Select the Best Bonded Abrasive

This article serves as a primer
on the fundamentals of
selecting the best conventional
grinding wheel.

Abrasives grits and grains

Grinding wheels and other bonded abrasives have only two major com­ponents. The abrasive grains that do the actual cutting, and the bond that holds the grains together and supports them while they cut. The percentage of grain and bond and their spacing in the wheel determines the wheel's structure.

The par­ticular abrasive used in a wheel is chosen based on the way it will interact with the work material. Each abrasive type is unique, with distinct properties for hardness, strength, fracture toughness, and resistance to impact.

Aluminum oxide is the most com­mon abrasive used in bonded prod­ucts. It is usually the abrasive chosen for grinding carbon steel, alloy steel, highspeed steel, annealed malleable iron, wrought iron, and bronzes and similar metals.

Zirconia alumina is another fam­ily of abrasives, each made from a dif­ferent percentage of aluminum oxide and zirconium oxide.

Silicon carbide is an abrasive used for grinding gray iron, chilled iron, brass, soft bronze, and aluminum, as well as stone, rubber and other nonferrous materials.

Ceramic aluminum oxide is the newest development in abrasives. This is a high purity grain manufac­tured in a gel sintering process. The result is an abrasive with the ability to fracture at a controlled rate at the submicron level, constantly creat­ing thousands of new cutting points. This abrasive is exceptionally hard and strong. It is primarily used for precision grinding in demanding ap­plications on steels and alloys that are the difficult to grind.

Every grinding wheel has a number that designates its grit size. Coarse grains are used for rapid stock re­moval where finish is not important. Fine grit wheels are suitable for im­parting fine finishes, for small areas of contact, and for use with hard, brittle materials.

Bonds types

There are three principal types of bonds used in conventional grind­ing wheels. Each type is capable of giving distinct characteristics to the grinding action of the wheel. The type of bond selected depends on such factors as the wheel operating speed, the type of grinding operation, the precision required, and the material to be ground.

Most grinding wheels are made with vitrified bonds . Grinding wheels made with vitrified bonds are very rigid, strong and porous. They remove stock material at high rates and grind to precise requirements. They are not affected by water, acid, oils or variations in tempera­ture.

Some bonds are made of organic substances. These bonds soften under the heat of grind­ing. The most common organic bond type is the resinoid bond, which is made from synthetic resin. Wheels with resin­oid bonds are good choices for applications that require rapid stock removal, as well as those where better finishes are needed.

Another type of organic bond is rubber . Wheels made with rub­ber bonds offer a smooth grinding action. Rubber bonds are often found in wheels used where a high quality of finish is required, such as ball bearing and roller bear­ing races. The hardness or strength of a bond is designated as the grade of the grinding wheel. The bond is said to have a hard grade if the bond posts or spans between each abrasive grain are very strong and can retain the grains against the grinding forces tending to pry them loose. A wheel is said to have a soft grade if only a small force is needed to release the grains. Therefore, it is the relative amount of bond in the wheel that determines its grade or hardness.

Hard grade wheels are used for lon­ger wheel life, for jobs on high horse­power machines, and for jobs with small or narrow areas of contact. Soft grade wheels are used for rapid stock removal, for jobs with large areas of contact, and for hard materials such as tool steels and carbides.

Selecting Conventional Bonded Abrasives

There are a number of factors that must be considered in order to select the best grinding wheel. The first is the material to be ground. This deter­mines the kind of abrasive that you will need in the wheel. For example, aluminum oxide or zirconia alu­mina should be used for grinding steels and steel alloys. The choice between those two will depend on the type of grinding being done. For grinding cast iron, nonferrous metals and nonmetallic materials, select a silicon carbide abrasive.

Hard, brittle materials generally require a wheel with a fine grit size and a softer grade. Hard materi­als resist the penetration of abrasive grains and cause them to dull quickly. Therefore, the combi­nation of finer grit and softer grade lets abra­sive grains break away as they become dull, exposing fresh, sharp cutting points. Wheels with the coarse grit and hard grade should be chosen for materials that are soft, ductile, and easily penetrated.

Another consideration is the amount of stock to be removed. Coarser grits give rapid stock removal, since they are capable of greater penetration and heavier cuts. However, if the work material is hard to penetrate, a slightly finer grit wheel will cut faster since there are more cutting points to do the work.

Wheels with vitrified bonds provide fast cutting. Resin, rubber or shellac bonds should be chosen if a smaller amount of stock is to be removed, or if the finish requirements are higher.

Another factor is the wheel speed in operation. Unless specifically designated for high speed grinding, vitrified wheels are used at speeds less than 6,500 surface feet per minute; at higher speeds, the vitrified bond may break. Organic bond wheels are generally the choice between 6,500 and 9,500 surface feet per minute.

Working at higher speeds usually requires specially designed wheels for high speed grinding. In any case, do not exceed the safe operating speed shown on the wheel or its blotter.

The next factor to consider is the area of grinding contact between the wheel and the workpiece. For a broad area of contact, use a wheel with coarser grit and softer grade. This ensures a free, cool cutting action under the heavier load im­posed by the size of the surface to be ground. Smaller areas of grinding contact require wheels with finer grits and harder grades to withstand the greater unit pressure.

Next, consider the severity of the grinding action. This is defined as the pressure under which the grinding wheel and the workpiece are brought and held together. Some abrasives have been designed to withstand severe grinding condi­tions when grinding steel and steel alloys.

Grinding machine horsepower must also be considered. If horse­power is less than wheel diameter, a softer grade wheel should be used. If horsepower is greater than wheel diameter, choose a harder grade wheel. In general, harder

Conclusion

Today's grinding processes can truly be considered engineered systems, made up of four key com­ponents: machine tool, abrasive product, work material and opera­tional factors. Manufacturers who want to optimize the productivity of their grinding systems look at all these variables and evaluate how changes to one impact the others when making decisions on which grinding wheel is bestsuited to their applications. TMD

By Bob Smith,

Director, Applications Engineering

SaintGobain Abrasives
nortonabrasives.com