CNC machining is a subtractive manufacturing technology where a cutting tool, attached to a spinning spindle, removes the material from the workpiece to perform the desired machining operation.
The high-speed spinning spindle comes in direct contact with the material, which generates a lot of heat around the cutting area.
This is where the cutting fluid comes into play. The cutting fluid provides the necessary heat dissipation and lubrication to ensure a smooth and efficient machining operation.
In this article, I have discussed CNC cutting fluids, their functions, types, and factors to consider before selecting a cutting fluid for your application.
What is CNC cutting fluid and Why is it needed?
Cutting fluid is a type of industrial liquid that primarily regulates the working temperature by taking away the heat generated during the cutting process and also serves the purpose of lubrication. Generally, a good cutting fluid should have specific properties like high heat capacity, high lubricity, and low viscosity.
If the heat generated due to the friction between the workpiece and the tool exceeds a certain limit, it might cause thermal expansion of the material and even weld the surfaces together.
Hence, it is essential to keep the temperature in check by lubricating the point of contact and absorbing the generated heat away from the cutting area.
Furthermore, the lubrication effect provided by the cutting fluid reduces the friction, thereby reducing wear and tear on the tools and increasing their lifetime.
Cutting fluid also prevents corrosion by creating an extremely thin protective layer over the machine parts and the workpiece.
In certain manufacturing applications, a high-pressure jet of cutting fluid facilitates the clearance of chips and debris from the cutting area, thereby enhancing the surface finish of the workpiece.
Properties of cutting fluid
Cutting fluid is an essential part of manufacturing process. It has several functions like cooling, lubricating, preventing corrosion, and cleaning debris.
Therefore, an ideal cutting fluid should have certain properties that facilitate it to perform the above-mentioned functions.
Thermal conductivity and heat capacity
Cutting fluid should have high thermal conductivity and high heat capacity.
High thermal conductivity allows the cutting fluid to absorb the heat quickly and prevent overheating of the workpiece and the tool.
A high heat capacity means that the cutting fluid can absorb a large amount of heat before it reaches its boiling point.
Hence, a cutting fluid with high heat capacity needs to be replaced less often than a one with low heat capacity.
It is recommended to use a low viscosity cutting fluid, as it can flood the workpiece and seep into the holes and grooves for efficient cleaning.
Low viscosity also prevents the formation of sludge when the cutting fluid mixes with the chips or debris.
Toxicity and corrosion
It is essential for the cutting fluid to be non-toxic and non-corrosive so that it doesn't corrode the workpiece or the tool and is not harmful to humans.
Cutting fluid should be inert and not react with the workpiece and the tool.
Being chemically reactive will damage the surface of the workpiece and degenerate the fluid.
Odor and color
It is desirable that the cutting fluid is odorless and relatively colorless to avoid any lingering smell or reduce the visibility of the machining operation.
factors to consider before choosing your cutting fluid
Apart from the properties of the cutting fluid, you must be wary of some other factors that affect the performance of cutting fluids.
Different metals have different requirements and selecting the appropriate cutting fluid will enhance the performance of the machining operation.
Cast iron is one of the most easily machinable metals, as the graphite content present in it helps with lubrication.
However, machining of cast iron can form small chunks which pose the risk of clogging the machine parts.
Hence, when machining cast iron, it is recommended to use a cutting fluid with excellent cleaning properties.
Hard machinable metals, like stainless steel, need better lubrication and anti-weld properties because of the extreme heat generated during the machining process.
Aluminum, which is a soft machinable material, is prone to staining due to the sulfur content in the cutting fluid. Hence, you should choose a non-staining cutting fluid for machining aluminum.
High-speed machining would require rapid cooling, and therefore, cutting fluids with low viscosity and good thermal conductivity should be used for this purpose.
What are The Types of cutting fluids?
The important functions of cutting fluid are cooling, lubrication, cleaning debris, and preventing corrosion.
Depending on what type of manufacturing operation you are performing, the need for the cutting fluid will vary, and hence, you should choose your cutting fluid accordingly.
Emulsions are soluble oils that are made by adding mineral oil to water in particular proportions (1 to 20% oil to water).
It also consists of some additives, such as sodium sulfonate for better mixing of oil and water, anti-corrosion additives to improve corrosion-resistant properties, biocides to prevent bacterial growth, and anti-wear additives to improve lubrication.
This type of cutting fluid is used where rapid cooling is required, such as in high-speed drilling, milling, turning, and grinding operations.
Emulsions provide better lubrication and anti-corrosion than synthetic cutting fluids.
Pure oils are derived from animals or plants. As a result, they are considered eco-friendly cutting fluids.
They provide excellent lubrication, and the lack of water content improves their rust prevention properties.
However, being organic makes them comparatively more costly than other cutting fluids.
This is why pure oils are only used as supplements to other cutting fluids during heavy cutting and drilling operations to improve lubrication.
Synthetic fluids are made by combining organic and inorganic chemical components in water along with certain additives to improve lubrication, rust, and corrosion prevention.
Synthetic fluids are very efficient in cooling and cleaning chips from the workarea.
Semi-synthetic fluids are used as cutting fluids. They are a mixture of synthetic, water-based, and emulsion fluids. Hence, offering properties of both emulsions and synthetic fluids.
