Coolant delivery methods are the ways cutting fluid, a lubricant aerosol or a cryogen reaches the tool and workpiece, and this reference groups them into 6 methods, from a flood to no liquid at all. How much fluid does each method put on the cut? Among cnc coolant delivery methods, MQL meters at most 50 mL (1.7 US fl oz) of lubricant per process hour (DGUV Information 209-024, November 2010).
The method decides what the cutting fluids (metalworking fluids) carried to the cut must do: resist foam under a jet, or flash high as MQL oil.
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 a Coolant Delivery Method?#
A coolant delivery method, also called a coolant application method, is the way a machine brings cutting fluid, a lubricant aerosol, cold gas or nothing to the cut, defined by medium, quantity per hour and pressure. DGUV Information 209-024 (November 2010), German occupational safety guidance, gives practice definitions for MQL, reduced-quantity cooling and dry machining.
On that scale, this independent reference on metalworking fluids and their additives groups delivery by the quantity that reaches the cut.
What separates one delivery method from another?#
Three attributes separate coolant delivery methods: the medium, the quantity and the pressure at the cut. The three attributes are listed below.
- Medium: water-miscible fluid, neat oil, lubricant aerosol, cryogen or air.
- Quantity: liters per minute in a flood, milliliters per hour in MQL.
- Pressure: around 20 bar (290 psi) for flood up to 350 bar (5,000 psi) for ultra-high-pressure jets (one tool maker).
MQL is total-loss; reduced-quantity cooling returns its fluid (DGUV Information 209-024).
The 6 Coolant Delivery Methods in 3 Groups#
This reference groups coolant delivery methods into 6 methods in 3 groups: recirculated liquid at the cut, small quantities of lubricant, and no liquid at the cut.
1. Recirculated liquid at the cut: flood and high-pressure jets (2 methods)#
Recirculated liquid at the cut covers 2 methods: flood coolant, pumped at around 20 bar (290 psi) according to one tool maker, and high-pressure jets above 20.7 bar (300 psi) in one equipment maker's definition.
Flood coolant is a continuous stream pumped from the sump and back. One supplier's technical bulletin sizes it at about 5.1 to 7.6 L/min per kW (1 to 1.5 US gpm per hp).
Typical high-pressure coolant runs at about 69 bar (1,000 psi) (two independent sources). In Ti-6Al-4V turning, Palanisamy, McDonald and Dargusch (2009) measured almost 3 times the tool life of conventional cooling, as cited by Struzikiewicz (2024).
2. Small quantities of lubricant: spray mist and MQL (2 methods)#
Small quantities of lubricant cover 2 methods: spray mist, which sprays fluid droplets onto the cut, and minimum quantity lubrication (MQL), which meters at most 50 mL per process hour and tool on average (DGUV Information 209-024, 2010).
One supplier's technical bulletin lists spray mist separately from MQL; a cnc mist coolant setup delivers droplets, not a stream. For mist coolant for aluminum parts, DGUV Information 209-024 gives MQL data: MQL works on aluminum alloys up to 1 % silicon.
IFA, the German social accident insurance's safety institute, puts MQL consumption at about 5 to 50 mL/h (0.17 to 1.7 US fl oz/h).
3. No liquid at the cut: dry and cryogenic cooling (2 methods)#
No liquid at the cut covers 2 methods: dry machining, which DGUV Information 209-024 counts as dry while less than 2 mass % of lubricant adheres to the chip, and cryogenic cooling.
Up to 80 mL/h may still be used under that definition, so the term includes MQL. One tool maker recommends air cooling for light high-speed cuts at 45 HRC and above.
Liquid nitrogen cools at -196 °C (-321 °F); carbon dioxide expands to snow and gas at -78.5 °C (-109.3 °F). In Ti-6Al-4V end milling, Shokrani and co-workers (Machining Science and Technology, 2016) measured almost 3 times the tool life of flood cooling with liquid nitrogen.
