Pushing and transforming into a lathe machine

The high-temperature cracking tube used in petrochemical plants is centrifugally cast and made of Cr25Ni35Nb alloy, with a length exceeding 4 meters and an outer diameter of 56 mm. It is crucial that the outer surface remains un-machined, while the inner surface must be precision-machined to achieve an inner diameter of Ø43±0.1 mm and 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 and ease of operation. However, when dealing with slender tubes with an inner diameter less than 50 mm, the boring bar becomes long and flexible, making it difficult to maintain rigidity during the process. This can lead to vibrations, poor chip removal, tool breakage, reduced tool life, and inconsistent wall thickness, which may even result in wall penetration. To address these challenges, the machining method was changed from push-pull to a ramming process. The original push-pull machine was modified into a lathe, allowing for better control over spindle speed using a frequency converter, enabling stepless speed adjustment. The guide rails and overall precision of the machine meet the requirements for accurate boring and machining. Figure 1 shows a schematic diagram of the modified lathe machine, highlighting key components such as the cooling hose, box, guide bush, workpiece, center frame, mast support, and bracket. Cooling and lubrication of the boring tool is essential for extending the tool's life. In this setup, a high-pressure cooling system was utilized, directing coolant through a slanted hole in the tool shank to cool and lubricate the cutting edge. Additionally, high-pressure liquid helps flush away chips from the tool head, ensuring smooth operation. The reamer and floating reamer in the pull rake play a critical role in achieving the required machining quality. The reamer cuts the workpiece to near final dimensions, leaving a small amount of material for the floating reamer to finish, improving surface finish and dimensional accuracy. Chip removal is particularly challenging in the machining of slender tubes due to limited space. If chips are not removed efficiently, it can lead to reduced tool life, poor surface quality, and blockages in the chip evacuation channel. To solve this, the mast was designed with a spiral groove (as shown in Figure 2), which allows for efficient chip evacuation by combining the spiral motion of the groove with high-pressure oil flushing. The direction of the spiral groove is opposite to the rotation of the tube, further enhancing chip removal efficiency. The material Cr25Ni35Nb has high viscosity, making chip evacuation even more difficult. Therefore, a trowel with a chip-breaker is necessary to manage the chips effectively. A guiding sleeve is also added at the front end of the feeding knife to ensure the boring bar is smoothly introduced into the tube. The inner diameter of the guiding sleeve matches the required machining size. To increase the rigidity of the mast and ensure machining accuracy, a floating support made of soft bakelite is installed at the rear end of the cutter head. When the bakelite wears out, it must be replaced to maintain precision. After switching from the push-pull method to the ramming process, the actual machining results were very satisfactory. The diameter error of the slender tubes was within ±0.1 mm, and the surface roughness reached Ra 3.2 μm, fully meeting the technical specifications. This improvement significantly enhances both the efficiency and quality of the machining process.

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