A Matter of Grit and Grade

Grit, grade, bond and grinding speed all affect the process of choosing the right grinding wheel for the job.
If you’re looking for a grinding wheel, you’ll soon discover there is no shortage of types from which to choose. The market has wheels of every size, style, manner and description. The problem is not a matter of “Does the right wheel exist?” The question is “Which one do you want?” The challenge is in differentiating between the countless configurations available. What follows are some fundamental guidelines to help find the right wheel for the job.

You can begin narrowing your options by deciding what type of grinding wheel is best suited to the job at hand. Grinding wheels have two criteria: the abrasive grains and the bond that holds those grains together. The grain a wheel employs is specifically geared towards a certain type of material. Some abrasives are better for certain materials than others. When properly matched to its material, the abrasive possesses the ability to stay sharp for a longer duration. Since there are so many different types of material to be matched, define the purpose of the wheel first, as each abrasive type is unique, with distinct properties in regards to hardness, strength, fracture toughness and resistance to impact.

Aluminum oxide is the abrasive most commonly used on grinding wheels, followed by silicon carbide, zirconia alumina and a newer material, ceramic aluminum oxide. For grinding carbon steel and alloyed steel as well as malleable and wrought iron, aluminum oxide is best. For grinding gray iron, chilled iron, brass, soft bronze and aluminum, a good choice is silicon carbide. Silicon carbide can also be used on stone, rubber and other non-ferrous materials. Zirconia alumina is a tough, durable abrasive that works well on a broad range of steels and steel alloys while the recently developed ceramic aluminum oxide is an optimum abrasive for steels and alloys that are considered the most difficult to grind. This is due to ceramic aluminum oxide’s peculiar ability to fracture at a controlled rate at the sub-micron level, constantly creating new cutting points, a property that is well-suited for difficult metals.

Grit size is another factor to consider. Grit size refers to the actual size of the individual grains used on the wheel. The lower the grit number, the coarser the grit and the higher the number, the finer the grit. Coarser grits are best used for rapid removal of material where the surface finish is less important. Finer grits are used in cases when the surface finish must be exceedingly smooth.

Next to consider is the bond of the wheel. This term refers to the adhesive material used to hold the abrasive together, creating the wheel’s surface. It is necessary for the wheel’s bond to gradually erode with the abrasive grains so that successive grain points can be exposed to grind the material. Presently, there are three types of bonds used on grinding wheels: vitrified, resinoid and rubber. A vitrified bond, like its name implies, is comprised of melted glass that suspends the grains. As the glass is ground, the grains it holds are revealed. The vitrified bond is ideal for removing stock material quickly, and has the additional advantage of being able to grind with a high degree of precision. The resinoid bond is made of organic substances that soften under the heat of grinding. This bond is proficient at speedily grinding through stock material and leaves a smooth finish. Lastly, there is the rubber bond. This bond is organic in nature as well. It offers a smooth grinding action and is known for its high quality of finish. Rubber bonds are ideal for jobs where a smooth finish is the priority, such as creating ball bearing races that need to be flawless in order to work correctly.

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