1. Introduction Deep hole machining is a challenging, technically demanding, and costly machining technique. To achieve chip removal, tool cooling, and lubrication, large quantities of cutting fluid (such as specialized deep hole cutting fluid or machine oil) are typically required in deep hole machining, especially for medium and large diameter deep holes (D≥30mm) using internal chip removal drills, where the consumption of cutting fluid is substantial (mainly carried away by the chips). This not only causes significant pollution to the machining area but also threatens the health of operators. Furthermore, handling oily chips increases production costs and contributes to environmental pollution. According to incomplete statistics, cutting fluid-related costs account for 15%–20% of the total machining cost in deep hole machining. Therefore, achieving dry machining without cutting fluid or near-dry machining using minimal cutting fluid is an important development direction and research topic for deep hole machining technology. 2. Sub-dry Deep Hole Machining System Due to the characteristics of deep hole drilling, a completely dry cutting method (i.e., without any cutting fluid) is generally difficult to implement in actual production. This is because deep hole drilling differs from ordinary turning and milling; it is performed in a closed environment, generating a large amount of cutting heat per unit time, with long chip removal channels, making it difficult to remove chips and cutting heat in a timely manner. Furthermore, in deep hole machining, the tool relies on guide blocks for centering and guidance. The guide blocks and the hole wall experience significant friction due to prolonged contact and compression. Cutting fluid (cutting oil) can form an oil film between the guide blocks and the hole wall, providing lubrication and reducing friction. Without this oil film, the guide blocks will quickly wear and tear, causing cutting vibration or tool breakage. Therefore, a sub-dry cutting method using a small amount of cutting fluid is more suitable for the actual conditions of deep hole machining. The sub-dry cutting process mainly utilizes compressed air for chip removal and cooling, and atomized cutting fluid for lubrication. The machining system mainly consists of an internal chip removal deep hole drilling machine, an air compressor, an atomizer, and a gas-liquid mixing nozzle. The working process is as follows: the air compressor provides air with a certain pressure (about 0.5 to 0.6 MPa), which is split into two paths at the outlet. One path carries a certain amount of cutting fluid through the atomizer to form a gas-liquid mixture, which meets the other path of compressed air at the nozzle. The gas-liquid mixture is accelerated as it passes through the nozzle and sprayed into the cavity of the air intake device, forming a high-pressure, high-speed atomized cutting fluid. Finally, it is transported to the cutting part of the drill bit through the channel between the outer wall of the drill rod and the hole wall, cooling and lubricating the tool and blowing the chips out from the inside of the drill rod. Because the sub-dry deep hole machining system uses atomized cutting fluid for lubrication and tool cooling, and compressed air for chip removal and cooling, it overcomes many drawbacks of traditional deep hole machining involving the large-scale recycling of cutting fluid, significantly reducing cutting fluid consumption. It also ensures a proper cutting fluid lubrication layer between the guide block and the inner hole surface, and between the tool's rake and flank faces and the chips. Simultaneously, the atomized cutting fluid absorbs heat sufficiently and provides uniform lubrication, allowing it to perform its function better. Using continuous compressed air for chip removal increases the chip removal space (compared to using high-pressure oil) and rapidly blows chips out of the cutting zone, shortening the chip heat transfer time and directly carrying away some heat, thus lowering the cutting zone temperature. 3. Design of Sub-Dry Deep Hole Drill Bits The design and development of deep hole drilling tools is one of the key technologies for successfully realizing sub-dry deep hole machining. It directly affects the stability and reliability of the entire machining system and is an important guarantee for the smooth progress of deep hole machining. The geometric parameters of the cutting tool play an important role in the tool's stress, chip breaking, durability, and surface quality; while the tool material has a decisive influence on the tool's cutting efficiency, working life, and the tool's affinity with the workpiece. The cutting characteristics of sub-dry deep hole machining are different from those of traditional deep hole machining, so the design requirements for deep hole drills are also special. (1) Requirements for cutting tools in sub-dry deep hole machining ① Since deep hole cutting is carried out in a closed state, and there is no circulating cutting oil to cool and lubricate the tool for heat exchange, the temperature in the cutting zone is high. This requires the cutting tool material to have high strength, hardness, good heat resistance, red hardness, and impact resistance. At the same time, the coefficient of