Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

Issue link:

Contents of this Issue


Page 24 of 707

Table II. Wheel Speeds for Hand Buffing, sfm Carbon and stainless steel Brass Nickel Aluminum Zinc and other soft metals Chromium 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 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 sus- pension. 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 stabil- ity 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 condi- tions. To measure viscosity, a representative sample from a batch is needed. This sam- ple must be in a state of equilibrium for a defined period and at a constant tem- perature. 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 feed- ing the spray guns, and the actual number of spray guns. With one or two spray guns close to the tank, 10 to 15 psig tank pressure may be sufficient, while 6 to 8 guns could require 40 to 45 psig tank pressure. A drum pumping system is inserted into a steel drum. The pump then trans- fers the compound through a fluid line or manifold that feeds the guns. Depending on the size of the system, the drum pump is operated at 10 to 40 psig air pressure. The spray gun is usually mounted in back of the buffing wheel so it will not interfere with the operator and is at a distance from the buffing wheel face so that complete coverage of the face of the buff is obtained with proper regulation of the spray gun. An opening in the dust collecting hood allows the compound to be sprayed from this position. Where buffing machines are totally enclosed, there are no hoods to interfere with the placement of the guns. The spray guns are actu- ated by air, which is released, in the case of manually operated lathes, by a foot valve that allows the buffer to keep both hands on the part being buffed. With 23

Articles in this issue

view archives of Metal Finishing Guide Book - 2011-2012 Surface Finishing Guidebook