1 Introduction Graphite has high high-temperature strength, low thermal expansion coefficient, good processability, and good heat and conductivity. Therefore, graphite electrodes are widely used in metallurgy, electric furnace, EDM and other fields. In EDM, the development of new graphite electrode materials and their processing technology has expanded the application of EDM and improved its performance. Compared with copper electrodes, graphite electrodes have the advantages of low electrode consumption, high processing speed, good mechanical processing performance, high processing accuracy, small thermal deformation, light weight, easy surface treatment, high temperature resistance, high processing temperature, and electrode adhesion. Although graphite is a very easy-to-cut material, the graphite material used as the EDM electrode must have sufficient strength to avoid damage during handling and EDM processing, while the electrode shape (thin-walled, small rounded, sharp) Such as the graphite electrode grain size and intensity requirements put forward higher, which led to the graphite workpiece during the processing process is easy to disintegrate, the tool is easy to wear. Therefore, how to prevent the workpiece from disintegrating, improve the quality of surface processing, and reduce the cost of processing tools have become an important issue in the processing of graphite electrodes. Conventional machining methods such as turning, milling, and grinding can be used to meet the requirements for machining simple-shaped electrodes. However, the requirements for the complexity of the electrode geometry have continued to increase in recent years. The high machining surface quality and high processing precision of high speed machining have made graphite electrode high speed machining a hot spot in the die EDM processing. Many manufacturers have already launched the graphite high speed machining center. For example, Makino SNC64 CNC high-speed graphite milling machine, Rö:ders RFM series machine tools, spindle speed is usually 10000 ~ 60000r/min, feed rate can reach 60m/min or more, processing wall thickness is less than 1mm, the minimum fillet radius is less than 0.2mm. Because high-speed machining of EDM graphite electrodes is still a new process, the processing performance of different graphite materials is also different. For the problems of workpiece disintegration, tool wear and processing strategy in the high-speed processing of graphite electrodes, few domestic literature mentions . This article introduced the research results of high-performance graphite electrode materials machining in Japan, Germany, etc., including the basic theory of cutting mechanism, cutting temperature, tool wear, chip processing and other aspects of graphite electrode materials, and the high-speed processing strategy and processing of graphite electrodes. Parameter selection and other content. 2 graphite electrode high-speed cutting mechanism
Fig. 1 Cutting process when turning sintered carbon and graphite
Figure 2 Graphite high-speed milling process and tool wear patterns
Figure 3 Cutting temperature of graphite electrode material
Fig. 4 Tool Wear Graphite Electrode High-Speed ​​Machining Process Masuda (1996) observed the sintering process of sintered carbon (sintering below 2000°C) and graphite (sintering above 2500°C) using high-speed photography (Fig. 1). Yes: When the cutting edge of the tool comes into contact with the workpiece, a crack propagates. A part of the workpiece breaks due to tool advancement and forms chips. Most of the cracks in the cutting of carbon-phase materials expand downwards, and the chips are scattered on the surface of the tool or accumulate on the rake face: the graphite cutting cracks extend in the cutting direction, and most of the chips slide along the rake face, thus causing crater wear of the tool. Kö:nig (1998) thought after studying the high-speed milling process of graphite: The formation of graphite chips is very similar to brittle materials such as ceramics. There are crushing at the blade tip to form fine chips and small pits. The crack will extend to the front of the tool tip and then spread to the free surface, forming a fracture pit, which can be explained by fracture mechanics: the state of contact between the chip and the rake face of the tool is divided into the cutting contact impact zone and the swarf rake face. The slip zones, respectively, lead to different tool wear patterns (Figure 2). The factors that affect the cutting process are: cutting speed, cutting feed, tool geometry, tool material and tool wear. Cutting force and cutting temperature for high-speed cutting of graphite electrodes The cutting force of graphite electrode materials is only about 10% of that of tough metals such as aluminum and copper, so cutting forces are usually not the focus of research. The experimentally measured turning temperature of graphite material is not high, when Vc = 500m/min, the maximum temperature is between 160 ~ 300 °C, and a linear relationship with the cutting speed. According to this, even if Vc = 500m/min, the cutting temperature will not exceed 500°C, and the influence on the cutting process will not be too great (Figure 3). 3 High-speed machining of graphite electrodes Tool wear Tool wear is the most important issue in the processing of graphite electrodes. The amount of wear not only affects the tool wear loss, processing time, and processing quality, but also affects the surface quality of the EDM-worked workpiece material, which is an important parameter for optimizing high-speed machining. High-speed processing of graphite electrode tool wear mechanism The main tool wear area of ​​graphite electrode material processing is rake face and flank face. On the rake face, the impact contact between the tool and the crushing chip area produces impact abrasive wear, and the sliding chips along the tool surface generate sliding frictional wear. There are two types of impacts produced by the rake face impact wear chip particles on the rake face of the tool. One is the impact with the rake face at an angle. The impact causes the surface layer to fall off or peel off: the other is the impact of cutting. That is, the graphite cuttings produce micro-cuts on the rake face of the tool. The maximum width of the groove marks reaches 150 nm. The wear of the impact zone forms the crater wear of the rake face (Figure 4). The tool wear of the rake face is in the sliding wear zone of the rake face, and the graphite debris has a certain protective effect on the rake face. The flank wear is mainly due to the mechanical friction and wear of the tool flank and the machined surface.
