March 2018 Issue
Toc

On average, the cost of abrasives for large metal fabrication firms is less than 2 percent. Conversely, the total cost of grinding and finishing is 10 to 15 percent. While cutting abrasive costs may seem attractive, a reduction in labor costs presents the real upside for the end user. To achieve those cuts, it’s important to remember that some grinding wheels perform at a higher level than others.

Grinding wheels for right-angle grinders used in metal fabrication have undergone dramatic increases in performance over the last 20 years. Some of the improvements have come in bond technology and manufacturing tolerances, but the majority of the performance increases have resulted from advances in abrasive grain technology.

An example of how abrasive grain technology helps to lower grinding wheel usage is shown in Figure 1. Initially, a large fabricator of cast parts went through 100 competitive aluminum oxide wheels for one job, but when the company switched to Norton | Saint-Gobain ceramic alumina wheels for the same job, wheel usage dropped to 20.

When adding in the reduced time for changing wheels and time spent grinding, the total cost of labor for the job was reduced by 80 percent – measurable savings for any shop or job site.

So why are manufacturers with labor rates at $24 per hour and upward still not using the best-performing wheels? One answer may be perception. Abrasives are seen as a tertiary expense for the operation. The idea of spending more for higher quality doesn’t even show up on the radar.

Abrasive Grain Overview

Aluminum oxide: Until the mid 1970s, aluminum oxide, seen in Figure 2, was the only choice for grinding all types of metal. Aluminum oxide 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 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: In the late 1970s, zirconia alumina, seen in Figure 3, was developed through a fusion of zirconia and aluminum oxide. It provides a sharp, tough grain that is still “friable” enough to break down in portable applications. Zirconia alumina has a harder, sharper grain engineered for controlled fracturing. The combination of the harder, sharper grain and better utilization of each grain provides a 50 percent improvement in cut rate and three times the life over aluminum oxide. The technology was first commercialized in depressed center grinding wheels in 1980 with the introduction of the Norton NorZon products.

Ceramic alumina: This technology, seen in Figure 4, was created in the 1990s based on doped alumina sol gels. The key characteristic of these high-performance abrasives is the unique combination of hardness and toughness, which is essential for controlling micro-fracturing. Micro-fracturing of the grains is crucial for providing a continuous supply of sharp cutting edges during grinding thereby raising the utilization of each grain to more than 80 percent.

Overall, grain shapes have become more sophisticated, especially with recently introduced precision-shaped grains (PSGs). In side-by-side studies, irregular-shaped ceramic grains cut through the workpiece with smooth almost effortless gliding motions. PSGs, on the other hand, were perceived to be rougher and required the operator to adjust his stance.

This situation stems from the fact that PSGs 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. The 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 the high-performance ceramic alumina abrasives come in the form of:

  • less machine maintenance
  • less metallurgical damage/improved part integrity
  • less operator hardship/fatigue

Abrasive grains are the key component of the depressed center wheel. They are the “cutting tool” of the product and, therefore, it’s important to match the performance characteristics with the needs of the job.

Resin Bond Overview

Premium-performance abrasive depressed center wheels are constructed using a precisely engineered iron, sulfur and chlorine-free resin technology to provide a uniform abrasive distribution throughout the wheel. This unique bond is designed for retaining the grains long enough during and after the grain fractures. This subtle but powerful bond characteristic enables excellent cut rates and exceptional wheel life.

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

In the second scenario, fractured grains are released prematurely, resulting in a fast cut but with a very short wheel life. This shortened life requires frequent wheel changes and downtime and generates additional operating costs.

Other variables, such as machine horsepower, air supply (pneumatic only), grinding angle, surface contact area, and type of material being shaped or cut, are also factors in the grain-to-bond ratio. However, the abrasive manufacturer must work on improving bond technology in conjunction with advancements in abrasive grain to maximize the total performance of the depressed center wheel.

Fiberglass Reinforcement Overview

The third component of every depressed center wheel is the fiberglass reinforcing layers. These are required for safety standards due to the fact that the wheels are used on a portable handheld power tool. In Figure 5, the standard orientation of the fiberglass reinforcing is shown. Here, two and a half layers are used to meet the ANSI B7.1 safety standard.

On certain products, especially for rugged applications like foundries, three full layers are used. Grinding with these wheels can be done on the face or the periphery of the wheel that is showing two and a half layers.

For 1/4-in. wheels where the grinding is only done on the face of the wheel, a fine, hard back/finer grit mixture in the top half of the wheel is available that limits premature shedding or spalling and allows for 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).

The Science of Abrasives

Norton | Saint-Gobain is committed to using the science of abrasives to offer fabricators the most productive solutions, including its Norton Quantum3 (NQ3) grinding wheel, which grinds faster with less pressure to significantly increase production while using fewer wheels. The best industry metal removal rate is gained from the larger grit size, such as the rate delivered from a proprietary Saint-Gobain grain.

Bond is matched to grain to produce the highest level of performance in terms of metal removal rates and wheel life. The new bond contains
a unique combination of fillers and bonding agents that allow for a much better mix quality in manufacturing. The better mix keeps the individual grains from clumping together, allowing even wear of the bond and abrasive structure because of the more homogenous distribution of grains within the wheel and better adhesion of the grains to bonding materials.

The design is also contaminate-free as it contains less than 0.1 percent iron, sulfur or chlorine. This produces rust-free and corrosion-free results that will also not contaminate stainless steel. Some of the other benefits of the wheel include a smoother feel, better control and operator comfort.

The Art of Performance

Smoother feel: Wheels that use a type of ceramic PSG tend to generate a different, variable operating feel than NQ3 wheels. This variable feel is characterized as vibration or jumping on the workpiece.

The Norton | Saint-Gobain ceramic grain, however, is a more robust, consistent grain due to the manufacturing process and is more rounded than pointed. Therefore, the NQ3 design permits an easier, smoother feel during grinding, promoting a faster cutting action at reduced pressure for substantially longer wheel life.

Better control: Typical PSG wheels do not allow sufficient control during the grinding cycle when used in different directions. More effort can also be required to grind with PSGs on the edge, so the operator has less control and may need to exert more energy/body support to keep the grinder under control.

NQ3 wheels, on the other hand, produce no grabbing or digging on the workpiece when used in any direction. Wheels glide through the workpiece while removing metal, thereby producing a very smooth grind at a faster rate for less downtime. Reduced landfill waste due to fewer wheels used is a positive side effect.

Improved operator comfort: The NQ3 wheel is exceptionally free cutting, enabling the wheel to boost the strength of force on its own with little or no pressure. This is due to the unique mix of bond and abrasive. The operator doesn’t have to apply excess pressure to get the job done when compared to using lower performing products, resulting in reduced hand fatigue from less cutting vibration.

Latest Tech

Norton recently introduced a new product using NQ3 technology: Norton Quantum3 combination wheels. In comparison tests, NQ3 combo wheels have several key benefits over competitive wheels. NQ3 combo wheels have 50 percent longer life, which significantly reduces the need for wheel changes, resulting in decreased production downtime and less labor cost. Due to more efficient and faster cutting and grinding capabilities, the new combo wheels increase productivity by 15 percent over competitive wheels, so users can finish more pieces in less time.

In addition to lasting longer and increasing productivity, users are reporting that NQ3 combo wheels are significantly easier to use and cut effortlessly compared to other wheels. They’re considered ideal tools in a broad range of applications, including general fabrication, oilrig building and repair, rail, container and pipeline manufacturing.