If you machine metal (shape it by cutting chips away with a tool, as on a lathe or a drill), you direct cutting fluid at the point where the tool meets the workpiece, to lubricate and to cool. There are four types: straight oil, which is pure oil, and three mixed with water (soluble, semi-synthetic and synthetic). Slow, heavy cutting calls for more oil; fast cutting calls for more water; and the metal of the workpiece decides the ingredients.
What happens where the tool cuts the metal?
The tool tears a chip from the metal, and that creates friction and heat in a very small spot, which is where the fluid does its work.
Two everyday scenes show what is at stake. Soap helps a tight ring slide off a finger, because it reduces friction. A stonecutter pours water on the cutting disc to carry the heat away.
Machining works the same way, with metal. The sharp edge of the tool, called the cutting edge, bites into the metal and tears off a sliver, called the chip. Heat is generated mainly in two places: where the metal is deformed and where the chip rubs against the tool. A little more appears where the tool scrapes the surface it has just cut.
At that spot, cutting fluid does four things:
Lubricates: lets the chip slide over the tool with less friction.
Cools: carries heat away from the tool, the chip and the workpiece.
Cleans: washes the chip out of the cutting point.
Protects: coats the workpiece and the machine against rust.
Without it, in most cuts, the tool wears out too soon and the workpiece comes out of size.
Figure 1. The four jobs of cutting fluid: lubricate, cool, clean and protect.
Oil and water, however, do not do these four things in the same way.
What types of cutting fluid are used in machining?
There are four, and what separates them is how much oil and how much water each one contains. Oil slides better; water carries more heat.
Two names come up in this conversation. Mineral oil is oil that comes from refined petroleum. An ester is an oil from the same chemical family as fats, made by the reaction between an alcohol and an acid taken from them.
Straight oil: oil used neat, just as it comes from the drum. Neat means without water: it usually carries additives, chemicals that reinforce what the oil does. The base can be mineral oil, vegetable oil or ester.
Soluble: oil spread through water in droplets that turn the mix milky white; the technical name for this is an emulsion. This is the familiar soluble oil of many lathes.
Semi-synthetic: carries less oil, in much smaller droplets, and the mix looks almost clear or slightly milky.
Synthetic: no mineral oil. Lubrication comes from synthetic ingredients that dissolve in water the way sugar dissolves, and the mix is usually transparent.
The three that use water arrive at the plant as a concentrate, a liquid that is mixed with water at the machine itself. The mix sits in a tank, the reservoir the fluid is pumped from to the cut and returns to.
From straight oil to synthetic, lubrication falls, while cooling, cleaning and how clearly you can see the workpiece through the fluid improve. No type is better in absolute terms.
Figure 2. From straight oil to synthetic: less oil, less lubrication and more cooling.
For those who need the standard: ISO 6743-7 divides metalworking fluids into MH, used neat, and MA, mixed with water. Within MA it separates emulsion, microemulsion and solution, which correspond, in market language, to soluble, semi-synthetic and synthetic.
If one lubricates more and the other cools more, which one weighs more in your case? The answer comes from the operation, the kind of cut the machine makes.
The most common names are these:
Turning: the workpiece rotates and the tool cuts it.
Milling: the tool rotates and cuts the workpiece, which feeds against it.
Drilling and tapping: making the hole and cutting the helical thread inside it.
Sawing: cutting with a saw.
Broaching: a long tool with many stepped teeth passes once through the workpiece and shapes a hole or a slot.
Why does the operation decide between oil and water?
Because cutting speed decides which of the two jobs weighs more: in slow cutting, lubricating; in fast cutting, cooling.
Slow, heavy cutting. In tapping and broaching, the chip moves out slowly. At that pace, the fluid has time to enter at the edge of the contact between the chip and the tool and lubricate.
The pressure there is high, and the metal of the workpiece tends to stick to the cutting edge. This stuck, almost welded metal is called a built-up edge, and it ruins the finish. Two additives fight it, at different moments:
At low pressure, the lubricity additive acts: a fat or an ester that clings to the metal and forms a slippery film.
