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  3. Temperature control equipment Oil chiller VSC series
  4. Oil chiller Technical Information
  5. 1. (Cooling) liquid to control temperature at the manufacturing site
  6. 1-3. Cutting oils

Oil chiller Technical Information 1-3. Cutting oils

1-3. Cutting oils

This section describes the applications of cutting oils, how they are used and the need to cool them.

What is a cutting oil?

Cutting oil is the lubricating oil used when cutting or grinding metals or plastics with machine tools, etc.
Applying cutting oil to the material being cut (metal, etc.) and the tool (blade for processing the material) has a number of effects.
As described in more detail below, cutting oils can roughly be divided into water-insoluble cutting oils that are used without diluting them in water and water-soluble cutting oils that are used diluted in water.
Water-insoluble cutting oils are mainly used for “① Lubrication” and “② Welding prevention”, for cutting operations requiring a high processing precision.
Water-soluble cutting oils are mainly used for “③ Cooling”, for cutting operations involving large quantities of material to be processed or requiring dimensional precision. They also offer the advantage that they reduce the risk of ignition. There are sometimes called water-soluble coolants. With cutting machines, the word “coolant” often refers to a water-soluble coolant (*).

* But not always. Water-insoluble cutting oils are also sometimes called “coolants”, so be careful not to confuse them.

Purpose of cutting oils and basic elements of performance

There are five basic elements of performance required of cutting oils: “① Lubrication”, “② Welding prevention”, “③ Cooling”, “④ Rust prevention” and “⑤ Cleaning”. Three of these effects are especially important: “① Lubrication”, “② Welding prevention” and “③ Cooling”. “② Welding prevention” is the effect by which the material being cut and the tool or the tool and chips are prevented from bonding due to high temperature and high pressure during cutting.

Effects of the basic elements of performance

“① Lubrication” effect: Improved cutting tool cutting quality

The lubricating effect of the cutting oil reduces the wear of the tool and workpiece. Let us explain this in more detail.
When the tool, workpiece and metal come into direct contact, because the surfaces of contact are actually slightly uneven and not completely flush against each other, a large force acts on the protruding parts of the uneven surfaces, resulting in a high amount of friction.
Cutting oil is therefore supplied to both the tool and the workpiece to create an oil film on the metal surface that prevents direct contact between the metals, reducing friction and wear. This is the lubricating effect of the cutting oil.

In addition, chips are generated during cutting, but the more the friction between the surface on the chip side of the tool (the rake surface) and the contact surface of the chip is reduced by the cutting oil, the smaller the radius of curvature of the chip.
The smaller the radius of curvature, the smaller the contact area between the tool and the chip, and the cleaner the cut.
The friction during cutting that causes the tool to wear out is mainly due to the chips, so cutting oil plays an important role in protecting the tool.

“② Welding prevention” effect: Protection of the cutting edge of the tool

When cutting, part of the chips melts and adheres to the cutting edge of the tool, becoming a hard, welded substance. This is called the built-up edge. Because it is welded onto the cutting edge, it reduces the precision of the workpiece's finish, and can peel off together with a part of the cutting edge during cutting, deteriorating the tool.
This tends to occur particularly with aluminum alloys, stainless steel, heat-resistant alloys, etc.
Cutting oil has the effect of protecting the cutting edge with an oil film, preventing adhesion of the built-up edge.


“③ Cooling” effect: Reduced tool deterioration and improved processing precision and work environment safety

It is important to reduce the processing heat at the cutting point as much as possible and to disperse the heat that is generated as soon as possible in order to cool the tool and the material being cut. There are three sources of heat, as shown on the diagram at the left.
 (a) Shearing heat when the material being cut chips. This is generated because of distortion and rupturing
  (about 70% of the cutting heat).

 (b) Frictional heat generated when the chip rubs against the rake surface of the tool as it is ejected.
 (c) Frictional heat generated when the tool's flank rubs against the material being cut.
  ((b) and (c) together account for about 30% of the cutting heat.)


