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Study on micro-grinding quality in micro-grinding tool for single crystal silicon
Journal of Manufacturing Processes 42 (2019) 246–256
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Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Study on micro-grinding quality in micro-grinding tool for single crystal silicon
T
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Yinghui Ren , Chenfang Li, Wei Li, Maojun Li, Hui Liu College of Mechanical and Vehicle Engineering, Hunan University, Changsha, 410082, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Micro-grinding Micro-grinding tools Single crystal silicon
Micro-grinding has a wide application for machining meso/micro parts using hard and brittle materials with high precision. In order to investigate the effect of grinding parameters and various micro-grinding tools on micro-grinding quality, experimental trials on side grinding of single crystal silicon (100 lattice orientation) were carried out using electroplated diamond micro-grinding tools. Based on experimental results, the effects of grinding parameters involving spindle speed, grinding depth and feed rate, together with various micro-grinding tools including different grain size and tip diameter on micro-grinding quality were discussed. The microgrinding quality was mainly evaluated in terms of surface roughness and the average edge-chipping width. The wear condition of micro-grinding tools under different tip diameter and grain size was analyzed. In addition, the effect of tool wear on micro-grinding quality was also discussed. From the experimental results, feed rate showed significant effect on micro-grinding quality. The wear of micro-grinding tools could be divided into initially fast wear stage and steady wear stage, which was crucial to influence the grinding quality.
1. Introduction
since the grinding thickness is usually less than grain size of the workpiece, the grinding grains cut through the grain of the workpiece or grain boundary [8]. The size effect has an obvious impact on the micro-grinding quality and the service life of micro-grinding tools. The grinding force must be higher than the atomic binding force inside the grains to remove material and form chips. In order to efficiently and accurately fabricate micro-structure, the fabrication technology of micro-grinding tools and micro-grinding quality is the research topic in recent years. Aurich et al. [1] fabricated micro-grinding tool with diameter varying 4–40 μm by electroplating process, and analyzed the effect of grain size, grain concentration and process parameters on machined surface quality and accuracy during micro-grinding of silicon. Chen et al. [9] proposed a method that combined “micro-EDM” with “precision composite electroforming” for fabricating micro-diamond tool with diameter of 100 μm, and obtained the machined surface roughness of 0.085 μm during micro-grinding of micro-ZrO2 ceramic ferrule. Morgan et al. [10] investigated polycrystalline diamond microgrinding tools with 50 μm in diameter by micro-EDM, and produced a groove with a bottom surface roughness of only 5.7 nm in microgrinding of ULE (ultra low expansion) glass. Cheng et al. [11] developed a new micro drilling-grinding tool for precision micro-drilling of hard and brittle materials, and proposed a ductile regime propelling technology of resin coating to reduce the size of edge chipping in micro
In recent years, the demand for precision meso/micro parts, particularly those made of hard and brittle materials, is continuously growing in various industries such as aerospace, biomedicine and microelectronics. The precision meso/micro parts are normally featured with micro-slot, micro-wall and micro-pin with dimensions of several microns to hundreds of microns, which requires high surface quality and geometrical accuracy [1,2]. Recently, micro-nano manufacturing technology such as LIGA (lithography, electroplating, and molding) and FIB (focused ion beam) based on the fabrication process of semiconductor is mainly used to manufacture micro-parts with large graphical surface, while both techniques are relatively expensive and timeconsuming, and only suitable for mass production [3]. Mechanical micro-machining process such as micro-cutting, micro-milling, microgrinding and micro-drilling is an emerging micro-manufacturing technique, which has great potential for fabricating three-dimensional micro-parts with various workpiece materials, complex shape and high geometrical accuracy [3–6]. Among them, micro-grinding process has attracted extensively concerns due to its advantages of high surface finish, high form accuracy and surface integrity, which is normally applied to manufacture complex three dimensional micro-structure using hard and brittle materials [7]. During micro-grinding process,
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Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Ren).
https://doi.org/10.1016/j.jmapro.2019.04.030 Received 3 January 2019; Received in revised form 9 April 2019; Accepted 28 April 2019 Available online 10 May 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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drill-grinding of soda-lime glass. Pratap et al. [12] machined a single groove and double perpendicular grooves on the flat end of the PCD tools using wire EDM process. Results showed that improved PCD tools significantly reduced surface roughness and edge chipping. Wen et al. [13] investigated the effect law of process parameters, tip diameters of the micro-grinding tools, and grain size on the surface roughness in micro-grinding of soda-lime glass. The micro-grinding tool is usually produced with electroplated diamond, polycrystalline diamond and PCD materials. Scholars are mainly concerned with the surface roughness of micro-structure [9,10,12,13]. A few reports mentioned edge chipping of micro-structure and its damage mechanism. However, the edge chipping not only severely affects geometric accuracy of the workpiece, but also results in failure of parts in service and shorts its service life, especially for microstructure parts manufactured with hard and brittle materials [14]. Therefore, the degree of edge chipping is also an important index to evaluate the quality following micro-grinding process. In addition, the grinding quality greatly depends on the kinematic interactions between workpiece and grinding tools [15]. Since the micro-grinding tools have a tip diameter of less than 1 mm (even down to 4 μm), their performance is more sensitive to tool wear compared with conventional grinding wheels [16]. This paper studied micro-grinding quality when machining single crystal silicon micro-structure under various grinding parameters and grinding tools. The surface roughness and edge chipping width of micro-structure were evaluated as grinding quality. The loss of tool tip diameter was measured to describe tool wear, and the effect of tool wear on micro-grinding quality was further discussed.