friction between the tool and the chip should be as small as possible. ② In deep hole machining, the chip removal channel of the tool is long, and the chips are not easy to be removed smoothly. This problem is more prominent for sub-dry machining that uses air chip removal. In order to ensure smooth chip removal and rapid heat dissipation, it is necessary to increase the chip removal outlet, reduce air resistance, speed up chip removal, and shorten chip residence time. ③ In the pre-dry deep hole machining, compressed air is used for chip removal and atomized cutting fluid for cooling, so the chip shape cannot be too wide or too long. To prevent chip clogging, it is best to form small "C" shaped chips; to make the chips detach from the tool face faster, the contact area between the tool and the chips should not be too large. For this reason, chip breaking edge or forced chip breaking measures can be used. In view of the above requirements, we have conducted structural analysis and optimization design of the tool to meet the machining requirements of the system. (2) Selection and optimization design of tool structure The types of deep hole machining tools are mainly divided into external chip removal and internal chip removal. External chip removal is used in gun drilling systems and is suitable for machining small diameter (generally φ<20mm) deep holes. Internal chip removal is often used in BTA system, jet suction drilling system and DF system. The jet suction drilling system and DF system have the same function, but because the structure of the jet suction drilling system is more complex and the chip removal space is limited by the double drill rod, it is now rarely used. The DF system mainly uses a double oil inlet device to push and suck the chips to facilitate their smooth discharge. While maintaining the same chip removal effect, the sealing pressure can be reduced to improve machining accuracy. BTA deep hole drills are available in single-tooth and multi-tooth versions. Single-tooth drills are suitable for hole diameters ranging from φ6 to φ25mm, while multi-tooth drills are suitable for machining deeper holes with larger diameters (above φ25mm). In this experiment, we selected the BTA single-tooth drill. Considering the characteristics of sub-dry deep hole machining, we optimized the traditional BTA single-tooth drill design, primarily in the following ways: ① We appropriately increased the clearance h (air inlet clearance) between the drill body and the hole wall to reduce air resistance, allowing compressed air and atomized liquid to quickly reach the cutting zone for lubrication and cooling. ② We appropriately increased the chip removal opening, forming an inverted cone shape, allowing chips to enter the chip removal channel quickly and smoothly, preventing chip blockage; simultaneously, a "jet suction effect" is created at the chip removal inlet, increasing the chip removal speed. ③ We increased the number of chip-breaking edges and widened the chip-breaking platform to achieve forced chip breaking and separation, making the chips narrower and easier to break, which helps with chip removal and heat dissipation. Several improvements were also made to the tool geometry, mainly in the following aspects: ① The rake angle γo was appropriately increased to reduce the contact area between the tool's rake face and the chip, ensuring that compressed air and atomized cutting fluid can fully enter the cutting zone to cool and lubricate the tool. Since the inner cutting edge mainly bears axial compressive force during cutting, the rake angle can be appropriately increased to increase the inner cutting edge strength. ② The clearance angle αo was appropriately increased to reduce the friction between the tool's clearance face and the machined surface, making it easier for the tool to cut into the workpiece, reducing tool wear, and improving tool durability. ③ The width of the chip breaker and the radius of the fillet were increased to lengthen the chip curling deformation time, reduce the friction between the chip and the rake face and the impact force of the chip on the chip breaker boss, allowing the chip to flow smoothly over the chip breaker. Additional deformation was added at the transition fillet, causing the material to lose some plasticity, then bend to the bottom, deform again under the action of bending moment, and finally break the chip. This reduces chip deformation, changes cutting heat, cutting impact force, and friction, and controls the formation of small "C"-shaped chips for discharge. (3) Selection of insert materials Currently, there are many tool materials used for dry machining, such as ultrafine grain cemented carbide, CBN, PCD, ceramics and cermets, and various coated hard tools. However, most of these tool materials are only used in ordinary dry turning and dry milling. At present, the most commonly used deep hole machining tools in China are welded deep hole drills, and their insert materials are mainly domestic inserts such as YD15, YG8, YW1, YT726, and YT798. These materials have different chemical compositions and physical properties, and their applicable ranges are also different. We attempted to select the insert material that is more suitable for sub-dry deep hole machining through cutting tests. The cutting