Grinding Journal - Summer 2007 - (Page 21)

Summer 2007 A grinding wheel manufacture will sometimes combine different types of aluminum oxide grains to get the proper “grain breakdown” for the grinding application. This allows the wheel to remain sharp throughout the entire grinding operation. You can use the following rule of them when considering the friability of a grain and how it affects the grinding process: Variable Wheel Life Material Removal Rate (MRR) Cutting Temperature Form Holding Most Friable Shorter Higher Decreased Worse Least Friable Longer Lower Increased Better the grains need high unit pressure to perform the microfracturing process. This is why ceramic does not work well when grinding soft materials like aluminum. The grains would never break down, which leads to dulling, rubbing, and ultimately loading of the wheel unless it was made exceedingly soft. On the other hand, if the wheel is too soft it will simply release the ceramic aluminum oxide grains and not allow the end user to fully utilize the higher performance grain. Using a ceramic wheel requires high quality diamonds in your truing device. Ceramic has the reputation of prematurely wearing diamond tools. That’s true under certain circumstances, but a high quality diamond tool or roll will greatly reduce this problem. Another point to remember is that due to the sharpness of the grain, ceramic will typically leave a deeper scratch than aluminum oxide grains of the same size. Therefore Spring 2007 it is sometimes necessary to decrease grit size by one to reach required finishes versus aluminum oxide. Remember the following when considering ceramic wheels for your grinding operation: • Horsepower and machine stiffness is critical to the performance of the grain • Material should be difficult to grind due to hardness and/or toughness • As a general rule, decrease dress depth by 50% when switching from aluminum oxide wheels • Due to sharpness of grain, it’s sometimes necessary to move to a finer grit size vs. aluminum oxide to reach required surface finishes • Pay attention to total process cost rather than wheel cost with ceramic grain wheels 3. Silicon Carbide (SiC) is used to grind non-ferrous materials. Traditionally, before diamond grinding wheels were readily available, silicon carbide wheels were the wheel of choice for grinding carbide cutting tools. Wheels containing silicon carbide grain are almost always either black or green in color and will be typically called out in the marking system by the letter “C.” Another way to easily identify silicon carbide is simply by looking at the wheel. Wheels containing silicon carbide will “sparkle” when tilted to the light. A number of factors make silicon carbide unique. Grinding Journal 21 Friability is also a factor in other abrasive grains and will be referenced throughout the article. 2. Ceramic aluminum oxide (ceramic) is a high performance version of aluminum oxide. Grain manufactures basically fire typical aluminum oxide grains to create an extremely hard resilient grain which has a property we refer to as “microfracturing.” As the name suggests, each grain micro-fractures, allowing it to hold its shape and remain sharper for a longer period of time than typical aluminum oxide. This type of grain is sometimes referred to as “SG,” which is Saint-Gobain/Norton’s trade name for this material. Typically, grinding wheel manufactures call out this type of grain by depicting a “G” in the first position of the marking system. Generally the “G” will be accompanied by a digit that designates the percentage of ceramic in the wheel. Typical mixes are 10, 30 and 50% ceramic. The balance is usually a type of friable aluminum oxide. There is also an extruded ceramic grain called “Targa” developed and patented by SaintGobain. It’s used in high Material Removal Rate (MRR) applications where coolant delivery and chip clearance are crucial. Ceramic should be used for materials that are difficult to grind due to hardness and/or toughness. Ceramic can also be used when you’re trying to extend wheel life, decrease cycle times by increasing material removal rates, or increase the form holding capability of a process. Ceramic performs best when pushed because

Table of Contents Feed for the Digital Edition of Grinding Journal - Summer 2007

Contents My Thoughts Exactly When Lives Are at Stake Walter Consolidating Production The Art of Grinding: Knowing the Source of Grinding Errors and How to Fix Them The How and Why of Conventional Vitrified Grinding Wheel Selection Grinding Glass Flat Problem Solver

Grinding Journal - Summer 2007

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