Metalworking Coolants

A Comprehensive Guide to Metalworking Coolants: Types, Uses, and Benefits

The Basic Types of Metalworking Fluids

Different types of metalworking coolant are used in various machining processes, such as turning, milling, drilling, and grinding. This provides lubrication, cooling and to remove debris generated during the machining process.

The basic properties of metalworking fluids include:

  • Lubrication: Metalworking fluids reduce friction and wear on cutting tools and machine components. It improves tool life and reduces heat generated during machining.
  • Cooling: Metalworking fluids help regulate tool temperatures and reduce thermal stress on cutting tools. It improves the quality and accuracy of machined parts.
  • Debris removal: Metalworking fluids help remove chips and debris generated during machining. It improves surface finish and preventing clogging of coolant systems.
  • Corrosion protection: Some metalworking fluids also provide corrosion protection to machine components and cutting tools. This extends their service life.
  • Biodegradability: Some metalworking fluids, such as synthetic fluids, are formulated to be more biodegradable and environmentally friendly than other fluids.

But what about the temperature?

All metal removal processes generate a tremendous amount of heat. This heat must be minimized in order to achieve productivity and parts quality. In addition, the cooling effect provided by a metalworking fluid gives the cutting tool or grinding wheel a longer life and helps to prevent burning and smoking.

When the tool is in contact with the part, lubrication is necessary to reduce friction between the tool and the part. This results in improved tool life and better finishes on the metal cut.

Metalworking fluids also provide corrosion protection for the newly machined part and machine tool. Water-miscible metalworking fluid formulations (those fluids that are meant to be diluted with water) include components that slow or prevent such corrosion. MWFs also help remove chips or swarf (an accumulation of fine metal and abrasive particles) from the cutting zone.

The type of metalworking fluid used will depend on the specific requirements of the machining operation. Examples are the material being machined, the cutting speed, and environmental concerns. Therefore, it’s important to carefully consider the benefits and drawbacks of different metalworking fluid. This will help you determine the best choice for a particular application.

What are the different types of metalworking coolant?

Metalworking fluids are used in CNC machines to provide lubrication, cooling and assist in removing debris generated during the machining process. There are four significant metalworking classes widely available:

  • Straight Oil
  • Soluble Oil
  • Semi-Synthetic
  • Synthetic

Many metalworking fluids, except straight oils, are mixed with water. Each has additives such as surfactants, biocides, extreme pressure agents, anti-oxidants, and corrosion inhibitors to improve performance and increase fluid life.

Straight Oil:

These mineral oils provide lubrication in high-speed machining operations. This metalworking fluid mostly comprises mineral (petroleum) or vegetable oils. Petroleum oils used for these fluids tend to be “severely solvent refined” or “severely hydrotreated”. This refining processes reduces cancer-causing substances called polynuclear aromatic hydrocarbons [PAHs] present in crude oil.

Other animals, marine, or synthetic oils can also be used singly or in combination with straight oils to increase the wetting action and lubricity.

An oily appearance and viscous feel can recognize straight oils. They may contain chlorinated and sulfur additives. This type of oil coolant is not diluted with water before use.

Straight-oil metalworking fluids are generally used for processes that require lubrication rather than cooling. They perform best when used at slow cut speeds, high metal-to-metal contact, or with older machines explicitly made for use with straight oils. In addition, straight-oil MWF systems may require fire protection.

Tips to remember:

  • Have a higher viscosity compared to other types of fluids
  • Provide good lubrication but limited cooling
  • Can leave a residue on machined parts
  • Have limited biodegradability and can be harmful to the environment

Soluble Oil:

Soluble oil is also called emulsifiable oil. These emulsions of oil in water provide both lubrication and cooling. They are often used for general-purpose machining operations. It comprises 30 to 85 percent of severely refined lubricant base oil and emulsifiers. This helps disperse the oil in water.

The fluid concentrate usually includes other additives to improve performance and lengthen the fluid’s life. Soluble oil products are supplied as concentrates with diluted water to obtain the working fluid. In addition, they may have colorants added.