Workpiece material: EK85: Grain size: 13μm: Tool: End mill, D=12mm, Z=2: Cutting conditions: fz=0.05mm, ap=3mm, ae=12mm
Fig. 5 Influence of cutting speed and tool material on tool wear Influence of high-speed machining tool wear The cutting speed increases. Although the large fracture area increases, KT decreases, KB is approximately constant, and the crescent tooth wear cross-sectional area decreases. small. With the increase of cutting speed, the graphite lubricating film produced on the friction surface is thickened and the surface wear coefficient is reduced, so the tool wear decreases rapidly, which is also an important reason for the superiority of high-speed processing of graphite. Tool material The tool material has a great influence on the impact wear. As the general tool material HV increases, the KT decreases. The effect of tool material on tool wear is shown in Fig. 5. Wear Mechanism of Cemented Carbide Cutting Tools for Graphite Electrode Materials: Wear in the Sliding Zone due to Micro-cutting and Surface Fatigue Failure: Eventually Causes Tool Cohesion Phase (Co) Abrasive Wear and Wear-Resistant Phase (WC) Wear, Generation Cracks and fractures fall off. Increasing WC particle size and reducing Co phase particle size can reduce tool wear. Tool surfaces sometimes have graphite bonds. Polycrystalline diamond tool wear consists of the wear of the binder phase by graphite chips and the secondary abrasive wear caused by diamond crushing. The surface of the cutter is usually a layer of lubricating film made of adhered graphite, and its wear resistance is 1 to 2 times that of the hard alloy K10. The surface of the diamond film cutter usually has strong graphite adhesion, and the diamond surface is broken and there is no crater wear. It belongs to macroscopic impact wear, not mechanical abrasive wear. The effect of the surface treatment of the coated substrate cemented carbide affects the coating efficiency and also has a great influence on the tool life. The lifetime of diamond film tools can reach 100 times that of K10 and is superior to PCD tools. Cemented carbide coated tools (TiN, etc.) can significantly improve tool wear resistance. Al2O3 ceramic tools are not suitable for cutting graphite materials.
Workpiece material: EK85: Grain size: 13μm: Tool: Flat-bottomed end mill, D=12mm, Z=2: Tool material: Cemented carbide K10: Dull grinding standard: VB=0.1mm
Fig. 6 Relationship between feed and milling width and tool wear
Tool: Flat-end milling cutter, D=6mm, Z=2: Tool material: Cemented carbide K10: Dull grinding standard: VB=0.12mm: Cutting condition: Vc=600m/min, fz=0.06mm, ap=6mm, Ae=1mm
Figure 7 Effect of tool angle on tool wear
Tool: Flat-end milling cutter, D=6mm, Z=2: Tool material: HM K10: Cutting conditions: fz=0.05mm, ap=3mm, ae=12mm
Figure 8 Effect of Graphite Electrode Material on Tool Wear Feedrate increases the feed per tooth to the cutter, KB, KL, and KT increase. Increasing the cutting width per tooth, ie, increasing the average chip thickness, increases the cutting impact and increases tool wear. It can be seen from FIG. 6 that the cutting condition changes after the milling width is larger than the milling cutter radius. Tool Angle The tool rake angle increases, changing the impact angle of chip particles, KT decreases, but KB does not change much: When the back angle increases, the sharpness of the tool increases, the wear of the flank reduces: the change of the main angle, change The cutting force direction and the actual cutting area, so as the main angle increases, the tool wear is also reduced, tool life is improved (Figure 7). Graphite electrode materials Graphite electrode materials have a great influence on tool wear. As shown in Fig. 8, the smaller the graphite grain size during high-speed machining, the higher the tool life, and the tool life is roughly proportional to the bending strength and Shore hardness. In addition, the degree of graphitization of the graphite electrode material, the composition of the impregnated material, and the particle size of the filler material also have a certain influence on tool wear. Tool structure High-speed milling Common ball-end cutters and flat-end cutters for machining. When a ball end mill is used to machine the surface, the cutting speed is reduced from outside to inside, so the top of the tool is prone to wear. The flat-bottom end mill can process the contour of the step, and the machining allowance fluctuates sharply. The tool contour fluctuates during machining and the finishing tool is strongly damaged. Compared with the two tools under the same conditions, the cutting distance of the flat end mill is longer than that of the ball end mill. Milling direction Milling direction in high-speed milling is very important. Under the same conditions for down milling and up milling, the tool life is not the same due to the different amount of broken particles and the actual cutting impact of the tool. The tool life for up-cut milling is higher than that for down-cut tools (Figure 9).
