Lesson 201 - Coolant Types

In this lesson, we explain the different types of coolant and why they matter.

Machining generates heat between the tool and the work piece. Metalworking fluid (“MWF” or “coolant”) was designed to provide lubrication and cooling. This extends tool life, makes for higher quality parts, and lessens the likelihood of everything getting welded together during the machining operation.

As our understanding of chemistry and the demands of manufacturing have increased, many kind of coolants have been created. The types discussed here are Straight Oil, Soluble Oil, Semi-Synthetic, and Synthetic. Generally, they are categorized by the amount of natural oils they contain. These ranges are: Straight oils ~ 100% — Soluble Oils ~ 80% — Semi-Synthetics ~ 35% — Synthetics ~ 0%

Straight Oils

Straight oils were the first metalworking fluids, and are the only type that is not mixed with water. They provide great lubrication for certain machining operations and are very stable in that they offer excellent resistance to bacterial attack and corrosion. Since they contain no water, they provide the least amount of cooling of the tool and work piece. They are still used in many operations where heat transfer is not an issue.

Soluble Oils

Soluble oils were the first oil/water blends, using a large percentage of mineral or vegetable oils as the base. They have a milky appearance when mixed with water. Soluble oils work well to both lubricate and cool. However, since they are composed mostly of natural oil, they tend to allow tramp oil to easily emulsify, and can spoil relatively easily.

In general, soluble oils have suspended particles that measure 25 microns in diameter (about .001 inches). That may sound small, but it’s rather large for a chemical particle. In order to better compare particle sizes between the different types of coolants, imagine that the soluble oil particle is the size of a baseball diamond. Later, we’ll compare this particle to the particle size of semi-synthetics. Partly because of the large size of these particles, the soluble oil coolants tend to destabilize when filtering is being performed for other particles, such as fine chips under 25µ in size.


These formulations are similar to soluble oils in that they contain natural oils, but relatively little. These coolant types also work well for both lubricity and cooling, but they emulsify tramp oils less than soluble oils. They are generally opaque, with a particle size of 0.01–0.1 microns. If our baseball diamond represents a single particle of a soluble oil, a particle of a semi-synthetic would be the size of a baseball. Because the particles are so much smaller, they also allow for finer chip filtration without chemical destabilization.


Synthetic blends contain no natural oils. Since they contain no base oil they easily reject tramp oil and are the least vulnerable to chemical destabilization due to chip filtration and other factors. Their weakness is in providing lubricity in difficult machining operations, and they tend to encourage rust and foam more than other types. These blends are more transparent when mixed with water and have a particle size of .003 microns. Using our baseball diamond to represent the soluble oil particle, and the baseball to represent a particle of a semi-synthetic, a single particle of a full synthetic would be the size of one of the threads on the baseball. Now that’s small!


There are tens of thousands of different coolant formulas on the market today, but these broad categories serve well to cover most of them. In general, coolants may also contain other chemicals such as dyes and biocides, stabilizers and corrosion inhibitors. When a chemist creates a coolant formula, he or she designs it to be used within a particular concentration range for a particular machining operation as well as for the material being machined. Make sure to check with the coolant supplier to get those details. Finally, consider coolant maintenance when choosing coolant, thinking about its reaction to tramp oil and chip contamination.