Soluble oil is the cutting fluid (the liquid sprayed where the tool cuts the metal) that you buy as a concentrate and mix with water; in general, it turns white like milk. The water cools the tool and the part; the oil, in tiny droplets, lubricates the cut; the additives protect the part from rust. To work well, the mix is made by pouring the oil into the water, never the reverse, and is checked often, because it changes with use.
What is soluble oil used for?
Soluble oil is used in machining where cooling matters as much as lubricating, or more: general work on lathes, milling machines, drills and saws.
Think of a drill bit that comes out of the wall hot after one hole. Machining is the same, only without stopping: the tool tears a chip from the metal, and the heat gathers at its edge, the part that cuts. Too much heat wears the tool out early and changes the size of the part.
The metals it works on:
Steel, the most common case.
Cast iron, with one caveat: the fine dust it gives off rusts and turns into sludge, so the fluid has to protect well and the tank needs good filtration.
Stainless steel, in turning (cutting on the lathe), drilling and milling; in threading and heavy, slow cutting, straight oil (used pure, without water) usually does better.
Non-ferrous metals, the metals without iron (aluminum, copper, brass and bronze). For these, the fluid chemistry has to be compatible: a very alkaline fluid (the opposite of acidic), for example, stains aluminum. The product sheet and the Technical Team say whether it fits.
Inside the mix, each part has a role. The water does the cooling, and the jet carries the chips away from the cut. The oil and the additives lubricate the contact between chip and tool and protect the part from the rust that the water itself would cause. These are the four jobs of every fluid (cooling, lubricating, cleaning and protecting), explained in the guide to cutting fluid for industrial machining.
It stops being the first choice in slow, heavy cutting, such as threading and broaching (cutting done by a long tool with many teeth that passes once through the part). There the pressure on the edge matters more than the heat, and straight oil usually takes over.
But if it is oil, how does it mix with water?
Does soluble oil dissolve in water?
Strictly speaking, no: it spreads in the water as droplets too small to see.
Think of a salad dressing made with olive oil and vinegar. Shaken, it turns cloudy for a moment; left alone, the oil rises and forms a layer, because the droplets touch and join.
Milk is also fat in droplets inside water, and still it does not separate in the glass. The difference is that each fat droplet has a coat that keeps it from joining the others.
This mix of oil droplets spread in water, with a coat on each droplet, is called an emulsion. In soluble oil, the coat comes from the emulsifier, an additive that works like dish soap on a greasy pan: each of its molecules has one end that likes water and another that likes oil.
The path, from drum to tank, goes like this:
The concentrate falls into the water under stirring and breaks into droplets.
The emulsifier covers each droplet, with the oil-loving end inward and the water-loving end outward.
With this coat, the droplets repel each other and do not join, which leads to a mix that stays uniform in the tank for a long time.
Droplets of this size scatter light in every direction, which leads to the milky white look.
When something damages the coat (hard water, with a lot of calcium and magnesium; bacteria; wear of the additives with use), the droplets merge, the oil rises and forms a layer, as in the dressing left standing. This is the broken emulsion, and it does not come back with stirring.
A contrast helps to recognize the problem in the tank. In raw milk, straight from the cow, the fat rises and forms cream when it stands; those are droplets that rose without merging, and a shake returns them to the mix. In a broken emulsion, the droplets have already become a single layer, and no shaking undoes that.
Figure 1. With the emulsifier coat, two oil droplets touch and move apart without joining; without the coat, they merge into a larger droplet, which is the start of a broken emulsion.
For those who need the number: in the ordinary cutting-fluid emulsion, the droplets measure a few micrometers, that is, a few thousandths of a millimeter (ScienceDirect Topics, "Microemulsion"; T. Norrby, STLE, 2017).
The emulsifier, however, is only one of the ingredients of the concentrate.
What is soluble oil made of?
In general, a mineral base oil, the refined petroleum oil that forms the base of almost every lubricant, plus an additive package. There are also soluble oils with an ester base, an oil of vegetable or synthetic origin. Base oil has its own article, about what base oils are.
These are the main groups in a concentrate (the full list varies from formula to formula):
Base oil (usually mineral): lubricates the cut and forms the droplets.
Emulsifiers: keep the droplets apart, with the coat you have just seen.
