Cutting Fluids (Metalworking Fluids): Types, Composition, Uses and How to Choose

Cutting fluids (metalworking fluids) are neat oils or water-mixed fluids applied at the cut to cool, lubricate, carry chips away and protect metal against rust. A water-mixed fluid runs at 5 to 10 % concentrate in 90 to 95 % water, according to an STLE webinar (April 2022). EU-OSHA OSHwiki widens the in-use range to 2 to 25 % by volume of concentrate in water.

The same product goes by machining oil, CNC coolant, cutting coolant, machine coolant and metalworking oil on shop floors, and HSE guidance in the UK adds suds, coolants, slurry and soap. Every one of these names points to the same family: a fluid between tool and workpiece that removes heat, reduces friction and leaves the part free of corrosion.

NIOSH Publication 98-116 (1998) splits that family into four types by the oil share of the concentrate, from straight oils used as supplied to synthetics with no petroleum oil, and this reference adds vegetable and ester-based fluids as a fifth row. A water-miscible concentrate holds 20 to 80 % base oil before dilution (EU-OSHA OSHwiki), the rest being water, emulsifiers and additives. ISO 6743-7:1986 codes the types as 8 neat and 9 water-mixed categories, the operation and the workpiece material decide which type fits, and weekly checks keep a sump usable for the 9 to 12 months a UK guide reports. For the aerosol, NIOSH recommends 0.4 mg/m³ thoracic particulate mass, while OSHA enforces no limit specific to metalworking fluids.

Standards, regulations and exposure limits are checked against their issuing bodies, product values against the named data sheet, and every figure carries its source and date; see our methodology and fact-checking.

What Is Cutting Fluid?#

Cutting fluid is a neat oil or a water-based fluid used while metal is machined or shaped, to lubricate and cool the tool and the workpiece (HSE, "About metalworking fluids"). That definition also answers what coolant for machining is: the diluted, water-based branch of the same family, pumped over the cut at a set concentration. OSHA's best-practices manual describes metalworking fluids as complex mixtures of oils, detergents, surfactants, biocides, lubricants, anti-corrosive agents and other potentially toxic ingredients, which is why one product name can hide very different chemistry. A straight oil and a synthetic solution both qualify as cutting fluid, yet the first contains no water and the second contains no petroleum oil.

NIOSH Publication 98-116 (1998) names four types in a fixed order. Straight oils are severely solvent-refined petroleum oils or animal, marine, vegetable or synthetic oils, used singly or combined, with or without additives, and not designed to be diluted with water. Soluble oils hold 30 to 85 % severely refined base oil in the concentrate plus emulsifiers. Semi-synthetics hold 5 to 30 % severely refined base oil and 30 to 50 % water in the concentrate. Synthetics hold no petroleum oil and dissolve or disperse in water. This reference groups vegetable and ester-based fluids as a fifth row by base fluid, because NIOSH counts vegetable and synthetic oils among straight-oil bases rather than as a type of their own. The same five-row order (straight, soluble, semi-synthetic, synthetic, bio-based) runs through every table, list and image on this page.

Coolant, Lubricant, Cutting Oil or Suds: What the Names Mean#

Coolant, cutting oil and lubricant name the same product family from different sides: coolant stresses heat removal, lubricant stresses friction at the cut, and cutting oil stresses the neat-oil form. HSE lists the UK shop names as suds, coolants, slurry or soap, and all four describe water-mixed fluids by their milky or soapy look. Cutting oil in trade use means a straight (neat) oil, applied as supplied. CNC coolant, machine coolant and industrial coolant name the diluted fluid in a machine-tool sump, whatever its type.

What is the difference between coolant and lubricant? The difference is one of emphasis, not of product. OSHA describes straight oils as best where a process needs lubrication rather than cooling, while synthetics give the best heat reduction of the four types. A soluble oil or a semi-synthetic sits between the two, carrying enough oil to lubricate and enough water to cool. A fluid sold as coolant therefore still lubricates, and a fluid sold as cutting oil still carries some heat away from the tool.

Machining, Forming and Protection: Where Metalworking Fluids Are Used#

This reference covers metalworking fluids in three jobs: cutting and grinding, forming, and short-term rust protection of machined parts. Cutting and grinding cover turning, milling, drilling, tapping, sawing, honing and grinding, where the fluid meets a chip and a hot tool edge. Forming covers stamping, drawing, wire drawing and rolling, where the metal is shaped rather than cut and the fluid is called a forming lubricant. Short-term protection covers temporary rust preventives that keep machined parts free of rust between operations or in storage. German usage draws the same border: DIN 51385 uses one umbrella term, Bearbeitungsmedien, for cutting fluids plus forming lubricants, and uses Kühlschmierstoffe for cutting fluids only. The three jobs share base oils, emulsifiers and additive chemistry, and OECD ESD No. 28 (2011) gives default in-use dilutions of 5 % concentrate in water for machining, 3 % for grinding and 10 % for pressure-impact forming.

