All You Wanted to Know About Coolants

Coolants are used in the metalworking industry whether it be cutting, grinding, bending, stretching or stamping to perform two functions:

  • Provide lubricity to reduce friction and the heat generated by it.
  • Dissipate and carry away the heat  from the tooling and the part.

To say that this is all it does, would be a gross over simplification. Material, tooling, and tolerances all pose their own unique requirements. The type of coolant and the additives in them address these specific and complex machining issues.

Types of machining coolants

Straight Oil (100% petroleum or mineral oil)

These may have additives designed to improve specific properties. For severe applications, straight oils may contain wetting agents (typically up to 20% fatty oils) and extreme pressure (EP) additives such as sulfur, chlorine, or phosphorous compounds.  These additives improve the oil’s wettability; that is, the ability of the oil to coat the cutting tool, work piece and metal fines. These additives also enhance the anti-welding properties to control BUE (build up edge on the tool).

Advantages – Excellent lubricity, good rust protection, good sump life, rancid resistant

Disadvantages – Poor heat dissipation, increased risk of smoking, misting, and fire, oil film on work piece requires subsequent wash operation, limited to low-speed, severe cutting operations

Soluble Oil

Petroleum oil (60%-90%) and water are mixed together with emulsifiers and other chemicals to form oil-in-water emulsion making a milky solution. Normal milky emulsions have particle size ~ 2 to 50 microns in diameter.

Advantages – Good lubrication, Improved cooling capabilities, General-purpose product for light to heavy-duty operations

Disadvantages – More susceptible to rust problems, Susceptible to tramp oil contamination and bacterial growth, Susceptible to evaporation losses and may form precipitates on machine



Synthetic Fluids contain no petroleum or mineral oil base and instead are formulated from alkaline inorganic and organic compounds along with additives for corrosion inhibition. They are generally used in a diluted form (3 to 10%). Synthetic fluids often provide the best cooling performance among all cutting fluids.

Advantages of synthetic fluids: very good cooling ability, good lubrication properties, good stability in hard water, good corrosion protection, low mist, easy handling, cleaning and maintenance.

 Disadvantages of synthetic fluids: some toxicity, easily contaminated by foreign oils, relatively high cost.

Semi Synthetic Fluids

These are similar to soluble oils since they are water-based emulsions. However, there is usually 5 to 20% mineral oil emulsified into the water to form a micro emulsion. The emulsion particle size is 0.1 to 0.01 microns in diameter.

Semi-synthetic fluids combine advantages (and disadvantages at some extent) of mineral emulsions and synthetic fluids: They have better corrosion protection than synthetic fluids and better cooling and wetting capabilities, easier handling and maintenance than mineral emulsions.

Disadvantages of semi-synthetic fluids: misting, relatively poor stability in hard water, contaminated by foreign oils, some toxicity.

Maintaining the cleanliness of your coolant will extend its life and prevent health hazards. Using the EXAIR Chip Trapper to pump out your machine sump on a scheduled basis will filter out problem causing debris. Watch a video of the chip trapper in use.

Joe Panfalone
Application Engineer

Phone (513) 671-3322
Fax   (513) 671-3363

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