grinding 20 times, the micro-grinding tool was online observed to evaluate its wear condition via an industrial microscope (JT-1400B) to avoid re-installation error. The observed positions of micro-grinding tool were marked to ensure the accuracy and reliability of sample data. The tip diameter was measured three times at different work positions and the average value was collected. In Fig. 2, the observed position showed a typical example such as the position d1, d2 and d3 on tool tip. The wear condition was evaluated through comparing the tip diameter loss of the micro-grinding tools and grain morphology at different stage. The surface roughness and edge chipping width of specimen were measured to evaluate the micro-grinding quality, as shown in Fig. 1. The digital microscope (VHX-5000, Keyence Inc.) and the 3D laser scanning confocal microscope (VK-X100, Keyenc Inc.) were applied to detect 3D surface topography and surface roughness. The edge chipping of micro-structure was characterized a random distribution and uneven characteristic in size and position. Fig. 3 shows a typical edge chipping pattern of a single crystal silicon machined by micro-grinding. The degree of edge chipping can be assessed using the average edge chipping width [17]. In this paper, the average edge chipping width W was calculated through microscopic edge morphology and spline curve fitting. Firstly, the edge chipping morphology of the specimen was observed by the digital microscope. Then the image showing edge chipping morphology was imported into AutoCAD software. The actual contour line of the morphology was fitted with a spline curve. The sampling length lAB, which runs over the peak point C and the peak point D of the edge profile, was equal to the microscopic observation length, as shown in Fig. 3. Therefore, the edge chipping area S could measure according to the sampling length lAB and the fitted edge contour line. The average edge chipping width W is calculated by
2. Micro-grinding experiment
W=
The side micro-grinding trials were conducted using electroplated diamond micro-grinding tools on a precision CNC grinding center (MK2945C, Ningjiang machine tools Inc.). The schematic of side microgrinding process is shown in Fig. 1 and the experimental setups are shown in Fig. 2. An air turbine spindle (HTS1501S-BT40, Nakanishi Inc.) was installed to provide high spindle speed up to 150,000 rpm with small runout down to 1 μm. The commercial electroplated diamond micro-grinding tools with diameter of 3 mm were employed, with detailed specification listed in Table 1. In order to analyze the effect laws of grinding parameters and the performance of micro-grinding tools on micro-grinding quality, experimental trials were designed and detailed in Table 2. The experiment was run in dry condition. The workpiece materials was single crystal silicon with 100 lattice orientation and dimensions of 15 mm × 10 mm × 3 mm. The specimen was firstly polished with Ra 0.5 nm to avoid impact on subsequent surface quality evaluation. A single side micro-channel with 10 mm in length and 500 μm in depth was carried out under the same grinding parameter, as shown in Table 2. The feed rate, spindle speed and grinding depth were set at 6 mm/min, 80,000 rpm and 10 μm, respectively. After side micro-
S lAB
(1)
In this study, the sampling length lAB was set at 2000 μm. The average edge chipping width W of the specimen was measured at three random positions for each specimen. The average value of W was used as an index to evaluate micro-grinding quality. 3. Results and discussion 3.1. Effect of feed rate on grinding quality Fig. 4 shows results of surface roughness and the average edge chipping width of samples processed with various feed rates, with spindle speed and grinding depth setting at 80,000 rpm and 10 μm, respectively. As the feed rate increased, the surface roughness (Fig. 4(a)) varied similarly to the average edge chipping width (Fig. 4(b)) in the micro-grinding experiment. In Fig. 4(a), specimens were ground by three types of micro-grinding tools with different grain sizes, and surface roughness showed an increasing trend with the increase of feed rate. The surface roughness increased from 0.746 to 1.062 μm using 600# micro-grinding tool with feed rate varying from 2 to 10 mm/min. Fig. 5 shows 3D surface topography of specimens under different processing parameters. With the same grinding depth and spindle speed, relatively more brittle fracture and severe peaks and troughs were observed on grinding surface when machining with higher feed rate of 10 mm/min compared with finding shown in Fig. 5(a) and (b). It was found that the maximum undeformed chip thickness hm significantly affected surface roughness. Based on geometry of the abrasive-workpiece interactions, the maximum undeformed chip thickness hm in traditional plane grinding can be calculated by [18]
hm = 2L
vw vs
ap ds
(2)
where L is the distance between two adjacent grits, vw is feed rate, vs is grinding speed, ap is grinding depth and ds is tool tip diameter. Due to
Fig. 1. Schematic of micro-grinding process. 247
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Fig. 2. Micro-grinding equipment and tool.
surface quality. Fig. 4(b) shows that the average edge chipping width ground by the three micro-grinding tools had an increasing trend with the increase of feed rate. When the feed rate increased from 2 to 10 mm/min, the average edge chipping width increased from 10.545 to 15.596 μm when ground by the 600# micro-grinding tool. Fig. 6 shows the edge chipping topography of specimens under different processing parameters. Compared with Fig. 6(a) and (b) using the same micro-grinding tool, grinding depth and spindle speed were relatively smaller and denser edge chipping was observed under the lower feed rate of 6 mm/min. It indicated that smaller edge chipping width and better grinding quality of specimens could be obtained when running with lower feed rate. The tangential grinding force Fet per grain is calculated by [20]
Table 1 Details of micro-grinding tools used in the experiment. Micro-grinding tool no.
1#
2#
3#
4#
5#
Tip diameter (mm) Grain size (#)/average grain diameter (μm)
0.3 600/24
0.5 600/24
0.7 600/24
0.5 200/74
0.5 1000/14
Table 2 Experimental parameters selected for micro-grinding.
1 2 3
Feed rate vw (mm/min)
Grinding depth ap (μm)
Spindle speed n (rpm)
2, 4, 6, 8, 10 6 6
10 5, 10, 15, 20, 25 10
80,000 80,000 20,000, 40,000, 60,000, 80,000, 100,000
Fet = (k
H 3 γ 2(1 − ε ) ) hm K c2
(4)
where H is workpiece hardness, Kc is workpiece fracture toughness, k is a constant and related to the physical–mechanical properties of the workpiece material, ε and γ are parameters that can be determined based on the workpiece properties and grinding parameters. According to Eq. (4), when the maximum undeformed chip thickness increases with higher feed rate, the grinding force between single grain and specimen also increases. As a result, higher grinding force is easier to produce more micro-cracks on the ground surface. Then the microcracks near the edge would easily extend towards the free surface and the phenomenon of large and wide edge chipping occurred [21]. It means that relatively low feed rate is preferred in the practical microgrinding process. Fig. 3. Measurement schematic of the average edge chipping width.
3.2. Effect of spindle speed on grinding quality Fig. 7 shows results of surface roughness and the average edge chipping width of specimens under different spindle speeds, with feed rate and grinding depth setting at 6 mm/min and 10 μm, respectively. In Fig. 7(a), specimens were ground by three different micro-grinding tools with different grain sizes, the surface roughness showed a slight decreasing trend with increasing spindle speed. When increasing spindle speed from 20,000 to 100,000 rpm, the surface roughness decreased only 0.084 μm using the 1000# micro-grinding tool. Compared with Fig. 5(a) and (c), the grinding scratches were more uniform and their height difference was small under higher spindle speed of 100,000 rpm, while morphology features between the two specimens were slightly different.
the influence of size effect in micro-grinding, hm can be calculated by [19]
hm = Md [2(
ap 1/2 Lvw ap 1/2 L2 vw 2 )( ) (1 − ) − ( )] vs ds ds ds vs
(3)
where Md is a component variable to describe the effects when grinding enters microscale. According to Eqs. (2) and (3), the maximum undeformed chip thickness hm increases with the feed rate vw, which results in more and severe brittle fracture in micro-grinding of hard and brittle specimen. Therefore, the removal way of single crystal silicon under a higher feed rate is mainly brittle fracture, which leads to poor 248
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Fig. 4. Effect of feed rate on micro-grinding quality.
0.1 μm. However, the grinding depth showed more obvious effect on the ground surface roughness when the grinding depth was higher than 15 μm. Fig. 5(a) and (d) show the 3D surface topography of specimens under different grinding depths. It could be seen that the height difference of grinding scratches was larger with the grinding depths of 25 μm (Fig. 5(d)). Additionally, relatively serious peaks and troughs were observed on the ground surface, which damaged the microgrinding quality of ground specimen. Different from groove grinding mode, the chip was easily and rapidly evacuated in side micro-grinding mode. Therefore, the chip was not blocked up to damage the ground surface roughness in side microgrinding. However, the tool stiffness was barely satisfactory since the micro-grinding tool had a tip diameter of 0.5 mm and a tip length of 6 mm. As the grinding depth was further increased, the radial offset of tool tip became the main factor affecting micro-grinding quality [21]. Therefore, the ground surface roughness showed a significant upward trend when the grinding depth was higher than 15 μm in Fig. 8(a). It should be noted that the critical grinding depth was related to the stiffness and the length-to-diameter ratio of micro-grinding tool. It was observed that limited influence on the average edge chipping width with the grinding depth below 15 μm for different types of microgrinding tools under the same parameters, as shown in Fig. 8(b). The curves of the average edge chipping width showed an upward trend with the further increase of grinding depth. Compared with Fig. 6(a) and (d), the average edge chipping width W was wider and the edge chipping area S was larger under the grinding depth of 25 μm, which produced greater extent ground surface damage and lower grinding quality. When the grinding depth was lower, the radial offset of tool tip was comparatively smaller, thus it had a litter effect on edge chipping of specimen. However, the grinding force was larger as the grinding depth increases further. And then the effect of stiffness of micro-grinding tools on tip radial motion error increases significantly. As a result, the average edge chipping width of specimen showed an increasing trend under higher grinding depth. In practical micro-grinding process, the optimal length-diameter ratio of micro-grinding tool should be considered, which is significant to improve tool stiffness and reduce edge damage of micro-structure.
