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Coolants and lubricants used in metalworking

Cutting fluid is a type of coolant and lubricant designed specifically for metalworking processes, such as machining and stamping. There are various kinds of cutting fluids, which include oils, oil water emulsions, pastes, gels, aerosols (mists), and air or other gases. Cutting fluids are made from petroleum distillates, animal fats, plant oils, water and air, or other raw ingredients. Depending on context and on which type of cutting fluid is being considered, it may be referred to as cutting fluid, cutting oil, cutting compound, coolant, or lubricant. Most metalworking and machining processes can benefit from the use of cutting fluid, depending on workpiece material. Common exceptions to this are cast iron and brass, which may be machined dry (though this is not true of all brasses, and any machining of brass will likely benefit from the presence of a cutting fluid). The properties that are sought after in a good cutting fluid are the ability to:
Keep the workpiece at a stable temperature (critical when working to close tolerances). Very warm is acceptable, but extremely hot or alternating hot and cold are avoided. Maximize the life of the cutting tip by lubricating the working edge and reducing tip welding. Ensure safety for the people handling it (toxicity, bacteria, fungi) and for the environment upon disposal. Prevent rust on machine parts and cutters.

Metal cutting generates heat due to friction and energy lost deforming the material. The surrounding air has low thermal conductivity (conducts heat poorly) meaning it is a poor coolant. Ambient air cooling is sometimes adequate for light cuts and low duty cycles typical of maintenance, repair and operations (MRO) or hobbyist work. Production work requires heavy cutting over long time periods and typically produces more heat than air cooling can remove. Rather than pausing production while the tool cools, using liquid coolant removes significantly more heat more rapidly, and can also speed cutting and reduce friction and tool wear. However, it is not just the tool which heats up but also the work surface. Excessive temperature in the tool or work surface can ruin the temper of both, soften either to the point of uselessness or failure, burn adjacent material, create unwanted thermal expansion or lead to unwanted chemical reactions such as oxidation.

Besides cooling, cutting fluids also aid the cutting process by lubricating the interface between the tool's cutting edge and the chip. By preventing friction at this interface, some of the heat generation is prevented. This lubrication also helps prevent the chips from being welded onto the tool, which would interfere with subsequent cutting. Cutting fluids may also assist in reducing cutting forces through the Rehbinder effect. Extreme pressure additives are often added to cutting fluids to further reduce tool wear.

Every conceivable method of applying cutting fluid (e. g., flooding, spraying, dripping, misting, brushing) can be used, with the best choice depending on the application and the equipment available. For many metal cutting applications the ideal has long been high pressure, high volume pumping to force a stream of liquid (usually an oil water emulsion) 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 maintenance, repair and overhaul or hobbyist metal cutting, 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. As technology continually advances, the flooding paradigm is no longer always the clear winner. It has been complemented since the 2000s by new permutations of liquid, aerosol, and gas delivery, such as minimum quantity lubrication and through the tool tip cryogenic cooling (detailed below). Through tool coolant systems, also known as through spindle coolant systems, are systems plumbed to deliver coolant through passages inside the spindle and through the tool, directly to the cutting interface. Many of these are also high pressure coolant systems, in which the operating pressure can be hundreds to several thousand psi (1 to 30 MPa)—pressures comparable to those used in hydraulic circuits. High pressure through spindle coolant systems require rotary unions that can withstand these pressures. Drill bits and endmills tailored for this use have small holes at the lips where the coolant shoots out. Various types of gun drills also use similar arrangements.

Cutting fluid may also take the form of a paste or gel when used for some applications, in particular hand operations such as drilling and tapping. In sawing metal with a bandsaw, it is common to periodically run a stick of paste against the blade. This product is similar in form factor to lipstick or beeswax. It comes in a cardboard tube, which gets slowly consumed with each application.

