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.

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.

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.

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.