Choosing a metalworking fluid means matching four inputs, the operation, the workpiece material, the machine and the mix water, to one of the four fluid types OSHA names: straight oil, soluble oil, semi-synthetic or synthetic. Health rules, legal limits and cost strike out candidates; a trial confirms the last one.
The four types of metalworking fluids differ in oil content and dilution: straight oils run neat, soluble oils hold 30 to 85 % severely refined base oil in the concentrate, and synthetics hold no petroleum oil (NIOSH 98-116, 1998). The four key figures for the choice are listed below.
- Four fluid types: straight oils used neat; soluble oils, semi-synthetics and synthetics diluted with water (OSHA, NIOSH)
- Dilution by volume: about 2.4 to 16.7 % for soluble oils, about 2.4 to 9.1 % for semi-synthetics and synthetics (NIOSH 98-116, 1998)
- NIOSH REL for metalworking fluid aerosol: 0.4 mg/m³ thoracic (about 0.5 mg/m³ total), up to 10 h/day, 40-h week (NIOSH Criteria Document 98-102, 1998)
- Cost in use: concentrate price x concentration, per liter of working fluid (our worked example)
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.
How Do You Choose a Metalworking Fluid?#
Choose a metalworking fluid in six steps: name the operation, list the workpiece materials, check the machine, test the mix water, strike out excluded chemistries, then trial one shortlisted fluid type. The six steps are listed below.
- Name the operation and its severity: slow, high-load cutting or fast, heat-driven cutting
- List the workpiece materials and any staining limits for aluminum and yellow metals
- Check the machine: individual sumps or a central system, pump type, pressure and delivery method
- Test the mix water for hardness, chloride, sulfate and conductivity
- Strike out the chemistries your health, legal or customer requirements exclude
- Shortlist one fluid type, compare straight oil vs water-based cutting fluid or soluble oil vs semi-synthetic vs synthetic coolant, and trial it
Step 6 needs a trial because one supplier's technical bulletin states that product cross-references are not one-to-one replacements.
Which Fluid Types Are You Choosing Between?#
You are choosing between four fluid types that OSHA and NIOSH define by oil content and dilution: straight oils used neat, and soluble oils, semi-synthetics and synthetics diluted with water. Straight oils are not designed to be diluted with water (NIOSH 98-116, 1998).
Four fluid types and their ISO 6743-7 categories#
The four types split into neat fluids and water-mix fluids made from a concentrate, which ISO 6743-7 groups as MH and MA categories (as quoted). The table below compares the four types by form in use, base oil, dilution and category.
| Fluid type | Form in use | Base oil in the concentrate (NIOSH 98-116, 1998) | Dilution (NIOSH, by volume) | ISO 6743-7 categories (as quoted) | Source |
|---|---|---|---|---|---|
| Cutting oils (straight oils) | Neat, not diluted | Severely solvent-refined petroleum oils or animal, marine, vegetable or synthetic oils, with or without additives; base oil usually over 95 % of the product (EU-OSHA OSHwiki) | None | MHA to MHF (oils), MHG (greases, pastes, waxes), MHH (soaps, powders, solid lubricants) | NIOSH 98-116; OSHwiki; secondary quotations of ISO 6743-7 |
| Soluble oils | Milky emulsion | 30 to 85 % severely refined base oil plus emulsifiers | 1 part concentrate to 5 to 40 parts water, about 2.4 to 16.7 % | MAA to MAD | NIOSH 98-116; secondary quotations of ISO 6743-7 |
| Semi-synthetic metalworking fluids | Transparent to milky | 5 to 30 % severely refined base oil, 30 to 50 % water | 1:10 to 1:40, about 2.4 to 9.1 % | MAE, MAF | NIOSH 98-116; OSHA; secondary quotations of ISO 6743-7 |
| Synthetic cutting fluids | Transparent solution | No petroleum oil; water-soluble or water-dispersible | 10 to 40 parts water, about 2.4 to 9.1 % (calculated) | MAG, MAH | NIOSH 98-116; secondary quotations of ISO 6743-7 |
ISO 6743-7:1986 categories as quoted by secondary sources, not verbatim ISO text; last confirmed 2020, 2025 review result pending.