Experimental result indicates that the surface roughness decreased slightly with higher spindle speed. Although the spindle speed increased five times, the grinding quality was superior while its improvement was small. That was mainly due to the fact that more grains involved in the micro-grinding at the same time with higher spindle speed. Therefore, the active distance between two adjacent grains L decreased with increasing spindle speed. And then, more active grains were involved in the contact zone, the maximum undeformed chip thickness was reduced based on Eq. (3). However, the spindle speed only had a weak effect on surface roughness, which was attributed to narrow range variation of grinding speed. The tip diameter of microgrinding tool was about 0.5 mm. Although the spindle speed increased from 20,000 to 100,000 rpm, the grinding speed just increased from 0.524 to 2.618 m/s. Fig. 7(b) shows the variations of average edge chipping width with different spindle speeds. The average edge chipping width of the specimens ground by the 600# micro-grinding tool decreased from 13.746 to 10.508 μm, with increasing of spindle speed from 20,000 rpm to 100,000 rpm. Compared Fig. 6(a) with (c), the largest size of edge chipping of specimen significantly reduced with increased spindle speed. The maximum edge chipping width under the spindle speed of 100,000 rpm was less than that running with spindle speed of 80,000 rpm. Accordingly, the area of edge chipping S and the average edge chipping width W decreased under higher spindle speed. According to the Eqs. (2) and (3), the material removal volume of single grain and the maximum undeformed chip thickness reduced with the increase of the grinding speed. Then the grinding force also decreased with the maximum undeformed chip thickness according to the Eq. (4). The grinding force decreased accordingly with increasing spindle speed of micro-grinding tool, which were helpful to restrain the generation of edge chipping. In addition, The radial motion error decreased with the increase of spindle speed of air turbine spindle [22]. Higher spindle speed was conducive to reduce surface roughness and restrain the generation of edge chipping, and it was preferred in practical micro-grinding.
3.3. Effect of grinding depth on grinding quality Fig. 8 shows results of surface roughness and the average edge chipping width of the specimen under different grinding depths, with feed rate and spindle speed setting at 6 μm/min and 80,000 rpm, respectively. Generally, the surface roughness of specimens showed an increasing trend with increased grinding depth, which were ground by the three types of micro-grinding tools, as shown in Fig. 8(a). For all micro-grinding tools, when the grinding depth was less than 15 μm, the grinding depth showed limited effect on surface roughness, where the maximum fluctuation of the ground surface roughness was within
3.4. Effect of grain size on grinding quality Regarding to Figs. 4(a), 7 (a) and 8 (a), it was seen that the surface roughness was improved by using the micro-grinding tools with small grain size (1000#). For example, in Fig. 4(a) the surface roughness of specimens ground by the 200#, 600# and 1000# micro-grinding tools are 1.258 μm, 0.840 μm and 0.664 μm, respectively at the feed rate 6 mm/min. Fig. 9(a) and (b) shows the ground surface topography of 249
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Fig. 5. 3D surface topography of ground specimens.
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Fig. 6. Edge chipping topography of specimens.
roughness decreased greatly and the surface quality was improved significantly. Figs. 4(b), 7 (b) and 8 (b) show that the average edge chipping width of specimens reduced with the decrease of grain size in the same grinding parameters. In Fig. 4(b), when the feed rate was set at 6 mm/ min, the average edge chipping width of specimen was 18.304 μm, 11.929 μm and 6.345 μm ground by the 200#, 600# and 1000# microgrinding tools, respectively. Compared with Fig. 6(a) and (f), the edge chipping was narrower and denser when using smaller grain size of 1000#, and the grinding quality was improved significantly. Since the distance of the abrasive grains from the free surface shortened effectively by using the tools with smaller grain size, which was conducive to reducing the width of edge chipping [23–25]. In addition, the grinding force applied in each grain also reduced due to the reduced maximum undeformed chip thickness. Therefore, among
specimens machined with different grain sizes tools. Compare with Fig. 9(a), relatively more uniform grooves were observed with ductile removal model on the ground surface, as shown in Fig. 9(b). The height difference between grooves under the grain size of 1000# was smaller than that under the grain size of 600#, as shown in Fig. 5(a) and (f). Therefore, the surface quality was best for all experimental trials when the micro-grinding tools with small grain size (1000#) was adopted. As more abrasive grains distributed on the surface of micro-grinding tools with a small grain size per unit area, the distance between two adjacent grains L decreased with smaller grain size. According to the Eqs. (2) and (3), the maximum undeformed chip thickness also decreased with smaller grain size. In addition, the smaller grain size of micro-grinding tools generally had more uniform height of protrusion, which was conductive to eliminating the height difference between tiny grooves on the ground surface to some extent. Therefore, the surface 251
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Fig. 7. Effect of spindle speed on micro-grinding quality.
Fig. 8. Effect of grinding depth on micro-grinding quality.
Fig. 9. Ground surface topography with different grain sizes.
the five types of micro-grinding tools, the performance of microgrinding tool with a tip diameter of 0.5 mm and the grain size of 1000# showed the best performance. An optimal grinding quality (Ra: 0.619 μm, W: 4.579 μm) could be obtained.
(e). When the feed rate was set at 6 mm/min and the micro-grinding tools was 600#, the surface roughness of specimen ground by the micro-grinding tools with tip diameters of 0.3 mm, 0.5 mm and 0.7 mm was 1.106 μm, 0.840 μm and 0.707 μm, respectively, as shown in Fig. 10(a). For the micro-grinding tool with large the tip diameter, the grinding depth had limited effect on ground surface roughness, as shown in Fig. 10(c). As the grinding depth increased from 5 μm to 25 μm, the surface roughness of specimen ground by the micro-grinding tools with tip diameters of 0.3 mm, 0.5 mm and 0.7 mm increased by 0.409 μm, 0.123 μm and 0.090 μm, respectively. Fig. 5(a) and (e) shows the 3D morphology of ground surface with different tool tip diameters under the same grinding parameters. It was seen that there was shallow
3.5. Effect of tool tip diameter on the micro-grinding quality Fig. 10 shows the effect of tool tip diameter on the ground surface roughness and the average edge chipping width under the tools with the same grain size. Experimental result shows that the surface roughness decreased with larger tool tip diameter under the same feed rate, spindle speed and grinding depth as shown in Fig. 10(a), (c) and 252
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Fig. 10. Effect of tool tip diameter on micro-grinding quality.
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Fig. 11. Tool tip diameter loss with the number of grinding times under different micro-grinding tools. Fig. 13. Effect of micro-grinding tool wear on surface roughness.
grinding scratches on the ground face by the micro-grinding tool with a larger tip diameter. As the abrasive grains on a large micro-grinding tool possessed faster grinding speed under the same spindle speed, and more abrasive grains were involved in the grinding process per unit time, the maximum undeformed chip thickness decreased according to Eq. (3). Additionally, the micro-grinding tool with larger tip diameter exhibited better stiffness performance. Although the grinding force increased significantly with higher grinding depth, the bending deformation of micro-grinding tool with larger tip diameter was smaller. Therefore, the grinding depth showed limited effect on the surface roughness when using smaller grain size micro-grinding tools. Fig. 10(b), (d) and (f) show that the average edge chipping width of specimens was small when using the micro-grinding tools with large tip diameter under the same grinding parameters. Smaller and denser chippings were found on the edge of the specimen ground by the microgrinding tool with larger tip diameter of 0.7 mm (Fig. 6(e)). The microgrinding tool with larger tip diameter presented faster grinding speed under the same grinding parameters, thus the maximum undeformed chip thickness decreased and the grinding force applied in grains was reduced. In addition, the micro-grinding tool with larger tip diameter showed better stiffness performance, which also greatly reduced the generation of edge chipping. Therefore, in the case of meeting the dimensional requirement of micro-structure, the micro-grinding tools with larger tip diameter should be selected as far as possible in practical
Fig. 14. Effect of micro-grinding tool wear on average edge chipping width.
