# Experimental Study on ELID Grinding of Silicon Nitride Ceramics for G5 Class Bearing Balls
## 4 Comparative Analysis of Surface Roughness
### Machining Conditions  
Determining the optimal grinding conditions based on the workpiece material and grinding wheel type requires a substantial investment of time, experience, and effort. The impact of grinding conditions on surface roughness was compared and analyzed. Figure 5 presents a graph summarizing the experimental results for each abrasive grit size. As the DOC increases, the surface roughness (Ra) also increases. The analysis reveals a direct correlation between the DOC and surface roughness across all three grit sizes, As the DOC increased, the surface roughness further increased. In the case of the #325 grinding wheel, the surface roughness (Ra and Rmax) increased proportionally with the increase in the DOC. The range of surface roughness was notably more prominent for the diamond abrasive than the cBN abrasive. For the #1200 grinding wheel, no significant difference in surface roughness (Ra and Rmax) was observed between a DOC of 1 \(\upmu\)m and 2 \(\upmu\)m. However, a substantial increase in surface roughness was observed at a DOC of 4 \(\upmu\)m. In this instance, the difference in surface roughness based on the type of abrasive was not prominently evident; however, Rmax was slightly higher for the diamond grinding wheel. For the #2000 grinding wheel, an increasing trend in surface roughness was observed with the increase in the DOC. The surface roughness based on the type of abrasive demonstrated significant superiority for the ground surface processed with the diamond grinding wheel compared to the cBN grinding wheel. This correlation can be attributed to a higher proportion of surfaces affected by ductile mode machining in the ground surface processed with the diamond grinding wheel, as observed through the analysis of ground surface images in Tables 3 and 4. Meanwhile, the range of surface roughness represents the difference between the maximum and minimum values of the measured data, which allows for the evaluation of ground surface variability and distribution. Figures 6-11 present graphs depicting the average values and ranges of surface roughness, Ra, based on the DOC. Figure 6 illustrates the measurement results for #325 diamond and cBN grinding wheels. For the #325 grinding wheel, the diamond abrasive showed significantly larger differences between the maximum and minimum surface roughness values compared to the cBN abrasive. In the case of the #1200 grinding wheel, no significant difference was observed in the average value and range of surface roughness. As for the #2000 grinding wheel, although the range of surface roughness showed no difference, the average value for the diamond abrasive ground surface was remarkably low. These outcomes can be attributed to the diamond abrasive's effective involvement in the complete cutting of silicon nitride particles, clearly revealing the traces of abrasive passing through the peaks and valleys. In contrast, the cBN abrasive generated a ground surface with a rubbing action on the silicon nitride surface, resulting in a lower proportion of involvement in ductile mode machining. Thus, significant improvement in the ground surface beyond the #1200 grit size using cBN abrasive is challenging to expect. The ground surface of the #325 cBN grinding wheel exhibited superior surface roughness compared to the diamond ground surface. This phenomenon is believed to be influenced by factors such as chip formation during machining and the cutting match between the abrasive and the workpiece material. However, the exact cause will be investigated further in subsequent research. Figure 9 compares the results of experiments conducted with the same DOC, 2 \(\upmu\)m, to analyze the impact of grit sizes #325, #1200, and #2000 on surface roughness. For the diamond abrasive, the surface roughness for grit sizes #325, #1200, and #2000 were in the ranges of 0.166-0.189 \(\upmu\)m Ra, 0.025-0.046 \(\upmu\)m Ra, and 0.0031-0.0069 \(\upmu\)m Ra, respectively. Particularly, for the #2000 grinding wheel, nanosurface machining with roughness below 7 nm Ra was achievable.