- Email: [email protected]
Precision grinding of optical glass with laser micro-structured coarse-grained diamond wheels
Journal of Materials Processing Technology 214 (2014) 1045–1051
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Precision grinding of optical glass with laser micro-structured coarse-grained diamond wheels Bing Guo a,∗ , Qingliang Zhao a,1 , Xiaoyan Fang b,2 a b
Center for Precision Engineering, School of Mechatronics Engineering, Harbin Institute of Technology, Harbin 150001, China Shanghai Machine Tool Works Co., Ltd., Shanghai 200093, China
a r t i c l e
i n f o
Article history: Received 18 September 2013 Received in revised form 12 December 2013 Accepted 19 December 2013 Available online 28 December 2013 Keywords: Precision grinding Coarse-grained diamond wheel Micro-structuring Laser machining Optical glass Subsurface damage
a b s t r a c t This paper presents a series of micro-structured coarse-grained diamond wheels for optical glass surface grinding aiming to improve the grinding performance, especially subsurface damage. The 150 m grit size, single layer electroplated diamond grinding wheels with different interval micro-groove arrays were manufactured by nanosecond pulsed laser, successfully. The influence of micro-structures on surface roughness and subsurface damage was investigated. Compared with conventional coarse-grained diamond wheel, the subsurface damage depth was reduced effectually from 5 to 1.5 m, although the better surface roughness was not obtained by the micro-structured coarse-grained diamond wheel. In addition, the surface roughness and subsurface damage depth were both reduced with the decreasing interval of micro-groove arrays. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Precision grinding of optical glasses has extended its applications in aerospace, automotive, semiconductor and communication industry. With the increasing of demand for high accuracy optical glass products, the precision grinding process not only need to meet the requirements of surface roughness in nanometer scale, form deviations in sub-micron level, and subsurface damage depth within micrometer, but also be required to have a high machining efficiency. Fine-grained (grit sizes from several micron down to sub-micron scale) diamond wheels are usually used for finishing of optical glasses with a low surface roughness. However, the wheel loading, high wear rate and periodic conditioning requirement will deteriorate the form accuracy of workpiece at a lower grinding efficiency. In order to solve the wheel wear problem existed in fine-grained diamond wheels, electroplated nickel mono-layer coarse-grained diamond wheels featuring grit size of 91 m was alternatively
∗ Corresponding author at: P.O. Box 413, Harbin Institute of Technology, Harbin 150001, China. Tel.: +86 451 86402683; fax: +86 451 86415244. E-mail addresses: [email protected] (B. Guo), [email protected] (Q. Zhao). 1 P.O. Box 413, Harbin Institute of Technology, Harbin 150001, China. 2 Shanghai Machine Tool Works Co., Ltd., 1146 Jun Gong Road, Shanghai 200093, China. 0924-0136/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2013.12.013
proposed by Brinksmeier et al. (2000) in precision grinding of optical glasses. Koshy et al. (2003) pointed out that the realization of precision grinding with coarse-grained diamond wheels is determined by the distribution of abrasive protrusion height. In order to obtain the coarse-grained diamond wheels with homogeneous grit protrusion, Aurich et al. (2003) optimized the grit pattern of coarse-grained electroplated grinding wheel assisted by kinematic simulation. Zhao et al. (2005) introduced a conditioning method for coarse-grained electroplated diamond wheels using a diamond cup dressing wheel and ELID technique. The applicability of the conditioned coarse-grained grinding wheels was demonstrated by grinding experiments on BK7 glass yielding a surface roughness in the nanometer range. Subsequently, Zhao et al. (2011) tried to apply copper-resin hybrid bonded coarse-grained diamond wheels in ductile grinding of optical glass. The results showed that nanometer scaled surface roughness and higher form accuracy can be obtained at a low wheel wear rate. The above mentioned investigations indicated that, compared with the fine-grained diamond wheels, precision conditioned coarse-grained diamond wheels could achieve the same nanometer scale roughness, higher form accuracy with a longer wheel life. However, the flat tops of diamond grits created in preconditioning process will lead to greater specific grinding normal force as presented by Heinzel and Rickens (2009). Guo et al. (2013) indicated that a serious subsurface damage will be introduced by coarse-grained diamond wheel in optical glass grinding process.
