Cutting fluid for industrial machining is the liquid that lubricates and cools the point where the tool meets the part. There are four types: straight oil, used neat, and three that are mixed with water (mineral soluble oil, semi-synthetic and synthetic). As a rule, slow, heavy cutting such as threading calls for straight oil, which lubricates better; fast cutting and grinding (finishing with an abrasive wheel) call for a water-based fluid, which cools better. The metal being machined sets the chemistry.
Think of two everyday scenes. Soap helps a tight ring slide off a finger by cutting friction. The water a stonemason pours on a cutting disc carries the heat away. Cutting fluid does both, in proportions that change with the type. It reduces friction between the tool and the chip (the sliver of metal removed from the part), carries away the heat generated in the cut, flushes chips out of the cutting zone and protects part and machine against corrosion. Tool, cutting parameters (speed, feed and depth of cut) and fluid work together: with the wrong fluid, the tool wears out early and the part comes out of tolerance, even with the right parameters. Kelpen Oil manufactures metal machining fluids of all four types.
What is the best cutting fluid for each operation?
The best cutting fluid is the one that handles the most demanding operation on the machine and the most sensitive metal that goes through it. A few terms before the table. The cutting edge is the sharp edge of the tool. Lubricity is the fluid's ability to reduce friction between chip and tool. EP stands for extreme pressure additive, which forms a protective layer on the cutting edge when the load becomes too high for the oil. Ferrous metals are steel and cast iron; non-ferrous metals contain no iron, such as aluminum, copper, brass and bronze (the last three are the yellow metals).
| If your plant has… | Type of fluid | Why |
|---|---|---|
| Threading, broaching (cutting slots and profiles with a broach, a tool with stepped teeth), slow and heavy cutting in steel | Straight oil, or a high-lubricity soluble oil with EP | In a slow cut the cutting edge works under high pressure. Lubrication decides, reinforced by the EP additive |
| Thread rolling in brass (thread formed by deformation, with no chip); machining copper and bronze | Copper-compatible straight oil, with no active sulfur | It does not stain or attack yellow metals. Active sulfur is what to avoid here; the copper strip test confirms it |
| General-purpose turning, drilling and sawing, in ferrous and non-ferrous metals | Mineral soluble oil or semi-synthetic | The operation calls for lubricating and cooling at the same time |
| Machining aluminum and its alloys | Ester-based soluble fluid (a lubricant made by the chemical reaction of an acid and an alcohol), boron-free | Lubricates aluminum well, with the cooling of water |
| Grinding and high-speed cutting, in most cases | Synthetic or semi-synthetic | Heat matters more than friction: maximum cooling and a part you can see through the fluid |
| Thread and gear grinding, grinding with a CBN wheel (cubic boron nitride, a superhard abrasive), honing (fine finishing of bores with abrasive stones), automatic lathe | Low-viscosity straight oil | Exceptions to the water rule: thread and tooth profile, wheel life and bore finish depend on lubrication. With oil at high speed, the machine needs mist extraction and fire protection |
| Grinding and light machining of carbide (tungsten carbide bound by cobalt, the material of inserts and drills) and cobalt alloys | Synthetic resistant to cobalt leaching | Avoids cobalt leaching (the fluid dissolving the cobalt), which weakens the tool |
| Storage that needs a fluid outside the flammable and combustible classes of NR 20 (the Brazilian standard for flammable and combustible liquids), or a workstation where the health hazard report under NR 15 Annex 13 (chemical agents) weighs | Straight oil with a high flash point, or a fluid without mineral oil | The first simplifies storage; the second takes mineral oil out of the report. Details in the safety section |
| End customer that has banned boron, nitrite or triazine | Fluid free of the banned substance | Each restriction has its own reason, explained in the safety section |
Kelpen Oil has a complete range of metalworking fluids (straight oil, soluble, semi-synthetic and synthetic), and the Kelpen Oil Technical Team recommends the right product for your operation and your metal.
