Cutting fluid is the liquid sprayed where the tool cuts the metal. It does four jobs: it cools the tool and the workpiece, lubricates the contact with the chip, carries the chips away and protects the workpiece from rust. Which job matters most depends on the pace of the cut: in fast cutting, cooling; in slow, forceful cutting, such as tapping a thread, lubrication. When a job fails, the sign shows up on the workpiece or the tool, and this article teaches you to read it.
What is cutting fluid?
Cutting fluid is the liquid that wets the exact point where the tool removes metal from the workpiece. Think of a drill bit that comes out of a wall hot, with dust all around: in a plant the same thing happens all day long.
This work is called machining: shaping metal by removing chips with a tool, as on the lathe, the milling machine and the drill press. The tool's cutting edge is the part that cuts. The piece of metal it removes is the chip, and the metal that stays is the workpiece.
The fluid arrives as a jet aimed at the spot where the edge meets the workpiece, runs into a tank under the machine, and a pump sends it back. It comes in two families:
Straight oil: oil used pure, with no water.
Water-mixed fluids: they arrive as a concentrate that is mixed with water at the machine. There are three: soluble oil, which turns white like milk; semisynthetic, with less oil, almost clear; and synthetic, with no petroleum oil, usually clear.
Oil slides better and water carries more heat, so the families split the jobs differently. The types are in the guide to cutting fluid for industrial machining, and how to prepare the white mixture is in the article on soluble oil for machining.
On the shop floor, "cutting oil" is usually the name for straight oil. Further on it will matter whether the oil is thin or thick, which is called viscosity (how much it resists flowing): water is thin, honey is thick.
If the tool only cuts, where does so much heat come from?
Why does cutting fluid need to cool?
Because cutting metal generates a lot of heat, and that heat softens the tool and throws the workpiece out of size.
Bend a wire back and forth and the bend heats up: deformed metal turns into heat. In cutting, heat is born in three places:
In the shear zone, the strip just ahead of the edge where the metal is crushed. One layer slides over another, like cards in a deck pushed sideways, until it breaks free and becomes a chip; this sliding of layers is called shear. Most of the heat is born here.
On the rake face of the tool, where the chip leaves scraping along it. This is the source that heats the tool itself the most.
On the flank, the part of the tool just below the edge, which rubs against the freshly cut surface.
In turning and milling, most of this heat leaves with the chip. In grinding it is the opposite: each grain of an abrasive wheel removes a tiny chip, and most of the heat stays in the workpiece. What stays in the tool and the workpiece is what the fluid has to carry away.
Figure 1. The three points where heat is born; in turning and milling most of it leaves with the chip, and the fluid carries away the rest.
The tool is usually made of high-speed steel, a special steel common in drill bits, or of carbide, an insert much harder than steel. What the heat does:
The hot edge softens, and a high-speed steel edge softens before a carbide one. This leads to fast wear: the flank wears down, and the rake face gets a crater, a shallow hollow behind the edge where the hot chip drags material away.
The hot workpiece expands, like the metal lid of a jar that loosens under hot water. This leads to a part that measures right on the machine and comes out smaller after cooling, outside the tolerance, the range of error the drawing allows.
The two together lead to scrap and to tool changes before their time.
Figure 2. Hot, the shaft fits the tolerance; cold, it shrinks and falls outside it. The red triangle marks where the shaft reading lands relative to the green band.
Here water-mixed fluid has the edge, because water stores and conducts more heat than oil. In fast cutting, the fluid barely enters the contact between the chip and the tool: it cools from the outside, wetting everything around. That is why flow rate and nozzle aim count as much as the product.
How this shows up in your plant: the workpiece size drifts from the start to the end of the shift, the high-speed steel tool comes out bluish, and the chip changes color from what is normal for that operation.
For those who need the number: when machining steel at high speed, the contact between the chip and the tool exceeds 700 °C (Trent and Wright, Metal Cutting; Machado et al., Teoria da Usinagem dos Materiais). Steel expands about 0.012 mm per meter for each degree Celsius. A 100 mm shaft measured 20 °C above room temperature shrinks 0.024 mm as it cools. That is more than the whole IT6 tolerance of ISO 286-1 for this diameter range (80 to 120 mm), which is 0.022 mm.
