What are Cutting Oils and What are They Used For?

Cutting Oils, also commonly referred to as coolant, cutting fluid or cutting compound, provides lubrication and cooling during machining processes, rust and corrosion protection as well as aiding in chip removal from the cutting area. This advantage provided by added lubrication and cooling can help extend tool life and improve the surface finishing. These oils come in a number of forms and choosing the right one with the right properties for you can be a complicated process and as such we have this below guide to provide you with a brief introduction to the way in which they are delivered, the benefits and disadvantages of each type, the uses they are best suited to and other factors that must be considered.

Coolant Delivery Systems

There are three main ways in which coolant is delivered to the cutting area. The most common of which is flooding in which the entire cutting area is covered at the rate specific to the operation. Through Spindle Cooling (TSC) is used in CNC machining and involves coolant being sprayed through the spindle to the cutting tool into the area of the operation. This provides more cooling and chip evacuation for the cutting tool in turn reducing the cycle time and increasing the life of the tool. Finally there is MQL (Minimum Quantity Lubrication), which is achieved by applying a light spray of an air/oil mixture to the cutting area to lubricate the cutting area. This method of lubrication uses significantly less coolant than traditional flood cooling but is not suitable for heavy cutting.

Types of Cutting Oil / What They are Best For

Neat Oils

These oils can be either petroleum or vegetable based oils with blended additives to improve particular aspects of their performance and typically offer the highest amount of lubrication, good rust and corrosion protection and stability although with the least cooling capacity. Insoluble in water, these oils must be kept clean as they have a tendency to smoke and offers a risk of fire during misting. Common uses for straight oils are in Fogging, MQL, heavy machining operations, milling, tapping, turning, hobbing and honing.

Soluble Oils

Soluble Oils form an emulsion when mixed with water and offers high levels of lubrication and adequate cooling capacity making it suitable for heavy cutting. The nature of the fluid remains stable even with the contamination from tramp oil, machining fines and other materials that may find their way into the machine sumps. Suitable for all cutting fluid applications, soluble oils are however more prone to bacterial problems. Soluble Oils are also the cheapest and most widely used type of cutting oil.

Semi-Synthetic Fluids

Semi-synthetic fluids are a combination of synthetic and soluble oils and its properties and cost are a middle ground between the two, therefore providing a good solution for both lubricity and heat transfer capability. The chemical makeup of the semi-synthetic fluids can be adjusted for different situations and processes and provide further added benefits such as bacteria resistance.

Synthetic Fluid

Synthetic oils do not contain a mineral or petroleum base, rather they are made from alkaline organic and inorganic compounds which function well both in diluted and undiluted forms. These synthetic oils offer high resistance to bacteria and the best cooling performance and are best suited for use in CNC mills, lathes and grinding operations where high cutting loads are achieved and where superior cutting performance and longer tool life are crucial. Alternatively though, synthetic oils are the most expensive and require the most maintenance to remain effective where careful attention must be paid to the quality of the water. However they lack the corrosion prevention of soluble and semi-synthetic oils due to the absence of a mineral oil base.

Other Factors & Considerations


The handling and usage of cutting fluids can expose workers to a number potential health hazards and proper safety measures and PPE need to be observed to limit these risks. The chemical makeup of the fluids will determine the particular health hazards and effects that range from dermatitis to respiratory issues and cancer. Proper use of PPE, including gloves, safety eye wear, aprons and face shields are recommended.


Contamination of the cutting oil can be caused by many different sources. Firstly, hydraulic oil and machine lubricant can end up in the cutting fluid which can interfere with the performance. Depending on the type of cutting oil in use and the contaminant oil, the way the contamination mixes together may alter and in turn the best way to separate them may be different. In some cases industrial centrifuges or other filtration systems may be necessary to filter out the contaminant particles. Contamination of the fluid can also cause an imbalance in the pH resulting in higher risk of bacteria growth, particularly in synthetic and semi-synthetic fluids.

Concentration, Usage & Water Quality

The concentration and flow rate of the cutting fluid are key factors in optimising the efficiency and cost effectiveness of the operation. Finding the correct concentration and rate of flow to provide the lubricity and cooling qualities for the extension of the tools life depends on the tools in use and the materials being worked with. Understanding the quality of the water is also incredibly important to the effectiveness of the cutting oil. Water Hardness and the pH level have significant effects upon the concentration level necessary for peak effectiveness and many CNC machining shops invest in RO and DI water systems and Brix Scale Refractometers to monitor and manage this. Usage and environmental factors also influence the coolant levels and concentration.

Normal concentration levels are generally between 5 and 10%. Maintaining an effective concentration level leads to a prolonged tool life, reduced cycle times and better quality surface finishing and potential reduction in per tool cost that can lead to significant increases in productivity and profitability. Having too lean of a concentration can cause rusting, poor surface finishing and excessive tool wear while having too rich of a concentration on the other hand can lead to skin irritation and excessive usage.


The waste and disposal of cutting fluids offers a conundrum both in terms of cost and socio-environmentally. Finding the best combination for your business in terms of limiting the impact on the environment, cost of disposal, cost of recycling as well as the usage rate and cost of new fluid is critical and depends on the particular circumstances of the business.


Contact our expert team today with any questions or queries you have or help deciding what the best cutting oil is for you and your businesses needs!