Corrosion inhibitors: protect the part, the table and the ways (the guides on which the machine carriage slides) from rust.
Lubricity and extreme-pressure (EP) additives: lubricity additives reduce friction; EP additives react with the hot metal and form a thin layer that protects the tool edge in the heaviest operations.
Alkaline part: ingredients that make the mix alkaline, the opposite of acidic. This helps the inhibitors against rust, slows bacteria and keeps the emulsion stable. This alkaline part works as a reserve, the alkaline reserve, which is used up with use; further on you will see how.
Defoamer: cuts the foam that the jet and the pump make.
Microbe control: biocides, that is, bactericide and fungicide, or a formula that resists them, because water is where bacteria and fungi grow.
In the machine, the ready mix is almost all water, and the concentrate is the small part. The proportion of oil, additives and water changes from one type of fluid to another, and this recipe is what separates the families in the next section.
For those who need the number: according to the OSHA best-practices manual for metalworking fluids (OSHA is the United States workplace safety agency), the soluble oil concentrate contains 30% to 85% of highly refined base oil. Along with it go emulsifiers, EP additives, corrosion inhibitors, defoamers and bactericides. The base oil is usually naphthenic or paraffinic, the two types explained in the article on base oils.
What is the best soluble oil for machining?
The best one serves the most demanding operation on the machine, the metal of the part and workplace safety, without the substances that your customer (the one who buys the part) prohibits. And it has to be well cared for in the tank.
On the shop floor, "soluble oil" became the name of three fluids that contain water, separated by recipe and droplet size. To see where each one sits, the list shows the four types of cutting fluid, with straight oil, which has no water, only for comparison:
Straight oil: pure oil, without water, the one that lubricates most.
Soluble: the soluble oil of this article, with more oil and droplets of a few micrometers; the mix turns white.
Semisynthetic: less oil and much smaller droplets, which is called a microemulsion; the mix goes from slightly milky to almost transparent.
Synthetic: no mineral oil, with ingredients that truly dissolve in water, like sugar. This is a solution, and the mix is usually transparent.
From straight oil to synthetic, lubrication drops, and cooling, cleaning and the view of the part go up. In grinding, the finishing done with the grinding wheel (an abrasive disc that works like a round sandpaper), the wheel gets very hot and cuts little at a time, so cooling and seeing the part matter more than lubricating. That is why most grinders use semisynthetic or synthetic.
Figure 2. Droplet size changes the color: in the soluble, the larger droplets scatter the light and the mix turns white; in the semisynthetic, much smaller droplets let part of the light through; in the synthetic, which has no droplets because it is dissolved, the light passes through.
The choice follows four steps:
The most demanding operation on the machine chooses the type of fluid.
The metal chooses the chemistry: aluminum, copper and brass call for a fluid formulated for them, because the wrong chemistry can stain the metal (a very alkaline mix, for example, stains aluminum), and the product sheet or the Technical Team confirms.
Workplace safety and your customer's list rule out options. The mineral oil in the soluble may enter the plant's unhealthy-work report (the "laudo de insalubridade," the document in which a Brazilian company records the agents that harm health at a workstation; NR 15 is the Brazilian standard that defines unhealthy activities). If it does, there may be hazard pay and a protection requirement for the operator; the plant's safety officer assesses this. Boron, chlorine, nitrite and triazine (a bactericide) are among the substances that customers most often prohibit. The reasons are in the cutting fluid guide, in the section "Which safety and chemistry requirements change the choice?".
A test on your own machine confirms it, backed by the measuring routine this article shows further on.
Kelpen Oil formulates the complete line of metalworking fluids: straight, soluble, semisynthetic and synthetic. The Keen Sol line has options free of boron, nitrite and triazine, and the Micro Corte line has a chlorine-free soluble oil.
Talk to the Technical Team to find the fluid for your operation. The options are on the page for metalworking fluids, part of the Kelpen Oil metalworking products.
For those who need the standard: in ISO 6743-7 (family M, machining), fluids used pure form group MH, and those mixed with water, group MA. Group MA separates the milky emulsion, the translucent emulsion (microemulsion) and the clear solution, which correspond, in market language, to the soluble, the semisynthetic and the synthetic. In the microemulsion, the droplets measure less than about 100 nanometers, usually a few tens, that is, millionths of a millimeter (IUPAC, Gold Book, "microemulsion").