Cooling takes care of the heat. The friction of the chip against the tool is still left.
How does cutting fluid lubricate the tool?
The fluid forms a film between the chip and the tool that separates the two metals and keeps the chip from sticking to the edge.
It is the drop of oil that stops a hinge from squeaking. On the tool, the chip is pushed against the rake face with enormous pressure, and the fluid has to enter from the outer edge of this contact, where the chip moves away from the face. It enters better in slow cutting, because it has time to get in before the chip passes.
There it reduces the area where the chip sticks; in the middle of the contact, where the chip is stuck to the face, the fluid does not reach at any pace.
A fact about every cut: the chip leaves harder than it went in, because it was crushed, like the paper clip that hardens after being bent many times. This happens with or without fluid.
That hardened metal is hard enough to cut in place of the tool. Without the film, in slow and forceful cutting, this is what happens:
Bits of this hardened metal stick to the edge, welded by pressure and heat, and form the built-up edge, a false tip that starts cutting in place of the real one.
It grows and breaks off in pieces, which stay stuck to the workpiece or tear chips out of the tool. This leads to a rough finish, burrs (leftover metal on the edge) and irregular size.
The built-up edge is typical of low to medium speed and of soft metal that sticks, such as aluminum and low-carbon steel, the softest common steel.
To avoid it, the fluid carries additives, chemical products that reinforce what the oil does. At low pressure, the oiliness additive, a fatty compound, sticks to the metal and forms a slippery film.
When heat and pressure break this film, the extreme-pressure (EP) additive takes over, usually sulfur-based (there are also phosphorus-based ones and, less and less, chlorine-based ones). It reacts with the hot metal and forms a thin layer that slides easily, renews itself on every pass and prevents welding.
Figure 3. Without a film, the built-up edge forms and the workpiece comes out rough; with the EP additive, the chip slides and the workpiece comes out smooth.
How this shows up in your plant: a thread with torn ridges (the ridge is the spiral crest of the thread), a tap (the tool that cuts a thread inside a hole) broken in the hole, a surface that looks torn, and metal stuck to the tool tip, very common in aluminum.
For those who need the reference: the built-up edge disappears when the cutting speed and the temperature of the cutting zone rise enough (Trent and Wright, Metal Cutting; Machado et al.).
And the chip, once it leaves, where does it go?
How does cutting fluid carry the chip away?
By the drag of the jet, like a hose washing sand off the sidewalk toward the drain. The fluid carries the chip to the tank, where a filter or a settling tank (a box where the dust settles to the bottom) separates it. This matters in three cases:
Chip in the way: it is cut again and scratches the workpiece.
Deep-hole drilling, a hole much longer than it is wide: a chip that does not get out clogs the drill flutes, which makes the drill jam and break. There the fluid, often oil, enters at high pressure through the inside of the drill itself and pushes the chip out.
Grinding, the finishing work with the wheel, a disc of hard abrasive grains that works like a round sandpaper: fine metal dust gets into the pores of the wheel, and the fluid washes it out.
Figure 4. With fluid through the inside of the drill, the chip leaves; without it, the flute clogs and the drill jams.
The tank has to hold back this dust, or the fluid goes back to the workpiece dirty.
How this shows up in your plant: a spiral scratch on the workpiece, chips piling up on the table, a drill broken in a deep hole and a clogging filter.
Water-mixed fluid cools and cleans. But doesn't water on top of steel make it rust?
How does cutting fluid protect against corrosion?
With additives that form a film over the metal and keep water and air away.
A wrench left wet in the yard is rusty by morning, because rust needs iron, water and oxygen from the air. Water-mixed fluid brings the water to the workpiece; without protection, it would be the cause itself. That is why every water-mixed fluid carries a corrosion inhibitor, an additive that sticks to the metal and forms a thin film against water and oxygen.
The mixture is also kept alkaline, the opposite of acidic (soap is alkaline; lemon is acidic), which helps the inhibitor. Like any standing liquid with water, the tank can grow bacteria, which remove the alkalinity of the mixture and weaken this protection.
