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Thin-wall milling of aluminum using a water-based cutting fluid on the milling cutter.

Cutting fluids are various fluids that are used in machining to cool and lubricate the cutting tool. There are various kinds of cutting fluids, which include oils, oil-water emulsions, pastes, gels, and mists. They may be made from petroleum distillates, animal fats, plant oils, 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.

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:

  • keep the workpiece at a stable temperature (critical when working to close tolerances). Very warm is OK, 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.


Mechanisms of action



Metal cutting operations involve generation of heat due to friction between the tool and the pieces and due to energy lost deforming the material. The surrounding air alone is a rather poor coolant for the cutting tool, because the rate of heat transfer is low. Ambient-air cooling is adequate for light cuts with periods of rest in between, such as are typical in maintenance, repair and operations (MRO) work or hobbyist contexts. However, for heavy cuts and constant use, such as in production work, more heat is produced per time period than ambient-air cooling can remove. It is not acceptable to introduce long idle periods into the cycle time to allow the air-cooling of the tool to "catch up" when the heat-removal can instead be accomplished with a flood of liquid, which can "keep up" with the heat generation.

Lubrication at the tool-chip interface

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 chip from being welded onto the tool, which interferes with subsequent cutting.

EP additives are often added to cutting fluids.

Delivery methods

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 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.

Types of cutting fluid


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.[1]

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, which consists of a small amount of oil emulsified into a larger amount of water through the use of a detergent.

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 concentration) of water soluble coolants. Numerous other test equipment are used to determine such things as acidity, and amount of conductivity.

Pastes or gels

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.


Some cutting fluids are used in mist (aerosol) form, although breathing such a lubricant in mist form is a severe and immediate health hazard.

Other fluids used (present and past)


  • Kerosene, rubbing alcohol, and 3-In-One Oil often give good results when working on aluminium.
  • Lard is suitable for general machining and also press tool work.
  • Mineral oil
  • WD-40
  • Dielectric fluid is the cutting fluid used in Electrical discharge machines (EDMs). It is usually deionized water or a high-flash-point kerosene. Intense heat is generated by the cutting action of the electrode (or wire) and the fluid is used to stabilise the temperature of the workpiece, along with flushing any eroded particles from the immediate work area. The dielectric fluid is nonconductive.
  • Liquid- (water- or petroleum oil-) cooled water tables are used with the plasma arc cutting (PAC) process.


  • In 19th-century machining practice, it was not uncommon to use plain water. This was simply a practical expedient to keep the cutter cool, regardless of whether it provided any lubrication at the cutting edge–chip interface. When one considers that high-speed steel (HSS) had not been developed yet, the need to cool the tool becomes all the more apparent. (HSS retains its hardness at high temperatures; other carbon tool steels do not.) An improvement was soda water, which better inhibited the rusting of machine slides. These options are generally not used today because better options are available.
  • Lard was very popular in the past.[2] It is used infrequently today, because of the wide variety of other options, but it is still 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. Lead is a health hazard, and excellent non-lead-containing options are available.
  • 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 CNS-depressing properties.

Safety concerns (toxicity, bacteria, fungi)

Cutting fluids have been associated with skin rashes, dermatitis, esophagitis, lung disease, and cancer. These problems result from either toxicity or bacterial or fungal contamination.

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.[3]

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.[4]

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.[4]

Environmental impact

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.

See also


  1. ^ OSHA (1999). Metalworking Fluids: Safety and Health Best Practices Manual. Salt Lake City: U.S. Department of Labor, Occupational Safety and Health Administration.
  2. ^ Hartness, James (1915). Hartness Flat Turret Lathe Manual: A Hand Book for Operators. Springfield, Vermont and London: Jones & Lamson Machine Company. pp. 153-155.  
  3. ^ NIOSH (2007). Health hazard evaluation and technical assistance report: HETA 005-0227-3049, Diamond Chain Company, Indianapolis, Indiana.
  4. ^ a b NIOSH (1998). Criteria for a recommended standard: occupational exposure to metalworking fluids. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health. DHHS (NIOSH) Pub. No. 98-102.


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