When the pressure breaks that film, the extreme pressure (EP) additive takes over, usually sulfur-based, sometimes chlorine or phosphorus. It reacts with the hot metal and forms a thin, soft layer that slides under pressure and is rebuilt with every pass of the chip; this way the chip does not weld to the tool.
Fast cutting. Here the chip moves past too quickly, and the fluid does not reach the contact. It works from outside, taking heat out of the tool, the chip and the workpiece. In that job water wins: it stores about twice as much heat per kilogram and conducts it about four to five times better than oil.
When the heat does not leave, this is the chain:
The cutting edge heats up and loses hardness, which leads to fast wear.
The workpiece heats up and expands, which leads to it falling outside the permitted dimension (the tolerance) once it cools.
Both together lead to scrap and to tool changes before their time.
Figure 3. The cutting point at two speeds: in slow cutting, the fluid enters at the edge of the contact between the chip and the tool and lubricates; in fast cutting, it has no time to enter and acts from outside, removing heat.
The rule that fast cutting calls for water has one exception: grinding, which works with an abrasive and usually uses neat, thin oil, because of the finish of the workpiece.
Thin means low viscosity, which is how much an oil resists flowing: honey is thick, water is thin.
Thread grinding and gear grinding are fast cutting and, even so, call for oil. Honing, which also uses an abrasive, works slowly and follows the slow-cutting rule: it calls for oil. Each has its reason:
Thread and gear grinding. Grinding is the finishing done with a grinding wheel, an abrasive disc that works like a round sandpaper. Oil wears the wheel less, so it keeps for longer the profile that shapes the thread or the tooth.
Honing. This is the fine finishing of a bore with abrasive stones. Thin oil washes the abrasive and keeps the stone from glazing, that is, from turning smooth and ceasing to cut.
Neat oil at high speed forms mist, a cloud of fine droplets in the air. That is why the machine needs mist extraction and fire protection.
How this shows up in your plant:
Torn threads, burrs (leftover metal on the edge of the workpiece) and metal stuck to the tool point to too little lubrication.
A hot workpiece that changes size as it cools, a cutting edge that loses its edge early and burn marks point to too little cooling.
Smoke and mist with straight oil point to oil that is too thin for the speed, low flow or missing extraction.
For those who need the numbers: the specific heat of water is about 4.2 kJ/(kg·K), and that of mineral oil about 2 kJ/(kg·K); per liter, water stores more than twice as much. Thermal conductivity is about 0.6 against 0.13 W/(m·K) (Incropera, Fundamentals of Heat and Mass Transfer). Cutting mechanism: Trent and Wright, Metal Cutting.
What is the best cutting fluid for each operation?
The best one is the one that suits the most demanding operation on the machine. With what you have seen so far, the choice fits in five lines:
| If your machine does… | Fluid type | Why |
|---|---|---|
| Tapping, broaching, deep drilling (a hole much longer than it is wide), slow, heavy cutting in steel | Straight oil, or a high-lubricity soluble, with EP additive | High pressure at the contact, and in a long hole the oil helps push the chip out: lubrication decides |
| General-purpose turning, milling, drilling and sawing | Soluble or semi-synthetic | Calls for lubricating and cooling at the same time |
| Grinding and high-speed cutting, in most cases | Synthetic or semi-synthetic | Heat weighs more than friction, and a clear fluid lets you see the workpiece |
| Thread and gear grinding | Thin straight oil | The exception from the previous section: fast cutting that calls for oil, because of the finish |
| Honing | Thin straight oil | Slow cutting, as the rule says: thin oil washes the abrasive |
When the same machine does different jobs from the same tank, the most demanding operation rules. The metal also counts: a single brass part in the routine is enough to change the chemistry of the fluid.
Figure 4. The choice in four steps: the operation, the metal, the safety rules and testing.
Kelpen Oil formulates a complete line of metalworking fluids: straight oil, soluble, semi-synthetic and synthetic. Talk to the Technical Team: they help weigh the operations on your machine and point to the right fluid.
