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High-speed processing of graphite electrodes

1. Introduction Graphite is known for its high-temperature strength, low thermal expansion coefficient, good processability, and excellent thermal and electrical conductivity. As a result, graphite electrodes are widely used in metallurgy, electric furnaces, and EDM (Electrical Discharge Machining) applications. In EDM, the development of advanced graphite electrode materials and their processing technologies has significantly expanded the scope of EDM and enhanced its performance. Compared to copper electrodes, graphite offers advantages such as lower electrode consumption, faster machining speed, better mechanical machinability, higher precision, minimal thermal deformation, lighter weight, easier surface treatment, and resistance to high temperatures. Although graphite is easy to machine, it must have sufficient strength to avoid damage during handling and EDM processes. The complexity of electrode shapes—such as thin walls, small radii, and sharp corners—requires higher grain size and strength, which often leads to workpiece disintegration and tool wear during machining. Therefore, preventing workpiece disintegration, improving surface quality, and reducing tool costs have become critical issues in graphite electrode processing. Conventional machining methods like turning, milling, and grinding can handle simple-shaped electrodes, but the increasing demand for complex geometries has made high-speed machining (HSM) a popular choice. HSM offers superior surface quality and precision, making it a key focus in die EDM. Many manufacturers now use high-speed graphite machining centers, such as the Makino SNC64 CNC machine and Röders RFM series, with spindle speeds ranging from 10,000 to 60,000 rpm and feed rates up to 60 m/min. These machines can process wall thicknesses less than 1 mm and fillet radii smaller than 0.2 mm. However, due to the novelty of high-speed graphite machining, material behavior varies, and few domestic studies address issues like workpiece disintegration, tool wear, and optimal machining strategies. This article explores research on high-performance graphite electrode machining from Japan, Germany, and other regions, covering cutting mechanisms, temperature, tool wear, chip formation, and high-speed machining strategies. 2. High-Speed Cutting Mechanism of Graphite Electrodes
Fig. 1: Cutting process when turning sintered carbon and graphite
Fig. 2: Graphite high-speed milling process and tool wear patterns
Fig. 3: Cutting temperature of graphite electrode material
Fig. 4: Tool Wear in Graphite Electrode High-Speed Machining Masuda (1996) observed the sintering process of sintered carbon (below 2000°C) and graphite (above 2500°C) using high-speed photography (Fig. 1). When the tool edge contacts the workpiece, cracks propagate, and parts of the material break off, forming chips. In carbon-phase materials, cracks expand downward, while in graphite, they extend along the cutting direction, causing crater wear on the tool. Kö:nig (1998) found that graphite chip formation resembles brittle materials like ceramics, with fine chips and pits formed at the blade tip. Cracks spread to the free surface, creating fracture pits, which aligns with fracture mechanics principles. The contact between chips and the tool rake face divides into impact and slip zones, leading to different wear patterns (Fig. 2). Factors affecting the cutting process include cutting speed, feed rate, tool geometry, tool material, and wear. The cutting force for graphite is about 10% of that for metals like aluminum or copper, so it is not the main concern. Experimental measurements show that at 500 m/min, the maximum temperature is between 160–300°C, with a linear relationship to cutting speed. Even at this speed, the temperature rarely exceeds 500°C, having limited impact on the process (Fig. 3). 3. High-Speed Machining of Graphite Electrodes – Tool Wear Tool wear is the most critical issue in graphite electrode machining. It affects tool life, processing time, and surface quality, making it essential for optimizing high-speed machining. High-Speed Tool Wear Mechanism The main wear areas are the rake and flank faces. On the rake face, impact abrasive wear occurs due to chip contact, while sliding friction causes wear. Two types of impacts occur: one at an angle, causing surface peeling, and another from micro-cutting, forming grooves up to 150 nm wide. These lead to crater wear on the rake face. Flank wear results from mechanical friction between the tool and the workpiece.
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 As cutting speed increases, the large fracture area expands, but KT decreases, and KB remains approximately constant. The crescent tooth wear cross-section shrinks. With higher cutting speeds, a thick lubricating film forms, reducing surface wear and decreasing tool wear. This explains why high-speed machining is more efficient for graphite. Tool Material Effects Cemented carbide tools exhibit impact wear, with KT decreasing as hardness increases. The wear mechanism involves micro-cutting and surface fatigue failure, leading to abrasive wear of Co and WC phases. Increasing WC particle size and reducing Co phase size can reduce tool wear. Polycrystalline diamond (PCD) tools experience binder phase wear and secondary abrasive wear from diamond crushing. Their surfaces often form a lubricating graphite layer, offering 1–2 times better wear resistance than K10. Diamond film tools typically show macroscopic impact wear, not mechanical abrasion. Surface treatments on coated substrates affect coating efficiency and tool life, with diamond film tools lasting up to 100 times longer than K10.