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It has been shown that the micro-structured surface of diamond solid would reduce the normal grinding force as compared with traditional electroplate grinding block provided by Butler-Smith et al. (2013). This is associated with the stability of active cutting elements and the integrity of micro-cutting edges composed by number of micro-structures. Besides, Axinte et al. (2013) indicated that the increased number of cutting edges would also leads to a decrease of specific cutting force in grinding process, which can be obtained by micro-structuring of flat top diamond grits. Therefore, micro-structured diamond surface seems to have the ability to improve grinding performance of coarse-grained diamond wheels and reduce the specific grinding normal force. Base on the above, this paper presented a kind of microstructured coarse-grained diamond wheels with micro-groove arrays for optical glass surface grinding aiming to improve the grinding performance, especially subsurface damage. Firstly, the strategy for micro-structuring of coarse-grained diamond wheels was provided. Some micro-structured coarse-grained diamond wheels with different interval parallel micro-groove arrays were manufactured by nanosecond pulsed laser. The morphology of the micro-structured wheels was analyzed by SEM. And then these wheels were properly conditioned by a metal bond diamond dressing wheel with ELID method. Finally, the micro-structured coarse-grained diamond wheels were used in optical glass grinding experiments. The ground surface quality and subsurface damage were characterized by profilometer and SEM, respectively. The influence of micro-structures of wheel surface on surface roughness and subsurface damage depth were investigated. 2. Surface micro-structuring of coarse-grained diamond wheel by laser Fig. 1 shows the schematic of the micro-structuring operation on coarse-grained diamond wheels’ cylindrical surface by laser. The micro-structures composed by parallel micro-groove arrays with various intervals on the diamond wheel can be created by individual setting of the laser scan feed step, showed in this figure. The kinematic parameters of wheel rotation speed and dwell time per laser scan feed step are designed to ensure the whole wheel surface could be micro-structuring. Furthermore, more complex microstructures can also be machined on wheel surface by adjustment of more parameters such as the laser scan feed direction, wheel rotation direction, laser scan feed rate, and so on.
Fig. 2. Experimental setup for coarse-grained diamond grinding wheel structuring.
In this experiment, the 1A1 type single layer electroplated diamond grinding wheels with 150 m grit size were used. The diameters of these grinding wheels are 85 mm, and the widths are 6 mm. A precision spindle (Dr. Kaiser C58F3) was used to rotate the grinding wheel under laser source. An UV nanosecond pulsed laser with 355 nm wavelength and 1–255 kHz pulse repetition frequency was selected as the laser source to machining micro-groove arrays on coarse-grained diamond wheels. The photo of the experimental setup for coarse-grained diamond grinding wheels structuring is shown in Fig. 2. Four micro-structured grinding wheels (No. 2–5) with different interval micro-groove arrays were machined by laser. The half peripheral area of No. 2 grinding wheel was structured with 70 m interval micro grooves. The intervals of No. 3–5 grinding wheel were 30, 90 and 150 m, respectively. The parameters of microstructuring are shown in Table 1. The photo of micro-structured wheels and SEM images of wheel morphology are shown in Fig. 3. On micro-structured surface, the continuous micro-grooves were obtained. The widths of grooves were 10–15 m. The protruding parts of most diamond grits were cut-through by one or two grooves. The broken diamond grit and falling off of grit was not found. Therefore, the coarse-grained diamond grinding wheels were micro-structured by UV nanosecond pulsed laser, successfully. 3. Grinding experimental setup and procedure
Fig. 1. Schematic view of laser micro-structuring (a) micro-structured coarsegrained diamond wheel (b) conventional coarse-grained diamond wheel.
The experiments to evaluate the effects of the micro-structured coarse-grained diamond wheels on the precision grinding of optical glass were conducted on a precision plane grinder from Hangzhou Machine Tool Group Co., Ltd., model “MUGK7120X5”. The workpiece material was optical glass BK7. The precision spindle used in laser machining process with micro-structured coarse-grained diamond wheel was fixed on this grinder as grinding spindle. The grinding parameters were 3000 r/min spindle speed, 2 m depth of grinding and 2 mm/min feed rate. Water-base emulsion was used as a coolant to improve the grinding process. The angle between the grinding feed direction and wheel axial direction was 45◦ , in order to reduce the ground residual height due to the parallel microgroove arrays of grinding wheel surface. The experimental setup and the kinematic illustration of the grinding process are shown in Fig. 4. Before grinding, the ELID assisted conditioning technique with metal-bond diamond truer was used to conditioning these coarsegrained diamond wheels. The truer was assembled on the main spindle of grinder. The grit size of truer is 90 m and the diameter
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Table 1 The parameters of micro-structuring. Laser energy density [J/cm2 ] Pulse repetition frequency [kHz] Focal shift [m] Laser dwell time [s] Wheel rotation speed [r/min] Laser scan feed step [m]
56 25 0 60 300 No. 1 Without micro-structures
No. 2 70 (half area)
No. 3 30
No. 4 90
Fig. 3. Photo of micro-structured wheels and morphology of micro-structured coarse-grained diamond grinding wheel.
Fig. 4. Grinding experimental setup and kinematic illustration of the grinding process.
is 250 mm. The spindle speed of grinding wheel was 3000 r/min. The conditioning parameters were 500 r/min truer spindle speed, 2–5 m depth and 2 mm/min feed rate. Fig. 5 shows the conditioning process.
4. Grinding experimental results and discusses 4.1. Conditioning of micro-structured coarse-grained diamond wheels For coarse-grained diamond wheel, the main purpose of pre-conditioning is to obtain the homogeneous grit protrusion in order to uniformize the grinding depth per grit. That is to say, the profile accuracy of contour circle which consists of wellconditioned diamond grit flat tops is the key for realization of precision grinding. However, in the run-out measure process of
Fig. 5. Conditioning process.
