During the metal cutting process, to increase cutting efficiency, improve workpiece precision, reduce surface roughness, extend tool life and achieve optimal economic results, it is vital to minimize the friction between the tool and the workpiece , as well as between the tool and the chips.
Furthermore, it is crucial to promptly dissipate the heat generated by the deformation of the material in the cutting zone.
To achieve these goals, on the one hand, advances have been made through the development of high-hardness, high-temperature-resistant tool materials and the refinement of tool geometries.
The introduction of materials such as carbon steel, high-speed steel, tungsten carbide and ceramics, as well as the use of interchangeable tools, have substantially accelerated metal cutting rates.
On the other hand, the use of high-performance cutting fluids often significantly increases cutting efficiency, reduces surface roughness, and extends tool life, leading to superior, cost-effective results. The functions of cutting fluids include:
I. Cooling effect
The cooling effect depends on heat transfer by convection and vaporization of the cutting fluid to remove heat from solids (tools, parts and chips), thereby lowering the temperature in the cutting area and reducing distortion of the part, maintaining hardness and the size of the tool.
The efficiency of this cooling effect depends on the thermal properties of the fluid, especially its specific heat capacity and thermal conductivity.
Furthermore, fluid flow conditions and heat exchange coefficients play essential roles. The heat exchange coefficient can be increased by adjusting the surfactant materials and latent heat.
Water, with its high specific heat capacity and impressive thermal conductivity, outperforms oil-based cutting fluids in terms of cutting performance. Modifying the flow conditions, such as increasing the flow speed and volume, can effectively increase the cooling effect of the cutting fluid.
This method is particularly beneficial for oil-based cutting fluids with inferior cooling effects. In deep drilling and high-speed gear machining, increasing fluid supply pressure and volume has shown improvements.
Spray cooling, which facilitates the vaporization of the liquid, also significantly increases the cooling effect.
The cooling effectiveness of a cutting fluid is influenced by its permeability. Fluids with good permeability cool the cutting edge more quickly. The permeability of cutting fluids is related to their viscosity and wettability. Low viscosity fluids have better permeability than high viscosity fluids.
Oil-based cutting fluids tend to have better permeability than water-based cutting fluids, but water-based cutting fluids containing surfactants have significantly improved permeability.
The wettability of a cutting fluid is related to its surface tension. When liquid has high surface tension, it tends to form droplets on solid surfaces, resulting in low permeability.
On the other hand, when the liquid has low surface tension, it spreads over the solid, with the solid-liquid-gas contact angle being minimal, or even zero. This leads to excellent permeability, allowing liquid to flow quickly into spaces where the tool comes into contact with the workpiece and chips, thus intensifying the cooling effect.
The quality of the cooling effect is also associated with foam formation. Because foam consists mainly of air, which has low thermal conductivity, cutting fluids with excess foam exhibit decreased cooling performance.
This is why synthetic cutting fluids containing surfactants typically include a small amount of emulsified silicone oil to serve as an antifoam agent.
Recent studies have shown that ionic water-based cutting fluids can quickly neutralize the static charge generated during cutting and grinding due to intense friction, preventing part overheating and offering exceptional cooling effects.
These ionic cutting fluids are now widely used as cooling lubricants for high-speed grinding and aggressive grinding processes.
II. Lubrication function
During machining, friction occurs between the cutting tool and the chips and between the tool and the workpiece surface. Cutting fluids act as lubricants to reduce this friction.
For the cutting tool, given its relief angle during machining, it contacts the material being machined less than the primary cutting face, resulting in reduced contact pressure.
The frictional lubrication condition on the relief face approaches a limit state of lubrication. Strongly adsorbing substances such as oily agents and extreme pressure (EP) agents with reduced shear strength effectively reduce this friction.
The situation on the primary cutting face is different; As the deformed chip is forced outward by tool pressure, the contact pressure increases and the chip, undergoing plastic deformation, heats up.
After application of cutting fluid, the chip contracts abruptly due to cooling, reducing the chip contact length on the primary cutting face and the metallic contact area between the chip and the tool.
This also reduces the average shear stress, resulting in a larger shear angle and reduced cutting force, improving the machinability of the workpiece material.
