Minimum Quantity Lubrication (MQL): Oils, Systems, Consumption and Where It Works

Minimum quantity lubrication (MQL) sprays a neat lubricant as an air-oil aerosol at the cutting edge, on average no more than 50 mL (1.7 US fl oz) per process hour and tool, and none of it returns to a sump. The method is also called near-dry machining, and German guidance calls it Minimalmengenschmierung (MMS) or Minimalmengenkühlschmierung (MMKS).

Unlike water-mix metalworking fluids that return to a sump, MQL oils are neat metalworking fluids that are used up in the cut and leave the machine on chips and parts. The Institute for Occupational Safety and Health of the German Social Accident Insurance (IFA) puts MQL consumption at about 5 to 50 mL (0.17 to 1.7 US fl oz) per hour, and no standard defines the method. The four key figures of MQL are listed below.

• Consumption: on average no more than 50 mL (1.7 US fl oz) per process hour and tool (DGUV Information 209-024), about 5 to 50 mL/h in practice (IFA)
• Oils: synthetic esters for lubrication-dominated work, fatty alcohols for separation-dominated work (DGUV Information 209-024)
• Low-emission criteria: viscosity above 10 mm²/s at 40 °C (104 °F), flash point above 150 °C (302 °F), Noack loss below 65 %
• Fire limit: up to 1,000 mL per process hour per m³ of work space with monitored extraction (DGUV FB HM-088, 09/2017)

Two oil classes carry MQL in German practice: synthetic esters for drilling, tapping, reaming and turning, and fatty alcohols for separation-dominated work on non-ferrous metals.

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 Minimum Quantity Lubrication?#

Minimum quantity lubrication is a total-loss method that meters milliliters of lubricant per hour into compressed air and sprays the resulting aerosol at the tool, so the lubricant leaves the machine on chips and parts.

Coolant delivery methods differ in whether the fluid returns to a tank. MQL is one of five coolant delivery methods this reference groups, next to flood, high-pressure, dry and cryogenic machining, and the one whose lubricant does not return.

How much lubricant counts as MQL#

MQL has no standard definition; DGUV Information 209-024 (November 2010) sets it at an average of no more than 50 mL (1.7 US fl oz) of lubricant per process hour and tool. Single operations, for example tools over 40 mm (1.57 in) in diameter, may briefly exceed 150 mL/h (5.1 US fl oz/h) under DGUV Information 209-024, formerly BGI/GUV-I 718.

The German statutory accident insurance rule for metalworking fluids, DGUV Regel 109-003, uses the same 50 mL average but words the basis per process hour and machining point and calls it total-loss lubrication. Reduced-quantity cooling lubrication, at up to 2 L (0.53 US gal) per process hour, is not MQL in German guidance because that fluid is recirculated instead of lost. The five figures below show how each source states the MQL quantity and on which basis.

MQL has no standard definition; each figure applies only on its stated basis.

How MQL lubricates the cut#

MQL lubricates the cut with an aerosol of droplets about 0.5 to 2 µm in size that carries the oil to the edge as a thin film, and at settings below 10 mL/h parts and chips leave the machine dry. The lubricant is neat, so there is no water phase: microbes grow only in an aqueous phase, and MQL oils need no biocide (DGUV Information 209-024).

The lubricating film is the whole mechanism, because no fluid circulates to carry heat and chips away. DGUV Information 209-024 marks the boundary at grinding, which it calls not yet economical with MQL, and at honing and lapping, which it calls difficult.

Which Oils Are Used for MQL?#

MQL oils are neat lubricants from two main classes, synthetic esters and fatty alcohols, chosen to meet low-emission criteria for viscosity, flash point and evaporation loss. The table below compares the five lubricant classes by evaporation, lubricity, flash point, preferred work and their status in German MQL guidance.

Evaporation, lubricity and flash point compare esters and fatty alcohols at equal viscosity (DGUV Information 209-024, Table 5).

Synthetic esters#

At equal viscosity, synthetic esters evaporate slowly, lubricate well and flash high, and they suit lubrication-dominated operations with abrasive wear such as tapping, drilling, reaming and turning (DGUV Information 209-024, Table 5). The same chemistry appears in formulated cutting fluids, where synthetic esters are also the ester carriers among base oils for metalworking fluids.

One supplier's data sheet describes an ester-based MQL series with higher boiling and flash points at low viscosity and less vapor, rated low-emission under the DGUV criteria. The two ester examples in DGUV Information 209-024 used grades of 30 mm²/s and 40 to 50 mm²/s at 40 °C (104 °F) in aluminum series machining.

