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Advances in Mechanical Engineering
Volume 2013 (2013), Article ID 768104, 7 pages
Vertical Spindle Grinding of Si and Granite with a New Abrasive Disk
The Ministry of Education Research Center for Machining of Brittle Materials, Huaqiao University, Xiamen 361021, China
Received 27 July 2013; Accepted 17 October 2013
Academic Editor: Gang Wang
Copyright © 2013 Yiqing Yu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
An investigation was reported of a new attempt in the fabrication of an ultrafine abrasive tool for vertical spindle grinding. The principle of sol-gel was applied to granulate ultra-fine abrasives in order to reduce their aggregation. The granulated abrasives were then added to resin bonds, thereby forming ultra-fine abrasive grinding disks. Before grinding, the disks were dressed to expected flatness using a brazed diamond pad. The dressed grinding disks were then used to grind silicon wafers and natural granite. Both dressing and grinding were conducted on a high precision vertical spindle grinding machine. After grinding, the morphologies of the workpiece materials were examined. With regard to the different concerns for silicon wafers and granite, surface roughness was measured for the silicon wafers and gloss readings were measured for the granite surfaces. It was found that the brazed diamond abrasives could dress the grinding disks with high efficiency and satisfactory flatness. The new ultra-fine abrasive disks were found to be able to process silicon wafers and granite slabs to acceptable results.
Grinding with ultrafine abrasive tools is one of the most widely used processes for precision machining of many materials such as ceramics, glass, natural stone materials, and novel materials used in the semiconductor industry [1–4]. In the production of ultrafine abrasive tools for grinding, resin and copper are two common bonds, in which case ultrafine abrasives are supposed to distribute uniformly in either resin or copper. However, ultrafine abrasives tend to agglomerate with the decrease of grit sizes due to the increase of abrasive surface energies . Accordingly, it is almost impossible to let ultrafine abrasives distribute uniformly in the bonds only by traditional powder metallurgical techniques. As a result of random distribution of ultrafine abrasives in the bonds, it is quite difficult to control the precision and surface quality of components during grinding with ultrafine abrasives.
Therefore, tremendous studies have been done during the past decades in order to improve the dispersing of ultrafine abrasives in resin or metal bonds. Nakamura et al. proposed a new method to fabricate ultrafine abrasive tools using the principle of sol-gel . Ikeno et al. developed an ultrafine silica abrasive tool by applying an electrophoretic deposition technology . In our previous study, the sol-gel technique was found to be effective in granulating ultrafine abrasives of diameter greater than 5 μm .
As an attempt to fabricate ultrafine abrasive tools for grinding, the present work was undertaken to combine the granulating ability of sol-gel with the bonding ability of resin. The main purpose of this work is to elucidate the possibility of combining the dispersing of sol-gel granulation and advantages of resin as a bonding matrix. It is hoped that the results obtained in this work will be of benefit to the development of high-performance ultrafine abrasive tools for high precision and cost-effective grinding.
2. Fabrication of a New Grinding Disk
In order to reduce the aggregation of ultrafine abrasives in the fabrication of resin-bonded grinding disks, the first step in the present work was to granulate ultrafine abrasives based on the principle of sol-gel. The setup for producing gel spheres containing ultrafine abrasives is illustrated in Figure 1. According to the principle of chemistry, the sodium salt solution of alginate can react with the metallic ion of Ca2+ and form gels. In the present work, ultrafine abrasives were added to the sodium alginate solution, indicated as solution 1 in Figure 1. It is well known that it is much easier to make ultrafine abrasives disperse uniformly in a liquid than in such powder bonds as resin and metal. Then, the sodium alginate solutions containing uniformly dispersed abrasives were dropped into the calcium solution, indicated as solution 2 in Figure 1, to produce gel spheres. The nozzle was carefully chosen to make gel spheres with average diameter of 3.5 mm. The concentration of calcium solution was determined through many tests and finally fixed at 2.0 wt%, whereas the sodium salt solution of alginate was 3.0 wt%.
After granulation, the spheres containing ultrafine abrasives were dried to small balls, as shown in Figure 1. These gel-bonded (granulated) ultrafine abrasives were then combined together by resin to form a grinding disk of 320 mm in diameter. To make the grinding disk, the resin was heated up to 80°C and the gel-bonded abrasives were stirred uniformly in the molten resin. Figure 2 shows the procedure to make the grinding disk in the present work.
3. Dressing of the Grinding Disk
In order to make the grinding disk flat enough to carry out grinding, an experimental setup (see Figure 3) was proposed to dress the grinding disk fabricated above. Based on our previous study , in which case dressing with loose abrasives, bonded Al2O3 wheel and brazed diamonds were compared and the brazed diamonds were found to be most effective; a brazed diamond pad was used here and the diamond abrasives of 70/80 US mesh were applied (see Figure 4). The grinding disk and dressing pad were installed on a vertical spindle grinding machine to facilitate dressing. The rotating speeds for the grinding disk and the dressing pad were 250 rpm and 50 rpm, respectively.
A micrometer was used to detect the unevenness of the grinding disk, whereas a Hirox KH-1000 optical microscope attached to a digital video system was used to observe the morphologies of the dressed grinding disk.
