Cutting fluid is also commonly referred to as "coolant". In the metal processing process, in addition to the functions of lubrication, cooling, chip removal and rust prevention, it also has anti-foaming, environmentally friendly and antibacterial functions. The selection of cutting fluid is related to the surface quality and processing accuracy of the workpiece, and is also closely related to the material being processed, the processing process and the tool material.
1. Typical materials of CNC tools
(1) High-speed steel
High-speed steel is a high-alloy tool steel with a large amount of alloying elements such as tungsten (W), molybdenum (Mo), chromium (Cr) and vanadium (V). High-speed steel has high hardness (62HRC ~ 67HRC) and heat resistance, and can still cut at cutting temperatures as high as 500℃~650℃; high-speed steel has high strength (bending strength is 2~3 times that of general cemented carbide and 5~6 times that of ceramics) and good toughness, and can be used in occasions with impact and vibration; it can be used to process a wide range of materials such as non-ferrous metals, structural steel, cast iron, and high-temperature alloys. High-speed steel has good manufacturing processability and is easy to sharpen. It is suitable for manufacturing various types of tools, especially for manufacturing tools with complex shapes such as drills, broaches, forming tools, gear tools, etc.
According to cutting performance, high-speed steel can be divided into ordinary high-speed steel and high-performance high-speed steel; according to manufacturing process, it can be divided into smelting high-speed steel and powder metallurgy high-speed steel.
(2) Cemented carbide
Cemented carbide is a powder metallurgy product sintered under high temperature conditions using high-hardness, refractory metal carbides (WC, TiC, etc.) and metal binders (Cr, Ni, etc.). The room temperature hardness of cemented carbide reaches 89HRA~93HRA, and its hardness is 77HRA~85HRA at 760℃. Cemented carbide can also be used for cutting at 800℃~1000℃. The tool life is several to dozens of times higher than that of high-speed steel tools. It can process a variety of materials including hardened steel. However, the strength and toughness of cemented carbide are inferior to those of high-speed steel. Its impact toughness at room temperature is only 18 to 130 of that of high-speed steel. Therefore, cemented carbide has poor ability to withstand cutting vibration and impact. Table 1 shows several types of cemented carbide.
(3) Ceramic tools
Ceramic tools have high hardness, wear resistance and good high-temperature mechanical properties. They have low affinity with metals, are not easy to bond with metals, and have good chemical stability. Ceramic tools are mainly used in the cutting of steel, cast iron and its alloys and difficult-to-cut materials. They can be used for ultra-high-speed cutting, high-speed cutting and hard material cutting.
(4) Superhard tools
Superhard materials refer to artificial diamond and cubic boron nitride (CBN), as well as polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) sintered with these powders and binders.
Superhard materials have excellent wear resistance and are mainly used in high-speed cutting and the processing of difficult-to-cut materials.
Common types of cutting fluids
To meet different processing requirements, there are many types of cutting fluids. According to their chemical composition and state, they can be divided into two categories: water-based cutting fluids and oil-based cutting fluids. Cutting fluids that are generally diluted with water are called water-based cutting fluids, and cutting fluids that do not need to be diluted with water are called oil-based cutting fluids. Water-based cutting fluids are mainly used for cooling and have poor lubrication performance. Commonly used water-based cutting fluids include: rust-proof emulsions, rust-proof lubricant emulsions, extreme pressure emulsions, microemulsions, etc.; oil-based cutting fluids are mainly used for lubrication and have poor cooling and cleaning effects. Commonly used oil-based cutting fluids include: pure mineral oil, fatty oil (or oily additives) + mineral oil, inactive extreme pressure cutting oil, active extreme pressure cutting oil, etc. The composition and application characteristics of several major cutting fluids are shown in Table 2.
Reasonable selection of cutting fluids
(1) High-speed steel tools
This material is a high-grade alloy steel based on chromium, nickel, tungsten, molybdenum, vanadium (some also contain aluminum). Their heat resistance is significantly higher than that of tool steel, and the maximum allowable temperature can reach 600 ℃. Compared with other high-temperature resistant metal and ceramic materials, high-speed steel has a series of advantages, especially it has higher toughness, suitable for workpieces with complex geometric shapes and continuous cutting processing, and high-speed steel has good machinability and is easily acceptable in price.
Due to the poor red hardness of high-speed steel tools, cutting fluid is required during use. When cutting at low and medium speeds, it is recommended to use oil-based cutting fluid or emulsion. When cutting at high speeds, due to the high heat generation, it is appropriate to use water-based cutting fluid. If oil-based cutting fluid is used, more oil mist will be generated, polluting the environment, and it is easy to cause workpiece burns, reduce processing quality, and increase tool wear. In addition, it is recommended to use extreme pressure aqueous solution or extreme pressure emulsion for rough processing, and extreme pressure emulsion or extreme pressure cutting oil for fine processing to reduce friction, improve surface quality and precision, and extend tool life.
(2) Cemented carbide tools
Cemented carbide is composed of tungsten carbide (WC), titanium carbide (TiC), tantalum carbide (TaC) and 5% to 10% cobalt. Its hardness is much higher than that of high-speed steel. The maximum allowable operating temperature can reach 1000°C. It has excellent wear resistance and can reduce the adhesion between chips when processing steel materials.
Due to the good red hardness of cemented carbide tools, when processing general materials, dry cutting is generally adopted without adding cutting fluid. However, during dry cutting, the temperature rise of the workpiece is high, which makes the workpiece prone to thermal deformation and affects the processing accuracy of the workpiece. Moreover, when cutting without lubricant, the cutting resistance is large, which increases the power consumption and accelerates the wear of the tool. Cemented carbide tools are expensive, so from an economic point of view, dry cutting is not cost-effective.
When selecting cutting fluid, the sensitivity of cemented carbide to sudden heat should be considered, and the tool should be heated evenly as much as possible, otherwise it will cause chipping. Generally, oil-based cutting fluids have poor thermal conductivity and are less likely to cause sudden cooling of the tool than water-based cutting fluids. Therefore, oil-based cutting fluids containing anti-wear additives are generally used. When using cutting fluids for cutting, it is important to cool the tool evenly. It is best to pre-cool the tool before starting cutting. For high-speed cutting, a large flow of cutting fluid should be used to spray the cutting area to avoid uneven heating of the tool and chipping of the blade. This can also reduce oil fume pollution caused by evaporation due to excessive temperature.
(3) Ceramic cutting tools
This material is made of alumina, metal and carbide sintered at high temperatures. Its high-temperature wear resistance is better than that of cemented carbide, so dry cutting is generally used. Considering uniform cooling and avoiding excessive temperatures, water-based cutting fluids are also often used, but it is best to pour continuously and fully without interruption.
(4) Diamond cutting tools
It has extremely high hardness and is generally used for dry cutting. To avoid excessive temperatures, like ceramic materials, water-based cutting fluids are used in many cases.
Conclusion
Reasonable selection of cutting fluid according to different tool materials can not only improve the surface quality and machining accuracy of the workpiece, but also reduce the thermal deformation of the workpiece and the wear of the tool, improve the durability of the tool, reduce production costs, and improve production efficiency. Therefore, it should be paid attention to in production.
