Polishing

Polishing is a finishing process used to improve the surface quality and luster of objects of metal and some other materials. Polishing is done with motor-driven belts or wheels coated with fine grains of an abrasive material, usually aluminum oxide or silicon carbide. The belts or wheels are made of flexible materials such as canvas, cloth, or leather. The flexibility of the belts and wheels makes possible the polishing of irregular or curved surfaces.

Polishing usually follows a coarser finishing process, such as grinding. Several steps, using increasingly finer abrasives, are required before the final polish is achieved. For an especially shiny surface, polishing may be followed by buffing. Buffing wheels are similar to polishing wheels, but they are made of cotton, hemp cloth, flannel, linen, or sheepskin. A fine abrasive, such as rouge, or synthetic iron oxide, is injected between the buffing wheel and the object being buffed.

Polishing Wheels

Some polishing wheels have blocks or sections of canvas, leather, or felt clamped between center plates to form the face of the wheel. Other polishing wheels are built up from a number of leather, felt, cotton, or canvas disks until the entire width of the wheel face is formed. Different degrees of wheel softness are obtained by the varying constructions and materials, thereby forming a wheel best suited for a particular polishing task.

After a wheel is formed, a hot glue or cold cement is brushed on the face of the wheel. Then the wheel is rolled in a grit consisting of grains of synthetic corundum, silicon carbide, emery, or other abrasive. Finally, the wheel is rolled on a flat surface to drive the abrasive into the wheel surface.

Abrasive Belts

Polishing also is done with an endless abrasive belt, one end of which runs over a flexible-faced wheel at the front of the machine, and the other end over a pulley at the rear. In polishing, the operator forces the work-piece against the rotating wheel.

Superfinishing

Superfinishing, the most precise method of fine polishing, was first developed in 1934 by Donald A. Wallace and was used for Chrysler automobile engine parts. In superfinish-ing, abrasive stone of extremely fine grit, usually shaped to match a cylindrical workpiece, is moved over the surface of the workpiece as it rotates underneath the stone. Superfinishing is done under very light to moderate pressure, and a flood of lubrication is used. A superfinished part is free from minute surface flaws and has a mirrorlike surface.

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