3.1 Sapphire tool fabrication and edge radius measurement
Figure 6 presents the schematic and SEM images of the flank face, cutting edge, and the indentation profile of the sapphire tool after each stage of lapping, polishing, and CMP. Initially, the sapphire tool underwent lapping using #400 boron carbide abrasives on a cast iron lap, resulting in a material removal rate of 10 µm/min. After this step, a sapphire tool featuring a 6mm width cutting edge and a 7° clearance angle was successfully fabricated. Figure 6(step1-b) illustrates the SEM image of the cutting edge that appears blunt. This is evident from the Fig. 6(step1-c) that depicts the SEM image of the indentation profile of sapphire tool, the edge radius measured was 13.5 µm. Hence, the need for further refinement in the polishing process to fabricate the sharp sapphire tool with lesser cutting-edge radius.
Subsequently, following the initial lapping process, the tool underwent polishing using a 3 µm diamond slurry applied on a spiral groove copper. The material removal rate in this process was 0.5 µm/min. The flank surface finish experienced a significant improvement, observed a decrease in surface roughness from 170 nm Ra to 13.2 nm Ra and the same is evident from Fig. 6(step2-a). The sharpness of the cutting tool is dependent on the edge chipping[13]. Higher MRR tends to cause increased chipping, consequently reducing sharpness and vice versa. The lapping process has high MRR compared to polishing process therefore, the sharpness of cutting edge improved in the polishing process as observed in Fig. 6(step2-b). From the sapphire tool’s indentation profile as shown in Fig. 6(step2-c), the edge radius measured was 2.8 µm.
To further eliminate polishing marks on the flank face of the sapphire tool and improve the sharpness the CMP process was employed. The flank face’s surface finish enhanced from 13.2 nm Ra to 2 nm Ra as shown in Fig. 6(step3-a). The MRR of the CMP process is in few nanometers per minute which is least among polishing and lapping. Hence, least edge chipping was observed in this process which resulted in a sharp cutting edge as depicted in Fig. 6(step3-b). From the sapphire tool’s indentation profile as shown in Fig. 6(step3-c), the cutting-edge radius measured was 430nm.
3.2 Performance of sapphire tool while machining OFHC copper
Thrust forces and cutting forces during the orthogonal cutting are measured from Kistler Type 9256 three-component piezoelectric force dynamometer. Figure 7a displays the plot of cutting and thrust force against cutting speed, at a constant UCT of 1 µm. Cutting force is greater than the thrust force indicating material removal is taking place in shear mode as UCT 1 µm. However, it is observed that the cutting force increases with an increase in cutting speed. Due to its high ductility and low yield strength, OFHC copper displays a greater effect of strain rate hardening with an increase in cutting speed[14], rather than thermal softening.
Figure 7b displays the plot of cutting and thrust force against UCT, at a constant cutting speed of 1 m/s. As the UCT is increased from 0.1 µm to 3 µm, both the cutting force and thrust force increase due to higher material removal rate. Ploughing material removal mode is predominant when UCT is less than 0.5 µm, as the thrust force is greater than the cutting force. However, at UCT greater than 0.5 µm, shear material mode dominates, as the cutting force becomes greater than the thrust force[15]. Consequently, increasing UCT results in improved surface finish.
The apparent coefficient friction (µ) between the rake face of the tool and chip is obtained from the Eq. 1[16]. Where, \({F}_{c }\)is cutting force, \({F}_{t }\)is thrust force and α is the effective rake angle.
$$\mu = \frac{{F}_{t}+ {F}_{c }\text{tan}\alpha }{{F}_{c}- {F}_{t }\text{tan}\alpha }$$
1
The effective rake angle (α) is the angle between the vertical axis and the tangent to the contact point between the cutting tool and the UCT as depicted in Fig. 8. When the uncut chip thickness is less than the edge radius, the effective rake angle is obtained from Eq. 2. Where, α is the effective rake angle, t is the uncut chip thickness and \({r}_{e}\) is the cutting-edge radius of the Sapphire tool.
$$\alpha ={\text{sin}}^{-1}\frac{({r}_{e}-t)}{{r}_{e}}$$
2
When the UCT is 100 nm, which is below the cutting-edge radius of 432 nm, the application of Eq. 2 results in an effective rake angle of 50°. In all other cases where UCT exceeds the edge radius i.e., 432 nm the effective rake angle (α) becomes zero degree because the rake angle of the Sapphire tool is also zero. The values of α are substituted in the Eq. 1 accordingly to determine the apparent coefficient of friction (µ). Figure 9a displays the relationship between coefficient of friction and cutting speed (m/s), plotted for a UCT of 1 µm. It is observed that the friction coefficient decreases from 0.88 at 1 m/s cutting speed to 0.815 at 2 m/s cutting speed and further increases to 0.822 at 3 m/s.
