Editing Cutting fluid (section)

==Types==

=== Liquids ===
There are generally three types of liquids: mineral, semi-synthetic, and synthetic. Semi-synthetic and synthetic cutting fluids represent attempts to combine the best properties of oil with the best properties of water by suspending emulsified oil in a water base. These properties include: rust inhibition, tolerance of a wide range of water hardness (maintaining pH stability around 9 to 10), ability to work with many metals, resist thermal breakdown, and environmental safety.<ref>OSHA (1999). [http://www.osha.gov/SLTC/metalworkingfluids/metalworkingfluids_manual.html Metalworking Fluids: Safety and Health Best Practices Manual]. Salt Lake City: U.S. Department of Labor, Occupational Safety and Health Administration.</ref>

Water is a good conductor of heat but has drawbacks as a cutting fluid. It boils easily, promotes rusting of machine parts, and does not lubricate well. Therefore, other ingredients are necessary to create an optimal cutting fluid.

[[Mineral oil]]s, which are petroleum-based, first saw use in cutting applications in the late 19th century. These vary from the thick, dark, sulfur-rich cutting oils used in heavy industry to light, clear oils.

Semi-synthetic coolants, also called ''soluble oil'', are an emulsion or [[microemulsion]] of water with mineral oil.   In workshops using British English, soluble oil is colloquially known as '''SUDS'''.<ref>{{Cite web | url=https://www.midlandslubricants.co.uk/metalworking/cutting-oils-suds/midlands-lubricants-general-soluble-cutting-oil-suds/ | title=General Soluble Cutting Oil – Water Soluble Cutting Oil – Midlands Lubricants Ltd| date=16 January 2016}}</ref> These began to see use in the 1930s. A typical CNC machine tool usually uses emulsified coolant, which consists of a small amount of oil emulsified into a larger amount of water through the use of a detergent.

[[Synthetic chemical|Synthetic]] coolants originated in the late 1950s and are usually water-based.

The official technique to measure oil concentration in cutting fluid samples is manual [[titration]]:<ref>{{cite book |doi=10.4324/9781351228213 |title=Metalworking Fluids, Third Edition |date=2017 |isbn=978-1-351-22821-3 |editor-first1=Jerry P. |editor-last1=Byers }}{{pn|date=March 2025}}</ref> 100ml of the fluid under test is titrated with a 0.5M HCl solution to an endpoint of pH 4 and the volume of titrant used to reach the endpoint is used to calculate the oil concentration. This technique is accurate and not affected by fluid contamination, but needs to be performed by trained personnel in a laboratory environment. A hand-held [[refractometer]] is the industrial standard used to determine the mix ratio of water-soluble coolants<ref>{{cite journal |last1=Fukuta |first1=Mitsuhiro |last2=Yanagisawa |first2=Tadashi |last3=Miyamura |first3=Satoshi |last4=Ogi |first4=Yasuhiro |title=Concentration measurement of refrigerant/refrigeration oil mixture by refractive index |journal=International Journal of Refrigeration |date=June 2004 |volume=27 |issue=4 |pages=346–352 |doi=10.1016/j.ijrefrig.2003.12.007 }}</ref> that estimates oil concentration from the sample refractive index measured in the [[Brix scale]]. The refractometer allows for in situ measurements of oil concentration within industrial plants. However, contamination of the sample reduces the accuracy of the measure. Other techniques are used to measure the oil concentration in cutting fluids, such as measure of the fluid [[viscosity]], [[density]], and [[ultrasound]] speed. Other test equipment is used to determine such properties as acidity and conductivity.

Others include:
* [[Kerosene]] and [[Isopropyl alcohol|rubbing alcohol]] often give good results when working on [[Aluminium|aluminum]].
* [[WD-40]] and [[3-In-One Oil]] work well on various metals. The latter has a citronella odor; if the odor offends, mineral oil and general-purpose lubricating oils work about the same.
* Way oil (the oil made for machine tool ways) works as a cutting oil. In fact, some screw machines are designed to use one oil as both the way oil and cutting oil. (Most machine tools treat way lube and coolant as separate things that inevitably mix during use, which leads to tramp oil skimmers being used to separate them back out.)
* [[Motor oil]]s have a slightly complicated relationship to machine tools. Straight-weight non-detergent motor oils are usable, and in fact SAE 10 and 20 oils used to be the recommended spindle and way oils (respectively) on manual machine tools decades ago, although nowadays dedicated way oil formulas prevail in commercial machining. While nearly all motor oils can act as adequate cutting fluids in terms of their cutting performance alone, modern multi-weight motor oils with detergents and other additives are best avoided. These additives can present a copper-corrosion concern to brass and bronze, which machine tools often have in their bearings and leadscrew nuts (especially older or manual machine tools).
* [[Dielectric]] fluid is used as a cutting fluid in [[electrical discharge machining|electrical discharge machines]] (EDMs). It is usually [[deionized water]] or a high-[[flash point|flash-point]] kerosene. Intense heat is generated by the cutting action of the electrode (or wire) and the fluid is used to stabilize the temperature of the workpiece, along with flushing any eroded particles from the immediate work area. The dielectric fluid is non-conductive.
* Liquid (water or petroleum oil) cooled water tables are used with the plasma arc cutting (PAC) process.
* [[Neatsfoot oil]] of the highest grade is used as a lubricant. It is used in metalworking industries as a cutting fluid for aluminum. For machining, tapping and drilling aluminum, it is superior to [[kerosene]] and various water-based cutting fluids.<ref>{{Cite web|url=https://www.britannica.com/topic/neats-foot-oil|title=Neat's-foot oil {{!}} lubricant|website=Encyclopedia Britannica|language=en|access-date=2019-01-26}}</ref>

=== Pastes or gels ===
Cutting fluid may also take the form of a paste or gel when used for some applications, in particular hand operations such as [[drill]]ing and [[taps and dies|tapping]]. In sawing metal with a [[bandsaw]], it is common to periodically run a stick of paste against the blade. This product is similar in form factor to lipstick or beeswax. It comes in a cardboard tube, which gets slowly consumed with each application.

