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Dry electro-contact discharge mutual-wear truing of micro diamond wheel V-tip for precision micro-grinding
International Journal of Machine Tools & Manufacture 60 (2012) 44–51
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International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Dry electro-contact discharge mutual-wear truing of micro diamond wheel V-tip for precision micro-grinding J. Xie n, H.F. Xie, M.J. Luo, T.W. Tan, P. Li School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 October 2011 Received in revised form 4 May 2012 Accepted 10 May 2012 Available online 19 May 2012
A novel V-tip truing of metal-bonded fine diamond wheel is proposed using a hybrid of electro-contact discharge (ECD) and mechanical mutual-wear between diamond wheel and electrode without any coolant. The objective is to explore an eco-efficient truing of micro diamond wheel V-tip for microgrinding. First, a V-tip truing of metal-bonded #600 diamond wheel was performed with the ideal V-tip angle of 601; then the effects of dry discharge variables were analyzed on wheel V-tip form error, wheel V-tip angle and wheel V-tip radius; next, truing ratio and truing efficiency were observed; finally, the micro-groove of quartz glass was ground by using the trued diamond wheel V-tip. It is shown that continuous spark discharges may be maintained during truing as the pulse voltage ranges 5 V to 7 V, leading to the least wheel V-tip form error, the least wheel V-tip angle and the least wheel V-tip radius, respectively. When the suitable discharge conditions are employed, the Dry ECD mutual-wear truing may achieve less wheel V-tip form error, less wheel V-tip angle and less wheel V-tip radius than mechanical truing. It may also improve truing ratio by about 350 times and truing efficiency by about 59 times against mechanical truing, respectively. After micro-grinding of quartz glass using the trued diamond wheel V-tip, the integrated micro-groove may be ground with the average absolute angle error of 0.591 and the average surface roughness of 0.128 mm. It is confirmed that the micro diamond wheel V-tip can be fabricated by Dry ECD mutual-wear truing for precision micro-grinding of hard and brittle material. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Diamond wheel Electro-contact discharge Dry V-tip truing Micro-grinding
1. Introduction A micro-structured surface can produce many predominant functions and features compared to smooth surface in micro-electro mechanical systems. Its fabrication mainly depends on photolithography, etching and laser techniques, but these approaches require extensive capital investment, process time and pollute liquid. Hence, researchers have focused on eco-effective mechanical micro-machining. The micro-milling was first considered using a micro-end tool. For example, the T-shaped micro-endmill with 101.6 mm in diameter and 480 mm in fluted axial depth was used to fabricate the micro-barbs with 68–174 mm in width and 84–460 mm in height for medical implants [1]. Moreover, the micro-cutting tool made of cemented tungsten carbide with 0.6 mm in grain size was fabricated by electrical discharge machining to realize the micro-drilling of diameter-3 mm hole [2]. A plough method was also used to fabricate micro-grooves on inner surface of copper tube [3]. However, these approaches easily produced burrs on micro-structured edges.
n
Corresponding author. Tel.: þ86 20 87114634; fax: þ86 20 87111038. E-mail addresses: [email protected], [email protected] (J. Xie).
0890-6955/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijmachtools.2012.05.006
It has been known that optimum micro-scale machining conditions may minimize burr size [4]. Moreover, the chatter can be suppressed with process damping in micro-end milling [5]. In order to eliminate these burrs completely, lapping and water jet should be employed to deburr after micro-milling [6]. An abrasive flow machining was also utilized to smooth the burrs of micro-channel surface of stainless steel [7]. However, these additional deburr approaches increased process time. A single-crystal diamond tool is an alternative to perform a micro-machining without any burrs. This is because the sharp edge of superhard diamond tool may deburr during mechanical machining. For example, a rotational single-crystal diamond tool of radius 0.2 mm was validly used to perform a CNC end milling of micro-inducer with the depth of cut of 20 mm and the spindle rotation of 35,000 rpm [8]. However, these approaches were only able to machine easy-cut metallic materials. A diamond grinding tool can be used to perform a micromachining of difficult-cut materials such as silicon, glass, ceramics and tungsten carbide. For example, a diamond grinding tool with 45 mm in diameter and 1–3 mm in grain size was used to machine the micro-groove with 24 mm in width and 5 mm in depth on tungsten carbide surface [9]. Moreover, a sharp diamond wheel V-tip was also employed to fabricate the micro-pyramidstructured Si surface [10].
J. Xie et al. / International Journal of Machine Tools & Manufacture 60 (2012) 44–51
In order to assure the ground form accuracy, a metal-bonded diamond wheel needs to be employed, but it is very difficult to remove wheel metal bond and sharpen fine diamond grains. Until now, ELID (electrolytic in-process dressing) [11], EDM (electro-discharge machining) dressing [12] and ECDM (electro-chemical discharge machining) dressing [13] have been successfully applied to sharpening metal-bonded diamond wheel for grinding of hard and brittle materials. However, they all need pollutive coolant and a complex system to control the discharge gap. Hence, dry electro-contact discharge (Dry ECD) dressing has been introduced without any coolant and special control system for dressing metal-bonded #600–1500 diamond wheels [14–16]. Further, the Dry ECD truing was developed to assure the profile accuracy of trued diamond wheel to be sub-micron-scale [17]. However, these Dry ECD dressing and truing were only used for plate grinding. Although a CNC mutual-wear V-tip truing was successfully applied to metal-bonded #600 diamond wheel [10], its production efficiency was very low. In this paper, a novel V-tip truing of metal bond diamond wheel is proposed using a hybrid of Dry ECD removal and mutual-wear between diamond wheel and electrode. In Dry ECD mutual-wear truing, continuous spark discharges were maintained between diamond wheel bond and electrode by controlling discharge variables. The objective is to realize an eco-efficient truing of micro diamond wheel V-tip for precision micro-grinding. First, a truing of metal-bonded #600 diamond wheel V-tip was performed with the ideal V-tip angle of 601; then truing ratio and truing efficiency were observed in contrast to mechanical truing; next, the effects of dry discharge variables were investigated on wheel V-tip angle, wheel V-tip radius and wheel V-tip form accuracy, finally, the microgroove of quartz glass was ground by the Dry ECD mutual-wear trued diamond wheel V-tip.
