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  6. 1-4. How to choose a cutting oil

Mist collector technical information 1-4. How to choose a cutting oil

1-4. How to choose a cutting oil

This section explains how to choose a cutting oil by purpose, material, and processing method.

(1) Selecting a water-soluble or water-insoluble cutting oil according to purpose

Selecting an appropriate oil according to the work purpose

Although I mentioned the last time that many different types of cutting oils are available, how should these be used for different purposes?
Cutting oils are broadly divided by their cooling performance and lubricity, but it is important to choose one based on their respective characteristics while considering the processing process, the type of work material, and the working environment.

Effects obtained by cooling

  • Workpiece temperature control……Maintain and improve dimensional accuracy
  • Cutting tool temperature control… Maintain and improve dimensional accuracy, maintain machine accuracy, extend tool life

Effects obtained by lubrication

  • Suppression of adhesion to cutting tools … Improved quality of machined surface and increased tool life

The oil used in a grinding wheel for grinding purposes is known as a grinding oil, but it is actually to counter the heat generated by friction. Depending on the purpose of the work, when selecting a lubricating oil, it is important to carefully consider whether you should focus on the lubricity that suppresses frictional heat or the cooling performance to eliminate the heat generated.

Choosing between a water-soluble or water-insoluble oil

Water-insoluble oils are mainly chosen when lubricity and anti-welding resistance are more important, while water-soluble oils are mainly selected when cooling performance is more important. To put it simply, if sharpness is more important, select a water-insoluble oil, or if cooling is more important, then select a water-soluble oil.
In addition, water-soluble oils containing surfactants have excellent permeability and detergency, but water-insoluble oils that do not contain water are superior in terms of their anti-rust resistance and deterioration resistance etc.

How to choose a water-insoluble oil

Water-insoluble oils can be broadly divided into oil-based, inert hypoid, and active hypoid types. Inert types are used when the life of the tool is important, while the active type is suitable for improving the quality of the machining surface.
In addition, it is necessary to select a water-soluble oil according to the work purpose by the work material as shown in the diagram below. For example, oil-based types with a moderate lubricity are suitable for carbon steel, alloy steel, cast iron, and aluminum alloys, while hypoid types with excellent anti-welding resistance are suitable for materials that are prone to welding, such as stainless steel and heat-resistant steel etc.

How to choose a water-soluble oil

On top of having an excellent cooling performance, water-soluble oils are also available in a wide range of selection depending on whether cooling performance or lubricity is more important. If cool performance is more important, a soluble type or solution type is used, and if lubricity is more important, then an emulsion type is used.
In addition, it is necessary to select a water-soluble oil according to the purpose by work material too. For example, an emulsion type is suitable for steels which require lubricity, while a soluble type is suitable for cast iron which requires cooling performance. For work material that adheres easily such as stainless steel and heat-resistant steel etc., an emulsion type containing a hypoid additive to enhance its anti-welding resistance is suitable.

(2) Selecting a cutting oil for each workpiece

Steel

  • Water-insoluble oil containing a hypoid additive with high lubricity if machining precision is required.
  • Emulsion type with a high lubricity when using a water-soluble oil.
  • Emulsion type containing a hypoid additive for heavy cutting.



     

Cast iron

  • Inert hypoid oil when using a water-insoluble oil to prevent rust.
  • An oil with an excellent anti-rust performance when using a water-soluble oil.
  • In the case of ductile cast iron, an oil with an excellent hard water resistance in consideration of the increase in the hardness of a water-soluble oil due to long-term use.



     

Aluminium/aluminium alloy

  • Oil-based cutting oil if it is a water-insoluble oil.
  • Water-soluble oil of the emulsion type with a high lubricity because the work material is soft and adhesion occurs easily.
  • In the case of a water-soluble oil, an oil that suppresses discoloration.




     

Copper/copper alloy

  • Caution is required because corrosion occurs with oils that contain sulfur-based hypoid additives.
  • An anti-corrosion oil if it is a water-soluble oil.