No. 5 150
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Fig. 6. Illustration of conditioned coarse-grained diamond wheel and the relation between grinding wheel run out and depth of cut.
Fig. 7. The morphology of conditioned grinding wheel.
coarse-grained diamond wheel, not only the diamond grits but also the bond surface between grits will contribute to the measured results. When the diamond grit tops have been truncated into a satisfactory contour circle, the radial run out of grinding wheel is mainly determined by the variation of grit protrusion height from plated bonding surface, as shown in Fig. 6. Therefore, the radial run out value of conditioned coarse-grained diamond wheel would not be limited in microns like the conventional fine-grained wheel, unless the coarse-grained diamond grits are totally truncated off along the lowest plating surface resulting in a minimized run-out error within 2 m, as presented by Brinksmeier et al. (2000) and Zhao et al. (2005). In this experiment, the average diamond grits protrusion height of original wheel was about 50 m (one-third of the grit size). In pre-conditioning process, the minimum grit remove height was about 30 m, thus the maximum residual protrusion height of conditioned grits from plating surface was 20 m. In order words, the coarse-grained diamond wheels with
radial run out of 20 m (measured by Keyence LK-G5000 laser displacement sensor) were generated to assure obtain homogeneous grit protrusion and reduce conditioning time. Although the radial run-out of 20 m is magnitude higher than the depth of cut, the parts which engage in grinding process are only the small top areas of diamond grits. Therefore these conditioned wheels with 20 m radial run-out which caused by the plating surface profile error are acceptable in this experiment, as shown in Fig. 6 enlarged view. The surface morphology of conditioned grinding wheels was examined using the SEM, as shown in Fig. 7. Most of diamond grits were involved in the truncating process regardless of whether the wheel was micro-structured. The SEM image of micro-structured wheels show a detail of well-conditioned diamond grits featuring clear micro grooves while being still strongly held by the electroplated bonding material. As compared with the flattened tops of non-structured coarse-grained diamond grits, more chipping off, fractures and cleavage were found on micro-structured diamond
Fig. 8. The SEM images of BK7 ground surfaces.
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Table 2 The experimental results of subsurface damage by different diamond wheels.
Sample 1 Sample 2 Sample 3 Average
Original wheel
Conditioned wheel without micro-structures
Structured wheel (150 m interval)
Structured wheel (90 m interval)
Structured wheel (70 m interval)
Structured wheel (30 m interval)
7.1 m 12.5 m 11.7 m 10.4 m
5.3 m 3.4 m 6.1 m 4.9 m
2.5 m 2.5 m 3.5 m 2.8 m
2.2 m 2.1 m 2.7 m 2.3 m
2.3 m 1.7 m 2.3 m 2.1 m
1.1 m 0.9 m 1.3 m 1.1 m
grits, which resulting in more cutting edges would generate on grinding wheel. 4.2. The influence of micro-structures of wheel surface on ground surface quality The morphology of ground surfaces by different grinding wheels is shown in Fig. 8. It can be found that the ground surface by conventional coarse-grained diamond wheel was smoother than that by micro-structured wheel with 150 m interval micro-groove arrays. Moreover, when the interval of micro grooves was decreased to 30 m, the grinding quality was improved. The improved ground surface was as smooth as that by conventional coarse-grained diamond wheel, as shown in Fig. 8(c). In order to further evaluate the influence of the micro grooves interval on ground surface quality, the surface roughness Ra values were measured parallel to and vertical to the feed direction, respectively, by means of a contact probe profilometer (Talysurf PGI 1240), as shown in Fig. 9. The results showed the surface roughness Ra was improved when the interval decreased. This can be due to the increasing of active cutting edges, when smaller interval grooves array was formed on diamond wheel. In other words, the chip thickness becomes relatively smaller when the grooves interval was decreased. However, it is clear indicated that the better surface quality was not obtained by micro-structured coarse-grained diamond wheel compared with non-structured coarse-grained diamond wheel in this experiment. One possible reason for higher roughness generated by micro-structured wheel attributes to the modified profile of conditioned diamond grit tops. According to the experimental results of conditioning, most of the diamond grit tops’ profiles exhibited definite relationship with having micro grooves or not, as shown in Fig. 7. As compared with the flattened tops of non-structured coarse-grained diamond grits, more chipping off, fractures and cleavage were found on micro-structured diamond grits. In other words, the flattened diamond grit tops of conditioned coarse-grained were replaced with the jagged grit tops through laser induced micro-grooves. Subsequently, the profiles of jagged grit tops were replicated on the ground surface to form the more
Fig. 9. The influence of micro grooves interval on surface roughness.
textured profile in grinding operation. Therefore the higher roughness was obtained by micro-structured wheels. Furthermore, the number of micro grooves cut through grit (determined by the interval of the micro grooves) seems having not obviously effect on the profile of diamond jagged grit tops, as shown in Fig. 10. Thus, for the micro-structured wheels with different interval of grooves, the variation of roughness was mainly effected by varied uncut chip thickness. 4.3. The influence of micro-structures of wheel surface on subsurface damage The influence of micro-structured surface on subsurface damage was investigated by angle polishing methods (Zhao et al., 2007). The polished surfaces were etched 3 min by NH4 HF2 solution in order to expose the subsurface cracks. For each kind of grinding wheel, three samples were ground and measured. The measured results and their averaged values of subsurface crack depth corresponding
Fig. 10. The micro-structured diamond grits with different number of micro grooves.