Fig. 12. Surface topography of worn micro-grinding tools. (O abrasive grain with high protrusion O attrition, O fracture, O pull-out). 254
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chipping width of specimen ground by the tool with a tip diameter of 0.5 mm and the grain size of 1000# was minimum down to 5.521 μm. In the initially wear stage, the protrusion height of grains varied greatly and only a small number of grains participated in grinding. The high grains resulted in deep grinding scratches on ground surface. The specimen was mostly removed in brittle mode, which led to poor surface quality. Besides, the high grains had large maximum undeformed chip thickness and grinding force. As a result, the edge of specimen occurred serious chipping and the average edge chipping width was large. With the progress of micro-grinding, the grains with higher protrusion height wore seriously and the height difference between grains decreased. Consequently, the tools wear became slowly and gone into slowly steady wear stage. At this stage, more number of grains were involved in grinding and the grinding force became more stably. Therefore, both of the surface roughness and edge chipping were decreasing and the grinding quality improved significantly.
micro-grinding. 3.6. Effect of tool wear on the micro-grinding quality Fig. 11 shows the curves of tool tip diameter loss with different grinding times. It was seen that the curves of five types of microgrinding tools had the same tendency with grinding time. At the beginning stage, the tip diameter losses of five micro-grinding tools increased rapidly. After about 60 grinding times, the tip diameter losses began to increase slowly. Therefore, the wear of micro-grinding tools could be divided into two stages. One was featured with initial fast wear, and the other was characterized with slow and steady wear. At the beginning, some abrasive grains with high protrusion distributed unevenly on the surface of micro-grinding tools, as shown in Fig. 12(a). The high protruding grains were easy to be rapidly fractured due to continuous mechanical impact, which resulted in rapid tool tip diameter loss. In addition, the protrusion height of grains differed greatly, and only a few grains were involved in the micro-grinding process. The grains with higher protrusion height had deeper grinding depth, so that the maximum undeformed chip thickness and the grinding force acting on these grains increased. The wear form of the grains is mainly fracture at initially quick wear stage as shown in Fig. 12(b). Therefore, the tip diameter loss of all the five types of micro-grinding tools increased sharply at initially wear stage. For continuous grinding process, the tools gradually entered slow and steady wear stage. At this stage, since the grains became dull and their protrusion heights were relatively unified, the number of active grains gradually increased and the grinding force decreased accordingly. The wear form was mainly attrition wear at this stage, as shown in Fig. 12(c). Therefore, the tool tip diameter loss varied slowly at the second stage. It was found that the micro-grinding tool with the small grain size manifested less tip diameter loss under the same grinding times in Fig. 11. The tip diameter loss of micro-grinding tools with grain size of 200#, 600# and 1000# was 10.02 μm, 7.03 μm and 6.58 μm, respectively. There were more active grains participated in grinding contact zone when the micro-grinding tool with small grain size was used. Hence, the maximum undeformed chip thickness reduced and grinding force acting on grains also decreased accordingly. Therefore, the phenomenon of grains fracture infrequently occurred and tool tip diameter loss was less by using the tools with a smaller average grain size. The micro-grinding tool with larger tip diameter shows less tip diameter loss under the same grinding times and grain size. When the grinding times was 100 and the grain size of micro-grinding tool was 600#, the loss of tools with diameter of 0.3 mm, 0.5 mm, and 0.7 mm, respectively, increased by 7.76 μm, 7.03 μm, and 5.75 μm. The tool with larger tip diameter presented higher grinding speed under the same spindle speed. Thus both of the maximum undeformed chip thickness and grinding force acting on grains were decreased according to Eqs. (3) and (4). Therefore, there was relatively less grains fractured or pulled out with larger tip diameter. Fig. 13 shows the specimens ground by five types of micro-grinding tools have the same variation trend with different grinding time. At the beginning, the ground surface roughness decreased rapidly. After 60 grinding times, the ground surface roughness fluctuated within a certain range. For example, when the micro-grinding tool with a tip diameter of 0.5 mm and the grain size of 1000# was used, the ground surface roughness decreased from 1.063 μm (20 grinding times) to 0.613 μm (60 grinding times), then it fluctuated between 0.613 and 0.678 μm. Fig. 14 shows the variation curve of average edge chipping width with the number of grinding times under different micro-grinding tools. It was seen that the average edge chipping width machined by the five micro-grinding tools decreased rapidly at the beginning, then it fluctuated within a certain range with the number of grinding times. Among the five types of micro-grinding tools, the average edge
4. Conclusions Based on comprehensive investigation and analysis, the following conclusions can be drawn: (1) The impact of edge chipping width on surface integrity of microstructure cannot be neglected. Both surface roughness and the average edge chipping width were proposed to evaluate microgrinding quality. (2) Surface roughness and the average edge chipping width increased at a near-liner trend with the increase of feed rate, while the spindle speed exhibited limited effect. When the grinding depth was greater than a certain depth about 15 μm, both of them significantly increased with higher grinding depth. The critical grinding depth was related to the stiffness and the length-to-diameter ratio of microgrinding tool. (3) Among the five types of micro-grinding tools, the micro-grinding tool with a tip diameter of 0.5 mm and the grain size of 1000# presented superior grinding performance. With the same grinding process parameters, the surface roughness was recorded down to 0.619 μm and the average edge chipping width was optimal (4.579 μm). (4) The wear of micro-grinding tools could be divided into initial and fast wear stage together with slow and steady wear stage. The surface roughness and average edge chipping width decreased rapidly at the beginning and then fluctuated within a certain range with the increase number of grinding times. Acknowledgements This work is funded by the National Natural Science Foundation of China (51675170, 51875192). I would like to express my gratitude to all those who helped me during the writing of this manuscript. References [1] Aurich JC, Carrella M, Walk M. Micro grinding with ultra small micro pencil grinding tools using an integrated machine tool. CIRP Ann Manuf Technol 2015;64(1):325–8. https://doi.org/10.1016/j.cirp.2015.04.011. [2] Walk M, Aurich JC. Integrated desktop machine tool for manufacturing and application of ultra-small micro pencil grinding tools. Procedia CIRP 2014;14:333–8. https://doi.org/10.1016/j.procir.2014.06.003. [3] Pratap A, Patra K, Dyakonov AA. Manufacturing miniature products by microgrinding: a review. Procedia Eng 2016;150:969–74. https://doi.org/10.1016/j. proeng.2016.07.072. [4] Francis NK, Viswanadhan KG, Paulose MM. SAFBM of softer materials: an investigation into the micro-cutting mechanisms and the evolution of roughness profile. Mater Manuf Process 2016;31(7):969–75. https://doi.org/10.1080/ 10426914.2015.1090585. [5] Burton G, Goo CS, Zhang Y. Use of vegetable oil in water emulsion achieved through ultrasonic atomization as cutting fluids in micro-milling. J Manuf Processes 2014;16(3):405–13. https://doi.org/10.1016/j.jmapro.2014.04.005.
