The definition and specific varieties of difficult-to-machine materials vary from generation to generation and from professional fields. For example, superalloys, titanium alloys and composite materials containing carbon fibers commonly used in the aerospace industry are difficult materials in this field. The engineering and technical personnel of the aerospace industry have carried out research and development work on processing technology, and have developed cutting tools and processing methods suitable for use in this field. In recent years, the development of multi-function and high-functionality of mechanical products has been very strong, requiring parts to be miniaturized and miniaturized. In order to meet these requirements, the materials used must have high hardness, high toughness and high wear resistance, and materials having these characteristics are particularly difficult to process, and new difficult-to-machine materials have emerged. This is why the difficult-to-machine materials have emerged with the development of the times and the professional fields. Their unique processing technologies have also been developed with the research and development of the times and various professional fields. 
On the other hand, with the advent of the information society, information on difficult-to-machine material cutting technology can be exchanged via the Internet. Therefore, information on the processing of difficult-to-machine materials will be more substantial, and processing efficiency will inevitably increase. In this paper, the cutting process of difficult-to-machine materials is taken as the core, and the development trend of this technology in recent years is introduced. 
In the cutting process, the tool wear that usually occurs includes the following two forms: (1) wear due to mechanical action, such as chipping or abrasive wear; (2) wear due to thermal and chemical effects, such as Wear such as bonding, diffusion, corrosion, etc., as well as breakage, thermal fatigue, thermal cracking, etc. caused by softening and melting of the cutting edge.
When cutting difficult materials, the above-mentioned tool wear occurs in a short time, which is due to the fact that there are many factors in the material to be processed to promote tool wear. For example, most difficult-to-machine materials have the characteristics of low thermal conductivity, and the heat generated during cutting is difficult to spread, resulting in a high temperature at the tip of the tool and a significant influence on the cutting edge. As a result of this effect, the bond strength of the binder in the tool material is lowered at high temperatures, and particles such as WC (tungsten carbide) are easily separated, thereby accelerating tool wear. In addition, components in difficult-to-machine materials and certain components in the tool material react under high-temperature cutting conditions, appear to be analyzed, detached, or generate other compounds, which accelerates the formation of tool wear such as chipping. 
When cutting high-hardness, high-toughness materials, the temperature of the cutting edge is high, and tool wear similar to that of cutting difficult materials can occur. For example, when cutting high-hardness steel, the cutting force is larger than that of cutting general steel. Insufficient rigidity of the tool will cause chipping and the like, which will make the tool life unstable and shorten the tool life, especially the workpiece that generates short chips. In the case of materials, crater wear occurs near the cutting edge, and the tool breakage often occurs in a short time. 
When cutting superalloys, due to the high temperature hardness of the material, the stress during cutting is concentrated at the tip of the blade, which will cause plastic deformation of the cutting edge; at the same time, the boundary wear due to work hardening is also serious. 
Due to these characteristics, users are required to carefully select the tool type and cutting conditions when cutting difficult materials, in order to obtain the desired processing results. 
Machining is roughly divided into turning, milling, and centering-based cutting (bit cutting of end bits, end mills, etc.), and the cutting heat of these cuttings has different effects on the cutting edge. Turning is a kind of continuous cutting. The cutting force of the cutting edge does not change significantly. The cutting heat acts continuously on the cutting edge. Milling is a kind of intermittent cutting. The cutting force acts intermittently on the cutting edge and will vibrate during cutting. The heat effect on the tip is alternated between heating during cutting and cooling during non-cutting, and the total heat received is less than that during turning. 
The heat of cutting during milling is an intermittent heating phenomenon, and the teeth are cooled when they are not cut, which will contribute to the extension of tool life. The Japan Institute of Physics and Chemistry has conducted a comparative test on the tool life of turning and milling. The tool used for milling is a ball end mill, and the turning is a general turning tool. Both are in the same material to be processed and cutting conditions (due to different cutting methods, cutting The depth, feed rate, cutting speed, etc. can only be achieved in a consistent manner. Under the same environmental conditions, the cutting comparison test shows that the milling process is more advantageous for extending the tool life.
When cutting with a tool such as a drill with a center edge (ie, a cutting speed = 0 m/min) or a ball end mill, the life of the tool near the center edge is often low, but it is still stronger than that during turning. 
When cutting difficult materials, the cutting edge is greatly affected by heat, which often reduces the tool life. If the cutting method is milling, the tool life will be relatively longer. However, difficult-to-machine materials cannot be milled from start to finish. There is always a need for turning or drilling. Therefore, corresponding technical measures should be taken for different cutting methods to improve processing efficiency.
CBN's high temperature hardness is the highest among existing tool materials and is best suited for cutting difficult materials. The new coated cemented carbide is made of ultra-fine grain alloy as the matrix and coated with a coating material with high temperature and hardness. This material has excellent wear resistance and is also an excellent tool material for cutting difficult materials. one. 
Titanium and titanium alloys in difficult-to-machine materials have high chemical activity and low thermal conductivity, and diamond tools can be used for cutting. CBN sintered body cutters are suitable for cutting high hardness steel and cast iron. The higher the CBN content, the longer the tool life and the higher the cutting amount. It has been reported that a CBN sintered body which does not use a binder has been developed. 
The diamond sintered body tool is suitable for cutting of aluminum alloy, pure copper and other materials. The diamond cutter has a sharp cutting edge, high thermal conductivity, and low heat retention at the tip of the blade, which can control the occurrence of deposits such as built-up edge to a minimum. When cutting pure titanium and titanium alloy, the single crystal diamond tool is more stable and can extend the tool life. 
