Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 33 of 707

Nonferrous products made of copper, nickel, chromium, zinc, brass, alu- minum, etc., frequently are buffed with compounds containing silica (generally amorphous, often "tripoli"). "Tripoli" is found in a small area of Oklahoma and is shipped all over the world. Steel products are normally buffed with com- pounds of fused aluminum oxide, which is available in DCF collector fines and as graded aluminum oxide in a range of grit designations. Special abrasives are available for other purposes. For example, chromium oxide is widely used to give stainless steel, chromium- and nickel-plated products high reflectivity. Iron oxides are used to color buff gold, silver, copper, and brass. Lime-based buffing compounds are used to generate mirror finishes on nickel products. Skilled buffing engineers can help manufacturers select the optimum equip- ment, buffing compounds, wheels, and buffing techniques. Cleaners and clean- ing processes must be matched to the soil to be removed. BUFFING WHEELS Fabrics used in buffing are designated by thread count and fabric weight. Count is measured by threads per inch; weight by the number of linear yards per pound of 40-inch-wide fabric. Heavier materials have fewer yards per pound. Lower thread count and lighter weight materials are used for softer metals, plastics, and final luster. More closely woven, heavier, and stiffer materials are used on hard- er metals for greater cut and surface defect removal. Stiffness is a result of heav- ier weight, higher thread count fabrics, more material, specialized treatments, sewing, and overall buff design. Buff wheel construction determines the action of the buff by making it hard- er or softer, usually by varying convolutions of the face of the wheel. This influ- ences aggressiveness. Part configuration dictates buff design, construction, thread count, etc. Conventional buffs employ a circular disk of cloth cut from sheeting and sewn into a number of plies. For example, some materials require from 18 to 20 plies to make a -in.-thick section. Multiple sections are assembled on a spindle to build the required face width. The density of these types of buffs is also controlled by spacers that separate the plies of fabric or adjacent faces from one another. Industry standards for the inside diameter of airway-type buff wheels are 3, 5, 7, and 9 in. As a rule, productivity and buff wheel life increase as outside diam- eter increases and thread count and material content increases. Larger buffs and higher shaft rotation speeds also increase productivity and buff life. The choice of buff center size depends on how far the buff material can be worn before the surface speed reduces to a point of inefficiency, or flexibility declines to a point where contours cannot be followed. Airway buff flexibility decreases with use as wear progresses closer to the steel center. Most airway buffs are designed with as much material at the inside diameter as the outside diameter. Flanges Buffing wheels require flanges for safe operation. Flanges must be sized for the specific inside diameter of each buffing wheel. It is important for all buffs that the flange be designed with sufficient strength to withstand the tremendous forces and pressures exerted in buffing. If buffs are not well designed and fabricated, cen- trifugal forces at higher speeds and the shock from operations can cause failure of clinching teeth, breakage of rings, and breakdown of buff sections. MUSLIN BUFFS The most commonly used fabrics for buffs are cotton muslins. As previously not- 32

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