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2. Dry ECD mutual-wear truing of diamond wheel V-tip Fig. 1 shows electro-contact discharge (ECD) dressing principle of metal-bonded diamond wheel. In this study, ECD dressing was employed without any coolant. This is because the dry impulse discharge removal was less than the wet one [16], thus it may sharpen finer diamond grains from wheel working surface [15]. In dressing, the conductive chip is first gradually formed as soon as a diamond grain cuts electrode (see Fig. 1a). As the rolled-up chip approaches the metal bond to a certain extent, an impulse spark discharge occurs between the diamond wheel and the electrode (see Fig. 1b). After this impulse spark discharge, a discharge crater is formed on the metal bond surface of diamond wheel (see Fig. 1c). Gradually, the metal bond around grain is removed, and then the micron-scale diamond grain is protruded from wheel working surface. Fig. 2 shows Dry ECD mutual-wear truing of diamond wheel V-tip. In mechanical mutual-wear truing, the rotary diamond wheel was driven with the wheel speed N to grind a GC dresser along the crossed V-shaped tool paths with the feed speed vf in CNC grinding system (see Fig. 2a), and then a wheel V-tip profile was gradually produced with the depth of cut a through the CNC mutual-wear between diamond wheel and dresser, whose wheel V-tip angle at is equal to the angle of CNC V-shaped tool paths [10]. In Dry ECD mutual-wear truing, the dresser was replaced by a conductive material, called electrode, and then continuous pulse voltages were applied to diamond wheel (positive pole) and electrode (negative pole) (see Fig. 2b). In this study, the wheel V-tip angle at was designed as 601, thus the crossed angle of V-shaped tool paths was chosen as 601 in truing experiments. Fig. 3 shows the Dry ECD mutual-wear truing setup in experiment. It was composed of CNC system, grinder, diamond wheel, working table, straight polarity and pulse power supply. The
Fig. 1. Dry ECD dressing principle: (a) electrode chip formation, (b) spark discharge occurrence and (c) metal bond removal.
Fig. 2. Dry ECD mutual-wear truing of diamond wheel V-tip: (a) truing mode and (b) truing process.
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Fig. 3. Dry ECD mutual-wear truing setup.
Table 1 Dry ECD mutual-wear truing conditions. CNC grinder
SMRART B818
Diamond wheel CNC tool paths Electrode Truing Discharge variables
SD600, Metal-bonded, N ¼3826 rpm V-shaped linear interpolation movement Bronze and GC grains vf ¼ 200 mm/min, a ¼3 mm/Sa ¼150 mm Duty cycle ¼50% E ¼1 V, 3 V, 5 V, 7 V, 10 V f ¼100 Hz, 500 Hz, 1000 Hz
The wheel V-tip profile point error en was defined as a normal deviation of measured point relative to fitted side line l is shown in Fig. 4(c). In order to evaluate form-truing accuracy, wheel V-tip form error PV was introduced, which was defined as the peak and valley distance of the wheel V-tip profile point errors distribution (see Fig. 4c). Moreover, the wheel V-tip angle at was regarded as the angle of two fitted side lines (see Fig. 4b). The wheel V-tip radius rt is regarded as the arc radius of trued diamond wheel V-tip (see Fig. 4b) decreasing the wheel V-tip radius rt may increase its sharpness.
4. Diamond wheel removal and electrode wear Table 2 Mechanical truing conditions. CNC grinder
SMRART B818
Diamond wheel Tool paths Dresser Rough truing Fine truing Coolant
SD600, Bronze bonded, N ¼3826 rpm V-shaped linear interpolation movement #600 GC, Ceramic bond vf ¼500 mm/min, a ¼10 mm/Sa ¼4.95 mm vf ¼50 mm/min, a ¼5 mm/Sa ¼50 mm Water
electrode was composed of a hybrid of bronze and #600 GC grains. The GC grains on electrode working surface may cut off arc discharge and prevent short circuit between diamond wheel and electrode during truing. Detailed Dry ECD mutual-wear truing conditions are shown in Table 1. Moreover, a mechanical truing without ECD was also employed to sharpen the diamond wheel V-tip using a CNC mutual-wear approach [10]. The objective is to compare their wheel V-tip truing performances. Detailed mechanical truing conditions are shown in Table 2.
3. Evaluation of trued diamond wheel V-tip Fig. 4 shows the measured results of trued diamond wheel V-tip. The wheel V-tip profile was defined as the section V-tip profile specialized by wheel-axis. In measurement, the trued wheel V-tip profile was on-machine replicated on a carbon plate using a rotary diamond wheel [10]. Then, a white light interferometer BMT SMS Expert 3D was employed to measure the replicated plate flank profile. Because the noise parts were produced on replicated wheel V-tip profile boundary (see Fig. 4a), the profile extracted from the boundary noises was regarded as the trued wheel V-tip profile (see Fig. 4b).
Fig. 5 shows the volumetric removal model of wheel V-tip in truing. After truing, the wheel profiles were worn to the dashed parts. According to geometrical relationship, the initial diamond wheel volume V is described as follows: pa p pa p p t t V ¼ WD2 W 2 D tan þ W 3 tan2 ð1Þ 4 4 2 12 2 where W is the thickness of diamond wheel and D is the initial outer diameter of diamond wheel (see Fig. 5). After truing, the diamond wheel diameter was reduced from D to Dw. The trued diamond wheel volume Vw is described as follows: pa p pa p p t t W 3 tan2 V w ¼ WD2w W 2 Dw tan þ ð2Þ 4 4 2 12 2 Hence, the removal volume DV of trued diamond wheel can be calculated as follows:
DV ¼ VV w ¼
p 4
WðD2 D2w Þ
p 4
W 2 ðDDw Þ tan
pa t
2
ð3Þ
Considering the wheel radial worn height DR¼(D Dw)/2 is much less than D, D þDw approximates 2D. Hence, it may be approximately given as follows: h pa i 1 t DV pW DR 2DW tan ð4Þ 2 2 The wheel radial worn height DR was regarded as the segment height deviation of the on-machine replicated wheel profiles on carbon plate before truing and after truing [16]. Because the wheel profile was replicated with a rotating wheel, the spindle mounting error does not influence the measurement of wheel radial worn height DR. Fig. 6 shows the volumetric removal model of electrode or dresser in truing. The electrode or dresser would also be worn
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Fig. 6. Volumetric removal model of electrode (dresser) in truing: (a) electrode profile and (b) YZ section specialized by point Oe.
