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Understanding The Use Of Cutting Fluid in CNC Machining

CNC machining has revolutionized the manufacturing industry by providing precise, efficient, and automated machining processes. In CNC machining, cutting fluid plays a crucial role in ensuring the quality of the final product, prolonging tool life, and enhancing machining efficiency. This article aims to delve into the significance, types, and applications of cutting fluid in CNC machining.

What is Cutting Fluid?

The nomenclature for cutting fluids can vary, with terms like cutting oil, cutting compound, coolant, or lubricant used interchangeably depending on the specific context or type of fluid being discussed. Is a substance used in machining processes to enhance the efficiency and quality of metal cutting operations. It is applied directly to the cutting tool and workpiece during machining processes such as milling, turning, drilling, and grinding.

In most cases, utilizing cutting fluid proves advantageous in metalworking and machining processes, particularly based on the material of the workpiece. However, there are exceptions such as cast iron and brass, which can often be machined dry. It’s worth noting that while some brasses can be machined without cutting fluid, the majority of brass compositions benefit from its presence during machining.

Cutting Fluid

Properties of a Cutting Fluid

The following are some of the properties a cutting fluid should have.

Thermal ConductivityMeasure of how well the cutting fluid can conduct heat away from the workpiece and the tool.
Heat CapacityAbility of the cutting fluid to absorb heat before it gets hot or boils, influencing its cooling ability.
ViscosityFluid’s resistance to flow, with lower viscosity allowing for easier flow and preventing sludge formation.
CorrosivenessCutting fluid should not corrode or attack the workpiece or tool.
Corrosion ResistanceAbility to prevent corrosion of parts, enhancing tool and workpiece longevity.
Oxidation ResistanceAbility to prevent oxidation of surfaces, maintaining part integrity.
Non-toxicityCutting fluid should not pose health hazards to humans or the environment.
Chemical InertnessShould not react chemically with surfaces, preventing damage to components and fluid degradation.
OdorlessAbsence of strong smells to avoid unpleasant odors in the workplace.
TransparencyClearness of the fluid, ensuring it does not obstruct the view of the workpiece during machining.
StabilityResistance to degradation over time, maintaining performance during use and storage.
Lubricating PropertiesAbility to reduce friction between the tool and workpiece for smoother machining and tool longevity.

Basic Types of Cutting Fluids

In CNC machining, cutting fluid is indispensable as it can significantly improve machining efficiency and working environment. Each cutting fluid has its unique characteristics and applicable scenarios.

Emulsions (Water-based Cutting Fluids)

Emulsions are a common type of cutting fluid, composed of water, emulsified oil, and various additives. The advantages of this type of cutting fluid include low cost and excellent cooling performance, making it particularly suitable for operations that require substantial heat dissipation. However, emulsions are prone to bacterial growth and have a certain degree of corrosiveness, necessitating strict control of dilution ratios and regular maintenance during use.

Synthetic Fluids

Synthetic fluids are cutting fluids that do not contain petroleum oil bases and are primarily composed of various chemical additives. Their main advantages include exceptional cooling and lubrication performance, as well as resistance to bacterial growth, making them highly popular in applications requiring a very clean processing environment, such as precision parts manufacturing. However, synthetic fluids are relatively expensive, and some chemical components may be corrosive to machinery, so careful consideration is needed when selecting them.

Semi-synthetic Fluids

Semi-synthetic fluids fall between emulsions and synthetic fluids, containing a certain proportion of petroleum oil base and water. This type of cutting fluid combines the cooling effects of emulsions with the lubrication properties of synthetic fluids, attempting to balance the advantages of both. In general machining scenarios, semi-synthetic fluids can provide good performance, but attention must still be paid to balancing biological stability and corrosiveness to ensure their applicability in various processing environments.

Straight Oils (Oil-based Cutting Fluids)

Straight oils, made from mineral or vegetable oils, do not contain water at all. They offer superior lubrication performance and do not cause rust on the workpiece, making them particularly suitable for machining operations with high lubrication requirements, such as turning and boring. However, straight oils have lower heat capacity and do not cool as effectively as water-based cutting fluids, which can be a limitation in high-speed or high-temperature machining.

Soluble Oils

Soluble oils, also known as emulsions, are stable emulsions formed by mixing mineral oil with water in specific proportions (usually 1-20% oil to water). They are one of the cheapest and most common types of cutting fluids used in machining. Soluble oils offer good cooling performance and average lubrication properties, making them most suitable for light cutting operations. As a mixture of oil-based and water-based cutting fluids, they form an emulsion by adding water. They provide both good lubrication and cooling performance, suitable for various machining needs. However, the use of soluble oils requires strict control of dilution ratios and water quality to prevent corrosion and bacterial growth, ensuring the safety and efficiency of the machining process.

