Metal Finishing Guide Book


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Page 29 of 843

Table II. Wheel Speeds for Hand Buffing, sfm Carbon and stainless steel Brass Nickel Aluminum Zinc and other soft metals Chromium LIQUID SPRAY BUFFING Cutting Down 8,000-9,000 6,000-9,000 6,000-9,000 6,000-9,000 5,000-8,000 Luster Buffing 7,000-9,000 6,000-9,000 6,000-8,000 6,000-7,000 6,000-7,000 7,000-8,000 Liquid spray buffing compositions have largely replaced bar buffing compositions on automatic buffing machines. Unlike the bar compound previously discussed, liquid buffing compound is a water-based product. The liquid buffing compound has three main constituents: water, binder, and abrasive. Water is used as the vehicle to transport the binder and abrasive to a buffing wheel through a spray system. This water-based liquid is an oil/water emulsion. In this emulsion the abrasive particle is suspended and could be thought of as particles coated with a binder material. The emulsifying materials act as a device to hold the oil-soluble molecules onto the water molecules. Larger abrasive particles offer less surface area (when compared with the weight of that particle) than several smaller particles. Surface area and density play an important role in the suspension of any liquid emulsion. Stability is the ability to keep the abrasive particle in suspension. When the abrasive particles tend to fall out of suspension, their weight factor is greater than the ability of the emulsified material to maintain stability. Viscosity, therefore, plays an important role in a suspension. A totally unstable emulsion will settle out under all circumstances. The flow characteristics of a liquid buffing compound are controlled generally by the viscosity of that compound as well as its degree of slip. The viscosity stability of any emulsion is established by its thixotropic nature, which means the viscosity becomes lighter in direct proportion to the amount of shear to which the compound is subjected. As the degree of slip is increased, the flow characteristics of the compound will also increase in direct proportion to the resultant change in slip or the resultant change in the coefficient of friction. The gel-type property of an emulsion is broken down by the action of the pump, thus producing viscosity changes. The changes are determined by the amount of shearing action of the pump and the length of time. This breakdown is necessary to allow the transfer of the buffing compound from the pump to the spray gun, which often requires a significant distance. The viscosity of a liquid compound is measured under a constant set of conditions. To measure viscosity, a representative sample from a batch is needed. This sample must be in a state of equilibrium for a defined period and at a constant temperature. A viscometer is used with a specific spindle. This reading multipled by a factor will give a viscosity reading in centipoise. A deviation of 25% is normal. The control of viscosity of a compound is somewhat difficult. Variations in raw materials or the method of blending are two reasons for viscosity changes. Viscosity is an arbitrary measurement. Liquid compounds are supplied to the spray guns by means of either air pressure feed tanks or drum pumping equipment. Air pressure is varied depending on the viscosity of the liquid compound, the length and diameter of the fluid lines feeding the spray guns, and the actual number of spray guns. With one or two spray guns 26

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