MQL (Minimum Quantity Lubrication) and Dry Machining: When Is Work Carried Out Without Fluid?

Cutting fluid is an input that is most often accepted without question in machining. Yet when the purchase, maintenance, disposal and part cleaning costs of the fluid are added together, an item several times larger than tooling cost emerges. This article addresses the MQL method, which reduces fluid use to the order of millilitres, fully fluid-free dry machining, and the limits of both.

1. What is MQL and what is the order of consumption?

MQL (minimum quantity lubrication) is the atomisation of a very small quantity of oil within compressed air and its spraying directly into the cutting zone. In the German literature it is known as Minimalmengenschmierung, and in the industry also as "near-dry machining". The chemical and system side of the method is classified together with other machining media in the VDI 3397 Blatt 1 guideline.

The decisive point is this: MQL is not a cooling method but a lubrication method. The quantity of fluid applied does not have the mass to carry heat away; its function is to reduce friction and adhesion. The removal of heat from the cutting zone is left to the chip.

The definition of MQL has not yet been tied to a standard. DGUV Information 209-025 gives the definition established in practice as follows: an oil consumption not exceeding an average of 50 mL per machining hour and per tool. The same guideline also states that on tools above 40 mm diameter, figures above 150 mL/h can be reached for short periods. Low-flow applications going up to 2 litres per hour, on the other hand, count not as MQL but as reduced quantity lubrication; in this method the fluid is recirculated, so there is no total-loss system involved.

In the same guideline, "dry machining" too is defined not by the complete absence of fluid but by the proportion of fluid adhering to the chip: if the fluid remaining on the chip is below 2%, the process counts as dry. By this definition MQL also falls under the heading of dry machining.

2. Delivery methods: external nozzle and through-tool

External (nozzle) delivery

The aerosol is directed into the cutting zone by one or several nozzles attached to the spindle from outside. Installation is cheap, it can be retrofitted to an existing machine, and the tool need not have a coolant hole. On the other hand the nozzle has to be re-aimed at every tool change; in deep pockets, with long-reach tools and in multi-axis movements, the aerosol reaching the cutting point is not reliable.

Internal (through-tool) delivery

The aerosol is fed through the spindle and toolholder into the internal channels of the tool and emerges right beside the cutting edge. In work where a nozzle cannot provide access, in particular hole drilling above 3×D and deep slot machining, it is the only applicable solution. The tool must have internal coolant channels and the holder and drawbar must be permeable. In two-channel systems an HSK holder with an internal fluid tube and a matching nozzle arrangement is additionally sought; sealing on the holder side becomes decisive in system selection.

1-channel and 2-channel systems

The comparison below is based on the system tables in DGUV Information 209-025.

In single-channel systems the aerosol volume in the line behaves like a buffer; when the tool changes, the arrival of the new dose in the cutting zone is delayed. In twin-channel systems the oil and air travel separately as far as the spindle, so this delay disappears and a different dose can be defined for each tool. In return, feeding more than one spindle from a single unit becomes difficult in the twin-channel solution, and the pump–valve group enters the maintenance list as a wearing part.

3. Air pressure and aerosol particle size

MQL units operate on compressed air; DGUV Information 209-025 regards an air pressure above 4 bar as necessary for both single- and twin-channel systems. The pressure determines two outcomes at once: the speed at which the aerosol reaches the cutting zone and the droplet size. As the pressure rises, atomisation becomes finer, the mean droplet diameter decreases and wetting becomes more homogeneous.

A small droplet is not always an advantage. A very fine aerosol may be carried away and dispersed into the environment before reaching the cutting zone, and it falls into the respirable size range. Particle fractions are defined in ISO 7708:1995 according to aerodynamic diameter: the 50% cut point is 4 µm for the respirable fraction, 10 µm for the thoracic fraction and 100 µm for the inhalable fraction. Which fraction a limit value is given for is therefore decisive.

Pressure, fluid flow rate and nozzle diameter are set together; the cabin is kept closed and the extraction system is interlocked with the machine control. The same guideline requires that the flow velocity in the extraction duct does not fall below 20 m/s; at lower velocities dust deposits form inside the duct. For a detailed health assessment, the section on occupational health and safety with metalworking fluids can be consulted.