During grinding, the addition of grinding fluid forms a lubricating film between the grinding grain, the workpiece and the chips. This layer of lubrication reduces friction, prevents abrasive wear on the edges and improves the surface finish.
Generally, oil-based cutting fluids outperform water-based cutting fluids, with the best results coming from oil-based fluids containing oily and EP additives. These oily additives are typically long-chain organic compounds with polar groups, such as fatty acids, alcohols, and vegetable or animal fats.
They form a lubricating layer on the metal surface, reducing friction between the tool and the part and chips, aiming to reduce cutting resistance, extend tool life and improve surface finish.
Oil additives work better at lower temperatures; above 200°C, its adsorption layer is compromised, losing its lubricating properties. Hence, oil-containing cutting fluids are used for low-speed precision cutting, while high-speed, heavy-duty cutting requires cutting fluids with EP additives.
EP additives contain elements such as sulfur, phosphorus and chlorine, which chemically react with metals at high temperatures to form compounds such as iron sulfide, iron phosphate and iron chloride, all of which have low shear strength.
This reduces cutting resistance and friction between the tool, the workpiece and the chips, facilitating the cutting process. Cutting fluids containing EP also prevent chip build-up and improve surface finish.
Iron chloride has a layered crystalline structure, providing the lowest shear strength. Compared to iron sulfide, it has a lower melting point and loses its lubricating properties at around 400°C.
Iron phosphate is between iron chloride and iron sulfide in properties. Iron sulfide withstands temperatures of up to 700°C and is typically used in cutting fluids for heavy cutting and machining difficult-to-cut materials.
In addition to forming low-shear lubricating layers on ferrous metals such as steel and iron, EP additives also fulfill this function on non-ferrous metals such as copper and aluminum. However, for cutting non-ferrous metals, reactive EP additives should be avoided to prevent corrosion of the workpiece.
The lubricating effect of cutting fluids is also linked to their penetrating properties; those with good penetration allow lubricants to quickly access the interfaces between chips, tools and parts, forming lubricating films that reduce friction coefficients and cutting resistance.
Recent studies suggest that, in addition to the lubrication effects mentioned above, cutting fluids can penetrate tiny cracks in metal surfaces, altering the physical properties of the material being machined, thus reducing cutting resistance and facilitating the machining process.
III. Cleaning action
During metal cutting processes, chips, metal powders, grinding debris and oil residues can easily adhere to the surface of the workpiece, cutting tools and grinding wheels. This affects cutting performance and dirty both the workpiece and the machine tool.
Therefore, cutting fluids must have excellent cleaning properties. For oil-based cutting fluids, the lower the viscosity, the greater the cleanability. Cutting fluids that contain light components such as diesel and kerosene offer superior penetration and cleaning performance.
Water-based cutting fluids containing surfactants produce better cleaning results.
On the one hand, surfactants can adsorb various particles and oily sludge, forming an adsorption film on the surface of the part, preventing adhesion on the part, tools and grinding wheels.
On the other hand, they can penetrate the interface where particles and oil residues adhere, separating them and removing them with the cutting fluid.
The cleaning ability of cutting fluids must also be evident in the effective separation and sedimentation of debris, grinding particles, metal powders and oil residues.
Recycled cutting fluids should quickly settle particles such as metal chips, powders, grinding debris and microparticles to the bottom of the container after returning to the cooling tank, while oil residues float to the surface.
This ensures that the cutting fluid remains clean even after repeated use, ensuring processing quality and extending its useful life.
4. Rust prevention
Throughout the machining process, if the part comes into contact with corrosive substances produced by the decomposition or oxidation of water and cutting fluids, such as sulfur, sulfur dioxide, chloride ions, acids, hydrogen sulfide and alkalis, it becomes susceptible to corrosion.
Machine parts in contact with cutting fluids can also corrode. If the cutting fluid does not have rust prevention capabilities, the workpiece may suffer from chemical and electrochemical corrosion due to moisture and corrosive substances in the air during post-processing storage or between operations, leading to rust.
Therefore, cutting fluids must have superior rust prevention properties, which is one of their fundamental characteristics.
Cutting oils generally have some rust prevention features. If the storage period between operations is not long, there is no need to add rust inhibitors. Adding rust inhibitors, such as barium sulfonates and petroleum, to cutting oil can decrease its anti-wear properties.