Workpiece material: V1364: Particle size: 7μm: Tool: Flat-bottomed end mill, D=6mm: Tool material: Cemented carbide K10: Cutting conditions: Vc=600m/min, fz=0.074mm, ap=3mm, ae=0.35mm ,Rth=5μm
Figure 9 Effect of milling direction on tool wear
Machine tool cost: 200DM/h: Tool change time: 30s: Workpiece material: EK85: Grain size: 13μm: Cutting condition: fz=0.05mm, ap=3mm, ae=12mm: Grinding standard: VB=0.1mm
Fig. 10 Roughing costs of typical tool materials 4 Basic principles of milling strategy for graphite electrodes The electrodes in the traditional electro-machining process move vertically, so the corresponding three-axis milling width is not limited. On NC controlled EDM machines, multi-axis and vertical feed EDM can be performed to meet specific requirements. Electrode geometry is very complex and difficult to classify based on geometry. It can be roughly classified into two categories based on cavity surface features: 1) free surface forming electrode: with fillet adjustment surface, no sharp cavity and corner, can use ball Head cutter machining, long milling path, and time-consuming: 2) Prismatic surface electrode: Consisting of prismatic surfaces, no rounded corners, the simplest curved surface, cylindrical surface, and sharp-edged edge, usually with flat-bottomed end mills . The economics of high-speed processing of graphite are related to the effects of cutting and wear mechanisms. Therefore, it is necessary to optimize cutting parameters and machining strategies for the geometric characteristics of tools and machining tools, machine tools, and chip processing. Optimization of the processing conditions In the foregoing, several factors affecting the processing mechanism and tool wear of graphite were analyzed in detail. The following is a brief description of how to optimize the processing conditions. When the tool material is roughed, the higher the cutting speed, the smaller the tool wear and the lower the machining cost. When the cutting speed is greater than 900m/min, the cost per unit cut length for cemented carbide tools, diamond coated tools, and polycrystalline diamond tools is not much different. It is therefore recommended to use coated diamond tools at low speeds (Figure 10). In the finishing process, when calculating the tool cost per blade, the diamond-coated tool is the best, the polycrystalline diamond tool is the second, and the carbide tool is the worst: but if the individual tool cost is calculated, the polycrystalline diamond tool has a better heavy edge. Grinding, so the polycrystalline diamond tool has the lowest tool cost. Small-diameter tools are recommended for use with diamond-coated tools, where machining is most economical. Tool geometry parameters The increase of the tool front and back angles increases the chip space. For roughing, the rake angle is better around 6°, and the relief angle should be less than 15°. The tool declination angle is not related to the wear of the side edge. In the finish machining, the rake angle is between 6 and 10°. Although the tool wear amount decreases when the main deflection angle is large, this will cause the surface roughness fluctuation to increase. Therefore, the main deflection angle is less than 30°. Large is not suitable for finishing. The cutting amount per tooth is related to tool wear, and the maximum value of feed rate and cutting speed is related to the machine tool characteristics. At the same time, the dynamic vibration of the arbor load bearing and fine arbor machining is also related to the machine tool performance. The cutting data selection principle is: 1) Determine the number of teeth of the tool according to the given conditions such as the machine tool and tool chuck to prevent the tool from vibration: 2) Calculate the maximum allowable feed per tooth in the range of cutting tool strength, cutting depth and cutting width: 3 ) According to the machine tool feed and the machine feed acceleration characteristics, the maximum cutting spindle speed is determined at constant feed per tooth: 4) The stable maximum spindle speed is finally selected and adapted to the feed per tooth. The processing parameters of the polycrystalline diamond tools recommended for turning are: Vc=200-400m/min, fz=0.02-0.04mm/tooth, and depth of cut less than 1.5mm during rough machining: Vc=25 to 100m/min during finishing Fz=0.02-0.1mm/tooth, cutting depth less than 0.5mm. Graphite Electrode High-speed Processing Strategy The strategy of high-speed roughing and finishing of graphite electrodes is different. Normal roughing should leave less allowance for finishing, so high-feed (cutting feed and pass feed) should be used when using small-diameter tools. Under the premise of minimizing the amount of tool wear, a high unit cutting volume and effective single-blade cutting amount are obtained, and the residual cutting amount meets the requirement of finishing machining: the goal of finishing is to obtain the highest processing quality with the shortest processing time. The ratio of optimum surface quality to minimum tool wear should be optimized. During processing, the processing speed should be increased, the processing time should be shortened, and the instability of the machining process caused by the change in cutting amount should be minimized to maximize the tool life. The high-speed machining strategy of the free surface forming electrode is mainly to optimize the cutting machining path taking into account the local machining allowance.
