a large area of contact, you should probably use a softer grade and coarser grit wheel. Finer grit and harder grade wheels are more suitable for a smaller area of grinding contact.

There are also wheels that are designed to bear a lot of pressure that created when the wheel and the workpiece are pressed together, plus, grinding machine horsepower should not be overlooked neither. A useful guideline would be: if the horsepower is smaller than the diameter of the grinding wheel, then a softer-grade wheel should be used, and harder-grade wheel should be used in opposite situations.