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Study on micro-grinding quality in micro-grinding tool for single crystal silicon
T
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Yinghui Ren , Chenfang Li, Wei Li, Maojun Li, Hui Liu College of Mechanical and Vehicle Engineering, Hunan University, Changsha, 410082, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Micro-grinding Micro-grinding tools Single crystal silicon
Micro-grinding has a wide application for machining meso/micro parts using hard and brittle materials with high precision. In order to investigate the effect of grinding parameters and various micro-grinding tools on micro-grinding quality, experimental trials on side grinding of single crystal silicon (100 lattice orientation) were carried out using electroplated diamond micro-grinding tools. Based on experimental results, the effects of grinding parameters involving spindle speed, grinding depth and feed rate, together with various micro-grinding tools including different grain size and tip diameter on micro-grinding quality were discussed. The microgrinding quality was mainly evaluated in terms of surface roughness and the average edge-chipping width. The wear condition of micro-grinding tools under different tip diameter and grain size was analyzed. In addition, the effect of tool wear on micro-grinding quality was also discussed. From the experimental results, feed rate showed significant effect on micro-grinding quality. The wear of micro-grinding tools could be divided into initially fast wear stage and steady wear stage, which was crucial to influence the grinding quality.
1. Introduction
since the grinding thickness is usually less than grain size of the workpiece, the grinding grains cut through the grain of the workpiece or grain boundary [8]. The size effect has an obvious impact on the micro-grinding quality and the service life of micro-grinding tools. The grinding force must be higher than the atomic binding force inside the grains to remove material and form chips. In order to efficiently and accurately fabricate micro-structure, the fabrication technology of micro-grinding tools and micro-grinding quality is the research topic in recent years. Aurich et al. [1] fabricated micro-grinding tool with diameter varying 4–40 μm by electroplating process, and analyzed the effect of grain size, grain concentration and process parameters on machined surface quality and accuracy during micro-grinding of silicon. Chen et al. [9] proposed a method that combined “micro-EDM” with “precision composite electroforming” for fabricating micro-diamond tool with diameter of 100 μm, and obtained the machined surface roughness of 0.085 μm during micro-grinding of micro-ZrO2 ceramic ferrule. Morgan et al. [10] investigated polycrystalline diamond microgrinding tools with 50 μm in diameter by micro-EDM, and produced a groove with a bottom surface roughness of only 5.7 nm in microgrinding of ULE (ultra low expansion) glass. Cheng et al. [11] developed a new micro drilling-grinding tool for precision micro-drilling of hard and brittle materials, and proposed a ductile regime propelling technology of resin coating to reduce the size of edge chipping in micro
In recent years, the demand for precision meso/micro parts, particularly those made of hard and brittle materials, is continuously growing in various industries such as aerospace, biomedicine and microelectronics. The precision meso/micro parts are normally featured with micro-slot, micro-wall and micro-pin with dimensions of several microns to hundreds of microns, which requires high surface quality and geometrical accuracy [1,2]. Recently, micro-nano manufacturing technology such as LIGA (lithography, electroplating, and molding) and FIB (focused ion beam) based on the fabrication process of semiconductor is mainly used to manufacture micro-parts with large graphical surface, while both techniques are relatively expensive and timeconsuming, and only suitable for mass production [3]. Mechanical micro-machining process such as micro-cutting, micro-milling, microgrinding and micro-drilling is an emerging micro-manufacturing technique, which has great potential for fabricating three-dimensional micro-parts with various workpiece materials, complex shape and high geometrical accuracy [3–6]. Among them, micro-grinding process has attracted extensively concerns due to its advantages of high surface finish, high form accuracy and surface integrity, which is normally applied to manufacture complex three dimensional micro-structure using hard and brittle materials [7]. During micro-grinding process,
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Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Ren).
https://doi.org/10.1016/j.jmapro.2019.04.030 Received 3 January 2019; Received in revised form 9 April 2019; Accepted 28 April 2019 Available online 10 May 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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drill-grinding of soda-lime glass. Pratap et al. [12] machined a single groove and double perpendicular grooves on the flat end of the PCD tools using wire EDM process. Results showed that improved PCD tools significantly reduced surface roughness and edge chipping. Wen et al. [13] investigated the effect law of process parameters, tip diameters of the micro-grinding tools, and grain size on the surface roughness in micro-grinding of soda-lime glass. The micro-grinding tool is usually produced with electroplated diamond, polycrystalline diamond and PCD materials. Scholars are mainly concerned with the surface roughness of micro-structure [9,10,12,13]. A few reports mentioned edge chipping of micro-structure and its damage mechanism. However, the edge chipping not only severely affects geometric accuracy of the workpiece, but also results in failure of parts in service and shorts its service life, especially for microstructure parts manufactured with hard and brittle materials [14]. Therefore, the degree of edge chipping is also an important index to evaluate the quality following micro-grinding process. In addition, the grinding quality greatly depends on the kinematic interactions between workpiece and grinding tools [15]. Since the micro-grinding tools have a tip diameter of less than 1 mm (even down to 4 μm), their performance is more sensitive to tool wear compared with conventional grinding wheels [16]. This paper studied micro-grinding quality when machining single crystal silicon micro-structure under various grinding parameters and grinding tools. The surface roughness and edge chipping width of micro-structure were evaluated as grinding quality. The loss of tool tip diameter was measured to describe tool wear, and the effect of tool wear on micro-grinding quality was further discussed.