Which cutting fluid should you use for each metal?
The operation chooses the type of fluid, and the metal chooses its chemistry. There are seven cases:
Steel. Accepts almost every type; the operation decides.
Cast iron. Gray cast iron, the most common, is often machined dry, because graphite, the carbon in flakes found inside the metal itself, helps lubricate.
When a fluid with water is used, the fine dust rusts and turns to sludge. That is why the fluid must protect well against rust, and the tank must be well filtered.
Copper, brass and bronze, the yellow metals. With straight oil, the risk is the sulfur of the EP additive. Active sulfur, loosely bound in the molecule, breaks free with heat and reacts with copper: it stains and attacks the metal, like the egg that tarnishes silverware. Inactive sulfur, tightly bound, reacts little.
With a fluid mixed with water, the risk comes from amines, ingredients that protect against rust and make the mix alkaline, the opposite of acidic (soap is alkaline).
pH measures this: 7 is neutral, and above 7 is alkaline. Amines keep the pH high, and that combination dissolves copper and turns the mix greenish or bluish. That is why a fluid for yellow metals carries its own protector for copper and brass.
Figure 5. Active sulfur darkens copper; inactive sulfur barely reacts.
Aluminum. A soft metal that sticks to the tool. It calls for a water-mixed fluid with good lubrication, usually a soluble, that is not too alkaline, because a very alkaline mix stains aluminum.
Cemented carbide. This is the material of cutting tools, such as inserts (replaceable plates held in the tool holder) and drills, made of hard tungsten carbide grains bonded by cobalt. It appears as a tool and also as a workpiece, when the tool is sharpened. A wrong water-mixed fluid dissolves the cobalt, which is called leaching: the tool weakens and the cobalt contaminates the fluid.
The problem is greater in sharpening, where the dust is fine. There straight oil is widely used, and anyone who uses a water-mixed fluid asks for a version with protection against leaching. Cobalt that passes into the fluid can also cause skin allergy.
Stainless steel. It hardens as it is cut, conducts heat poorly (so heat stays concentrated at the cutting edge) and sticks to the tool. In slow, heavy operations, such as tapping, broaching and deep drilling, it calls for strong EP. In turning with a carbide tool, a soluble or a semi-synthetic usually does the job.
Magnesium, titanium and nickel alloys. In a water-mixed fluid, magnesium reacts and releases hydrogen, a gas that is a hazard if it builds up, and the wet fine chip becomes dangerous to store. That is why it is usually machined with thin mineral oil or dry.
Emulsion only enters with its own precautions (control of hydrogen and of the wet chip) and, well controlled, can give good fire protection, better than dry cutting.
Titanium and nickel alloys, common in aerospace, usually carry a limit on chlorine in the fluid; nickel alloys, on sulfur as well. The reason is the residue of these elements left on the part: it can crack or embrittle the metal when the part runs hot or goes through heat treatment. The restriction is imposed by the customer's specification. For the heat treatment itself, read quench in oil or water.
For those who need the standard: attack on copper is measured by the copper strip corrosion test, NBR 14359 (equivalent to ASTM D130; NBR is the Brazilian standards series), usually 3 hours at 100 °C for oil, or at the time and temperature that the product specification requires.
The scale runs from class 1 (slight tarnish, divided into 1a and 1b) to class 4 (corrosion); the closer to 1a, the smaller the attack, which matters above all for yellow metal. Designed for petroleum products, the test suits straight oil well; for a water-mixed fluid, it applies to the concentrate, as an indication.
For magnesium, the references are the guidance of DGUV (the German statutory accident insurance) and SAE paper 910415, which shows the use of emulsion with hydrogen control.
Besides performance, there are rules that take a fluid off the list before the first cut.
Which safety and chemistry requirements change the choice?
Three requirements come before performance and can eliminate a type of fluid: fire risk in storage, occupational health hazard and substances the customer has banned.