Hence, the electrode or dresser removal volume Dv may be approximately calculated as follows: \
Dv wDhAB ¼ wDhD arcsin
b D
ð6Þ
where w and Dh are the thickness and the worn height of electrode or dresser, respectively (see Fig. 6b). Because the profile wear size of superhard diamond wheel was much less than that of electrode or dresser during mutual-wear, the Dh is approximately equal to Sa. In order to evaluate the truing ability, the truing ratio g was introduced, which was defined as the ratio of wheel removal volume DV to the electrode or dresser removal volume Dv. Hence, the truing ratio g is described as follows:
g¼
DV Dv
¼
Fig. 4. Measurement of trued diamond wheel V-tip: (a) measured data of replicated plate flank and (b) wheel V-tip profile and (c) wheel V-tip form errors.
pW DR 2DW tan 2wð
P
pa
aÞD arcsin
2 b D
t
ð7Þ
Moreover, the truing efficiency Z was defined as the ratio of volumetric wheel removal DV to truing time T. Hence, it is described as follows: pDR 2DW tan p2at W ð8Þ Z¼ 2T
5. Results and discussions 5.1. Discharge power P
Fig. 5. Volumetric removal model of diamond wheel in V-tip truing: (a) diamond wheel shape and (b) YOZ section.
after truing (see Fig. 3a). Because the arc AB of electrode or dresser is the outer profile of diamond wheel, it is described as follows: \
AB ¼
D b=2 b 2 arcsin ¼ D arcsin 2 D=2 D
where b is the length of electrode or dresser (see Fig. 6a).
ð5Þ
Fig. 7 shows the discharge power P versus pulse voltage E and pulse frequency f in Dry ECD mutual-wear truing. It is shown that an increase in the pulse voltage E led to an increase in discharge power P. When the pulse voltage ranged 1–3 V and 7–10 V, the discharge power P increased slowly. The former is because the discharge power was too low to break down the air between diamond wheel and electrode. The latter is because an arc discharge greatly lowered discharge voltage so that the shortcurrent could happen between diamond wheel and electrode. It is identical to the results of impulse discharge machining [15]. When the pulse voltage E was increased from 3 V to 7 V, the discharge power P, however, increased linearly. Hence, the discharge voltage might approximate the pulse discharge voltage so that the pulse discharge current might be very little and the pulse duration might be very short. This means that the spark discharges continuously happened between diamond wheel and electrode. It contributed to micron-scale grain protrusion from wheel working surface along with micron-scale impulse removals of wheel metal bond [14].
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Fig. 7. Discharge power P versus pulse voltage E and pulse frequency f. Fig. 9. Wheel V-tip angle at versus pulse voltage E and pulse frequency f.
Fig. 8. Wheel V-tip form error PV versus pulse voltage E and pulse frequency f.
Fig. 10. Wheel V-tip radius rt versus pulse voltage E and pulse frequency f.
5.2. Wheel V-tip form error PV versus dry discharge variables Fig. 8 shows the wheel V-tip form error PV versus dry discharge variables such as pulse voltage E and pulse frequency f. It is shown that the wheel V-tip form error PV mainly ranged 10–40 mm, whereas it was 94 mm in the case of mechanical truing. This also means that Dry ECD mutual-wear truing may achieve higher wheel V-tip form accuracy than mechanical truing. It is also found that the least wheel V-tip form error PV may be achieved as the pulse voltage E ranged 5–7 V. This is because the spark discharges may assure continuous micron-scale removal of metal bond around diamond grain and sharpen diamond wheel V-tip. Moreover, the wheel V-tip form error PV had little relations with pulse frequency.
shown that the wheel V-tip radius rt decreased with decreasing pulse frequency f. Although the wheel V-tip radius rt had an increase trend with increasing pulse voltage E as the E was not larger than 5 V, it reached the least value of 26, 31 and 35 mm at E¼7 V for f ¼100, 500 and 1000 Hz, respectively. This is because the wheel V-tip would be easily burned in high impulse discharge power and high pulse frequency. Moreover, the least wheel V-tip radius of 26 mm in the case of E¼7 V and f¼ 100 Hz was much less than the one of 42 mm in the case of mechanical truing. This also means that Dry ECD mutualwear truing with suitable discharge variables may make the wheel V-tip sharper than mechanical truing.
5.3. Wheel V-tip angle at versus dry discharge variables
5.5. Truing ratio and truing efficiency
Fig. 9 shows the wheel V-tip angle at versus dry discharge variables such as pulse voltage E and pulse frequency f. It is shown that in the case of f¼500–1000 Hz, the wheel V-tip angle at decreased with increasing pulse voltage E as the E was less than or equal to 7 V, but it increased as the E was larger than 7 V. In contrast, the wheel V-tip angle at has no relations with the E in the case of f ¼100 Hz. This is because the pulse frequency was so low that the mechanical truing was mainly performed in truing. Moreover, the trued wheel V-tip angles may approach the ideal wheel V-tip angle of 601 in the case of E¼5–7 V and f¼500 Hz, leading to the least wheel V-tip angle.
Fig. 11 shows the truing ratio g versus pulse voltage E and pulse frequency f. The truing ratio g was calculated according to Eq. (7). It is shown that in the case of high pulse frequency in 1000 Hz, the largest truing ratios of 5.8–6.2 mm3/mm3 were achieved at E¼ 3–5 V. However, the least truing ratios of 0.5–1.4 mm3/mm3 were achieved at E¼5 V in the cases of f¼ 500 Hz and 100 Hz. This means that low pulse frequency produced much less truing ratio at E¼5 V than high pulse frequency, but they achieved less wheel V-tip angle and less wheel V-tip radius (see Figs. 9 and10). The truing ratios and truing efficiencies for Dry ECD mutualwear truing and mechanical truing are shown in Table 3. The truing efficiency Z was calculated according to Eq. (8). The truing ratio g and truing efficiency Z were regarded as their average values for all experiments, respectively. It is shown that the Dry ECD mutual-wear truing improved the truing ratio by about 350 times and truing efficiency by about 59 compared to mechanical
5.4. Wheel V-tip radius rt versus dry discharge variables Fig. 10 shows wheel V-tip radius rt versus dry discharge variables such as pulse voltage E and pulse frequency f. It is
J. Xie et al. / International Journal of Machine Tools & Manufacture 60 (2012) 44–51
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truing. As a result, Dry ECD mutual-wear truing can produce a remarkable truing ability of fine diamond wheel V-tip.
metal bond around diamond grain on wheel working surface. This contributed to sharpening of diamond wheel V-tip during truing.