Some cutting fluids are used in aerosol (mist) form (air with tiny droplets of liquid scattered throughout). The main problems with mists have been that they are rather bad for the workers, who have to breathe the surrounding mist tainted air, and that they sometimes don't even work very well. Both of those problems come from the imprecise delivery that often puts the mist everywhere and all the time except at the cutting interface, during the cut—the one place and time where it's wanted. However, a newer form of aerosol delivery, (minimum quantity of lubricant), avoids both of those problems. The delivery of the aerosol is directly through the flutes of the tool (it arrives directly through or around the insert itself—an ideal type of cutting fluid delivery that traditionally has been unavailable outside of a few contexts such as gun drilling or expensive, state of the art liquid delivery in production milling). MQL's aerosol is delivered in such a precisely targeted way (with respect to both location and timing) that the net effect seems almost like dry machining from the operators' perspective.

Ambient air, of course, was the original machining coolant. Compressed air, supplied through pipes and hoses from an air compressor and discharged from a nozzle aimed at the tool, is sometimes a useful coolant. The force of the decompressing air stream blows chips away, and the decompression itself has a slight degree of cooling action. The net result is that the heat of the machining cut is carried away a bit better than by ambient air alone. Sometimes liquids are added to the air stream to form a mist (mist coolant systems, described above). Liquid nitrogen, supplied in pressurized steel bottles, is sometimes used in similar fashion. In this case, boiling is enough to provide a powerful refrigerating effect. For years this has been done (in limited applications) by flooding the work zone. Since 2005, this mode of coolant has been applied in a manner comparable to MQL (with through the spindle and through the tool tip delivery). This refrigerates the body and tips of the tool to such a degree that it acts as a "thermal sponge", sucking up the heat from the tool–chip interface. This new type of nitrogen cooling is still under patent. Tool life has been increased by a factor of 10 in the milling of tough metals such as titanium and inconel. These are used infrequently today, because of the wide variety of other choices, but remain as an option. Old machine shop training texts speak of using red lead and white lead, often mixed into lard or lard oil. This practice is obsolete due to the toxicity of lead. From the mid 20th century to the 1990s, 1,1,1 trichloroethane was used as an additive to make some cutting fluids more effective. In shop floor slang it was referred to as "one one one". It has been phased out because of its ozone depleting and central nervous system depressing properties.

Cutting fluids degrade over time due to contaminants entering the lubrication system. A common type of degradation is the formation of tramp oil, also known as sump oil, which is unwanted oil that has mixed with cutting fluid. It originates as lubrication oil that seeps out from the slideways and washes into the coolant mixture, as the protective film with which a steel supplier coats bar stock to prevent rusting, or as hydraulic oil leaks. In extreme cases it can be seen as a film or skin on the surface of the coolant or as floating drops of oil. Skimmers are used to separate the tramp oil from the coolant. These are typically slowly rotating vertical discs that are partially submerged below the coolant level in the main reservoir. As the disc rotates the tramp oil clings to each side of the disc to be scraped off by two wipers, before the disc passes back through the coolant. The wipers are in the form of a channel that then redirects the tramp oil to a container where it is collected for disposal. Floating weir skimmers are also used in these situations where temperature or the amount of oil on the water becomes excessive. Since the introduction of CNC additives, the tramp oil in these systems can be managed more effectively through a continuous separation effect. The tramp oil accumulation separates from the aqueous or oil based coolant and can be easily removed with an absorbent. Old, used cutting fluid must be disposed of when it is fetid or chemically degraded and has lost its usefulness. 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. Modern cutting fluid disposal involves techniques such as ultrafiltration using polymeric or ceramic membranes which concentrates the suspended and emulsified oil phase. Chip handling and coolant management are interrelated. Over the decades they have been improved, to the point that many metalworking operations now use engineered solutions for the overall cycle of collecting, separating, and recycling both chips and coolant. For example, the chips are graded by size and type, tramp metals (such as bolts and scrap parts) are separated out, the coolant is centrifuged off the chips (which are then dried for further handling), and so on.