Cutting fluid is a type of coolant and lubricant designed specifically for metalworking processes, such as machining and stamping. There are various kinds of cutting fluids, which include oils, oil-water emulsions, pastes, gels, aerosols (mists), and air or other gases. They may be made from petroleum distillates, animal fats, plant oils, water and air, or other raw ingredients. Depending on context and on which type of cutting fluid is being considered, it may be referred to as cutting fluid, cutting oil, cutting compound, coolant, or lubricant.

Most metalworking and machining processes can benefit from the use of cutting fluid, depending on workpiece material. Common exceptions to this are cast iron and brass, which may be machined dry (though this is not true of all brasses, and any machining of brass will likely benefit from the presence of a cutting fluid).

The properties that are sought after in a good cutting fluid are the ability to:
  • keep the workpiece at a stable temperature (critical when working to close tolerances). Very warm is acceptable, but extremely hot or alternating hot-and-cold are avoided.
  • maximize the life of the cutting tip by lubricating the working edge and reducing tip welding.
  • ensure safety for the people handling it (toxicity, bacteria, fungi) and for the environment upon disposal.
  • prevent rust on machine parts and cutters.
Emulsified oil (sometimes called soluble oil), semi-synthetic, and synthetic coolants are the three most common alternatives. All of these alternatives require additional components for improved machining as well as foam control, biological control, pH stability and corrosion control. Occasionally, other additives will be used to reduce staining of sensitive metals and lesson degradation of carbide. In order to properly manage your coolants, understanding that additives are often used up at different rates is extremely important. This process is called selective depletion. One of the most common examples of selective depletion can be seen when corrosion prevention additives are depleted in cast iron applications before Steel alloy applications.

Soluble Oils - These types are very common and generally form a milky, opaque fluid. Typically soluble oils are 50% oil, or more, and balanced by emulsifier and additives. Most heavy duty soluble oils have added chlorine to enhance extreme pressure machining.

Oil provides lubricity for much of the machining as well as excellent corrosion control for the machine and its parts. Unfortunately, oil emulsifiers are known as a favorite food source for bacteria, and often impede the heat dissipation process.

Semi-Synthetics - This type of coolant contains both oil and a synthetic (polymer). A semi-synthetic coolant can contain anywhere from 5% oil to 35% oil. The smaller percentage of oil in semi-synthetics allows for heat to be dissipated much faster than with soluble oils, improving tool life and finish. Much like soluble oils, chlorine is sometimes added to improve heavy machining performance.

Synthetic - Synthetics often contain as many as 17 ingredients, including polymers, which are used to replace the oil and cutting additives found in soluble oils and semi-synthetics. Without oil, additives are required to control corrosion. The historic problems of corrosion control and hard deposits on the machine tool have improved with the development of modern coolants. Some modern formulas actually emulsify a limited amount of oil to help improve performance. Without oil however, synthetic coolants can offer superior cooling with extended tool and sump life.