Machinery's Handbook, 31st Edition
CUTTING FLUIDS 1247 cutting fluid are: cooling of the tool, workpiece, and chip; reducing friction at the sliding contacts; and reducing or preventing welding or adhesion at the contact surfaces, which forms the “built-up edge” on the tool. Two other functions of cutting fluids are flushing away chips from the cutting zone and protecting the workpiece and tool from corrosion. The relative importance of the functions is dependent on the material being machined, the cutting tool and conditions, and the finish and accuracy required on the part. For exam ple, cutting fluids with greater lubricity are generally used in low-speed machining and on most difficult-to-cut materials. Cutting fluids with greater cooling ability are generally used in high-speed machining on easier-to-cut materials. Types of Cutting and Grinding Fluids.— In recent years a wide range of cutting fluids has been developed to satisfy the requirements of new materials of construction and new tool materials and coatings. There are four basic types of cutting fluids; each has distinctive features, as well as advantages and limitations. Selection of the right fluid is made more complex because the dividing line between types is not always clear. Most machine shops try to use as few different fluids as possible and prefer fluids that have long life, do not require constant changing or modifying, have reasonably pleasant odors, do not smoke or fog in use, and, most important, are neither toxic nor cause irritation to the skin. Other issues in selection are the cost and ease of disposal.The major divisions and subdivisions used in classifying cutting fluids are: Cutting Oils, including straight and compounded mineral oils plus additives. Water-Miscible Fluids , including emulsifiable oils; chemical or synthetic fluids; and semichemical fluids. Gases. Paste and Solid Lubricants. Since cutting oils and water-miscible types are the most commonly used cutting fluids in machine shops, discussion will be limited primarily to these types. It should be noted, however, that compressed air and inert gases, such as carbon dioxide, nitrogen, and Freon, are sometimes used in machining. Paste, waxes, soaps, graphite, and molybdenum disul fide may also be used, either applied directly to the workpiece or as an impregnant in the tool, such as in a grinding wheel. Cutting Oils.— Cutting oils are generally compounds of mineral oil with the addition of animal, vegetable, or marine oils to improve the wetting and lubricating properties. Sulfur, chlorine, and phosphorous compounds, sometimes called extreme pressure (EP) additives, provide for even greater lubricity. In general, these cutting oils do not cool as well as water-miscible fluids. Water-Miscible Fluids.— Emulsions or soluble oils are a suspension of oil droplets in water. These suspensions are made by blending the oil with emulsifying agents (soap and soaplike materials) and other materials. These fluids combine the lubricating and rust- prevention properties of oil with water’s excellent cooling properties. Their properties are affected by the emulsion concentration, with “lean” concentrations providing better cooling but poorer lubrication, and with “rich” concentrations having the opposite effect. Additions of sulfur, chlorine, and phosphorus, as with cutting oils, yield “extreme pres sure” (EP) grades. Chemical fluids are true solutions composed of organic and inorganic materials dis solved in water. Inactive types are usually clear fluids combining high rust inhibition, high cooling, and low lubricity characteristics with high surface tension. Surface-active types include wetting agents and possess moderate rust inhibition, high cooling, and moderate lubricating properties with low surface tension. They may also contain chlorine and/or sulfur compounds for extreme pressure properties. Semichemical fluids are combinations of chemical fluids and emulsions. These fluids have a lower oil content but a higher emulsifier and surface-active-agent content than
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