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Deburring/Finishing

Grain & grit

By Debbie Gaspich

Above: Norton developed abrasive grinding media from grains of zirconia alumina.

New abrasives development improves performance and user ergonomics

September 2016 - Abrasives for large metal fabricators averages less than 2 percent of job cost and the total cost of grinding and finishing is 10 to 15 percent. Reducing abrasive costs may seem attractive, but lowering labor costs presents the real upside. 

Fabricators may want to compare cost and performance of competing products. Grinding wheels for right angle grinders have had dramatic increases in performance over the last 20 years. Some of the improvements have come in bond technology and tolerances, but the majority of the performance gains resulted from advances in abrasive grain technology.

A large fabricator of cast parts reduced wheel usage from 1,000 competitive aluminum oxide wheels to 296 Norton ceramic alumina wheels to accomplish the same job. When calculating the lower rates of labor required to change wheels and the reduced time spent actually grinding, labor cost for the job fell 70 percent. The table on Page 3 shows the measurable savings that can be realized at any shop or job site.

So why are manufacturers that pay wages of $50 per hour and up still not using the best-performing wheels? One answer may be perception. Abrasives are seen as a tertiary expense for the operation.

Abrasive grain 

Aluminum Oxide (AO): Until the mid-1970s, AO was the only choice for grinding all types of metal. AO is a strong, blocky-shaped abrasive grain. While the initial edge of the grain is sharp, it rapidly dulls. With broad cleavage lines, the grain continues to wear until enough pressure fractures the grain to 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.

FFJ 0916 deburr image1

Norton developed abrasive grinding media from grains of aluminum oxides (above) and ceramic alumina (below) to find the right combination of toughness and hardness.

FFJ 0916 deburr image3

Zirconia Alumina (ZA): Engineered for controlled fracturing in the late 1970s, ZA grain was developed by fusing together zirconia and aluminum oxide. ZA provides a sharp, tough grain that’s still friable enough to break down in portable applications. The combination of the harder, sharper grain, and better use of each grain, provides a 50 percent improvement in cut rate and three times the life over AO wheels. This technology was first commercialized in depressed center wheels in 1980 when Norton introduced the NorZon products.

Ceramic Alumina (CA): Created in the 1990s, this technology is based upon doped alumina sol gels. The key was to combine hardness and toughness, which is essential for controlling microfracturing, which provides a continuous supply of sharp cutting edges during grinding, thereby raising the utilization rate of each grain to over 80 percent.

More recently, manufacturers developed precision-shaped grain (PSG). In side-by-side studies, irregular-shaped ceramic grains cut through the workpiece with smooth, almost effortless gliding motions. Conversely, precision-shaped grains were perceived to be rougher and required the operator to adjust his or her stance. 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 provides a more uniform cutting action throughout the wheel life and, therefore, gives operators a better overall grinding experience.

Potential additional benefits of high-performance abrasives are less machine maintenance, less metallurgical damage, improved part integrity and less operator hardship or fatigue.

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.

FFJ 0916 deburr image2

Resin bond 

Premium performance abrasive depressed center wheels are made using iron, sulfur and chlorine-free resin technology to provide a uniform abrasive distribution throughout the wheel. Precisely engineered, this bond was designed for retaining the grains long enough both during and after grain fractures, enabling both excellent cut rates and exceptional wheel life.

Non-optimal bond technologies can result in either retaining the grain too long or not long enough. In the first scenario, fractured grains are retained in the wheel. This results in swarf build-up on the wheel face followed by glazing, slower cutting and, ultimately, greater effort by the operator to force the cut.

In the second scenario, fractured grain is released prematurely. This causes a fast cut, but results in a very short wheel life. This shortened life requires frequent wheel changes, downtime and creates higher operating costs.

Machine horsepower, air supply (pneumatic only), grinding angle, surface contact area and the type of material being shaped or cut are also factors in the abrasive grain to resin bond ratio. But to maximize the total performance of the depressed center wheel, abrasive manufacturers must work on improving bond technology in conjunction with advancements in abrasive grain.

Fiberglass reinforcement

The third component of every depressed center wheel is the fiberglass reinforcing layers required for use on portable power tools. The layers provide a measure of mandated operator safety. 

   The standard orientation of the fiberglass reinforcing is shown in the photo above. Two and a half layers are used to meet ANSI B7.1 standards. On certain products, especially for rugged applications like foundries, three layers are used. Grinding with these wheels can be done on the face or the periphery of wheels showing 21⁄2 layers.

For 1⁄4-in. wheels, where grinding is done on the face of the wheel only, a fine hard-back grit mixture at the top half of the grinding wheel can limit premature spalling and promote a smoother grinding action from the periphery to the hub.

The correct orientation of the reinforcing layer is required for safety. Choosing the fine hard-back can help improve grinding performance by reducing premature spalling or spitting (loss of grain), which is especially critical when using premium abrasives like NorZon or blended Ceramic Alumina NorZon.

FFJ 0916 deburr image4

Grinding wheels include fiberglass reinforcing layers (shown) to protect operators of portable handheld power tools.

Quantum3

Norton Quantum3 grinds faster with less pressure, which boosts production rates while using fewer wheels. The best industry metal removal rate is gained from the larger grit size of a proprietary Saint-Gobain grain. Bond is matched to grain to enhance metal removal rates and extend wheel life. 

This new bond combines fillers and bonding agents that allow for much better mix quality, so individual grains don’t clump together, which gives more even wear of the bond and abrasive structure. The product is also contaminate-free, as it contains less than 0.1 percent iron, sulfur or chlorine, for corrosion-free results on finished stainless steel.

Wheels that use a type of ceramic PSG tend to generate a variable operating feel characterized as vibration or jumping on the workpiece. The Saint-Gobain ceramic grain is more robust and consistent due to the manufacturing process and is more rounded than pointed.

Norton Quantum3 permits an easier, smoother feel during grinding and promotes a faster cutting action at reduced pressure, for substantially longer wheel life.

Typical PSGs don’t allow sufficient grinder/grinding wheel control when used in different directions. An operator may need to exert more energy or body support to keep the grains on edge. Quantum3 wheels won’t grab or dig on the workpiece when used in any direction. Wheels glide through the workpiece, producing a smooth grind at a faster rate.

As a free-cutting wheel, the Norton Quantum3 boosts amperage on its own with little or no pressure, compared with lower performing products, resulting in reduced hand fatigue from less cutting vibration. FFJ

Debbie Gaspich, Director of Product Management, Thin Wheels and Construction Products-North America

Sources

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ARKU

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