4. MQL fluids: ester oils, fatty alcohols and products not regarded as suitable

The fluid used in MQL has a different chemistry from the emulsion in a flood system. Since the fluid does not return, the amount of film remaining on the part and the chip, the evaporation behaviour and the toxicological profile become decisive. Two groups stand out in industrial practice:

• Synthetic ester oils. Despite their low viscosity they have a high boiling point and flash point; for this reason the quantity of vapour released into the working volume is markedly lower than with mineral oils. They are preferred in operations where the lubricating effect is decisive, that is where abrasive wear has to be prevented: tapping, drilling, reaming and turning. Their biodegradability is high and their water hazard classes are low.
• Fatty alcohols. Compared with ester oils of the same viscosity, both the flash point and the lubricating power are lower; in return they evaporate more quickly and leave a practically dry surface on the part. They are used in the machining of non-ferrous metals, where the main expectation is not lubrication but a separating effect, that is where the formation of built-up edge (BUE) has to be prevented.

Both groups have high biodegradability and are assessed as not hazardous to water (water hazard class 1). For the selection of a low-emission MQL oil, three guide values are given in DGUV Information 209-025: a viscosity above 10 mm²/s at 40 °C, an open-cup flash point above 150 °C and a Noack evaporation loss below 65% at 250 °C. In addition to these, odour and sprayability are also among the assessment criteria.

Products not regarded as suitable

The same guideline separately lists the products that should not be used in MQL. The list directly counters the assumption that "a vegetable-based oil is the safer option under all conditions":

When making a selection, the classification of constituents in the product safety data sheet and the registration status under KKDİK (the Turkish REACH equivalent) are checked. Since bacterial growth is not an issue in water-free products, the biocide content and the sensitisation risk associated with it also disappear. That removes at a stroke the greatest burden on the cutting fluid maintenance side.

5. Where does MQL work and where does it fall short?

The distinction is made with a single question: does the heat leave with the chip, or does it build up in the part and the tool? If the greater part of the heat is removed with the chip, MQL is sufficient; if it builds up, cooling capacity is required.

In deep slot and pocket machining the source of the problem is not lubrication but the inability of the chip to leave the slot. In this case, before switching to MQL, strategies that reduce the chip load and the fullness of the slot, such as trochoidal milling, have to be assessed. The mention in Sandvik Coromant's milling guide of micro-lubrication systems consisting of compressed air and low-flow oil for chip evacuation in deep pockets also belongs in this framework. For the ISO S group, the same guide recommends that heat-resistant alloys be machined with fluid at low cutting speeds; the reason is both lubrication and cooling of the part. For this group the conditions in the section on milling titanium and heat-resistant alloys apply.

6. Dry machining and thermal shock in hardened steel

Milling is an interrupted operation. The cutting edge enters the cutting zone on every revolution, heats up, then cools in the air. This cycle creates cyclic thermal stress in the edge. When cutting fluid is applied, the hot–cold difference grows and thermal cracks perpendicular to the cutting edge (comb cracks) form along the edge. As the cracks progress, the edge breaks away in particles and tool life ends earlier than expected.

For this reason the default approach for milling in manufacturers' guides is dry working. Sandvik Coromant's milling guide takes rough milling operations always being carried out dry as the basis and ties fluid to a handful of exceptions: in finishing operations on stainless steel and aluminium to prevent metal smearing on the surface texture, in heat-resistant alloys at low cutting speeds, in cast iron to suppress dust, and on thin-walled parts to prevent geometric distortion. The same guide states that if fluid is to be used it has to be applied in copious quantity. These exceptions are reflected in the ISO groups as follows:

In hardened steel (above 45 HRC) heat is not a problem but part of the process: the high temperature in the cutting zone lowers the local yield strength of the material and eases chip formation. Cooling disrupts this mechanism, raises the cutting force and triggers thermal cracking. For detail, the section on milling hardened and tool steel can be consulted. The condition for dry working is a heat-resistant coating and a suitable carbide grade; dry working with an uncoated or high-cobalt tool does not deliver results.

7. Cryogenic cooling: liquid nitrogen and carbon dioxide

In cryogenic cooling, liquid nitrogen (LN₂, −196 °C) or liquid carbon dioxide (CO₂, which sublimes at −78.5 °C at atmospheric pressure) is delivered to the cutting zone. After doing its work the fluid turns into gas and leaves the environment; it leaves no residue whatsoever on the part or the chip.