What Do Cutting Fluids Do? The Four Functions at the Cut#

Cutting fluids do four jobs at the cut: they cool the tool and workpiece, lubricate the contact zone, flush chips and swarf away, and protect the part and the machine tool from corrosion (OSHA Metalworking Fluids Best Practices Manual). The four functions of a metal cutting fluid are listed below in the order this page uses.

• Cooling of tool and workpiece, which removes the heat that softens a cutting edge.
• Lubrication of the contact zone, which lowers friction between tool, chip and workpiece.
• Removal of chips and swarf, which keeps the cut and the machine clear.
• Corrosion protection of the part and the machine tool, during and after the cut.

The balance of the four functions shifts with the fluid type. OSHA names slow cutting speeds and high metal-to-metal contact as the best use of straight oils, which lubricate well but cool less, and it rates synthetics as giving the best heat reduction and excellent rust control. Semi-synthetics, in OSHA's description, give good lubrication, heat reduction and rust control together. Which function matters most depends on the cut: cutting fluids in metal cutting at high speed carry heat, while slow, heavily loaded cuts need film strength.

The four functions hold only at the right concentration. OSHA lists decreased tool life, bacteria or fungus problems and possible corrosion as effects of a concentration that runs too low, and dermatitis, foaming and heavy residues as effects of a concentration that runs too high; one supplier's bulletin adds excess fluid use to the high side. A cutting fluid that drifts from its range therefore loses one function before the operator notices the others.

What Is Inside a Cutting Fluid Concentrate? Base Fluid, Water and Additives#

A cutting fluid concentrate is a base fluid (mineral oil, synthetic lubricant or water) plus additives: most straight oils are over 95 % base oil, while a water-miscible concentrate holds 20 to 80 % base oil (EU-OSHA OSHwiki). NIOSH Publication 98-116 (1998) narrows the base-oil share by type: 30 to 85 % severely refined base oil in a soluble-oil concentrate, 5 to 30 % in a semi-synthetic concentrate and none in a synthetic. The base fluid is the carrier, and water is added at the machine, so the concentrate in the drum is only a fraction of what flows over the cut. A semi-synthetic, for example, already holds 30 to 50 % water before dilution.

Additives turn the carrier into a working fluid. STLE (TLT, September 2006) defines an additive as a chemical component or blend used at a specific treat rate to give one or more functions, at levels from below 1 % to about 35 % of the formulation per component. STLE's 2006 list names boundary lubricity additives, EP additives, corrosion inhibitors, reserve alkalinity boosters, metal deactivators, emulsifiers, couplers, chelating agents, antimist additives, antimicrobial pesticides, antifoamers and defoamers, and dyes. OECD ESD No. 28 (2011) lists biocides at below 2 wt % of soluble-oil, semi-synthetic and synthetic concentrates. This reference groups the components into 11 classes, and the guide to metalworking fluid additives lists every additive class with its treat rate and basis. In short, a cutting fluid is made of a base oil or water-miscible concentrate plus additives that lubricate, protect and control the fluid.

Dated EU rules reshape what goes into a concentrate. The formaldehyde releasers MBM (N,N'-methylenebismorpholine, index 607-721-00-5), MBO (3,3'-methylenebis(5-methyloxazolidine), 612-290-00-1) and HPT (612-291-00-7) are classified Carc. 1B H350 and Muta. 2 H341 in CLP Annex VI by ATP10 (Reg. (EU) 2017/776) since 2018-12-01, while the triazine HHT (613-114-00-6) carries Acute Tox. 4 H302 and Skin Sens. 1 H317 only.

Short-chain chlorinated paraffins (SCCP) are prohibited under EU POPs Regulation (EU) 2019/1021, Annex I Part A. Commission Delegated Regulation C(2026) 6262 on medium-chain chlorinated paraffins (MCCP), adopted by the Commission on 11 Sep 2026 and scheduled to apply from 16 Dec 2026, provides derogation 3(a) for EP use in heavy-duty metalworking fluids with collection systems until 2036-12-31.