The classification behind the codes is ISO 6743-7:1986, Family M (Metalworking), last confirmed 2020; 2025 review result pending. In the United States, ASTM D2881-19(2025) classifies metalworking fluids and related materials. The categories and their codes are explained under ISO 6743-7.
What each fluid type contains, with treat rates#
Each fluid type is a base fluid plus an additive package, and additives run from under 1 % to 35 % of the formulation (STLE TLT article, September 2006). The table below compares the main components by fluid type, rate and basis.
| Component | Fluid types | Rate and basis | Source |
|---|---|---|---|
| Base oil share | Straight oils | Usually over 95 % of the product | EU-OSHA OSHwiki |
| Base oil share | Water-miscible concentrates | 20 to 80 % of the concentrate | EU-OSHA OSHwiki |
| Additives as a group | All types | Under 1 % to 35 % of the formulation | STLE TLT article, September 2006 |
| Emulsifier blend | Soluble oils | 1 to 50 wt % of the concentrate (preferred 5 to 20 wt %) | One patent, EP 1 819 804 B1 |
| Alkanolamines (monoethanolamine, triethanolamine, aminomethylpropanol) and boric acid | Water-mix fluids | Monoethanolamine and triethanolamine 1 to 5 %, aminomethylpropanol 1 to 10 %, boric acid 1 to 5 %, all in the concentrate | Review of 209 safety data sheets, Gerulova et al. (2017) |
| Triazole yellow-metal inhibitors (benzotriazole, tolyltriazole) | Water-mix fluids | 50 to 1,000 ppm at use dilution, commonly 250 to 500 ppm | One patent, US8722592B2 |
| Polyisobutylene anti-mist polymer | Straight oils | 20 to 100 ppm in use for 70 to 90 % mist reduction | Plant trials, SAE paper 950245 (1995) |
| Vegetable and ester-based cutting fluids | Neat or water-mix | Vegetable or synthetic oils are allowed bases of straight oils; a biodegradable base does not make used fluid drain-disposable | NIOSH 98-116 (1998); one supplier's bulletin (2025) |
Treat rates are in the concentrate unless the row says use dilution or in use; single-source values name their source.
The OSHA best practices manual (Appendix 2) lists oiliness agents, EP additives and antioxidants for straight oils, adds petroleum sulfonates, alkanolamines and biocides for soluble oils, EDTA for semi-synthetics, and EO-PO polymers and glycol ethers for synthetics.
Those lists give an outline only. What a cutting fluid is made of, class by class, is on metalworking fluid additives.
How Cooling, Lubrication and Corrosion Protection Decide the Choice#
Slow cutting needs lubrication and points to a straight oil, while fast cutting needs cooling and points to a water-mix fluid, the speed rule in one lubricant maker's cutting-fluids manual. OSHA lists four jobs for every fluid: cooling, lubrication, corrosion protection of part and machine tool, and chip and swarf removal.
The same manual rates cutting oil best for lubrication and corrosion protection, emulsion good on all counts, and synthetic fluid best for cooling and cleaning but weak on lubrication. OSHA credits synthetics with the best heat reduction.
Choosing by Operation: Severity and Cutting Speed#
The operation sets the first filter: OSHA places straight oils in work that needs lubrication rather than cooling, such as slow cutting speeds and high metal-to-metal contact, and water-mix fluids where heat removal leads. Fluid notes for each process are on cutting fluids by machining operation.
Low-speed, high-load work: tapping, threading, broaching and deep holes#
Neat oils suit grinding hard metals, deep-hole drilling, threading difficult materials, gear cutting and rotary table machines in one lubricant maker's manual, and slow, high-contact work in the OSHA manual. For drilling, that list names deep-hole drilling as neat-oil work, and OSHA notes that straight oils may require fire protection.
In form tapping of hardened steel, a less effective oil needed 53 % higher torque than a highly effective oil, as cited in a doctoral thesis by Demmerling (University of Duisburg-Essen). Cutting oils for tougher machining in steel, aluminum and titanium almost always contain sulfur, phosphorus or chlorine EP additives (same manual).