When cutting, the temperature reaches 800 ℃ and in some cases as much as 1000 ℃. Most of this heat is carried away with the chips, but some of it is also conducted to the tool, increasing the temperature of the cutting edge.
The cooling effect of the cutting oil keeps the temperature from rising, protecting the tool from deformation and softening caused by the heat, thereby reducing wear and extending the tool's service life.
The workpiece also becomes hot, but the cooling effect prevents expansion and deformation due to the heat, thereby maintaining processing precision.
In addition, hot chips are extremely dangerous as they may cause a fire or result in burns when touched. The cooling effect also plays an important role in improving the safety of the work environment.


“④ Rust prevention” effect: Protection of the device, tool and material being cut

After cutting, the workpiece reacts with water and oxygen and can rust easily.
Using a cutting oil creates a protective film of oil on the surface, preventing contact with water and air. In addition, use of water-soluble cutting oils is increasing as a result of environmental issues. Since they can easily affect not only the workpiece but also the tool and machine, it is necessary to closely examine the types and properties of cutting oils available.


“⑤ Cleaning” effect: Reduced tool damage

When cutting, the cutting oil flushes the chips away, preventing them from getting caught between the tool and workpiece or sticking.
In addition, the cutting oil is also effective in keeping the work environment clean and safe because it prevents the fine chips generated during cutting from flying about.


Types and characteristics of cutting oils

Selecting a cutting oil

Why does the cutting oil need to be cooled?

The oil tanks of most machining devices using cutting oil are equipped with a cooling mechanism. The temperature of the cutting oil rises due to the heat generated by processing and operation, but the oil is cooled to avoid a number of problems that can be caused by this temperature rise.

(1) Maintaining processing precision

When the temperature of the cutting oil rises due to the processing heat, the temperature of the material being cut, of the machine itself and of the tool also rises.
Matter has the property of expanding when the temperature rises. Steel for example expands more than 0.1 mm per meter with a temperature rise of 10 ℃. This decreases processing precision, and could lead to a decrease in quality or the need for extra processing for compensating.
Cooling the cutting oil and keeping the temperature constant contributes to improving product quality and reducing the workload.

(2) Protecting the machine itself and the tool

When the temperature of the machine itself or of the tool rises, an excess load is applied due to thermal expansion, the cutting edge of the tool can weld more easily, and this could shorten the tool's service life. Maintaining the cutting oil at a suitable temperature is important for protecting the machine itself and the tool.

(3) Reducing cutting oil evaporation and decay

For water-soluble cutting oils in particular, the oil evaporates more easily when the temperature rises, and because bacteria can proliferate easily at temperatures between 30 and 40 ℃, the oil can decay rapidly in this temperature range.
Reducing evaporation loss and decay leads to a decreased maintenance workload and lower oil purchasing expenses.

Cutting oil cooling methods

There are two common methods of cooling cutting oil, as described below.

(1) Liquid cooled type heat exchanger

With this method, a liquid cooled type heat exchanger in which the liquids exchange heat indirectly is used to cool the cutting oil indirectly with cooling water.
The cooling effect differs greatly depending on the type of cooling water.
For example, when factory circulation water or cooling tower water is used as the cooling water, the temperature change is great, and in summer in particular the temperature of the cooling water itself is high so it is difficult to achieve stable processing quality. On the other hand, when well water with a stable temperature year round or the water of a chiller whose temperature can be set is used, it is easier to achieve stable processing precision throughout the year, so these cooling waters can be considered superior.

2-2. Liquid-cooled heat exchanger (liquid to liquid)
3-1. Types and characteristics of cooling water

(2) Coolant chiller

This refers to chillers that are exclusively for coolants. They can be divided into two major types: immersion type chillers and direct circulation type chillers. If there is chip powder or sludge in the coolant, with circulation type chillers the piping can get clogged or abraded, but an immersion type chiller (①) can be used because the heat exchange unit can be immersed directly in the coolant. With direct circulation type chillers (②), the coolant flows into the machine from the piping, and this type of chiller is used by connecting it to a clean tank with little chip powder or dust, or by using strainers and filters. They often provide higher temperature control precision than immersion type chillers (①).

2-3. Direct cooling by chiller


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