Selecting the right abrasives for your operation

The Nitty Gritty

For each weld configuration and material there is an abrasive wheel that ensures optimal grinding results.

The first step in determining the right wheel for the job is to identify the material being welded. Most abrasives manufacturers offer wheels made of three abrasive materials:

1. Aluminum oxide. For ferrous metals and high-tensile-strength alloys, an aluminum oxide wheel is the best choice.
   
   
2. Zirconia alumina. If the grinder is high-torque or the application is severe, use a zirconia alumina wheel to grind ferrous metals and high-tensile-strength alloys.
   
   
3. Silicon carbide. Select a silicon carbide wheel for nonferrous metals such as copper, aluminum, bronze, and low-tensile-strength materials such as cast iron and ductile iron.
   
   

When choosing a wheel for an application, also consider the hardness of the material you'll be grinding.

To understand this, let's look at how abrasive grains and bonding agents work on a wheel. During grinding, dulled grains in the abrasive wheel create friction and heat that melt the resin bond that holds the abrasive. This releases the dulled grain and exposes a new, sharp one.

A soft-grade abrasive is designed to release new grains more quickly than a hard-grade abrasive, which means that a soft grade will remove the maximum amount of stock while creating the least amount of heat in the base material.

Conversely, a hard-grade abrasive sheds its dulled grains more slowly, so it removes less stock. The rubbing of the dulled grains against the base material increases the amount of heat in the base material.

The softness of the steel makes the stock easier to remove with less heat buildup. Soft steel is less resistant to the abrasive and, as a result, doesn't cause as much friction.

A hard abrasive wheel lasts longer than a soft one, so using the hardest-grade abrasive contributes to longer wheel life. Therefore, when grinding soft steels, harder abrasives can be used effectively.

A zirconia alumina abrasive wheel works somewhat differently. Instead of being released by friction, the grain in a zirconia alumina wheel actually fractures to expose sharp, new facets that continue to cut. The stock removal rate of these wheels is similar to a soft-grade abrasive while offering the extended life of a hard grade.

In addition to grain type, match the grain size, or grit, to the work. Use a large grain for soft material such as mild steel and a small grain for harder material. Use large grain for more aggressive stock removal and a small grain for hard metals to avoid loading the grit, or clogging the grinding wheel's grit with the metal being ground.

Get in Shape

The application also determines the type and shape of the wheel you select. Many abrasive manufacturers offer wheels for most types of cutting, slicing, and grinding operations.

Common types include depressed-center wheels (type 27), flat wheels (type 1), cup wheels (type 11), semiflexible wheels (type 29), and an assortment of special-purpose wheels.

The appropriate choice often depends on the geometry of the work. You can use cup wheels and depressed-center wheels, for example, to create a bevel. Semiflexible wheels are more suited for medium to light stock removal or blending on contoured surfaces, such as inside a pipe or the welded joint on a handrail assembly.

Another shape consideration is the size. Always match the wheel size and revolutions per minute (RPM) to your grinder. For optimal performance and safety, never use a wheel that is smaller or larger than the grinder manufacturer specifies.

Always verify that the RPM of the grinder doesn't exceed the RPM marked on the wheel. For example, a 14-inch chop-saw wheel will fail explosively if installed on a 14-in., gas-powered saw.

Moreover, don't even think about running an oversized wheel on a grinder with the guard removed. It's dangerous to run any grinder without proper guarding installed.

How to Read a Wheel

Figure 1 This grinding wheel is a ZA24-T: zirconia alumina, 24 grit, toward the harder side of the scale. Since the wheel is a depressed-center type, its phenolic resin bond and fiberglass reinforcement are implied and not actually indicated on the wheel.

Most abrasives manufacturers use standard codes to identify wheel properties. The wheel's number offers information about grain type, grain size, hardness of bond, type of bond, and the reinforcing material.

Grain type A is aluminum oxide. ZA is zirconia alumina. C is silicon carbide.

The number that follows the grain type is the grain size, or grit; the smaller the number, the larger the grit.

A letter grade indicates the hardness of the bond, with A being the softest and Z the hardest. Wheels usually fall between M and V. The last two letters, if used, indicate the type of resin used for bonding and the reinforcing material used in the wheel. B indicates a phenolic resin, and F indicates fiberglass reinforcement.

As an example, a wheel designated ZA24-T is zirconia alumina, 24 grit, toward the harder side of the scale. Many types of wheels have a phenolic resin bond and fiberglass reinforcement as standard, so a BF designation may not appear on the wheel itself, as in Figure 1.

Like all tools, an abrasive wheel must be matched to the application if it is to perform as designed. If you have any doubt about proper wheel selection for your grinder or application, always contact either the tool or abrasives manufacturer for assistance.

Proper use of the right grinder and abrasive can give you a one-up on the competition and won't wear you down. Following these guidelines can help you get the most out of your grinder and abrasive wheel.

Richard Lawson is product manager of abrasives and accessories at Metabo Corp., 1231 Wilson Drive, West Chester, PA 19380, 800-638-2264, fax 610-436-1829, rlawson@metabousa.com, www.metabousa.com.