grinding 20 times, the micro-grinding tool was online observed to evaluate its wear condition via an industrial microscope (JT-1400B) to avoid re-installation error. The observed positions of micro-grinding tool were marked to ensure the accuracy and reliability of sample data. The tip diameter was measured three times at different work positions and the average value was collected. In Fig. 2, the observed position showed a typical example such as the position d1, d2 and d3 on tool tip. The wear condition was evaluated through comparing the tip diameter loss of the micro-grinding tools and grain morphology at different stage. The surface roughness and edge chipping width of specimen were measured to evaluate the micro-grinding quality, as shown in Fig. 1. The digital microscope (VHX-5000, Keyence Inc.) and the 3D laser scanning confocal microscope (VK-X100, Keyenc Inc.) were applied to detect 3D surface topography and surface roughness. The edge chipping of micro-structure was characterized a random distribution and uneven characteristic in size and position. Fig. 3 shows a typical edge chipping pattern of a single crystal silicon machined by micro-grinding. The degree of edge chipping can be assessed using the average edge chipping width [17]. In this paper, the average edge chipping width W was calculated through microscopic edge morphology and spline curve fitting. Firstly, the edge chipping morphology of the specimen was observed by the digital microscope. Then the image showing edge chipping morphology was imported into AutoCAD software. The actual contour line of the morphology was fitted with a spline curve. The sampling length lAB, which runs over the peak point C and the peak point D of the edge profile, was equal to the microscopic observation length, as shown in Fig. 3. Therefore, the edge chipping area S could measure according to the sampling length lAB and the fitted edge contour line. The average edge chipping width W is calculated by
2. Micro-grinding experiment
W=
The side micro-grinding trials were conducted using electroplated diamond micro-grinding tools on a precision CNC grinding center (MK2945C, Ningjiang machine tools Inc.). The schematic of side microgrinding process is shown in Fig. 1 and the experimental setups are shown in Fig. 2. An air turbine spindle (HTS1501S-BT40, Nakanishi Inc.) was installed to provide high spindle speed up to 150,000 rpm with small runout down to 1 μm. The commercial electroplated diamond micro-grinding tools with diameter of 3 mm were employed, with detailed specification listed in Table 1. In order to analyze the effect laws of grinding parameters and the performance of micro-grinding tools on micro-grinding quality, experimental trials were designed and detailed in Table 2. The experiment was run in dry condition. The workpiece materials was single crystal silicon with 100 lattice orientation and dimensions of 15 mm × 10 mm × 3 mm. The specimen was firstly polished with Ra 0.5 nm to avoid impact on subsequent surface quality evaluation. A single side micro-channel with 10 mm in length and 500 μm in depth was carried out under the same grinding parameter, as shown in Table 2. The feed rate, spindle speed and grinding depth were set at 6 mm/min, 80,000 rpm and 10 μm, respectively. After side micro-
S lAB
(1)
In this study, the sampling length lAB was set at 2000 μm. The average edge chipping width W of the specimen was measured at three random positions for each specimen. The average value of W was used as an index to evaluate micro-grinding quality. 3. Results and discussion 3.1. Effect of feed rate on grinding quality Fig. 4 shows results of surface roughness and the average edge chipping width of samples processed with various feed rates, with spindle speed and grinding depth setting at 80,000 rpm and 10 μm, respectively. As the feed rate increased, the surface roughness (Fig. 4(a)) varied similarly to the average edge chipping width (Fig. 4(b)) in the micro-grinding experiment. In Fig. 4(a), specimens were ground by three types of micro-grinding tools with different grain sizes, and surface roughness showed an increasing trend with the increase of feed rate. The surface roughness increased from 0.746 to 1.062 μm using 600# micro-grinding tool with feed rate varying from 2 to 10 mm/min. Fig. 5 shows 3D surface topography of specimens under different processing parameters. With the same grinding depth and spindle speed, relatively more brittle fracture and severe peaks and troughs were observed on grinding surface when machining with higher feed rate of 10 mm/min compared with finding shown in Fig. 5(a) and (b). It was found that the maximum undeformed chip thickness hm significantly affected surface roughness. Based on geometry of the abrasive-workpiece interactions, the maximum undeformed chip thickness hm in traditional plane grinding can be calculated by [18]
hm = 2L
vw vs
ap ds
(2)
where L is the distance between two adjacent grits, vw is feed rate, vs is grinding speed, ap is grinding depth and ds is tool tip diameter. Due to
Fig. 1. Schematic of micro-grinding process. 247
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Fig. 2. Micro-grinding equipment and tool.
surface quality. Fig. 4(b) shows that the average edge chipping width ground by the three micro-grinding tools had an increasing trend with the increase of feed rate. When the feed rate increased from 2 to 10 mm/min, the average edge chipping width increased from 10.545 to 15.596 μm when ground by the 600# micro-grinding tool. Fig. 6 shows the edge chipping topography of specimens under different processing parameters. Compared with Fig. 6(a) and (b) using the same micro-grinding tool, grinding depth and spindle speed were relatively smaller and denser edge chipping was observed under the lower feed rate of 6 mm/min. It indicated that smaller edge chipping width and better grinding quality of specimens could be obtained when running with lower feed rate. The tangential grinding force Fet per grain is calculated by [20]
Table 1 Details of micro-grinding tools used in the experiment. Micro-grinding tool no.
1#
2#
3#
4#
5#
Tip diameter (mm) Grain size (#)/average grain diameter (μm)
0.3 600/24
0.5 600/24
0.7 600/24
0.5 200/74
0.5 1000/14
Table 2 Experimental parameters selected for micro-grinding.
1 2 3
Feed rate vw (mm/min)
Grinding depth ap (μm)
Spindle speed n (rpm)
2, 4, 6, 8, 10 6 6
10 5, 10, 15, 20, 25 10
80,000 80,000 20,000, 40,000, 60,000, 80,000, 100,000
Fet = (k
H 3 γ 2(1 − ε ) ) hm K c2
(4)
where H is workpiece hardness, Kc is workpiece fracture toughness, k is a constant and related to the physical–mechanical properties of the workpiece material, ε and γ are parameters that can be determined based on the workpiece properties and grinding parameters. According to Eq. (4), when the maximum undeformed chip thickness increases with higher feed rate, the grinding force between single grain and specimen also increases. As a result, higher grinding force is easier to produce more micro-cracks on the ground surface. Then the microcracks near the edge would easily extend towards the free surface and the phenomenon of large and wide edge chipping occurred [21]. It means that relatively low feed rate is preferred in the practical microgrinding process. Fig. 3. Measurement schematic of the average edge chipping width.
3.2. Effect of spindle speed on grinding quality Fig. 7 shows results of surface roughness and the average edge chipping width of specimens under different spindle speeds, with feed rate and grinding depth setting at 6 mm/min and 10 μm, respectively. In Fig. 7(a), specimens were ground by three different micro-grinding tools with different grain sizes, the surface roughness showed a slight decreasing trend with increasing spindle speed. When increasing spindle speed from 20,000 to 100,000 rpm, the surface roughness decreased only 0.084 μm using the 1000# micro-grinding tool. Compared with Fig. 5(a) and (c), the grinding scratches were more uniform and their height difference was small under higher spindle speed of 100,000 rpm, while morphology features between the two specimens were slightly different.
the influence of size effect in micro-grinding, hm can be calculated by [19]
hm = Md [2(
ap 1/2 Lvw ap 1/2 L2 vw 2 )( ) (1 − ) − ( )] vs ds ds ds vs
(3)
where Md is a component variable to describe the effects when grinding enters microscale. According to Eqs. (2) and (3), the maximum undeformed chip thickness hm increases with the feed rate vw, which results in more and severe brittle fracture in micro-grinding of hard and brittle specimen. Therefore, the removal way of single crystal silicon under a higher feed rate is mainly brittle fracture, which leads to poor 248
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Fig. 4. Effect of feed rate on micro-grinding quality.
0.1 μm. However, the grinding depth showed more obvious effect on the ground surface roughness when the grinding depth was higher than 15 μm. Fig. 5(a) and (d) show the 3D surface topography of specimens under different grinding depths. It could be seen that the height difference of grinding scratches was larger with the grinding depths of 25 μm (Fig. 5(d)). Additionally, relatively serious peaks and troughs were observed on the ground surface, which damaged the microgrinding quality of ground specimen. Different from groove grinding mode, the chip was easily and rapidly evacuated in side micro-grinding mode. Therefore, the chip was not blocked up to damage the ground surface roughness in side microgrinding. However, the tool stiffness was barely satisfactory since the micro-grinding tool had a tip diameter of 0.5 mm and a tip length of 6 mm. As the grinding depth was further increased, the radial offset of tool tip became the main factor affecting micro-grinding quality [21]. Therefore, the ground surface roughness showed a significant upward trend when the grinding depth was higher than 15 μm in Fig. 8(a). It should be noted that the critical grinding depth was related to the stiffness and the length-to-diameter ratio of micro-grinding tool. It was observed that limited influence on the average edge chipping width with the grinding depth below 15 μm for different types of microgrinding tools under the same parameters, as shown in Fig. 8(b). The curves of the average edge chipping width showed an upward trend with the further increase of grinding depth. Compared with Fig. 6(a) and (d), the average edge chipping width W was wider and the edge chipping area S was larger under the grinding depth of 25 μm, which produced greater extent ground surface damage and lower grinding quality. When the grinding depth was lower, the radial offset of tool tip was comparatively smaller, thus it had a litter effect on edge chipping of specimen. However, the grinding force was larger as the grinding depth increases further. And then the effect of stiffness of micro-grinding tools on tip radial motion error increases significantly. As a result, the average edge chipping width of specimen showed an increasing trend under higher grinding depth. In practical micro-grinding process, the optimal length-diameter ratio of micro-grinding tool should be considered, which is significant to improve tool stiffness and reduce edge damage of micro-structure.
