# Surface Quality Evolution Model and Consistency Control Method of Large Shaft Multi-Pass Grinding
## 2 Surface Quality Indexes and Influencing Factors
### Analysis of Surface Quality Influencing Factors
Therefore, it is necessary to further analyze the actual grinding depth at different positions of the same grinding pass and its influence on the surface roughness and glossiness. The actual grinding depth \(d^{\prime}_{p}\) is calculated as:  
\[{a^{\prime}}_{p}=\frac{1}{2}\big{(}D_{\text{shaft}}-{D^{\prime}}_{\text{shaft }}\big{)} \tag{2}\]  
where \(D_{\text{shaft}}\) and \({D^{\prime}}_{\text{shaft}}\) are diameters of the shaft before and after a grinding pass. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \(N_{\text{wheel}}\) **(t/min)** & **550** & **700** & **850** & **1000** \\ \hline Standard deviations (A) & 1.35 & 0.64 & 0.73 & 1.23 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The spindle current standard deviations of the grinding passes in Figure 5d. Figure 4: The influence of three grinding parameters on surface roughness and glossiness: (**a**) grinding depth, (**b**) shaft speed, (**c**) pallet speed, (**d**) grinding wheel speed. Figure 5 shows the actual grinding depth, surface roughness, and glossiness of a single grinding pass. The actual grinding depth is larger on both sides of the shaft and smaller in the middle due to grinding wheel wear. With the decrease of actual grinding depth, the roughness value decreases and the glossiness increases, which is consistent with the effect of nominal grinding depth on surface quality. This reveals the mechanism of inconsistent surface quality. The actual grinding depth along the \(z\)-direction is inconsistent due to grinding wheel wear, so that the surface roughness and glossiness are inconsistent along the \(z\)-direction. In addition, the deflection of the shaft and grinding wheel under grinding force will also reduce the actual grinding depth. However, in this study, the shaft size is large and the grinding depth is small (less than 5 \(\upmu\)m), so the influence of grinding force can be ignored. The grinding process of large shaft parts usually includes four stages: rough grinding, semi-finish grinding, finish grinding, and spark-out grinding. Each stage includes several grinding passes with the same parameters. In a grinding stage, even if the grinding parameters are the same, the surface roughness and glossiness after grinding are different. Figure 6 shows the surface roughness and glossiness of four consecutive passes with the same grinding parameters. The data in Figure 6 are the test values only at the measuring point at z = 800 mm. As can be seen from Figure 6, these two surface quality indicators change with the grinding passes. This indicates that the surface quality of large shaft parts in multi-pass grinding is affected by the quality of the previous grinding pass, showing evolutionary characteristics, rather than a simple correlation only with grinding parameters. It should be pointed out that the above conclusions are obtained when the material of workpiece is 92 CrMo alloy steel. Considering that the material of the large shaft is essentially alloy steel, this conclusion applies to other large shafts made of different kinds of alloy steel. Figure 5: The actual grinding depth, surface roughness, and glossiness of a single grinding pass: (**a**) actual grinding depth, (**b**) surface roughness and glossiness. Figure 6: The surface roughness and glossiness of four consecutive passes.