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 Feed rate increases cause KB, KL, and KT to rise. Larger cutting widths increase chip thickness, leading to higher tool wear. As shown in Fig. 6, when the milling width exceeds the cutter radius, cutting conditions change. Tool Angle Increasing the rake angle reduces KT but has little effect on KB. A larger relief angle improves tool sharpness and reduces flank wear. Main angles influence cutting force direction and actual cutting area, with increased angles reducing tool wear and extending life (Fig. 7). Graphite Electrode Materials Smaller grain sizes improve tool life, which is proportional to bending strength and Shore hardness. Degree of graphitization, impregnation composition, and filler particle size also affect wear. Tool Structure Ball-end and flat-end cutters are commonly used. Ball-end mills experience top wear due to reduced cutting speed, while flat-end mills face fluctuating machining allowances, leading to tool damage. Under similar conditions, flat-end mills travel farther than ball-end mills. Milling Direction Milling direction significantly affects tool life. Up-milling provides longer tool life compared to down-milling due to differences in chip breaking and impact (Fig. 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
Fig. 9: Effect of milling direction on tool wear Cost Analysis 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 Traditional EDM electrodes move vertically, limiting three-axis milling. Modern NC-controlled EDM allows multi-axis and vertical feed. Complex electrode geometries can be roughly categorized into two types: 1) Free-surface forming electrodes: Use ball-end cutters for long, time-consuming paths. 2) Prismatic surface electrodes: Use flat-end cutters for simpler, sharper edges. Economics of high-speed machining depend on cutting and wear mechanisms. Optimizing cutting parameters, machining strategies, and tool geometry is essential. Optimization of Processing Conditions At higher cutting speeds, tool wear decreases, and machining costs drop. At speeds over 900 m/min, the cost per unit length is similar for cemented carbide, diamond-coated, and PCD tools. Coated diamond tools are recommended for low-speed operations. For finishing, diamond-coated tools offer the best cost per blade, while PCD tools provide better heavy-edge grinding. Small-diameter tools are most economical when using diamond-coated options. Tool Geometry Parameters Increasing rake and relief angles enhances chip space. For roughing, a 6° rake angle is ideal, with a relief angle under 15°. For finishing, a 6–10° rake angle is preferred. Large main deflection angles increase surface roughness, so they should be less than 30° for finishing. Feed per tooth relates to tool wear, with values depending on machine characteristics. Dynamic vibrations and arbor load also affect machining performance. Cutting data selection principles: 1) Determine the number of teeth based on machine and chuck conditions to prevent vibration. 2) Calculate the maximum allowable feed per tooth based on tool strength, depth, and width. 3) Set the maximum spindle speed according to feed and acceleration capabilities. 4) Select a stable spindle speed adapted to the feed per tooth. Recommended parameters for PCD tools: Roughing: Vc=200–400 m/min, fz=0.02–0.04 mm/tooth, cutting depth <1.5 mm. Finishing: Vc=25–100 m/min, fz=0.02–0.1 mm/tooth, cutting depth <0.5 mm. Graphite Electrode High-Speed Processing Strategy High-speed roughing and finishing require different strategies. For roughing, use high-feed (cutting and pass feed) with small-diameter tools to minimize tool wear and maximize material removal. Finishing aims to achieve the highest quality in the shortest time, balancing surface quality and tool wear. Increase processing speed, shorten time, and minimize instability caused by changing cutting amounts to maximize tool life. For free-surface forming electrodes, optimize the machining path considering local allowance.
Workpiece material: EK85, Grain 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
Fig. 12: Pulling and Drilling and Finishing Strategies
Figure 13: Processing Example - Roughing In general, graphite electrodes are machined from monolithic material. The machining allowance is straightforward, with the goal being to remove the most material in the shortest time. Roughing can be done via copy milling or contour milling (Fig. 11). Profile milling uses ball-end cutters, with variable cutting depth and width, leading to faster tool wear and longer machining times. Contour milling with flat-end cutters reduces tool wear and shortens machining time. During profiling, milling can follow an envelope trajectory, allowing zigzag movement with fixed cutting width and rapid acceleration. Traditional contour milling follows the contour line, processing local surfaces sequentially. The pros and cons of roughing depend on NC programming based on the tool’s surface profile function, enabling quick and efficient contour milling. Finishing requires stability, minimal shape error, and good surface quality, with low tool wear. Tool wear and cost are key considerations. During finishing, the milling direction affects accuracy and surface quality, influenced by tool bearing and machine vibration. Up-milling or down-milling can cause tool deformation, leading to workpiece contour deviation (Fig. 12). Down-milling shows less deviation than up-milling, and reverse milling performs better. A combination of up- and face-contour milling is optimal for planar contours, considering tool quality and process stability. During crush milling, enveloping contour milling extends tool life compared to drilling and milling, with similar performance when reversed. For prismatic surface processing, corner fracture is a major issue. The cutting force direction must be considered. For example, when machining the bottom plate and vertical plate (Fig. 13), up-milling increases surface roughness and degrades edge quality. To achieve high-quality corners, side milling is recommended. The force applied to the two sides differs, requiring a change in feed direction when cutting corners. The top surface corner should be processed by adjusting the cut-in point during up-milling. Main deflection angles around 30° help prevent collapse. To avoid missing corners during processing, consider the following measures: (1) Soak the electrode in machining fluid before processing. (2) Use a wear-resistant tool. (3) Apply a down-milling method. (4) Reduce cutting amount. (5) Ensure the tool pitch is less than half the diameter. (6) Decelerate when machining both ends. (7) Use a backing plate to enhance rigidity. (8) Process the upper part first if fine cracks occur during corner machining. Chip Processing Although graphite is stable and harmless, its dust can pollute the environment and harm health. Vacuum cleaners and masks are recommended during processing. Chip shape depends on cutting media, chips, and broken chips. Wet cutting increases tool wear, while dry cutting improves tool life. Strong blow prevents secondary wear. Electrolyte-impregnated graphite reduces tool wear. Cleaning graphite powder is crucial, requiring wet devices for fine powder extraction. During roughing, clean cycles and intermittent filtration are necessary.

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