3.1 Experiment platform design and development
The three-dimensional structure of the experiment platform was designed and the corresponding experimental system was built to conduct lapping experiment. The frame of the four cup lapping machine mainly includes a table and a column, and the table is made of marble; the linear feed adopts a manual module; the four cups rotation adopts an integrated motor. Figure 6 shows the physical diagrams of four cup lapping machine.
Four-axis motion is the key technology to four cup lapping. The rotation of the shaft is driven by an integrated motor, which is controlled by GTS400 motion control card. The four-axis motion combination and periodic adjustment are controlled by the host computer software based on LabVIEW. Figure 7 shows the user interaction interface of the four-axis motion control system.
3.2 Lapping process experiment
The material used in the rotor of the ESG is metallic beryllium. In order to obtain the lapping process of the beryllium rotor, and study the four cup lapping technology, the characteristics of the beryllium material and the relationship between process parameters and lapping quality must be studied. Therefore, this paper conducted an experimental study on the influence of the surface quality of beryllium rotors under different cup materials, granularity and the other parameters.
3.2.1 Removal efficiency of different materials
In order to study the removal efficiency of different materials for beryllium materials, this paper designs and develops cups of different specifications (the material is steel). The three materials of diamond, SiC and Al2O3 are sprayed and vapor deposited on the surface of the cups to obtain cups with different surface materials, as shown in Fig. 8.
By installing the above three kinds of cups on the experiment platform, the removal efficiency of different materials on beryllium materials was studied under the condition of the same other lapping process parameters (cup diameter is φ24mm, lapping speed 120r/min, lapping pressure 0.1 MPa). Figure 9 shows the result of the removal efficiency of different materials to beryllium.
3.2.2 Influence of granularity
It can be seen from Fig. 9 that the removal efficiency of diamond is significantly better than Al2O3 and SiC. Therefore, this paper further uses vapor deposition on the steel cups to prepare diamond powder films with different granularity (W5ཞW40). Cups with different sizes of diamond particles were developed, and lapping experiments were carried out for the beryllium rotor. Figure 10 shows the micro-morphology and roughness of the workpiece surface after lapping for 1 hour with diamond cups of different granularity.
It can be seen from Fig. 10 that the smaller the particle size, the better the grinding surface quality of the beryllium rotor. When the diamond granularity is W7, after 1 h of precision lapping, a beryllium rotor with a surface roughness of 80 nm is obtained, as shown in Fig. 11.
However, it must be emphasized that the smaller the granularity is, the lower the removal efficiency is. Therefore, the roughing, semi-finishing, and finishing process routes are adopted in the grinding process. The diamond cups with granularity of W28 is used for rough lapping, Semi-finishing uses cups with granularity of W14, and finishing uses a cups with granularity of W7 to ensure the grinding efficiency on the basis of obtaining a beryllium rotor with excellent surface quality.
3.2.3 Influence of lapping speed and pressure
Lapping speed and pressure are important process parameters in the lapping action. In order to obtain the influence of the lapping speed and pressure on the surface quality of the beryllium rotor, especially the surface roughness, cups with granularity of W7 (diameter φ24mm) is used to evaluate the surface roughness of the beryllium rotor under the same lapping time (1 h). Figure 12 shows the surface roughness of a solid beryllium rotor at different lapping speeds (the lapping pressure is 5N) and lapping pressures (the lapping speed is 180r/min).
It can be seen from Fig. 14 that with the increase of the lapping speed and pressure, the surface roughness of the beryllium rotor is lower under the same process conditions. The fundamental reason is that when the lapping speed and pressure are higher, the removal is greater which makes the surface quality of the sphere better.