Optimization of parameters in cylindrical and surface grinding for improved surface finish

Surface integrity has attracted the attention of researchers for improving the functional performance of engineering products. Improvement in surface finish, one of the important parameters in surface integrity, has been attempted by researchers through different processes. Grinding has been widely used for final machining of components requiring smooth surfaces coupled with precise tolerances. Proper selection of grinding wheel material and grade with grinding parameters can result in an improved surface finish and improved surface characteristics. The present work reports the study of the effect of grinding parameters on surface finish of EN8 steel. Experiments were performed on surface grinding and cylindrical grinding for optimization of grinding process parameters for improved surface finish. Grinding wheel speed, depth of cut, table feed, grinding wheel material and table travel speed for surface grinding operation, and work speed for cylindrical grinding operation were taken as the input parameters with four types of grinding wheels (Al2O3 of grades K and L, and white alumina of grades J and K). The surface roughness was taken as an output parameter for experimentation. The grinding wheel material and grade have been observed to be the most significant variables for both cylindrical grinding and surface grinding. Surface roughness in the case of surface grinding is better compared to that of cylindrical grinding, which can be attributed to vibrations produced in the cylindrical grinding attachment. Surface roughness (Ra) values of 0.757 µm in cylindrical grinding and 0.66 µm in surface grinding have been achieved.

DG, 0000-0002-7761-6925 Surface integrity has attracted the attention of researchers for improving the functional performance of engineering products. Improvement in surface finish, one of the important parameters in surface integrity, has been attempted by researchers through different processes. Grinding has been widely used for final machining of components requiring smooth surfaces coupled with precise tolerances. Proper selection of grinding wheel material and grade with grinding parameters can result in an improved surface finish and improved surface characteristics. The present work reports the study of the effect of grinding parameters on surface finish of EN8 steel. Experiments were performed on surface grinding and cylindrical grinding for optimization of grinding process parameters for improved surface finish. Grinding wheel speed, depth of cut, table feed, grinding wheel material and table travel speed for surface grinding operation, and work speed for cylindrical grinding operation were taken as the input parameters with four types of grinding wheels (Al 2 O 3 of grades K and L, and white alumina of grades J and K). The surface roughness was taken as an output parameter for experimentation. The grinding wheel material and grade have been observed to be the most significant variables for both cylindrical grinding and surface grinding. Surface roughness in the case of surface grinding is better compared to that of cylindrical grinding, which can be attributed to vibrations produced in the cylindrical grinding attachment. Surface roughness (R a ) values of 0.757 µm in cylindrical grinding and 0.66 µm in surface grinding have been achieved.

Selection of workpiece material
The workpiece material used was EN8 steel, which is widely used in industrial applications like engine shafts, spindles, connecting rods, studs and screws due to its good mechanical properties. It is medium carbon steel usually supplied untreated and having good tensile strength. The tensile strength varies in the range of 500-800 N mm −2 .

Experimental set-up
The experiments were conducted on a surface grinding machine with a cylindrical grinding attachment at different combinations of grinding process parameters. For surface grinding, the machine allowed r.p.m. as well as table feed variations, whereas for cylindrical grinding, only the fixed r.p.m. mechanism was available. The existing fixed r.p.m. mechanism was modified for obtaining r.p.m. variations at three levels.

Selection of grinding process parameters
The grinding wheel speed, grinding wheel grade, depth of cut, grinding wheel material and feed rate are the important parameters that affect the surface finish, which in turn affects the productivity and cost of the component.

Experimentation
The experiments were conducted on the surface grinding machine with a cylindrical grinding attachment at different combinations of grinding process parameters. Experiments were performed at two values of grinding wheel speed, i.e. 1400 and 2800 r.p.m., for both surface and cylindrical grinding operations. A total of 32 experiments were performed. For each test run, three trials were performed to increase the accuracy of results for a better surface finish result. The average of these three trial values has been used for experimental analysis. A total of 96 test runs were made during experimentation.

Cylindrical grinding
For cylindrical grinding, a round bar 110 mm long and 20.4 mm in diameter was used. Experiments were conducted on the surface grinding machine with a cylindrical grinding attachment by using watersoluble coolant. Grinding wheel speed, depth of cut, table feed, grinding wheel material and work speed for cylindrical grinding operation were taken as the input parameters with four types of grinding wheels (Al 2 O 3 of grades K and L, and white alumina of grades J and K).

Surface grinding
A flat plate of 132 × 28.5 × 6.15 (L × B × W) mm was used for the surface grinding experiments using the same lubricant. The input parameters used for cylindrical grinding were kept the same, except that the work speed was replaced by the table travel speed for surface grinding operation. The surface roughness values for all the experiments for surface and cylindrical grinding operations were measured. The signal-to-noise (S/N) ratio and analysis of variance (ANOVA) were used to study the performance characteristics of the grinding operation. Confirmation tests were carried out to compare the results of predicted values with the experimental value. ANOVA was carried out to identify the significant factors affecting the surface roughness.

Results
The results obtained from experimental work for the optimization of grinding process parameters are given in annexure A. The results obtained from the experimental data are discussed below.

Evaluation of S/N ratios
At each set of input variables, three experiments were conducted and the average of these three trial values has been taken for analysis. The mean surface roughness values and the corresponding S/N ratio of each test run obtained for both grinding operations are shown in annexure A.

Level mean response analysis
The level mean values of S/N ratios calculated for four levels of grinding parameters of surface grinding and cylindrical grinding operations are as shown in annexure A. The level mean response S/N ratios help in analysing the trend of the quality characteristics with respect to the variation of the grinding input parameters. The level mean response plots based on the S/N ratios are used in optimizing the surface roughness.
The rank of grinding process parameters used in both grinding processes is given in table 3 based on S/N ratios. Delta, the value calculated for ranking the grinding process parameters, was used in both the grinding processes. The value of λ was calculated by taking the difference in the maximum value from the minimum value of S/N ratios. The parameter having a larger difference of S/N ratios is ranked first; similarly, other differences were compared and ranked accordingly.