When one machine runs different jobs from the same tank, the choice follows the most critical job. A single brass part in the routine is enough to require a copper-compatible fluid.
What are the types of cutting fluid in machining?
There are four: straight oil, used neat, and three water-miscible fluids, which are mineral soluble oil, semi-synthetic and synthetic. The international classification of metalworking fluids (ISO 6743-7) uses the same split between fluids used neat and fluids mixed with water.
| Type | What it is | Where it works best | In the Kelpen Oil range |
|---|---|---|---|
| Straight oil | Neat oil, no water | Severe and slow operations, fine finishing, threading and broaching, where lubrication decides | Keen Cut line, with chlorine-free options |
| Mineral soluble oil | Mineral oil that forms a milky emulsion when mixed with water | General machining, where the fluid must lubricate and cool at the same time | Micro Corte and Keen Sol lines, with chlorine-free soluble oils |
| Semi-synthetic | Part mineral oil, part chemical; in water it forms a microemulsion, with much smaller droplets | Balance between lubricity and cleanliness; machining and grinding on a range of machines | Keen Sol line |
| Synthetic | Chemical solution, no mineral oil | Grinding and high speed, where cooling and part visibility decide | Keen Sol line |
From mineral soluble oil to synthetic, the amount of oil in the mix goes down. Lubricity goes down with it, while cooling, cleanliness and transparency go up. No type is better than the others in absolute terms: the right one is the one the operation calls for. Mineral soluble oil is the familiar soluble oil, the white emulsion seen on many lathes.
Quenching oil, used after machining to harden steel, belongs to a different family with a different job: cooling the part at a controlled rate. That topic is covered in which oil to use for quenching.
Why does the operation decide between oil and water?
Because, in most cases, cutting speed decides whether the fluid can reach the point of friction: in a slow cut it gets in and lubricates; in a fast cut it can do little more than cool from the outside.
The cutting edge removes the chip by deforming the metal, and the chip slides under pressure along the face of the tool, which leads to heat in two zones: where the metal deforms and where the chip rubs against the tool (Figure 1).
In a slow cut, as in threading and broaching, the fluid has time to enter at the edge of the contact between chip and tool, which leads to less friction, less built-up edge (metal from the part sticking to the cutting edge) and a better finish. Here lubrication decides, and straight oil, being neat oil, lubricates best.
When the pressure is too high for the oil film (the thin layer that separates the two surfaces), the EP additive reacts with the hot metal and forms a protective layer at the contact point, which leads to less welding and tearing at the cutting edge.
In a fast cut, as in high-speed turning and most grinding, the chip moves too quickly for the fluid to enter the contact, which leads the fluid to work mainly from the outside, taking heat away from the tool, the chip and the part. Here cooling usually decides.
Water holds about twice as much heat per kilogram as mineral oil (specific heat of 4.18 versus about 1.9 kJ per kilogram per degree) and conducts heat about four times better (thermal conductivity of 0.61 versus about 0.145 W per meter per degree), which leads water-based fluids to cool better.
If that heat is not removed, the cutting edge loses hardness and the part expands, which leads to a worn tool, dimensions out of tolerance, scrap and early tool changes.
The rule has well-known exceptions. In thread and gear grinding and in grinding with CBN wheels, low-viscosity straight oil remains the choice even at high speed, because lubrication preserves the profile shape and wheel life. In honing, which runs at low speed, the thin oil lubricates the abrasive stones and flushes the particles out of the bore. On automatic lathes, which perform several operations in the same cycle, threading included, straight oil covers the most demanding operation. Straight oil in high-speed grinding forms mist: the machine needs an enclosure, mist extraction and fire protection.
How this shows up in your plant: torn thread flanks (the flank is each side of a thread turn), burrs (leftover metal on the edge of the part) and metal stuck to the cutting edge usually point to a lack of lubrication. A part that comes out hot and changes size as it cools, a tool that loses its edge early and burn marks (discoloration) on a ground part usually point to a lack of cooling. Smoke and mist rising from straight oil usually point to low flow, oil that is too thin or volatile for the operation, or neat oil where the speed calls for water.