The 4 Types of Cutting Fluids, Plus Bio-Based Fluids#

NIOSH Publication 98-116 (1998) defines four types by oil content: straight oils, soluble oils with 30 to 85 % severely refined base oil in the concentrate, semi-synthetics with 5 to 30 % and synthetics with no petroleum oil. This reference adds vegetable and ester-based fluids as a fifth row, grouped by base fluid rather than by oil share. A buyer asking for cutting oil wants a straight (neat) oil, used as supplied. A buyer asking for water soluble coolant wants a soluble oil, which forms an emulsion of oil in water and turns milky white in use. For the fifth row, the EU Ecolabel for lubricants (Decision (EU) 2018/1702) sets a biodegradability rule for Accidental Loss Lubricants: more than 90 % w/w readily biodegradable, at most 10 % inherently biodegradable, plus at least 25 % bio-based carbon (EN 16807) for a bio-based claim.

The table reads left to right: what each type is, how much base oil the concentrate holds, how far it is diluted, which ISO 6743-7 codes fluid makers quote for it, which additives OSHA's best-practices manual lists, and where the type is strong or weak. Each type has its own page in this independent guide to metalworking fluids and their additives, and the five type pages of this reference on metalworking fluids and their additives are linked from the rows of the table below.

The five cutting fluid types compared by base-oil share, dilution, ISO codes, additives and trade-offs.

Oil shares are % of the undiluted concentrate (NIOSH Publication 98-116, 1998; straight-oil share from EU-OSHA OSHwiki); dilutions are parts water per part concentrate and % by volume of concentrate in the diluted fluid; ISO 6743-7 codes as quoted by fluid makers, not verbatim ISO text; additives from the OSHA Best Practices Manual, Appendix 2. Row 5 is this reference's grouping and carries no NIOSH oil share.

Water-Mixed or Neat: The Two Main Groups of Cutting Fluid#

Cutting fluids fall into two groups: neat oils, used as supplied, and water-miscible fluids, diluted to 2 to 25 % by volume of concentrate before use (EU-OSHA OSHwiki). The two main types of coolant a shop meets are therefore a straight oil in the sump or a water-mixed fluid in the sump, never both in one machine.

DIN 51385 draws the same line in its terms: non-water-miscible fluids are used as supplied, water-miscible concentrates are diluted before use, and water-mixed fluids are those dilutions at use concentration. Water dominates the second group. STLE (webinar, April 2022) puts the diluted fluid at 90 to 95 % water and 5 to 10 % concentrate, so a 1,000 L (264 US gal) sump at 5 % holds 50 L (13.2 US gal) of concentrate.

Oil still wins where lubrication outweighs cooling. OSHA names slow cutting speeds, high metal-to-metal contact and older machines built for straight oil as the best use of straight oils, and it notes that they may require fire protection. The comparison of straight oil vs water-based cutting fluid weighs both groups criterion by criterion.

Emulsion, Microemulsion or Synthetic Solution?#

A water-mixed cutting fluid is an emulsion, a microemulsion or a solution, and the oil share of the concentrate decides which: 30 to 85 %, 5 to 30 % or none (NIOSH Publication 98-116, 1998). A soluble oil forms an emulsion, milky white with no free oil layer, and OSHA notes it can show poor corrosion control, dirty residues, smoke, poor mix stability or short sump life.

A semi-synthetic forms a microemulsion, transparent to milky, with 5 % or less oil once mixed, and OSHA rates it cleaner and longer-lived in the sump than a soluble oil. A synthetic forms a true solution: transparent, the cleanest of the four types in OSHA's description, with the best heat reduction, excellent rust control and largely unaffected by hard water.

The trade-off runs along the oil share. More oil gives more lubricity and less cooling, less oil gives more cooling, cleaner machines and longer sump life. The three-way comparison of soluble oil vs semi-synthetic vs synthetic coolant covers lubricity, life and cost.

How Are Metalworking Fluids Classified?#

ISO 6743-7 (Family M, metalworking) sorts metalworking fluids into 17 categories, as quoted by fluid makers: 8 neat-oil codes from MHA to MHH and 9 water-mixed codes from MAA to MAI. The standard is ISO 6743-7:1986, last confirmed 2020; 2025 review result pending. Its codes map onto the five rows of the type table, and three codes (MHG, MHH and MAI) cover forming products rather than cutting fluids.

The table below maps each ISO 6743-7 category, as quoted, to the fluid type on this page.

ISO 6743-7:1986, last confirmed 2020; 2025 review result pending. Category wording as quoted by fluid makers; the ISO text is not reproduced.

