High-angle bevel laser cutting process
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Laser cutting is typically performed on flat or slightly inclined surfaces. However, there has been no prior study on cutting at large angles. Figure 1 shows a schematic of a cavity with a slotted hole in a steam turbine. The material thickness is 6mm, and the round table contains approximately 60 holes. The diameter of the circular table exceeds 3000mm. After processing, the pitch error for each hole must not exceed ±0.05mm, and the total cumulative pitch error should be within 0.5mm. The tilt angles are a=31°, b=0.5°, and q=41.37°, with an angular tolerance of at least ±0.005°. Additionally, the cavity surface must remain smooth. Previously, wire cutting was used, but it resulted in low efficiency, high workload, and an unguaranteed production schedule. Therefore, this project aimed to replace conventional wire cutting with laser cutting to meet batch production requirements.
To achieve a bevel cut with a total inclination of 31.5°, a high-precision tooling system, as shown in Figure 2, was developed. The pitch dimension tolerance of the tooling was controlled within ±0.01mm, and the shroud was machined after sub-section processing to address the challenge of the large perimeter, which exceeds 3000mm.
Figure 2: Tooling Schematic
Next, the laser processing parameters were optimized. At a power of 1260W, a cutting speed of 1.4m/min, using ordinary oxygen (99.8% purity) as the assist gas at a pressure of 0.08MPa, a large-angle bevel cut could be achieved. The kerf width was approximately 0.18mm.
The distance between the laser head nozzle and the workpiece, as well as the amount of coke input, significantly affected the cut quality. If the nozzle was too far from the workpiece, the cut would widen and the surface roughness would increase. Conversely, if the nozzle was too close, it might collide with the workpiece. A distance of 0.5–0.8mm was found to produce the best results. The coke input, or the position of the laser focal point within the workpiece, was set to about 1mm, ensuring a smooth cut without any chipping at the entrance, leading to excellent surface finish.
Since the process involves three-dimensional closed-surface machining, careful attention was given to both the accuracy of the surface model and the linear interpolation step length in the program. Experiments showed that setting the linear interpolation step to 0.02mm yielded optimal results.
Mass production of over 10,000 holes confirmed that laser cutting on large-angle bevels is feasible, provided that the laser parameters, nozzle-to-workpiece distance, coke input, and program interpolation are properly controlled. This approach enabled precise machining of a 31.5° bevel on a 6mm thick plate, achieving a surface roughness of Ra 3.2μm. The time required to cut a single hole was reduced from 90 minutes using wire cutting to just 34 seconds with laser cutting, improving efficiency by more than 100 times.