CNC cutting fluid delivery System
Depending on the type of CNC operation being performed, the cutting fluid can be delivered to the machining area through various different methods.
Using compressed air is a better alternative to using ambient air as a coolant.
Compressed air is one of the easiest, cheapest, and mess-free coolants that can be used to regulate the temperature of the cutting process.
The high-pressure jet of the compressed air blows the chips away and provides cooling action.
Compressed air is an ideal coolant for applications such as plastics, that are subjected to thermal shock, the phenomenon of rapid contraction and expansion of material, under the action of liquid coolants.
Liquid nitrogen can also be used as an effective coolant. The nitrogen freezes the machined surfaces and quickly absorbs the heat away from the workpiece-tool point of contact.
Carbon dioxide is another option that can be used as a cutting fluid.
Compressed liquid CO2 is made to expand, converting it to solid and causing a drop in temperature. These CO2 crystals are then targeted at the machined surface, thereby producing the cooling effect.
Mist Delivery System
Mist-type delivery is where tiny droplets of the cutting fluid are sprayed over the workarea.
This type is suitable for applications that don't have a major cooling or chip cleaning requirement and the workpiece just needs lubrication.
Although mist cooling is easier to use and reduces the mess, it is not the ideal system as most of the fluid is scattered in the air and wasted.
Furthermore, the scattered mist can be inhaled by the operator, making it unsafe for use on busy shop floors.
Mist delivery is generally used for milling and lathe coolant systems.
Flood Delivery System
Flooding of the cutting fluid with pressure under 1000 psi is done where chip removal is a major concern to prevent chip recutting.
Flood-type cutting fluid is used where chip removal is a major concern to prevent chip recutting.
In this system, a cutting fluid with pressure under 1000 psi is used to flood the machining area, thereby producing cooling effect along with chip clearance away from the cutting area.
This type of delivery is commonly used in CNC milling operations where the chips can get accumulated around the cutting area and jammed in the helix of end mills.
Haas is one of the leading CNC manufacturers that provides efficient lubrication delivery systems in their machines.
High-Pressure Flood Delivery System
High-pressure flood system delivers the fluid at a pressure greater than 1000 psi. It is very effective for rapid chip removal and cooling.
Therefore, this delivery system is used commonly in CNC milling and drilling operations.
Specialized tools that would allow the coolant delivery through the tools' built-in nozzles could also be used in this type of cutting fluid delivery.
However, using a high-pressure delivery poses the risk of causing breakage to miniature radius tools.
Minimum Quantity Lubricant (MQL)
MQL is a type of aerosol that is generally delivered through specially designed tools.
This type of delivery system is used for applications where you cannot use water-based coolant.
In MQL, the fluid consists of a minimal quantity of oil, atomized with high-pressure air. This mixture is sprayed over the cutting area and can be seen as a fine layer over the workpiece.
Unlike flood delivery system, MQL is sprayed only at the point of contact of the workpiece and cutting tool.
MQL can also be delivered through special cutting tools that have a channel through which the fluid is directly inserted onto the cutting area.
This type of delivery greatly increases the efficiency of fluid delivery.
MQL may not be the most efficient coolant, but it is an effective lubricant.
The root cause of heat generation is the friction between the workpiece and the tool.
Hence, MQL reduces heat generation by efficiently lubricating the point of contact, thereby minimizing the need for a coolant.
Using MQL as the cutting fluid also keeps the chips dry, which eliminates the need for them to be drained.
Problems related to cutting fluid
Cutting fluids have various additives to enhance their functionality.
However, these additives are not always effective and can even lead to a bad odor of the cutting fluid.
Cutting fluids have a lot of bacteria that reside in the minerals and the oil leaked from the machines.
These bacteria multiply and they eventually release sulfur dioxide gas, which gives out the smell of rotten eggs.
Bacterial growth can be controlled by following a few simple steps, such as minimizing the oil leak from the machines and maintaining a pH level of 8.3–9.2.
Keeping the work area clean and installing an oil slick filtering device to remove the dirt will also help keep the odor in check.
Furthermore, the pH of the cutting fluid can affect its corrosive properties.
Therefore, in order to prevent corrosion, cutting fluids' pH should always be adjusted according to the metal that is being machined.
For example, when machining aluminum, it is advised to use cutting fluids with a pH lower than 9.2 because anything higher than that can corrode the aluminum workpiece.
Cutting fluid vapors can be detrimental to the health of the operators, especially when cutting fluids are delivered in the form of mist instead of using a targeted approach like MQL.
All these factors make it essential for the cutting fluid to be chosen appropriately, keeping in mind the type of operation to be performed, the material to be worked on, and the potential health risks that it could pose.
Frequently asked questions (FAQ)
What are the properties required in cutting fluid while machining aluminum?
The properties required in the cutting fluid while machining aluminum are: good cleaning action, good cooling ability, good lubrication, pH between 8 and 8.5, and good anti-rust properties. Soluble oil is one of the best cutting fluids for machining aluminum.
What are some of the cutting fluid accessories used?
Some of the cutting fluid accessories used are cutting fluid filter, mist collector, bubbler, and coalescer.
What is the most common type of coolant delivery used?
The most common type of coolant delivery used is flooding (in almost 50% of applications).