Figure 1. The cutting zone at two speeds: in a slow cut, the fluid enters at the edge of the contact between chip and tool and lubricates; in a fast cut, it has no time to get in and works from the outside, taking the heat away.
Which cutting fluid should you use for each metal?
The metal sets the chemistry of the fluid, and this is the rule that prevents the most expensive losses.
Copper, brass and bronze: fluid with active sulfur (sulfur free to react with the metal) stains and attacks yellow metals. The objective check is the copper strip corrosion test (NBR 14359, a Brazilian ABNT standard equivalent to ASTM D130). The scale runs from class 1 (slight tarnish, split into 1a and 1b) to class 4 (corrosion): for yellow metals, the closer to 1a, the milder the attack. Inactive sulfur (chemically bound) reacts little with copper. At Kelpen Oil, the Keen Cut line has straight oils compatible with copper and its alloys, free of chlorine and heavy metals; the Kelpen Oil Technical Team recommends the one that fits your operation.
Aluminum: ester-based soluble fluids, free of boron, are the usual choice. With extreme pressure additives, in many cases they can take the place of a straight oil while keeping the cooling of water. The Keen Sol line has boron-free options.
Carbide and cobalt alloys: the wrong fluid can leach cobalt, that is, dissolve the metal that binds the grains of carbide. The tool loses strength and the dissolved cobalt contaminates the fluid. For grinding and light machining of these materials, the choice is a synthetic fluid resistant to cobalt leaching; the Kelpen Oil Technical Team recommends the right fluid.
Which safety and chemical requirements change the choice?
Two Brazilian occupational regulations and the end customer's chemical restrictions can rule out whole types of fluid before performance even enters the equation.
NR 20: the Brazilian regulatory standard for flammable and combustible liquids. It defines a flammable liquid as one with a flash point (the lowest temperature at which the liquid's vapor ignites for an instant when a flame is brought close) up to 60 °C, and a combustible liquid as one with a flash point above 60 °C and up to 93 °C. The standard sets storage and handling rules for facilities. The thinner the oil, the lower its flash point tends to be: very thin straight oils, such as honing oils, can fall into the combustible range. Check the flash point on the technical data sheet and safety data sheet of each fluid.
NR 15 Annex 13: the annex of the Brazilian health hazard standard that covers chemical agents. It lists handling mineral oils as a maximum-grade health hazard. A fluid without mineral oil falls outside that item of the annex; how the workstation is rated still depends on the health hazard report.
Boron, nitrite and triazine: each has its own reason for restriction. Boric acid is classified in the European Union as toxic to reproduction under the CLP regulation, which governs the classification and labeling of substances. Nitrite can react with the amines in the fluid (compounds used to control pH and corrosion) and form nitrosamines, which are carcinogenic; for that reason the German technical rule TRGS 611 bans nitrite in water-miscible fluids. Triazine is a biocide that releases formaldehyde, which is classified as carcinogenic in the European Union. The Kelpen Oil Keen Sol line has options free of boron, nitrite and triazine, for plants that need to eliminate these substances.
For the plant's occupational safety and health team, these facts carry as much weight as cutting performance.
What can replace cutting fluid?
In industrial machining, no alternative delivers the same overall result. Dry machining and MQL (minimum quantity lubrication, an oil mist applied directly at the cutting edge) are legitimate strategies for specific operations and materials, with dedicated tooling and parameters. The home-made recipes found online, such as kerosene, motor oil and improvised mixtures, trade a controlled input for corrosion, uncontrolled mist and skin risk for the operator.
People who ask what replaces cutting fluid usually want to know something else: how to make the fluid cost less. The answer lies in control. A fluid whose concentration, pH and contamination are monitored lasts longer in the machine and protects the tool better.
How do you know the cutting fluid you chose is right?