test scheme adopted is as follows: using the drills of the above 5 insert materials, 45 steel (tempered hardness HRC28) is machined under the same tool structure and geometric parameters. The stress and chip state of each tool under sub-dry cutting conditions are detected, and the wear of the tool flank and the surface quality of the inner hole are measured to select the sub-dry tool material most suitable for machining 45 steel. The cutting test conditions were as follows: machining hole diameter: 20.2 mm, workpiece length: 1000 mm, spindle speed v = 800 r/min, tool axial feed rate f = 0.01 mm/r, and cutting fluid: German Arlens emulsion No. 5. From the cutting performance and external tooth flank wear of deep hole drills with various insert materials, it can be seen that among these five insert materials, YD15 has the lowest flank wear value, axial force, torque, and internal hole surface roughness values; followed by YG8. This reflects that YD15 inserts have high heat resistance and good wear resistance, as well as good machining stability and quality. Therefore, YD15 is more suitable for semi-dry deep hole machining of 45 steel workpieces; YG8's machining effect is relatively slightly worse; while the other materials are not suitable for semi-dry deep hole machining due to severe wear. The test results are also consistent with the tool material properties: YD15 has good red hardness, good wear resistance, and high bending strength, making it particularly suitable for machining high-temperature alloy materials. Furthermore, the machining process shows that the semi-dry deep hole machining system has excellent chip removal performance, with almost no chip clogging. The discharged chips are also low in temperature (no burning sensation) and silvery-white, indicating good atomization and cooling effects, which meet the requirements of semi-dry deep hole machining. 4. Performance Comparison of Semi-Dry Deep Hole Machining and Traditional Deep Hole Machining Through the above experimental analysis, it can be determined that the YD15 insert material is more suitable for semi-dry deep hole machining of medium carbon steel. However, whether its machining performance and tool durability differ from wet deep hole machining will be key to the widespread application of semi-dry deep hole machining technology. Therefore, we conducted a comparative cutting test, namely, drilling a 45 steel workpiece of the same length using YD15 and YG8 deep hole drills (with identical tool geometry parameters) under traditional deep hole machining conditions (v=800r/min, f=0.01mm/r, 10# machine oil cooling and lubrication). The measured experimental data show that the tool flank wear and cutting force are lower under traditional machining methods than under semi-dry machining conditions. This indicates that the friction between the tool and workpiece, and between the tool and chips, is greater under semi-dry machining, meaning the cutting fluid's lubrication performance is poorer, which is related to the cutting fluid used. Semi-dry machining uses a water-based emulsion (whose atomization is superior to cutting oil), while traditional machining uses cutting oil (which has better lubrication). This also points to a direction for our future research: finding a cutting fluid that has both good atomization and good lubrication. Furthermore, the difference in tool wear between the two machining methods is not significant, indicating that a certain level of tool durability can be guaranteed under semi-dry deep hole machining conditions. After dissecting and testing the specimens machined by both methods, it was found that the surface hardness of the inner hole was almost the same, while the surface roughness value of the inner hole machined by semi-dry machining was lower, indicating better surface quality. This also shows that the cooling effect on the inner hole surface is good in semi-dry deep hole machining, and the cutting temperature is not high. In addition, due to the use of a specially formulated emulsion, which has a certain resistance to extrusion, the guide block of the drill bit did not experience tearing wear, thus ensuring the surface finish of the inner hole. 5 Conclusions Through the above cutting tests and comparative analysis of the results, the following conclusions can be drawn: (1) The designed sub-dry deep hole machining system can complete the deep hole machining process well and achieve satisfactory machining results, which has certain practical application value. (2) Through cutting tests on several commonly used deep hole machining tool materials, it was determined that under the sub-dry deep hole machining method, YD15 material is more suitable for machining 45 steel workpieces and has a better machining effect. For other workpiece materials, suitable tool materials can also be found through experimental methods. (3) Through the cutting performance tests and analysis of tools under dry and wet machining methods, it can be seen that sub-dry deep hole machining can achieve higher tool durability and better machining results. (4) In view of the problem of large tool wear in sub-dry deep hole machining, low temperature cold air or increasing the oiliness of coolant can also be used to improve the cooling and lubrication effect and reduce tool wear.