This section discusses the results obtained for the different MWFs application proposed (CN, ME and ME + CA) in through-feed centerless grinding process of bearing steel SAE 52100.
3.1. Analysis of surface roughness and GWAS condition
The Fig. 4 presents the mean surface roughness (Ra) values for different lubri-cooling techniques (CN – conventional nozzle, ME – multitubular with emulsion and ME + CA – multitubular with emulsion and compressed air) and different flow rates employed.
By Fig. 4 and irrespective of nozzle used, it can be seen that the reduction of surface roughness values is related to the increase of flow rate. As reported by Ramesh et al. [34] and Bianchi [10], a higher flow rate results in higher coolant velocity, improving the lubrication effect and contributing to a better grinding heat dissipation characteristic. In general, the results obtained for ME and ME + CA nozzles were lower than those obtained for CN.
Regarding to application of ME + CA technique, it was noticed that this technique resulted in lower values of surface roughness and variability in comparison to ME and CN. For tests with ME + CA nozzles at the flow rate of 10 L/min, the surface roughness obtained was 73 and 28% lower than, respectively, CN and ME nozzles.
For tests with ME + CA nozzles at the flow rate of 20 L/min, the surface roughness obtained was 24 and 18% lower than, respectively, CN and ME nozzles. For tests with ME + CA nozzles at the flow rate of 30 L/min, the surface roughness obtained was 30% lower than CN nozzle and no mathematical difference in comparison to ME nozzle. For tests with ME + CA nozzles at the flow rate of 40 L/min, the surface roughness obtained was 17 and 9% lower than, respectively, CN and ME nozzles.
The lowest surface roughness values obtained for ME + CA can be justified by the fact that compressed air increases the velocity of cutting fluid in the nozzle end, producing a higher force in the cutting zone. Higher is the force of fluid jet, higher is the pressure of cutting fluid in the cutting zone; promoting a better cleaning of chips generated in the cutting zone during the grinding process and consequently increasing the lubricant effect of lubri-cooling method. According to Bianchi et al [10], the use of wheel cleaning jet in MQL with different flow rates (30, 60 and 120 mL/h) significantly lowered the surface roughness Ra for AISI 4340 steel grinding process, producing results similar to flood MWF method.
The best surface roughness values were obtained for the highest flow rates of the cutting fluid, that is, 30 L / min and 40 L / min using the ME + CA nozzle, thus confirming a better grinding condition. Unlike the CN, the ME + CA nozzle promotes a directional jet, with low dispersion at the nozzle outlet and with a speed close to the peripheral speed of the grinding wheel, due to the use of compressed air as an auxiliary fluid. With this nozzle, the fluid penetrates easily in the aerodynamic barrier around the wheel and guarantees a greater amount of fluid in the cutting zone, thus ensuring a better lubricating and cooling effect. The greater dilution of the cutting fluid provides a faster removal of the chips, preventing them from affecting the quality of the piece. Bianchi et al. [10] state that the occurrence of clogging phenomenon on GWAS is hindered by pure oil application because of higher fluid viscosity results in solid suspension, and consequently grout formation which lodges in the wheel pores.
Yoshimura et al. [35] infers that the addition of water in MQL grinding generates lower plastic deformation caused by higher coolant effect by water addition, i.e., higher heat dissipation in the cutting zone by phase transition (from liquid water to vapor). According to Sato et al. [36], this reduces the wheel clogging phenomenon and keeps the wheel grit sharp [37].
According to Ramesh [34], the increase of fluid speed promotes a lubricating effect with better heat dissipation effect in the grinding and, thus, lower surface roughness values. According to Daniel et al. [38], the MWF heat transfer increases with the fluid flow velocity. The Turkey method was employed in order to compare the surface roughness values for ME + CA technique in different flow rate, considering a significance level of 5%. Regarding to flow rate values, the differences in surface roughness were small and were significant between 40 L / min − 10 L / min and 40 L / min − 20 L / min, as shown in Fig. 5.
The high values of surface roughness Ra for the tests using the CN and ME application, for flow rates of 10 and 20 L / min, as shown in Fig. 4, are related to the low efficiency in heat dissipation and in removing the chip from the cutting zone, which may cause the clogging phenomenon on GWAS. Figures 6 (a) and 6 (b) depict evidence of the occurrence of this phenomenon. For the other conditions, no wheel clogging was observed. According to Malkin [39], the presence of chips in the cutting region hinders the cutting performance, increases efforts during material removal and, consequently, increases the surface roughness value. According to Walker et al. [40], the eco-efficient application of a cleaning jet by compressed air diminishes the clogging phenomenon, further decreasing the volume of grout formation on GWAS and, consequently, minimizing the effects of the rubbing and ploughing effects on the ground surface. Thus, Rodriguez et al. [41] state that the surface roughness results can be reduced by 45%. Additionally, the authors infer that excessive temperatures in the contact zone result in reduction of yield strength and increasing of material strain from the ground material, increasing the lodging and adherence of chips in the grinding wheel pores.
Comparing the surface conditions of the grinding wheel using the CN (Fig. 6a) and those obtained for ME (Fig. 6b) at 10 L / min, it is possible to observe in both cases a probable clogging of the GWAS. For the condition employed in the current tests, the surface roughness values generated for CN and ME application did not show mathematically significant differences.