1. Fire and storage (NR 20). Alcohol catches fire with a match; cold cooking oil does not. The difference lies in the flash point, the lowest temperature at which the vapor of a liquid briefly ignites when a flame is brought close. The thinner the oil, the lower its flash point usually is.
NR 20 is the Brazilian regulation (Norma Regulamentadora) on flammable and combustible liquids. It sorts liquids by this number: flammable, with a flash point up to 60 °C, and combustible, above 60 °C and up to 93 °C. Each class has its own fire-safe storage rules.
A thin straight oil can land in the combustible class; the concentrate of a water-mixed fluid is usually outside both. The safety data sheet of each fluid gives the flash point.
Figure 6. The NR 20 classes by the flash point of the liquid.
2. Occupational health hazard (NR 15). NR 15 is the Brazilian regulation on unhealthy work activities, the basis for the hazard-pay supplement paid to workers. Its Annex 13 treats the handling of mineral oils as a maximum-grade health hazard. For a buyer in Brazil, this can mean a hazard-pay supplement for the operator, if the workplace assessment report classifies the fluid under it.
Straight mineral oil, soluble and semi-synthetic fluids usually contain mineral oil; the synthetic and the vegetable- or ester-based ones fall outside this item.
3. Substances the customer has banned. Four appear often:
Boron: boric acid and borates are classified as toxic for reproduction (CLP, the European regulation).
Chlorine: chlorinated paraffins, used as EP, persist in the environment and accumulate in living things; that is why they are restricted.
Nitrite: together with the fluid's amines, it forms nitrosamines, which cause cancer.
Triazine: a biocide (a product that kills bacteria) that releases formaldehyde, a substance that causes cancer.
The list comes from the customer, such as the carmaker or the aerospace company, or from the company's own occupational safety team (in Brazil, the SESMT).
At Kelpen Oil, the Keen Cut line has straight oils free of chlorine and heavy metals, compatible with copper and its alloys; the Keen Sol line has options free of boron, nitrite and triazine. Talk to the Technical Team to find the version that fits your customer's list.
For those who need the numbers: under CLP (Regulation EC 1272/2008), boric acid and borates are toxic for reproduction and formaldehyde is carcinogenic, both in category 1B.
Short-chain chlorinated paraffins were added to the elimination list of the Stockholm Convention in 2017, and medium-chain ones in 2025, both with temporary exemptions. In Germany, the technical rule TRGS 611 restricts the use of nitrite in water-mixed fluids, because it can form nitrosamines.
With so many rules and so much care, many people ask whether the fluid can simply be done without.
What can replace cutting fluid?
In industrial machining, nothing delivers the same overall result. There are two legitimate alternatives for specific operations and metals, with their own tooling and settings: dry machining, as with cast iron, and MQL (minimum quantity lubrication), a little oil blown as a mist straight onto the cutting edge.
The homemade recipe (kerosene, engine oil, cooking oil, an improvised mix) trades a controlled input for rust, mist and a risk to the skin. Cooking oil, without the additives of a formulated fluid, also reacts with oxygen in the air, turns rancid and gets sticky.
People who ask this almost always want something else: to spend less. The answer is control.
How can you make a water-mixed fluid last longer?
In the tank of a water-mixed fluid, five things need monitoring:
Concentration: how much concentrate goes into the water.
pH: if it drops, the mix loses its protection against rust and bacteria grow; if it rises too high, it irritates the skin and stains aluminum.
Dirt and fine chips piling up in the tank.
Tramp oil: oil that leaks from the machine itself, such as hydraulic oil or slideway oil, and floats in the tank.
Bacteria: they grow in the mix and cause a bad smell.
In the plant, concentration is checked with a handheld refractometer, an instrument that measures it by how the mix bends light; the reading is corrected by the factor the fluid supplier provides. pH is checked with a strip or a meter. The right range for each comes from the technical data sheet; outside it, contact the supplier.
The water of the mix also counts. Hard water, with a lot of calcium (the kind that leaves a white crust in a kettle), forms calcium soaps, breaks the emulsion and leaves sludge, a sticky mud, at the bottom. Water that is too soft makes foam.
