Cutting fluid is a type of coolant and lubricant designed specifically for metalworking processes, such as machining and stamping. There are various kinds of cutting fluids, which include oils, oil–water emulsions, pastes, gels, aerosols (mists), air, and other gases. Cutting fluids are made from petroleum distillates, animal fats, plant oils, water and air, or other raw ingredients. Depending on context and on which type of cutting fluid is being considered, it may be referred to as cutting fluid, cutting oil, cutting compound, coolant, or lubricant.
Most metalworking and machining processes can benefit from the use of cutting fluid, depending on workpiece material. Common exceptions to this are cast iron and most types of brass, which may be machined dry.
The properties that are sought after in a good cutting fluid are the ability to: Keep the workpiece at a stable temperature (critical when working to close tolerances). Very warm is acceptable, but extremely hot or alternating hot-and-cold are avoided. Maximize the life of the cutting tip by lubricating the working edge and reducing tip welding. Ensure safety for the people handling it (toxicity, bacteria, fungi) and for the environment upon disposal. Prevent rust on machine parts and cutters.
Function
Cooling Metal cutting generates heat due to friction and energy lost deforming the material. The surrounding air has low thermal conductivity (conducts heat poorly) meaning it is a poor coolant. Ambient air cooling is sometimes adequate for light cuts and low duty cycles typical of maintenance, repair and operations (MRO) or hobbyist work. Production work requires heavy cutting over long time periods and typically produces more heat than air cooling can remove. Rather than pausing production while the tool cools, using liquid coolant removes significantly more heat more rapidly, and can also speed cutting and reduce friction and tool wear.
However, it is not just the tool which heats up but also the work surface. Excessive temperature in the tool or work surface can ruin the temper of both, soften either to the point of uselessness or failure, burn adjacent material, create unwanted thermal expansion or lead to unwanted chemical reactions such as oxidation.
Regulating the heat created during machining processes is necessary to extend tool life, prevent the alteration of the workpiece's heat treatment, and prevent warping of the piece. The use of cutting fluids allows machinists to cut faster than they would be capable of if relying on surrounding air to cool the workpiece.
Studies on alloyed structural steels used in power engineering have demonstrated that lubricating liquids may significantly modify chip formation mechanisms and the morphology of cutting products during turning operations. Further investigations have shown that technological modification of 38KhN3MFA rotor steel can alter its structural-phase state and affect chip morphology under variable machining conditions.
Lubrication Besides cooling, cutting fluids also aid the cutting process by lubricating the interface between the tool's cutting edge and the chip. By preventing friction at this interface, some of the heat generation is prevented. This lubrication also helps prevent the chips from being welded onto the tool, which would interfere with subsequent cutting.
Rehbinder effect The Rehbinder effect is the reduction in strength of a material when a surfactant, available in cutting fluids, is applied. Cutting fluid coats the cutting surface and reduces its surface energy, effectively weakening the material. This decreases the force required to make cuts, the wear on tools, and the time required for machining processes. The Rehbinder effect is complicated, relying on the chemical properties and structures of both the cutting fluid and working material, and is therefore difficult for machinists to factor into the planning of machining processes. Experienced machinists may utilize "rules of thumb" or trial and error methods if they consider the effect at all.
Extreme pressure additives Extreme pressure additives in cutting fluids create a barrier between the cutting tool and working material. This barrier prevents contact between the two. If the cutting tool and working material were to make contact, particles from the working material could be welded to the cutting tool. These added particles would decrease the accuracy of the tool and its cuts, increase the friction in cutting, and lower the quality of the surface finish.
Delivery methods
There are several different delivery methods of cutting fluids that are used in machine cutting, this includes:
Dry Machining MQL Machining Cryogenic Machining Pressure or Jet Cooling Flooded Cooling Cryogenic MQL Machining Dry Machining is exactly like it sounds; it doesn't use any lubrication during the machining process. Dry machining has fewer economic and ecological impacts than other delivery systems; however, it will often cause temperature increases, which can affect tool life.
Minimum Quantity Lubrication (MQL) Machining is a delivery system where a small amount of lubrication is used during machine cutting instead of using large amounts of fluid. MQL is regularly used due to its ability to extend tool life while remaining eco-friendly.
Cryogenic Machining uses cryogenic liquids as coolant with a regulated pressure and volume. This pressure and volume increase the tool life of the machine while simultaneously improving the surface finish on the products. Cryogenic Machining is also considered to be a sustainable option due to its environmentally friendly nature.
Pressure or Jet Cooling uses high-pressure water/liquid that sprays directly on the machine cutting surface.
Flood cooling is widely used in machining and typically involves an oil-water mixture as the cutting fluid. For machining additively manufactured metals, biodegradable vegetable-based oils are often chosen by companies. The fluid will cover, flood, the cutting zone during machining. This method combines water's cooling ability with oils lubrication, but the flow rate must be regulated properly for effective temperature control and performance.
