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Understanding Coolants In CNC Machining For Optimal Performance

May 23, 2023



CNC coolants optimize the machine's performance by cooling, lubricating, and flushing residue.

•Coolant systems can be single or central. A single system assists one machine, while a central system caters to multiple devices.

•The coolant is delivered to the machining area, where pipes aid in circulation and control valves regulate the flow.

•The four types of coolants are there: synthetic fluids, semi-synthetic fluids, soluble oils, and straight oils.

•Coolant concentration is essential to prevent foaming, bad odor, and staining. Check concentration regularly using hand refractometers.

•The coolant can be delivered through air, mist, flood, or high pressure.

•Follow manufacturer recommendations and inspect regularly to ensure machine longevity.


What Are Coolants And Their Role In CNC Machining?


Coolants are used in CNC (computer numerical control) machining, or cutting fluids, to help accurately and efficiently cut materials, e.g., plastics, metals, or high-density plastics.

The terms "coolants" and "lubricants" are often confused. Still, it is essential to note that not all coolants can function as lubricants, but some specifically designed coolants also perform lubrication tasks and enhance the machine's life.

For CNC machines, coolant serves three purposes:



•Flushing of residue 


Coolants are crucial for optimizing the tool's performance and extending its life. They play their role by optimizing the below-mentioned factors during the machining process. 


•Heat Dissipation: During the machining process, heat is generated on the workpiece and cutting tool, and coolant helps dissipate that heat generated during the process.

•Improved Surface Finishing: Coolants are known to remove debris and contaminants from the workpiece to help achieve a better finishing surface on the machined parts.

•Chips Evacuation: They ensure optimized machining by flushing away all the remains of chips and abrasive materials from the work area.

•Lubrication and Tool Life Expansion: Coolants reduce the friction between tools by providing lubrication and extending the tool life.

•Protection Against Corrosion: Coolants protect the machine parts and the material you are working on from corrosion, as some coolants may contain corrosion inhibitors. If it has a limited corrosion protection inhibitor, additives can be added to the coolants to enhance their efficiency.

•Controls The Tool Temperature: Coolants balance the tool's temperature by increasing it if you are working at a very low cutting speed. Or by decreasing it if the machine is cutting at high speed to save it from thermal damage.


How Do Machine Coolant Systems Work?


A coolant system is a separate system attached to the CNC machine for applying the fluid into the machine and workpiece. The systems can be single or central, either one system providing cooling and lubrication to only a single machine or a central management cooling system offering services to multiple devices simultaneously.



The only difference between their mechanism is that a single system contains only one pipe. In contrast, a central coolant system comprises many distribution pipes and control valves because it has to deliver fluids to multiple machines. 

The primary mechanism of a CNC coolant system involves the circulation and delivery of coolant to the machining area to achieve its purpose. It typically consists of coolant reservoirs, pumps, filters, pipes, nozzles, and control valves.


•Firstly, during machining, the pumps circulate the coolant mixture from the reservoir over the work area.

•Coolant filters help in washing away chips and particles from the work area.

•Pipes aid in circulating fluid from the machinery to make a cooling effect.

•Nozzles are arranged on the area where the material cutting is going on. Fluid flows from the nozzle to the workpiece to have a lubricant and cooling effect.

•At last, control valves help to regulate the flow of coolant fluid. 

•Coolant collects in a sump located at the bottom of the machine.

•The coolant is pumped from the sump and recirculated back to the work area.


Types Of Coolant Used In CNC Machining



Coolants are of different compositions and have different working based upon the machine in which you will use these coolants. CNC machine tools typically use one of these four types of coolants, namely:


1. Synthetic Fluids

These are water-based coolants containing organic and inorganic compounds with no mineral and petroleum oils. In short, synthetic fluids are a mixture of synthetic oils, additives, and water. These are designed for high cooling capacity, corrosion prevention, lubricity, and easy maintenance. They are available as a concentrate, and you can mix them with water to create metalworking fluid.

They are further classified as complex, simple, and emulsifiable synthetics. Simple concentrates are best for light grinding operations, while complex synthetics are for moderate to heavy-duty operations. Emulsifiable synthetics are opaque or translucent and can deal with high-temperature alloys.




