CUTTING oils are coolants and lubricants. As lubricants, they lubricate the area between the tool and the chip, and are tool and power preservers. Aside from these functions, they are useful in washing away the accumulated chips, giving a bright and smooth finish and acting as rust preventives.There are two general types of cutting oils, the so-termed soluble or emulsifiable oil and the straight cutting oil. The soluble oils mix or emulsify in all proportions in water; they are primary coolants but also have some lubricating action. There are two types of soluble oils, those which are principally or entirely of mineral-oil origin and those which contain an appreciable amount of saponifiable materials or soaps. The straight cutting oils offer a greater range of composition, and likewise a greater field for individual preference.Mr. Huffman emphasizes the importance of laboratory investigation and control, and says that the laboratory should cooperate in the selection and inspection of cutting oils. Summarized, the fundamental characteristics governing the selection of cutting oils are, for the soluble cutting oils, ability to emulsify, a tendency to remain stable without separation, and a tendency to prevent corrosion and give satisfactory finish. For the non-emulsifiable cutting oils, the characteristics are the quantity and nature of the saponifiable material and the quantity of combined sulphur not in natural combination.