Studying the effect of the grinding parameters of CBN wheel on the bearing internal surface roughness by the orthogonal experiments can provide the reference for the adjustment of grinding parameters in the production process, ensuring the production efficiency and bearing quality.
Bearing processing accuracy is one of the key factors affecting product quality. The grinding of bearing internal, external and roller directly determines the bearing accuracy. The grinding wheel is the key tool for bearing grinding, which determines the bearing accuracy and surface quality, as well as the energy saving, environmentally friendly and efficiency of the grinding process.
Compared with conventional grinding wheels( silicon carbide and corundum), the machining efficiency of CBN wheel can be nearly doubled, the service life is increased by about 100 times, the dressing interval is extended by 200 times, the waste residue is reduced by about 90%, and the labor intensity of workers is greatly reduced.
The application technology of the CBN grinding wheel affects the effect of the workpiece to some extent. The optimization of grinding parameters is one of the important contents of the application technology, which can affect the machining precision of the bearing and machining efficiency.
The main technical analysis indicators of internal grinding of bearings include surface roughness, taper, ellipticity, wall thickness difference, chatter mark, and burns. It was found that the surface roughness is the most difficult to overcome compared with other parameters such as no burn, no chatter mark, high production efficiency, and good durability. Moreover, the surface roughness is also one of the important factors affecting bearing performance.
There are many factors affecting the surface roughness of grinding, such as cooling technology, grinding wheel performance, machine tool parameters, and etc. The grinding parameters of the machine tool have a great effect on the surface roughness and the adjustment method is simple and quick.
The effect of the single grinding parameter on the surface roughness has a law, and cannot simply combine the grinding parameters according to this law. Taking into account the efficiency and effect, in order to make the surface roughness meet the requirements, how to adjust various parameters? How much adjustment is right? This study shows the effect law of the various grinding parameters on the surface roughness of internal bearing by orthogonal experiment in the grinding process of CBN wheel. It provides references for the adjustment of the grinding parameters under different working conditions in the production process.
1 Experimental conditions:
Low temperature and high strength microcrystalline vitrified bond are used for vitrified bond CBN grinding wheel. CBN abrasive grain size is 100/120 and the concentration is 150%. The microscopic morphology of vitrified bond CBN wheel is shown in Fig. 1. It can be seen from Fig.1., the interface of the bond with CBN abrasive, the porous state of the grinding wheel, abrasive falling off, the abrasion of the abrasive and the crushed bond bridge.
The workpiece is bearing with hardness HRC63. The orthogonal experiment design is 4 factors and 3 levels of L9, the total is 9 groups of experiments. The 4 factors include finishing time, oscillating frequency, single feed, and grinding wheel linear velocity. The grinding experimental equipment uses CNC bearing inner diameter grinding machine of 3MZK208A. The coolant is water based emulsion. German Marsurf PSI instrument is used for the surface roughness test. Observe the structure of the grinding wheel with FEI QUANT 200 scanning electron microscope.
2 Results and analysis
2.1 The effect of individual grinding parameters on surface roughness
There are many factors that affect the surface roughness, and the most obvious is the abrasive grain size. Fig.2 shows the relationship that the effect of the abrasive grain size on the bearing internal surface roughness. From Fig.2, the surface roughness increases with the single feed increase. The larger the single feed, the smaller change of the surface roughness varies with the abrasive grain size.
This is because the amount of plow per unit time unit abrasive on the bearing surface is different. In the actual applications, rough-grained CBN abrasive is often chosen under the conditions meeting the surface roughness of the workpiece, which is good for improving the grinding efficiency and wear ratio.
It is unlike the effect of grain size on surface roughness. The grinding parameters have a cross effect on grinding efficiency and grinding effect. For example, the effects of finishing time, oscillating frequency, single feed and linear velocity on surface roughness are more complicated.
In the grinding process, when the parameters are at extreme values, the best results are usually achieved. For example, the longer the finishing time, the smaller the oscillating frequency, the smaller the single feed, the larger the linear velocity, and the smaller the bearing internal surface roughness is. As shown in Fig.3, the principle of the effect of these grinding parameters on the surface roughness is the same, that is, the smaller the depth of the plow on the bearing surface of CBN wheel in the unit time and unit abrasive, the smaller the surface roughness is.
But in the actual production process, should consider the limitations of efficiency and machine tool parameters, and it is impossible to set each parameter to the limit value. The design applied orthogonal experiment method to optimize the relationship of grinding parameters to achieve a better comprehensive grinding effect.
2.2 Orthogonal experimental analysis
The level of each factor in the orthogonal experiment has referred to the results in Fig.3. The effect of each factor on the surface roughness has a change process. Choose the inflection point of the figure as the horizontal area for each factor study. The design results are shown in Table 1.
Table 1 Design of factor level
|Level||A Finishing time s||B Oscillating frequency min-1||C Single feed μm||D Grinding wheel linear velocity m/s|
Orthogonal experiment with 4 factors 3 levels, the total is 9 group grinding experiments. The results of the orthogonal experiment analysis are shown in Table 2.
Table 2 Result analysis of orthogonal experiments
The average range of repeatability experiments shown in Table 2 is 0.042. The experimental response table is calculated according to Table 2, as shown in Table 3.
Table 3 Response of orthogonal experiments
The response values in Table 3 correspond to the contribution value of different levels of surface roughness. The difference in response results corresponds to the difference between the 3 levels of each factor. Compared with the average range, it can be concluded that:
- There are significant differences between the 3 optional conditions of A and D, which indicates that the finishing time and CBN wheel linear velocity have a great effect on the surface roughness of the workpiece. It is good for getting good surface roughness for choosing A3 and D3. It can be seen that A2 and D2 are not significantly affected by the increase in surface roughness compared with A3 and D3. If the processing efficiency is a key factor in the production process, choose A2. If the machine parameters cannot use the higher speed and cannot reach the linear velocity of 50m/s, choose D2.
- The 2 factors B and C have little effect on the surface roughness of the workpiece. Efficiency can be considered first when choosing parameters in the production process.
- The best experimental combination is A3, B3, C2, and D3. It is not based on the analysis results of Fig.3 that A3, B1, C1, and D3 can be used to achieve the best surface roughness.
- It can be seen that various factors on the surface roughness by the orthogonal experiment analysis. And provide the reference basis when choosing parameters.
The best combination results predicted:
0.9119-(0.9119-0.8764)-(0.9119-0.8994)-(0.9119-0.8946)-(0.9119-0.8773)=0.812μm, of which 0.9119 is the average of 9 groups of experiments, the actual experimental results are fluctuating between 0.7754-0.8375. The theoretical analysis is similar to the experimental results. It can be seen that the effect of 4 factors on the bearing surface roughness does not change linearly. When it needs to consider the limitations of the effect and facilities, the situation becomes more complicated. The orthogonal experiment is a scientific, simple research method which can provide valuable references for the production process.
3.1 The finishing time and the linear velocity have a significant effect on the bearing internal surface roughness. The effect of the oscillating frequency and the feed rate on the surface roughness is relatively small.
3.2 The orthogonal experiment can quantify the effect of various parameters on the surface roughness to a certain extent. Optimize the grinding parameters by this effect rule in the actual production process.
3.3 When the orthogonal experimental theory analyzing the parameter combination, the roughness is 0.8245μm, and the actual grinding result is 0.7754-0.8375μm, and they are basically consistent. It is proved that the orthogonal experiment can provide the reference data for bearing internal ground by CBN wheel.