Two US standards and one German standard sit beside the ISO scheme. ASTM D2881-19(2025), active and last updated on 18 Nov 2025 by subcommittee D02.L0.01, covers terminology, nomenclature and classification of metalworking fluids and related materials, including removal and forming fluids. ASTM E2523-23 (November 2023, subcommittee E34.50) gives terminology for metalworking fluids and operations, fluid types included. DIN 51385 defines the German terms for non-water-miscible, water-miscible and water-mixed fluids.

A category code appears on a product data sheet as a short declaration: one maker's data sheet for a neat cutting oil states "ISO 6743-7: L-MHE", MHE being one of the neat-oil categories. Reading that code against the table tells a buyer which group and which performance additions the maker claims. The page on the ISO classification of metalworking fluids lists every category code.

Which Fluid Type Fits Which Job?#

The right cutting fluid type follows from four inputs, the operation, the workpiece material, the machine and the mix water, and OSHA names slow, heavily loaded cuts as the best use of straight oils. Water-mixed fluids take over where heat removal matters more than film strength. ASTM E1497-23, "Selection and Safe Use of Water-Miscible and Straight Oil Metal Removal Fluids" (18 Apr 2023), is the US practice that covers this choice for both groups. The machine sets limits of its own: OSHA lists older machines built for straight oil among the cases where a neat oil stays, and a neat oil may require fire protection.

One supplier's cutting fluids manual states the speed rule in two lines: a slow operation needs lubrication and therefore a cutting oil, a fast operation needs cooling and therefore an emulsion. The same manual ranks workpiece materials from easy to severe to machine: cast iron, copper alloys, free-cutting steel, low-alloy steel, high-alloy steel, aluminum alloys, stainless steel, nickel alloys and titanium. A severe material on that list asks more lubricity from the fluid than an easy one. Mix water is the fourth input, since it makes up 90 to 95 % of a diluted fluid (STLE, April 2022): the UKLA/HSE Good Practice Guide v2.5 (February 2025) names 80 to 200 ppm as CaCO3 as the hardness range, formulation-dependent, and notes that soft water foams.

The operation also sets the starting dilution: OECD ESD No. 28 (2011) uses default in-use dilutions of 3 % concentrate in water for grinding, 5 % for machining and 10 % for pressure-impact forming. Workpiece materials such as aluminum, cast iron, stainless steel and titanium, and delivery methods such as flood, high-pressure coolant and minimum quantity lubrication, each have their own page in this reference. The guide on how to choose a metalworking fluid walks through operation, material, machine and water.

Keeping a Cutting Fluid in Specification#

A water-mixed cutting fluid is kept in specification by checking its concentration, pH, microbial count, tramp oil and temperature at least weekly against the supplier's range, as the HSE MW5 sheet (03/21) sets out for UK shops. OSHA's best-practices manual names a sump 30 % below its full mark as the signal to check concentration. Concentration drifts as water evaporates and fluid leaves on chips, pH falls as bacteria grow, and leaking machine oils float up as tramp oil. HSE MW5 (03/21) sets out the checks listed below.

• Concentration by refractometer, weekly.
• pH, weekly.
• Dip slide for bacteria and fungi, weekly.
• Tramp oil, weekly.
• Sump temperature, weekly.
• Appearance and odor, daily.

A refractometer gives a °Brix reading, and concentration equals that reading times the product's own refractometer factor, zeroed on the mix water at about 20 °C (68 °F), per the UKLA/HSE Good Practice Guide v2.5 (February 2025); no single factor fits every product. The same guide adds the concentrate to the water, never water into concentrate, which can form an inverse emulsion (cream). EU-OSHA OSHwiki gives 8.5 to 9.5 as the pH of water-mixed fluids, against about 5.5 for the skin surface. HSE grades dip slides below 10,000 CFU/mL as good control, 10,000 to below 1,000,000 CFU/mL as reasonable (review and adjust) and 1,000,000 CFU/mL or more as poor, needing immediate action, normally drain and clean.

Tramp oil stays below 2 % by volume, checked at least weekly (HSE MW5, 03/21; UKLA/HSE Good Practice Guide v2.5). Sump temperature stays at or below 30 °C (86 °F), since above it microbial growth is optimal and evaporation raises concentration (UKLA/HSE Good Practice Guide v2.5, Table 3). Held in that window, a water-mix fluid lasts 9 to 12 months in UK practice, according to the UKLA and Pera disposal guide, which puts UK spent water-mix fluid at about 400,000 t per year.

Each check hands over to a routine of its own: top-ups, tramp-oil removal, filtration, sump cleaning and recharge, recycling and disposal. The coolant management hub explains how to keep metalworking fluids in specification and extend sump life.