=== Aerosols (mists) ===
Some cutting fluids are used in [[aerosol]] (mist) form (air with tiny droplets of liquid scattered throughout). The main problems with mists have been that they are rather bad for the workers, who have to breathe the surrounding mist-tainted air, and that they sometimes don't even work very well. Both of those problems come from the imprecise delivery that often puts the mist everywhere and all the time except at the cutting interface, during the cut—the one place and time where it's wanted. However, a newer form of aerosol delivery, {{Visible anchor|MQL}} (minimum quantity of lubricant),<ref name="Z">{{Citation |last=Zelinski |first=Peter |date=2006-08-28 |title=Toward more seamless MQL |journal=Modern Machine Shop |url=http://www.mmsonline.com/articles/toward-more-seamless-mql }}</ref><ref name="K">{{Citation |last=Korn |first=Derek |date=2010-09-24 |title=The many ways Ford benefits from MQL |journal=Modern Machine Shop |url=http://www.mmsonline.com/articles/the-many-ways-ford-benefits-from-mql }}</ref> avoids both of those problems. The delivery of the aerosol is directly through the flutes of the tool (it arrives directly through or around the [[tipped tool|insert]] itself—an ideal type of cutting fluid delivery that traditionally has been unavailable outside of a few contexts such as [[gun drill]]ing or expensive, state-of-the-art liquid delivery in production milling). MQL's aerosol is delivered in such a precisely targeted way (with respect to both location and timing) that the net effect seems almost like dry machining from the operators' perspective.<ref name="Z"/><ref name="K"/> The chips generally seem like dry-machined chips, requiring no draining, and the air is so clean that machining cells can be stationed closer to inspection and assembly than before.<ref name="Z"/><ref name="K"/> MQL doesn't provide much cooling in the sense of heat transfer, but its well-targeted lubricating action prevents some of the heat from being generated in the first place, which helps to explain its success.

=== Carbon dioxide ===
[[Carbon dioxide]] (chemical formula CO<sub>2</sub>) is also used as a [[coolant]]. In this application pressurized liquid CO<sub>2</sub> is allowed to expand and this is accompanied by a drop in temperature, enough to cause a change of phase into a solid. These solid crystals are redirected into the cut zone by either external nozzles or through-the-spindle delivery, to provide temperature controlled cooling of the cutting tool and work piece.<ref>{{Citation |date=2011-09-26 |title=CO2 Cooling System reduces friction |journal=Modern Machine Shop Online |url=http://www.mmsonline.com/products/co2-cooling-system-reduces-friction}}</ref>

=== Air or other gases (e.g., nitrogen) ===
Ambient air, of course, was the original machining coolant. Compressed air, supplied through pipes and hoses from an [[air compressor]] and discharged from a nozzle aimed at the tool, is sometimes a useful coolant. The force of the decompressing air stream blows chips away, and the decompression itself has a slight degree of cooling action. The net result is that the heat of the machining cut is carried away a bit better than by ambient air alone. Sometimes liquids are added to the air stream to form a mist (mist coolant systems, [[#Aerosols (mists)|described above]]).

[[Liquid nitrogen]], supplied in pressurized steel bottles, is sometimes used in similar fashion. In this case, boiling is enough to provide a powerful refrigerating effect. For years this has been done (in limited applications) by flooding the work zone. Since 2005, this mode of coolant has been applied in a manner comparable to [[#MQL|MQL]] (with through-the-spindle and through-the-tool-tip delivery). This refrigerates the body and tips of the tool to such a degree that it acts as a "thermal sponge", sucking up the heat from the tool–chip interface.<ref name="Z2">{{Citation |last=Zelinski |first=Peter |date=2011-01-28 |title=The 400° difference |journal=Modern Machine Shop |volume=83 |issue=10 |url=http://www.mmsonline.com/articles/the-400-difference }}</ref> This new type of nitrogen cooling is still under patent. Tool life has been increased by a factor of 10 in the milling of tough metals such as [[titanium]] and [[inconel]].<ref name="Z2"/>

Alternatively, using airflow combined with a quick evaporating substance (ex. alcohol, water etc.) can be used as an effective coolant when handling hot pieces that cannot be cooled by alternate methods.

=== Past practice ===
* In 19th-century machining practice, it was not uncommon to use plain water. This was simply a practical expedient to keep the cutter cool, regardless of whether it provided any lubrication at the cutting edge–chip interface. When one considers that [[high-speed steel]] (HSS) had not been developed yet, the need to cool the tool becomes all the more apparent. (HSS retains its hardness at high temperatures; other carbon tool steels do not.) An improvement was soda water ([[sodium bicarbonate]] in water), which better inhibited the rusting of machine slides. These options are generally not used today because more effective alternatives are available.
* Animal fats such as [[tallow]] or [[lard]] were very popular in the past.{{sfn|Hartness|1915|pp=153–155}} These are used infrequently today, because of the wide variety of other choices, but remain as an option.
* Old machine shop training texts speak of using [[Lead tetroxide|red lead]] and [[white lead]], often mixed into lard or lard oil. This practice is obsolete due to the toxicity of lead.
* From the mid-20th century to the 1990s, [[1,1,1-Trichloroethane|1,1,1-trichloroethane]] was used as an additive to make some cutting fluids more effective. In shop-floor slang it was referred to as "one-one-one". It has been phased out because of its [[Ozone depletion|ozone-depleting]] and [[central nervous system]]-depressing properties.