Fatty alcohols#

Fatty alcohols evaporate faster, lubricate less and flash lower than esters of equal viscosity, which suits separation-dominated work and built-up-edge control on non-ferrous metals (DGUV Information 209-024). The camshaft example in the same guidance step-drilled 16MnCr5 steel with a 10 to 20 mm²/s fatty alcohol, and one supplier's data sheet describes its fatty-alcohol MQL series as lower in flash point and quick to dry.

Fatty alcohols in MQL products are long-chain aliphatic alcohols whose chain length the products do not disclose; 1-dodecanol (lauryl alcohol) and oleyl alcohol are example members. Once ethoxylated, the same alcohols become the nonionic emulsifiers covered under fatty alcohol ethoxylates in water-mix fluids.

Both suitable MQL classes fall into German water hazard class nwg (not hazardous to water) or WGK 1 (DGUV Information 209-024, Table 5). The table below compares the identity and regulatory status of the two example members, neither of which has a CLP Annex VI entry.

Example members only: MQL products do not disclose the chain length of the fatty alcohol they contain.

Vegetable oils and other native esters#

Native esters such as rapeseed oil are unsuitable for MQL under DGUV Information 209-024 because they oxidize and resinify, although research and products still use vegetable oils in MQL. One turning study on hardened stainless steel at 48 HRC, published in Procedia CIRP, applied castor oil at 50 mL/h (1.7 US fl oz/h).

One supplier's data sheet lists a vegetable-derived MQL oil at 16 mm²/s (cSt) at 40 °C (104 °F), a viscosity above the 10 mm²/s criterion, while the oxidation concern of the German guidance remains. Oxidation and biodegradability are covered under vegetable and ester-based cutting fluids.

Lubricants unsuited to MQL#

German MQL guidance rules out the six lubricant groups listed below.

• Native esters such as rapeseed oil: they oxidize and resinify
• Water-miscible fluids and concentrates: their biocides would enter the aerosol
• Lubricants with organic chlorine or zinc additives
• Lubricants carrying hazard labels
• Mineral oils with more than 3 ppm benzo[a]pyrene
• Low-boiling liquids such as ethanol: fire, explosion and health risk (DGUV FB HM-088, 09/2017)

What Properties Must an MQL Lubricant Have?#

A low-emission MQL lubricant has a kinematic viscosity above 10 mm²/s at 40 °C (104 °F), an open-cup flash point above 150 °C (302 °F) and a Noack evaporation loss below 65 % at 250 °C (482 °F). These are the three criteria of DGUV Information 209-024, Table 6, each tied to a test method: DIN 51562-1 for viscosity, DIN EN ISO 2592 for flash point and DIN 51581-1 for evaporation loss. The table below lists each criterion with its requirement, test method and source.

Why viscosity decides mist and vapor#

Below 10 mm²/s, MQL lubricants produced strong mist in DGUV test-rig drilling, while above 20 mm²/s emissions were clearly lower. The test drilled with internal supply at 30,000 min⁻¹, and a 3 mm²/s sample emitted far more than the others, mostly as vapor (DGUV Information 209-024).

The best-practice range of 15 to 50 mm²/s keeps the oil above the strong-mist zone of that test, and the upper end depends on the system: one-channel systems handle up to 50 mm²/s and two-channel systems up to 100 mm²/s. How aerosol forms and is captured is covered under metalworking fluid mist.

How Much Lubricant Does MQL Consume?#

MQL consumes about 5 to 50 mL (0.17 to 1.7 US fl oz) of lubricant per hour, the range the IFA gives, and EU-OSHA's OSHwiki puts the order of magnitude at about 1 L (0.26 US gal) per 8-hour shift. Each figure holds only on its own basis: per tool, per machining point, per hour or per shift.

The IFA lexicon entry on MQL states the hourly range, and the DGUV definition adds the per-tool ceiling. The table below lists each consumption figure with its value, basis and source.

Dosing settings and the one-shift balance#

A system should allow settings below 10 mL/h (0.34 US fl oz/h) for dry parts and chips, while the one-channel maximum of about 200 mL/h and the two-channel range above 400 mL/h set the upper end.

Worked example: 50 mL/h x 8 h = 400 mL (13.5 US fl oz) of lubricant per tool and shift at the definition ceiling.

How Do MQL Systems Work?#

MQL systems fall into two classes: one-channel systems form the aerosol in the unit and send it through one line, while two-channel systems carry oil and air separately and mix them at or near the tool. Both consist of a dosing unit, lines and nozzles or tool channels, and both need mains air above 4 bar (58 psi). The table below compares the two system classes attribute by attribute.

