How to Select and Maintain Machine Coolant

March 29, 2019 1:40 pm

Coolant is a fundamental factor in the machining equation. Although not always the most pleasant substance to deal with, coolant serves three very important purposes:

  • Cooling tools
  • Lubricating the cutting process
  • Clearing chips

Investing in a good cutting fluid will allow you to machine parts smoothly and avoid issues but selecting the right one can be challenging. Let’s look at the different types of fluids available and their characteristics.

Types of Coolant

Coolant can be divided into two main categories: sulfur-based oil coolants (aka “neat” oils) and water-based coolants. Unless you are dealing with challenging machining operations that require very high cutting forces, you will most likely be using water-based coolant. This type of coolant is generally more pleasant to work with and is often more effective – as lubricant does a better job at removing heat than oil, especially when machining aluminum and other non-ferrous alloys.

Depending on the amount of oil they contain before they get mixed with water, coolants are then divided into three different categories: soluble oils, synthetic coolants, and semisynthetic coolants. Let’s discuss their differences.

Soluble Oils

This type of coolant typically contains 50% oil. It is the most natural type of coolant and therefore tends to be gentler on the skin. However, if not properly maintained, it can develop a bad smell due to the growth of bacteria in the sump.

Synthetic Coolants

Synthetics fluids, as the name suggests, contain no oil. They work well on exotic materials and are usually used for aerospace applications. They tend to be the clearest of all cutting fluids but are often the least lubricious and can be a little harsh on the skin due to their chemical composition.

Semisynthetic Fluids

Semisynthetic fluids are a combination of the first two types. They contain a mix of mineral oils and chemical compounds. They are easier to maintain, less prone to develop bad odor and have good lubricity.

Selecting the Right Coolant

Choosing the right coolant for your operations generally depends on several different factors. The material, tooling, and type of cut are all important things that you should consider when selecting a coolant to use in your machine.

The “human interaction” with the liquid is also something to keep in mind. In the end, you will be dealing with this liquid every day, so working with a coolant that is particularly harsh on the skin or has an unpleasant odor might not be the best choice.

Here are some of  coolants we recommend.

Jancy Slugger Water Soluble Cutting Fluid

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Aimed at improving surface finish and reducing heat buildup when drilling, this water-soluble concentrate can make up to 10 quarts of cutting fluid.

Sta-Lube SL2513 Soluble Oil 

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Sta-Lube is great for multiple machine shop applications, particularly grinding. This soluble oil is non-corrosive and used for improved wetting, cleaning and lubricating during metal cutting and grinding applications.

JEI Soluble Oil Cutting Fluid

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This is a mixture of synthetic lubricants and other additives that readily mixes in water to form a milky solution that lubricates, cools and provides excellent rust protection. It is great for extending tool life on difficult-to-machine alloys.

Rustlick 74905 Ultracut

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This is a heavy-duty, biostable, semisynthetic fluid with chlorinated EP additives. It allows for a long sump life and can be used in a wide range of operations, from milling to drilling.

Houghton Hocut 795B

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Mobil Mobilcut 100 Aqueous Metalworking Fluids

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This is a conventional milky soluble oil that readily mixes with different waters to form stable emulsions. Its versatile performance makes it suitable for a wide range of metal cutting and grinding operations for both ferrous and non-ferrous materials. It offers excellent emulsion stability and long-term corrosion protection for both components and machine tools.

JEI Full Synthetic Multi-Purpose Coolant – VERSAKUT 7227

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Versakut 7227 is a multi-purposeful synthetic coolant for grinding, machining, milling, and other CNC applications. Made with a biostatic lube package that will not break down or be subject to selective depletion, Versakut 7227 is the economical solution for a long CNC sump life.

TRIM Cutting & Grinding Fluids MS685/5 MicroSol 685 High Lubricity Semisynthetic Metalworking Fluid

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TRIM MicroSol 685 is a high-lubricity, semisynthetic microemulsion coolant. The formula offers the performance of a heavy-duty soluble oil with the cleanliness of a semisynthetic. It provides excellent cooling and mechanical lubricity, along with the machine friendly characteristics you expect from a premium TRIM coolant.

TRIM Cutting & Grinding Fluids SC520/5 General Purpose Semisynthetic

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A low-odor, low-cost semisynthetic, SC520 is a general-purpose, cutting and grinding fluid concentrate for the multi-material, multi-operational shop. SC520 uses a proven EP-additive package to control built-up edges, improve tool life, and has the wetting and cooling characteristics necessary for superior machining results on high-speed milling and turning operations. It also controls chip welding on soft, gummy materials like aluminum.

Kool-All Semisynthetic Cutting & Grinding Fluid

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KOOL-ALL is a clear, dark blue semisynthetic cutting and grinding fluid. Designed primarily for light to medium machining of ferrous and non-ferrous metals, it is operator and environmentally safe, exhibits good detergency, and rejects tramp oil. It also offers long sump life and is easily maintained.

Coolant Maintenance

The next step after selecting your cutting fluid is maintenance. Well-maintained coolant will last longer and improve performance. Coolant maintenance is required for three main reasons

  • Keeps the correct balance between water and coolant
  • Maintains the pH
  • Prevents the oil from going rancid (in case you are using soluble or semisynthetic fluid

Machine Issues

When the makeup of coolant breaks down, the concentration of the coolant ingredients become weaker and less effective. That low concentration can cause damage to tool and machine life.

To check the concentration of coolant you’ll need to use a refractometer. This device uses a special scale, called Brix scale, to show the density of the water sample. Some types of coolant might require you to apply a specific correction factor when measuring the concentration in the refractometer. If the correction factor listed in the coolant manufacturer’s spec sheet is higher or lower than 1, all you have to do is multiply the value that you see on the Brix scale by the conversion factor.

Measuring alkalinity is the second important aspect of coolant maintenance. As the fluid ages, its pH will start to slowly degrade, making it eventually too acidic and potentially exposing your machine to rust problems. To monitor the pH of your coolant, all you need to have are pH test strips. The recommended pH level varies depending on the coolant but usually ranges from 8.0 to 9.0.

The last maintenance item that you’ll have to address is oil skimming. This process is fundamental to avoid the development of anaerobic bacteria that will make the coolant go rancid. The best and easiest way to combat coolant rancidity is to keep it skimmed well and always moving. Oil skimmers are the best way to carry on this operation, although they can be quite expensive. Here you can find a creative way to make your own oil skimmer.

Additionally, dirt that gets into the coolant can also become abrasive and scratch paint and surfaces and even damage machine windows. Dirt in coolant can also damage high pressure pumps.

Health Issues

Beyond the machine issues that can arise, there are a host of health problems that can come from neglecting your coolant. Coolant is notoriously tough on skin, causing rashes and irritation. All of this is amplified if the coolant is dirty. Abrasions to the skin are frequent in machining, so if the microorganisms that develop in dirty coolant get into those abrasions, you’ll be more prone to infection.

Try to avoid inhaling coolant when you can, even though that fresh coolant can smell nice, it is highly damaging to the lungs.

In conclusion, choosing the right coolant and keeping it “healthy” will allow you to machine parts better, avoid ruining your tools and keep your machine running smoothly. And it’s more economical to maintain clean coolant than fix machinery and tools that were damaged by the dirty coolant.


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