Pushing and transforming into a lathe machine

The high-temperature cracking tube, commonly used in petrochemical plants, is centrifugally cast and made from Cr25Ni35Nb alloy. It has a length exceeding 4 meters and an outer diameter of 56 mm. The outer surface of the pipe remains unprocessed, while the inner surface requires machining. After processing, the inner diameter must be maintained at Ø43±0.1 mm with a surface roughness of Ra 3.2 μm. For tubes with an inner diameter larger than 50 mm, the push-pull method is typically used due to its simple structure, ease of feeding, and efficient processing. However, when dealing with tubes having an inner diameter less than 50 mm, the boring bar becomes long and slender. In such cases, using the push-pull method can lead to insufficient rigidity of the tool, resulting in vibrations, poor chip removal, rapid tool wear, reduced machining accuracy, and even potential wall penetration due to uneven wall thickness. To address these challenges, the process was modified by converting the original push-pull machine into a lathe. This transformation allowed for more precise control over the spindle speed through a frequency converter, enabling stepless speed adjustment. The machine’s guide rails and overall precision were also upgraded to meet the requirements of internal boring and machining. Figure 1 shows a schematic of the modified lathe setup, including components such as the cooling hose, box, guide bush, workpiece, center frame, mast support, and bracket. Cooling and lubrication play a critical role in maintaining the tool's life. The modified system uses a high-pressure cooling system from the original machine to cool and lubricate the cutting edge via a slanted hole in the tool shank. At the same time, the high-pressure fluid helps flush away chips from the tool head. The reamer and floating reamer are key elements in ensuring machining quality. The reamer cuts the blank to near final dimensions, leaving a small amount of material for the floating reamer to finish, reducing surface roughness and improving dimensional accuracy. Chip removal is particularly challenging during the boring of slender tubes. If chips are not effectively removed, it can significantly reduce tool life, degrade surface quality, and cause blockages that halt the process. Given the small inner diameter of the tube, the available space for chip evacuation is limited. To solve this, the mast was designed with a spiral groove (as shown in Figure 2), which ensures both sufficient rigidity and a large chip evacuation channel. In practice, the combination of the spiral groove and high-pressure oil flushing allows for smooth chip removal. The direction of the spiral groove is opposite to the rotation of the tube, further aiding in chip evacuation. The Cr25Ni35Nb material is known for its high viscosity, making chip evacuation difficult with standard tools. Therefore, a trowel with a chip-breaker was used. Additionally, a guiding sleeve was added at the front end of the boring bar to ensure smooth entry into the tube. The inner diameter of the guiding sleeve matches the required machining size. To enhance the rigidity of the mast and improve machining accuracy, a floating support made of soft bakelite was installed at the rear end of the cutter head. When the bakelite wears out, it must be replaced to maintain precision. After implementing the push-to-pull conversion, the actual machining results were highly satisfactory. The diameter error of the slender tube remained within ±0.1 mm, and the surface roughness reached Ra 3.2 μm, fully meeting the technical specifications.

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