The daily grind

Understanding bonded abrasives will help you get more out of them

An angle grinder can turn a solid, yet splattered, sloppy weld into a smooth, shiny finish ready for coating. Based on the correct grinding wheel, the job can go even faster and produce an even better finish.

Terry Sands, manufacturing and industrial engineer at The M.K. Morse Co., has worked with many contractors in his career helping them select and use the right bonded abrasive for the application. When users understand how a grinding wheel does its work, it’s more likely they will be able to select the best bonded abrasive for the job, he says.

The anatomy of a grinding wheel
Grinding wheels are made up of primarily two materials: an abrasive grain and a binding material, says Sands.

The abrasive grains may come from a variety of materials. Natural materials such as diamonds, emery, sandstone, corundum and quartz can be used, but often contain impurities that can greatly affect the consistency of the abrasive, he says.

“That’s the reason why most bonded abrasives use manufactured aluminum oxide or silicon carbide. The manufacturing process controls the quality and characteristics of these materials, so the manufacturer can select specific characteristics to meet specific grinding conditions,” he says.

The abrasive’s grain size also affects the grinding wheel’s performance. It is graded according to grain size, using an industry-wide and accepted scale that is based on the number of meshes per linear inch in the screen used to separate the various grains.

Typically, coarse grain sizes range from 10 to 24; medium grain sizes range from 30 to 100 and fine grain sizes range from 120 to 600, says Sands.

Bond agents vary
A variety of materials, such as clay, glass, silicate of soda, synthetic resins, rubber or shellac are used to bond the grains to form a wheel. Depending on the job the wheel must do, the bonding agent must allow the abrasive grain to cut and eventually break off to expose new abrasive.

The strength of the bond that holds the abrasive grains together governs how the wheel is graded. Manufacturers can control this, allowing them to offer wheels ranging from very soft (A) to very hard (Z). This grade helps users match the wheel’s hardness for best performance and finish on the job.

Structure determines finish
The wheel’s structure, or porosity, also affects its grinding ability. The structure is the relative spacing between each grain of abrasive and affects its ability to remove material quickly. An open structure will remove material quickly but leave a rougher finish; a tighter structure will remove less material, but leave a much more finished surface.

“The manufacturer has complete control over how dense or open the wheel will be, which affects chip clearance. If the wheel is too dense on certain materials, it can’t clear the chips out adequately and the chips will plug, clog or load up on the wheel. This creates a dull cutting surface, reducing cutting efficiency,” Sands says.

If wheel loading is a problem, look at the material being ground and consider grinding wheels with a different grade or structure.

Generally, when grinding soft, ductile metals, an open-structured wheel with wide grain spacing should be used. They can provide enough clearance so chips can be removed rapidly from the wheel face.

When grinding harder materials, a closer spacing in the abrasive structure can be used because harder metals produce small, clean-cut chips in the grinding process which are easily removed. Finally, a more open-structured wheel tends to cut cooler, and if it is of the proper hardness, the less likely it will load or fill, depending on the metal being ground.

Grinding tips
Use the right wheel. The most critical mistake most people make is choosing the incorrect abrasive grain for the application, says Sands. “Use aluminum oxide for metal applications, such as cutting or grinding general-purpose pipe, angle iron, structural materials, mild steels and decking. Use silicon carbide abrasives for masonry applications such as brick, block, concrete and ceramic tile.”

Secure the work. For best and safest grinding results, properly secure the work piece before starting to grind.

Respect the speed limit. “Safety should be the main concern of any abrasive user. Never exceed the maximum rpm specification for a wheel. You will find this information printed on every wheel. Also, the user should ensure all machine guards are in place before operating the wheel and always wear proper safety equipment when using abrasive wheels,” he adds.

Keep wheels dry and protected when not in use. The biggest favor you can do for grinding wheels is to store them correctly. “Keep them in a dry area. Moisture will ruin an abrasive wheel. Store wheels flat, keeping the edges of the wheel protected,” he concludes.

Published in the November/December 2002 issue of Contractor Tools and Supplies magazine.

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