5.6. Grain protrusion topography for Dry ECD mutual-wear truing
5.7. Micro-grinding of micro-groove on quartz glass substrate
Fig. 12 shows the SEM photos of grain protrusion topography before and after Dry ECD mutual-wear truing (E¼7 V and f¼500 Hz). It is shown that micro diamond grains were enchased with wheel metal bond, and their cutting edges were worn to be a near plate before truing (see Fig. 12a). After truing, the diamond grains were well protruded from the metal bond surface, and their cutting edges were also sharpened (see Fig. 12b). This means that Dry ECD mutual-wear truing is valid to sharpen the metalbonded fine diamond wheel V-tip. Fig. 13 shows the SEM photo of electrode working surface. It is shown that many micro GC grains were dispersedly distributed on electrode working surface. These GC grains not only made a mutual wear with diamond wheel but also prevented an arc discharge between diamond wheel and electrode. This may lead to continuous impulse spark discharges for a table micron-scale removal of
Fig. 14 shows SEM photos of ground micro-groove of quartz glass substrate. Before micro-grinding, the Dry ECD mutual-wear trued #600 diamond wheel V-tip (E¼7 V and f¼500 Hz) was performed. In experiments, 9 micro-grooves were ground. The micro-groove depths ranged 615–807 mm. The micro-grinding conditions were given by the wheel speed of 3000 rpm, the table speed of 100 mm/ min and the depth of cut of 1 mm and water coolant. It is shown that integrated micro-groove was produced without any brittle crack edges after micro-grinding (see Fig. 14a). However, micro-shape deformation (see Fig. 14a) and surface damage (see Fig. 14b) happened on diamond wheel V-tip. This is because the diamond wheel V-tip was easily worn when the micro-groove depth was large. This problem may be resolved through on-machine wheel V-tip truing during micro-grinding. Fig. 15 shows the measurement of ground micro-groove quality. In measurement, a depth-of-field microscope KEYENCE VHX-600E
Fig. 11. Truing ratio g versus pulse voltage E and pulse frequency f.
Fig. 13. SEM photo of electrode working surface.
Table 3 Truing ratio and truing efficiency. Truing method
Dry ECD mutual-wear truing
Mechanical truing
Electrode/Dresser Diamond wheel Wheel radial worn height DR Electrode/Dresser worn height Dh Truing ratio g Truing efficiency Z
Thickness w¼ 8 mm Width b ¼29 mm Diameter D ¼ 150 mm Thickness W ¼5 mm 20.8 mm 0.15 mm 2.82 mm3/mm3 12.25 mm3/min
Thickness w¼24.7 mm Width b ¼ 48.6 mm Diameter D ¼ 150 mm Thickness W ¼5 mm 10.2 mm 5 mm 0.008 mm3/mm3 0.21 mm3/min
Fig. 12. SEM grain protrusion topography: (a) before Dry ECD V-tip mutual-wear truing and (b) after Dry ECD mutual-wear V-tip truing.
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Fig. 14. SEM photos of micro-grooved quartz glass: (a) groove profile and (b) groove tip surface.
Fig. 15. Measurement of ground micro-groove quality: (a) measured micro-groove profile parameters (micro-groove angle aw and micro-groove tip radius rw) and (b) measured micro-groove angles distribution.
was employed. It is shown that the micro-groove tip radius rw was 46 mm for the micro-groove depth of 630 mm. It is larger than the one in the case of mechanical truing for micro-groove depth of about 150 mm [10]. This is because diamond wheel V-tip was easily worn when deep micro-groove was ground. It is also seen that the micro-groove angle aw was dispersedly distributed around the ideal V-tip angle of 601. The average value was 59.951; the average absolute angle error was only 0.591. This also means that the form accuracy of ground micro-groove may be controlled by the truing accuracy of diamond wheel V-tip. The surface roughness of ground micro-groove was measured by using a roughmeter TR200. The average surface roughness Ra was 0.128 mm for 12 measured points. It is identical to the plate-ground surface roughness Ra of optic glass (BK10) using the Dry ECD dressed #600 diamond wheel [16]. However, it is much less than the micro-grooved Si surface roughness Ra of 0.26– 0.30 mm using the mechanical-trued #600 diamond wheel V-tip [10] and the plate-ground Al2O3 surface roughness Ra of 0.25 mm using the Dry ECD trued #600 diamond wheel [15], respectively. As a result, Dry ECD mutual-wear truing is valid to precisely sharpen fine diamond wheel V-tip of for precision micro-grinding of hard and brittle material.
6. Conclusions A Dry Electro-contact Discharge (ECD) mutual-wear truing is developed to sharpen the #600 diamond wheel V-tip with an ideal V-tip angle of 601 for precision micro-grinding of hard and brittle material. The detailed results are included as follows: 1. The continuous spark discharges may be maintained when the pulse voltage ranges 5–7 V. However, the arc discharge occurs
when it is larger than 7 V. The spark discharge conditions contribute to good grain protrusion on wheel working surface, leading to the least wheel V-tip form error, the least wheel V-tip angle and the least wheel V-tip radius. 2. Dry ECD mutual-wear truing with suitable discharge variables may achieve less wheel V-tip form error, less wheel V-tip angle and less wheel V-tip radius than mechanical truing. It may improve the truing ratio by about 350 times and truing efficiency by about 59 times against mechanical truing, respectively. 3. The integrated micro-groove of quartz glass may be achieved without any brittle crack edges after micro-grinding using the Dry ECD mutual-wear trued diamond wheel V-tip. The average absolute error of wheel V-tip angle is 0.591. The surface roughness Ra of ground micro-groove averagely reaches 0.128 mm. Acknowledgments The project was sponsored by the Natural Science Foundation of China (Grant No. 51075156).