The ME + CA application presented a more efficient condition than the other two lubri-cooling methods, resulting in lower surface roughness values. This can be explained by the high outlet speed of the cutting fluid jet using this technique. According to Webster and Grün [17], the fluid outlet speed associated with the flow rate and the type of nozzle have a significant influence on the coolant effect, as it increases the cleaning efficiency in cutting zone. Moreover, it facilitates the removal of material and results in better surface quality. Thus, the process performance was improved by the application of the cutting fluid associated with the high speed provided by the auxiliary fluid using ME + CA technique and, thus, greater pressure force in the cutting zone. For all conditions tested for ME + CA, no evidence of clogging on GWAS was observed. Figure 6 (c) shows the condition of GWAS for the lowest flow, i.e., 10 L / min.
Figure 6. Wheel topography for (a) CN, (b) ME and (c) ME+CA at 10 L/min.
As can be seen in Fig. 6, higher wheel wear could be seen for ME application. According to Rodriguez et al. [41], the abrasive tool wear occurs by the following mechanisms: grain detachment from bond, grain fracture and wear. As reported by Martini et al. [42], excessive temperatures promote the reduction of bond material strength, what increases tool wear. As observed for ME + CA application, the cutting force is distributed on a larger area of grain protrusion (larger GWAS), what results in higher MRR and tool wear minimization [43].
Figure 7 shows the topography of GWAS for all conditions tested. By the analysis of this figure, it can be observed clogging on GWAS for CN and ME at flow rates of 10 L / min and 20 L / min.
Figure 8 shows the Ra surface roughness recorded, for depth of cut of 0.03 mm and different values of flow rate.
In general, it can be seen in Fig. 8 that the Ra surface roughness values for all conditions tested were less than the maximum tolerance specified for the grinding process of cylindrical rollers for SAE 52100 steel bearings, i.e., 0.15 µm. According to Malkin and Guo [44], the allowable range of Ra roughness values for the grinding process is between 0.2 to 1.6 µm. Thus, all values obtained using the CN method and the optimized methods (ME and ME + CA) are within the tolerance range for the grinding process.
Analyzing Fig. 8, it can be observed that lower values of surface roughness at 40 L / min when the ME + CA method is employed are related to the higher speeds of the cutting fluid and the directional output of the cutting fluid promoted by multitubular nozzle. Despite presenting lower values of surface roughness when compared to ME at 40 L / min, the multiple comparison test, as shown in Fig. 9, does not present mathematically significant differences between both techniques at 40 L / min. The higher speeds of the fluid at the outlet of the nozzle promote the reduction of friction, due to greater presence of lubricant in the cutting region.
3.2. Roundness Deviation
The values of the roundness deviation obtained for the tested lubricooling conditions and material removal of 0.10 mm are shown in Fig. 10. Comparing the results of the roundness from Fig. 10, it is observed that the increase in flow rate resulted in lower roundness deviation, in particular, conditions tested with the CN and ME. In general, the values of the roundness deviation for ME + CA application were lower in comparison to CN application.
The reduction of roundness deviation is due to the fact that the higher flow rate promotes a better cooling effect in the cutting zone. According to Demeter and Hockenberger [45] and Malkin [39], greater is the difficulty of the fluid penetration in the cutting zone, greater is the amount of heat distributed to workpiece and consequently producing thermal expansion and causing an increase in the form deviation, i.e., mainly amplifying the roundness deviation. The best result of the roundness deviation can be seen in Fig. 10 for ME and ME + CA technique at 40 L / min.
The results of the mean values and variability of the roundness deviations (for total removal of 0.03 mm) are shown in Fig. 11.
By Fig. 11, it can be seen that higher values of the roundness deviations were recorded for the tests ME + CA at 20, 30 and 40 L / min. For the flow rate of 10 L / min, the value of the roundness deviation was lower in relation to the tests with CN and ME application.
For all tested conditions, the mean values of the roundness deviations were kept below the maximum allowed tolerance (1µm) for the ground rolling rollers. The roundness deviation during the centerless grinding process is mainly influenced by the geometric variables (inclination angle of regulating wheel, rest blade) and dynamics of the grinding process (natural frequency of machine, wheel rotation, among others). In addition, it is influenced by the magnitude of wheel wear, the MRR (material removal rate) and specific energy.
In addition, the roundness profile of the machined samples was evaluated and concluded that the deviation was not influenced by the dynamic variables of grinding process. The roundness deviations for all conditions tested did not show deviations in the form of periodic undulation, therefore, not due to the influence of dynamic factors in grinding process.
3.3. Residual Stress
The results of the residual stress measurements in “mp” (elastic magnetic parameter) using the Barkhausen noise method for all tests with CN, ME and ME + CA techniques with material removal of 0.03 mm, as shown in Fig. 20.
The mean values of the residual stress results for the lubri-cooling conditions with CN, ME and ME + CA, as shown in Fig. 12.
Barkhausen residual stress analysis is a non-destructive quantitative control method. Workpieces with high values of residual compressive stresses indicate low Barkhausen noise intensity (mp). In order to define the limits between the residual compressive and tensile stresses, an investigation of the ground surface using the X-ray diffraction technique was performed. Through the stress values (MPa) by the X-ray diffraction and Barkhausen noise (mp) technique it is possible to define a factor to convert the values in “mp” to “MPa”.
As shown in Fig. 20, it can be seen that the lubri-cooling method using the ME + CA presented, in general, residual stresses lower than those generated through the methods using the CN and ME + CA techniques. Thus, it can be observed that workpieces ground with ME + CA technique suffered fewer thermal effects.
Comparing the residual stress values (Fig. 12) with the results of the metallographic analysis, a maximum residual stress value can be admitted for the rectification of SAE 52100 steel of 80 mp, due the fact that the conditions using the ME with a flow rate of 40 L / min and for the ME + CA with a flow rate of 30 and 40 L / min, did not produce changes in the microstructure, i.e., the ground workpieces without grinding burns.