is the fluid, usually petroleum-based, for cooling and lubricating the tool and work in metal cutting operations. Some fluids are fortified with EP additives to facilitate cutting of hard metals, to improve finishes, and to lengthen tool life. Cutting fluids that react chemically with metal surfaces are called active oils; sulfurized oils, for example, have improved load-carrying properties, but may stain non-ferrous metals. Some cutting oils are transparent to provide a better view of the work. Most cutting fluids fall into four basic categories: l) straight oils, mineral oils blended with fatty oils for good wetting and penetrating characteristics and a good machined finish; 2) emulsifiable (soluble) oils, mineral oils dispersed as minute droplets in water to combine the lubricating properties of the oil with the cooling properties of water (since emulsifiable oils are subject to bacterial action and resultant odors, they may contain a bactericide ); 3) synthetic fluids, blends of chemical agents in water for improved machining speed, cooling, and tool life; and 4) semi-synthetic fluids, water-dilutable fluids that combine the lubricity of soluble oils with the advantages of synthetics.

Also see metalworking lubricant.