Appendix A. Supplementary Information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ijmachtools. 2012.05.006.
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Contents lists available at SciVerse ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Dry electro-contact discharge mutual-wear truing of micro diamond wheel V-tip for precision micro-grinding J. Xie n, H.F. Xie, M.J. Luo, T.W. Tan, P. Li School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 October 2011 Received in revised form 4 May 2012 Accepted 10 May 2012 Available online 19 May 2012
A novel V-tip truing of metal-bonded fine diamond wheel is proposed using a hybrid of electro-contact discharge (ECD) and mechanical mutual-wear between diamond wheel and electrode without any coolant. The objective is to explore an eco-efficient truing of micro diamond wheel V-tip for microgrinding. First, a V-tip truing of metal-bonded #600 diamond wheel was performed with the ideal V-tip angle of 601; then the effects of dry discharge variables were analyzed on wheel V-tip form error, wheel V-tip angle and wheel V-tip radius; next, truing ratio and truing efficiency were observed; finally, the micro-groove of quartz glass was ground by using the trued diamond wheel V-tip. It is shown that continuous spark discharges may be maintained during truing as the pulse voltage ranges 5 V to 7 V, leading to the least wheel V-tip form error, the least wheel V-tip angle and the least wheel V-tip radius, respectively. When the suitable discharge conditions are employed, the Dry ECD mutual-wear truing may achieve less wheel V-tip form error, less wheel V-tip angle and less wheel V-tip radius than mechanical truing. It may also improve truing ratio by about 350 times and truing efficiency by about 59 times against mechanical truing, respectively. After micro-grinding of quartz glass using the trued diamond wheel V-tip, the integrated micro-groove may be ground with the average absolute angle error of 0.591 and the average surface roughness of 0.128 mm. It is confirmed that the micro diamond wheel V-tip can be fabricated by Dry ECD mutual-wear truing for precision micro-grinding of hard and brittle material. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Diamond wheel Electro-contact discharge Dry V-tip truing Micro-grinding
1. Introduction A micro-structured surface can produce many predominant functions and features compared to smooth surface in micro-electro mechanical systems. Its fabrication mainly depends on photolithography, etching and laser techniques, but these approaches require extensive capital investment, process time and pollute liquid. Hence, researchers have focused on eco-effective mechanical micro-machining. The micro-milling was first considered using a micro-end tool. For example, the T-shaped micro-endmill with 101.6 mm in diameter and 480 mm in fluted axial depth was used to fabricate the micro-barbs with 68–174 mm in width and 84–460 mm in height for medical implants [1]. Moreover, the micro-cutting tool made of cemented tungsten carbide with 0.6 mm in grain size was fabricated by electrical discharge machining to realize the micro-drilling of diameter-3 mm hole [2]. A plough method was also used to fabricate micro-grooves on inner surface of copper tube [3]. However, these approaches easily produced burrs on micro-structured edges.
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Corresponding author. Tel.: þ86 20 87114634; fax: þ86 20 87111038. E-mail addresses: [email protected], [email protected] (J. Xie).
0890-6955/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijmachtools.2012.05.006
It has been known that optimum micro-scale machining conditions may minimize burr size [4]. Moreover, the chatter can be suppressed with process damping in micro-end milling [5]. In order to eliminate these burrs completely, lapping and water jet should be employed to deburr after micro-milling [6]. An abrasive flow machining was also utilized to smooth the burrs of micro-channel surface of stainless steel [7]. However, these additional deburr approaches increased process time. A single-crystal diamond tool is an alternative to perform a micro-machining without any burrs. This is because the sharp edge of superhard diamond tool may deburr during mechanical machining. For example, a rotational single-crystal diamond tool of radius 0.2 mm was validly used to perform a CNC end milling of micro-inducer with the depth of cut of 20 mm and the spindle rotation of 35,000 rpm [8]. However, these approaches were only able to machine easy-cut metallic materials. A diamond grinding tool can be used to perform a micromachining of difficult-cut materials such as silicon, glass, ceramics and tungsten carbide. For example, a diamond grinding tool with 45 mm in diameter and 1–3 mm in grain size was used to machine the micro-groove with 24 mm in width and 5 mm in depth on tungsten carbide surface [9]. Moreover, a sharp diamond wheel V-tip was also employed to fabricate the micro-pyramidstructured Si surface [10].
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In order to assure the ground form accuracy, a metal-bonded diamond wheel needs to be employed, but it is very difficult to remove wheel metal bond and sharpen fine diamond grains. Until now, ELID (electrolytic in-process dressing) [11], EDM (electro-discharge machining) dressing [12] and ECDM (electro-chemical discharge machining) dressing [13] have been successfully applied to sharpening metal-bonded diamond wheel for grinding of hard and brittle materials. However, they all need pollutive coolant and a complex system to control the discharge gap. Hence, dry electro-contact discharge (Dry ECD) dressing has been introduced without any coolant and special control system for dressing metal-bonded #600–1500 diamond wheels [14–16]. Further, the Dry ECD truing was developed to assure the profile accuracy of trued diamond wheel to be sub-micron-scale [17]. However, these Dry ECD dressing and truing were only used for plate grinding. Although a CNC mutual-wear V-tip truing was successfully applied to metal-bonded #600 diamond wheel [10], its production efficiency was very low. In this paper, a novel V-tip truing of metal bond diamond wheel is proposed using a hybrid of Dry ECD removal and mutual-wear between diamond wheel and electrode. In Dry ECD mutual-wear truing, continuous spark discharges were maintained between diamond wheel bond and electrode by controlling discharge variables. The objective is to realize an eco-efficient truing of micro diamond wheel V-tip for precision micro-grinding. First, a truing of metal-bonded #600 diamond wheel V-tip was performed with the ideal V-tip angle of 601; then truing ratio and truing efficiency were observed in contrast to mechanical truing; next, the effects of dry discharge variables were investigated on wheel V-tip angle, wheel V-tip radius and wheel V-tip form accuracy, finally, the microgroove of quartz glass was ground by the Dry ECD mutual-wear trued diamond wheel V-tip.