Choose the Appropriate Cutting Fluid

The selection of cutting fluid depends on many factors, including cutting tools, materials, and machining operations.

Cutting Fluid Selection for Different Materials

The material of the workpiece being machined influences the choice of cutting fluid due to variations in machining characteristics and material compatibility.

  • Steel: Mineral oils containing lubricant additives are suitable for machining steel.
  • Alloy Steels: Sulphur brass oil or mineral oil is recommended for machining alloy steels.
  • Aluminium: Soluble oils or mineral oils without active sulphur are preferred for aluminium machining to prevent staining.
  • Copper: Soluble oils are suitable for machining copper.
  • Stainless Steels: Mineral oils containing excellent extreme pressure additives are recommended for stainless steel machining.
  • Cast Iron: Generally, no cutting fluid is required for machining cast iron.

Cutting Fluid Selection for Different Cutting Tool Type

Different types of cutting tools have varying heat generation rates and susceptibility to thermal shock. Selecting the right cutting fluid based on the tool type is essential for efficient machining.

  • Carbide Tools: Carbide tools generate significant heat and require cutting fluids with high cooling capabilities to prevent thermal shocking. Synthetic cutting fluids with excellent cooling properties are recommended for carbide tools.
  • High-Speed Steels: Although high-speed steels also generate heat, they are less susceptible to thermal shock compared to carbide tools. Soluble oils and semi-synthetic fluids are suitable choices for high-speed steel tools.

Cutting Fluid Selection for Different Machining Operation

The specific machining operation being performed, such as turning, milling, drilling, or broaching, also influences the choice of cutting fluid.

  • Easy Machining Operations (e.g., turning, milling): Synthetic fluids with high cooling capabilities are preferred for high-speed operations. Soluble oils may also be used.
  • Difficult Machining Operations (e.g., broaching, thread cutting): These operations require good lubrication due to low speeds and high pressures. Mineral oils containing extreme pressure additives are the best choice for such operations.
Cutting Fluid Production

Production Process and Steps for Cutting Fluids

  1. Raw Material Preparation: According to the specific formulation of the cutting fluid, prepare all necessary raw materials in advance. These include base fluids (such as water, mineral oil, synthetic esters), emulsifiers, rust inhibitors, lubricants, extreme pressure additives, corrosion inhibitors, biocides, defoamers, fragrances, and other functional additives.
  2. Pretreatment:
    • Water-Based Cutting Fluids: Treat the water first by using deionized or softened water to reduce mineral content and hardness, ensuring high water quality.
    • Oil-Based Cutting Fluids: Dehydrate and remove impurities from the base oil to ensure the oil is pure.
  3. Initial Mixing: Add the treated water or base oil into a reaction kettle or mixing tank. Sequentially add various additives according to specific order and temperature conditions. For instance, some formulations require adding emulsifiers or dispersants first for thorough mixing before adding rust inhibitors, lubricants, and other additives.
  4. Reaction Phase: Conduct reactions at a specific temperature (e.g., 70-90°C). Some components need to be fully dissolved or react at particular temperatures, such as triethanolamine, sebacic acid, and rust inhibitors. Adjust the pH level, if necessary, using substances like sodium carbonate or acids to ensure the stability and efficacy of the cutting fluid.
  5. Thorough Stirring and Mixing: Utilize professional stirring equipment for prolonged and even mixing to ensure all components are uniformly combined, forming a stable emulsion or solution.
  6. Cooling and Filtration: After the reaction is complete, gradually cool the mixture to a temperature suitable for storage and use, generally below 40°C. Filter the mixture to remove any undissolved solid particles and impurities, ensuring the cutting fluid is clear and free of sediment.
  7. Quality Inspection: The finished product must undergo various performance tests, including pH value, concentration, cooling performance, lubrication performance, rust prevention performance, stability, and microbial content.
  8. Addition of Functional Components: Based on the inspection results, add necessary components like biocides and mold inhibitors to further enhance the cutting fluid’s lifespan and safety.
  9. Filling and Packaging: Fill the qualified cutting fluid into dedicated packaging containers, such as IBC totes or smaller drums. Label the containers with product name, specifications, batch number, expiration date, and other relevant information.
  10. Storage and Transportation: Store the cutting fluid in a cool, ventilated place, avoiding direct sunlight and freezing conditions. During transportation, comply with chemical transportation regulations to ensure the product is safely delivered to customers.

Key Functions of Cutting Fluids in CNC Machining

Understanding the specific functions of cutting fluids can help in selecting the right type for different machining tasks, leading to improved performance and extended tool life. Below are the key functions of cutting fluids in CNC machining processes.

Cutting Fluids in CNC Machining


During metal cutting processes, significant heat is generated in the workpiece, chips, and cutting tool due to the friction between the cutting tool and the workpiece surface. Additionally, heat results from the plastic deformation of the metal in the shear zone. This heat can lead to undesirable effects such as thermal expansion, chemical reactions like oxidation, and welding of surfaces.


Friction is the primary cause of heating during machining, and the combination of heat and friction can cause surfaces to weld together. Cutting fluids reduce friction by forming a thin lubricating layer between the chip and the tool, which decreases their contact. This lubrication minimizes the abrasion of the cutting tool and reduces the energy consumption of the machining process. By lowering friction, cutting fluids also contribute to smoother cutting action, which can improve the surface finish of the machined part.

Prevention of Corrosion

Rust and corrosion inhibitors in cutting fluids protect machine parts and the machined surface from corrosion. Mineral oil-based cutting fluids prevent oxidation by forming a very thin protective layer over exposed surfaces. This protective layer is particularly important in humid environments or when machining reactive metals that are prone to oxidation. Preventing corrosion ensures the longevity of the machine tools and maintains the integrity and appearance of the machined parts.

Improvement of Tool Life

Cutting fluids significantly reduce tool wear by dissipating heat, reducing friction and abrasion, and preventing corrosion and rust. By maintaining lower temperatures at the cutting interface, cutting fluids prevent thermal degradation of the tool material. Reduced friction minimizes mechanical wear, while corrosion inhibitors protect the tool from chemical damage. As a result, the overall tool life is extended, reducing the frequency of tool changes and downtime, and improving the cost-effectiveness of the machining process.

Removal of Chips

In machining processes such as milling and drilling, chips can accumulate around the cutting zone, potentially hindering cutting efficiency and damaging the workpiece surface. Cutting fluids help to flush away chips from the cutting area, maintaining an unobstructed cutting zone. This chip removal is critical for maintaining the quality of the machined surface and preventing chip re-cutting, which can lead to surface imperfections and tool damage.

Enhancement of Surface Finish

By preventing thermal expansion and property changes in the workpiece, cutting fluids contribute to achieving a good surface finish on machined parts. Consistent cooling helps maintain the workpiece dimensions within tight tolerances, while lubrication reduces the formation of micro-cracks and surface irregularities. The result is a smoother, more precise surface that meets the required specifications and reduces the need for additional finishing processes.

Reduction of Buildup Edge Formation

Cutting fluids help to reduce the formation of a buildup edge on the cutting tool. This phenomenon occurs when material adheres to the cutting edge, leading to poor surface finish and increased tool wear. By minimizing adhesion, cutting fluids help maintain the integrity of the cutting edge and improve machining accuracy. Reducing buildup edge formation also contributes to more consistent cutting performance and prolongs the intervals between tool maintenance.

Control of Particulate Emissions

Cutting fluids assist in controlling particulate emissions generated during the machining process. These particulates can include fine metal dust and other debris that are harmful if inhaled or if they settle on surfaces and machinery. Cutting fluids help to capture and wash away these particulates, keeping the work environment cleaner and reducing inhalation risks for operators.

Cutting Fluid Application in CNC Machining Process

he methods of applying cutting fluids vary across different machining operations, each tailored to the specific requirements and challenges of the process.

Cutting Fluid Usage

CNC Milling

In CNC milling operations, where the workpiece remains stationary while the cutting tool moves, flooding is a common method of cutting fluid application. A cutting fluid is continuously pumped onto the machining area, creating a flood of fluid that helps to dissipate heat, lubricate the cutting action, and flush away chips. This method ensures consistent cooling and lubrication throughout the milling process, improving tool life and surface finish.

CNC Drilling

In CNC drilling, the primary purpose of cutting fluids is chip removal and cooling. Due to the nature of drilling operations, lubrication is not typically required as the risk of built-up edges forming is low. Instead, a jet application method is often used, where a directed stream of cutting fluid is aimed precisely at the drilling area. This targeted application helps to efficiently remove chips and provide cooling to the cutting tool and workpiece, enhancing drilling efficiency and accuracy.

CNC Turning

CNC turning involves high-speed rotation of the workpiece, making cooling a crucial aspect of the process. Jet application of cutting fluid is well-suited for CNC turning operations. A stream of cutting fluid is directed towards the cutting zone, effectively cooling the cutting tool and workpiece. This method helps to prevent overheating, reduce tool wear, and maintain dimensional accuracy during high-speed turning operations.

CNC Grinding

In CNC grinding, where precision and surface finish are critical, mist application of cutting fluid is commonly employed. Cutting fluid is atomized into fine mist particles and directed towards the grinding wheel and workpiece. This method provides effective cooling and lubrication while minimizing fluid consumption and waste. The mist also helps to control airborne particles and reduce the risk of inhalation by machine operators.

CNC Electrical Discharge Machining (EDM)

In CNC EDM, where material is removed through electrical discharges, a submersion method is often used for cutting fluid application. The workpiece and electrode are submerged in a dielectric fluid, which serves as both a coolant and an electrical insulator. This submersion ensures efficient heat dissipation, prevents arcing, and flushes away debris, enabling precise and controlled material removal.

CNC Laser Cutting

For CNC laser cutting, where a laser beam is used to melt or vaporize material, nozzle application of cutting fluid is employed. A fine stream of cutting fluid is directed through a nozzle onto the cutting zone, providing cooling and preventing material from overheating or burning. This method also helps to remove molten material and maintain consistent cutting quality throughout the process.

These methods of cutting fluid application are tailored to the specific requirements and challenges of each CNC machining process, ensuring optimal performance, efficiency, and quality in metalworking operations.

Advantages and Disadvantages of Cutting Fluid

Enhances cutting efficiencyEnvironmental pollution
Provides cooling and lubricationPotential for skin allergies
Increases tool lifespanLiquid spills may cause slipping hazards
Reduces workpiece deformationHigh usage costs
Lowers cutting temperatureRequires special treatment of waste liquid
Improves machining surface qualityPotential health hazards for operators

Precautions for Cutting Fluid Usage

When selecting and using cutting fluids, it is essential to consider their performance, safety, and environmental impact to maximize cost-effectiveness.

Here are some key points to consider:

  1. Environmental Friendliness: Environmentally friendly cutting fluids are made from biodegradable materials, reducing pollution. These fluids typically do not contain harmful chemicals such as sulfates and chlorides, minimizing environmental impact during use. Choosing environmentally friendly cutting fluids helps companies fulfill their social responsibility and promote green production awareness.
  2. Safety: Traditional cutting fluids may contain harmful chemicals such as volatile organic compounds and heavy metals, which can cause skin irritation, respiratory issues, and other health problems. Therefore, choosing low-toxic or non-toxic cutting fluids is crucial. Ensuring that cutting fluids meet relevant safety standards and providing a safe working environment is essential for employee health.
  3. Cost-effectiveness: The cost-effectiveness of cutting fluids is another crucial consideration for companies. Although environmentally friendly and high-performance cutting fluids may have higher initial costs, they can save more costs in the long run by reducing tool wear, improving machine efficiency, and lowering waste disposal fees. Choosing high-quality cutting fluids can prolong equipment life, reduce production downtime, and further optimize production costs.
  4. Applicability: Different cutting fluids are suitable for different machining materials and processes. When selecting cutting fluids, it is essential to consider their applicability to ensure optimal machining performance. Some cutting fluids may be particularly suitable for machining specific materials such as steel, aluminum alloys, while others may be more suitable for specific machining processes such as drilling, milling, etc.

Addressing Issues with Cutting Fluid

Understanding Cutting Fluid

Cutting fluid is extensively utilized in metal cutting processes for its multifunctional properties including cooling, lubrication, cleaning, and corrosion prevention. However, issues such as unpleasant odors, corrosion, foaming, and allergic reactions may arise during its use. Drawing from our practical experience, we address common problems encountered in cutting fluid applications and propose effective strategies for mitigation.

1. Odor and Deterioration of Cutting Fluid


  • Presence of bacteria in the cutting fluid, including aerobic and anaerobic strains, leading to degradation and odor formation.
  • Contamination during fluid preparation, transport, or through improper storage and handling.
  • Inadequate cleanliness of machining environment and equipment.

Preventive Measures:

  • Employ high-quality and stable cutting fluids to minimize bacterial growth.
  • Use distilled water for fluid preparation to reduce microbial contamination.
  • Maintain recommended concentration levels of cutting fluid to deter bacterial proliferation.
  • Regularly monitor pH levels and adjust as needed to inhibit bacterial growth.
  • Ensure cleanliness of machining environment and equipment to prevent contamination.

2. Corrosion Issues


  • Inappropriate pH levels of the cutting fluid.
  • Contact between dissimilar metals.
  • Improper storage or handling leading to fluid contamination.
  • Elevated humidity levels in the working environment.

Preventive Measures:

  • Maintain cutting fluid pH within recommended ranges for specific metal types.
  • Utilize rust inhibitors or corrosion-resistant fluids when necessary.
  • Control humidity levels in the working environment to minimize corrosion risks.
  • Prevent contact between dissimilar metals to avoid galvanic corrosion.
  • Ensure proper fluid storage and handling procedures to prevent contamination.

3. Foaming Phenomenon


  • Inadequate fluid levels in the sump.
  • Excessive flow rates leading to air entrapment.
  • Poor sump design with sharp angles or inadequate baffles.

Preventive Measures:

  • Maintain adequate fluid levels in the sump to minimize foaming.
  • Regulate flow rates to prevent excessive aeration.
  • Design sumps with rounded corners and proper baffling to reduce foaming tendencies.
  • Use anti-foaming agents as needed, following manufacturer recommendations.

4. Operator Skin Sensitivity


  • High pH levels or chemical composition of the cutting fluid.
  • Contact with insoluble metals or oils used in machining.
  • Incorrect concentration of cutting fluid or presence of contaminants.
  • Formation of protective surface layers or residues.

Preventive Measures:

  • Provide operators with protective gear, including gloves and workwear, to minimize skin exposure.
  • Adhere to recommended concentration levels and fluid specifications to reduce skin sensitization.
  • Use biocides in accordance with recommended dosages to control microbial growth.
  • Choose fluorinated or nitrile rubber seals for machine components to minimize fluid-induced deformations.

Growth Prospects and Market Size of Cutting Fluid

Cutting fluid is an industrial liquid used in metal cutting processes to lubricate the tool-workpiece interface, reduce cutting temperature, flush away chips, and prevent workpiece corrosion. It is designed to improve machining precision, extend tool life, enhance production efficiency, and ensure surface quality and workpiece integrity during machining.

Globally, the outlook and scale of the cutting fluid market maintain a stable and positive growth trend, especially in manufacturing powerhouses like China. With a strong manufacturing base and evolving demand for advanced manufacturing, China presents significant growth potential in the cutting fluid market.

As of 2024, the global cutting fluid market size has reached approximately $3.622 billion. It is projected to grow at a compound annual growth rate (CAGR) of around 4.31% by 2026, reaching an estimated market size of around $4.969 billion. This growth is primarily attributed to sustained development and upgrades in various sectors such as automotive manufacturing, aerospace, machinery manufacturing, mold processing, shipbuilding, and electronics manufacturing. These industries exhibit increasing demand for metal cutting fluids while also raising the performance standards for cutting fluids, thereby driving market growth.


In CNC machining, the use of cutting fluid is indispensable for achieving optimal performance, extending tool life, and maintaining part quality. Understanding its role and selecting the appropriate type of cutting fluid are critical for successful machining operations.

When it comes to selecting the ideal cutting fluid for your CNC machining processes, making the right choice can significantly impact the efficiency and quality of your production. At BOYI, we understand the complexities involved in choosing the perfect cutting fluid for your specific manufacturing needs. Our team of professional technicians is here to provide you with expert guidance and support.

Whether you are dealing with challenging materials, intricate designs, or specific performance requirements, BOYI offers tailored CNC machining services that cater to your unique demands. Don’t leave your machining success to chance—contact BOYI today to consult with our knowledgeable professionals. Let us help you enhance your machining operations, improve tool life, and achieve superior surface finishes with the right cutting fluid solutions.

Reach out to our experts now and take your manufacturing to the next level.


What is the primary purpose of using cutting fluid in CNC machining?

Cutting fluid serves several essential functions in CNC machining. Its primary purposes are to cool the cutting tool and workpiece, reduce friction, and flush away chips from the cutting zone. This enhances machining efficiency, prolongs tool life, and improves the quality of the finished product.

How does cutting fluid improve the surface finish of machined parts?

Cutting fluid helps achieve a smoother surface finish by reducing friction between the cutting tool and the workpiece, which minimizes the formation of built-up edge and tool wear. Additionally, it aids in chip removal, preventing re-cutting of chips that can mar the surface. The lubrication provided by cutting fluid ensures a more consistent and finer finish on the machined part.

Catalog: CNC Machining Guide

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