Cutting Fluids: Types, Uses, and Selection Criteria for Optimal Machining Performance

What Are Cutting Fluids?

Cutting fluids are liquids that are commonly applied while machining (or cutting) operation is taking place. The machining operation can be anyone, i.e., milling, turning, drilling, etc.

Sometimes a cutting fluid is also called a coolant, but this is incorrect. A cutting fluid performs many functions, and cooling the equipment and workpiece is one of them.

Cutting fluid is a fluid used primarily to remove heat generated during metal cutting & other machining processes. It is also used’s as a lubricant in some cases. They are also used to improve cutting conditions and tool life.

The cutting fluids reduce friction between the tool and the chip and between the tool and the workpiece. If the correct amount of cutting fluid is applied, heat can be removed as soon as it is generated. So cutting fluids is necessary to increase production efficiency.

Prior to the discovery of HSS, there was no problem in selecting the cutting fluid, and water was used as a coolant because machining was done at a very slow rate.

But after the discovery of HSS, water was not appropriate because the heat generated during machining was high due to high-speed machining. Therefore, different cutting fluids were discovered & made for different machining processes & materials.

Cutting fluids can be in liquid or gaseous forms. It is mainly used in metal processes such as machining and stamping.

Cutting fluids are of various types such as oil, oil-water emission, pastes, gels, etc. The cutting fluid may be referred to as the cutting fluid, cutting oil, coolant, or lubricant, depending on the type of cutting fluid and its use. Cutting fluids can be beneficial for most metal and machining processes.

Types of Cutting Fluids:

Cutting fluid is classified into the following types: –

#1. Straight Oil

Stroth oils are non-emulsifying. These oils are used without diluting them with water. The compositions of this type of oil are base minerals or petroleum oil. Examples of straight oil are paraffin oil, Naphthenic oil, vegetable oil.

In systems where environmentally friendly oil is required, vegetable oil is used because it is biodegradable. Straight oils are best for lubricating, but they cannot serve as a good coolant because they have very cool properties.

#2. Soluble Oil

Soluble oils are made by mixing mineral oil, water, & coupling agents. It provides good lubrication between water-inaccurate liquids. Soluble oils are used in the machining of both ferrous and non-ferrous metals when high cooling quality is required & chip bearing capacity is not very high.

This soluble oil works well in its diluted form and provides great lubrication in addition to heat transfer. Soluble oils are the most widely used liquids in the industry, and also they are very cheap.

#3. Mineral Oil

Mineral oils are typically found in high production machines that have high metal removal rates. They are used in heavy cutting operations as they have very good lubricating properties. A disadvantage of these oils is that they are corrosives and therefore are not used for copper or its alloys.

#4. Synthetic Liquids

As these are synthetic liquids, they do not contain mineral oil or petroleum. These are water-based liquids, and water provides very good cooling properties. The problem with synthetic fluids is that it is not a good lubricant and also causes corrosion.

Corrosion or corrosion can be prevented by adding corrosion inhibitors to synthetic liquids. Typically, synthetic fluids are used in grinding liquids.

#5. Semi-Synthetic Fluids

Semi-synthetic fluid is made from a combination of synthetic fluid and soluble oils. For semi-synthetic liquids, approximately 5 to 20% of mineral oil is emitted with water to produce microabrasion. The size of microalgae particles varies from 0.01 to 0.1 mm, which can easily transmit all light.

Semi-synthetic fluids have very good cooling properties and cost between synthetic fluid and soluble fluid costs. Some examples are molybdenum disulfides, graphite, wax stick, etc.

#6. Solid and Paste Lubricants

These lubricants are in the solids phase or as a paste. These lubricants are placed directly on the workpiece or tool. Some examples of this are molybdenum disulfide, graphite, wax stick, etc.

#7. Cutting Oil

Cutting oil is made by mixing minerals oil & fatty oil. It is used as both a coolant and a lubricant.

Uses of Cutting Fluids:

The cutting fluid can be used for different purposes. Some of its uses are listed below:

Used as a coolant:- The main task for which the fluid was discovered is to cool the equipment and workpiece. By cooling the equipment and workpiece, we can reduce tool wear, thermal expansion of the tool, and a good surface finish.
Used for lubrication:- Cutting fluids are also used for lubrication. By lubricating, chips & other contaminants can be easily removed from the workpiece.
Used for cleaning the machining area:- Cutting fluids are also used as a cleansing agent and remove dust near and inside the cutting area. This dust removal is important because dust can cause uneven bites.
Is used to reduce friction:- The cutting fluid also reduces friction between the tool and the workpiece, which keeps the cutting forces down and also reduces heat generation.
It is used to improve the surface’s finish by protecting the newly formed surface from oxidation and corrosion.
It is used to prevent rust.
This allows the use of higher cutting speeds and larger metal removal rates.
It prevents the formation of built-up edges (BUE).
It is also used to extract chips from the cutting zone.

Selection of Cutting Fluid:

The choice of cutting fluid in machining processes depends on various factors. Cutting fluids are selected according to which the main factors are given below:

#1. Machining Process Type

Typically, more difficult machining processes more cutting fluids. Heavy cutting oil is required for heavy machining processes such as broking or screwing with a tap.