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2. Dry ECD mutual-wear truing of diamond wheel V-tip Fig. 1 shows electro-contact discharge (ECD) dressing principle of metal-bonded diamond wheel. In this study, ECD dressing was employed without any coolant. This is because the dry impulse discharge removal was less than the wet one [16], thus it may sharpen finer diamond grains from wheel working surface [15]. In dressing, the conductive chip is first gradually formed as soon as a diamond grain cuts electrode (see Fig. 1a). As the rolled-up chip approaches the metal bond to a certain extent, an impulse spark discharge occurs between the diamond wheel and the electrode (see Fig. 1b). After this impulse spark discharge, a discharge crater is formed on the metal bond surface of diamond wheel (see Fig. 1c). Gradually, the metal bond around grain is removed, and then the micron-scale diamond grain is protruded from wheel working surface. Fig. 2 shows Dry ECD mutual-wear truing of diamond wheel V-tip. In mechanical mutual-wear truing, the rotary diamond wheel was driven with the wheel speed N to grind a GC dresser along the crossed V-shaped tool paths with the feed speed vf in CNC grinding system (see Fig. 2a), and then a wheel V-tip profile was gradually produced with the depth of cut a through the CNC mutual-wear between diamond wheel and dresser, whose wheel V-tip angle at is equal to the angle of CNC V-shaped tool paths [10]. In Dry ECD mutual-wear truing, the dresser was replaced by a conductive material, called electrode, and then continuous pulse voltages were applied to diamond wheel (positive pole) and electrode (negative pole) (see Fig. 2b). In this study, the wheel V-tip angle at was designed as 601, thus the crossed angle of V-shaped tool paths was chosen as 601 in truing experiments. Fig. 3 shows the Dry ECD mutual-wear truing setup in experiment. It was composed of CNC system, grinder, diamond wheel, working table, straight polarity and pulse power supply. The
Fig. 1. Dry ECD dressing principle: (a) electrode chip formation, (b) spark discharge occurrence and (c) metal bond removal.
Fig. 2. Dry ECD mutual-wear truing of diamond wheel V-tip: (a) truing mode and (b) truing process.
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Fig. 3. Dry ECD mutual-wear truing setup.
Table 1 Dry ECD mutual-wear truing conditions. CNC grinder
SMRART B818
Diamond wheel CNC tool paths Electrode Truing Discharge variables
SD600, Metal-bonded, N ¼3826 rpm V-shaped linear interpolation movement Bronze and GC grains vf ¼ 200 mm/min, a ¼3 mm/Sa ¼150 mm Duty cycle ¼50% E ¼1 V, 3 V, 5 V, 7 V, 10 V f ¼100 Hz, 500 Hz, 1000 Hz
The wheel V-tip profile point error en was defined as a normal deviation of measured point relative to fitted side line l is shown in Fig. 4(c). In order to evaluate form-truing accuracy, wheel V-tip form error PV was introduced, which was defined as the peak and valley distance of the wheel V-tip profile point errors distribution (see Fig. 4c). Moreover, the wheel V-tip angle at was regarded as the angle of two fitted side lines (see Fig. 4b). The wheel V-tip radius rt is regarded as the arc radius of trued diamond wheel V-tip (see Fig. 4b) decreasing the wheel V-tip radius rt may increase its sharpness.
4. Diamond wheel removal and electrode wear Table 2 Mechanical truing conditions. CNC grinder
SMRART B818
Diamond wheel Tool paths Dresser Rough truing Fine truing Coolant
SD600, Bronze bonded, N ¼3826 rpm V-shaped linear interpolation movement #600 GC, Ceramic bond vf ¼500 mm/min, a ¼10 mm/Sa ¼4.95 mm vf ¼50 mm/min, a ¼5 mm/Sa ¼50 mm Water
electrode was composed of a hybrid of bronze and #600 GC grains. The GC grains on electrode working surface may cut off arc discharge and prevent short circuit between diamond wheel and electrode during truing. Detailed Dry ECD mutual-wear truing conditions are shown in Table 1. Moreover, a mechanical truing without ECD was also employed to sharpen the diamond wheel V-tip using a CNC mutual-wear approach [10]. The objective is to compare their wheel V-tip truing performances. Detailed mechanical truing conditions are shown in Table 2.
3. Evaluation of trued diamond wheel V-tip Fig. 4 shows the measured results of trued diamond wheel V-tip. The wheel V-tip profile was defined as the section V-tip profile specialized by wheel-axis. In measurement, the trued wheel V-tip profile was on-machine replicated on a carbon plate using a rotary diamond wheel [10]. Then, a white light interferometer BMT SMS Expert 3D was employed to measure the replicated plate flank profile. Because the noise parts were produced on replicated wheel V-tip profile boundary (see Fig. 4a), the profile extracted from the boundary noises was regarded as the trued wheel V-tip profile (see Fig. 4b).
Fig. 5 shows the volumetric removal model of wheel V-tip in truing. After truing, the wheel profiles were worn to the dashed parts. According to geometrical relationship, the initial diamond wheel volume V is described as follows: pa p pa p p t t V ¼ WD2 W 2 D tan þ W 3 tan2 ð1Þ 4 4 2 12 2 where W is the thickness of diamond wheel and D is the initial outer diameter of diamond wheel (see Fig. 5). After truing, the diamond wheel diameter was reduced from D to Dw. The trued diamond wheel volume Vw is described as follows: pa p pa p p t t W 3 tan2 V w ¼ WD2w W 2 Dw tan þ ð2Þ 4 4 2 12 2 Hence, the removal volume DV of trued diamond wheel can be calculated as follows:
DV ¼ VV w ¼
p 4
WðD2 D2w Þ
p 4
W 2 ðDDw Þ tan
pa t
2
ð3Þ
Considering the wheel radial worn height DR¼(D Dw)/2 is much less than D, D þDw approximates 2D. Hence, it may be approximately given as follows: h pa i 1 t DV pW DR 2DW tan ð4Þ 2 2 The wheel radial worn height DR was regarded as the segment height deviation of the on-machine replicated wheel profiles on carbon plate before truing and after truing [16]. Because the wheel profile was replicated with a rotating wheel, the spindle mounting error does not influence the measurement of wheel radial worn height DR. Fig. 6 shows the volumetric removal model of electrode or dresser in truing. The electrode or dresser would also be worn
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Fig. 6. Volumetric removal model of electrode (dresser) in truing: (a) electrode profile and (b) YZ section specialized by point Oe.