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Fig. 11. The SEM images of BK7 ground subsurface damage (Sample 1).
to all three samples are provided in Table 2. And the SEM images of Sample 1 are shown in Fig. 11 for revealing the detail of subsurface damage. The subsurface damage depth was improved form about 10 m (by original coarse-grained wheel, as shown in Fig. 11(a)) to 5 m because of the uniform grit protrusion height due to preconditioning. The subsurface damage depth by micro-structured wheel with 150 m interval groove arrays was further decreased to about 3 m. Compared with the conventional coarse-grained diamond wheel, the subsurface damage depth was reduced effectually when using the micro-structured coarse-grained diamond wheel. The effect of micro grooves interval on subsurface damage could be observed from Fig. 11(c)–(f). The subsurface damage depth was reduced with the decreasing interval. At the interval of 30 m, the subsurface damage depth of 1.5 m was obtained. This is mainly caused by the increase of active cutting edges and hence decreased individual chip load when using smaller interval.
5. Conclusions A novel micro-structured coarse-grained diamond wheel was invented for precision grinding of optical glass in order to improve the grinding performance, especially subsurface damage. The strategy of coarse-grained diamond wheels micro-structuring based on laser machining was presented. And the effect of micro-structures topography on the optical glass ground quality was investigated. The main conclusions of this research are: (1) The coarse-grained diamond grinding wheels were microstructured by UV nanosecond pulsed laser, successfully. Continuous micro-groove arrays with 10–15 m width were obtained on the peripheral surface of grinding wheel. The protruding parts of most diamond grits were cut-through by micro grooves. The broken diamond grit and falling off of grit was not found during laser machining.
(2) Although the better surface quality would not be obtained by micro-structured coarse-grained diamond wheel compare with conventional coarse-grained diamond wheel, the subsurface damage depth could be reduced effectually when using the micro-structured coarse-grained diamond wheel. The surface roughness and subsurface damage depth were both reduced with the decreasing interval. For future research, the micro-structured coarse-grained diamond grinding wheels, which will be conditioned by ELID before laser machining will be investigated. Acknowledgements The authors would like to thank the Joint Lab for Ultra-Precision Grinding Technology (JLUPG) of Harbin Institute of Technology and Hangzhou Machine Tool Group Co., Ltd. This work was supported by National Natural Scientific Foundation of China (grant number 51075093) and China Postdoctoral Science Foundation funded project (2013M541361). References Aurich, J.C., Braun, O., Wamecke, G., 2003. Development of a superabrasive grinding wheel with defined grain structure using kinematic simulation. CIRP Annals – Manufacturing Technology 52 (1), 275–280. Axinte, D., Butler-Smith, P., Akgun, C., Kolluru, K., 2013. On the influence of single grit micro-geometry on grinding behavior of ductile and brittle materials. International Journal of Machine Tools and Manufacture 74, 12–18. Brinksmeier, E., Malz, R., Preuss, W., 2000. Investigation of a novel tool concept for ductile grinding of optical glass. In: Proceedings of ASPE 2000 Annual Meeting, Scottsdale, US, pp. 74–77. Butler-Smith, P.W., Axinte, D.A., Pacella, M., Fay, M.W., 2013. Micro nanometric investigations of the effects of laser ablation in the generation of micro-tools from solid CVD diamond structures. Journal of Materials Processing Technology 213, 194–200. Guo, B., Zhao, Q., Zhang, W., 2013. Optical glass grinding with laser structured coarse-grained diamond wheels. In: Proceedings of EUSPEN, Berlin, Germany, pp. 31–34.
B. Guo et al. / Journal of Materials Processing Technology 214 (2014) 1045–1051 Heinzel, C., Rickens, K., 2009. Engineered wheels for grinding of optical glass. CIRP Annals – Manufacturing Technology 58 (1), 315–318. Koshy, P., Iwasaki, A., Elbestawi, M.A., 2003. Surface generation with engineered diamond grinding wheels: insights from simulation. Annals of the CIRP 52 (1), 271–274. Zhao, Q., Brinksmeier, E., Riemer, O., Dong, S., 2005. Ultraprecision grinding of optical glass using super abrasive diamond wheel. In: Proceedings of ASPEN, Shenzhen, China, pp. 710–714.
