Abrasive Grain Overview
Abrasive grains are the key component of the depressed center wheel. They are the &#;cutting tool&#; of the product and it is important to match their performance characteristics with the needs of the job to ensure right choices for the user.
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Aluminum Oxide (AO)
Until the mid-s, aluminum oxide (AO) was the only choice for grinding all types of metal. Aluminum oxide is a strong, blocky-shaped abrasive grain (see Figure 3). While the initial edge of the grain is sharp, it rapidly dulls. With broad cleavage lines, the grain continues to wear until enough pressure is created to fracture the grain and create a new sharp cutting edge. When the fracture does occur, a large percentage of the grain is lost. The result is a slow cutting abrasive where only 25 percent of the utility of the grain is realized.
Zirconia Alumina (ZA)
In the late s, zirconia alumina (ZA) grain was developed through a fusion of zirconia and aluminum oxide to provide a sharp, tough grain that would still be &#;friable&#; enough to breakdown in portable applications. Zirconia alumina grain is a harder, sharper grain that has been engineered for controlled fracturing. The combination of the harder, sharper grain and better utilization of each grain provided a 50 percent improvement in cut rate and three times the life over aluminum oxide (see Figure 4). This technology was first commercialized in depressed center wheels in with the introduction of our Norton NorZon products.
Ceramic Alumina (CA)
This technology, created in the s, is based upon doped alumina sol gels. The key characteristic of these high performance abrasives is the unique combination of hardness and toughness that is essential for controlling micro-fracturing of the grains, which is crucial for providing a continuous supply of sharp cutting edges during grinding that results in raising the utilization of each grain to over 80 percent (see Figure 5).
Grain shapes have become more sophisticated with recently introduced precision shaped grain (PSG). In side-by-side studies, irregular-shaped ceramic grains cut through the workpiece with smooth almost effortlessly &#;gliding&#; motions. Precision-shaped grains, on the other hand, were perceived to be &#;rougher&#; and required the operator to adjust his stance. This situation stems from the fact that precision-shaped grains break down to form geometric shapes that are significantly less efficient in cutting than the original geometric form. Irregular-shaped grains fracture and wear to form more irregular-shaped geometries. This consistency of irregular starting and intermediate fractured geometries provide a more uniform cutting action throughout the wheel life and hence a better overall grinding experience. Additional benefits of these high performance abrasives can be:
&#; Less machine maintenance.
&#; Less metallurgical damage/improved part integrity.
&#; Less operator hardship/fatigue.
Every abrasive product has unique qualities that make it a good or bad fit for certain applications. Determining which abrasive grain and bond combinations are best suited to your metal finishing needs is key to optimizing workflow. Selecting the wrong consumable can have the shop floor clocking extended hours to produce average quality parts.
Choose the Best Grain for Your Operation
With the introduction of dozens of new fillers and proprietary bonding agents, shop managers tasked with selecting abrasive wheels can be left overwhelmed by the options and choose what they are familiar with. Knowing the qualities of the wheel composition can help with these decisions. Start with the following characteristics:
- Grain Friability
- Grain Hardness
- Bond Type
The abilities of an abrasive depend on the combination of all three factors used on a specific material. Reassess the shop&#;s abrasive cabinet by looking directly at the grains and bonds used in an abrasive wheel to determine the best match for the job.
Breakdown and Friability
Every abrasive grain has the ability to self sharpen as it breaks down. That rate of breakdown is called friability. Garnet is an example of a grain with very low friability. Its natural breakdown results in plenty of dust, making it the right choice for abrasive blasting or waterjet cutting.
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More friable grains such as black and green silicon carbide are better suited for grinding and finishing very hard materials. For instance, when deburring titanium parts, black silicon carbide will do the trick. As the wheel works, the grain will break and expose new sharp edges, allowing for continuous cutting and avoiding surface burn.
Combine Friability with Grain Hardness
Looking at grain characteristics together will give you a better idea of their capabilities. Grains with high hardness and a sharp natural breakdown will create a more aggressive grain. If the grain is not hard enough for the specific job, you could find yourself going through grinding wheels too quickly. Or vice versa, if the grain is too aggressive, you might remove more metal than you intended, scrapping expensive parts.
The hardness of abrasive grains is rated on the Mohs Hardness Scale. A score of 9 and above is considered very hard, most abrasive grains meet this measurement. Yet, the differing levels of friability make each grain individually suited to different tasks. Aluminum Oxide, Alumina Zirconia, and Ceramic alumina all measure in at 9, yet:
- Aluminum oxide is a tough grain with low friability. But when it does shatter, it exposes sharp edges, making it a workhorse on various metals.
- Alumina Zirconia has a microstructure that allows this grain to break off into smaller, sharper edges than aluminum oxide, making it excellent for heavy grinding.
- Ceramic alumina is a microcrystalline grain meaning it consists of millions of smaller crystals. The grain fractures over and over during the grinding process making it a very aggressive grain.
Every grain and abrasive wheel has a unique hardness. When in doubt, consult the manufacturer&#;s documentation to see if you are using the best grain for the job.
Bonds to Boost Performance
The bond used in manufacturing an abrasive wheel has a remarkable impact on the abrasive wheel performance. The same abrasive grain can have different actions pending on what bond the manufacturer used.
- Resinoid wheels are tough and are used for rough grinding applications. The abrasive grain is combined with phenolic resin to create resinoid products.
- Vitrified wheels are strong and rigid, manufactured with finely ground clay and fluxes with abrasive grain mixed throughout. Vitrified wheels can also have poor shock resistance.
- Cotton fiber abrasive wheels embed abrasive grains in non-woven cotton fabric. As the cotton wears down the wheel will constantly reveal more abrasive grains allowing for a consistent finish. Cotton fiber is best known for controlled metal removal.
- Unitized wheels are a web material impregnated with abrasive grain that is combined into a resin slab. They often used for very light metal removal.
After considering each grain and the bond options available, selecting the most viable options and beginning testing is the next step. Taking time to hone in on the correct abrasive for specific applications can drastically increase productivity
Multitasking Abrasive Wheels
Changing out tooling too quickly or too frequently can be a waste of time and energy for operators.
Metalworkers from pipefitters to aerospace machinists are following the trend and turning to Cotton Fiber Abrasive Wheels to chamfer, deburr and finish in one step. Fewer stops to change out abrasive wheels saves precious time and significantly reduces errors.
Efficiently grinding and finishing metal is dependent on the material bond of the abrasive wheel and on the inherent effectiveness of the cutting grain being used. Learn more about abrasive grains to find the perfect match for your shop.
Learn more about abrasive grains. Get your copy of our grain chart.