Hence, the electrode or dresser removal volume Dv may be approximately calculated as follows: \
Dv wDhAB ¼ wDhD arcsin
b D
ð6Þ
where w and Dh are the thickness and the worn height of electrode or dresser, respectively (see Fig. 6b). Because the profile wear size of superhard diamond wheel was much less than that of electrode or dresser during mutual-wear, the Dh is approximately equal to Sa. In order to evaluate the truing ability, the truing ratio g was introduced, which was defined as the ratio of wheel removal volume DV to the electrode or dresser removal volume Dv. Hence, the truing ratio g is described as follows:
g¼
DV Dv
¼
Fig. 4. Measurement of trued diamond wheel V-tip: (a) measured data of replicated plate flank and (b) wheel V-tip profile and (c) wheel V-tip form errors.
pW DR 2DW tan 2wð
P
pa
aÞD arcsin
2 b D
t
ð7Þ
Moreover, the truing efficiency Z was defined as the ratio of volumetric wheel removal DV to truing time T. Hence, it is described as follows: pDR 2DW tan p2at W ð8Þ Z¼ 2T
5. Results and discussions 5.1. Discharge power P
Fig. 5. Volumetric removal model of diamond wheel in V-tip truing: (a) diamond wheel shape and (b) YOZ section.
after truing (see Fig. 3a). Because the arc AB of electrode or dresser is the outer profile of diamond wheel, it is described as follows: \
AB ¼
D b=2 b 2 arcsin ¼ D arcsin 2 D=2 D
where b is the length of electrode or dresser (see Fig. 6a).
ð5Þ
Fig. 7 shows the discharge power P versus pulse voltage E and pulse frequency f in Dry ECD mutual-wear truing. It is shown that an increase in the pulse voltage E led to an increase in discharge power P. When the pulse voltage ranged 1–3 V and 7–10 V, the discharge power P increased slowly. The former is because the discharge power was too low to break down the air between diamond wheel and electrode. The latter is because an arc discharge greatly lowered discharge voltage so that the shortcurrent could happen between diamond wheel and electrode. It is identical to the results of impulse discharge machining [15]. When the pulse voltage E was increased from 3 V to 7 V, the discharge power P, however, increased linearly. Hence, the discharge voltage might approximate the pulse discharge voltage so that the pulse discharge current might be very little and the pulse duration might be very short. This means that the spark discharges continuously happened between diamond wheel and electrode. It contributed to micron-scale grain protrusion from wheel working surface along with micron-scale impulse removals of wheel metal bond [14].
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Fig. 7. Discharge power P versus pulse voltage E and pulse frequency f. Fig. 9. Wheel V-tip angle at versus pulse voltage E and pulse frequency f.
Fig. 8. Wheel V-tip form error PV versus pulse voltage E and pulse frequency f.
Fig. 10. Wheel V-tip radius rt versus pulse voltage E and pulse frequency f.
5.2. Wheel V-tip form error PV versus dry discharge variables Fig. 8 shows the wheel V-tip form error PV versus dry discharge variables such as pulse voltage E and pulse frequency f. It is shown that the wheel V-tip form error PV mainly ranged 10–40 mm, whereas it was 94 mm in the case of mechanical truing. This also means that Dry ECD mutual-wear truing may achieve higher wheel V-tip form accuracy than mechanical truing. It is also found that the least wheel V-tip form error PV may be achieved as the pulse voltage E ranged 5–7 V. This is because the spark discharges may assure continuous micron-scale removal of metal bond around diamond grain and sharpen diamond wheel V-tip. Moreover, the wheel V-tip form error PV had little relations with pulse frequency.
shown that the wheel V-tip radius rt decreased with decreasing pulse frequency f. Although the wheel V-tip radius rt had an increase trend with increasing pulse voltage E as the E was not larger than 5 V, it reached the least value of 26, 31 and 35 mm at E¼7 V for f ¼100, 500 and 1000 Hz, respectively. This is because the wheel V-tip would be easily burned in high impulse discharge power and high pulse frequency. Moreover, the least wheel V-tip radius of 26 mm in the case of E¼7 V and f¼ 100 Hz was much less than the one of 42 mm in the case of mechanical truing. This also means that Dry ECD mutualwear truing with suitable discharge variables may make the wheel V-tip sharper than mechanical truing.
5.3. Wheel V-tip angle at versus dry discharge variables
5.5. Truing ratio and truing efficiency
Fig. 9 shows the wheel V-tip angle at versus dry discharge variables such as pulse voltage E and pulse frequency f. It is shown that in the case of f¼500–1000 Hz, the wheel V-tip angle at decreased with increasing pulse voltage E as the E was less than or equal to 7 V, but it increased as the E was larger than 7 V. In contrast, the wheel V-tip angle at has no relations with the E in the case of f ¼100 Hz. This is because the pulse frequency was so low that the mechanical truing was mainly performed in truing. Moreover, the trued wheel V-tip angles may approach the ideal wheel V-tip angle of 601 in the case of E¼5–7 V and f¼500 Hz, leading to the least wheel V-tip angle.
Fig. 11 shows the truing ratio g versus pulse voltage E and pulse frequency f. The truing ratio g was calculated according to Eq. (7). It is shown that in the case of high pulse frequency in 1000 Hz, the largest truing ratios of 5.8–6.2 mm3/mm3 were achieved at E¼ 3–5 V. However, the least truing ratios of 0.5–1.4 mm3/mm3 were achieved at E¼5 V in the cases of f¼ 500 Hz and 100 Hz. This means that low pulse frequency produced much less truing ratio at E¼5 V than high pulse frequency, but they achieved less wheel V-tip angle and less wheel V-tip radius (see Figs. 9 and10). The truing ratios and truing efficiencies for Dry ECD mutualwear truing and mechanical truing are shown in Table 3. The truing efficiency Z was calculated according to Eq. (8). The truing ratio g and truing efficiency Z were regarded as their average values for all experiments, respectively. It is shown that the Dry ECD mutual-wear truing improved the truing ratio by about 350 times and truing efficiency by about 59 compared to mechanical
5.4. Wheel V-tip radius rt versus dry discharge variables Fig. 10 shows wheel V-tip radius rt versus dry discharge variables such as pulse voltage E and pulse frequency f. It is
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truing. As a result, Dry ECD mutual-wear truing can produce a remarkable truing ability of fine diamond wheel V-tip.
metal bond around diamond grain on wheel working surface. This contributed to sharpening of diamond wheel V-tip during truing.