•They are long-lasting, with no foaming, and are more stable than other metalworking fluids.

•Their initial cost is high, but that is compensated due to their less consumption and improved efficiency.

•Mostly synthetic fluids are preferred for high-heat and high-velocity metalworking. 

•The microbial control is great and can resist rancidity.

•The coolants are nonflammable with superior corrosion-resistant properties.




•They may cause fine mists in moderate to high agitation scenarios.

•Lubricating oils can contaminate the coolants, and a thorough inspection is needed.


2. Semi-Synthetic Fluids

These fluids contain 2% to 30%of petroleum oil. These are a mixture of some mineral oils and chemical compounds that can dissolve in water, and they offer the properties of both soluble oils and synthetic fluids, making them practical and efficient options for achieving clean and effective machining operations in CNC machines.

There is high emulsifier content helping in suspending small oil globules. Also, the refracted fluid light is decreased. They are transparent with a slight haze to translucent with high heat sensitivity. The lubrication is great as oil molecules gather around the cutting tool and redisperse when the solution cools.




•They are easier to maintain than soluble oils.

•The lubrication is great for moderate and heavy-duty tasks.

•They generate less oil mist and smoke with rancidity control.

•The better wetting properties allow faster feed rates and cutting at higher speeds.

•The low viscosity allows for better settling and efficient cleaning.




•Hard water can impact their stability and lead to scum formation.

•Due to cleaning additives, foam formation capacity is higher than other types.


3. Soluble Oils

Soluble oils are used for general-purpose machining and are the most common coolants made by mixing 60‑90 percent mineral oil into water. Also, emulsifiers are added to maintain the solution mixture. That's why they are also known as emulsifiable oils or water-soluble oils. 

The oil can be dispersed upon mixing, forming an "oil-in-water" emulsion. The oils will cling to the workpiece during machining, and the emulsifier will refract light. It’ll lead to a fluid milky, and opaque appearance.




•They offer improved cooling capabilities and good lubrication due to the mixture of oil and water.

•The coolants are suitable for a range of light and medium-duty operations.

•The lubricity can be improved by using chlorine, sulfur, and phosphorus additives.




•They leave a protective oil film on machine tool components, making cleaning difficult from the workpiece.

•The presence of water makes them prone to rust, tramp oil contamination, and bacterial growth.

•The maintenance costs are high, and misting may lead to the unsafe work environment.


4. Straight Oils

Straight oils are not based on water; they are petroleum or mineral oils, so they are used directly without dilution. They are particularly suitable for heavy-duty machining applications and materials requiring high lubricity, such as stainless steel or titanium. However, straight oils require regular maintenance and can produce more oil mist and residues than other coolant options.


They may contain additives (not needed for ferrous and nonferrous metals) and wetting agents. Wetting agents contain extreme pressure additives (sulfur and chlorine) and 20% fatty oils for severe applications. They improve wettability; oil can coat the cutting tool, metal fines, and workpiece properly.

There’ll be enhanced lubrication, management of metal fines, and microscopic welding guarding in heavy machining. Sulfurized fatty oils and chlorine may exceed 20% to enhance anti-welding properties in extreme conditions.




•Straight oils offer cushioning effect between the cutting tool and the workpiece. It’s great for a quality finish and low clearance operations at low speed.

•The tool life is enhanced, and the best option for broaching, tapping, crush grinding, deephole drilling, and hard metal cutting (like super alloys and stainless steel).

•Due to high lubrication, they are the best option for honing operations.

•They offer extended sump life, rust protection, and easy maintenance.

•They’ll prevent rancidity; bacteria can only thrive if oil is contaminated.




•The heat-dissipating properties of straight oils are bad, leading to fire risk.

•Mist and smoke from them cause an unsafe work environment with insufficient shielding or vent systems.

•They are limited to low-speed operations, and the oily film on the workpiece is difficult to clean.

•Highly viscous fluids have more coolant loss as they cling to the workpiece. Opt for low-viscosity oil for enhanced results.


Find The Right Coolant Concentration


The correct concentration of coolant is essential to maintain the optimal performance of CNC machining. Most manufacturers like Yangsen already provide detailed instructions about the type and amount of coolant. Follow these proportions and use deionized water for mixing purposes. It’ll prevent corrosion for metals like steel and aluminum.