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Zhao, Q., Liang, Y., Stephenson, D., Corbett, J., 2007. Surface and subsurface integrity in diamond grinding of optical glasses on Tetraform ‘C’. International Journal of Machine Tools and Manufacture 47 (14), 2091–2097. Zhao, Q., Chen, J., Yao, J., 2011. Grinding damage of BK7 using copper-resin bond coarse-grained diamond wheel. International Journal of Precision Engineering and Manufacturing 12 (1), 5–13.
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Precision grinding of optical glass with laser micro-structured coarse-grained diamond wheels Bing Guo a,∗ , Qingliang Zhao a,1 , Xiaoyan Fang b,2 a b
Center for Precision Engineering, School of Mechatronics Engineering, Harbin Institute of Technology, Harbin 150001, China Shanghai Machine Tool Works Co., Ltd., Shanghai 200093, China
a r t i c l e
i n f o
Article history: Received 18 September 2013 Received in revised form 12 December 2013 Accepted 19 December 2013 Available online 28 December 2013 Keywords: Precision grinding Coarse-grained diamond wheel Micro-structuring Laser machining Optical glass Subsurface damage
a b s t r a c t This paper presents a series of micro-structured coarse-grained diamond wheels for optical glass surface grinding aiming to improve the grinding performance, especially subsurface damage. The 150 m grit size, single layer electroplated diamond grinding wheels with different interval micro-groove arrays were manufactured by nanosecond pulsed laser, successfully. The influence of micro-structures on surface roughness and subsurface damage was investigated. Compared with conventional coarse-grained diamond wheel, the subsurface damage depth was reduced effectually from 5 to 1.5 m, although the better surface roughness was not obtained by the micro-structured coarse-grained diamond wheel. In addition, the surface roughness and subsurface damage depth were both reduced with the decreasing interval of micro-groove arrays. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Precision grinding of optical glasses has extended its applications in aerospace, automotive, semiconductor and communication industry. With the increasing of demand for high accuracy optical glass products, the precision grinding process not only need to meet the requirements of surface roughness in nanometer scale, form deviations in sub-micron level, and subsurface damage depth within micrometer, but also be required to have a high machining efficiency. Fine-grained (grit sizes from several micron down to sub-micron scale) diamond wheels are usually used for finishing of optical glasses with a low surface roughness. However, the wheel loading, high wear rate and periodic conditioning requirement will deteriorate the form accuracy of workpiece at a lower grinding efficiency. In order to solve the wheel wear problem existed in fine-grained diamond wheels, electroplated nickel mono-layer coarse-grained diamond wheels featuring grit size of 91 m was alternatively
∗ Corresponding author at: P.O. Box 413, Harbin Institute of Technology, Harbin 150001, China. Tel.: +86 451 86402683; fax: +86 451 86415244. E-mail addresses: [email protected] (B. Guo), [email protected] (Q. Zhao). 1 P.O. Box 413, Harbin Institute of Technology, Harbin 150001, China. 2 Shanghai Machine Tool Works Co., Ltd., 1146 Jun Gong Road, Shanghai 200093, China. 0924-0136/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2013.12.013
proposed by Brinksmeier et al. (2000) in precision grinding of optical glasses. Koshy et al. (2003) pointed out that the realization of precision grinding with coarse-grained diamond wheels is determined by the distribution of abrasive protrusion height. In order to obtain the coarse-grained diamond wheels with homogeneous grit protrusion, Aurich et al. (2003) optimized the grit pattern of coarse-grained electroplated grinding wheel assisted by kinematic simulation. Zhao et al. (2005) introduced a conditioning method for coarse-grained electroplated diamond wheels using a diamond cup dressing wheel and ELID technique. The applicability of the conditioned coarse-grained grinding wheels was demonstrated by grinding experiments on BK7 glass yielding a surface roughness in the nanometer range. Subsequently, Zhao et al. (2011) tried to apply copper-resin hybrid bonded coarse-grained diamond wheels in ductile grinding of optical glass. The results showed that nanometer scaled surface roughness and higher form accuracy can be obtained at a low wheel wear rate. The above mentioned investigations indicated that, compared with the fine-grained diamond wheels, precision conditioned coarse-grained diamond wheels could achieve the same nanometer scale roughness, higher form accuracy with a longer wheel life. However, the flat tops of diamond grits created in preconditioning process will lead to greater specific grinding normal force as presented by Heinzel and Rickens (2009). Guo et al. (2013) indicated that a serious subsurface damage will be introduced by coarse-grained diamond wheel in optical glass grinding process.
