The Complete Guide to Choosing the Right Cutting Fluid for CNC Machining

Table of Contents

Synthetic oil cutting fluids
Synthetic oil cutting fluids


CNC machining is a critical step in many manufacturing processes including injection molding where it is used in tooling and mold production. However, the heat generated as a result of the immense friction between the machining tool and the piece can make the whole process both inefficient and frustrating.

Thankfully, the use of a cutting fluid in CNC machining does the magic. It helps in lubricating, cooling, and removal of chips from the surface of the machined piece.

Nevertheless, identifying and choosing the correct cutting fluid is more important than recognizing its need since there are different types of cutting fluids for different machining scenarios.

But not to worry. We have written this comprehensive guide to help you navigate the cutting fluid terrain, including the different types, critical factors to consider in your choice of a suitable cutting fluid, and other practical tips.

So, without wasting any of your time, let’s dive right in.

Understanding Different Types of Cutting Fluids:

Coolant properties of cutting fluids
Coolant properties of cutting fluids

Generally, there are two major challenges machinists encounter when using CNC machining. The first is the excessive heat generated from the friction between the machine bit and the piece.

This heat can damage both the bit and the piece. Next are the chips generated from removing small parts of the piece. These chips can heap up at the tip of the cutting bit, blocking the vision of the machine operator.

Either way, an experienced CNC machinist understands the pivotal role of cutting fluids. More importantly, they understand the different types of cutting fluids which helps them to choose the most appropriate for the material and task at hand.

We’ve highlighted these different cutting fluid types so you also can understand how to put them to good use in your next CNC machining project.

1. Oil-Based Fluids:

For starters, there is the oil-based cutting fluid which is suitable for heavy-duty applications where there is a critical need for superior lubrication than cooling. They are especially required when working with hard metals like high-yield steel.

For instance, operations like cutting gear threads or drilling very deep holes in metals require that the CNC machine bit in continuous contact with the machined piece for a significant duration of time.

Essentially, these processes generate tremendous heat as the friction from the bit action can be immense. Hence, oil-based fluids are needed to cool this heat and ensure that there is no corrosion on the metals involved.

Also, oil-based cutting fluid helps achieve a good surface finish as it reduces the impact of friction on the machined piece’s surface. Consequently, CNC machine operators often employ oil-based cutting fluid where the surface finish is a critical consideration.

Unfortunately, one drawback of oil-based cutting fluids is the risk of fire. Naturally, the continuous friction generates more heat which can spark a fire when in direct contact with the oil base during the machining process. It can also leave a messy deposit in the machine and in the environment where it was used.

Still, oil-based cutting fluids remain a darling of CNC machinists who carry out heavy machining like grinding hard metals given its advantages, including preventing welding between the piece and the tool. 

emulsion cutting oil
Emulsion cutting oil

2. Water-Miscible Fluids (Emulsions and Solutions):

By far the most popular cutting fluids are water-based including water-miscible emulsions and solutions both of which carry out the functions of cooling and lubricating the work surface adequately. Basically, emulsions are oil mixed in water but unlike oil-based cutting fluids, they have a very low chance of sparking a fire.

Solutions on the other hand are chemicals mixed in water. These chemicals are designed to protect the material from corrosion while they cool the machined piece.

Overall, water miscible cutting fluids are very effective for high-speed CNC machining where cooling the tool and the workpiece surface is of utmost importance. For instance, during turning, milling, and grinding operations of thin and relatively soft metals water-miscible cutting fluids are best.

Nevertheless, solutions have better cooling effects than emulsions which do a better job at lubricating the workpiece surface. Still, water miscible cutting fluids are notorious for the mess they create after the cutting operation leaving the operator with a lot of cleaning to do.

Also, they require high maintenance and care which requires more time and effort for the operator and can result in allergies for some individuals. However, water-miscible cutting fluids are still the most common ones when compared to other alternatives in the machining process.

3. Synthetic Fluids

This third cutting fluid type uses inorganic and organic compounds as a replacement for oils in water solution giving them even more resistance to fire ignition during a CNC machining process.

Synthetic fluids offer higher cooling effects than other cutting fluids which is why they’ve become a fluid of choice where cooling is essential. Additionally, synthetic fluids come with less smoke production even during high-speed CNC machining which often generates significant heat.

Still, they are the best when a clean machine and environment are the critical considerations for operation as they require less cleaning and maintenance since they don’t leave a mess behind. Overall, synthetic cutting fluids offer a balanced advantage relative to other cutting fluids as machinists still benefit from their lubrication and anti-corrosion properties in addition to their main advantages.

Factors to Consider When Choosing Cutting Fluids:

Aluminum materials are softer
Aluminum materials are softer

Choosing the ideal cutting fluids for a specific CNC machining operation can be tricky considering the many factors you need to consider. These considerations can include the type and properties of the material to be cut, the speed required for the operation, and the environmental requirements. We have highlighted these considerations below to help you understand what you must do to select the most suitable cutting fluids.