High coolant concentration can lead to issues like:


•Foaming: Hindering operations and reducing tool life and efficiency

•Wastage: Excessive coolant will be wasted, leading to increased cost

•Less Lubrication: More the amount, less the lubrication effectiveness, and more changes of friction and machine damage

•Bad Odor: Bacteria can develop due to the instability of the coolant, and there will be a bad smell after the machine shuts down

•Staining: Concentrated coolant can impact the physical appearance of the machine, causing surface damage

•Residue Formation: Excess coolant can leave residue, negatively affecting tool lifespan and machining quality

•Skin Irritation: High concentration is usually toxic and irritates if they come in contact with human skin

The operators should check the concentration daily and maintain appropriate levels.

•Hand refractometers can help check the refractive index of the CNC machining coolant and cutting fluid concentrations. To maintain control, do wipe them between different coolants checking.


There may be a 20% change in coolant concentrations due to splashing, evaporation, and misting. Maintain a daily log of levels, and you can notice the difference and changes daily. The right amount can enhance the machining efficiency and extend the life of the coolant.


Machine Coolant Concentration Chart


For sustained high-quality products, the coolant should be appropriate for the machining and metal type. The manufacturer recommendations vary for each machine, and here’s a general concentration idea for machining operations:








Soluble oil with 96% water

Use 10-20% lard oil with mineral oil

Use mineral oil containing 10% fat

Use of soluble oil

Alloy Steels

Use 10% lard oil with 90% mineral oil

Use 30% lard oil with 70% mineral oil

Use 25% sulfur base oil with 75% mineral oil

Use of soluble oil


96% water (soluble oil) or mineral oil

25% sulfur-based oil and mix it with mineral oil

Use soluble oil or mineral oil with 10% fat

Soluble oil containing 70-90% water


Use of soluble oil

30% lard with mineral oil

Use of soluble oil

Use of soluble oil


Use of soluble oil

Use of soluble oil

Use of soluble oil

Use of soluble oil

Cast Iron


Use 25% lard oil with 80% mineral oil



Malleable Iron

Use of soluble oil

Use of soluble oil

Use of soluble oil

Use of soluble oil

Low Carbon and Tool Steels

Use of soluble oil

25-40% lard oil with mineral oil

25% lard oil with 75% mineral oil

Use of soluble oil


Types of Coolant Delivery



CNC machining coolant is delivered in several forms, including:


•Air: Air is used for clearing and cooling but offers no lubrication. It’s great for sensitive materials like plastic, where direct coolant may cause thermal shock.

•Mist: Misting systems, utilized in high-speed working, provide superior lubrication by releasing atomized coolant as a fog or mist onto the workpiece. The pressure is adequate, and there’s no chance that the tool undergoes stress.

•Flood: During machining, flooding systems effectively manage debris by pouring large amounts of coolant onto the workpiece and machine surface.

•High Pressure: Jetting coolant systems are designed to release coolant or cutting fluid at high pressure. It is then directed towards the cutting edge to enhance lubrication and cooling efficiency. A pressure greater than 1000 psi aids in evacuation and chip removal. Though good for immediate cooling, sometimes it may break miniature diameter tooling. So, only deliver it through cooling grooves and use it in deep pockets or drilling operations.


Proper Maintenance And Replacement


•Regularly inspect the coolant concentration and contamination levels.

•Check for any odor, color, or consistency changes, as they indicate potential issues.

•Use pH test strips to monitor the pH levels and maintain recommended range.

•For heat dissipation and chip evacuation, use high-pressure coolant systems.

•Mix and apply the coolant as per manufacturer recommendations.

•Handle and dispose of following the safety procedures.



A thorough understanding of coolants is essential for the optimal machining process. Select the right coolant type, delivery method, and concentration to ensure dimensional accuracy and maintain the process stability of the final product. With regular maintenance and coolant management, CNC machines' productivity will improve with full potential work and less downtime.

For further help, Yangsen experts are here. With 20 years of experience and a 57,000 square meters factory, our dedicated team ensures high-quality products with strict quality control. We export to over 20 countries; you can rely on us for high-quality tools at competitive prices.

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