5.6. Grain protrusion topography for Dry ECD mutual-wear truing
5.7. Micro-grinding of micro-groove on quartz glass substrate
Fig. 12 shows the SEM photos of grain protrusion topography before and after Dry ECD mutual-wear truing (E¼7 V and f¼500 Hz). It is shown that micro diamond grains were enchased with wheel metal bond, and their cutting edges were worn to be a near plate before truing (see Fig. 12a). After truing, the diamond grains were well protruded from the metal bond surface, and their cutting edges were also sharpened (see Fig. 12b). This means that Dry ECD mutual-wear truing is valid to sharpen the metalbonded fine diamond wheel V-tip. Fig. 13 shows the SEM photo of electrode working surface. It is shown that many micro GC grains were dispersedly distributed on electrode working surface. These GC grains not only made a mutual wear with diamond wheel but also prevented an arc discharge between diamond wheel and electrode. This may lead to continuous impulse spark discharges for a table micron-scale removal of
Fig. 14 shows SEM photos of ground micro-groove of quartz glass substrate. Before micro-grinding, the Dry ECD mutual-wear trued #600 diamond wheel V-tip (E¼7 V and f¼500 Hz) was performed. In experiments, 9 micro-grooves were ground. The micro-groove depths ranged 615–807 mm. The micro-grinding conditions were given by the wheel speed of 3000 rpm, the table speed of 100 mm/ min and the depth of cut of 1 mm and water coolant. It is shown that integrated micro-groove was produced without any brittle crack edges after micro-grinding (see Fig. 14a). However, micro-shape deformation (see Fig. 14a) and surface damage (see Fig. 14b) happened on diamond wheel V-tip. This is because the diamond wheel V-tip was easily worn when the micro-groove depth was large. This problem may be resolved through on-machine wheel V-tip truing during micro-grinding. Fig. 15 shows the measurement of ground micro-groove quality. In measurement, a depth-of-field microscope KEYENCE VHX-600E
Fig. 11. Truing ratio g versus pulse voltage E and pulse frequency f.
Fig. 13. SEM photo of electrode working surface.
Table 3 Truing ratio and truing efficiency. Truing method
Dry ECD mutual-wear truing
Mechanical truing
Electrode/Dresser Diamond wheel Wheel radial worn height DR Electrode/Dresser worn height Dh Truing ratio g Truing efficiency Z
Thickness w¼ 8 mm Width b ¼29 mm Diameter D ¼ 150 mm Thickness W ¼5 mm 20.8 mm 0.15 mm 2.82 mm3/mm3 12.25 mm3/min
Thickness w¼24.7 mm Width b ¼ 48.6 mm Diameter D ¼ 150 mm Thickness W ¼5 mm 10.2 mm 5 mm 0.008 mm3/mm3 0.21 mm3/min
Fig. 12. SEM grain protrusion topography: (a) before Dry ECD V-tip mutual-wear truing and (b) after Dry ECD mutual-wear V-tip truing.
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Fig. 14. SEM photos of micro-grooved quartz glass: (a) groove profile and (b) groove tip surface.
Fig. 15. Measurement of ground micro-groove quality: (a) measured micro-groove profile parameters (micro-groove angle aw and micro-groove tip radius rw) and (b) measured micro-groove angles distribution.
was employed. It is shown that the micro-groove tip radius rw was 46 mm for the micro-groove depth of 630 mm. It is larger than the one in the case of mechanical truing for micro-groove depth of about 150 mm [10]. This is because diamond wheel V-tip was easily worn when deep micro-groove was ground. It is also seen that the micro-groove angle aw was dispersedly distributed around the ideal V-tip angle of 601. The average value was 59.951; the average absolute angle error was only 0.591. This also means that the form accuracy of ground micro-groove may be controlled by the truing accuracy of diamond wheel V-tip. The surface roughness of ground micro-groove was measured by using a roughmeter TR200. The average surface roughness Ra was 0.128 mm for 12 measured points. It is identical to the plate-ground surface roughness Ra of optic glass (BK10) using the Dry ECD dressed #600 diamond wheel [16]. However, it is much less than the micro-grooved Si surface roughness Ra of 0.26– 0.30 mm using the mechanical-trued #600 diamond wheel V-tip [10] and the plate-ground Al2O3 surface roughness Ra of 0.25 mm using the Dry ECD trued #600 diamond wheel [15], respectively. As a result, Dry ECD mutual-wear truing is valid to precisely sharpen fine diamond wheel V-tip of for precision micro-grinding of hard and brittle material.
6. Conclusions A Dry Electro-contact Discharge (ECD) mutual-wear truing is developed to sharpen the #600 diamond wheel V-tip with an ideal V-tip angle of 601 for precision micro-grinding of hard and brittle material. The detailed results are included as follows: 1. The continuous spark discharges may be maintained when the pulse voltage ranges 5–7 V. However, the arc discharge occurs
when it is larger than 7 V. The spark discharge conditions contribute to good grain protrusion on wheel working surface, leading to the least wheel V-tip form error, the least wheel V-tip angle and the least wheel V-tip radius. 2. Dry ECD mutual-wear truing with suitable discharge variables may achieve less wheel V-tip form error, less wheel V-tip angle and less wheel V-tip radius than mechanical truing. It may improve the truing ratio by about 350 times and truing efficiency by about 59 times against mechanical truing, respectively. 3. The integrated micro-groove of quartz glass may be achieved without any brittle crack edges after micro-grinding using the Dry ECD mutual-wear trued diamond wheel V-tip. The average absolute error of wheel V-tip angle is 0.591. The surface roughness Ra of ground micro-groove averagely reaches 0.128 mm. Acknowledgments The project was sponsored by the Natural Science Foundation of China (Grant No. 51075156).
Appendix A. Supplementary Information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ijmachtools. 2012.05.006.
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