Machinery's Handbook, 31st Edition
Joining and Edging 1443 lubricant is critical to the success of the operation. Under such pressure, the drawing lu- bricant should cool the die and the workpiece, provide boundary lubrication between the die and the workpiece, prevent metal-to-metal adhesion or welding, and cushion the die during the drawing operation. Lubricants work by forming lubricating films between two sliding surfaces in contact with each other. When these metal surfaces are viewed under magnification, peaks and valleys become apparent, even on finely-ground surfaces. The lubricating film needs to prevent the asperities (peaks) on the two surfaces in sliding contact with each other from damaging the mating surface. Under hydrodynamic or full-film lubrication, two surfaces are completely separated by a fluid film, with no contact between the asperities. This condition could change as speeds vary during start- and-stop modes or if the pressure and temperature increase beyond the lubricant’s film strength. Boundary lubricants work up to a certain temperature and pressure, and then the boundary additive breaks down and metal contacts metal. The working temperature varies with the type and amount of additive used and its interaction with other additives. Three types of drawing lubricants are used: 1) drawing oils; 2) emulsions; and 3) lubri cants containing both oil and solid substances. Drawing oils become an absorbed film, and they take the form of light or soluble oils such as straight mineral oil or emulsions of soluble oil and soap, or of heavy oils, fats, and greases such as tallow or lard oil. Aqueous solutions of non-oily lubricants containing some suspended solids are called emulsions. These lubricants are not widely used in deep drawing because they contain little or no oil. Lubricants containing both oil and solid substances are used in applications involving severe drawing; these lubricants contain oily components that reduce friction and heat. The combination of the oil and the solids produces enough lubrication for severe drawing applications such as deep drawing. Deep drawing often involves ironing or thinning the wall by up to 35 percent, and lubricant containing high proportions of chemically-active components. Dry soaps and polymer films are frequently used for these purposes. Alumi num can be shallow drawn with oils of low to medium viscosity, and for deep drawing, tallow may be added, as well as wax or soap suspensions for very large reductions. Lubricant Removal.— Removing lubricant from a formed part after the deep drawing operation is important because any lubricant left behind can interfere with subsequent steps in the manufacturing of the part. Mineral oils, animal fat, and vegetable oils can be removed with an organic solvent by emulsification or saponification, or with an aqueous alkaline cleaner. Greases can also be removed from sheet metal with an organic solvent or an alkaline cleaner. Solids are more difficult to remove because they are not readily soluble. The presence of solids often requires that additional cleaning methods be used. Petroleum oils can raise special issues from removal through disposal. These oils require the use of alkaline cleaners for removal, which can then contaminate cleaner tanks with oil, leading to potential disposal challenges. Vegetable oils can be removed with hot water if the parts are cleaned immediately, and with mildly to moderately alkaline cleaners if the parts have been left standing for a few days. Joining and Edging A duct system is an assembly whose main function is to convey air. Elements of the duct system are sheets, transverse joints, longitudinal seams, and reinforcements. The sheets must be able to withstand deflection caused by both internal pressure and vibration due to turbulent air flow. Transverse joints must be able to withstand 1.5 times the maximum operating pressure without failure. Transverse joint designs should be consistent with the static pressure class, sealing requirements, materials involved, and support interval distances. Notching, bending, folding, and fit up tolerances shall be appropriate for the proper class. Longitudinal seams also must be able to withstand 1.5 times the operating pressure without deformation. Seams must be formed and assembled with proper dimen- sion and proportion for tight and secure fit up. Seams may be a butt, corner, plug, or spot
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