With the fluid chosen, it still has to be prepared.
How to use soluble oil?
Always diluted, with the oil poured onto the water.
The amount is given as concentration (how much concentrate goes into the mix, in percent): 5% means 5 liters of concentrate in every 100 liters of mix. Preparation has five steps:
Clean the tank and the mixing container. Remains of old emulsion, chips and sludge (the sticky mud that gathers at the bottom) bring bacteria into the new charge, the new mix that fills the clean tank. Do not mix different products in the same tank.
Calculate the concentrate: tank volume times the concentration the supplier indicates for the operation.
Use clean water of low to moderate hardness, in the range the product sheet indicates. The reason comes right below.
Put the water in the tank first and pour the concentrate slowly, under stirring: a mixer, a recirculation pump or a proportioner, the device that draws concentrate and water already in proportion.
Check the concentration before releasing the machine. How to check is in the next section.
Water matters because not all water is the same. The white crust in a kettle is the limescale that the calcium and magnesium of hard water leave behind. These salts react with a good part of the emulsifiers and form a soap that does not dissolve, which leads to an unstable or broken emulsion, to sludge in the filter and at the bottom of the tank, and to less rust protection. At the other extreme, water that is too soft, with very little calcium and magnesium, makes foam.
The order also has a reason. With the water underneath, the oil that falls in is always the small part and turns into droplets. When water is poured over the concentrate, at first there is little water inside a lot of oil, and the mix comes out the other way around: water droplets trapped in oil. This is the inverted emulsion, thick and greasy like butter, which is precisely water in tiny droplets trapped inside fat. Afterwards, not all of it breaks up properly, and lumps and floating oil remain on top of a mix weaker than the calculation predicted.
Figure 3. On the left, oil poured onto water becomes fine droplets and the mix turns white and uniform; on the right, water thrown onto oil forms the inverted emulsion, with cream, lumps and floating oil.
With the machine running, the tank loses mix in two ways: the water evaporates on its own, and the whole mix leaves stuck to the part and the chips.
Since only water evaporates, the mix left in the tank becomes more concentrated, stronger; that is why what you add is a mix weaker than the charge, and the concentration is checked every day, as the next section shows.
If the measured concentration falls below the target, raise it by topping up with a stronger mix prepared separately, never with pure concentrate, which forms cream and lumps where it lands.
The salts in the water also stay behind when it evaporates and build up with each top-up; that is why the makeup water must be within the range on the sheet.
Figure 4. Evaporation removes only water and leaves the salts in the tank; the part and the chips take the whole mix. That is why the top-up is weaker than the charge and the makeup water also counts.
For those who need the number: a 200-liter tank at 5% takes 10 liters of concentrate and 190 of water (an example calculation; the concentration comes from the fluid in use). The 1:20 ratio has two readings: 1 liter of concentrate in 20 liters of mix (5%) or 1 liter of concentrate for 20 liters of water (about 4.8%). The supplier's calculation and the operator's must be the same. The hardness of the water in the tank can reach nearly ten times that of the makeup water (Lubes'N'Greases, "Dos and Don'ts of Emulsifying Coolant", 2018; the OSHA manual also asks for makeup at the proper dilution).
The same verb appears in all these steps: check.
How to measure soluble oil concentration with a refractometer?
With a pocket device, the refractometer, which reads one drop of the mix, and a simple calculation that turns that reading into a percentage.
Put a straw in a glass of water: it looks broken, because light changes direction when it goes from air to water. The more is mixed into the water, the more the light bends. The handheld refractometer measures this bending in one drop: the more concentrated the mix, the higher the reading.
The device scale is in °Brix, a unit created to measure sugar in juices and syrups and used here as a reference reading. Since each fluid bends light in its own way, the reading is multiplied by the refractometer factor, the number for each product that turns °Brix into a percentage. It comes from the fluid supplier, in the technical sheet.
The measurement, step by step:
Zero the refractometer with the water used for the mix, with the device and the sample at the same temperature, or use a model with automatic temperature compensation (ATC), because the bending of light changes with heat.
Take the sample from the running tank, below the surface, and clean the prism, the glass window where the drop goes.
Read the line between the blue field and the white one.