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It has been shown that the micro-structured surface of diamond solid would reduce the normal grinding force as compared with traditional electroplate grinding block provided by Butler-Smith et al. (2013). This is associated with the stability of active cutting elements and the integrity of micro-cutting edges composed by number of micro-structures. Besides, Axinte et al. (2013) indicated that the increased number of cutting edges would also leads to a decrease of specific cutting force in grinding process, which can be obtained by micro-structuring of flat top diamond grits. Therefore, micro-structured diamond surface seems to have the ability to improve grinding performance of coarse-grained diamond wheels and reduce the specific grinding normal force. Base on the above, this paper presented a kind of microstructured coarse-grained diamond wheels with micro-groove arrays for optical glass surface grinding aiming to improve the grinding performance, especially subsurface damage. Firstly, the strategy for micro-structuring of coarse-grained diamond wheels was provided. Some micro-structured coarse-grained diamond wheels with different interval parallel micro-groove arrays were manufactured by nanosecond pulsed laser. The morphology of the micro-structured wheels was analyzed by SEM. And then these wheels were properly conditioned by a metal bond diamond dressing wheel with ELID method. Finally, the micro-structured coarse-grained diamond wheels were used in optical glass grinding experiments. The ground surface quality and subsurface damage were characterized by profilometer and SEM, respectively. The influence of micro-structures of wheel surface on surface roughness and subsurface damage depth were investigated. 2. Surface micro-structuring of coarse-grained diamond wheel by laser Fig. 1 shows the schematic of the micro-structuring operation on coarse-grained diamond wheels’ cylindrical surface by laser. The micro-structures composed by parallel micro-groove arrays with various intervals on the diamond wheel can be created by individual setting of the laser scan feed step, showed in this figure. The kinematic parameters of wheel rotation speed and dwell time per laser scan feed step are designed to ensure the whole wheel surface could be micro-structuring. Furthermore, more complex microstructures can also be machined on wheel surface by adjustment of more parameters such as the laser scan feed direction, wheel rotation direction, laser scan feed rate, and so on.
Fig. 2. Experimental setup for coarse-grained diamond grinding wheel structuring.
In this experiment, the 1A1 type single layer electroplated diamond grinding wheels with 150 m grit size were used. The diameters of these grinding wheels are 85 mm, and the widths are 6 mm. A precision spindle (Dr. Kaiser C58F3) was used to rotate the grinding wheel under laser source. An UV nanosecond pulsed laser with 355 nm wavelength and 1–255 kHz pulse repetition frequency was selected as the laser source to machining micro-groove arrays on coarse-grained diamond wheels. The photo of the experimental setup for coarse-grained diamond grinding wheels structuring is shown in Fig. 2. Four micro-structured grinding wheels (No. 2–5) with different interval micro-groove arrays were machined by laser. The half peripheral area of No. 2 grinding wheel was structured with 70 m interval micro grooves. The intervals of No. 3–5 grinding wheel were 30, 90 and 150 m, respectively. The parameters of microstructuring are shown in Table 1. The photo of micro-structured wheels and SEM images of wheel morphology are shown in Fig. 3. On micro-structured surface, the continuous micro-grooves were obtained. The widths of grooves were 10–15 m. The protruding parts of most diamond grits were cut-through by one or two grooves. The broken diamond grit and falling off of grit was not found. Therefore, the coarse-grained diamond grinding wheels were micro-structured by UV nanosecond pulsed laser, successfully. 3. Grinding experimental setup and procedure
Fig. 1. Schematic view of laser micro-structuring (a) micro-structured coarsegrained diamond wheel (b) conventional coarse-grained diamond wheel.
The experiments to evaluate the effects of the micro-structured coarse-grained diamond wheels on the precision grinding of optical glass were conducted on a precision plane grinder from Hangzhou Machine Tool Group Co., Ltd., model “MUGK7120X5”. The workpiece material was optical glass BK7. The precision spindle used in laser machining process with micro-structured coarse-grained diamond wheel was fixed on this grinder as grinding spindle. The grinding parameters were 3000 r/min spindle speed, 2 m depth of grinding and 2 mm/min feed rate. Water-base emulsion was used as a coolant to improve the grinding process. The angle between the grinding feed direction and wheel axial direction was 45◦ , in order to reduce the ground residual height due to the parallel microgroove arrays of grinding wheel surface. The experimental setup and the kinematic illustration of the grinding process are shown in Fig. 4. Before grinding, the ELID assisted conditioning technique with metal-bond diamond truer was used to conditioning these coarsegrained diamond wheels. The truer was assembled on the main spindle of grinder. The grit size of truer is 90 m and the diameter
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Table 1 The parameters of micro-structuring. Laser energy density [J/cm2 ] Pulse repetition frequency [kHz] Focal shift [m] Laser dwell time [s] Wheel rotation speed [r/min] Laser scan feed step [m]
56 25 0 60 300 No. 1 Without micro-structures
No. 2 70 (half area)
No. 3 30
No. 4 90
Fig. 3. Photo of micro-structured wheels and morphology of micro-structured coarse-grained diamond grinding wheel.
Fig. 4. Grinding experimental setup and kinematic illustration of the grinding process.
is 250 mm. The spindle speed of grinding wheel was 3000 r/min. The conditioning parameters were 500 r/min truer spindle speed, 2–5 m depth and 2 mm/min feed rate. Fig. 5 shows the conditioning process.