Experimental result indicates that the surface roughness decreased slightly with higher spindle speed. Although the spindle speed increased five times, the grinding quality was superior while its improvement was small. That was mainly due to the fact that more grains involved in the micro-grinding at the same time with higher spindle speed. Therefore, the active distance between two adjacent grains L decreased with increasing spindle speed. And then, more active grains were involved in the contact zone, the maximum undeformed chip thickness was reduced based on Eq. (3). However, the spindle speed only had a weak effect on surface roughness, which was attributed to narrow range variation of grinding speed. The tip diameter of microgrinding tool was about 0.5 mm. Although the spindle speed increased from 20,000 to 100,000 rpm, the grinding speed just increased from 0.524 to 2.618 m/s. Fig. 7(b) shows the variations of average edge chipping width with different spindle speeds. The average edge chipping width of the specimens ground by the 600# micro-grinding tool decreased from 13.746 to 10.508 μm, with increasing of spindle speed from 20,000 rpm to 100,000 rpm. Compared Fig. 6(a) with (c), the largest size of edge chipping of specimen significantly reduced with increased spindle speed. The maximum edge chipping width under the spindle speed of 100,000 rpm was less than that running with spindle speed of 80,000 rpm. Accordingly, the area of edge chipping S and the average edge chipping width W decreased under higher spindle speed. According to the Eqs. (2) and (3), the material removal volume of single grain and the maximum undeformed chip thickness reduced with the increase of the grinding speed. Then the grinding force also decreased with the maximum undeformed chip thickness according to the Eq. (4). The grinding force decreased accordingly with increasing spindle speed of micro-grinding tool, which were helpful to restrain the generation of edge chipping. In addition, The radial motion error decreased with the increase of spindle speed of air turbine spindle [22]. Higher spindle speed was conducive to reduce surface roughness and restrain the generation of edge chipping, and it was preferred in practical micro-grinding.
3.3. Effect of grinding depth on grinding quality Fig. 8 shows results of surface roughness and the average edge chipping width of the specimen under different grinding depths, with feed rate and spindle speed setting at 6 μm/min and 80,000 rpm, respectively. Generally, the surface roughness of specimens showed an increasing trend with increased grinding depth, which were ground by the three types of micro-grinding tools, as shown in Fig. 8(a). For all micro-grinding tools, when the grinding depth was less than 15 μm, the grinding depth showed limited effect on surface roughness, where the maximum fluctuation of the ground surface roughness was within
3.4. Effect of grain size on grinding quality Regarding to Figs. 4(a), 7 (a) and 8 (a), it was seen that the surface roughness was improved by using the micro-grinding tools with small grain size (1000#). For example, in Fig. 4(a) the surface roughness of specimens ground by the 200#, 600# and 1000# micro-grinding tools are 1.258 μm, 0.840 μm and 0.664 μm, respectively at the feed rate 6 mm/min. Fig. 9(a) and (b) shows the ground surface topography of 249
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Fig. 5. 3D surface topography of ground specimens.
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Fig. 6. Edge chipping topography of specimens.
roughness decreased greatly and the surface quality was improved significantly. Figs. 4(b), 7 (b) and 8 (b) show that the average edge chipping width of specimens reduced with the decrease of grain size in the same grinding parameters. In Fig. 4(b), when the feed rate was set at 6 mm/ min, the average edge chipping width of specimen was 18.304 μm, 11.929 μm and 6.345 μm ground by the 200#, 600# and 1000# microgrinding tools, respectively. Compared with Fig. 6(a) and (f), the edge chipping was narrower and denser when using smaller grain size of 1000#, and the grinding quality was improved significantly. Since the distance of the abrasive grains from the free surface shortened effectively by using the tools with smaller grain size, which was conducive to reducing the width of edge chipping [23–25]. In addition, the grinding force applied in each grain also reduced due to the reduced maximum undeformed chip thickness. Therefore, among
specimens machined with different grain sizes tools. Compare with Fig. 9(a), relatively more uniform grooves were observed with ductile removal model on the ground surface, as shown in Fig. 9(b). The height difference between grooves under the grain size of 1000# was smaller than that under the grain size of 600#, as shown in Fig. 5(a) and (f). Therefore, the surface quality was best for all experimental trials when the micro-grinding tools with small grain size (1000#) was adopted. As more abrasive grains distributed on the surface of micro-grinding tools with a small grain size per unit area, the distance between two adjacent grains L decreased with smaller grain size. According to the Eqs. (2) and (3), the maximum undeformed chip thickness also decreased with smaller grain size. In addition, the smaller grain size of micro-grinding tools generally had more uniform height of protrusion, which was conductive to eliminating the height difference between tiny grooves on the ground surface to some extent. Therefore, the surface 251
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Fig. 7. Effect of spindle speed on micro-grinding quality.
Fig. 8. Effect of grinding depth on micro-grinding quality.
Fig. 9. Ground surface topography with different grain sizes.
the five types of micro-grinding tools, the performance of microgrinding tool with a tip diameter of 0.5 mm and the grain size of 1000# showed the best performance. An optimal grinding quality (Ra: 0.619 μm, W: 4.579 μm) could be obtained.
(e). When the feed rate was set at 6 mm/min and the micro-grinding tools was 600#, the surface roughness of specimen ground by the micro-grinding tools with tip diameters of 0.3 mm, 0.5 mm and 0.7 mm was 1.106 μm, 0.840 μm and 0.707 μm, respectively, as shown in Fig. 10(a). For the micro-grinding tool with large the tip diameter, the grinding depth had limited effect on ground surface roughness, as shown in Fig. 10(c). As the grinding depth increased from 5 μm to 25 μm, the surface roughness of specimen ground by the micro-grinding tools with tip diameters of 0.3 mm, 0.5 mm and 0.7 mm increased by 0.409 μm, 0.123 μm and 0.090 μm, respectively. Fig. 5(a) and (e) shows the 3D morphology of ground surface with different tool tip diameters under the same grinding parameters. It was seen that there was shallow
3.5. Effect of tool tip diameter on the micro-grinding quality Fig. 10 shows the effect of tool tip diameter on the ground surface roughness and the average edge chipping width under the tools with the same grain size. Experimental result shows that the surface roughness decreased with larger tool tip diameter under the same feed rate, spindle speed and grinding depth as shown in Fig. 10(a), (c) and 252
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Fig. 10. Effect of tool tip diameter on micro-grinding quality.
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Fig. 11. Tool tip diameter loss with the number of grinding times under different micro-grinding tools. Fig. 13. Effect of micro-grinding tool wear on surface roughness.
grinding scratches on the ground face by the micro-grinding tool with a larger tip diameter. As the abrasive grains on a large micro-grinding tool possessed faster grinding speed under the same spindle speed, and more abrasive grains were involved in the grinding process per unit time, the maximum undeformed chip thickness decreased according to Eq. (3). Additionally, the micro-grinding tool with larger tip diameter exhibited better stiffness performance. Although the grinding force increased significantly with higher grinding depth, the bending deformation of micro-grinding tool with larger tip diameter was smaller. Therefore, the grinding depth showed limited effect on the surface roughness when using smaller grain size micro-grinding tools. Fig. 10(b), (d) and (f) show that the average edge chipping width of specimens was small when using the micro-grinding tools with large tip diameter under the same grinding parameters. Smaller and denser chippings were found on the edge of the specimen ground by the microgrinding tool with larger tip diameter of 0.7 mm (Fig. 6(e)). The microgrinding tool with larger tip diameter presented faster grinding speed under the same grinding parameters, thus the maximum undeformed chip thickness decreased and the grinding force applied in grains was reduced. In addition, the micro-grinding tool with larger tip diameter showed better stiffness performance, which also greatly reduced the generation of edge chipping. Therefore, in the case of meeting the dimensional requirement of micro-structure, the micro-grinding tools with larger tip diameter should be selected as far as possible in practical
Fig. 14. Effect of micro-grinding tool wear on average edge chipping width.
