The most overlooked condition: cutting fluid

In previous articles introducing built-up edge, the author repeatedly mentioned the influence of cutting fluid on built-up edge. In actual work, many people will ignore the role of cutting fluid, and when designing the machining process, they will also ignore the selection of cutting fluid. It seems that it is enough to choose dry cutting or wet cutting.

This may be because everyone does not fully understand the role of cutting fluids. The author reads and refers to a variety of materials to systematically introduce the definition, classification and functional characteristics of cutting fluid for everyone. Please pay attention to reading.

First, the definition and type of cutting fluid

What is cutting fluid?

Cutting fluid, that is, metal cutting fluid, is a process lubricant used in the process of cutting metals and their alloys.

There are many varieties of metal cutting fluids. ASTM D2881 classifies metalworking fluids into three categories:

1. Oils and oil-based liquids

2. Water-based emulsions and dispersions

3. Chemical solution (true solution and colloidal solution)

In recent years, the development and changes of metal cutting fluids are mainly in the field of water-soluble liquids, namely ② and ③ categories.

Since this type of liquid is based on water, its heat transfer rate is high, and the heat transfer rate of water is 2.5 times that of oil. After the same amount of water absorbs a certain amount of heat, the temperature rises much slower than oil, thereby improving the cooling effect and eliminating the influence of "oil mist" on the processing system. Therefore, water-based cutting fluids are more widely used. Taking the UK as an example, water-based cutting fluids account for about 60% of the entire cutting fluid market.

However, compared with oil, water-based cutting fluid has the disadvantage of poor lubricity, followed by corrosion, colloidal stability, chemical stability, biological stability, filterability, foaming and other problems. These issues are critical to the "Sump Life" of cutting fluids in machine tool applications.

Therefore, in daily work, cutting fluids are subdivided into two categories: water-soluble cutting fluids based on cooling and pure oil-based cutting fluids based on lubrication.

2. Subdivided categories of water-soluble cutting fluids

Water-soluble cutting fluid, as the name suggests, more than 95% of which is dilution water. The other ingredients are:

Lubricant (5～70%)

Rust inhibitor (2～25%)

Emulsifier (2～25%)

Preservatives (below 1%)

Defoamer (below 1%)

Water-soluble cutting fluid is a new generation of semi-synthetic micro-emulsion water-soluble cutting fluid prepared by scientific methods from extreme pressure agent, rust inhibitor, mineral oil and various surfactants. Water-soluble cutting fluid is suitable for cutting steel, cast iron, stainless steel, copper, aluminum and its alloys and other metals, and has a long service life, so it is widely used in large machining centers and automatic lathes.

Water-soluble cutting fluids can be divided into the following three types:

Emulsion cutting fluid

Its main components are mineral oil, emulsifier, rust inhibitor, defoamer and so on.

Semi-synthetic cutting fluid

Its main components are mineral oil, fatty acid, extreme pressure agent, surfactant, rust inhibitor, etc.

Fully synthetic cutting fluid

Its main components are surfactants, amino alcohols, rust inhibitors, etc.

The difference between three water-soluble cutting fluids

Emulsion cutting fluid contains a large amount of mineral oil (50-80%).

Semi-synthetic cutting fluid components contain a small amount (5-30%) of mineral oil.

Fully synthetic cutting fluid components do not contain mineral oil.

Third, the performance and characteristics of water-soluble cutting fluid

Water-soluble cutting fluid not only has the lubricity and extreme pressure of emulsified oil, but also has the environmental protection performance, excellent cleaning performance and long service life of synthetic cutting fluid.

Its role is mainly reflected in the following three aspects:

1. Cooling performance

Cooling the tool removes the cutting heat from the tool surface and reduces thermal damage to the tool, of course after the cutting is complete.

Cool the workpiece, suppress the thermal expansion of the material, and improve the machining accuracy.

2. Lubrication performance

Reduce cutting force, extend tool life and improve machining accuracy.

3. Chip treatment and rust prevention

Promote the breaking and discharge of chips and prevent rusting of equipment and workpieces.

Fourth, the selection principle of cutting fluid

Oily or water soluble?

First of all, it is necessary to preliminarily judge the selection of pure oil-based metal cutting fluid or water-soluble metal cutting fluid according to the technological conditions and requirements of cutting.

Usually we can choose according to the recommendation of the machine tool supplier; secondly, we can also choose according to conventional experience.

For example, when using high-speed steel tools for low-speed cutting, pure oil-based metal cutting fluid is usually used; when using cemented carbide tools for high-speed cutting, water-soluble metal cutting fluid can usually be used.

For another example, when the fluid supply is difficult or the cutting fluid is not easy to reach the cutting area, pure oil-based metal cutting fluid (such as tapping, inner hole broaching, etc.) is used, and in other cases, water-soluble metal cutting fluid is usually used.

In short, according to the specific cutting conditions and requirements. According to the different characteristics of pure oil-based metal cutting fluids and water-soluble metal cutting fluids, and considering the different actual conditions of each factory, such as the ventilation conditions of the workshop, the capacity of waste liquid treatment, and the use of cutting fluids in the preceding and following processes, the specific Type of cutting fluid.

Machining requirements for cutting fluid

Secondly, after selecting the type of cutting fluid, it is necessary to preliminarily select the type of cutting fluid according to the cutting process, the material of the workpiece and the requirements for the machining accuracy and roughness of the workpiece.

For example, when selecting cutting fluid for grinding, we must not only consider the conditions of ordinary cutting, but also consider the characteristics of the grinding process itself

We all know that grinding is actually a multi-tool simultaneous cutting process. The feed of grinding is small, and the cutting force is usually not large, but the grinding speed is high (30-80m/s), so The temperature in the grinding area is usually high, which can be as high as 800-1000 °C, which is easy to cause local burns on the surface of the workpiece. The thermal stress of grinding will deform the workpiece, and even crack the surface of the workpiece. A large amount of metal grinding chips and grinding wheel sand will affect the surface roughness of the workpiece; therefore, when selecting a water-soluble metal cutting fluid for grinding, we require the cutting fluid to have good cooling, lubricity and Cleaning scourability.

Requirements of material properties for cutting fluid

Finally, according to the different materials of the workpiece, when selecting water-soluble metal cutting fluids, different cutting fluid products should be selected from the different characteristics of different materials.

For example, when cutting high-hardness stainless steel, it is necessary to select an extreme-pressure water-soluble metal cutting fluid with good extreme pressure performance according to its characteristics of high hardness, high strength and difficult cutting to meet the extreme pressure lubrication performance requirements of the cutting fluid during the cutting process. .

For materials such as aluminum alloys and copper alloys. Due to the high toughness and high activity of the material itself, when choosing a water-soluble metal cutting fluid, the lubricity and cleaning properties of the cutting fluid are more required, and at the same time, the workpiece cannot be corroded.

After the preliminary selection of the metal cutting fluid above, comprehensive consideration should be given to the irritation, toxicity, service life and waste liquid treatment of the cutting fluid product to the human body, and finally the correct type and concentration of metal cutting fluid should be selected.

Article summary

In this paper, the author starts from the varieties and their practical fields, focuses on the introduction of water-soluble cutting fluids and their characteristics, and briefly introduces the selection principles of cutting fluids.

In the next article, the author will introduce how to properly use and maintain cutting fluid. Stay tuned!