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Machining 10: What is a coolant?

April 5, 2023


1. What is a coolant?

2. Types of coolant

2-1. Oil-based coolant

2-2. Water-based coolant

3. Dry and semi-dry

4. Chiller

Key Points of a coolant

・Coolant has three main functions: lubrication, cooling and chip removal.

・There are two types of coolant: oil-based, which uses crude oil as it is, and water-based (water-soluble), which uses water to dilute the fluid.

・A chiller is a device to maintain a constant coolant temperature.

1. What is a coolant?

Coolant is a substance, usually fluid, that is supplied to the cutting point to make the cutting or grinding process run smoothly.

Coolant has three main functions: (1) lubrication, (2) cooling, and (3) chip removal.

The lubricating function is to reduce friction between the cutting tool and the workpiece, which reduces tool wear and extends tool life.

The cooling function is to cool the heat generated during machining. In general, it is said that the temperature at the cutting point rises to 600 to 1,000 degrees Celsius during the cutting process. The heat not only affects the hardness of the tools, but also causes thermal expansion of the workpiece, which affects dimensional accuracy. Therefore, it is important to cool the heat source with coolant. Since cooling is one of its main functions, the name coolant is widely used.

The chip removal function refers to the action of water pressure to blow away chips generated by cutting and remove them from the machining point.

Furthermore, coolant also functions to prevent built-up edge (BUE). BUE is a phenomenon in which some of the workpiece material is melted by heat and adheres to the tool, causing a buildup of material on the tool. When machining under BUE conditions, the same material as the workpiece is pressed directly against the workpiece, preventing precise cutting. Coolant flows between the tool and the workpiece to suppress weld adhesion and BUE, and also to stabilize machining accuracy.

There are two types of coolant supply methods: the external lubrication method, in which coolant is supplied to the machining point from the outside of the tool with a hose, and the internal lubrication method, sometimes called “center-through”, in which coolant is supplied through the inside of the tool.

2. Types of coolant

There are two types of coolant: (1) oil-based coolant, which is used undiluted, and (2) water-based (water-soluble) coolant, which is used after dilution with water.

2-1. Oil-based coolant

Based on the composition and corrosiveness to copper plates, the Japanese Industrial Standards (JIS) classify oil-based coolants into four types (Class N1 to Class N4).

Class N1 is effective for machining non-ferrous metals such as copper and cast iron; Class N2 is widely used for general metal cutting; Class N3 and Class N4 are suitable for machining difficult-to-machine materials and machining that requires high precision of the finished surface.

Compared to water-based coolants, oil-based coolants are less degrading and easier to handle. However, many of them are classified as hazardous materials under the Japanese Fire Service Act, so care must be taken when handling them, such as taking fire precautions.

2-2. Water-based coolant

Water-based coolants are usually used after being diluted to a concentration of 2-10%. Unlike oil-based coolants, water-based coolants have a lower risk of flaming and are suitable for unmanned machining. However, because they are used diluted with water, care must be taken to maintain and control the proper concentration, take measures to prevent rust, and dispose of waste fluid.

Based on the composition and color of the fluid when diluted with water, JIS specifies three types of water-based coolants: Class A1 to Class A3; Class A1 is also called “emulsion,” Class A2 is called “soluble,” and Class A3 is called “solution”.

The emulsion is milky white when diluted with water. It is used for all cutting operations. It has the highest lubricity of the water-based coolants.

The soluble is translucent or transparent in appearance when diluted. It has superior cooling and cleaning properties compared to emulsions. It is used primarily in cutting and grinding operations.

The solution becomes transparent when diluted with water. It has the highest cooling property among water-based coolants.

3. Dry and semi-dry

Coolant is necessary for high quality cutting operations. However, coolant has a negative impact on the environment, as fine coolant particles become oil mist during machining and are dispersed. Disposal of the waste fluid is also a problem. Energy conservation is also an issue because the power consumption of the pumps that circulate the coolant is significant.

With the growing awareness of energy conservation and global environmental concerns at manufacturing sites, environmentally friendly machining methods that use little or no coolant at all have recently attracted attention. The machining method that does not use any coolant at all is referred to as “dry machining,” while the machining method that uses a very small amount of coolant is referred to as “semi-dry machining”. Semi-dry machining is also called “MQL machining”, which stands for Minimum Quantity Lubrication.

4. Chiller

A chiller is a device that maintains the temperature of a liquid at a constant level.

The temperature of the coolant fluid can change due to heat generation from the coolant pump. Changes in fluid temperature affect not only machining accuracy, but also tool and grinding wheel life. Keeping the coolant temperature constant is therefore preferable.

In addition, as the fluid temperature rises, the coolant becomes rancid and bacteria and other germs grow. Since this can cause bad odors, it is also important to use a chiller to cool the coolant and keep the fluid temperature constant to improve the working environment and reduce corrosion and bacterial growth.

There are two main types of coolant chillers, depending on how they work: the immersion type and the recirculating type.

The immersion type has a bare coiled radiator that is ready to use by simply placing it directly into the coolant tank. However, depending on the depth of the radiator, the radiator cannot be directly submerged in a shallow coolant tank, and an external deep tank must be provided separately.

On the other hand, the recirculating type has a coolant suction pump and cooler built into the chiller. It has the ability to adapt to any coolant tank, regardless of the depth of the chiller.

Traditionally, most chillers have been air-cooled, using outside air to remove heat from the machine to the outside. However, with growing environmental awareness, there has been a recent demand for “water-cooled chillers” that use cooling water to transfer heat into the factory building and save energy.


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