1 Introduction
Deep hole machining is a kind of hole processing technology with high difficulty, high technical content, high professionalism and high processing cost in machining. In deep hole machining, a large amount of cutting fluid (such as special deep hole cutting fluid or engine oil) is usually used for the purposes of chip evacuation, cooling and lubricating the tool, especially for the medium and large diameters of the deep hole drill with internal chip removal. Hole drilling (D≥30mm) consumes a large amount of cutting fluid (mainly taken away by chips), which not only causes great pollution to the processing site, but also threatens the health of the operators. At the same time, the treatment with oil chips is also It will increase production costs and cause environmental pollution. According to incomplete statistics, in the deep hole processing, the related cost of the cutting fluid accounts for 15% to 20% of the total processing cost. Therefore, the realization of dry machining without using a cutting fluid or sub-dry machining using a small amount of a cutting fluid is an important development direction and research subject of the deep hole processing technology.
2 Sub-dry deep hole processing system
According to the characteristics of deep-hole drilling, it is generally difficult to achieve full dry cutting (ie, no use of cutting fluid) in actual production. Because the deep hole drilling process is different from ordinary turning and milling processes, it is carried out under closed conditions. The cutting heat generated per unit time is large, the chip removal channel is long, the chip and cutting heat are not easily discharged in time; in addition, the tool is deep. In the hole processing, the centering and guiding action is performed by means of the guide block. Large friction occurs between the guide block and the hole wall due to prolonged contact pressure, and the cutting fluid (cutting oil) can be between the guide block and the hole wall. A layer of oil film is formed, which acts as a lubricant and anti-friction. If there is no oil film, the guide block will be quickly worn and torn, resulting in cutting vibration or knife. Therefore, the use of a small amount of cutting fluid sub-dry cutting method is more suitable for the actual situation of deep hole processing.
The adopted sub-dry cutting machining program mainly uses compressed air for chip evacuation and cooling, and uses atomized cutting fluid for lubrication. The processing system is mainly composed of internal ramming deep hole drilling machine, Air Compressor, atomizer, and gas-liquid mixing. The nozzles and other components are composed of: The air compressor provides air with a certain pressure (about 0.5-0.6 MPa) and is divided into two at the outlet. One of them is used to bring a certain amount of cutting fluid through the atomizer to form a vapor-liquid mixture. At the nozzle, it meets the other compressed air. When the vapor-liquid mixture passes through the nozzle, it is accelerated and ejected into the cavity of the air intake device to form a high-pressure, high-speed atomized cutting fluid. Finally, it passes through the outer wall of the drill pipe and the hole wall. The channel between them is transmitted to the cutting part of the drill, cooling, lubricating the tool and blowing the chips back from the inside of the drill pipe.
As the sub-dry deep-hole machining system uses atomized cutting fluid to lubricate and cool tools, the use of compressed air for chip evacuation not only overcomes many drawbacks caused by the large number of circulating fluids used in traditional deep-hole machining, and greatly reduces cutting. The amount of liquid used also ensures the lubricating layer of cutting fluid between the guide block and the surface of the inner hole, between the front and rear rake faces, and the cutting chips. At the same time, the cutting fluid absorbs heat sufficiently and evenly in the atomized state, and can be better played. Role; the use of continuous compressed air chip removal can increase the chip space (as opposed to the use of high-pressure oil), but also quickly cut the chip from the cutting area, shorten the chip heat transfer time, and some of the heat directly out, Reduced cutting zone temperature.
3 Sub-dry deep-hole drill bit design
The design and development of deep-hole drilling tools is one of the key technologies to successfully realize sub-dry deep-hole machining, directly affecting the stability and reliability of the entire machining system, and is an important guarantee for the smooth operation of deep-hole drilling. The geometric parameters of the tool play an important role in the force, chip breaking, durability, and the quality of the machining surface of the tool. The tool material plays a decisive role in the cutting efficiency, working life, and the affinity of the tool and the workpiece. The cutting characteristics of sub-dry deep-hole machining are different from those of traditional deep-hole machining. Therefore, the design requirements for deep-hole drill bits also have their own particularities.
(1) Requirements for Sub-Dry Deep-hole Machining of Cutting Tools 1 As the deep-hole cutting processing is performed in a closed state, there is no circulation cutting oil cooling, lubrication of tools, heat exchange, so the cutting zone temperature is high, which requires The blade material must have high strength, hardness and good heat resistance, red hardness and impact resistance, and the coefficient of friction between the tool and the chip should be as small as possible.