Are Cutting Fluids Hazardous? Health Effects and Exposure Limits#

Metalworking fluids can cause dermatitis, occupational asthma, hypersensitivity pneumonitis, bronchitis and upper airway irritation, the conditions HSE lists for skin contact with the fluid and breathing its mist ("About metalworking fluids", updated 4 December 2024). Skin contact with the fluid and breathing the mist are the two routes. NIOSH Publication 98-116 (1998) links straight oils with folliculitis, oil acne and keratoses, and finds the asthma evidence strongest for synthetic metalworking fluid aerosol. NIOSH estimates 1.2 million US workers potentially exposed, a figure on its topic page last reviewed on 16 August 2013.

The table below sets out the exposure limits and recommendations for metalworking fluid aerosol by jurisdiction.

The limits differ in kind, not only in value. The NIOSH figure of 0.4 mg/m³ is a recommendation based on the thoracic fraction, while the OSHA values of 5 mg/m³ and 15 mg/m³ are enforceable but written for mineral oil mist and nuisance particulates, not for metalworking fluids. HSE backs its approach with COSHH essentials sheets for machining shops, among them MW1 for CNC machining, MW2 for control of skin risks and MW5 for managing fluid quality, plus G402 and G403 for health surveillance for asthma and dermatitis (page updated 15 January 2026). The 1999 advisory committee proposed the NIOSH-level value for OSHA, and OSHA did not adopt it. In the UK, HSE sets no workplace exposure limit for the fluids and instead expects health surveillance, carried out by a responsible person, wherever workers are exposed to fluid or mist.

Individual health questions go to an occupational physician, and questions about legal duties go to the competent authority. The hub on metalworking fluid health hazards covers skin, lungs, mist and exposure control.

Related Fluids and Terms Around Cutting Fluids#

Three neighbors of the four cutting-fluid types share their chemistry or their name: bioconcept fluids, forming lubricants and rust preventives. Each one uses the same base oils, emulsifiers or additives as a cutting fluid but answers a different job or a different upkeep model.

Bioconcept Fluids#

A bioconcept fluid is a water-mixed metalworking fluid claimed to be colonized by non-pathogenic waterborne bacteria that keep harmful bacteria out, without biocide additions (HSE, page updated 23 December 2024). HSE notes that such fluids need close control of pH and concentration, that they are not made in the UK, and that a bioconcept fluid is not the same as a biostable or bioresistant fluid. The dip-slide bands and weekly checks above still apply to the sump.

Forming Lubricants, Rust Preventives and Fluids This Reference Leaves Out#

Forming lubricants are metalworking fluids for stamping, drawing, wire drawing and rolling, where the metal is shaped rather than cut. Fluid makers quote three ISO 6743-7 codes for them: MHG for greases, pastes and waxes, MHH for soaps, powders and solid lubricants, and MAI for greases and pastes applied mixed with water. The hub on metal forming lubricants covers stamping, drawing, wire drawing, rolling and vanishing oils.

Temporary rust preventives protect machined parts between operations and in storage, and they belong to this reference as the third job of metalworking fluids. Quenchants for heat treatment share some base oils but are not covered here.

FAQ#

What is MWF?#

MWF is the abbreviation for metalworking fluid, the neat oil or water-mixed fluid that cools and lubricates metal during machining and shaping (HSE). HSE adds the shop names suds, coolants, slurry and soap. The abbreviation covers straight oils, soluble oils, semi-synthetics and synthetics alike.

How much water is in a water-mixed cutting fluid?#

A water-mixed cutting fluid is 90 to 95 % water and 5 to 10 % concentrate, according to an STLE webinar (April 2022). A 5 % mix is 1 part concentrate in 20 parts of mixture, or 19 parts water, and a 10 % mix is 1 part in 10, or 9 parts water. OECD ESD No. 28 (2011) uses 5 % for machining as its default.

Does every water-mixed cutting fluid contain a biocide?#

Not every one: OECD ESD No. 28 (2011) lists biocides at below 2 wt % of soluble-oil, semi-synthetic and synthetic concentrates, while a bioconcept fluid runs without biocide additions (HSE). A bioconcept fluid relies on its bacterial population and on close pH and concentration control instead.

How many people work with metalworking fluids?#

NIOSH estimates 1.2 million US workers potentially exposed to metalworking fluids, a figure on its topic page last reviewed on 16 August 2013. The estimate is not a current-year count. It covers machining, grinding and other operations where workers touch the fluid or breathe its mist.