Multiply by the factor of your product. The factor of another fluid gives the wrong concentration.
Figure 5. On the refractometer display, the line between the blue and white fields marks 4.0 °Brix; times the example factor of 1.25, it gives 5.0% concentration.
The most common error comes from tramp oil (also called foreign oil), the oil that leaks from the machine itself and falls into the tank. Part of it floats and escapes the sample, but part is spread into droplets by the mix itself, and that part bends the light in the refractometer: the reading comes out higher than the real one, and you end up dosing too little concentrate.
Off target, the damage shows in both directions:
Below: too little additive against rust and bacteria, which leads to rust, tools wearing out early and a tank that rots.
Above: foam, sticky residue, more risk of irritating the operator's skin and concentrate spent for nothing.
The second measurement is pH, which tells how acidic or alkaline the mix is, on a scale where 7 is neutral and above 7 is alkaline. Lemon is acidic; soap is the opposite, alkaline. The new mix is alkaline, like soap.
This helps the inhibitors hold back rust, slows bacteria and keeps the emulsion stable; when the pH falls, all three get worse together. In the other direction, a pH above the product range irritates the skin and stains aluminum.
pH is measured with a strip or a meter. The pH of the new charge, measured at working concentration and noted on the first day, becomes the reference for that tank. Compare today's pH with that of the new charge, always with both mixes at the same concentration, because pH changes a little with it. The right range for each product is on its sheet.
As for frequency: look at and smell the tank every day, and measure the concentration every day. pH is measured at least once a week, and more often when the tank gives a sign.
For those who need the number: concentration = reading in °Brix × product factor; in the example in Figure 5, 4.0 °Brix × 1.25 = 5.0% (illustrative factor, from no product). Old mix also reads higher than the real value, because it accumulates salts, tramp oil and residues. In that case, the reference is a chemical measurement in the laboratory: titration, which measures the alkalinity of the mix and, compared with that of the new product, gives the concentration; or acid splitting, which separates and measures the oil. The fluid supplier or a laboratory does it. Minimum frequency (HSE, the British health and safety agency, guide MW5): visual and smell inspection daily; concentration, pH and bacteria count weekly.
A day comes, however, when getting the concentration right no longer solves it.
When to replace soluble oil emulsion?
When the correction does not bring the tank back: pH that keeps falling after the concentration is right, a strong smell that returns, a mix that breaks, or rust with the concentration on target.
Most of the time, pH falls because of bacteria. As they feed on the fluid, they release acids, and those acids use up the alkaline reserve, the capacity of the alkaline part of the formula to hold the pH high even when acid comes in. While the reserve lasts, the pH hardly moves; when it runs out, the pH plunges.
Figure 6. While the alkaline reserve lasts, the pH stays near that of the new charge; when it runs out, the pH falls, and if correcting the concentration does not hold it, it is time to replace the emulsion.
Each sign has a measurement, a probable cause and an answer; the last column says when correcting is enough and when it is time to replace. The culture slide, mentioned in the table, is a small plate dipped into the tank that shows the bacterial colonies after a day or two.
| Sign in the tank | What to measure | Probable cause | What to do |
|---|---|---|---|
| pH falling day after day | pH compared with the new charge, concentration and bacteria count (culture slide) | Bacterial acids using up the alkaline reserve | Correct the concentration; if the pH keeps falling, replace |
| Rotten-egg smell after the weekend | pH, concentration and bacteria count (culture slide) | Bacteria without air, in the idle tank, release hydrogen sulfide (the rotten-egg smell) | Recirculate the tank during stops; if the smell returns, replace with cleaning |
| Oil layer that covers the tank again soon after being removed | Surface appearance | Too much tramp oil (see "How to measure") | Remove it with the skimmer (a device that collects oil from the surface) and find the leak; does not call for replacement |
| Oil on top that does not return to the mix when stirred | Concentration and water | Broken emulsion, from hard water, wrong concentration or bacteria | Remove the separated oil, correct the concentration with a stronger mix prepared separately and check the water; replace if it keeps separating |
| Rust with the concentration on target | pH and water | Bacteria, accumulated salts from the water (chloride and sulfate speed up rust) or used-up additive | Correct; if the rust returns, replace |
| Slime or sludge on walls, hoses and filter | Appearance of the circuit | Fungus and bacterial colonies, which clog the circuit and return if it is not cleaned | Replace with circuit cleaning |
At replacement time, the order is this:
Before draining, circulate a system cleaner in the old mix, a circuit-cleaning product that loosens the sludge from walls and hoses, for the time the supplier indicates.