     

(3) Selecting a cutting oil for each processing method

Continuous/intermittent cutting

Previously, we introduced mainly the work materials in choosing a cutting oil, but it is also necessary to consider which one to use according to the machining method.
For example, when machining with a lathe, if a cutting tool with a single blade is continuously in contact with the rotating work material, the process is called continuous cutting. If a cutting tool with multiple blades acts on a fixed work material repeatedly (intermittently), the process is called intermittent cutting.
For continuous cutting, the heat caused by friction accumulates easily in the cutting edge because the cutting tool is constantly in contact with the work material, so an oil with an excellent cooling performance is suitable.
On the other hand, for intermittent cutting, an oil with an excellent lubricity is suitable because a large impact is generated when the cutting tool bites into the work material.

Optimal oil for each processing method

Turning

Previously, we introduced mainly the work materials in choosing a cutting oil, but it is also necessary to consider which one to use according to the machining method.
For example, when machining with a lathe, if a cutting tool with a single blade is continuously in contact with the rotating work material, the process is called continuous cutting. If a cutting tool with multiple blades acts on a fixed work material repeatedly (intermittently), the process is called intermittent cutting.
For continuous cutting, the heat caused by friction accumulates easily in the cutting edge because the cutting tool is constantly in contact with the work material, so an oil with an excellent cooling performance is suitable.
On the other hand, for intermittent cutting, an oil with an excellent lubricity is suitable because a large impact is generated when the cutting tool bites into the work material.

Milling

Milling is a method of machining that uses a mill to rotate a cutting tool against the work material. Cutting tools such as end mills, front mills, and flat mills are used.
Cutting is intermittent and the impact is large because the cutting tool has multiple blades, resulting in a large change in the temperature (thermal shock) at the cutting edge, and chipping also occurs more easily at the cutting edge. To prevent this, dry cutting without using cutting oil or a water-insoluble oil is suitable for use.



 

Drilling

When drilling holes with a drill, the discharge direction of the chips is opposite to the inflow direction of the cutting oil, making it difficult for the cutting oil to reach the cutting point.
As a result, a highly permeable cutting oil is required to reach the depth of the hole against the chips that are discharged.
An oil-based type is suitable with water-insoluble oils while a soluble or synthetic type is suitable with water-soluble oils.



 

Reaming

Reaming is the process of finishing a hole made by a drill etc. with a higher accuracy.
As a high precision is required, cutting oils with excellent lubricity are used, while low viscosity water-soluble oils containing hypoid additives are generally used to prevent tool wear and built-up edges.
In particular, oil-based types are suitable for aluminum alloys while active hypoid types containing active sulfur are suitable for alloy steels and stainless steel.
For water-soluble oils, emulsions high in lubricity are common, but emulsions containing hypoid additives are suitable for alloy steels and stainless steel.


 

Tapping

Tapping is the machining of screw holes with a tap (cutting tool). As the friction between the tap and work material increases, a water-insoluble oil with an excellent lubricity and anti-welding resistance is suitable.
The details are the same as the cutting oil recommended for reaming.






 

Gear cutting

Gear cutting is the process of cutting the teeth of a gear.
The friction between the cutting tool and chips increases, so a high lubricity is required. In general, a dry machining or water-insoluble oil is suitable.
In addition, a low viscosity water-insoluble oil is used for finishing (shaving) work as precision is required.



 

Broaching

Broaching is a process to cut the inner surface of a hole or the surface of a workpiece using a cutting tool called a broach that has a large number of circular blades lined up in a sawtooth shape on a rod-shaped shaft.
Since both tool life and machining accuracy are required, water-insoluble oils containing a hypoid additive are used typically.





 

Grinding

Grinding is a process to polish the surface of a workpiece with a grinding wheel rotating at a high speed. Since a lot of the heat generated is stored in the work material, the cutting oil needs to have an excellent lubricity and cooling performance to reduce frictional heat, and permeability (cleaning performance) to prevent clogging of the grinding wheel is also required.
In general, water-soluble oils suitable for cooling that have an excellent lubricity and permeability are used.
 

(4) Summary of how to select a cutting oil

When choosing a cutting oil, factors such as the ● machining method of the work material ● material of the tool ● machining accuracy ● tool life work environment ● global environment (eco) are involved in a complex relationship. Therefore, it is important to formulate a basic policy and define which factor should be prioritized, be it “precision”, “tool life”, or “work environment”.

 

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