1. Selection Based on Machine Tool Requirements

When selecting cutting fluids, it is essential to consider the compatibility with the machine tool’s structure. Generally, the cutting fluid specified in the machine tool manual should be used unless there are specific reasons to change it to avoid damage to the machine tool. Some machine tools are designed to use oil-based cutting fluids and lack special mechanisms to protect internal structures from water and vapor ingress. Such machine tools rely on oil-based cutting fluids to lubricate moving parts near the cutting area and, therefore, must use oil-based fluids.

 Using water-based cutting fluids can lead to water penetration into bearings and internal structures, causing parts to lose oil and corrode. Water-based fluids can also emulsify hydraulic oil in hydraulic systems, altering its properties and affecting normal operation. Therefore, when switching from oil-based to water-based fluids, caution and possibly modifications are needed to avoid damaging the machine tool.

1. Selection Based on Tool MaterialTool Steel Tools

Tool steel tools have a heat resistance temperature of about 200-300°C and are suitable for cutting general materials. They lose hardness at high temperatures. Due to poor heat resistance, good cooling effects are needed, typically achieved with emulsions.

1、High-Speed Steel

High-speed steel is an advanced alloy steel based on chromium, nickel, tungsten, molybdenum, and vanadium (sometimes aluminum). Its heat resistance is significantly higher than tool steel, with a maximum permissible temperature of up to 600°C. High-speed steel has a series of advantages over other high-temperature metals and ceramic materials, particularly its high toughness, making it suitable for complex geometric workpieces and continuous cutting. It also offers good machinability and affordable pricing. For low and medium-speed cutting, oil-based cutting fluids or emulsions are recommended. For high-speed cutting, where heat generation is substantial, water-based cutting fluids are preferred to avoid smoke pollution and workpiece burns, which can degrade machining quality and increase tool wear.

2、Cemented Carbide Tools

Cemented carbide used in cutting tools consists of tungsten carbide (WC), titanium carbide (TiC), tantalum carbide (TaC), and 5%-10% cobalt. Its hardness exceeds high-speed steel, with a maximum working temperature of up to 1000°C. It offers excellent heat resistance and reduces chip bonding during steel material machining. When selecting cutting fluids, consider the sensitivity of cemented carbide to thermal shock, ensuring even tool heating to prevent chipping. Dry cutting is common for general materials, but high workpiece temperatures can cause thermal deformation, affecting machining accuracy. Without lubricants, cutting resistance increases power consumption and tool wear. Oil-based cutting fluids with anti-wear additives are typically chosen due to their better thermal conductivity, reducing the risk of sudden cooling compared to water-based fluids. When using coolants, ensure even tool cooling, preferably pre-cooling the tool with cutting fluid before starting. For high-speed cutting, use high-flow cutting fluids to prevent uneven heating and tool chipping while minimizing pollution from excessive evaporation.

3、Ceramic Tools

Made from sintering alumina (Al2O3), metals, and carbon at high temperatures, ceramic tools offer better high-temperature wear resistance than cemented carbide. They are generally used for dry cutting, but water-based cutting fluids are often used for even cooling and avoiding excessive temperatures.

4、Diamond Tools

Diamond tools are extremely hard and generally used for dry cutting. Like ceramic materials, water-based cutting fluids are used in many cases to avoid excessive temperatures.

III. Selection Based on Processing Type

Different cutting processes have varying metal removal characteristics, with more challenging machining requiring higher standards for cutting fluids. The difficulty of cutting processes, ranked from hardest to easiest, is: internal broaching, external broaching, thread tapping, threading, gear hobbing, deep hole drilling, boring, threading with forming tools, high-speed low-feed threading, milling, drilling, planing, turning (single-edge tools), sawing, and grinding.

1、Turning and Boring

(1) Rough Turning: Rough turning involves large machining allowances, resulting in significant cutting depth and feed rates, increased cutting resistance, and heat generation, causing severe tool wear. It is important to select water-based cutting fluids with cooling effects, cleaning, lubrication, and rust prevention to dissipate cutting heat and reduce temperatures, thus improving tool durability. Extreme pressure emulsions are preferred for better cooling and lubrication, extending tool life and enhancing cutting efficiency. When using water-based fluids, maintain machine tool guideways by wiping off cutting fluid and applying lubricating oil at the end of the day.

(2) Finish Turning: Finish turning involves small machining allowances, with cutting depths of only 0.05-0.8mm and small feed rates, requiring precision and surface finish. Due to low cutting force and temperature, high-concentration (10% or more) emulsions and oil-based cutting fluids with additives are suitable. For high-precision turning, such as fine threading, rapeseed oil, soybean oil, or similar products should be used for lubrication to meet accuracy requirements. Due to the instability and ease of oxidation of vegetable oils, some factories use a mix of 15% JQ-1 precision cutting lubricant and 85% L-AN32 system oil for precision cutting oil, achieving excellent results.

(3) Boring: Boring shares similar mechanics with turning but involves internal hole machining with limited cutting and speed and poor heat dissipation. Emulsions are recommended, with increased cutting fluid flow and pressure during use.

2、Threading

During threading, tools contact the material in a wedge shape, with cutting edges surrounded by the material. The cutting torque is high, chip removal is challenging, and heat is not efficiently dissipated, causing tool wear and vibration. Particularly in thread cutting and tapping, conditions are harsher, sometimes resulting in tool chipping and tap breakage. Cutting fluids must have low friction coefficients and high extreme pressure to reduce tool friction and extend tool life. Cutting fluid permeability is crucial in tapping, affecting tap durability. Low-viscosity oils offer better penetration, sometimes supplemented with diesel or kerosene. In cases like blind hole tapping, where fluid penetration is difficult, high-viscosity, adhesive cutting fluids are more effective.

3、Reaming

Reaming is a precision hole-finishing process requiring high accuracy. It involves low-speed, small-feed cutting, where the tool is pressed against the hole wall. Chip fragments can remain in tool grooves or adhere to cutting edges, affecting the cutting band’s extrusion, damaging accuracy and surface roughness, increasing torque, and causing built-up edge and tool wear. Reaming involves boundary lubrication, with high-concentration extreme pressure emulsions or cutting oils providing optimal results. For deep-hole reaming, deep-hole drilling cutting oils with good lubrication meet process requirements.

4、Deep Hole Drilling

Deep hole drilling (gun drilling) is a recently developed deep-hole machining technique. Traditional deep-hole machining (hole depth-to-diameter ratio greater than 5) requires multiple processes like drilling, boring, rough reaming, and grinding to achieve high precision and low surface roughness. New techniques using specialized tools and high-pressure cooling lubrication systems consolidate these steps into a single continuous operation, achieving high-depth, high-precision, low-surface roughness hole machining efficiently and economically. High-performance deep-hole drilling fluids are key to this technique, requiring:

Good cooling to eliminate heat from deformation and friction, suppressing built-up edge formation.

Excellent high-temperature lubrication to reduce tool edge and support friction and wear, maintaining lubrication under high cutting zone temperatures.

Good permeability and chip removal, ensuring cutting fluid reaches tool edges and chips are expelled. Therefore, deep-hole drilling fluids require high extreme pressure and low viscosity.

Conclusion

Selecting the wrong cutting fluid can damage tools and materials, failing to achieve desired results. Conversely, using high-quality cutting fluids can improve machining performance, product quality, and tool longevity, yielding significant economic benefits despite higher costs.