• Liquid nitrogen. It provides deeper cooling and a marked increase in tool life has been reported in ISO S alloys. Because of the high temperature difference, film boiling may occur; this creates an insulating gas layer and lowers the heat transfer coefficient. It requires an insulated line and a rotary union.
• Liquid carbon dioxide. On leaving the nozzle it expands and turns into a solid–gas mixture ("snow"). Since it can be supplied from standard bottles its infrastructure is simpler, but its cooling power is lower than that of nitrogen.

Neither fluid lubricates. For this reason, in practice a cryogenic fluid and a low-flow oil aerosol are frequently delivered together. Because gases that displace oxygen in an enclosed volume create an asphyxiation risk, ventilation of the machine cabin and ambient gas monitoring are mandatory complements.

8. Cost, disposal and the recycling value of the chip

The share of cutting fluid in cost is assumed in most workshops to be smaller than it is. In Klocke and Eisenblätter's study published in CIRP, the part-related manufacturing costs arising from the use of cutting fluid are given as being at a level of 7–17%, whereas the share of cutting tool cost is given as only 2–4%. This ratio shows that an improvement made on the fluid side is worth several times one made on the tool side.

The items that disappear when MQL and dry machining are adopted are the following:

In Turkey, waste cutting fluid is managed as hazardous waste. In the waste list annexed to the Atık Yönetimi Yönetmeliği (the Turkish Waste Management Regulation), halogen-free machining emulsions and solutions are classified under code 12 01 09* as hazardous, with an asterisk. Mineral-oil-based waste oils additionally fall within the scope of the Atık Yağların Yönetimi Yönetmeliği (the Turkish Waste Oil Management Regulation, Official Gazette No. 30985 of 21.12.2019). In both cases an obligation arises to hand the waste over to a licensed company and to keep records and make declarations. The disappearance of this burden on the emulsion side when MQL is adopted is the most concrete gain, with a direct monetary equivalent, of the method.

The value of the chip staying dry

A wet chip carries a marked proportion of its mass as fluid. This situation has three consequences: the fluid carried away has been purchased but lost without being used; scrap buyers take chips with a high residual fluid content at a reduced price or refuse them; and drying the chips before melting requires an additional operation.

Only a very thin oil film remains on chips machined with MQL, and in cryogenic machining there is no residue at all. The chips can be briquetted directly and valued in a higher scrap class. In materials with a high scrap value such as aluminium, this difference is large enough on its own to shorten the payback period of an MQL investment; the typically high chip volume in aluminium milling magnifies this effect.

9. Decision summary

Method selection is not reduced to a single criterion; material group, operation type and part geometry are assessed together. A rough ranking can be set up as follows: if the heat leaves with the chip, dry or MQL; if the heat builds up at the edge, fluid; if the heat both builds up and fluid is not tolerated, cryogenic. If emulsion use is to be continued, the classification in the section on cutting fluid types and selection can be examined together with the chemicals category.

Sources

• Klocke, F.; Eisenblätter, G. — Dry Cutting, CIRP Annals, 46(2), 1997, 519–526. DOI
• VDI 3397 Blatt 1:2020-03 — Processing media (PM) for forming and machining — Metal working fluids, forming fluids, minimum quantity fluids, multifunctional oils. VDI
• DGUV Information 209-025 — Minimum quantity lubrication for machining operations, Deutsche Gesetzliche Unfallversicherung. DGUV Publikationen
• NIOSH — Criteria for a Recommended Standard: Occupational Exposure to Metalworking Fluids, DHHS (NIOSH) Publication No. 98-102, 1998. CDC Stacks
• OSHA — Metalworking Fluids: Safety and Health Best Practices Manual. OSHA
• ISO 7708:1995 — Air quality — Particle size fraction definitions for health-related sampling.
• Sandvik Coromant — Dry milling or with cutting fluid. Sandvik Coromant
• Atık Yönetimi Yönetmeliği (waste list, 12 01 09) and Atık Yağların Yönetimi Yönetmeliği, Official Gazette No. 30985 of 21.12.2019. mevzuat.gov.tr