Cryogenic MQL Machining is a combination of Cryogenic Machining and MQL Machining. Created to address the issues of Cryogenic and MQL, its Cryogenic cooling helps reduce the temperature of the cutting area while the MQL keeps the area lubricated. Combining the two methods can help address issues found in both.
Every conceivable method has its strength and weaknesses; however, the best choice will depend on the application and the equipment available. For many metal cutting applications, the ideal has long been high-pressured, high-volume pumping to force a stream of liquid (usually an oil-water emulsion) directly into the tool-chip interface, with walls around the machine to contain the splatter and a sump to catch, filter, and recirculate the fluid (Flooded Cooling). This type of system is commonly employed, especially in manufacturing. It is often not a practical option for maintenance, repair and overhaul or hobbyist metal cutting, where smaller, simpler machine tools are used. Fortunately, it is also not necessary in those applications, where heavy cuts, aggressive speeds and feeds, and constant, all-day cutting are not vital.
Through-tool coolant systems, also known as through-spindle coolant systems, are systems plumbed to deliver coolant through passages inside the spindle and through the tool, directly to the cutting interface. Many of these are also high-pressure coolant systems, in which the operating pressure can be hundreds to several thousand psi (1 to 30 MPa)—pressures comparable to those used in hydraulic circuits. High-pressure through-spindle coolant systems require rotary unions that can withstand these pressures. Drill bits and endmills tailored for this use have small holes at the lips where the coolant shoots out. Various types of gun drills also use similar arrangements.
Types
Liquids There are generally three types of liquids: mineral, semi-synthetic, and synthetic. Semi-synthetic and synthetic cutting fluids represent attempts to combine the best properties of oil with the best properties of water by suspending emulsified oil in a water base. These properties include: rust inhibition, tolerance of a wide range of water hardness (maintaining pH stability around 9 to 10), ability to work with many metals, resist thermal breakdown, and environmental safety.OSHA (1999). Metalworking Fluids: Safety and Health Best Practices Manual. Salt Lake City: U.S. Department of Labor, Occupational Safety and Health Administration.
Water is a good conductor of heat but has drawbacks as a cutting fluid. It boils easily, promotes rusting of machine parts, and does not lubricate well. Therefore, other ingredients are necessary to create an optimal cutting fluid.
Mineral oils, which are petroleum-based, first saw use in cutting applications in the late 19th century. These vary from the thick, dark, sulfur-rich cutting oils used in heavy industry to light, clear oils.
Semi-synthetic coolants, also called soluble oil, are an emulsion or microemulsion of water with mineral oil. In workshops using British English, soluble oil is colloquially known as SUDS. These began to see use in the 1930s. A typical CNC machine tool usually uses emulsified coolant, which consists of a small amount of oil emulsified into a larger amount of water through the use of a detergent.
Synthetic coolants originated in the late 1950s and are usually water-based.
The official technique to measure oil concentration in cutting fluid samples is manual titration: 100ml of the fluid under test is titrated with a 0.5M HCl solution to an endpoint of pH 4 and the volume of titrant used to reach the endpoint is used to calculate the oil concentration. This technique is accurate and not affected by fluid contamination, but needs to be performed by trained personnel in a laboratory environment. A hand-held refractometer is the industrial standard used to determine the mix ratio of water-soluble coolants that estimates oil concentration from the sample refractive index measured in the Brix scale. The refractometer allows for in situ measurements of oil concentration within industrial plants. However, contamination of the sample reduces the accuracy of the measure. Other techniques are used to measure the oil concentration in cutting fluids, such as measure of the fluid viscosity, density, and ultrasound speed. Other test equipment is used to determine such properties as acidity and conductivity.
Others include: Kerosene and rubbing alcohol often give good results when working on aluminum. WD-40 and 3-In-One Oil work well on various metals. The latter has a citronella odor; if the odor offends, mineral oil and general-purpose lubricating oils work about the same. Way oil (the oil made for machine tool ways) works as a cutting oil. In fact, some screw machines are designed to use one oil as both the way oil and cutting oil. (Most machine tools treat way lube and coolant as separate things that inevitably mix during use, which leads to tramp oil skimmers being used to separate them back out.) Motor oils have a slightly complicated relationship to machine tools. Straight-weight non-detergent motor oils are usable, and in fact SAE 10 and 20 oils used to be the recommended spindle and way oils (respectively) on manual machine tools decades ago, although nowadays dedicated way oil formulas prevail in commercial machining. While nearly all motor oils can act as adequate cutting fluids in terms of their cutting performance alone, modern multi-weight motor oils with detergents and other additives are best avoided. These additives can present a copper-corrosion concern to brass and bronze, which machine tools often have in their bearings and leadscrew nuts (especially older or manual machine tools).