How grinding wheel is made – Background, Raw Materials
Grinding wheels are made of natural or synthetic abrasive minerals bonded together in a matrix to form a wheel. While such tools may be familiar to those with home workshops, the general public may not be aware of them because most have been developed and used by the manufacturing industry. In this sector, grinding wheels have been important for more than 150 years.
For manufacturers, grinding wheels provide an efficient way to shape and finish metals and other materials. Abrasives are often the only way to create parts with precision dimensions and high-quality surface finishes. Today, grinding wheels appear in nearly every manufacturing company in the United States, where they are used to cut steel and masonry block; to sharpen knives, drill bits, and many other tools; or to clean and prepare surfaces for painting or plating. More specifically, the precision of automobile camshafts and jet engine rotors rests upon the use of grinding wheels. Quality bearings could not be produced without them, and new materials such as ceramic or material composites would be impossible without grinding wheels to shape and finish parts.
Sandstone, an organic abrasive made of quartz grains held together in a natural cement, was probably the earliest abrasive; it was used to smooth and sharpen the flint on axes. By the early nineteenth century, emery (a natural mineral containing iron and corundum) was used to cut and shape metals. However, emery’s variable quality and problems with importing it from India prior to its discovery in the United States prompted efforts to find a more reliable abrasive mineral.
By the 1890s, the search had yielded silicon carbide, a synthetic mineral harder than corundum. Eventually, manufacturers figured out how to produce an even better alternative, synthetic corundum or aluminum oxide. In creating this bauxite derivative, they developed an abrasive material more reliable than both natural minerals and silicon carbide. Research into synthetic minerals also led to production of the so-called superabrasives. Foremost in this category are synthetic diamonds and a mineral known as cubic boron nitride (CBN), second in hardness only to the synthetic diamond. Today, development continues, and a seeded-gel aluminum oxide has just been introduced.
Throughout the grinding wheel’s history, the bond that holds the abrasive grains together has proven as important as the grains themselves. The success of grinding wheels began in the early 1840s, when bonds containing rubber or clay were introduced, and by the 1870s a bond with a vitrified or glass-like structure was patented. Since then, bonds used in grinding wheels have been continually refined.
Grinding wheels are available in a wide variety of sizes, ranging from less than .25 inch (.63 centimeter) to several feet in diameter. They are also available in numerous shapes: flat disks, cylinders, cups, cones, and wheels with a profile cut into the periphery are just a few. Although many techniques, such as bonding a layer of abrasives to the surface of a metal wheel, are used to make grinding wheels, this discussion is limited to wheels composed of vitrified materials contained in a bonding matrix.
Two important components, abrasive grains and bonding materials, make up any grinding wheel. Often, additives are blended to create a wheel with the properties necessary to shape a particular material in the manner desired.
Abrasive grains constitute the central component of any grinding wheel, and the hardness and friability of the grinding materials will significantly affect the behavior of a given wheel. Hardness is measured in terms of a relative scale developed in 1812 by a German mineralogist named Friedrich Mohs. On this scale, extremely soft talc and gypsum represent hardnesses of one and two, and corundum and diamond represent hardness of nine and ten.
Friability refers to how easily a substance can be fractured or pulverized. People who design grinding wheels consider the friability of their abrasives—which can differ with the nature of the materials being ground—very carefully. For example, while diamond is the hardest known material, it is an undesirable steel abrasive because it undergoes a destructive chemical reaction during the cutting process; the same is true of silicon carbide. On the other hand, aluminum oxide cuts irons and steels better than diamond and silicon carbide, but it is less effective for cutting nonmetallic substances.
If selected correctly, an abrasive chosen to shape a particular substance will retain its friability when ground against that substance: because the grinding will cause the abrasive to continue fracturing along clean, sharp lines, it will maintain a sharp edge throughout the grinding process. This gives the grinding wheel the unique characteristic of being a tool that sharpens itself during use.
Although bonded abrasives began as tools made from natural minerals, modern products are made almost exclusively with synthetic materials. A bonding material holds the abrasive grits in place and allows open space between them. Manufacturers of grinding wheels assign a hardness to the wheel, which should not be confused with the hardness of the abrasive grain. Bonds that allow abrasives grains to fracture easily are classified as soil bonds. Bonds that restrict the fracturing of the grains and allow a wheel to withstand large forces are classified as hard bonds. Generally, soil wheels cut easily, produce poor surface finishes, and have a short useful life. On the other hand, harder wheels last longer and produce finer surface finishes, but cut less well and produce more heat during grinding.
The bonding matrix in which the abrasive grains are fixed may include a variety of organic materials such as rubber, shellac or resin; inorganic materials such as clay are also used. Inorganic bonds with glass-like or vitreous structures are used on the tool-sharpening wheels for the home workshop grinder, while resin bonds are used in masonry or steel-cutting wheels. Generally, vitrified bonds are used with medium to fine grain sizes in wheels needed for precision work. Resin bonds are used generally with coarse grains and for heavy metal removal operations such as foundry work.
In addition to their abrasive and bonding materials, grinding wheels often contain additional ingredients that produce pores within the wheel or assist chemically when a particular abrasive is used to grind a special material. One important aspect of a grinding wheel that can be created or altered through additives is porosity, which also contributes to the cutting characteristics of the grinding wheel. Porosity refers to the open spaces within the bond that allow room for small chips of metal and abrasive generated during the grinding process. Porosity also provides pathways that carry fluids used to control heat and improve the cutting characteristics of the abrasive grains. Without adequate porosity and spacing between abrasive grains, the wheel can become loaded with chips and cease to cut properly.
A variety of products are used as additives to create proper porosity and spacing. In the past, sawdust, crushed nut shells, and coke were used, but today materials that vaporize during the firing step of manufacturing (for example, napthaline-wax) are preferred. Some grinding wheels receive additional materials that serve as aids to grinding. These include sulfur and chlorine compounds that inhibit microscopic welding of metal particles and generally improve metal-cutting properties.
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