4. Grinding experimental results and discusses 4.1. Conditioning of micro-structured coarse-grained diamond wheels For coarse-grained diamond wheel, the main purpose of pre-conditioning is to obtain the homogeneous grit protrusion in order to uniformize the grinding depth per grit. That is to say, the profile accuracy of contour circle which consists of wellconditioned diamond grit flat tops is the key for realization of precision grinding. However, in the run-out measure process of
Fig. 5. Conditioning process.
No. 5 150
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Fig. 6. Illustration of conditioned coarse-grained diamond wheel and the relation between grinding wheel run out and depth of cut.
Fig. 7. The morphology of conditioned grinding wheel.
coarse-grained diamond wheel, not only the diamond grits but also the bond surface between grits will contribute to the measured results. When the diamond grit tops have been truncated into a satisfactory contour circle, the radial run out of grinding wheel is mainly determined by the variation of grit protrusion height from plated bonding surface, as shown in Fig. 6. Therefore, the radial run out value of conditioned coarse-grained diamond wheel would not be limited in microns like the conventional fine-grained wheel, unless the coarse-grained diamond grits are totally truncated off along the lowest plating surface resulting in a minimized run-out error within 2 m, as presented by Brinksmeier et al. (2000) and Zhao et al. (2005). In this experiment, the average diamond grits protrusion height of original wheel was about 50 m (one-third of the grit size). In pre-conditioning process, the minimum grit remove height was about 30 m, thus the maximum residual protrusion height of conditioned grits from plating surface was 20 m. In order words, the coarse-grained diamond wheels with
radial run out of 20 m (measured by Keyence LK-G5000 laser displacement sensor) were generated to assure obtain homogeneous grit protrusion and reduce conditioning time. Although the radial run-out of 20 m is magnitude higher than the depth of cut, the parts which engage in grinding process are only the small top areas of diamond grits. Therefore these conditioned wheels with 20 m radial run-out which caused by the plating surface profile error are acceptable in this experiment, as shown in Fig. 6 enlarged view. The surface morphology of conditioned grinding wheels was examined using the SEM, as shown in Fig. 7. Most of diamond grits were involved in the truncating process regardless of whether the wheel was micro-structured. The SEM image of micro-structured wheels show a detail of well-conditioned diamond grits featuring clear micro grooves while being still strongly held by the electroplated bonding material. As compared with the flattened tops of non-structured coarse-grained diamond grits, more chipping off, fractures and cleavage were found on micro-structured diamond
Fig. 8. The SEM images of BK7 ground surfaces.
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Table 2 The experimental results of subsurface damage by different diamond wheels.
Sample 1 Sample 2 Sample 3 Average
Original wheel
Conditioned wheel without micro-structures
Structured wheel (150 m interval)
Structured wheel (90 m interval)
Structured wheel (70 m interval)
Structured wheel (30 m interval)
7.1 m 12.5 m 11.7 m 10.4 m
5.3 m 3.4 m 6.1 m 4.9 m
2.5 m 2.5 m 3.5 m 2.8 m
2.2 m 2.1 m 2.7 m 2.3 m
2.3 m 1.7 m 2.3 m 2.1 m
1.1 m 0.9 m 1.3 m 1.1 m
grits, which resulting in more cutting edges would generate on grinding wheel. 4.2. The influence of micro-structures of wheel surface on ground surface quality The morphology of ground surfaces by different grinding wheels is shown in Fig. 8. It can be found that the ground surface by conventional coarse-grained diamond wheel was smoother than that by micro-structured wheel with 150 m interval micro-groove arrays. Moreover, when the interval of micro grooves was decreased to 30 m, the grinding quality was improved. The improved ground surface was as smooth as that by conventional coarse-grained diamond wheel, as shown in Fig. 8(c). In order to further evaluate the influence of the micro grooves interval on ground surface quality, the surface roughness Ra values were measured parallel to and vertical to the feed direction, respectively, by means of a contact probe profilometer (Talysurf PGI 1240), as shown in Fig. 9. The results showed the surface roughness Ra was improved when the interval decreased. This can be due to the increasing of active cutting edges, when smaller interval grooves array was formed on diamond wheel. In other words, the chip thickness becomes relatively smaller when the grooves interval was decreased. However, it is clear indicated that the better surface quality was not obtained by micro-structured coarse-grained diamond wheel compared with non-structured coarse-grained diamond wheel in this experiment. One possible reason for higher roughness generated by micro-structured wheel attributes to the modified profile of conditioned diamond grit tops. According to the experimental results of conditioning, most of the diamond grit tops’ profiles exhibited definite relationship with having micro grooves or not, as shown in Fig. 7. As compared with the flattened tops of non-structured coarse-grained diamond grits, more chipping off, fractures and cleavage were found on micro-structured diamond grits. In other words, the flattened diamond grit tops of conditioned coarse-grained were replaced with the jagged grit tops through laser induced micro-grooves. Subsequently, the profiles of jagged grit tops were replicated on the ground surface to form the more
Fig. 9. The influence of micro grooves interval on surface roughness.