Figures from DGUV Information 209-024, Tables 8 and 9; individual systems differ.

One-channel systems#

One-channel systems produce an aerosol of about 0.5 to 2 µm by venturi and reach about 30,000 min⁻¹, with flows up to 200 mL/h (6.8 US fl oz/h) and viscosities up to 50 mm²/s. DGUV Information 209-024 prefers tool channel bores of at least 0.5 mm (0.020 in) for them.

Two-channel systems#

Two-channel systems reach about 40,000 min⁻¹, respond in about 0.1 s and handle flows above 400 mL/h (13.5 US fl oz/h) and viscosities up to 100 mm²/s. Tool channels below 0.5 mm (0.020 in) are possible with them (DGUV Information 209-024).

External and internal supply#

External nozzles retrofit easily at low cost for sawing, drilling, milling and turning, but tool length and diameter, shadowing and spray loss limit them.

Internal supply carries the aerosol through spindle and tool to the edge, which suits deep holes and high speeds but needs special tools and costs more (DGUV Information 209-024).

Where Does MQL Work?#

MQL works especially well in cutting with a geometrically defined edge, such as drilling, reaming, tapping, milling, turning and sawing, on gray cast iron, non-ferrous metals and steels up to 800 N/mm² (about 116,000 psi). That scope is documented in DGUV Information 209-024.

Drilling, milling, turning and sawing with MQL#

Esters suit tapping, drilling, reaming and turning, fatty alcohols suit separation-dominated work, and external nozzles cover sawing, drilling, milling and turning. Fluid choice for twist drills and through-tool supply is covered under cutting fluid for drilling.

Deep holes need internal supply through spindle and tool, since external nozzles are limited by tool length and shadowing. The crankcase example in DGUV Information 209-024 deep-drilled 5,000 holes per tool in AlSi9Cu3 with a 40 to 50 mm²/s synthetic ester.

Cast iron, aluminum and steel workpieces#

MQL works especially well on gray cast iron, because graphite lubricates, on non-ferrous metals and aluminum up to 1 % Si and on steels up to 800 N/mm² (about 116,000 psi). DGUV Information 209-024 gives the cast-iron range as GG 25 to GGG 40 in the older German designations.

The series examples in the same guidance include the cast aluminum alloys AlSi9Cu3 and AlSi10Mg, both machined with synthetic esters. Staining and built-up edge are covered under cutting fluid for aluminum.

For steels, the guidance names free-cutting steel and C45 (CK 45) as standard cases and X90CrMoV18 as a difficult steel that is still possible.

Series production examples#

German series production reached tool lives from 2,400 holes to about 105,000 parts with MQL, as the three examples in DGUV Information 209-024, Table 7 show.

Examples from DGUV Information 209-024, Table 7; they show what was achieved, not what every shop will achieve.

Where Does MQL Fail? Grinding, Honing and Lapping#

MQL fails in grinding, where DGUV Information 209-024 calls it not yet economical, and is difficult in honing, lapping and other processes with geometrically undefined cutting edges. For grinding, the low-volume route in the same guidance is reduced-quantity cooling lubrication, where shoe nozzles cut the grinding fluid volume by up to 90 %.

That route recirculates up to 2 L (0.53 US gal) per process hour. Fluid choice for surface, cylindrical and creep-feed work is covered under grinding fluids.

External nozzles also fail where the tool is long or shadowed, and the thin MQL film may not protect finished parts against rust.

What Health and Fire Precautions Does MQL Need?#

MQL needs three precautions: a low-emission lubricant and extraction to keep the aerosol low, a dosing and extraction limit that keeps the enclosure below an explosive mixture, and a check that the thin film protects parts against rust.

Aerosol exposure and exposure limits#

MQL exposure measurements in the DGUV survey found formaldehyde only in traces, and lubricants below 10 mm²/s produced strong mist in the test-rig work. No biocide enters the MQL aerosol because the oils carry no water, and German guidance excludes water-miscible fluids from MQL for exactly that reason.

In the United States, NIOSH recommends an exposure limit for metalworking fluid aerosol of 0.4 mg/m³ thoracic particulate mass (about 0.5 mg/m³ total particulate mass) as a time-weighted average for up to 10 h/day in a 40-hour week (NIOSH Criteria Document 98-102, 1998). OSHA has no standard specific to metalworking fluids. Values by jurisdiction are listed under metalworking fluid exposure limits.