2 In deep hole machining, the chip evacuation channel of the tool is long and the chips are not easily discharged. This problem is even more pronounced for sub-dry machining using air chip removal. In order to ensure the smooth and rapid heat dissipation of the chips, it is necessary to increase the chip outlet, reduce the air resistance, speed up the chip removal speed, and shorten the chip retention time.
3 Sub-dry deep-hole machining uses compressed air chip removal and atomized cutting fluid cooling, so the chip shape cannot be too wide or too long. In order to prevent the clogging of chips, it is better to form small “C†chips; in order to make the chips get out of the knife face faster, the contact surface between the tool and the cuttings should not be too large. Therefore, it is possible to use a chipping edge or forced chip breaking measures.
In response to the above requirements, we performed structural analysis and optimized design of the tool to meet the processing requirements of the system.
(2) Selection of tool structure and optimization design The types of deep hole machining tools are mainly divided into two types: external chip removal and internal chip removal. The chip removal method is used in gun drilling systems and is suitable for machining deep holes of small diameter (generally φ<20mm). The internal chip removal method is commonly used in BTA systems, jetting and suction drilling systems and DF systems. The function of the ejector and suction system is the same as that of the DF system. However, due to the complicated structure of the ejector and suction system, the space for chip evacuation is limited by the double drill pipe, so it has been used less frequently. The DF system mainly utilizes the double oil feeding device to push and suck the swarf, which promotes smooth discharge. When the chip removal effect is the same, the sealing pressure can be reduced and the machining accuracy can be improved. BTA deep hole drill bit is divided into single tooth and multi-teeth two kinds, single-tooth tool suitable for the scope of the hole diameter φ6 ~ φ25mm, multi-tooth tool is suitable for machining larger diameter (φ25mm or more) deep hole.
In this experiment, we chose the BTA single tooth bit. In the design of the drill bit structure, taking into account the characteristics of the sub-dry deep-hole machining, a partial optimization design based on the traditional BTA single-drill bit is mainly embodied in the following points:
1 Properly increase the gap h between the cutter body and the hole wall (ie, the air intake gap) to reduce the air resistance, so that the compressed air and atomized liquid can quickly reach the cutting area for lubrication and cooling.
(2) Properly increase the chip evacuation port, and the chip evacuation port forms an inverted cone type, so that the chip can enter the chip evacuation channel quickly and smoothly, and it is not easy to plug the swarf; at the same time, an “injection effect†is formed at the entrance of the chip evacuation and the chip removal is increased. speed.
3 Increase the number of chipping edges, widen the chip breaker table, realize forced chip separation and chip breaking, make the chips narrow and easy to break, and help to remove chips and dissipate heat.
Some improvements have also been made in the tool geometry, mainly as follows:
1 Properly increase the blade front angle γo to reduce the contact area between the rake face of the tool and the chip, and ensure that the compressed air and atomized cutting fluid can fully enter the cutting area to cool and lubricate the tool. Since the inner blade mainly bears the axial compression force during cutting, in order to increase the inner edge strength, the inner edge rake angle can be appropriately increased.
2 Appropriately increase the blade back angle αo to reduce the friction between the tool flank and the machined surface, and also make it easier for the tool to cut into the workpiece, which can reduce the tool wear and improve the tool durability.
3 Increase the width of the chip breaker table and the radius of the fillet, lengthen the time for chip curl deformation, reduce the friction between the chip and the rake face, and the impact of the chip on the breaker boss, so that the chip flows smoothly through the breaker table in the transition circle Adding a part of additional deformation at the corner causes the material to lose some plasticity, then bends the top to the bottom, deforms again under the action of the bending moment, and finally breaks the chips, which can reduce the chip deformation and make the cutting heat, cutting impact, friction and so on occur. Change, control chip formation fine "C" type discharge.
(3) Selection of blade materials There are many tool materials currently used for dry machining, such as ultra-fine grain cemented carbide, CBN, PCD, ceramics and cermets, and various coated hard tools, but most of these tool materials Only used in ordinary dry turning and dry milling. At present, domestic use of deep-hole processing is mainly welded deep-hole drills, and its blade materials mainly use domestic blades such as YD15, YG8, YW1, YT726, YT798, etc. These materials have different chemical compositions and physical properties. The scope of application is also different. We tried to select the blade material that is more suitable for sub-dry deep hole machining through cutting test.
