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Dry, semi dry and low temperature cold air

[Abstract] this paper introduces the development trend of dry and semi dry cutting, cutting mechanism and the characteristics of low temperature cold air cutting technology

[Key words] clean machining, dry cutting, semi dry cutting, low temperature cold air cutting

I. Development of dry and semi dry cutting

1. Main topics of metal cutting processing industry

with the popularization of numerical control and the improvement of tool manufacturing technology, the machining efficiency of metal cutting machine tools is getting higher and higher, and the amount of cutting fluid is also increasing; The method that simply depends on increasing the amount of cutting fluid to reduce the cutting temperature rise can no longer meet the needs of production. Cutting fluid is the main pollution source of metal cutting processing industry. The random discharge of these harmful liquids, the outdoor placement adhered to the chips, the splashing, rising and long-term leakage during processing will eventually pollute the drinking water source

In the early 1990s, experts from developed countries conducted serious discussions on the impact of metal cutting on the environment. Governments of all countries allocated special funds to support the research on dry cutting technology for the purpose of replacing oil cutting

after the adoption of ISO14000 environmental standard in Europe in September 1996, governments of all countries have increased the punishment for cutting agent pollution, and formulated very strict management and restriction measures for cutting waste liquid. Even the spilling of cutting fluid on the outside of machine tools is not allowed. Therefore, the costs related to cutting fluid in these countries are increasing year by year. For example, the professional cutting waste liquid treatment department in Japan has charged 1.5 yuan per liter of waste liquid treatment, making the processing cost of mechanical parts rise year by year. In this situation, the processing technology without cutting agent has become the main topic faced by the metal cutting and processing industry in developed countries; People are looking for processing methods that can not only improve the processing environment, save energy, but also improve efficiency and reduce costs

2. Dry cutting method

from the day of the birth of metal cutting technology, there have been methods of dry cutting and wet cutting. Because wet cutting can obtain ideal cutting effects, it is widely used by people to solve the problems that traditional dry cutting is not easy to solve. It is generally believed that cutting agents mainly have three functions: lubrication, cooling and chip removal. Modern dry cutting technology, without the use of cutting agents, we must find a method that can not only replace these three functions, but also meet the processing requirements and improve the cutting conditions

at present, the internationally popular cutting methods without cutting agents mainly include:

a, full dry cutting without cutting agents (high-speed dry cutting method)

b, semi dry cutting with gas mixed with trace lubricant instead of cutting agents

3, the characteristics of full dry cutting method

full dry cutting method, also known as high-speed dry cutting method, It is the development history of the instrument industry that allows the spindle to run at a very high speed (usually 20000 ~ 60000 RPM). It uses high-strength tools and a small amount of cutting to carry out ultra-high speed cutting. More than 90% of the cutting heat is taken away by chips in this method. It completes oilless machining through ultra-high speed operation of the machine tool and the change of cutting conditions. The efficiency is very high. At present, Germany is at the international leading level, and European and American countries are more popular. According to statistics in 2001, about 8% of enterprises in Germany have adopted this technology, and it is expected that more than 20% of enterprises in Germany will adopt this technology by 2003

4. Characteristics of semi dry cutting method

semi dry cutting method is a cutting method that uses gas plus a small amount of harmless oil to replace cutting agent. At present, common cutting methods include MQL (micro lubrication) cutting, nitrogen flow cutting and low temperature cold air cutting. This method is widely used in Japan. According to statistics: at the 19th (1998) Tokyo International Machine Tool Exhibition, 43 of the 69 machine tools exhibited in Japan have semi dry cutting power. Let's "test" it. The following are several common semi-dry cutting methods

a, MQL (micro lubrication) cutting

a relatively popular semi-dry cutting method abroad, which mixes a small amount of pollution-free oil mist into the compressed air to replace a large amount of oil to cool, lubricate and remove chips at the cutting points

b, nitrogen flow cutting can be achieved theoretically

is a cutting method that sends the nitrogen generated by the nitrogen generator into the cutting point under the action of pressure, replacing a large amount of oil

c, low temperature cold air cutting

reduce the temperature of compressed air to

-20 ℃ ~ -30 ℃, and mix a small amount of lubricant to cool, lubricate and remove chips at the cutting point. This method is different from MQL cutting in that it can reduce the temperature of the cutting point

II. Cutting and temperature rise

1. Cutting temperature rise is the main cause of tool wear

it is generally believed that tool wear is related to the following factors:

a, mechanical scratch

during cutting, the carbide, debris and other hard points on the workpiece or chip, the mechanical scratch and wear on the edge of the tool, and the material is relatively soft in the high temperature state of the tool, which will aggravate this kind of wear

b, adhesion between tool and workpiece material

in the cutting process, especially when cutting plastic metal materials, the chip tearing process, the rake face and chip of the tool, and the flank and workpiece surface will produce large pressure and heat. These pressure and heat will cause adsorption between the tool material and the tool material, and the particles with low strength at the edge of the tool are easy to be taken away by the chip or workpiece, Cause tool wear or cracking

c, mutual diffusion of alloy elements

when the cutting temperature is high, the alloy elements between the tool and the workpiece will diffuse mutually, which will reduce the physical and mechanical properties of the tool material and accelerate the wear of the blade

d, change of metallographic structure

when the tool temperature is higher than the transformation temperature of the material, the metallographic structure of the high-temperature part will change, for example, martensite will be transformed into austenite, which will reduce the hardness of the tool. The transformation temperature of alloy tool steel is 300 ℃ ~ 350 ℃, and the transformation temperature of high-speed steel is 550 ℃ ~ 600 ℃. When the blade temperature exceeds this range, the cutting performance of the tool will decline rapidly and intensify wear

the above four factors are related to the increase of cutting temperature. Therefore, controlling the temperature rise of cutting point is of great significance to alleviate tool wear

2. Use of cutting agent

in order to control the rapid rise of cutting temperature, the traditional method is to inject a large amount of cold start machine cooling agent into the cutting area, and implement the hot spots through these liquids: cooling, lubrication and chip removal. However, excessive cutting agent will cause a series of problems such as environment, cost and inconvenient processing

in fact, the use of cutting agents also has disadvantages for cutting tools. The research of Michigan University of technology in the United States shows that when the cutting speed reaches 130 m/min

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