The average surface roughness Ra on the machined surface across the cutting direction is measured using Olympus confocal microscope. Figure 9b displays the relationship between average surface roughness Ra (nm) and cutting speed (m/s), plotted for a UCT of 1 µm. It is observed that the surface roughness tends to decrease from 43 nm to 12 nm as the cutting speed is increased from 1 m/s to 2 m/s. This is due to the decrease in the friction coefficient from 0.88 at 1 m/s cutting speed to 0.815 at 2 m/s as displayed in Fig. 9a. The reduction in the coefficient of friction causes less adhesion between the tool rake face and the chip thus improving the chip separation and achieving better surface finish on the surface. Subsequently, there is a slight increase in average surface roughness to 14 nm at a cutting speed of 3 m/s. This can be attributed to an increase in the friction coefficient from 0.8148 at 2 m/s to 0.822 at 3 m/s as shown in Fig. 9a. As the adhesion between tool and chip increases it deteriorates the chip separation hence increase in the surface roughness.
Figure 10a illustrates the plot of the apparent coefficient of friction plotted against UCT (µm) for a constant cutting speed of 1 m/s. It is observed that the friction coefficient decreases from 3.14 at 0.1 µm UCT to 0.78 at 2 µm UCT and further increases to 0.805 at 3 µm UCT. Figure 10b displays the relationship between average surface roughness Ra (nm) and UCT (µm), plotted for a constant cutting speed of 1 m/s. The surface roughness Ra decreased from 69 nm to 9 nm with increase in UCT from 0.1 µm to 2 µm. This is due to the decrease in the apparent friction coefficient from 3.14 at 0.1 µm UCT to 0.78 at 2 µm because the material removal mode changes from ploughing at 0.1 µm to shearing at 2 µm. Consequently, the average surface roughness increased to 36 nm at 3 µm UCT as friction coefficient increased to 0.805. As, the increase in depth of cut caused increased the tool-chip contact length increasing the adhesion as result of which poor surface finish on workpiece surface[17].
After machining OFHC copper the chips are collected and observed under a scanning electron microscope. The use of sapphire tool used for machining OFHC copper produced small, discontinuous, and straight chips which indicates the material removal is occurred through fracture. Figure 11 displays the chip morphology at cutting speeds of 1m/s and 3m/s. It is observed that at the higher speed of 3m/s, the edges of the chips are torn, suggesting that brittle fracture is the dominant mechanism due to strain rate hardening.
Figure 12 displays the chip morphology at UCT of 0.1 µm and 3 µm. It is seen that at UCT 0.1 µm chips produced are long and continuous implies that material removal is through plastic deformation than fracture, because the UCT is less than the cutting-edge radius. On contrary at UCT 3 µm chips are short, thick, and discontinuous because fracture material removal mode is dominant as UCT is much greater than cutting-edge radius.
Figure 13 displays the sapphire tool’s cutting edge after machining OFHC copper with cutting distance of 10 m. From the image it is clear that tool wear is negligible because the sapphire being many times harder than the copper. Hence, sapphire can be considered as tool material for ultraprecision turning of the OFHC copper.
To summarize the performance of sapphire tool while machining OFHC copper. It was observed that increasing the cutting speed from 1 m/s to 3 m/s resulted in an overall increase in both the cutting and thrust forces. This can be attributed to the strain rate hardening behaviour of OFHC copper due to its high ductility and low yield strength. Evidently the chips generated at 3 m/s had more torn edges compared to those produced at 1 m/s indicating presence of brittle fracture due to the strain hardening. The cutting force value increased significantly than the thrust force as the cutting speed increased from 1 m/s to 3 m/s indicating shear mode of fracture being dominant. Therefore, a corresponding reduction in surface roughness was observed, decreasing from 43 nm to 14 nm.
