Objective The dynamic pressure air bearing has the characteristics of complex structure and demands of high manufacturing accuracy. To improve the bearing capacity, it is necessary to machine a series of equiangular spiral grooves with depths of (4±1) μm and a helix angle of 22° around the spherical surface on the hemispherical bearing. At present, ion-etching methods are employed to realize such processing but must through high-precision mask, as well as with high environmental requirements, complex processes, and low efficiency. Processing of high-precision mask has its own challenges. Laser processing technology is an effective supply of maskless processing. The cooperation of a laser beam and a five-axis linkage computer numerical control machine tool can solve a three-dimensional space processing problem but with complex processes and high cost. For a normal laser processing system using a common two-dimensional (2D) galvanometer, the key point is to seek a breakthrough in the processing track to achieve high-precision machining of curved surfaces with great curvature variation, such as the grooving of the dynamic pressure air bearing. In this study, we present a processing method of an equiangular spiral groove on a spherical surface by picosecond laser using a 2D galvanometer. The basic strategy and findings of this study may be helpful for high-precision grooving of parts with curved surfaces by just using a common 2D galvanometer ultrafast laser system.
Methods A 1064-nm wavelength picosecond (ps) laser processing system equipped with a 2D scanning galvanometer was employed to process equiangular spiral grooves on the curve surface of TN85 cermet spherical parts. The etching size and roughness were measured by a laser confocal microscope and a profiler. By the experiments of ps laser etching of the flat surface of the cermet parts, the process parameters that can meet the depth requirements with (4±1) μm microgroove were determined. Then, based on the process parameters, the TN85 cermet spherical surface was etched with equiangular spiral grooves via the laser projection milling method. Uneven etching depths and roughness on spherical surfaces by the method were found and analyzed. Based on this, the spiral groove was divided by Bézier surface segmentation. Next, a processing method for compensating spot overlap rate is proposed. The curved surface spiral groove laser high finishing is realized. Finally, the single spiral groove processing technology is applied to the spiral groove array processing.
Results and Discussions The process parameters of 1064-nm picosecond laser ablation depth with (4±1) μm for TN85 cermet flat parts were explored. The relationship between laser power and scanning speed and etching depth and width was studied. The laser etching width was similar to the laser spot diameter and remained basically unchanged with power changes. The laser etching depth increased with power, showing an approximately linear relationship (Fig. 3). The processing results showed that the picosecond laser could obtain regular grooves on the surface of the flat TN85 cermet. Groove etching experiments showed that the picosecond laser could obtain high-precision grooves on the surface of the planar TN85 cermet (Fig. 4). The projection milling method was used to process spherical spiral grooves. However, owing to the different overlap rates of the adjacent paths, the roughness of the grooves in different areas of the spherical surface was quite different (Fig. 6). Extracting the information in spiral grooves u and v, using Bézier's surface segmentation method and the binary space division algorithm of the surface normal vector direction cone to divide the spiral grooves, a surface spiral groove processing method with mutual compensation of the spot overlap ratio was proposed (Fig. 11). The laser processing of the equiangular spiral groove using the 2D galvanometer curved surface was realized. The well-controlled groove depth at (4±1) μm (Fig. 12) with uniform roughness (Fig. 13) of the processed spiral groove was obtained by the method. The laser process method of a single spiral groove was successfully applied to the 12 series spiral groove array processing on the spherical surface (Fig. 14).
Conclusions Based on the Bézier surface and binary space partition segmentation methods, an equiangular spiral groove is split. Then, the laser incident angle meets the requirements of the maximum allowable change of processing precision. The curved equiangular spiral groove processing via picosecond laser using a two-dimensional galvanometer is achieved. The problem of the vector direction of the incident laser beam constantly changing in curved surface processing by picosecond using the two-dimensional galvanometer was solved. Based on this method, 12 equiangular spiral grooves on the TN85 spherical cermet surface are processed via laser. The depth of the processed spiral grooves is well controlled at (4±1) μm; the surface roughness Ra is ~65 nm and is uniform everywhere. The advantage of this processing method is that the design requirements of complex trajectories for curved surface machining are reduced, and the height of the laser focal plane and normal vector direction do not need to be adjusted in real time.
