Understanding Grinding Wheels
To learn more about how to choose the right abrasive wheel for any job, maintenance tips, and safety best practices, check out Abrasive wheel Selection and Applications.
What Are Grinding Wheels?
An abrasive wheel creates a high-quality finish on surface materials, such as steel, glass, copper, stone, even concrete, with shape and dimension.An Abrasive Wheel is a precision cutting tool with an abrasive surface coated with thousands of hard, sharp grains that cut, chip and grind away metal, steel, copper, cast iron, stainless steel, stone, concrete, and other tough materials. The sharp grains are bound with a special adhesive to a backing, usually in the shape of a wheel.
As the wheel rotates on a grinder, the wheel “grinds” the surface, causing the sharp grains of the wheel to break off, exposing new sharp edges. When the grains wear down, they fall off the bonded backing, exposing sharper new grains in their place. As the grains grind away, the surface material is removed in small chips or thin ribbons. The process continues until the desired surface finish or shape is achieved.
Available in a wide variety of types, shapes, patterns, sizes, and abrasives, there are several factors to consider when choosing an abrasive wheel.
Grinding Wheel Types
Abrasive wheels come in a variety of shapes and sizes. The most common and easily recognizable one is the straight abrasive wheel. The part of any abrasive wheel that does the actual grinding work is called the “grinding face.” The grinding face may be on the center of the wheel, recessed, only on the edges, or be broken into segments, designed to funnel lubricants, which keep equipment and materials cool. Here are the most common wheel types.
The Five Primary Types of Abrasive Grains
Let’s review the four main types of grains used on abrasive wheels. These can also be blended for specific applications and performance.
1. Aluminum oxide grains are tough and hard-wearing; however, after a sharp, fast initial cut, the grains dull and lack the cut-rate and life span of other grains. Good for grinding metals such as steel, iron, bronze alloys, stainless steel and other ferrous metals. Great value and cost effective with excellent quality and consistent performance.
2. Zirconia alumina grains provide fast cuts, are very heat resistant, self-sharpening, and deliver rapid, consistent grinding on steel and stainless steel. They are also good for high-pressure machining and grinding because the pressure makes the grains break down quickly, exposing sharper edges. Zirconia alumna costs more than aluminum oxide, but it lasts longer.
3. Ceramic alumina is a newer abrasive type with a micro-grain structure that breaks down into smaller pieces, giving it the longest operating life in moderate to high pressure, generates less friction meaning it stays cooler in use, self-sharpening, fast cut rate, and minimizes heat discoloration on your workpiece. Ideal for hard-to-grind metals: armored steel, titanium, hard nickel alloys, tool steel, and stainless steel.
4. Silicon carbide grains are extremely hard, very sharp, fast cutting but break off easily under high pressure. Harder than aluminum oxide and versatile for grinding soft metals like aluminum, copper, or cast iron as well as hard materials such as cemented carbide, stone and concrete.
5. Silicon carbide/aluminum oxide blend creates a wheel ideal for grinding aluminum and other soft alloys. These grains offer extended life spans and fast, consistent cut rates.
Grit/Grain Size
Grit, or grain size, ranges from coarse to fine. and is indicated by a number shown on the abrasive wheel's label The larger the grit number, the smaller the grain, and the finer the finish. Large/coarse grains grind larger chips resulting in a rougher finish.
Bond Material/Matrix
The bond (sometimes called a matrix) is the “glue” that holds the key elements to a grinding wheel - the abrasive grains. Bonds help to determine the type, the characteristics, and the performance of a grinding wheel. The stronger the bond, the longer the grains adhere to the wheel. This means that sometimes a weaker bond is a better choice when cutting strong, tough metals that need razor-sharp abrasive grains.
Bond types:
With different degrees of strength, there are six standard bonds. There is not a best-to-worse ranking or “best” bond because each offers such unique characteristics that the best bond is one that suits your current particular grinding operation.- Vitrified: (V) Made from clays and fusible materials. Not susceptible to water, oils, acids, or temperature variations. Ideal for high stock removal and precision grinding projects.
- Resinoid: (B) Resins are mixed with abrasive grains to produce a soft, fast, and cool bond for wet or dry grinding, good cutting performance, and usually the least expensive bond. With a low curing temperature, resinoid wheels are tougher and less rigid than vitrified, making them good for all kinds of abrasives, especially heavy duty projects. However, they are affected by acid, humidity and weather extremes.
- Silicate: (S) Releases abrasive grains easily, creating mild, cool cutting actions good for projects that require minimum heat and good for sharpening edged tools.
- Shellac: (E) Made from natural and synthetic shellac, this bond has exceptional cool cutting and suited for grinding copper and other soft materials. Highly recommended for special applications that require a high-surface finish such as razor blade and roll grinding.
- Rubber: (R) Made from natural and synthetic rubber and used mostly in centerless and control abrasive wheels. Good for precision and fine surface finishes. Using wet grinding and thin cut-off wheels produce burr- and burn-free cuts.
- Metallic: Have limited use when compared to vitrified and organic bonds. Most use a diamond abrasive which removes material slowly and usually with high speed. Good for glass grinding, abrasive wheel shaping, concrete or stone sawing, and has a long-life span. Can be used with aluminum oxide or diamond abrasives to produce conductive abrasive wheels for electrolytic grinding.
Grinding Wheel Safety - Best Practices
- Always wear protective safety glasses or face shield with impact-resistant lenses and side guards.
- Install shatterproof safety eye shields over all stationary grinders.
- Before mounting, check abrasive wheels for cracks, chips, and damage. Repair or replace as necessary.
- Make sure operating speed does not exceed rated maximum wheel speed.
- Inspect and ring test wheels for cracks before each use.
- Be sure the abrasive wheel hole fits the machine arbor properly.
- Check that flanges are clean, flat, and the proper type and size for the wheel you’re mounting.
- Tighten nuts just enough to hold wheel firmly. Do not use excessive pressure.
- Standing to the side, run new wheels at full operating speed in a protected area at least one minute before grinding. Most defective wheels break when started.
- Use wheel guard provided with grinding machine.
- Grind a wheel at a rated speed LESS than rated grinder speed.
- Only mount ONE wheel per single arbor.
- Use wheels and grinding machines only for the purposes they are intended.
- Be aware of excessive vibration. It’s an indicator wheel is out of round.
- Never grind towards yourself or another worker. Always try to grind towards a wall.
- Be sure the third wire is grounded to avoid shock.
- Make sure that a portable grinder has completely stopped before setting it down. Use a stand if possible. Rest grinder on its guard if there is no stand.
- ALL grinders should be used ONLY with a protective hood over the wheel.
- ALL grinders should be inspected every 30 days and necessary adjustments and repairs made.
- Don’t stand directly in front of a grinding/cutting wheel whenever a machine is in operation.
- Don’t grind or cut material for which the wheel is not designed.