1. Introduction The main functions of cutting fluid in cutting or grinding processes are cooling and lubrication. To improve its performance, extreme pressure additives containing chemical elements such as sulfur (S), phosphorus (P), and chlorine (Cl) are often added. These substances pollute the environment and are harmful to human health. Moreover, used cutting fluid must be treated, increasing costs. Therefore, dry cutting and semi-dry cutting without cutting fluid have become research hotspots in green cutting processes. However, under certain conditions, dry cutting without any cooling measures cannot meet the machining requirements, and the tool still needs a certain degree of cooling and lubrication. Therefore, air-cooled machining technology has emerged. This article focuses on air-cooled machining technology and equipment. 2. Air-Cooled Machining System The composition of an air-cooled system is shown in Figure 1. It generally consists of a compressed air supply source, an air dehumidifier, an air cooler, an insulating pipe, a micro-lubrication device, air nozzles, a dust extraction pipe, and a dust collector. Air from the air supply source is dehumidified and then sent to the air cooler to be cooled to -30°C. The cold air is then delivered to the cutting area through the air nozzles via the insulating pipe. Simultaneously, a small amount of harmless vegetable oil is sprayed onto the processing point to prevent rust and provide some lubrication. A dust collection device is installed opposite the air nozzle to collect waste chips and dust, and the chips are filtered out through the filter inside the dust collector. Figure 1 Composition of the air cooling system 3 Air cooling methods and requirements There are many refrigeration methods, and the common refrigeration principles can be summarized into four types: liquid vaporization refrigeration, gas expansion refrigeration, vortex tube refrigeration, and hot spot refrigeration. In principle, these refrigeration methods can all be used for air cooling. However, currently, the methods used for cooling air for cutting include indirect cooling using low-boiling-point media such as liquid nitrogen, indirect cooling using compressor circulation with refrigerant, direct cooling using the adiabatic expansion of compressed air, and vortex tube refrigeration. 1) Indirect cooling using low-boiling-point media Figure 2 is a schematic diagram of the principle of indirect cooling using low-boiling-point media. This refrigeration method uses low-boiling-point gases such as nitrogen, which are harmless to humans and do not pollute the environment. These gases can be liquefied in relevant factories and then added to the air cooling system. They evaporate and absorb heat at normal temperature and pressure, thus cooling the air. Figure 2 shows an indirect cooling method using a low-boiling-point medium. In this method, liquid nitrogen is liquefied externally, and heat exchange only occurs with the air inside the system, resulting in a simple cooling structure. Furthermore, liquid nitrogen has a vaporization temperature of -180℃, allowing air to be cooled to below -100℃. The temperature can be controlled by the flow rate of liquid nitrogen, and subcooling can be addressed with a heater. However, this method requires a large-capacity liquid reservoir for continuous actual cutting, making it impractical. Additionally, the external preparation of low-boiling-point liquids like liquid nitrogen increases the overall system operating cost. 2) Cyclic Compression Indirect Refrigeration: Figure 3 shows a cyclic compression indirect refrigeration method. This method uses a low-boiling-point refrigerant and consists of a compressor, evaporator, condenser, and expansion valve forming a closed-loop cooling system. The refrigerant gas is selected based on refrigeration efficiency and evaporation temperature. This refrigeration method is widely used in household refrigerators and cold storage facilities, offering ideal temperature control and energy efficiency. Figure 3 shows a cyclic compression indirect refrigeration method. The gas evaporated in the evaporator is pressurized to a set pressure by the compressor, liquefied in the condenser, and stored in a liquid receiver. The high-pressure liquid in the receiver, below the set pressure, is depressurized by the expansion valve and enters the evaporator. Evaporation absorbs heat, cooling the air, and then the liquid returns to the compressor as gas. The cold air temperature can be set by the pressure in the evaporator and the liquid supply. This refrigeration method is divided into evaporation temperature (pressure) and condensation temperature (pressure); according to energy consumption, it is divided into single-stage compression cycle, multi-stage compression binary refrigeration cycle, etc. 3) Direct refrigeration via adiabatic expansion of air. Figure 4 shows a direct refrigeration method using adiabatic expansion of high-pressure air to lower the temperature of the cooling gas itself. This is an open-type direct refrigeration method. Figure 4 shows a direct refrigeration method using adiabatic expansion of air. At room temperature and high pressure air from an air compressor or pipeline enters the expander, causing it to expand below the set pressure. The energy of the air is consumed through power generation, external machine drive, etc., causing the temperature to drop. The cold air outlet temperature is determined by the inlet gas pressure, the outlet gas pressure, and the performance of the expander. Currently, equipment capable of generating -90℃ cold air is already in practical use. 4) Direct cooling with vortex tubes. Figure 5 shows the principle diagram of vortex tube cooling. When high-pressure air passes through the vortex tube, it will generate vortex motion. Due to the pressure difference (density difference) between the gas inside and outside the vortex, a temperature difference is generated. The gas in the center is low-temperature gas, and the gas on the outside is high-temperature gas. The temperature of the cold air is related to the inlet gas pressure and the exhaust gas flow rate. This cooling method does not require additional power, only a vortex tube is needed, and the structure is simple. However, since some gas needs to be discharged as hot gas, the cooling efficiency is somewhat poor. 1. Nozzle 2. Orifice plate 3. Vortex chamber 4. Control valve Figure 5 Direct cooling with vortex tubes 5) Requirements for air cooling systems in implementing air-cooled cutting technology The key to implementing air-cooled cutting technology is to control the four elements of air cooling: temperature, pressure, flow rate, and direction. Regardless of the refrigeration method, the following requirements should be met: • It should be compatible with existing air pressure in general factories; • The delivery pipe diameter should be as large as possible to reduce flow loss caused by the pipeline; • The cold air temperature and air volume should be adjustable; • The output of cold air should be able to be paused; • The cold air temperature should reach the required level in a short time; • The air nozzle should be as close as possible to the cooling point; a distance of more than 20mm from the cooling point results in a greater increase in cold air temperature. A comparison of the above four refrigeration methods is shown in the table below. Currently, Japan uses them most extensively, and the compressor cycle refrigeration method has been commercialized. 4. Advantages, Disadvantages, and Research Directions of Air-Cooled Cutting Advantages of air-cooled cutting: Because no cutting fluid is used, no elements such as Cl, S, and P are produced, resulting in no chemical pollution and benefiting the environment; it saves on the purchase cost of cutting fluid and related equipment and maintenance costs; it eliminates the need for cutting fluid treatment, preventing global warming and saving treatment costs; it saves Earth's resources by not using cutting fluid; and the chips can be directly utilized because no cutting fluid is used. However, air-cooled cutting also has the following problems: chip collection; tool lubrication during pure air cooling; rust prevention of machined workpieces; and noise from the cooling air. Current research directions in air-cooled cutting mainly include: improving cooling performance and reducing cooling air volume; reducing nozzle noise; developing tools with good lubrication; and researching efficient chip collection methods. Authors: Liu Xianli, Harbin University of Science and Technology; Wen Donghui, Dalian University of Technology; Zhong Jia, Shenzhen Yian Industrial Co., Ltd.