Tapping and threading load the fluid hardest at the lowest speed. Tap type, fluid form and torque are on tapping and threading fluids.
Grinding and high-speed machining#
Grinding needs heat removal, and OSHA rates synthetics best for heat reduction. A 2008 Cutting Tool Engineering article on M-50 steel puts 10 % of the heat into the fluid and 72 % into the workpiece with a conventional wheel, against 5 % and 45 % with a CBN wheel.
The same article gives about 10.2 L/min per kW (2 US gpm per hp) of grinding power for conventional abrasives. OSHA notes that high-pressure delivery may create mist, while low-pressure flood delivered to the cut reduces it.
Choosing by Workpiece Material#
The workpiece material sets the second filter, because harder-to-machine alloys need more lubrication and EP chemistry, and nonferrous metals limit the pH and sulfur a fluid may carry. Fluid choice for each metal is on cutting fluids by workpiece material.
Machinability order from cast iron to titanium#
One lubricant maker's manual ranks nine material groups from cast iron, the least demanding on the fluid, to titanium, the most demanding. The nine groups are listed below in that order.
- Cast iron
- Copper alloys
- Free-cutting steel
- Low-alloy steel
- High-alloy steel
- Aluminum alloys
- Stainless steel
- Nickel alloys
- Titanium
Steel sits in the middle of the order, and for tough steel jobs the same manual names cutting oils with EP additives.
Aluminum and copper alloys: pH and staining limits#
A pH above 9 protects ferrous metals but adversely affects aluminum, brass and bronze (a fluid maker's article, Production Machining, 15 May 2003), so in-use pH limits the choice for these alloys. EU-OSHA OSHwiki gives 8.5 to 9.5 for water-mixed fluids. Active-sulfur oils can discolor yellow metals (one lubricant maker's manual), while mercaptobenzothiazole, tolyltriazole and benzotriazole prevent staining of copper and brass (STLE TLT article, September 2006).
Choosing a cutting fluid for aluminum starts with those two limits, pH and sulfur. Alloy-safe chemistry and staining tests are on cutting fluid for aluminum.
Stainless steel, nickel alloys and titanium: heat and EP chemistry#
Stainless steel, nickel alloys and titanium need coolant, because titanium, heat-resistant superalloys and austenitic and duplex stainless steels fail in dry cutting (one tooling maker, Cutting Tool Engineering, 2016). Stainless steel with more than 12 % chromium work-hardens and generates a lot of heat, as presented by one tool maker.
For titanium, tool makers recommend high-pressure coolant at 50 to 300 bar (725 to 4,350 psi) according to Struzikiewicz (2024). Titanium alloy fines and turnings from machining are flammable: use a Class D dry powder extinguisher and do not spray water on burning particles (Titanium Industries safety data sheet for titanium alloys, revised 20 June 2019, section 5). One nickel alloy producer's bulletin asks for parts free from oil and shop soil before the furnace.
Cast iron and magnesium: dry, oil or water-mix#
Cast iron is traditionally machined dry and can be machined wet (one tool maker's guide), and its fine particles demand strong cleaning properties (one lubricant maker's manual). A fluid maker's article (2003) gives about 48 h of in-process rust protection for cast iron at proper concentration.
One machining supplier machines magnesium without coolant where possible and avoids water-based coolants because they react with chips in storage. Fire risk and fluid choice are on machining magnesium.
Choosing for the Machine: Sump, Pump, Pressure and Delivery#
The machine sets the third filter: one supplier's technical bulletin sizes flow at 3.8 to 5.7 L/min (1 to 1.5 US gpm) per horsepower at the cut and allows gear and vane pumps only with straight oils. The table below lists the machine checks from that bulletin and one trade article.
