Graphite electrodes are essential consumables in the electric arc furnace steelmaking process, and they constitute a significant portion of production costs. During manufacturing, multiple graphite electrodes are connected using tapered threads. Once the previous section is consumed, the remaining end is reconnected for continued use. However, graphite electrodes are fragile and can suffer damage, breakage, or tripping during transportation, lifting, and operation, rendering them unusable. Given their high cost, it is crucial to process and reuse damaged electrodes effectively. Among the key steps in electrode processing is the machining of tapered internal threads.
Figure 1 shows the tapered thread of a graphite electrode. There are two standard diameters for the 301 electric furnace graphite electrode: Ø450mm and Ø350mm. The taper ratio (K) is 1:3, and each diameter has different pitch, hole diameter, and depth. Machining these tapers on conventional lathes presents several challenges. Standard lathes lack the capability to machine tapered threads directly. Additionally, the large workpiece diameter and length (up to 1800mm) make it difficult to mount using traditional methods. The uneven end surface of broken electrodes and short chuck jaws further complicate clamping. The soft material of the electrode also causes issues with center frame support and friction. Moreover, the large screw hole diameter means there is no suitable twist drill available.
To address these challenges, a special mechanism was designed. This system allows for axial movement independently and simultaneously with the large carriage, enabling radial movement to achieve taper threading. It utilizes the existing turning tools and threading functions of the lathe. By fixing the workpiece (not rotating or moving axially), the mechanism solves the issues of clamping and small center holes. If the bed length is insufficient, an additional bracket is added at the tailstock end to secure the workpiece. Special tools and drills were also developed for this purpose.
Figure 2 illustrates the dedicated mechanism used for machining the tapered internal threads on an ordinary lathe. The spindle 3 features a chute aligned with the workpiece's angle and a sliding keyway to transmit rotational power. The main spindle 7 drives the three carriages, extension cylinder 12, and clamp block 13. When the front locking screw 2 engages, the spindle 3 rotates without axial movement. Meanwhile, the spindle 7 rotates and follows, allowing the large lathe to move toward the bed’s end. As the spindle 3 slides within its hole, the chute forces the two-way carriage 9 to move radially. This motion is transferred to the fine-adjusting carriage 11, which controls the extension tube 12, clamp block 13, and cutting tool to produce the taper thread.
When the front locking screw 2 is loosened and the rear locking screw 4 is engaged, the spindle 3 rotates and moves axially in sync with the spindle 7, enabling face and cylindrical hole machining. The trimming carriage 11 adjusts the radial feed precisely.
The mechanism was installed by removing the feed block, small carriage, tool holder, and tailstock from the lathe. The spindle box was mounted on the middle carriage, ensuring the rotation axis of the mechanism aligns with the lathe spindle. The middle carriage was secured to prevent movement due to cutting forces or vibration. Before tightening the locking screws, the front face and cylindrical hole must be loosened. After tightening, the cone hole and thread processing occur in reverse order. The lathe was operated by positioning the tool toward the bed’s end, reversing the tool, and first machining the Ø450mm face. After drilling to the required depth, the inner hole surface was machined using a 200mm-long tool. A common reverse-spindle was used to shape the taper hole to the desired size, followed by thread processing.
The electrode processing conditions involved using an old C620×1000mm lathe with limited accuracy. A special mechanism was installed on the large carriage, and the described method was applied. During the process, all components of the mechanism functioned accurately, reliably, and were easy to operate. Since November 1998, graphite electrodes have been processed in batches. Post-inspection confirmed that the geometric dimensions met the drawing requirements. Trials in electric furnace smelting showed no tripping issues, and the connections were strong enough to meet production needs. Ongoing improvements include enhancing the two-way carriages and fine-adjusting carriages to increase the scale, making feed rate control more intuitive and convenient. The two-way dovetail drag slots have been improved to enhance lubrication, and the fine-tuning screw and handwheel have been further refined for better flexibility.
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