As the UCT increased from 0.1 µm to 3 µm, there was a raise in both the cutting force and thrust force due to the greater amount of material being removed during the machining process. Initially, up to a UCT of 0.5 µm, the thrust force exceeded the cutting force as the UCT value approached the edge radius. However, once the UCT exceeded 0.5 µm, the cutting force surpassed the thrust force. Consequently, at a UCT of 0.1 µm, the chips produced exhibited a straight and continuous morphology, indicating that plastic deformation was the dominant mechanism of material removal. On the other hand, at a UCT of 3 µm, the chips were shorter, thicker, and discontinuous, indicating that fracture was the predominant mode of material removal. This was reflected the surface roughness, where an increase in UCT from 0.1 µm to 3 µm resulted in a decrease in average surface roughness (Ra) from 69 nm to 36 nm.
3.3 Performance of sapphire tool while machining of free cutting brass
The cutting and thrust forces (N) against cutting speed (m/s) for a constant UCT (uncut chip thickness) of 1 µm are shown in Fig. 14a. It is notable that the thrust force exceeds the cutting force, which is attributed to the high ductility and severe plastic deformation exhibited by brass during machining. Subsequently, a decrease in both thrust and cutting forces is observed as the cutting speed is increased from 1 m/s to 3 m/s. This decrease can be attributed to the effect of thermal softening, which is more prominent in free cutting brass due to its high ductility and plasticity[19].
Figure 14b displays the plot of cutting and thrust force (N) against UCT (µm), at a constant cutting speed of 2 m/s. It can be observed that the cutting force continuously increases as the UCT is increased from 0.1 µm to 3 µm, due to the corresponding increase in material removal rate. Additionally, it is noted that until a UCT of 2 µm, the thrust force exceeds the cutting force, suggesting that ploughing material removal mode is dominant during this range. However, after the 2 µm UCT mark, the cutting force becomes greater than the thrust force, indicating the dominance of the shear material removal mode. Consequently, an increase in UCT leads to an improvement in surface finish.
Figure 15a illustrates the plot of apparent coefficient of friction against cutting speed for constant uncut chip thickness of 1 µm. The friction coefficient declines from 2.18 at 1 m/s cutting speed to 1.28 at 3 m/s cutting speed. The average surface roughness Ra on the machined surface across the cutting direction is measured. Figure 15b displays the relationship between average surface roughness Ra (nm) and cutting speed (m/s), plotted for a UCT of 1 µm. It is observed that the surface roughness tends to decrease from 91 nm to 38 nm as the cutting speed is increased from 1 m/s to 3 m/s. The decline in the roughness is due to the decrease in the friction coefficient from 2.18 at 1 m/s cutting speed to 1.28 at 3 m/s as displayed in Fig. 15a. Thermal softening is more prominent in the machining of free cutting brass with an increase in the cutting speed that leads to decrease in apparent coefficient of friction. The chip separation becomes easy at higher cutting speeds as a result we achieved better surface finish.
Figure 16a displays the plot of apparent coefficient of friction plotted against uncut chip thickness (µm) for a constant cutting speed of 2 m/s. The friction coefficient declines from 4.35 at 0.1 µm UCT to 0.59 at 3 µm UCT. Figure 16b displays the relationship between average surface roughness Ra and UCT, plotted for a constant cutting speed of 2 m/s. The surface roughness Ra decreased from 71 nm to 34 nm as UCT increases from 0.1 µm to 3 µm. This reduction in roughness is due to the decrease in the apparent friction coefficient from 4.35 at 0.1 µm UCT to 0.59 at 3 µm as shown in Fig. 16a. The change in friction coefficient is a result of shift in material removal mode ploughing at 0.1 µm to shearing at 3 µm. The interaction between the tool and chip changes from sticking to sliding.
After machining free cutting brass, the chips are collected and observed under a scanning electron microscope (SEM). The use of sapphire as a tool material in machining free cutting brass results in the production of continuous, and ribbon-like chips, which suggests that material removal occurs primarily through plastic deformation. Figure 17 shows the chip morphology at cutting speeds of 1m/s and 3m/s. It is noted that the curl of the chip increases at the higher speed of 3m/s. Additionally, the chip surface in contact with the cutting tool has fewer scratches and pits (is smoother) and appears shiny in the case of a 3 m/s cutting speed compared to that produced with a 1 m/s cutting speed. This suggests that thermal softening occurs at high cutting speeds.
The morphology of chips produced at UCT of 0.1 µm and 3 µm is presented in Fig. 18. It can be observed that when the UCT is 0.1 µm, the chips produced are long, straight, and exhibit torn edges, indicating that brass behaves as a brittle material. This phenomenon occurs when the UCT is less than the cutting-edge radius. Conversely, at UCT of 3 µm, the chips are continuous and ribbon-like. This condition favors a ductile mode of machining in brass, as the UCT is greater than the edge radius.