| Check | Value | Source |
|---|---|---|
| Flow, big-chip machining | 1 to 1.5 US gpm per hp at the cut (3.8 to 5.7 L/min per hp) | One supplier's technical bulletin |
| Flow, grinding | 1.5 to 2 US gpm per hp (5.7 to 7.6 L/min per hp) | One supplier's technical bulletin |
| Flow added for chip transport | 1.5 to 2 gpm/hp per 3 m (10 ft) for chips; 1 to 1.5 gpm/hp per 3 m (10 ft) for grinding swarf | One supplier's technical bulletin |
| Tank size | 3 to 10 times pump capacity, divided about 25 % and 75 % | One supplier's technical bulletin |
| Pump type | Gear and vane pumps only with straight oils | One supplier's technical bulletin |
| Delivery method | Dry, MQL, spray mist, flood, high pressure | One supplier's technical bulletin |
| Entrained air | Neat oil under high-pressure coolant can cavitate the pump; fast air release needed | Fluid maker quoted in a trade article |
High-pressure delivery changes the fluid question: a fluid maker quoted in a trade article finds semi-synthetics foam less than soluble oils, and synthetics least, at the price of lower lubricity.
Our request checklist records sump or central system, volume in L and US gal, filtration rating, coolant pressure in bar and psi, and paint and seal compatibility.
How Mix Water Narrows the Fluid Choice#
Mix water narrows the choice before any fluid is bought: the UKLA/HSE Good Practice Guide v2.5 (February 2025) gives 80 to 200 ppm hardness as CaCO3 as the ideal range. An STLE webinar (April 2022) sets pretreatment triggers at total hardness above 125 ppm, chloride above 50 ppm, sulfate above 100 ppm and conductivity above 500 µS/cm.
Water quality cuts both ways: soft, deionized or reverse-osmosis water foams more, according to a fluid maker quoted in a trade article. Hardness, chlorides and deionized water are on water quality for metalworking fluids.
OSHA describes synthetics as largely unaffected by hard water, and one supplier's 2025 brochure allows 100 to 200 ppm as CaCO3 initially, at most 600 ppm in a central system, for one product run at 6 to 8 %.
Concentration and Mixing for the Chosen Fluid#
Water-mix fluids run at 5 to 10 % concentrate by volume in 90 to 95 % water in an STLE webinar (April 2022). Concentrate goes into water, never water into concentrate, which can form an invert emulsion (UKLA/HSE Good Practice Guide v2.5, February 2025).
The same guide defines the check: concentration in % by volume equals the Brix reading multiplied by the product's own refractometer factor, zeroed on the mix water at about 20 °C (68 °F). Measuring and correcting it is covered under coolant concentration.
Cost in Use and Upkeep by Fluid Type#
Coolant costs more to run than to buy: one tooling maker estimates liquid coolant at 16 to 17 % of part cost against about 3 % for tooling (Cutting Tool Engineering, 2016). A 2024 PLOS One article cites an estimated 16 % of manufacturing cost for handling, maintenance and disposal of cutting fluid.
Quotes are compared on cost in use, never on concentrate price alone. Our 2026 worked example: concentrate at USD 10 per liter used at 5 % costs USD 0.50 per liter of working fluid, about USD 1.89 per US gallon (arithmetic, not a standard or a market price).
OSHA credits synthetics and semi-synthetics with longer sump life, and a UKLA/Pera disposal guide puts UK water-mix service life at 9 to 12 months.
Health and Regulatory Limits That Rule Out a Fluid#
No fluid choice changes the exposure limits: NIOSH recommends 0.4 mg/m³ thoracic (about 0.5 mg/m³ total) for metalworking fluid aerosol, TWA up to 10 hours a day in a 40-hour week (NIOSH Criteria Document 98-102, 1998). OSHA has no fluid-specific standard, only PELs for mineral oil mist (5 mg/m³, 8-hour TWA) and particulates not otherwise regulated (15 mg/m³, 8-hour TWA) in 29 CFR 1910.1000 Table Z-1. Values by body and fraction are on metalworking fluid exposure limits.
NIOSH 98-116 (1998) finds asthma evidence strongest for synthetic fluid aerosol and links straight oils to folliculitis, oil acne and keratoses; an occupational physician assesses individual symptoms.
The OSHA best practices manual advises avoiding PAH-containing oils, chlorinated paraffins, alkanolamines, nitrites and formaldehyde-release biocides; it is guidance, not a rule.