The protection covers the workpiece, the machine table and the ways, the guides along which the machine carriage slides. Straight oil, with no water, protects through the oil itself covering the metal.
Figure 5. Without an inhibitor, the drop rusts the steel; with the film, the metal stays clean.
Copper, brass and bronze, the yellow metals, need a fluid that does not stain them. In straight oil, the risk is active sulfur, the kind in the EP additive that reacts easily with the metal. In water-mixed fluid, it is the amines, ingredients that make the mixture alkaline and protect steel, but can attack copper.
This protection covers the time inside the factory, between one operation and the next. A workpiece that goes to storage or shipping usually gets its own protective oil.
How this shows up in your plant: rust on the workpiece hours after machining, a stain on the table, and a dark stain on a brass part.
For those who need the standard: attack on copper is measured by the copper strip corrosion test, NBR 14359 (equivalent to ASTM D130). It was written for petroleum products, so it applies to straight oil; on the soluble concentrate it works only as an indication. For rust from the fluid mixed with water, the public reference is the cast iron chip test on filter paper, DIN 51360-2 (similar to ASTM D4627).
With the four jobs clear, what is left is which one weighs most on your machine.
What is cutting fluid for in each operation?
In every operation the fluid performs the four jobs, but the weight changes with the pace: the faster the cut, the more cooling leads; the slower and more forceful the cut, the more lubrication leads. Clearing the chip and protecting always apply.
In turning, the workpiece rotates and the tool cuts it; in milling, the tool rotates and the workpiece moves against it.
| Operation | Pace of the cut | Job that leads | Why |
|---|---|---|---|
| Common grinding, in most cases | Fast | Cooling and washing the wheel | The wheel grains cut little at a time and rub a lot, and most of the heat stays in the workpiece |
| General-use turning, milling and drilling | Medium to fast | Cooling and clearing the chip, with some lubrication | Heat matters more than pressure on the edge |
| Tapping, reaming (finishes a hole that already exists) and broaching (a long tool with stepped teeth that passes once) | Slow and forceful | Lubricating and avoiding the built-up edge | High pressure on the edge; the finish depends on a clean edge |
| Honing (finishing a hole with abrasive stones) | Slow | Washing the stone and lubricating just enough | The stone loads up with metal if it is not washed, and glazes if the oil is too thick |
Figure 6. From slow to fast cutting, lubrication loses weight and cooling gains; thread and gear grinding is an exception.
Thread and gear grinding is an exception: fast, but it needs lubrication, with thin straight oil.
Another exception is milling with carbide. The edge enters and leaves the cut on every turn, heating inside and cooling outside; a jet that does not cover it evenly increases this shock and can crack it. That is why this operation often runs dry or with very little oil mist.
The job that leads pulls the family with it: where cooling leads, usually a water-mixed fluid; where lubrication leads, straight oil or a high-lubricity water-mixed fluid, with EP. The choice by metal and by safety is in the guide to cutting fluid for industrial machining.
Kelpen Oil formulates a complete line of metalworking fluids for machining (straight, soluble, semisynthetic and synthetic), with options free of chlorine, boron and nitrite (substances some plants want to avoid in their fluids). The Technical Team helps weigh the operations on your machine.
Honing also uses an abrasive. There, what does the fluid do?
What is the function of cutting fluid in the honing process?
In honing, the fluid washes the abrasive stone and lubricates, just enough, its contact with the bore.
Honing is the fine finishing of precision bores, such as the engine cylinder and the hydraulic valve body. Abrasive stones held on a tool rotate and move up and down inside the bore, remove very little metal and leave fine cross-hatch marks. It is the wet-and-dry sandpaper of the auto body shop, used wet so it does not clog.
Here viscosity matters. The two jobs that lead:
Washing. Metal dust and loose grains leave the pores of the stone. Without washing, the pores fill with metal (loading), and the stone stops cutting and starts rubbing, which leads to heat, scratches and a bore out of shape.
Lubricating just enough. The oil controls the friction and the heat between the stone and the bore. The stone sharpens itself: worn grains break loose and expose new, sharp grains. With oil that is too thick, it cushions the contact, the worn grains do not break loose, and the stone becomes smooth and stops cutting (glazing).
