What is the manufacturing process of grinding wheels, and what are some of their uses?
The manufacturing process for grinding wheels begins with the selection of raw materials. The next step is mixing the raw materials and calibration of the proportions of each needed. This is followed by molding the mixture in a four-part mold form. After this, pressure force is applied to the mixture as it rests in the mold, forcing the materials together. Next, the discs of the grinding wheel are removed from the mold and, after final shaping, are fired in a kiln. The firing process melts the binding material and changes it to a resistant form.
Selecting Raw Materials
The two major components of all grinding wheels are the (1) abrasive mineral grains or synthetic mineral grains and the (2) bonding material. The earliest grinding material is thought to be sandstone containing (1) abrasive quartz mineral crystals held together in (2) natural earthen cement (e.g., clay minerals, calcite, silica or iron oxides). In the 1800s, emery was imported from India to use in Europe, England and the United States as grinding wheel abrasive grains. The cost of emery importation led to a search for less costly abrasives and, before emery was found to be plentiful in the United States, the search led to the development of chemically produced synthetic silicon carbide and synthetic corundum, an aluminum oxide, which is a bauxite derivative (bauxite: impure mixture of earthy hydrous aluminum oxides and hydroxides).
corundum: a very hard mineral that consists of aluminum oxide occurring in massive and crystalline forms, that can be synthesized, and that is used for gemstones (as ruby and sapphire) and as an abrasive (Merriam Webster Dictionary)
These synthetics proved more reliable and effective than abrasives in natural minerals. This success in the synthesis process spurred on further research that eventually led to the discovery of superabrasives, such as synthetic diamonds, synthetic cubic boron nitride and the newer seeded-gel aluminum oxide having a nano micro-structure built from sub-micron crystals. While these abrasives make grinding wheels effective, of equal importance is the bonding material that holds the grains together: without effective bonding the friability is undependable making the wheel unreliable or even useless (friability: the degree to which material is easily crumbled or reduced to powder, made crumbly as in friable rock). When rubber and clay were introduced as components of abrasive bonds in 1840, the success of grinding wheels was firmly established because the friability was reduced and controlled. In the 1870s another step forward in bond material occurred when vitrified bond structure was patented (vitrified : converted by heat into glass). Vitrified bonds continue to berefined yet grinding wheels composed of vitrified materials contained in a bonding matrix continue to be of great importance even though other processes of manufacturing grinding wheels are available, for example, "bonding a layer of abrasives to the surface of a metal wheel" (Theodore L....
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The basic component materials of grinding wheels are abrasive mineral grains and bonding material. The abrasive mineral or synthetic mineral grains must be selected with respect to material that need to be cut or finished. The abrasive grains can be made from diamond, silicon carbide or aluminum oxide. The bonding material in which the abrasive grains are fixed is usually made either from organic materials (rubber, resin), or inorganic materials (clay). The inorganic materials allows medium to fine sizes of grains, while the organic material allows large sizes of grains.
The stages of manufacturing of the grinding wheel usually are the followings:
- step 1: the abrasive grains and the bonding material are mixed together;
- step 2: the mixture is poured into a 4 pieces mold and then is compressed by a hydraulic press;
- step 3: the wheel is fired at a temperature that depends on the bonding material: up to 200^o Celsius for organics and up to 1260^o Celsius for inorganic materials;
- step 4: the circumference of wheel is made concentric to its center in order to correct relative position tolerances.
The grinding manufacturing process is an alternative to the chip-removing manufacturing process and it is successfully used when very-hard materials are machined.
Hence, the grinding wheels tools are used either to cut very hard materials, or to finish the surface of parts. The grinding wheels recommended for cutting steel are the ones whose bonding material is resin, while grinding wheels recommended for finishing surfaces are the ones whose bonding materials are vitrified bonds.