Machinery's Handbook, 31st Edition
GRINDING WHEEL SAFETY 1311 Work Speeds: In diamond grinding, work rotation and table traverse are usually estab lished by experience, adjusting these values to the selected infeed so as to avoid excessive wheel wear. Infeed per Pass: Often referred to as downfeed and usually a function of the grit size of the wheel. The following are general values which may be increased for raising the productivity, or lowered to improve finish or to reduce wheel wear. Wheel Grit Size Range Infeed per Pass 100 to 120 0.001 inch (0.025 mm) 150 to 220 0.0005 inch (0.013 mm) 250 and finer 0.00025 inch (0.006 mm) Grinding Wheel Safety Safety in Operating Grinding Wheels.— Grinding wheels, although capable of excep tional cutting performance due to hardness and wear resistance, are prone to damage caused by improper handling and operation. Vitrified wheels, comprising the major part of grinding wheels used in industry, are held together by an inorganic bond which is actu- ally a type of pottery product and therefore brittle and breakable. Although most of the organic bond types are somewhat more resistant to shocks, it must be realized that all grinding wheels are conglomerates of individual grains joined by a bond material whose strength is limited by the need of releasing the dull, abrasive grains during use. It must also be understood that during the grinding process very substantial forces act on the grinding wheel, including the centrifugal force due to rotation, the grinding forces resulting from the resistance of the work material, and shocks caused by sudden contact with the work. To be able to resist these forces, the grinding wheel must have a substantial minimum strength throughout that is well beyond that needed to hold the wheel together under static conditions. Finally, a damaged grinding wheel can disintegrate during grinding, liberating dormant forces which normally are constrained by the resistance of the bond, thus presenting great hazards to both operator and equipment. To avoid breakage of the operating wheel and, should such a mishap occur, to prevent damage or injury, specific precautions must be applied. These safeguards have been for mulated into rules and regulations and are set forth in the American National Standard ANSI B7.1-2017, entitled the American National Standard Safety Requirements for the Use, Care, and Protection of Abrasive Wheels. Handling, Storage and Inspection.— Grinding wheels should be hand carried, or trans ported, with proper support, by truck or conveyor. A grinding wheel must not be rolled around on its periphery. The storage area, positioned not far from the location of the grinding machines, should be free from excessive temperature variations and humidity. Specially built racks are recommended on which the smaller or thin wheels are stacked lying on their sides and the larger wheels in an upright position on two-point cradle supports consisting of appropriately spaced wooden bars. Partitions should separate either the individual wheels, or a small group of identical wheels. Good accessibility to the stored wheels reduces the need of undesirable handling. Inspection will primarily be directed at detecting visible damage, mostly originating from handling and shipping. Cracks which are not obvious can usually be detected by “ring testing,” which consists of suspending the wheel from its hole and tapping it with a non-metallic implement. Heavy wheels may be allowed to rest vertically on a clean, hard floor while performing this test. A clear metallic tone, a “ring”, should be heard; a dead sound being indicative of a possible crack or cracks in the wheel. Machine Conditions.— The general design of the grinding machines must ensure safe operation under normal conditions. The bearings and grinding wheel spindle must be dimensioned to withstand the expected forces and ample driving power should be pro vided to ensure maintenance of the rated spindle speed. For the protection of the operator, stationary machines used for dry grinding should have a provision made for connection to an exhaust system and when used for off-hand grinding, a work support must be available. Wheel guards are particularly important protection elements and their material specifica tions, wall thicknesses and construction principles should agree with the Standard’s speci fications. The exposure of the wheel should be just enough to avoid interference with the grinding operation. The need for access of the work to the grinding wheel will define the boundary of guard opening, particularly in the direction of the operator.
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