Tool wear is the most critical issue in the machining of graphite electrodes. The amount of wear not only affects the cost of tool consumption, machining time, and machining quality, but also impacts the surface quality of the electrode workpiece material. The primary areas of tool wear in the machining of graphite materials are the rake and the relief surface. On the rake, impact abrasive wear occurs due to the impact contact between the tool and the fractured chip area, and sliding friction wear is generated by the chips sliding along the tool surface.

The main factors influencing tool wear include:
  1. Tool Material Tool material is the fundamental factor determining the cutting performance of the tool, significantly impacting processing efficiency, quality, cost, and tool durability. Harder tool materials have better wear resistance, but higher hardness also means lower impact toughness and greater brittleness. Balancing hardness and toughness is a key challenge for tool materials. For graphite tools, standard TiAlN coatings can be used with materials that have relatively better toughness, such as fine-grained carbide with a cobalt content of 8-10%. For diamond-coated graphite tools, materials with relatively higher hardness, such as medium-grained carbide with a cobalt content of 6-8%, are recommended.

  2. Tool Geometry Selecting the appropriate geometry for graphite-specific tools helps reduce tool vibration, which in turn reduces the risk of chipping the graphite workpiece.

    2.1 Rake Angle Using a negative rake angle when machining graphite provides good edge strength and resistance to impact and friction. As the absolute value of the negative rake angle decreases, the wear area on the flank face shows a general decreasing trend. In contrast, with a positive rake angle, as the angle increases, the edge strength weakens, leading to increased flank wear. Negative rake angles increase cutting resistance and vibration, while large positive rake angles cause severe tool wear and high cutting vibration.

    2.2 Relief Angle Increasing the relief angle reduces edge strength and gradually increases the wear area on the flank face. Excessively large relief angles also enhance cutting vibration.

    2.3 Helix Angle A smaller helix angle results in a longer cutting edge engaging with the graphite workpiece simultaneously, leading to higher cutting resistance and greater impact forces on the tool, thus increasing wear, milling forces, and vibration. Larger helix angles direct milling forces away from the workpiece surface but increase impact forces due to graphite material chipping, also resulting in increased wear, milling forces, and vibration. Therefore, the combined effects of rake, relief, and helix angles on tool wear, milling forces, and vibration must be carefully considered.

  3. Tool Coating Diamond-coated tools offer high hardness, excellent wear resistance, and low friction. Currently, diamond coatings are the best choice for graphite machining tools, demonstrating superior performance. Diamond-coated carbide tools combine the hardness of natural diamond with the strength and fracture toughness of carbide. However, the technology for diamond coatings is still developing in China, and high costs limit its widespread use. Optimizing tool angles, material selection, and standard coating structures can still make significant contributions to graphite machining.

    The geometry of diamond-coated tools differs significantly from that of ordinary coated tools. Due to the special requirements of graphite machining, diamond-coated tools can have larger geometric angles and chip grooves without compromising edge wear resistance. While TiAlN coatings provide better wear resistance than uncoated tools, their geometry should be slightly reduced when machining graphite to enhance wear resistance. Many global companies are investing heavily in developing diamond coating technologies, but currently, mature and cost-effective technologies are mainly found in European companies.

  4. Tool Edge Reinforcement Edge dulling of tools is a critical issue that is often overlooked. After grinding carbide tools with diamond wheels, microscopic defects such as micro-chipping and serrations can appear on the cutting edge. High-speed cutting of graphite demands high performance and stability from tools, especially diamond-coated ones, which require edge dulling treatment before coating to ensure coating adhesion and tool life. The purpose of edge dulling is to smooth out these microscopic defects, enhancing durability and wear resistance.

  5. Cutting Conditions Cutting conditions have a significant impact on tool life.

    5.1 Cutting Methods (Climb Milling and Conventional Milling) Climb milling generates less vibration than conventional milling. In climb milling, the cutting thickness decreases from maximum to zero, minimizing the risk of tool rebound due to uncut chips. Conventional milling increases cutting thickness from zero to maximum, causing the tool to scrape the workpiece surface initially, which can lead to tool rebound or chatter if encountering hard spots or residual chips in the graphite material.

    5.2 Blowing (or Dust Extraction) and Immersion in EDM Fluid Timely cleaning of graphite dust from the workpiece surface helps reduce secondary tool wear, extend tool life, and minimize the impact of graphite dust on the machine’s screws and guides.

    5.3 Other Considerations Selecting an appropriate high spindle speed and corresponding high feed rate is crucial.

In summary, tool material, geometry, coating, edge reinforcement, and cutting conditions all play vital roles in tool life and are interdependent. A good graphite tool should have smooth chip evacuation grooves, long life, the ability to perform deep engraving, and cost-saving capabilities in machining.