Cylindrical grinding
The level mean response plots for various quality characteristics based on the S/N ratios in cylindrical grinding are shown in figure 1a-e. Figure 1a shows that the S/N ratio corresponding to 1400 r.p.m. of grinding wheel was larger, which is desirable for a better surface finish. The S/N ratio corresponding to 2800 r.p.m. of the grinding wheel was lower. Figure 1b shows the variation of S/N ratios for workpiece speed. It has been noted that the S/N ratio corresponding to 278 r.p.m. of workpiece speed was larger,     which is advantageous for achieving a good surface finish. The S/N ratio at the 656 r.p.m. workpiece speed was the lowest. Also, the S/N ratio increases slightly as the workpiece speed is increased from 128 to 278 r.p.m.; then, the S/N ratio decreases constantly on further increasing the workpiece speed. Figure 1c shows the graph of S/N ratios for different depths of cut. It has been observed that the S/N ratio corresponding to the 30 µm depth of cut was larger, which signifies improved surface finish. The S/N ratio at the 20 µm depth of cut was the lowest. Initially, the S/N ratio decreases from the 10 µm to the 20 µm depth of cut, then it increases slowly for higher values of depth of cut. The graphs in figure 1b and figure 1d depict similar trend in the effect of workpiece speed and material and grade of grinding wheel on S/N ratio, respectively. The S/N ratio corresponding to the second grinding wheel was found to be larger, which is desirable for a better surface finish. The S/N ratio corresponding to the third grinding wheel was the lowest. It has been concluded that the aluminium oxide grinding wheel gave better performance than the white alumina grinding wheel. It is found from figure 1e that the S/N ratio increases as the table cross feed increases. The surface finish is better at higher values of table cross feed. The graph shows almost the same trend as that of the depth of cut.

Surface grinding
The level mean response plots for various quality characteristics in surface grinding based on the S/N ratios are shown in figure 2a-e. In figure 2a, the S/N ratio corresponding to 1400 r.p.m. of the grinding wheel is larger, which is desirable for a better surface finish. The S/N ratio corresponding to 2800 r.p.m. of grinding wheel was lower. Figure 2b shows the graph of S/N ratios for table travel speed. The value of the S/N ratio at 10 m min −1 of table travel speed was the largest, which is desirable for a better surface finish. The value of the S/N ratio at 18 µm of depth of cut was the lowest. The S/N ratio decreases as the table travel speed is either increased or decreased from 100 m min −1 . Figure 2c shows the graph of S/N ratios with respect to depth of cut. The S/N ratio corresponding to 10 µm of depth of cut was larger, which is desirable for a better surface finish. The S/N ratio at 20 µm of depth of cut was the lowest. The S/N ratio initially decreases from the 10 µm to the 20 µm depth of cut, after which it increases slowly for higher values of the depth of cut. It is concluded from figure 2d that the aluminium oxide grinding wheel gave better performance than the white alumina grinding wheel. It was also observed from the graph that the S/N ratio corresponding to the first grinding wheel was larger, which is desirable for a better surface finish. The S/N ratio corresponding to the third grinding wheel was the lowest. It is observed from figure 2e that the value of the S/N ratio was highest at the 0.03 m min −1  ratio increases slowly. The surface finish is better at higher values of table cross feed. The graph shows the same trend as that of the depth of cut.  Table 4 shows that the depth of cut and the grinding wheel speed are the least significant parameters in surface grinding and their contribution is negligible.

Mathematical modelling
After obtaining the optimal level of design parameters, the final step is to predict and verify the surface roughness using the optimal level of the design parameters.

Cylindrical grinding
At optimum setting conditions, S/N ratios of cylindrical grinding were determined by using the following equation: where η G is the S/N ratio calculated at the optimum levels and η G is the mean S/N ratios of all parameters; and A 0 (−2.09713), B 0 (−0.94795), C 0 (−1.58109), D 0 (0.38327) and E 0 (−2.04855) are the mean S/N ratio values for A, B, C, D and E parameters, respectively, at the mean S/N ratio when these parameters are at the optimum surface roughness level. The predicted S/N ratio of 2.51139 dB is transformed to R a = 0.748 µm.

Surface grinding
At optimum setting conditions, the S/N ratios of surface grinding were determined by using the following equation:

Confirmation test
The predicted optimum surface roughness value of means was validated using a confirmation test.

Conclusion
In this paper, the effect of grinding parameters on surface finish on EN8 steel has been analysed. Experiments were performed on surface and cylindrical grinding for optimization of grinding process parameters for improved surface finish. The following conclusions have been obtained from the experimentation and analysis of results: (a) Surface roughness in the case of surface grinding is better when compared with cylindrical grinding. (b) The predicted surface roughness (R a ) for cylindrical and surface grinding was evaluated to be 0.748 µm and 0.660 µm, whereas the roughness value from the confirmation experiments for both operations was 0.757 µm and 0.655 µm, respectively. The percentage error of surface roughness for cylindrical and surface grinding is found to be 1.2% and 0.75%, respectively. (c) The material and grade of the grinding wheel have been found to be most prominent factors influencing surface roughness for both grinding operations. (d) The optimum conditions of cylindrical and surface grinding parameters for lower surface roughness for EN8 steel have been determined.
As the conclusions are based on the effect of a set of input parameters on surface roughness, it is expected that the proposed parameters will give the desired results on any other grinding machine with controlled vibrations, which can affect the results significantly.