textured profile in grinding operation. Therefore the higher roughness was obtained by micro-structured wheels. Furthermore, the number of micro grooves cut through grit (determined by the interval of the micro grooves) seems having not obviously effect on the profile of diamond jagged grit tops, as shown in Fig. 10. Thus, for the micro-structured wheels with different interval of grooves, the variation of roughness was mainly effected by varied uncut chip thickness. 4.3. The influence of micro-structures of wheel surface on subsurface damage The influence of micro-structured surface on subsurface damage was investigated by angle polishing methods (Zhao et al., 2007). The polished surfaces were etched 3 min by NH4 HF2 solution in order to expose the subsurface cracks. For each kind of grinding wheel, three samples were ground and measured. The measured results and their averaged values of subsurface crack depth corresponding
Fig. 10. The micro-structured diamond grits with different number of micro grooves.
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Fig. 11. The SEM images of BK7 ground subsurface damage (Sample 1).
to all three samples are provided in Table 2. And the SEM images of Sample 1 are shown in Fig. 11 for revealing the detail of subsurface damage. The subsurface damage depth was improved form about 10 m (by original coarse-grained wheel, as shown in Fig. 11(a)) to 5 m because of the uniform grit protrusion height due to preconditioning. The subsurface damage depth by micro-structured wheel with 150 m interval groove arrays was further decreased to about 3 m. Compared with the conventional coarse-grained diamond wheel, the subsurface damage depth was reduced effectually when using the micro-structured coarse-grained diamond wheel. The effect of micro grooves interval on subsurface damage could be observed from Fig. 11(c)–(f). The subsurface damage depth was reduced with the decreasing interval. At the interval of 30 m, the subsurface damage depth of 1.5 m was obtained. This is mainly caused by the increase of active cutting edges and hence decreased individual chip load when using smaller interval.
5. Conclusions A novel micro-structured coarse-grained diamond wheel was invented for precision grinding of optical glass in order to improve the grinding performance, especially subsurface damage. The strategy of coarse-grained diamond wheels micro-structuring based on laser machining was presented. And the effect of micro-structures topography on the optical glass ground quality was investigated. The main conclusions of this research are: (1) The coarse-grained diamond grinding wheels were microstructured by UV nanosecond pulsed laser, successfully. Continuous micro-groove arrays with 10–15 m width were obtained on the peripheral surface of grinding wheel. The protruding parts of most diamond grits were cut-through by micro grooves. The broken diamond grit and falling off of grit was not found during laser machining.
(2) Although the better surface quality would not be obtained by micro-structured coarse-grained diamond wheel compare with conventional coarse-grained diamond wheel, the subsurface damage depth could be reduced effectually when using the micro-structured coarse-grained diamond wheel. The surface roughness and subsurface damage depth were both reduced with the decreasing interval. For future research, the micro-structured coarse-grained diamond grinding wheels, which will be conditioned by ELID before laser machining will be investigated. Acknowledgements The authors would like to thank the Joint Lab for Ultra-Precision Grinding Technology (JLUPG) of Harbin Institute of Technology and Hangzhou Machine Tool Group Co., Ltd. This work was supported by National Natural Scientific Foundation of China (grant number 51075093) and China Postdoctoral Science Foundation funded project (2013M541361). References Aurich, J.C., Braun, O., Wamecke, G., 2003. Development of a superabrasive grinding wheel with defined grain structure using kinematic simulation. CIRP Annals – Manufacturing Technology 52 (1), 275–280. Axinte, D., Butler-Smith, P., Akgun, C., Kolluru, K., 2013. On the influence of single grit micro-geometry on grinding behavior of ductile and brittle materials. International Journal of Machine Tools and Manufacture 74, 12–18. Brinksmeier, E., Malz, R., Preuss, W., 2000. Investigation of a novel tool concept for ductile grinding of optical glass. In: Proceedings of ASPE 2000 Annual Meeting, Scottsdale, US, pp. 74–77. Butler-Smith, P.W., Axinte, D.A., Pacella, M., Fay, M.W., 2013. Micro nanometric investigations of the effects of laser ablation in the generation of micro-tools from solid CVD diamond structures. Journal of Materials Processing Technology 213, 194–200. Guo, B., Zhao, Q., Zhang, W., 2013. Optical glass grinding with laser structured coarse-grained diamond wheels. In: Proceedings of EUSPEN, Berlin, Germany, pp. 31–34.
B. Guo et al. / Journal of Materials Processing Technology 214 (2014) 1045–1051 Heinzel, C., Rickens, K., 2009. Engineered wheels for grinding of optical glass. CIRP Annals – Manufacturing Technology 58 (1), 315–318. Koshy, P., Iwasaki, A., Elbestawi, M.A., 2003. Surface generation with engineered diamond grinding wheels: insights from simulation. Annals of the CIRP 52 (1), 271–274. Zhao, Q., Brinksmeier, E., Riemer, O., Dong, S., 2005. Ultraprecision grinding of optical glass using super abrasive diamond wheel. In: Proceedings of ASPEN, Shenzhen, China, pp. 710–714.
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