Figure 7. What to watch in the tank of water-mixed fluid.
Monitored, these points make the fluid last longer and protect the tool better. What remains is how to check that the fluid you chose is right.
How do you know the cutting fluid you chose is right?
By checking at two moments: before approving the fluid, through laboratory testing, and during use, through the concentration and pH check at the machine itself, as seen above.
Kelpen Oil has its own laboratory in Arujá (SP), with 25 analyses described in the laboratory's tests, standards and equipment. Four of them answer the questions in this text:
| Test | What it answers | Standard (equipment) |
|---|---|---|
| Copper strip corrosion | In straight oil and in the concentrate: whether the fluid attacks copper, brass and bronze | NBR 14359 (ASTM D130) |
| Water by the Karl Fischer method, a chemical reaction that detects only water, even in minimal amounts | Whether the straight oil took in water, which makes it lubricate worse and protect less against rust | Coulometric Karl Fischer |
| Boron by ICP, an instrument that measures metals in very small amounts (the same test also measures barium, calcium, phosphorus and zinc) | In the concentrate, as an indication: whether a fluid sold as boron-free shows no boron in the test | NBR 14786 (Optima 7000 DV) |
| Viscosity at 40 °C | Whether the straight oil is thin or thick for the operation | ASTM D7042 (SVM 4001) |
For the conversation with the Technical Team, bring these answers:
Which metals the machine cuts.
Which operations it performs and which is the most demanding.
Where the mixing water comes from and whether it leaves a crust.
Which substances your customer bans.
Whether a fluid is already in use, and for how long it has been in the tank.
Frequently asked questions
What is the difference between oil and cutting fluid? Cutting fluid is the name of the whole group, with and without water. Cutting oil usually names straight oil, used neat; soluble is also called soluble oil, because it is an oil made to be mixed with water.
Does cutting fluid expire? Yes. The sealed concentrate has a shelf life, shown on the label or on the product data sheet. In service, the mix wears out: hard water, tramp oil, bacteria, dirt and the consumption of additives spoil it, often with no visible sign. Concentration and pH help track it, but the time to change it comes from the five tank checks taken together.
Is cutting fluid bad for the skin? It can be. According to the UK Health and Safety Executive (HSE), frequent contact with the fluid can cause dermatitis, an irritation that leaves the skin red and cracked. By the same source, the mist of contaminated fluid harms those who breathe it. Cobalt dissolved during the sharpening of cemented carbide can also cause skin allergy; gloves, a controlled fluid and extraction reduce the risk.
Is lathe oil the same as cutting fluid? No. The machine has its own oils: the way oil, which lubricates the guides the carriage slides on; the hydraulic oil; and the gearbox oil. What leaks from them and falls into the fluid tank is tramp oil, which has to be removed.
Glossary
Chip: the sliver of metal the tool tears from the workpiece.
Straight oil: cutting fluid used neat, without water; in the plant, also called cutting oil.
Concentrate: the fluid you buy and mix with water at the machine.
Emulsion: oil spread through water in droplets, which makes the mix milky; with much smaller droplets, a microemulsion.
Extreme pressure (EP) additive: an additive that reacts with the hot metal and forms a thin, soft layer that slides under pressure and keeps the chip from welding to the tool.
Active and inactive sulfur: sulfur loosely bound in the molecule, which breaks free with heat and stains copper (active), or tightly bound, which reacts little (inactive).
Cobalt leaching: the dissolving, by a water-mixed fluid, of the cobalt that binds the grains of cemented carbide.
Flash point: the lowest temperature at which the vapor of a liquid briefly ignites when a flame is brought close.
Talk to the Technical Team
The Kelpen Oil Technical Team evaluates the metals you machine, the operations of the machine and the most demanding of them, the dilution water and the substances your customer bans, and points to the right fluid for your case. The Arujá (SP) laboratory also performs analyses for other companies, as a paid service. Save this article or send it to whoever chooses the machining fluid at your company.