Coated cemented carbide tools are suitable for the machining of a variety of difficult-to-machine materials, but the properties of the coating (single coating and composite coating) vary greatly. Therefore, suitable coatings should be selected according to different processing objects. Tool material. It has been reported that diamond-coated cemented carbide and DLC (Diamond Like Carbon) coated cemented carbide have recently been developed, which has further expanded the application range of coated tools and has been used in high-speed machining. 
When cutting difficult materials, the shape of the tool can be optimized to maximize the performance of the tool material. Choosing tool geometries such as rake angle, back angle, and plunging angle that are suitable for the characteristics of difficult-to-machine materials and proper treatment of the cutting edge have a great influence on improving cutting accuracy and extending tool life. Therefore, in terms of tool shape Can't be taken lightly. However, with the popularization and application of high-speed milling technology, a small depth of cut has been gradually adopted to reduce the tooth load, and the up-cut milling is used to increase the feed speed. Therefore, the design idea of ​​the shape of the cutting edge has also changed. 
When drilling difficult-to-machine materials, increasing the angle of the drill and performing cross-shaped grinding is an effective way to reduce the torque and heat of cutting. It can control the contact area between the cutting and the cutting surface to a minimum. It is very beneficial to extend tool life and improve cutting conditions. When the drill bit is drilled, the cutting heat is easily trapped near the cutting edge, and the chip removal is also difficult. These problems are more prominent when cutting difficult materials, and sufficient attention must be paid. 
In order to facilitate chip evacuation, a coolant discharge port is usually provided on the rear side of the cutting edge of the drill bit, and sufficient water-soluble coolant or mist coolant can be supplied to make the chip removal smoother. The cooling effect is also ideal. In recent years, some coating materials with good lubricating properties have been developed. These materials can be applied to the surface of the drill bit, and when they are used to process shallow holes of 3 to 5D, dry drilling can be used. 
Hole finishing has traditionally been done by boring, but it has recently been changed from conventional continuous cutting to discontinuous cutting with contour cutting, which is more advantageous for improving chip evacuation performance and extending tool life. Therefore, the boring tool for such intermittent cutting is designed and used immediately for CNC machining of automotive parts. In terms of threaded hole machining, spiral cutting interpolation is also currently used, and end mills for thread cutting have been put on the market in large quantities.
As described above, this conversion from the original continuous cutting to the intermittent cutting is performed as the understanding of CNC cutting is deepened, which is a gradual process. When cutting such difficult-to-machine materials by this type of cutting, the smoothness of the cutting can be maintained and the tool life can be extended. 
The cutting conditions of difficult-to-machine materials have always been set relatively low. With the improvement of tool performance, the emergence of high-speed and high-precision CNC machine tools, and the introduction of high-speed milling methods, at present, cutting of difficult-to-machine materials has entered high-speed machining and cutting tools. The period of long life. 
Nowadays, it is the best way to cut difficult materials by using a small depth of cut to reduce the cutting edge of the tool and thus improve the cutting speed and feed rate. Of course, it is also extremely important to choose tool materials and tool geometries that are adapted to the unique properties of difficult-to-machine materials, and to optimize tool cutting trajectories. For example, when drilling materials such as stainless steel, since the thermal conductivity of the material is very low, it is necessary to prevent the cutting heat from being largely retained on the cutting edge. For this reason, intermittent cutting should be used as much as possible to avoid friction between the cutting edge and the cutting surface. Will help to extend tool life and ensure stable cutting. When roughing a difficult-to-machine material with a ball end mill, the tool shape and the clamp should be well matched, which improves the wobble accuracy and clamping rigidity of the cutting part of the tool, so that each tooth can be guaranteed under high-speed rotation conditions. Feeds are maximized while extending tool life. 
As mentioned above, the best cutting method for difficult-to-machine materials is constantly evolving, new difficult-to-machine materials are constantly appearing, and the processing of new materials has always plagued engineers and technicians. Recently, new machining centers, cutting tools, jigs, and CNC cutting technologies have developed rapidly. In addition to cutting, CNC grinding and CNC machining have also achieved unprecedented development. The selection of processing technology for difficult-to-machine materials has been Great expansion. 
Of course, the collection of information about the processing of difficult-to-machine materials and the in-depth understanding of the technology are not satisfactory. Because of this, and the continuous emergence of difficult-to-machine materials, people always feel that processing technology is not enough. 
For example, the above-mentioned turning processing is converted from continuous cutting to intermittent cutting, which is beneficial to prolong the life of the tool, and the use of the new coated cemented carbide tool further improves the cutting technology level of the difficult-to-machine material. In the machining of difficult-to-machine materials, special attention should be paid to the stability of the tool life. Not only the workpiece material should be properly matched with the tool performance, but also the requirements for machining size, surface roughness and shape accuracy are extremely strict. Therefore, it should not only be special. Attention should be paid to the selection of tools, the clamping of the workpiece and other related technologies.
In the future, the processing of difficult-to-machine material parts will adopt computer-controlled production methods such as CAD/CAM and CNC cutting. Therefore, the improvement of the tool management system such as database construction, tool design and production is extremely important. In the machining of difficult-to-machine materials, the data on cutting conditions, such as the applicable tools, fixtures, process schedules, tool trajectories, etc., should be accumulated as basic data, so that the production methods of parts can be developed in the direction of IT-based. In this way, the cutting technology of difficult-to-machine materials can enter a new stage faster.
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