Complete Table of Coolant Delivery Methods#
The table lists all 6 coolant delivery methods with the medium at the cut, pressure or quantity with source, fit, main limit and fluid need.
| # | Method | Medium at the cut | Pressure or quantity (source) | Where it fits | Main limit | Fluid need |
|---|---|---|---|---|---|---|
| 1 | Flood coolant (this page, group 1) | Recirculated water-miscible fluid or neat oil from the sump | Around 20 bar (290 psi), one tool maker; flow 5.1 to 7.6 L/min per kW (1 to 1.5 US gpm per hp) at the cut for big-chip machining, one supplier's technical bulletin | Traditional low-pressure delivery (one tool maker); the baseline for MQL emission tests (DGUV Information 209-024) | Sump and fluid upkeep; tank 3 to 10 times pump capacity per minute, one supplier's technical bulletin | Low foam at the working pressure; see the fluid section below |
| 2 | High-pressure coolant | Recirculated fluid as a jet, also through the tool | Above 20.7 bar (300 psi), one equipment maker; about 69 bar (1,000 psi) in general machining (two sources); ultra-high 350 bar (5,000 psi), one tool maker | Titanium: 50 to 300 bar (725 to 4,350 psi) recommended by tool makers (Struzikiewicz, 2024); chip control | Mist concentration about 200 % higher at 55 bar (800 psi) through the tool (peer-reviewed study) | Low-foam fluid (one supplier's technical bulletin; trade-journal report); neat oil with fast air release (one fluid maker) |
| 3 | Spray mist (this page, group 2) | Fluid droplets sprayed onto the cut | No figure given on this page | Named as a separate application method in one supplier's technical bulletin | Aerosol in the breathing zone (see the exposure section) | Water-miscible fluids are not MQL lubricants (DGUV Information 209-024) |
| 4 | Minimum quantity lubrication (MQL) | Lubricant aerosol, total loss | At most 50 mL per process hour and tool on average (DGUV Information 209-024, 2010); about 5 to 50 mL/h (0.17 to 1.7 US fl oz/h) in practice (IFA) | Gray cast iron, aluminum up to 1 % Si, steels up to 800 N/mm² (DGUV Information 209-024) | Grinding not yet economical; honing and lapping difficult (DGUV Information 209-024) | Viscosity above 10 mm²/s at 40 °C, flash point above 150 °C (302 °F), Noack loss below 65 % at 250 °C |
| 5 | Dry machining | Air or nothing | Below 2 mass % lubricant on the chip; up to 80 mL/h still counts as dry (DGUV Information 209-024) | Processes in the DGUV dry matrix; air cooling for light high-speed cuts at 45 HRC and above, one tool maker | Titanium, superalloys, austenitic and duplex stainless steels need coolant, one tool maker | None |
| 6 | Cryogenic cooling | Liquid nitrogen, or CO2 snow and gas | Liquid nitrogen -196 °C (-321 °F); CO2 -78.5 °C (-109.3 °F) | Ti-6Al-4V at high cutting speed, from 125 m/min in one milling study (Albertelli et al., 2021) | Below 19.5 % oxygen by volume is oxygen-deficient (29 CFR 1910.134(b)) | No cutting fluid at the cut; CryoMQL adds MQL oil (Pereira et al., 2020) |
Each cell names its source; one maker's figures are marked as such.
Dry and cryogenic cooling put no cutting fluid on the cut, so the request form below covers the other 4 methods.
How Much Fluid Reaches the Cut? Pressure and Quantity by Method#
Coolant delivery methods span pressures from around 20 bar (290 psi) for flood to 350 bar (5,000 psi) for ultra-high-pressure jets, both figures from one tool maker. For titanium, tool makers recommend 50 to 300 bar (725 to 4,350 psi) (Struzikiewicz, 2024).
Quantity falls in the same order: reduced-quantity cooling uses up to 2 L (0.53 US gal) per process hour (DGUV Information 209-024), and MQL uses about 1 L (0.26 US gal) per 8-hour shift according to EU-OSHA's OSHwiki.
What Each Delivery Method Asks of the Fluid#
Each delivery method sets different demands on the fluid: flood and high-pressure systems need low-foam fluids, MQL needs a low-emission oil, and dry and cryogenic machining use no cutting fluid.