Workpiece material: EK85: Stone size: 13μm: Tool: Ball end mill, D=10mm, Z=2
Fig. 11 Comparison of contour milling and profile milling
Workpiece material: EK85: Grain size: 13 μm: Graphite grain size: D=6 mm, Ik=50 mm: Tool material: Cemented carbide K10: Cutting conditions: Vc=600 m/min, fz=0.044 mm, Rth=10 μm
Figure 12 Pulling and Drilling and Finishing Strategies
Figure 13 Processing Example Roughing In general, the graphite electrode is performed on a monolithic material. The machining allowance is easily described. The processing target is to remove the largest amount of material in the shortest time. Roughing can be done by copy milling or contour milling (Figure 11). Profiling milling uses ball-end milling cutters, cutting depth and cutting width are all changing, the cutting depth is small, the tool wears fast, and the machining time is long: the contour milling uses flat-bottom milling cutters, the machining time is short, and the tool wear is small. In profile milling, milling can be performed along the envelope trajectory, that is to machine the machining surface in a zigzag manner before milling. The cutting width is fixed, there is not much reciprocating motion, and a large amount of feed can be achieved by rapid acceleration. . Processing along the contour trajectory is a traditional processing method, in which the local contour surface is processed in turn. The pros and cons of the roughing process depend on NC programming based on the surface profile function of the tool, allowing quick and easy milling along contour contour lines. Finishing finishing machining should make the machining stable, have small shape error and good surface quality, and at the same time, the tool wear amount is small. Tool wear and processing costs are the main considerations. In the finishing process, the processing of the angle must take into account the influence of the milling direction on the machining accuracy and the surface quality, the latter being related to the tool bearing and the machine vibration. When milling along the surface infeed, pull milling (upward pass) or Drilling (down pass) will occur. The tool deformation will cause workpiece contour deviation (Figure 12). Drilling and milling of the contour deviation is less than pull milling, and the inverse milling contour deviation is also better than the milling. Therefore, taking into account the critical conditions of the tool quality and the stability and reliability of the machining process, the best strategy for milling along planar contours should be a combination of up- and face-contour milling. In addition, during crush milling, the cutting tool life for enveloping contour milling is longer than that for drilling and milling, and when the milling is reversed, they are almost the same. The main problem of prismatic processing of prismatic surfaces is the fracture of the corners of the mold. The direction of the cutting force should be mainly considered. The following is an example of processing each side of the bottom plate and the vertical plate in Fig. 13. The roughness of the surface is not changed much when machining the edge of the bottom plate, and the quality of the bottom edge is good, but the surface roughness value increases during the up milling and the quality of the bottom edge decreases (Figure 13a). In order to obtain high-quality corners, the sides should be subjected to milling. The direction of the force applied to the two sides of the processing uprights during crush milling differs from one another, i.e. one side is pressed in and the other side is pressed out (Figure 13b). Therefore, the actual feed direction must be changed when cutting the corners. The top surface corner of the processing plate should be changed to avoid cutting edges of the workpiece by changing the position of the cut-in point during the up-cut milling. Tool angles such as the main declination angle have a great influence on this collapse and are generally controlled at about 30°. In order to prevent the occurrence of missing corners during processing, the following measures can also be taken: (1) Soaking in the machining fluid before electrode machining: (2) Using a tool with good wear resistance: (3) Adopting milling milling (downward walking Knife) method of processing: (4) reduce the cutting amount of cutting tool: (5) the cutting pitch of the cutting tool is less than 1/2 of the tool diameter: (6) Deceleration processing when machining both ends: (7) Use the backing plate to enhance the rigidity of the terminal surface when machining: (8) When processing the corners between the upper bending part and the side surface, if fine cracks are easily generated, the upper bending part should be processed. Finish the side. Chip Processing Although graphite is a very stable material, it has no direct adverse effects on human health and is easily cleaned with soap. However, graphite chips may affect the environment in the form of dust, pollutants, etc. In addition, dust is harmful to the human body. Therefore, it is best to use vacuum cleaners and masks when processing. The chip shape is influenced by the cuttings, chips, broken chips, and the cutting media used. Studies have shown that: graphite particles in wet cutting cause the tool to wear due to flow, while the external dry cutting tool life is higher than ordinary dry cutting processing. Strong blow can avoid secondary wear of graphite particles. When the graphite impregnated with the electrolyte is processed, the tool wear drops sharply. In addition, the cleaning of graphite powder must be highly regarded. Equipment for drawing finely-ground graphite powder into a wet device should be provided. During rough processing, a clean cycle and intermittent filtration are required.

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