Figure 19 displays the sapphire tool’s cutting edge after machining free cutting brass with cutting distance of 10 m. From the image it is clear that tool wear is negligible because the sapphire being many times harder than the copper. Hence, Sapphire is a suitable material for cutting tool in ultraprecision turning of the Free cutting brass.
To sum up the performance of the sapphire tool in machining of free cutting brass. It was observed that the cutting and thrust forces decreased as the cutting speed was increased from 1 m/s to 3 m/s. The higher ductility and significant plastic deformation exhibited by brass resulted in a thrust force higher than the cutting force. This was evident from the chip morphology, which showed continuous and ribbon-like chips, indicating that material removal predominantly occurred through plastic deformation. Additionally, both cutting forces exhibited a decrease as the cutting speed increased due to the occurrence of thermal softening. Furthermore, an increase in chip curling and a smoother contact surface of the chip were observed as the cutting speed increased, indicating the influence of thermal softening. Therefore, the surface finish improved from 91 nm to 38 nm as the cutting speed was increased from 1 m/s to 3 m/s.
The increase in UCT from 0.1 µm to 3 µm led to an increase in cutting force due to higher material removal, while thrust force decreased. The dominance of ploughing mode of material removal was indicated by the higher thrust force until a UCT of 2 µm, after which shear mode material removal prevailed. Chip morphology also reflected this behavior, with long and straight chips with torn edges at 0.1 µm UCT, indicating the brittle behavior of the brass during ploughing. In contrast, at 3 µm UCT, continuous and ribbon-like chips were observed, signifying shear mode material removal. As a result, the average surface roughness decreased from 71 nm at 0.1 µm UCT to 34 nm at 3 µm UCT.
3.4 Performance of Sapphire tool while machining of Al 6061
Figure 20a illustrates the relationship between cutting speed (m/s) and the cutting and thrust forces (N) at a constant UCT of 2 µm. The cutting force surpasses the thrust force, indicating the predominant material removal mechanism in shear mode. Moreover, as the cutting speed rises from 1 m/s to 3 m/s, both the thrust and cutting forces demonstrate a decrease. This reduction can be attributed to the decline in tool-chip contact area and the decrease in shear strength within the flow zone, which occurs as the cutting speed increases, resulting in elevated temperatures[20]. Figure 20b exhibits the plot of cutting and thrust forces (N) against UCT (µm) at a constant cutting speed of 2 m/s. It can be observed that the thrust and cutting forces continuously increase as the UCT is increased from 0.5 µm to 3 µm, due to the corresponding increase in material removal rate.
Figure 21a displays the plot of apparent coefficient of friction against cutting speed for constant uncut chip thickness of 2 µm. The friction coefficient decreases from 0.842 at 1 m/s cutting speed to 0.802 at 3 m/s cutting speed. The average surface roughness Ra on the machined surface across the cutting direction is measured. Figure 21b displays the relationship between average surface roughness Ra (nm) and cutting speed (m/s), plotted for a UCT of 2 µm. It is observed that the surface roughness tends to decrease from 50 nm to 14 nm as the cutting speed is increased from 1 m/s to 3 m/s. The reduction in the surface roughness is caused due to a decline in the friction coefficient from 0.842 at 1 m/s to 0.802 at 3 m/s as depicted in Fig. 21a. The thermal softening at higher cutting speed causes the reduction apparent friction coefficient.
Figure 22a illustrates the plot of apparent coefficient of friction plotted against uncut chip thickness (µm) for a constant cutting speed of 2 m/s. The friction coefficient declines from 0.936 at 0.5 µm UCT to 0.827 at 3 µm UCT. Figure 22b displays the relationship between average surface roughness Ra and UCT, plotted for a constant cutting speed of 2 m/s. The surface roughness Ra decreased from 90 nm to 34 nm as UCT increases from 0.5 µm to 3 µm. This reduction in roughness is due to the decrease in the apparent friction coefficient from 0.936 at 0.5 µm UCT to 0.827 at 3 µm. The change in friction coefficient is attributed to change in material removal mechanism from ploughing at 0.5 µm to shearing at 3 µm. Additionally, built-up edge (BUE) formation is more prominent at lower UCT like causing the increase in the apparent coefficient of friction. The adhesion between the chip and tool rake face increases the BUE formation resulting in poor surface finish. Therefore, surface finish improved from 90 nm at 0.5 µm UCT to 34 nm at 3 µm UCT.