The unevenness values of the grinding disk before and after dressing are compared in Figure 5. It should be noted that “outer,” “middle,” and “inner” here are corresponding to three different positions in diameter 277.5 mm, 235 mm, and 192.5 mm, respectively. For the position at a specified diameter, measurements were carried out for 20 times at an equal interval of 18 degrees along the circumference, thereby leading to 20 numbers for each curve in Figure 5. The morphologies of the grinding disk before and after dressing are shown in Figure 6. Both Figures 5 and 6 indicate that the dressing is effective in obviously reducing the unevenness of the grinding disk.
4. Grinding of Silicon Wafer and Granite with the Dressed Disk
In order to evaluate the grinding performance of the grinding disk, silicon wafer and granite were chosen as the workpiece materials. The diameter of the silicon wafer was 76 mm with its original surface roughness of 0.54 μm (Ra). Two typical granites, red one and black one, were used, in which case the red one is much harder than the black according to the factory records. The values of shore hardness for the red and black granites are 95 and 76, respectively.
Ultrafine Al2O3 abrasives of 5 μm were used for fabricating the grinding disks. For grinding of silicon wafers, Al2O3 abrasives of 20 μm and 10 μm were also used to make a comparison.
The experimental setup for the grinding of both silicon wafer and granite is shown in Figure 7. The rotating speed of the silicon wafer in grinding was 250 rpm and the grinding disk rotated at 200 rpm with a pressure of 13.7 × 104 Pa. The grinding time was set to 30 min.
For the grinding of granite, the speed of workpiece was 250 rpm and the grinding disk rotated at 100 rpm with a pressure of 13.7 × 104 Pa.
The morphologies of ground silicon and granite were checked by the Hirox KH-1000 optical microscope. In addition to microobservations, the morphological features of the ground silicon wafer were quantitatively evaluated in terms of surface roughness using a Mahr Perthometer M1 profiler. For roughness values (Ra), a traverse length of 4 mm, with the standard 0.8 mm cut-off, was chosen. During measuring, 20 points were taken randomly on the ground surface of the workpiece in order to reduce the influence of individual abnormal data, and then, the mean values were calculated.
Since the glossiness is one of the most important quality criteria to evaluate the grinding of granite, a WGG60-S digital gloss meter was used to measure the gloss readings of ground granite surfaces.
The morphologies of silicon wafer after grinding were compared with three abrasive sizes in Figure 8. It can be seen that the silicon wafer becomes smoother as the size of abrasives decrease and the surface scratches (see Figure 8(a)) were basically removed after grinding with ultrafine abrasive of 5 μm (see Figure 8(c)). This was also supported by the results of surface roughness, as shown in Figure 9. It needs to note that the Ra is as low as 15 nm for the grinding with 5 μm abrasives, which might be comparable to the results reported by some published studies on the grinding of silicon wafers with fixed abrasives.
The morphologies of the red granite after grinding are shown in Figure 10 and the changes of gloss readings at different grinding stages are plotted in Figure 11. The gloss readings for the black granite are also given in Figure 11, which will be addressed later on. It can be seen that the granite surface becomes smoother with the increasing of grinding time, and accordingly, the gloss readings increase with the grinding time. In our previous studies , gloss readings were found to increase with the increasing surface roughness in an exponential curve for the grinding of granite. This can be used to account for the relationship between Figures 10 and 11.
Similar results can also be seen for the grinding of the black granite (see Figures 11 and 12). However, only half of the time was used to reach same gloss readings on the black granite as compared to the red one, indicating that the cutting of black granite by Al2O3 is easier than the cutting of the red one. Moreover, higher gloss readings were achieved on the black granite, which is in agreement with factory records. By the way, the gloss readings for both granites mentioned here are comparable to those by traditional processing methods, which deserves further comparison in future studies.
The combination of sol-gel granulation and resin bonding was found to be possible in manufacturing ultrafine abrasive tools for fine grinding of silicon wafer and granite. In order to facilitate effective grinding, the grinding disks need to be dressed before grinding and brazed diamond disks can be used to dress the grinding disks to expected flatness. The grinding with abrasives of 5 μm can obviously remove the scratches on silicon wafer, thereby leading to a smooth surface of 15 nm (Ra).
For granite grinding, the rapid increase of gloss readings with the grinding time indicated the effectiveness of grinding process in surface smoothing. But the long time to achieve acceptable gloss readings on the granite surfaces indicates the low efficiency of the grinding disk in material removal, which might be due to the weak ability of Al2O3 in cutting hard brittle materials such as granite.
Future work might be planned with the incorporation of ultrafine diamond abrasives in the manufacture of the grinding disks in order to enhance the grinding efficiency. Other efforts will focus on the application of the gel-resin coupled disks in the grinding of more kinds of materials.
Conflict of Interests
All authors declare that there is no conflict of interests regarding the publication of this paper.
This research was supported by Grant nos. 51075159 and U1034006 from the National Natural Science Foundation of China. Thanks are also given to the financial support (IRT1063) from the Changjiang Scholars and Innovative Research Team in University, The Ministry of Education, China. The research was also supported by Program for Innovative Research Team in Science and Technology in Fujian Province University (Program IRTSTFJ), China.
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