How It Works – Cutting Fluids

Unless you’re doing dry machining, you’ll use some kind of cutting oil or fluid in your machines. Cutting fluids and oils provide lubrication and cooling. They also help remove chips from the cutting area. Selecting from the hundreds of available cutting fluids can be a real challenge. Experts in the business offer some guidelines on selecting and maintaining this important part of the machining process. Usually, you’ll choose either a straight oil or a water-miscible (dilutable) fluid.

Straight oils

These are “mineral oils,” petroleum products made fromcrude oil. Straight oils offer the maximum amount of lubrication, and the least cooling capacity. Blended with additives to improve performance, these oils are often used in screw machines and in heavy cutting operations like broaching and gun drilling.

Vegetable oils can be used instead of, or in addition to, mineral oils in many applications. They tend to have better lubricating qualities, and higher flash point, which is the temperature at which their vapor will ignite. In one cutting oil product line, for example, the flash points range from 200F to over 460F, with the vegetable oils on the higher end.

Water-miscible fluids

In addition to straight oils, three kinds of water-miscible cutting fluids are widely used. They give good tool life and help to produce a good surface finish. When properly mixed, the fluid is mostly water, with a few percent of the fluid concentrate, which is made up of oil and/or synthetics, plus additives.

Soluble oil

This is oil dispersed in water, making a milky-looking mixture. It offers the greatest amount of lubrication among the water-miscible fluids. Emulsifiers and surfactants let the oil mix with the water and remain stable despite contamination from tramp oil, machining fines, and other materials that find their way into machine sumps.


These don’t contain oil. They are made up of various chemical compounds such as phosphate esters. Synthetics fully dissolve into water. They are often transparent and may look like water or have a colorant added. They tend to be the most stable of the water-miscible fluids, and are often used for applications such as fine grinding, where a fluid is needed to keep the wheel open and clean.


These blend oil and synthetics to give a combination of lubrication, stability and cooling performance. The concentrate usually contains 30 percent, or less, of mineral oil.