Soluble oils, in general provide good lubrication and are better at cooling than straight oils. Drawbacks in using soluble oils, however, are that they sometimes have poor corrosion control. They are also sometimes “dirty”. As an example, machine tool surfaces and nearby areas become covered with oil or difficult-to-remove product residues. They also may smoke, may not cool as well as semi-synthetics and synthetics, and may have poor mix stability or short sump life.

Tips to remember:

  • Are an emulsion of oil and water
  • Provide both lubrication and cooling
  • Can leave a residue on machined parts and may require more frequent cleaning
  • Generally, these types of metalworking coolant have a higher biodegradability than straight oils


Semi-synthetic fluids are a type of metalworking fluid that combines the properties of mineral oil and synthetic fluids. They are formulated by blending mineral oils with synthetic additives. This enhances performance, such as improved cooling and reduced residue.

Semi-synthetic fluids offer some of the benefits of both mineral oils and synthetic fluids, such as good lubrication and improved cooling performance. They can also be more cost-effective compared to synthetic fluids. However, they may provide a different level of performance than synthetic fluids. Also, their properties may not be as consistent as fully synthetic fluids.

Semi-synthetic fluids will depend on the specific requirements of the machining operation and the trade-off between cost and performance.

This type of metalworking fluid contains a lower amount of severely refined base oil, for example, 5-30 percent in the concentrate. Semi-synthetics offer good lubrication, sound heat reduction, reasonable rust control, longer sump life, and cleaner than soluble oils. They are comprised of many of the same ingredients as soluble oils and contain a more complex emulsifier package.

Tips to remember:

  • Improved Lubrication: Semi-synthetic fluids contain a blend of mineral oil and synthetic additives. This provides improved lubrication compared to straight oils, reducing friction and wear during machining operations.
  • Enhanced Cooling: Semi-synthetic fluids provide improved cooling compared to straight oils. This helps to regulate tool temperatures and reduce thermal stress on cutting tools.
  • Reduced Residue: The synthetic additives in semi-synthetic fluids can help reduce the residue left on machined parts. This makes post-processing operations easier and improving surface finish.
  • Cost-Effectiveness: Compared to fully synthetic fluids, semi-synthetic fluids offer a good balance between performance and cost. This makes them a more economical option for many machining operations.

It’s important to note that the benefits of using semi-synthetic fluids will depend on the specific requirements of the machining operation and the trade-off between cost and performance. As a result, a semi-synthetic fluid may not be the best choice for every application. It is essential to carefully consider the specific benefits and drawbacks in your decision-making process.


These types of metalworking coolant are made from chemically-derived compounds. They are formulated to provide specific properties such as improved cooling, reduced residue, and reduced odor. Often, these are used for demanding machining operations or in applications where environmental concerns are a priority.

These metalworking fluid formulations do not contain any petroleum oil. They contain detergent-like components to help “wet” the part and other additives to improve performance. Like the other classes of water-miscible fluids, synthetics are designed to be diluted with water.

Among the four types of fluids, synthetic metalworking fluids generally are the cleanest. They also offer the best heat reduction, have excellent rust control, and offer longer sump life. This type of metalworking fluid is transparent (allowing the operator to see the work) and are largely unaffected by hard water.

Tips to remember:

  • Chemically derived and have consistent properties that are formulated to meet specific requirements
  • Provide good lubrication and cooling
  • Leave minimal residue on machined parts
  • Often have a low odor and biodegradability compared to other types of fluids
  • Can be more expensive than other types of fluids.

The type of fluid you use will depend on the specific requirements of the machining operation. Factors include the material being machined, the cutting speed, and environmental concerns. Our best tip for you is to consult with your coolant distributor to find the best available coolant for your job type.

Want a few questions to ask them? Check out our Tips to Selecting Your Perfect CNC Coolant.



About Zebra Skimmers:

Our mission has always been simple. It is to provide the metal working industry with the oil skimmers and metalworking fluid management systems they need to complete jobs quickly and efficiently. Innovative and affordable solutions have gained us many accolades. Our continued growth is due in no small part to our excellent customer service… Not to mention our satisfaction guarantees and the performance of our tools.

Among our diverse options of oil skimmers and metal working fluid management systems, you can expect to find: