The Functions, Types, and Selection of Cutting Fluids

A Comprehensive Guide for Machining Professionals

Published: August 2026 | Category: Machining Technology | Reading Time: 8 min

In metal cutting operations, the proper selection and application of cutting fluids play a critical role in minimizing friction, reducing cutting forces and temperatures, and limiting thermal deformation of workpieces. When used correctly, cutting fluids directly contribute to improved surface finish, higher machining accuracy, and significantly extended tool life.

This article provides an in-depth look at the four core functions of cutting fluids, the three main categories available on the market, and practical guidelines for selecting the right fluid for your specific machining application.

Why This Matters: Studies show that cutting fluid and related management costs account for approximately 15–17% of total CNC machining costs. Choosing the wrong fluid can lead to premature tool failure, poor surface quality, and increased scrap rates.

1. Key Functions of Cutting Fluids

Cutting fluids serve four essential purposes: cooling, lubrication, cleaning, and rust prevention. Understanding each function is the foundation for making informed selection decisions.

1.1 Cooling Function ❄️

When applied to the cutting zone, cutting fluids reduce the temperature of both the cutting tool and the workpiece through heat transfer and vaporization. The primary objective is to lower the maximum temperature on the rake face of the tool — the area most susceptible to thermal degradation.

The effectiveness of cooling depends on several factors:

• Thermal conductivity — how efficiently heat is transferred away

• Specific heat capacity — how much heat the fluid can absorb

• Heat of vaporization — cooling potential through phase change

• Evaporation rate, flow rate, and flow velocity — delivery efficiency

In general, water-based solutions offer the best cooling performance, oils the poorest, with emulsions falling in between. Triethanolamine solutions deliver cooling performance close to that of pure water-based fluids.

Pro Tip: Experimental results in turning show that directing the cutting fluid upward from the flank face toward the cutting edge yields better cooling efficiency than spraying downward from the rake face. To maximize effectiveness, expand cooling coverage using techniques such as mist cooling and internal cooling.

1.2 Lubrication Function

Cutting fluids penetrate the interface between the tool and workpiece, forming a boundary lubrication film that reduces friction. Under the high pressure and temperature conditions of cutting, parts of this film may thin or break down, causing asperities on the metal surfaces to come into direct contact. The remaining film, however, significantly reduces the area of direct metal-to-metal contact and lowers the friction coefficient.

The performance of this lubrication film depends on its adhesion strength, which is achieved through two mechanisms:

Under extreme pressure (EP) conditions, additives containing sulfur, phosphorus, chlorine, or iodine react with the metal surface to form compounds such as iron sulfide (FeS), iron chloride (FeCl₂), and iron phosphate (FePO₄). These films provide robust lubrication when standard oils would fail.

1.3 Cleaning Function

Cutting fluids flush away fine chips and abrasive particles generated during machining, keeping the workpiece surface, machine tool guideways, and fixtures clean. This prevents secondary damage to the machined surface and helps maintain machine accuracy over time.

1.4 Rust Prevention Function

Rust-inhibiting additives — such as sodium nitrite, trisodium phosphate, triethanolamine, and barium petroleum sulfonate — form a protective film on metal surfaces. This film shields machine tools and workpieces from corrosion caused by air, moisture, and acidic media.

2. Common Types of Cutting Fluids and Their Selection

There are three main categories of cutting fluids: water-based solutions, emulsions, and cutting oils. Each has distinct characteristics that make it suitable for specific machining scenarios.

2.1 Water-Based Solutions

Water is the primary component, supplemented with rust inhibitors. These fluids excel in cooling performance, making them ideal for high-heat operations.

A typical surface-active water solution — used for precision turning and reaming — can be formulated with:

2.2 Emulsions

Emulsions are prepared by diluting emulsified oil with water. Emulsified oil consists of mineral oil, emulsifiers, and additives (e.g., triethanolamine oleate, 69-1 anti-rust emulsified oil, extreme-pressure emulsified oil).

• Low-concentration emulsions (e.g., 3–5%) prioritize cooling and are suitable for rough machining.

• High-concentration emulsions (e.g., 10–20%) provide enhanced lubrication and are recommended for finish machining and complex operations.

2.3 Cutting Oils

Cutting oils include both mineral and natural options:

Selection pointers:

• General turning and tapping → machine oil

• Finish machining of non-ferrous metals and cast iron → low-viscosity kerosene + mineral oil blend

• Automatic machine tools → light diesel oil (low viscosity, good flow)

• Difficult-to-cut materials → EP cutting oil with sulfur, chlorine, or phosphorus additives

3. Selection Guidelines

The selection of cutting fluids should be based on four key factors:

3.1 Quick-Reference Selection Table

3.2 Material-Specific Recommendations

4. Best Practices for Application

4.1 Application Methods

• Flood cooling: The most common method — fluid is flooded over the cutting zone. Ensure adequate flow rate and pressure.

• Mist/spray cooling: Atomized fluid is sprayed at high velocity. Excellent for hard-to-reach areas and high-speed operations.

• Internal cooling: Fluid is delivered through channels inside the tool. Ideal for deep-hole drilling and boring.

4.2 Maintenance Tips

Extend Fluid Life: Regularly monitor concentration with a refractometer, maintain pH between 8.0–9.5, filter out chips daily, and control bacterial growth with appropriate additives. Replace fluid immediately if you detect foul odors or visible discoloration.

• Concentration control: Keep water-based fluids at 5–12% concentration

• pH monitoring: Maintain 8.0–9.5 to prevent bacterial growth and corrosion

• Filtration: Daily circulation filtering to remove chips and contaminants

• Replacement cycle: Emulsions (3–6 months), semi-synthetics (6–12 months), full synthetics (12–24 months)

Conclusion

Cutting fluids are far more than a simple coolant — they are a critical process variable that directly impacts tool life, surface quality, machining accuracy, and overall production costs. The key to success lies in matching the fluid to the specific combination of workpiece material, tool material, machining method, and quality requirements.

Remember: when in doubt, prioritize cooling for rough machining and lubrication for finish machining. And always consult with your cutting fluid supplier for application-specific recommendations — a small adjustment in fluid selection can yield significant improvements in productivity and part quality.