Every kind of machining (e.g., turning, boring, drilling, milling, broaching, grinding, sawing, shaping, planing, reaming, tapping) can potentially benefit from one kind of cutting fluid or another, depending on workpiece material. (Cast iron and brass are usually machined dry. Interrupted cuts such as milling with carbide cutters are usually recommended to be used dry due to damage to the cutters caused by thermoshock.)
The properties that are sought after in a good cutting fluid are the ability to:
EP additives are often added to cutting fluids.
Every conceivable method of applying cutting fluid (flooding, spraying, dripping, misting, etc.) can be used, with the best choice depending on the application and the equipment available. For many metalcutting applications the ideal would be high-pressure, high-volume pumping to force a stream of fluid directly into the tool-chip interface, with walls around the machine to contain the splatter and a sump to catch, filter, and recirculate the fluid. This type of system is commonly employed, especially in manufacturing. It is often not a practical option for MRO or hobbyist metalcutting, where smaller, simpler machine tools are used. Fortunately it is also not necessary in those applications, where heavy cuts, aggressive speeds and feeds, and constant, all-day cutting are not vital.
There are generally three types of liquids: mineral, semi-synthetic, and synthetic. Semi-synthetic and synthetic cutting fluids try to blend the best properties of oil into the best properties of water. They basically achieve this by allowing oil to emulsify into water. Some of these properties are: rust inhibition, tolerance of a wide range of water hardness (maintain pH stability around 9 to 10), ability to work with many metals, resist thermal breakdown, and environmental safety.
Water is a great conductor of heat but has drawbacks as a cutting fluid. It boils easily, promotes rusting of machine parts, and does not lubricate well. Therefore, other ingredients are necessary to create an optimal cutting fluid.
Mineral coolants, which are petroleum-based, began in the late 1800s. They vary from the thick, dark, sulfur-rich cutting oils used in heavy industry to light, clear oils.
Semi-synthetic coolants are an emulsion or microemulsion of water with mineral oil. They began in the 1930s. A typical CNC usually uses emulsified coolant. One way to understand it is to think of "pots-and-pans dishwater". It is a fair amount of oil emulsified into a larger amount of water using a detergent. Therefore it is roughly analogous to your dishwater after you have washed the vegetable oil off your pans (although more oily and less soapy).
Synthetic coolants originated in the late 1950s and are usually water-based.
A hand-held refractometer is used to determine the mix ratio (also called strength) of water soluble coolants to verify effectiveness. Numerous other test equipment are used to determine such things as acidity, and amount of conductivity.
Some cutting fluids are used in mist (aerosol) form and some of them are used in drilling aluminium
Metalworking fluids often contain substances such as biocides, corrosion inhibitors, metal fines, tramp oils, and biological contaminants. Inhalation of cutting fluid aerosols may cause irritation of the throat, nose, and lungs and has been associated with chronic bronchitis, asthma, hypersensitivity pneumonitis (HP), and worsening of pre-existing respiratory problems. Skin exposure may result from touching contaminated surfaces, handling parts and equipment, splashing fluids, and aerosol mist settling on the skin. Skin contact with cutting fluids may cause allergic contact dermatitis, irritant contact dermatitis, and occupational ("oil") acne.
Safer formulations provide a natural resistance to tramp oils allowing improved filtration separation without removing the base additive package. Ventilation, splash guards on machines, and personal protective equipment can mitigate hazards related to cutting fluids.
Bacterial growth is predominant in semi-synthetic and synthetic fluids. Tramp oil along with human hair or skin oil are some of the debris during cutting which accumulates and forms a layer on the top of the liquid, anaerobic bacteria proliferate due to a number of factors. An early sign of the need for replacement is the "Monday-morning smell" (due to lack of usage from Friday to Monday). Antiseptics are sometimes added to the fluid to kill bacteria. Such use must be balanced against whether the antiseptics will harm the cutting performance, workers' health, or the environment. Maintaining as low a fluid temperature as practical will slow the growth of microorganisms.
Old, used cutting fluid must be disposed of when it is fetid or when it is chemically degraded and has lost its performance. As with used motor oil or other wastes, its impact on the environment should be mitigated. Legislation and regulation specify how this mitigation should be achieved. Enforcement is the most challenging aspect. Modern cutting fluid disposal may involve techniques such as ultrafiltration using polymeric or ceramic membranes which concentrates the suspended and emulsified oil phase.