Grinding
Sandpapers (coated abrasive) are the next most significant abrasive product. They consist, basically, of a single layer of abrasive particles held to a flexible backing material by an adhesive bond. The cutting action of coated abrasive products is determined by the abrasive used, the grit size, the density or spacing of the grit, the strength of the adhesive, and the flexibility of the backing material.
Manufacture begins with huge rolls of backing material, either paper, cloth, or a combination of the two. The backing is fed to the making machine where the first layer of adhesive is applied. Next, the layer of abrasive is applied, either by gravity or electrostatically. The electrostatic method orients the slivery type of abrasive used, with the sharp ends facing out. The process can also control the spacing of the grains. This is an advantage in that wide spacing helps to alleviate loading problems when grinding soft, stringy, or gummy materials.
After being coated with the abrasive layer, the product is draped in long festoons in which it partially dries. Then it is run through another sizing operation and a second layer of adhesive is applied. The product is draped, allowed to dry thoroughly, and wound into large-diameter rolls.
The adhesives used to bond the abrasive to the backing are water-soluble, waterproof, or a combination of the two. Water-soluble types are used for dry grinding operations and in household sanding, and occasionally on wood-sanding commercial operations. Resin or resin-over-glue types currently in use have the flexibility associated with the soluble types. The all-resin type is best for severe operations; its properties are such that the heat of grinding actually increases the adhesive’s holding strength.
Sandpaper disks for right-angle grinders are die-cut. Sheets are cut in the standard lengths and widths used in production and household applications. Strips are slit, cut to length, and joined by their ends to make up the coated abrasive belts that have become an essential part of industry, replacing several kinds of abrasive wheels used previously.
Other products use the abrasive in the form of grains or powders. In addition to the sizing operation, many types are specifically treated, by calcining, acid, or heating, to make them more suitable for use as lapping abrasive or perhaps as sandblasting grain. For use in lapping and polishing, the abrasive is usually mixed with a vehicle such as mineral or seal oil. Polishing sticks consist of waxes or greases impregnated with various-sized abrasive grains, depending on the particular requirements of the work.
Two materials used for cleaning rather than grinding are still abrasive in nature. Glass beads, pressure blasted onto a surface, remove rust, scale, and carbon. These have replaced much hand cleaning with steel wool. Steel wool still has some applications.
Grinding, the most important abrasive application, is in some way involved in the manufacture of almost every product. This use may be direct, as when the product requires pieces that must be made within close dimensional tolerance limits, or a very smooth surface, or when used on materials too hard to be machined by conventional cutting tools; or indirect, as when, for example, grinding wheels are used to sharpen cutting tools. The materials that are used to make cutting tools must of course be hard in order for the cutting tool to cut and retain its sharp edge. Abrasive grinding wheels are the only means for sharpening the dull edges of such tools.
Grinding wheels in use in industry today rotate at peripheral speeds of almost 300 kilometres (180 miles) per hour. The abrasive wheel may throw a long stream of bright yellow sparks and remove upwards of half a ton of metal per hour while grinding the imperfections from the surface of a bar of stainless steel. Or the grinding wheel may be as small as 0.55 millimetre (0.022 inch) in diameter, may rotate at 150,000 revolutions per minute, and may grind miniature precision ball bearings to accuracies measured in micrometres.
In the automotive industry, only abrasives can produce the tight fit required between piston rings and cylinders to prevent the escape of compressed gasoline vapours. Valves and valve seats are ground. Bearing surfaces in the engine, transmission, and wheels need specific finish, size, and roundness to assure frictionless rotation. These can be achieved only with abrasive tools.
Abrasive machining, the use of abrasives rather than high-speed steel or tungsten carbide cutting tools, makes use of the self-sharpening grinding wheel and eliminates tool sharpening costs. The ability to grind hardened materials without the previously necessary prehardening machining saves intermediate part-handling operations.
Speed and improved grinding systems, machines, and grinding tools are the main reason for the increased importance of grinding. One-hundred-horsepower motors, automatic loading equipment, high-speed grinding wheels removing large amounts of hard-to-grind materials, ultrafine tolerances, and costly machines are part of the new abrasives systems that are capable of extremely high rates of production. Yet the abrasive products portion of the total cost to grind the part may be as low as 5 percent, even though the grinding wheel, with diamond as the abrasive, may cost thousands of dollars. Or the wheel may cost a few pennies and be used to shape a die used in the manufacture of tableware.
Relatively new products, such as plastic bonded tumbling media for mass finishing a multitude of parts, have eliminated time-consuming hand-deburring operations, a plague to the aircraft industry, in which the high cost of labour makes handwork prohibitively expensive. Parts with rough edges are tumbled in a rotating barrel of loose abrasive or preshaped abrasive pieces. As the mass slides, burrs are ground away, surfaces are finished, and edges are smoothed. When the same parts and media are mixed in a vibrating tub, the process becomes even more productive.