1. Material Compatibility:

The materials that must go under the cutting tool are often considered based on their thickness and their hardness both of which indicate how easy it is to machine them.

Metals like high-tensile steel and aluminum alloys are some of the most common metals used for many operations including molds in the injection molding process. Consequently, considering their compatibility with the different cutting fluids available goes a long way to ensure the success of the production process.

For instance, high-tensile steel used in making many molds in injection molding require heavy machining and as a result rely on oil-based cutting fluids to achieve cooling and lubrication. However, there is the need to take precautions which might involve enhancing the fluid with some additives to avoid thermal damages or igniting a fire due to the significant heat generated in the process.

Conversely, aluminum which is relatively softer rely on water-miscible fluids for workpiece cooling and lubrication. But then there is the issue of staining with some oils used in cutting fluids especially those oils that contain chlorine and sulfur. It is important that fluids containing such oils are not employed when working with metals like brass, copper, and aluminum as it results in staining the workpiece.

Also, it is important to critically consider the chips generated by the materials in the choice of a cutting fluid as these materials generate different chip sizes. As an example, materials like steel generate bigger chips than copper alloys which makes cleaning easier when working with steel than copper alloys. 

 High-speed machining operation
High-speed machining operation

2. Machining Operation Type:

The nature of the machining operations is another important factor to consider when choosing a suitable cutting fluid for your CNC machine. The truth is different operations determine an efficient speed for the CNC machine which in turn determines the intensity and difficulty of the operation.

For instance, operations like milling and grinding often happen at very high speeds and therefore need a cutting fluid that can effectively cool down the heat generated. Consequently, synthetic fluids which have the best cooling effect of the three types are often very effective.

Conversely, machine operations requiring low speeds generally require a more viscous fluid to take care of lubrication which is the critical consideration in these processes. For example, gear-cutting and threading processes are done at low speeds and require oil-based cutting fluids to take care of the lubrication needs of the operation.

Still, as machinists, you should consider the properties of the workpiece and the most essential machining requirements between cooling and lubrication when choosing a suitable cutting fluid.

These considerations will help save your workpiece from corrosion and thermal damage when you choose the right cutting fluids.

Chip removal
Chip removal

3. Environmental and Health Considerations:

Taking the health of individuals and the environment into consideration is also very important when choosing your cutting fluids. Certain fluids leave a mess that is often hard to clean and therefore pose a risk of contaminating the environment.

Also, the smoke generated when using specific cutting fluids in CNC machining can cause some health hazard to individuals. Again, specific water miscible fluids have been known to cause allergies for individuals making it uncomfortable for these individuals to stay around CNC machine operations.

Hence, ensuring that the cutting fluid chosen for the project at hand is both environmentally friendly while also safe for workers should be at the forefront when making your decision. The result is not just a flawless machined piece but a safe workspace for individuals and a healthy non-toxic environment.

Emulsions miscible coolant filled in sump
Emulsion miscible coolant filled in sump

Practical Tips for Using and Maintaining Cutting Fluids:

With all that has been said about choosing the right CNC cutting fluid, adding practical tips that can help you handle, store, and recycle cutting fluid will go a long way in maximizing their effectiveness.

For starters, it is important to install a filtration system that sieves out all that swarf that is often present in recycled cutting fluid. This not only prolongs the machine’s life but also increases the emulsion as it reduces the growth of organic matter like bacteria.

Also, you should consider using skimmers and separators as an important measure to minimize the presence of tramp oil in the cutting fluid. This ensures that bacteria do not thrive in the cutting fluid thereby reducing the effectiveness of the cutting fluid. These skimmers are better removed every day for better results.

Again, carefully monitoring the cutting fluid levels is a good way of ensuring that the water or concentrate level is adequate for maximum efficiency. Additionally, it is advisable you check the pH of the fluid weekly and keep it slightly alkaline as this discourages the growth of bacterial and other organic matter.

Furthermore, ensure that you use an effective system cleaner at the point of changing the cutting fluid to loosen all the stuck sludge and remove bacteria from the system. Nevertheless, ensure that there is no contact with the skin during this operation as it can be a source of skin irritation.

Still, it is advisable to do a thorough machine cleanout by disposing off of any cutting fluid that has lasted up to 12 months in the machine.


In a nutshell, cutting fluid is the lifeblood of any CNC machining operation as it ensures that machining hazards including thermal damage by cooling the tool and workpiece interface.

Hence, it is critical to choose the best suitable cutting fluid from the different ones for the CNC project to get the best machining result.

Care should be taken such that the selected cutting fluid matches the material properties and the operational requirements including the speed of operation. Furthermore, the fluid should not provide adequate cooling, lubrication, and corrosion protection for the workpiece.

Ultimately, the cutting fluid should be eco-friendly and it should be safe for the individuals working with it causing no havoc to the persons and the environment.


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

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