The cutting test scheme adopted was: using the drills of the above five kinds of blade materials, 45 steels (quenched and weighed hardness HRC28) were respectively machined under the condition of the same tool structure and geometrical parameters, and the influence of the tools under sub-dry cutting conditions was examined. Force and chip condition, tool back face wear amount, inner hole surface quality, etc. are selected to select the most suitable dry steel tool material for machining 45 steel.
The cutting test conditions used were: machined aperture: 20.2mm, workpiece length: 1000mm, spindle speed v=800r/min, axial feed of the tool f=0.01mm/r, cutting fluid: Germany Aerns emulsion No. 5 .
It can be seen from the cutting performance of the deep-hole drill bits of various insert materials and the wear condition of the external tooth flank. Among the deep-hole drill bits of the five types of insert materials, the flank wear of the YD15 is the smallest, the axial force, the torque and The inner surface roughness value is also the smallest; followed by YG8. Reflects YD15 blade has high heat resistance and good wear resistance, processing stability and processing quality is good. Therefore, YD15 is more suitable for sub-dry deep hole machining of 45 steel workpiece materials; YG8's machining effect is relatively less, while other materials are not suitable for sub-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, and is especially suitable for processing high-temperature alloy materials. In addition, it can be seen from the processing process that the sub-dry deep-hole machining system has good chip evacuation, almost no chip swarf, and low chip temperature (no burning sensation) and silvery white indicating mist Good cooling and cooling effect can meet the requirement of sub-dry deep hole machining.
4 Comparison of Sub-dry Deep-hole Machining and Conventional Deep-hole Machining
Through the above test analysis, it can be confirmed that the YD15 insert material is more suitable for sub-dry deep hole machining of medium carbon steel. However, whether the processing performance and tool life is different from the wet deep hole processing will be the key to whether the sub dry deep hole processing technology can be popularized and applied. For this purpose, we conducted a comparative cutting test, in which the same length of 45 steel workpieces (v=800r/min,f) were drilled with YD15 and YG8 deep-hole drills (with the same tool geometry) under the traditional deep-hole machining conditions. = 0.01mm/r, 10# mechanical oil cooling lubrication). From the measured experimental data, it can be seen that when machining with the traditional method, the flank wear and cutting force of the tool are smaller than in the sub-dry machining conditions, which indicates that the tool and the workpiece during sub-dry machining The friction between the tool and the swarf is greater than in conventional machining, ie, the lubricating performance of the cutting fluid is poor, which is related to the used cutting fluid. Sub-dry processing uses water-based emulsions (which have better atomization than cutting oils), while conventional machining uses cutting oils (which have better lubricity). This also determines the direction for our future research, that is, we need to find a cutting fluid that has both good atomization and good lubricity. In addition, the difference in tool wear between the two machining methods is not large, indicating that a certain degree of tool life can also be guaranteed under sub-dry deep-hole machining conditions.
The test pieces that were processed in two ways were cut and examined. It was found that there was almost no difference in the surface hardness of the inner hole, while the surface roughness value of the sub hole in the inner hole was smaller, that is, the surface quality was better. This also shows that in sub-dry deep hole machining, the inner hole surface has a good cooling effect and the cutting temperature is not high. In addition, due to the use of a specially formulated emulsion, it has a certain resistance to extrusion, so the guide block of the drill does not appear to tear wear, but also to ensure the quality of the inner surface of the hole.
5 Conclusion
Through the above cutting test and comparative analysis of the results, the following conclusions can be drawn:
(1) The designed sub-dry deep hole machining system can better complete the deep hole machining process, and can get satisfactory processing results, and has certain practical application value.
(2) Through the cutting test of several commonly used deep-hole machining tool materials, it was determined that YD15 material is more suitable for machining 45 steel workpiece materials in the sub-dry deep-hole machining mode, and has a better processing effect. For other workpiece materials, suitable tool materials can also be found by test methods.
(3) Through the cutting performance test and analysis of dry and wet tools, it can be seen that sub-dry deep-hole machining can achieve higher tool durability and better machining results.
(4) For the problem of large tool wear in sub-dry deep-hole machining, low-temperature cooling air or increasing the oiliness of coolant can also be used to improve the cooling and lubricating effects and reduce tool wear.
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