After machining Al 6061, the chips are collected and observed under a SEM. Figure 23 illustrates the chip morphology at cutting speeds of 1m/s and 3m/s. It is observed that as the cutting speed increases, the chip thickness decreases. Machining of Al 6061 with sapphire tool results in the production of continuous chips, no curling of chips is observed. The chip formed where of continuous shape, with a relatively uniform thickness and smooth surface. The formation of such continuous chips can be attributed to the more ductile behavior exhibited by Al 6061.
At lower cutting speeds, the rate of deformation and strain on the material being machined is reduced. This slower cutting speed provides more time for the material to undergo plastic deformation and flow smoothly along the shear plane. Consequently, the chip formed during machining exhibits a continuous shape with a relatively uniform thickness and smooth surface. The lower cutting speeds result in lower forces acting on the material, creating a more stable and predictable chip formation process. The material can undergo substantial plastic deformation before reaching its failure point, leading to the formation of a continuous chip rather than fragmentation or serration.
The chip morphology at different UCT of 0.5 µm and 3 µm is depicted in Fig. 24. When the UCT is 0.5 µm, the chips formed are discontinuous due to inadequate tool-workpiece contact length. However, as the UCT is increased from 0.5 µm to 3 µm, the chips transition to a continuous form with regular lamella formation. This transformation is attributed to the higher UCT, which causes the material ahead of the tool to be moved in the cutting direction, leading to the creation of a shear band. As a result, the work material is removed through a conventional shearing mechanism, resulting in the generation of long continuous chips. Moreover, the increased UCT reduces the surface fragmentation of microchips by expanding the layer of material removal.
Figure 25 showcases the cutting edge of the sapphire tool after machining Al 6061 with a cutting distance of 10 m. The presence of significant wear on the rake surface and the absence of wear on the flank surface indicate the formation of a built-up edge. Oishi[8] observed similar phenomena during machining of Aluminium 2014 T6 alloy with alumina tool. The built-up edge occurs due to the affinity of the alumina towards the aluminum in Al 6061. During the machining process, increase in temperature at tool-chip interface causes welding between alumina at cutting and Al6061 this results in built-up edge formation[21]. Furthermore, Al 6061 contains precipitation hardening Mg-Si alloys which are hard and brittle cause irregular machining conditions[22]. This leads to high tool wear and poor surface roughness.
To summarize the machining of Al 6061 using Sapphire tool. It was observed that the cutting and thrust forces decreased as the cutting speed increased from 1 m/s to 3 m/s. This reduction was attributed to a smaller tool-chip contact area and a decrease in shear strength within the flow zone at elevated temperatures. Evidently, the chip morphology exhibited a decrease in thickness as the cutting speed increased from 1 m/s to 3 m/s. The chips generated during the machining process displayed a continuous and uniform thickness, which can be attributed to the ductile behavior of Al 6061. Consequently, an improvement in surface finish was observed, with the surface roughness (Ra) decreasing from 50 nm to 14 nm as the cutting speed increased from 1 m/s to 3 m/s.
As the UCT increased from 0.5 µm to 3 µm, there was a raise in both the cutting force and thrust force due to the greater amount of material being removed during the machining process. The cutting force was higher than the thrust force indicating shear mode material removal. Consequently, at a UCT of 0.5 µm, the chips formed are discontinuous due to inadequate tool-workpiece contact length. However, at 3 µm the chips were continuous with regular lamella formation, and thicker. Furthermore, the surface finish improved from 90 nm at 0.5 µm UCT to 34 nm at 3 µm UCT. However, during the machining of Al 6061 using the Sapphire tool, the presence of built-up edge formation was observed. This was evident from the examination of the cutting edge, where significant wear was observed on the rake face, while the flank surface showed no signs of wear.
3.5 Performance of sapphire tool while machining Stavax ESR steel.
Stavax ESR, known for its high hardness, toughness, and yield strength, posed challenges when machined with a sapphire tool. During the machining observed chatter and severe tool wear. As depicted in Fig. 26 edge chipping was observed on both the rake and flank faces of the tool. Consequently, the surface finish of the workpiece was poor, and didn’t have the desired optical quality finish. Consequently, we were unable to conduct an in-depth analysis of the impact of process parameters such as uncut chip thickness and cutting speed on critical factors like cutting thrust forces, surface roughness, and chip morphology. As a result, it can be concluded that sapphire cutting tool is unsuitable for ultra-precision machining of Stavax ESR steel.