By testing, before and during use. Kelpen Oil's in-house laboratory in Arujá (SP) runs 25 tests under ASTM, NBR and ISO standards, described in lab tests, standards and instruments. Six of them answer the questions in this article:
| What we measure | What it reveals | Standard (instrument) |
|---|---|---|
| Copper strip corrosion, 3 h at 100 °C | Whether the oil stains copper, brass and bronze: the verdict for a straight oil that will machine yellow metals | NBR 14359 (ASTM D130) |
| Separation of oil and water | How long the oil takes to separate from water after being stirred; in a straight oil, it shows whether the oil sheds the water that gets into the tank | NBR 14172 (ASTM D1401) |
| Foaming characteristics, sequences I, II and III (three stages, at 24 °C, 93.5 °C and 24 °C again) | The oil's tendency to foam and how quickly the foam collapses; how the mix behaves in the machine also depends on the water and the concentration | NBR 14235 |
| Boron, barium, calcium, phosphorus and zinc content, by ICP (atomic emission spectrometry, which measures metals and additives in parts per million) | The presence of boron, useful for checking that a fluid sold as boron-free really is | NBR 14786 (Optima 7000 DV) |
| Sulfur content, by X-ray fluorescence | How much sulfur the straight oil carries; whether that sulfur attacks copper is shown by the copper strip | ASTM D6481 (Epsilon 1) |
| Viscosity at 40 °C | The grade of the straight oil (its viscosity class), which decides drag-out on parts and chips, mist and penetration into the cut | ASTM D7042 (SVM 4001) |
Kelpen Oil has manufactured lubricants and base oils since 1999, and the same laboratory that approves every batch produced serves outside companies through a paid analysis service. The analysis answers the question a brochure cannot: is the fluid running in your machine today still the one that was approved?
Frequently asked questions
What is cutting fluid used for? It reduces friction between tool and chip, removes heat from the cutting zone, flushes chips away and protects part and machine against corrosion. Without it, the tool lasts less and the finish gets worse.
What is the difference between cutting oil and cutting fluid? Cutting fluid is the name for the whole group, with and without water. Cutting oil usually refers to straight oil, used neat; mineral soluble oil is also called soluble oil because it is an oil made to be mixed with water.
Which cutting fluid should be used for stainless steel? Hard-to-machine alloys such as stainless steel call for strong extreme pressure additives: as a rule, straight oil with EP for threading and slow operations; soluble or semi-synthetic fluid with EP for turning and drilling. The right product depends on the material and the operation, and the Kelpen Oil Technical Team confirms the specification, including when the same machine also processes copper, brass or bronze.
Does cutting fluid go bad? Yes, in service. Contamination, additive depletion and microbial growth change performance with no visible warning. Regular checks of concentration and pH, combined with laboratory analysis, define the real service life.
Glossary
Chip: the sliver of metal the tool removes from the part.
Straight oil: cutting fluid used neat, with no water; in the plant, also called cutting oil.
Emulsion and microemulsion: oil dispersed in water as droplets; an emulsion (mineral soluble oil) looks milky, and a microemulsion (semi-synthetic) has smaller droplets.
Lubricity: the fluid's ability to reduce friction between chip and tool.
Extreme pressure (EP) additive: an additive that reacts with hot metal and forms a protective layer when the load breaks the oil film.
Built-up edge: metal from the part that sticks to the cutting edge, common in slow cutting, and spoils the finish.
Active and inactive sulfur: sulfur free to react with the metal (active, stains copper) or chemically bound, so it reacts little with copper (inactive).
Cobalt leaching: the fluid dissolving the cobalt that binds the grains of carbide.
Talk to the Technical Team
The Kelpen Oil Technical Team reviews the material being machined, the machine's operations, cutting severity, the hardness of the dilution water and the chemical restrictions of the process, and recommends the right product for each operation and material, with no obligation. If the fluid is already in use, the Arujá (SP) laboratory performs an analysis of the fluid in use, as a paid service, and delivers an objective diagnosis. Save this article or send it to whoever specifies machining fluids at your company.
