Metal Cutting Fluids/Oil for Machining (Simple Guide)

Metal Cutting Fluids

Cutting fluids: Cutting fluids (also known as cutting oil) are the liquids and gases that are applied to the cutting zone in order to improve the machining operation by removing the heat and reducing the friction between workpiece and cutting tool.

Cutting fluids are known as coolants when they are basically used for cooling purpose, and they are known as lubricants when their main function is to provide lubrication.

Well, this was just a simple overview. But there are a lot more things that you should know about metal cutting fluids used in machining.

So let’s dive right into it!

What is the purpose of cutting fluid?

The purpose of cutting fluids are listed below.

  1. Cutting fluids maintain the lower temperature of the cutting tool so that it can retain the hardness and resistant to abrasion.
  2. Cutting fluids reduces the friction and power consumption and hence the wear of tool reduces which increases the tool life.
  3. Heat generated in the cutting zone is removed by the help of cutting fluid.
  4. Cutting oil prevents the adhesion of chips with the tool.
  5. Cutting oil keeps the workpiece cool and hence it prevents expansion of workpiece.
  6. Cutting fluids also helps to keep the cutting surface bright by giving a protective coating against atmospheric oxygen. In this way it helps in preventing corrosion.
  7. Cutting fluids improves the machinability of the workpiece.

Examples of cutting fluids

The examples of cutting fluids are listed below.

#1) Active cutting oils: Active cutting oils react chemically with workpiece to improve the machining operations. Generally active cutting oils are used while machining ferrous metals.

#2) Inactive cutting oils: Inactive cutting oils are the straight mineral oils that are mixed with neat fatty oils. They are used for machining of non-ferrous metals.

#3) Gaseous type cutting fluids: Gaseous type cutting fluids includes;

  • Compressed air
  • Carbon dioxide
  • Argon, etc

Above mentioned gaseous cutting fluids are generally applied to the machining surface through a pressurized nozzle. Gaseous fluids are applicable for only few operations like grinding and polishing.

Gaseous fluids also help in removing the chips, dust, etc.

#4) Water: Water is considered as a best coolant where cooling is most urgent, because it has good wetting properties as well as high specific heat.

Water containing borax, salt or sodium carbonate are used for cooling and washing away of chips during the machining process.

#5) Straight mineral oils: Straight mineral oils are pure mineral oils which do not contain any additives.

These cutting oils are low in viscosity, lower in cost, they have good wetting properties and are also chemically stable.

Straight cutting fluids are suitable for light duty machining operations on non-ferrous metals like magnesium, brass, aluminum, etc.

#6) Fatty oils: Fatty oils are little costly, so they are not used nowadays. But previously they were widely used for heavy duty machining operations.

Fatty oil like lard oil is used for machining of tough non-ferrous alloys.

#7) Blended fatty-mineral oils: These cutting oils are the mixture of fatty oils and mineral oils of different viscosity.

The main advantage of using blended fatty-mineral oils is that they have less cost, they show good wetting properties, and show improved surface finish on non-ferrous alloys.

#8) Sulfurized blended of fatty mineral oils: These are the mineral oils blended with sulphurized fatty oils.

They shows very good lubrication properties during the process like metal drawing, metal forming, metal extrusion, etc.

Sulphurized oils are widely used nowadays, and they shows cooling properties as well as lubrication properties.

#9) Emulsion of soluble oils: Emulsion is a type of machining oil which consists of soluble oil emulsified with water. The ratio of oil to water can range from 1:10 to 1:100 depending upon the type of product and machining process.

Emulsions are less costly and that shows very good cooling properties.

For machining operations like threading, broaching and gear cutting, the emulsions having oil to water ratio of 1:10 is preferred.

How to apply cutting fluid during machining?

In machining operations, the cutting fluids are applied in following ways;

  1. Flood cooling
  2. Mist cooling
  3. High pressure refrigerated cooling

Flood Cooling: In flood cooling, about 10-20 litres/minute of fluid flow is made for a single point cutting tool, while upto 230 litre/min of flow is done for multi point cutting tool.
For such a high flow of machining oil, multiple nozzles can be used.

Mist cooling: In mist cooling, the small jet equipment is used to disperse water based cutting fluids.
Mist cooling system can pass the cutting fluid at high velocities.
The disadvantages of cooling method is that the venting is necessary and also the cooling capacity is limited.

High pressure refrigerated cooling: This is very efficient in removing the heat at high rates.
The cutting fluid is generally directed to the relief angle of the cutting tool at a pressure as high as 35,000 kPa.
Filtering of cutting oil is also necessary to eliminate the damage to workpiece.

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