The graphite electrode is a critical consumable in the electric arc furnace steelmaking process, and it constitutes a significant portion of the overall production cost. During manufacturing, multiple graphite electrodes are joined together using tapered threads. Once the previous section is consumed, the remaining end is reconnected and reused continuously. However, graphite electrodes are fragile, and they are prone to damage, breakage, or tripping during transportation, lifting, and use, which renders them unusable. Given their high cost, it is essential to process and reuse damaged electrodes effectively. Among the key steps in this process is the machining of the tapered internal thread.
Figure 1: Graphite electrode taper thread
As shown in Figure 1, there are two common diameters for 301 electric furnace graphite electrodes: Ø450mm and Ø350mm. The taper ratio (K) is 1:3, and each diameter has different pitch, hole diameter, and depth specifications. Machining these tapers on conventional lathes presents several challenges. Standard lathes lack the capability to machine tapered threads directly. Additionally, the large diameter and long length (up to 1800mm) make it difficult to mount the workpiece in a traditional way. The uneven end surface of the broken electrode further complicates clamping, as the chuck jaws are too short to hold the piece securely. Moreover, the soft nature of the graphite material makes it challenging to support the workpiece without causing friction or deformation.
There is a high demand for lathes with a larger swing radius and bed length. Furthermore, the large screw hole diameter requires specialized twist drills that are not commonly available. To address these issues, a custom mechanism was developed.
The design includes a special mechanism capable of both axial and radial movement. This system allows the lathe to perform taper threading by utilizing the existing turning tools and thread-cutting functions. Instead of rotating the workpiece, it is fixed in place, solving the problem of poor clamping and small center frame holes. If the lathe bed is insufficient in length, an additional bracket can be added at the tailstock end to secure the workpiece.
Special tools and drilling bits were also designed to meet the specific requirements of the process. These modifications enable efficient and accurate machining of the tapered internal threads.
Figure 2: Schematic drawing of the dedicated mechanism
This mechanism consists of several key components, including the headstock, locking screws, spindle, carriages, and clamping blocks. The spindle features a chute that aligns with the workpiece's taper, allowing for smooth rotation and sliding motion. The main spindle drives three carriages and an extension cylinder, enabling synchronized radial movement. When the front locking screw is engaged, the spindle rotates without moving axially, while the rear locking screw allows for combined rotational and axial movement, facilitating the machining of both the face and cylindrical hole.
The trimming carriage adjusts the radial feed control, ensuring precision during the threading operation. This setup significantly improves the efficiency and accuracy of the process.
During installation, the feed block, small carriage, tool holder, and tailstock are removed from the lathe, and the spindle box is mounted on the middle carriage. The calibration mechanism is aligned with the lathe's spindle axis and fixed in place. The middle carriage is designed to remain stable under cutting forces and vibrations, preventing misalignment. Before tightening the locking screws, the front face and cylindrical hole must be loosened to ensure proper alignment.
The lathe is operated by positioning the tool toward the end of the bed. The first step is to machine the Ø450mm face, followed by drilling to the required depth. A 200mm-long tool is then used to machine the inner surface of the hole, and a standard reverse-spindle is employed to shape the taper hole to the desired size. Finally, the threading operation is completed.
In terms of processing conditions, an old C620×1000mm lathe was used, equipped with a specially designed mechanism. All parts of the system were accurate, reliable, and easy to operate. Since November 1998, batch processing of graphite electrodes has been successfully carried out. The finished products met the required geometric dimensions, and after testing in the electric furnace, no tripping issues were observed. The connections were firm and suitable for industrial use.
Ongoing improvements focus on enhancing the two-way carriages and fine-adjusting carriages to increase their size and improve the ease of controlling the feed rate. The addition of lubrication grooves on the dovetail slides, along with enhanced fine-tuning mechanisms, aims to make the system more intuitive and user-friendly.
Powder coating is a type of coating that is applied as a free-flowing, dry powder. The main difference between a conventional liquid paint and a powder coating is that the powder coating does not require a solvent to keep the binder and filler parts in a liquid suspension form. The coating is typically applied electrostatically and then cured under heat to allow it to flow and form a "skin." The powder coating process is used to create a hard finish that is tougher than conventional paint, making it more resistant to scratches, chipping, and fading. It is commonly used on metals, such as aluminum, steel, and iron, as well as on household appliances, automotive parts, and outdoor furniture. Powder coating is an environmentally-friendly option as it produces less waste and emissions than traditional liquid coatings.
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