Drain and rinse.
Only then prepare the new charge, because a dirty tank contaminates the new mix.
Dispose of the spent emulsion properly. It is oily effluent, water with oil that cannot go to the sewer or to a river without treatment and without authorization. The destination is the plant's own treatment station or a company licensed by the environmental agency.
For those who need the number: according to the OSHA manual, if the oil layer, after being removed, covers the surface again in less than 5 to 8 seconds, there is too much tramp oil. Brazilian Resolution CONAMA 430/2011, from the National Council for the Environment, art. 16, limits mineral oils to 20 mg/L in effluent discharged directly into a river or stream. A 5% emulsion carries about a thousand times that limit (from about 650 to almost 2,000 times, using the OSHA 30% to 85% oil range and oil at about 0.87 kg/L).
When neither the refractometer nor the pH explains what is going on, the answer comes from somewhere else.
How does Kelpen Oil help control the fluid?
Through the machine routine and, when it is not enough, through the Technical Team.
At the machine, the routine of this article: concentration with the product factor and pH compared with the tank reference. The Kelpen Oil Technical Team helps set up this routine and read what it shows.
When the tank does not explain the symptom, talk to the Kelpen Oil Technical Team. The in-house laboratory in Arujá (SP) runs 25 analyses, described in the article on laboratory analysis; for cutting fluid, the analysis is done on the concentrate, as a paid service; for the mix already in the tank, the Technical Team gives guidance.
For those who need the standard: the laboratory has the copper strip corrosion test (NBR 14359, equivalent to ASTM D130), which gives an indication about the concentrate; the mix in the tank is followed through the machine routine.
Frequently asked questions
What is synthetic soluble oil used for? The name comes from the shop floor: with no mineral oil, it forms a transparent solution with water and serves most grinders and light machining; the types are in the section on the best soluble oil.
What is machinist's soluble oil used for? It is the same soluble oil of this article, under the name it earned in the workshop: the mix that cools and lubricates lathes, milling machines and drills. Preparation is the same, with the oil onto the water and the concentration checked.
What is the difference between soluble oil and straight oil? Straight oil is used pure, without water, and lubricates more; soluble oil works diluted in water and cools more. Slow, heavy cutting usually calls for straight oil; fast, hot cutting usually calls for a fluid with water (soluble, semisynthetic or synthetic), depending on the operation.
Can you make homemade soluble oil? Not with the same result. Without emulsifier, corrosion inhibitor, alkaline part and bacteria control in the right amounts, the mix breaks in the tank, rusts the part and exposes the operator's skin to uncontrolled products. The cutting fluid guide explains the alternatives, in the section "What can replace cutting fluid?".
Glossary
Emulsion: oil droplets spread in water, each with a coat that keeps it from joining the others; in soluble oil, the mix turns white.
Broken emulsion: a mix in which the droplets have merged and the oil has risen as a layer; it does not come back with stirring.
Concentration: how much concentrate goes into the mix, in percent; 5% is 5 liters in every 100 liters of mix.
Hard water: water with a lot of calcium and magnesium, which reacts with a good part of the emulsifiers and forms sludge.
Refractometer: a pocket device that measures the bending of light in one drop of the mix; the reading in °Brix times the product factor gives the concentration.
pH: a measure of how acidic or alkaline the mix is; 7 is neutral and above 7 is alkaline.
Alkaline reserve: the capacity of the alkaline part of the formula to hold the pH high even when acid comes in.
Talk to the Technical Team
The Kelpen Oil Technical Team looks at the metals, the operations and the most demanding of them, the tank, the water available and the substances your customer prohibits, and recommends the right fluid for your case. If the emulsion is already in the machine, the Technical Team helps read the tank notes; the analysis of the concentrate in the Arujá (SP) laboratory is a paid service.
Before changing fluid, note for one week the concentration of each day and the pH on at least two days, with the smell and appearance of the tank: most problems show up in these notes. Save this article or send it to whoever prepares the machining fluid at your company.