Heavy cutting oils should be used in horizontal broking, but in the vertical broking process, emulsions and solutions may be used. For the threading and drillings process, the cooling characteristics of cuttings fluids are more important.

#2. Workpiece Material

The next factor for the selection of cuttings fluids is the workpiece material. The cast iron and cast group of materials are brittle during machining, and therefore small chips are produced that do not cause too much friction. Therefore emulsion should be used as it increases the surface finish, and the concentration should be kept between 10 and 15 percent.

For steel and stainless steel workpiece materials, high-pressure cutting oils should be used.
For machinings of steel alloys, water-based cutting fluids are used because they are heat resistant & difficult to cut.

#3. Cutting Tool Material

Another’s a factor for the selection of cutting tool materials is cutting tool material. High-speed steel cuttings tools can be used with any type of cutting tool, but when the material is difficult to cut, the waterless cutting liquid is used with the HSS cutting tool.

For tungsten carbides, the cooling characteristics of cutting fluids are more important because higher heat is produced when it is used as a cutting tool material.

Properties of Good Cutting Fluid:

Cutting fluid should have good antivirus properties.
It should not foam easily.
It should be chemically neutral & should not harm the machine and operator.
It must have high specific heat or high heat-absorbing ability to disperse the heat generated during machining.
It should be transparent so that the operators can easily go through it in the cutting area.
It should have good lubricating properties so that it reduces friction during machining.
It should be nontoxic when it comes in contact with heating and should not create any toxic fumes.
It should be odorless and should not cause any odor at high temperatures.
It should have low viscosities so that it can flow easily over the workpiece.
It should be non-corrosive for workpieces and machines.
It should not glue parts of the machine.
It should have small molecular sizes to allow it to penetrate the chip tool interface and spread rapidly.
It should have a high flash point & should not burn due to heat generated during machining.

Applications of Cutting Fluid:

Cutting fluid can be used for different purposes, and those are:

As a Coolant.
It is used to cool the cutting tool, chip, and job.
As a Lubricant.
Used for more effortless movement of the tool on the job and also reduce the cutting force.
Used as Lubricant and Coolant.
Cutting fluid can also be used for coolant & lubrication purposes like Fatty Oil.
As a Flashing Action.
To wash off chips, to get better surfaces to finish.

FAQ: Cutting Fluids

What is the primary purpose of cutting fluids?

Cutting fluids are primarily used to remove heat generated during machining processes, provide lubrication between the cutting tool and workpiece, and improve overall cutting conditions to enhance tool life and production efficiency.

What are the different types of cutting fluids?

Cutting fluids are classified into several types, including:

Straight Oil: Non-emulsifying, used without dilution, good for lubrication.
Soluble Oil: Mixed with water, provides good lubrication and cooling.
Mineral Oil: Used in high production environments, good for lubrication but corrosive.
Synthetic Liquids: Water-based, excellent cooling properties but not a good lubricant.
Semi-Synthetic Fluids: Combination of synthetic and soluble oils, good cooling and lubrication.
Solid and Paste Lubricants: Applied directly to the workpiece or tool.
Cutting Oil: Mixture of mineral and fatty oils, used for both cooling and lubrication.

How do cutting fluids benefit machining operations?

Cutting fluids help in several ways:

Cooling: Reduces heat to prevent overheating of the tool and workpiece.
Lubrication: Reduces friction, which minimizes tool wear and improves surface finish.
Cleaning: Removes debris and contaminants from the cutting area.
Rust Prevention: Protects the workpiece and machine from rust and corrosion.
Improves Efficiency: Allows for higher cutting speeds and larger metal removal rates.

How do I choose the right cutting fluid for my machining process?

Selection of cutting fluid depends on:

Machining Process Type: Different processes may require different fluids for optimal performance.
Workpiece Material: The material being machined can influence the type of fluid needed.
Cutting Tool Material: The material of the cutting tool also affects fluid choice.

What properties should a good cutting fluid have?

A good cutting fluid should have:

High Heat Absorption: To effectively remove heat.
Good Lubrication: To reduce friction.
Chemical Neutrality: Should not harm the machine or operator.
Transparency: Allows visibility of the cutting area.
Non-Toxicity: Should not produce toxic fumes or odors.

Can cutting fluids be harmful?

Cutting fluids can be harmful if not handled properly. They should be non-toxic, non-corrosive, and safe for both the operator and equipment. Proper handling, disposal, and use of cutting fluids are essential to minimize risks.

How often should cutting fluids be replaced or refreshed?

The frequency of replacement or refreshing depends on factors like the type of fluid, the machining operation, and the volume of use. Regular monitoring of fluid condition and performance is important to ensure effective operation.

Are there environmental considerations with cutting fluids?

Yes, some cutting fluids can be harmful to the environment. There are biodegradable and environmentally friendly options available, such as vegetable oils. Proper disposal and management of cutting fluids are crucial to minimize environmental impact.