High-pressure coolant requires low-foam fluids, according to one supplier's technical bulletin and a trade-journal report. Neat oil under high pressure needs fast air release against pump cavitation, according to one fluid maker.
MQL lubricants meet three low-emission criteria in DGUV Information 209-024 (2010, Table 6), listed below.
- Viscosity: above 10 mm²/s at 40 °C (104 °F) by DIN 51562-1.
- Flash point: above 150 °C (302 °F) in the open cup by DIN EN ISO 2592.
- Evaporation: Noack loss below 65 % at 250 °C (482 °F) by DIN 51581-1.
The same guidance rules out native esters such as rapeseed oil, water-miscible fluids, organic chlorine or zinc additives and mineral oils above 3 ppm benzo[a]pyrene, and MQL oils need no biocide.
The cryogen does not lubricate; CryoMQL adds MQL oil and raised tool life 120 % over MQL alone in Inconel 718 milling (Pereira et al., 2020). Which metalworking fluid additives a fluid carries follows from the delivery method.
How Operation, Material and Machine Decide the Delivery Method#
The delivery method follows from the operation first, then the workpiece material and cutting speed, then the machine. DGUV Information 209-024 (2010) lists milling of aluminum cast alloys, steels and cast iron, turning of steels and cast iron, gear hobbing, and drilling of free-cutting steel and cast iron as processes run dry.
In Ti6Al4V milling, liquid nitrogen outlasts conventional cooling only from 125 m/min (Albertelli et al., 2021).
One tool maker puts liquid coolant at 16 to 17 % of part cost in batch production, against about 3 % for tooling. The guide to cutting fluids by machining operation then gives the fluid for each operation.
How Does the Delivery Method Change Exposure and Sump Behavior?#
The delivery method changes what reaches the operator's breathing zone and how hard the sump works: higher pressure raises mist and foam, while MQL and cryogenic cooling shift the hazard to aerosol and gas.
Mist and the delivery method#
Higher delivery pressure produces more mist: through-tool coolant at 55 bar (800 psi) raised mist concentration by about 200 % in a peer-reviewed study. MQL exhaust emissions were less than half those of flood in turning C45 steel (DGUV Information 209-024).
NIOSH recommends 0.4 mg/m³ thoracic particulate mass as a TWA for up to 10 hours a day in a 40-hour week (NIOSH Criteria Document 98-102, 1998). The guide to metalworking fluid mist covers its control.
Gas safety with liquid nitrogen and CO2#
Cryogenic cooling displaces oxygen, and OSHA treats an atmosphere below 19.5 % oxygen by volume as oxygen-deficient (29 CFR 1910.134(b)). For carbon dioxide, NIOSH sets a REL of 5,000 ppm TWA and 30,000 ppm STEL, OSHA a PEL of 5,000 ppm TWA, and the IDLH is 40,000 ppm. Neither gas has a CLP Annex VI entry; the ECHA C&L notified classification is H280 for nitrogen and H280 (70.5 %) for carbon dioxide, with a minority H281. Site rules come from the competent authority.
Sump volume, foam and filtration under high pressure#
High-pressure coolant works a smaller sump harder: 114 L (30 US gal) sumps in place of 379 L (100 US gal) turn the fluid over faster and raise foam (trade-journal report). One filter maker reports that particles above 40 µm hurt tool life, while filtration below 15 µm starts removing antifoam particles.
A coolant management routine adjusts concentration, foam control and filtration to the delivery pressure.
FAQ#
What is the difference between flood and mist coolant?#
Flood coolant pumps a continuous stream from the sump at around 20 bar (290 psi), according to one tool maker, while spray mist puts fluid droplets on the cut. MQL, the defined low-quantity method, uses at most 50 mL per process hour and tool (DGUV Information 209-024, 2010).
Which delivery method uses the least fluid?#
Cryogenic cooling puts no cutting fluid on the cut, and DGUV Information 209-024 counts a cut as dry below 2 mass % of lubricant on the chip. MQL uses about 5 to 50 mL/h (0.17 to 1.7 US fl oz/h) according to IFA.