Individual health questions, such as skin or airway complaints at an MQL machine, go to an occupational physician.

Fire and explosion limits in the machine enclosure#

MQL runs safely at up to 1,000 mL of lubricant per process hour per cubic meter of machine work space when a low-emission lubricant is used and extraction is monitored (DGUV FB HM-088, 09/2017). Exact conversion gives 16.7 mL/min per m³, which the leaflet rounds to about 15. Extraction moves 400 to 600 m³/h per m³ of work space, holds a slight negative pressure and stops the machine if it fails.

Vapors of MQL oils ignite at 200 to 350 °C (392 to 662 °F), aerosols only above 750 °C (1,382 °F). An earlier threshold expected no hazardous explosive mixture up to 100 mL/(m³·h), while practice ran at about 250 mL/(m³·h); the DGUV FB HM-088 leaflet, which replaced a 02/2017 draft, sets the higher limit under the conditions above. Low-boiling liquids such as ethanol are strongly advised against.

Rust protection of parts and machine cleaning#

Check whether the thin MQL film protects parts against rust; where it does not, a separate rust preventive is applied (DGUV Information 209-024). Film types and protection times are covered under rust preventive oils.

Machine cleaning uses low-pressure devices at 3 to 7.5 bar (44 to 109 psi), the range in DGUV Information 209-024.

How to Specify an MQL Oil for Supplier Quotes#

An MQL oil request names the lubricant class, viscosity at 40 °C (104 °F), flash point, Noack loss, system type, operations and materials, because those values decide whether the oil is low-emission and runs in the system. Our checklist for that request has seven items, listed below.

• Lubricant class: synthetic ester or fatty alcohol
• Viscosity at 40 °C (104 °F) in mm²/s, within the system's limit (up to 50 mm²/s one-channel, up to 100 mm²/s two-channel)
• Flash point (open cup) and Noack evaporation loss with their test methods
• System type: one-channel or two-channel, external or internal supply
• Operations and workpiece materials
• Excluded chemistries: organic chlorine and zinc additives, hazard-labeled lubricants
• Annual volume and packaging, quoted per liter of lubricant

Supplier data sheets carry viscosity, flash point and Noack loss with their test methods, so two offers are compared value by value on the data sheet.

How Does MQL Relate to Other Coolant Delivery Methods?#

MQL sits between cutting without fluid and recirculating coolant: it uses milliliters per hour as a total-loss lubricant, while reduced-quantity cooling lubrication uses up to 2 L (0.53 US gal) per process hour and returns to a tank. Consumption, tool life, chip evacuation and cost against flood are weighed under MQL vs flood coolant.

High-pressure coolant and cryogenic cooling complete the six delivery methods this reference groups, and cryogenic CO2 has a documented combination with MQL.

Can MQL Be Combined With CO2 Cooling?#

Yes: in milling of Inconel 718, Pereira and co-workers (Journal of Materials Research and Technology, 2020) combined internal CO2 cooling with external MQL and reported 120 % longer tool life than MQL alone. The result comes from one study on one nickel alloy.

Liquid nitrogen and CO2 are covered under cryogenic machining.

Is MQL a Dry Process?#

Under the DGUV definition, yes: chips count as dry below 2 mass % adhering lubricant, and up to 80 mL/h still qualifies, so the dry-machining term includes MQL.

Minimum Quantity Lubrication FAQ#

These five answers cover reader questions on the MQL process and its oils.

What does MQL stand for in machining?#

MQL stands for minimum quantity lubrication, a total-loss method using on average no more than 50 mL (1.7 US fl oz) of lubricant per process hour and tool. German guidance calls it Minimalmengenschmierung (MMS).

What is the MQL machining process?#

The MQL process meters oil into compressed air, in one aerosol line or in separate oil and air channels, and sprays the aerosol at the edge from outside or through the tool. One-channel systems reach about 30,000 min⁻¹ and two-channel systems about 40,000 min⁻¹.

Is MQL the same as near-dry machining?#

Yes, near-dry machining is another name for MQL, and under the DGUV definition chips with less than 2 mass % adhering lubricant still count as dry. Tool life and costs without any fluid are covered under dry machining.

Do MQL oils need a biocide?#

No: microbes grow only in an aqueous phase, and neat MQL oils have none. That is also why German guidance keeps water-miscible fluids, whose biocides would reach the aerosol, out of MQL.

Can water-mix coolant run through an MQL system?#

German MQL guidance rules out water-miscible fluids and concentrates because their biocides would enter the aerosol. An MQL system runs neat esters or fatty alcohols that meet the low-emission criteria.