Short-chain chlorinated paraffins (EC 287-476-5) have been on the REACH Candidate List since 2008-10-28 and medium-chain chlorinated paraffins since 2021-07-08 (list no. 799-971-8). California lists "Soots, tars, and mineral oils (untreated and mildly treated oils and used engine oils)" for cancer since 1987-02-27; severely refined base oils are not that entry. EP chemistry by chain length is on chlorinated paraffins in metalworking fluids.
Changing Fluids: Cross-References, Trials and Changeover#
A replacement fluid is chosen by trial, not by cross-reference chart: one supplier's technical bulletin calls for SDS comparison, laboratory tests and a field test before a change. Our checklist sets the trial terms: machines, duration, tool life, finish, foam, odor, skin complaints and who pays for the changeover clean.
For neat oils, one lubricant maker's manual mixes new and old oil at 10/90, 50/50 and 90/10 as a miscibility test.
Grades and Sourcing: What to Put in a Fluid Request#
A fluid request names the operations, materials, machines, mix-water analysis, excluded chemistries, commercial terms and trial, the core headings of our request checklist, since no ISO or ASTM document defines one. Packaging options are drum 208 L (55 US gal), IBC 1,000 L (264 US gal) or bulk. The five chemistries a request may exclude are listed below.
- Chlorine
- Boron
- Secondary amines
- Formaldehyde releasers
- Nitrite
Suppliers answer comparable requests in comparable terms. How suppliers are compared is on metalworking fluid suppliers.
What Can Replace a Cutting Fluid?#
A cutting fluid is replaced by a different delivery method: dry machining, minimum quantity lubrication and cryogenic cooling remove or shrink the fluid, while pastes, waxes and solid lubricants are ISO 6743-7 categories (as quoted).
Machining without fluid: dry, MQL and cryogenic cooling#
Dry machining works for milling of aluminum cast alloys, steels and cast iron, turning of steels and cast iron, gear hobbing and drilling of free-cutting and quenched-and-tempered steel and cast iron (DGUV Information 209-024, November 2010, Table 3). It fails on titanium, superalloys and austenitic and duplex stainless steels (one tooling maker, 2016).
Minimum quantity lubrication uses about 5 to 50 mL/h (0.17 to 1.7 US fl oz/h) according to the IFA, and cryogenic cooling uses liquid nitrogen at -196 °C (-321 °F). Each method is compared under coolant delivery methods.
Pastes, waxes and solid lubricants#
ISO 6743-7 has categories for MHG greases, pastes and waxes and for MHH soaps, powders and solid lubricants, as quoted by secondary sources.
Cutting Fluid Selection FAQ#
Five common selection questions are answered below.
What is the best cutting fluid?#
No single fluid is best: the best fluid is the type that matches the operation, its speed and the workpiece material, within the limits of the machine and the mix water. One lubricant maker's manual puts cutting oil on slow work and emulsion on fast work.
What is the best coolant for CNC machines?#
A CNC coolant is a water-mix fluid at 5 to 10 % concentrate by volume (STLE webinar, April 2022) unless the work is slow and high-contact (OSHA) or the pump is a gear or vane type. Both cases call for a straight oil.
Can WD-40 be used as cutting oil?#
This reference has no tapping, drilling or sawing test data for that product, so it does not rate it. A substitute would need EP additives and proven lubricity; formulated neat oils are described under cutting oils.
Does motor oil work as cutting fluid?#
Used motor oil is not a substitute this reference recommends: California lists used engine oils under "Soots, tars, and mineral oils" for cancer since 1987-02-27, and the OSHA manual advises avoiding PAH-containing oils. Straight cutting oils use severely refined or named natural and synthetic oils (NIOSH 98-116, 1998).
What is the ratio of cutting oil and water?#
A soluble oil is mixed at 1 part concentrate to 5 to 40 parts water, about 2.4 to 16.7 % by volume (NIOSH 98-116, 1998), always adding concentrate to water. The product data sheet sets the target; straight oils are not designed to be diluted.