That is why honing uses thin straight oil: it enters the pores, leaves with the debris, does not cushion the contact too much and stays less on the workpiece. In short: loading is pores full of metal, from too little washing; glazing is a smooth stone; on the fluid side, the cause is oil that is too thick.
Figure 7. The stone in the bore and its three states: thin oil, no washing and oil that is too thick.
For those who need the number: honing usually removes hundredths of a millimeter. The grade of the honing oil is read by the ISO VG class of ISO 3448, explained in the article on what ISO VG means.
And day to day, how do you tell which job is failing?
What happens when cutting fluid fails?
The sign shows up on the workpiece or the tool before any analysis, and each sign points to a job.
In water-mixed fluids, many causes go through concentration: how much concentrate goes into the water of the mixture, as a percentage. A concentration below what the supplier recommends takes away lubrication and protection at the same time. How to check it is in the article on soluble oil for machining.
| Symptom on the machine | Job that failed | Most common cause |
|---|---|---|
| Size that varies during the shift; burn mark on a ground workpiece | Cooling | Low flow, nozzle out of aim, weak mixture, or straight oil where the operation calls for water |
| Torn thread, burrs, metal stuck to the edge | Lubricating | Fluid without EP for the operation, mixture too weak |
| Spiral scratch, drill broken in a deep hole, clogged filter | Clearing the chip | Weak jet, tank with chips floating in the fluid, saturated filter |
| Rust on the workpiece or the table | Protecting | Weak mixture, water with a lot of dissolved salt, bacteria that remove the alkalinity of the mixture |
| Foam overflowing from the tank | Cooling and clearing the chip | Wrong mixture, pump pulling in air (foam is air, which carries little heat) |
| Smoke and mist in straight oil | Cooling | Oil too thin and volatile for the speed (it evaporates at the hot edge), low flow; without extraction, the mist stays in the air |
The symptom tells you where to look; the laboratory confirms.
How does Kelpen Oil check each function of cutting fluid?
Kelpen Oil formulates machining fluids and checks them in its own laboratory in Arujá (SP), which runs 25 analyses. Three tie in with the jobs in this article:
| What we measure | What it reveals | Standard |
|---|---|---|
| Viscosity at 40 °C (SVM 4001 viscometer) | Whether the straight oil is thin enough to enter the contact and wash, without leaving in excess stuck to the workpiece and the chip (lubricating, honing) | ASTM D7042 |
| Sulfur by X-ray fluorescence | The total sulfur of the new straight oil, base and additive together, only as an indication, and only in chlorine-free straight oil; it shows whether there is a sulfur additive, but does not say whether it is active, which stains copper and is seen on the copper strip (lubricating) | ASTM D6481 |
| Copper strip corrosion, 3 hours at 100 °C | Whether the fluid stains copper, brass and bronze; on the soluble concentrate, as an indication (protecting) | NBR 14359 |
For fluid already in use, the analysis is a paid service, with the tests the Technical Team recommends. The full list is in the article on the lubricant laboratory and its tests.
Frequently asked questions
What replaces cutting fluid? For general use, nothing delivers the same result. In specific operations and metals there is dry machining and MQL (minimum quantity lubrication, a little oil blown as a mist straight onto the edge); both lose the chip washing and most of the cooling.
Can you make homemade cutting fluid? For industrial use, not with the same result. An improvised mixture has no corrosion inhibitor, no EP additive and no bacteria control: it separates in the tank, rusts the workpiece and exposes the operator's skin.
Does all machining need cutting fluid? No. Gray cast iron is often machined dry, because graphite (carbon in flakes inside the metal) helps lubricate and the chip comes off as powder; and a lot of carbide milling runs dry so the edge does not crack. The decision weighs tool life, workpiece size and machine cleanliness.
Talk to the Technical Team
The Kelpen Oil Technical Team reviews the most demanding operation, the metal, the speed, the machine and the water available, and recommends the right fluid. If the fluid is already in use, the Arujá (SP) laboratory runs the analysis of the fluid in use, as a paid service.
Before changing fluids, note the symptom and the operation where it appears: the symptom table tells you which job failed and where to start. Save this article or send it to whoever looks after machining in your company.
