Types of cutting fluid

HSE puts the whole family in one line: “Metalworking Fluids (MWFs) are neat oils or water-based fluids used during the machining and shaping of metals”. The German classification, DIN 51385 as the IFA presents it, makes the same first cut, “zwischen nichtwassermischbaren und wassermischbaren Kühlschmierstoffen”, between fluids that are not water-miscible and fluids that are; a water-miscible fluid is a concentrate that becomes ready to use only once water is added. Within the water-miscible ones DIN 51385 separates those that form an oil-in-water emulsion from those that give colloidal or true solutions. OSHA’s manual sorts by composition instead: “There are four major classes of metal-working fluids widely available”, straight oil, soluble oil, semisynthetic and synthetic. This page follows OSHA’s four. How to measure and correct the mix of a water-miscible fluid is on the coolant concentration page.

Neat (straight) oiloily appearancenot diluted with watermostly mineral or vegetable oilSoluble oil (emulsion)milky whitemixed with water30–85 % base oil in the concentrateSemi-syntheticoften translucentmixed with watere.g. 5–30 % base oil in the concentrateSynthetictransparentmixed with waterno petroleum oil

Oil in the concentrate (OSHA)schematic; droplet sizes not to scale

order only, as OSHA and Lubrizol give it1234lubrication rather than coolingthe best heat reduction

Neat oils

OSHA: “This type of metalworking fluid is made up mostly of mineral (petroleum) or vegetable oils.”, and “This product is not diluted with water before use.” It is the lubricating end of the range: “Straight-oil metalworking fluids are generally used for processes that require lubrication rather than cooling.” Lubrizol puts them as “best suited for slower speed, severe operations that require the good lubricity that the base oil provides”, adds that they “often include extreme pressure additives”, and that they “also effectively prevent rusting of the workpiece.” OSHA: “These materials may contain chlorinated and sulfur additives.”

A neat oil lasts. FUCHS: “Cutting neat oils are particularly stable.”, and “A straight cutting oil rarely has to be changed as it is normally used in the process as it follows the machined parts and swarf”, unless it starts to oxidise or metal particles or sediment build up enough to cause problems. The price is a fire risk: “Straight-oil MWF systems may require fire protection.” (OSHA). One example from a Turkish data sheet: Petrol Ofisi’s PROCUT HD, a chlorine-free neat cutting oil for automatic lathes and screw machines, thread cutting, milling, broaching and cutting, has a viscosity of 35.7 mm²/s at 40 °C and a flash point (COC) of 206 °C.

Soluble oils (emulsions)

OSHA: “It is made up of from 30 to 85 percent of severely refined lubricant base oil”, with emulsifiers “to help disperse the oil in water.” FUCHS: “The concentrate consists of a mineral oil and possibly a synthetic ester, along with an emulsifier and other additives.” and “When the concentrate is mixed with water, a milky-white emulsion is formed.” In use it does both jobs: “Soluble oils in general provide good lubrication and are better at cooling than straight oils.” (OSHA).

OSHA also lists the drawbacks: they “sometimes have poor corrosion control”, are sometimes “dirty”, meaning that machine surfaces and nearby areas get covered with oil or with residues that are hard to remove, “may smoke (they may not cool as well as semisynthetics and synthetics)” and “may have poor mix stability or short sump life.” These are general statements; single products claim good corrosion protection, for example Mobil’s Turkish data sheet for its conventional milky Mobilcut 100, which promises long-term corrosion protection. The data sheet of the product in the machine is what counts. Turkish data sheets use their own names: Mobil’s product page calls Mobilcut 140 “su ile karışabilen” (water-miscible), Mobilcut 250 “yarı sentetik” and Mobilcut 320 “mineral yağ içermeyen sentetik” (synthetic with no mineral oil), while its brochure calls the range “suda çözünür” (water-soluble); Petrol Ofisi calls PROCUT HD a “saf kesme yağı” (neat cutting oil).

Semi-synthetics

OSHA: “This type of metalworking fluid contains a lower amount of severely refined base oil, for example, 5-30 percent in the concentrate.” “They are comprised of many of the same ingredients as soluble oils and contain a more complex emulsifier package.” It credits them with “good lubrication, good heat reduction, good rust control”, and writes that they “have longer sump life and are cleaner than soluble oils.” Lubrizol adds that as a concentrate they “contain a combination of oil, additives and water.”

They often look different from a soluble oil. FUCHS: “Such emulsions are clearer than normal and are called semi-synthetics or microemulsions.” OSHA is more careful: “semi-synthetics are often transparent to milky”. Norrby’s STLE study, measuring droplet sizes by light scattering at high dilution, found the droplets of (nearly) translucent semi-synthetic formulations in general smaller than those of the milky emulsions. The measured sizes belong to the formulations tested there, and in one of them a change in water hardness shifted the droplet size a hundredfold, so they are not class values.

Synthetics

OSHA: “These metalworking fluid formulations do not contain any petroleum oil.” “They contain detergent-like components to help ‘wet’ the part and other additives to improve performance.” FUCHS: “Synthetic fluids do not contain mineral oil but are often based on glycols.”, and “Unlike emulsions, such liquids are entirely soluble in water”. The result can be seen: OSHA, “this type of metalworking fluid is transparent (allowing the operator to see the work)”; FUCHS, “the resultant solution is normally very clear and transparent.”

They are the cooling end of the range. OSHA: synthetics “generally are the cleanest, offer the best heat reduction” and “offer longer sump life”. Lubrizol: “these fluids offer excellent cooling and are best suited for high-speed processes”, and “usually provide very low foaming”. FUCHS adds that “There are also synthetic fluids that do not contain any lubricating components.” and “These are used as grinding fluids.” Mobil’s Turkish data sheet lists its synthetic Mobilcut 320 for grinding steel and cast iron, and adds that it is not suitable for tungsten carbide because of cobalt leaching.

In short

OSHA’s type descriptions list no drawbacks for semi-synthetics and synthetics; its health section, however, puts water-based, synthetic and semi-synthetic fluids at the highest risk of contact dermatitis (below). None of the sources found ranks the classes for bacteria, tramp oil, foaming (apart from Lubrizol’s remark on synthetics), cost, or compatibility with machine paint and seals.

What all water-miscible fluids share

They are made to run at a set concentration: “All water based metalworking fluids are designed to be operated at a given concentration dissolved or emulsified with water.” (OSHA). They support bacterial growth: HSE writes that “Fluid systems that contain water or water-mixes can become highly contaminated with harmful bacteria.”, and NIOSH that water-based fluids “support microbial growth”. Every class carries additives; OSHA lists “surfactants, biocides, extreme pressure agents, anti-oxidants, and corrosion inhibitors”.

The health risks differ by class. OSHA: “People working with water based, synthetic, and semi synthetic MWFs are most at risk for developing contact dermatitis.”; “Straight oils are often associated with acne-like disorders”; and “Recent outbreaks of HP have been associated with exposure to aerosols of synthetic, semi synthetic, and soluble oil MWFs.” (HP: hypersensitivity pneumonitis.)