Fig. 12. Surface topography of worn micro-grinding tools. (O abrasive grain with high protrusion O attrition, O fracture, O pull-out). 254
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chipping width of specimen ground by the tool with a tip diameter of 0.5 mm and the grain size of 1000# was minimum down to 5.521 μm. In the initially wear stage, the protrusion height of grains varied greatly and only a small number of grains participated in grinding. The high grains resulted in deep grinding scratches on ground surface. The specimen was mostly removed in brittle mode, which led to poor surface quality. Besides, the high grains had large maximum undeformed chip thickness and grinding force. As a result, the edge of specimen occurred serious chipping and the average edge chipping width was large. With the progress of micro-grinding, the grains with higher protrusion height wore seriously and the height difference between grains decreased. Consequently, the tools wear became slowly and gone into slowly steady wear stage. At this stage, more number of grains were involved in grinding and the grinding force became more stably. Therefore, both of the surface roughness and edge chipping were decreasing and the grinding quality improved significantly.
micro-grinding. 3.6. Effect of tool wear on the micro-grinding quality Fig. 11 shows the curves of tool tip diameter loss with different grinding times. It was seen that the curves of five types of microgrinding tools had the same tendency with grinding time. At the beginning stage, the tip diameter losses of five micro-grinding tools increased rapidly. After about 60 grinding times, the tip diameter losses began to increase slowly. Therefore, the wear of micro-grinding tools could be divided into two stages. One was featured with initial fast wear, and the other was characterized with slow and steady wear. At the beginning, some abrasive grains with high protrusion distributed unevenly on the surface of micro-grinding tools, as shown in Fig. 12(a). The high protruding grains were easy to be rapidly fractured due to continuous mechanical impact, which resulted in rapid tool tip diameter loss. In addition, the protrusion height of grains differed greatly, and only a few grains were involved in the micro-grinding process. The grains with higher protrusion height had deeper grinding depth, so that the maximum undeformed chip thickness and the grinding force acting on these grains increased. The wear form of the grains is mainly fracture at initially quick wear stage as shown in Fig. 12(b). Therefore, the tip diameter loss of all the five types of micro-grinding tools increased sharply at initially wear stage. For continuous grinding process, the tools gradually entered slow and steady wear stage. At this stage, since the grains became dull and their protrusion heights were relatively unified, the number of active grains gradually increased and the grinding force decreased accordingly. The wear form was mainly attrition wear at this stage, as shown in Fig. 12(c). Therefore, the tool tip diameter loss varied slowly at the second stage. It was found that the micro-grinding tool with the small grain size manifested less tip diameter loss under the same grinding times in Fig. 11. The tip diameter loss of micro-grinding tools with grain size of 200#, 600# and 1000# was 10.02 μm, 7.03 μm and 6.58 μm, respectively. There were more active grains participated in grinding contact zone when the micro-grinding tool with small grain size was used. Hence, the maximum undeformed chip thickness reduced and grinding force acting on grains also decreased accordingly. Therefore, the phenomenon of grains fracture infrequently occurred and tool tip diameter loss was less by using the tools with a smaller average grain size. The micro-grinding tool with larger tip diameter shows less tip diameter loss under the same grinding times and grain size. When the grinding times was 100 and the grain size of micro-grinding tool was 600#, the loss of tools with diameter of 0.3 mm, 0.5 mm, and 0.7 mm, respectively, increased by 7.76 μm, 7.03 μm, and 5.75 μm. The tool with larger tip diameter presented higher grinding speed under the same spindle speed. Thus both of the maximum undeformed chip thickness and grinding force acting on grains were decreased according to Eqs. (3) and (4). Therefore, there was relatively less grains fractured or pulled out with larger tip diameter. Fig. 13 shows the specimens ground by five types of micro-grinding tools have the same variation trend with different grinding time. At the beginning, the ground surface roughness decreased rapidly. After 60 grinding times, the ground surface roughness fluctuated within a certain range. For example, when the micro-grinding tool with a tip diameter of 0.5 mm and the grain size of 1000# was used, the ground surface roughness decreased from 1.063 μm (20 grinding times) to 0.613 μm (60 grinding times), then it fluctuated between 0.613 and 0.678 μm. Fig. 14 shows the variation curve of average edge chipping width with the number of grinding times under different micro-grinding tools. It was seen that the average edge chipping width machined by the five micro-grinding tools decreased rapidly at the beginning, then it fluctuated within a certain range with the number of grinding times. Among the five types of micro-grinding tools, the average edge
4. Conclusions Based on comprehensive investigation and analysis, the following conclusions can be drawn: (1) The impact of edge chipping width on surface integrity of microstructure cannot be neglected. Both surface roughness and the average edge chipping width were proposed to evaluate microgrinding quality. (2) Surface roughness and the average edge chipping width increased at a near-liner trend with the increase of feed rate, while the spindle speed exhibited limited effect. When the grinding depth was greater than a certain depth about 15 μm, both of them significantly increased with higher grinding depth. The critical grinding depth was related to the stiffness and the length-to-diameter ratio of microgrinding tool. (3) Among the five types of micro-grinding tools, the micro-grinding tool with a tip diameter of 0.5 mm and the grain size of 1000# presented superior grinding performance. With the same grinding process parameters, the surface roughness was recorded down to 0.619 μm and the average edge chipping width was optimal (4.579 μm). (4) The wear of micro-grinding tools could be divided into initial and fast wear stage together with slow and steady wear stage. The surface roughness and average edge chipping width decreased rapidly at the beginning and then fluctuated within a certain range with the increase number of grinding times. Acknowledgements This work is funded by the National Natural Science Foundation of China (51675170, 51875192). I would like to express my gratitude to all those who helped me during the writing of this manuscript. References [1] Aurich JC, Carrella M, Walk M. Micro grinding with ultra small micro pencil grinding tools using an integrated machine tool. CIRP Ann Manuf Technol 2015;64(1):325–8. https://doi.org/10.1016/j.cirp.2015.04.011. [2] Walk M, Aurich JC. Integrated desktop machine tool for manufacturing and application of ultra-small micro pencil grinding tools. Procedia CIRP 2014;14:333–8. https://doi.org/10.1016/j.procir.2014.06.003. [3] Pratap A, Patra K, Dyakonov AA. Manufacturing miniature products by microgrinding: a review. Procedia Eng 2016;150:969–74. https://doi.org/10.1016/j. proeng.2016.07.072. [4] Francis NK, Viswanadhan KG, Paulose MM. SAFBM of softer materials: an investigation into the micro-cutting mechanisms and the evolution of roughness profile. Mater Manuf Process 2016;31(7):969–75. https://doi.org/10.1080/ 10426914.2015.1090585. [5] Burton G, Goo CS, Zhang Y. Use of vegetable oil in water emulsion achieved through ultrasonic atomization as cutting fluids in micro-milling. J Manuf Processes 2014;16(3):405–13. https://doi.org/10.1016/j.jmapro.2014.04.005.
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