Straight cutting oils are blended utilizing base oils and performance-enhancing additives. The most-common base oils used are paraffinic mineral oils, though there is a preference for naphthenic mineral oils in some cases. In any case, both types of mineral oils are widely used due to its availability in various viscosities, the primary factor in neat oil selection (taken up in the preceding blog). Mineral oils are ideal for machining and grinding processes also since it is inexpensive.
Esters are used both as a base oil in rare instances but may be combined with mineral oils, too. One can consider a certain neat cutting oil’s base oil as a combination of mineral oils and esters, or one can interpret esters, in such cases, as an additive. Either way, esters are preferred as it imparts excellent lubrication on the cutting interface, as it behaves in a magnetic manner towards metal, it is highly adherent. Moreover, esters degrade or oxidize at a lesser rate than mineral oils, and does not produce as much smoke. The addition or use of esters drives up the price of the cutting oils, though.
Various additives are added to the base oil to enhance performance. For improved cutting performance, which can prolong tool life and surface finish of machined and ground parts, EP or extreme pressure additives are used. The most common EP additives are based on sulfur, chlorine, and phosphorus, which at elevated temperatures, react with iron to form a chemical layer or film on the cutting tool, reducing friction. Care must be taken though to NOT use EP additives with sensitive non-ferrous metals such as aluminum, brass, or copper, as sulfur, chlorine and phosphorous may react to generate stains on the non-ferrous metal surface.
Other additives in use in neat cutting oils include anti-mist, anti-rust, non-ferrous corrosion inhibitors, and colorants.