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In-process evaluation of grit protrusion feature for fine diamond grinding wheel by means of electro-contact discharge dressing
Journal of Materials Processing Technology 180 (2006) 83–90
In-process evaluation of grit protrusion feature for fine diamond grinding wheel by means of electro-contact discharge dressing J. Xie a,∗ , J. Tamaki b a
College of Mechanical Engineering, South China University of Technology, Guangzhou 510640, China b Department of Mechanical Engineering, Kitami Institute of Technology, Kitami 090, Japan Received 2 November 2004; received in revised form 4 October 2005; accepted 9 May 2006
Abstract This paper introduces a new in-process evaluation method for grit protrusion feature on wheel surface by monitoring discharge current trace during electro-contact discharge (ECD) dressing of metal-bonded fine diamond grinding wheel. First an impulse discharge machining experiment was carried out to investigate the correlation between metal bond removal and discharge parameters, namely discharge current Ie and discharge pulse duration τ e . Then ECD dressing experiment for #600 diamond grinding wheel was conduced to analyses the quantitative effect of the discharge parameters (Ie and τ e ), derived from discharge current trace between wheel and dresser (electrode), on grit protrusion feature of wheel surface. The result shows that the grit protrusion feature is sensitive to the discharge parameters (Ie and τ e ) with reference to mean diamond grit size dgm . Further, the discharge parameters (Ie and τ e ) in ECD dressing should conform to the discharge variables’ requirement of 5.41 × Ie0.77 × τe0.19 < (dgm )/(2), by which the grit protrusion feature may be evaluated and the dressing process variables may be determined. Finally, the in-process evaluation method was successfully applied to ECD dressing of #1500 diamond grinding wheel for valid grinding of hard-brittle materials. © 2006 Elsevier B.V. All rights reserved. Keywords: Diamond grinding wheel; Electro-contact discharge dressing; Grit protrusion; Discharge current; Discharge pulse duration
1. Instruction Grit protrusion feature of diamond grinding wheel strongly affects integrity of ground surface [1]. Dressing is a prerequisite process to protrude fine grits from grinding wheel surface. Because grit protrusion feature of grinding wheel can not be precisely specified due to random nature of grit distribution in grinding wheel, the simulation of grinding wheel surface has often be carried out to explain the importance of dressing and grinding parameters [2,3]. However, it is very difficult to evaluate in-process grit protrusion feature of grinding wheel surface for determining process variables of dressing for actual grinding. In addition, the metal bond of high strength can hold firmly diamond grits in grinding wheel to realize valid grinding as compared with resin bond, whereas it obstructs fine grit protrusion due to its strength. In order to remove metal bond and protrude fine diamond grits from wheel surface, ELID (electrolytic
∗
Corresponding author. Tel.: +86 20 3333 0124; fax: +86 20 8711 4634. E-mail address: [email protected] (J. Xie).
0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.05.006
in-process dressing) [4], EDM (electro-discharge machining) dressing [5] and ECDM (electro-chemical discharge machining) dressing [6] have been introduced and successfully applied to grinding of hard-brittle materials. However, they all need a pollutive coolant to realize electrical erosion of metal bond and a complex system to control the discharge gap distance between wheel and electrode. Therefore, electro-contact discharge (ECD) dressing [7–9] has been considered to be a means for dressing metal-bonded diamond grinding wheel by plunge grinding of an electrode, dresser, without a special control system whose discharge gap may be filled with air instead of coolant. ECD dressing of metal-bonded diamond grinding wheel attributes to the action of thermal erosion derived from electric discharges between metal bond (anode) and electrode swarf (cathode) to which an open circuit voltage Ei is applied as shown in Fig. 1a. It may generate superior protrusion topography without damage to diamond grits [8,9], which is useful to eliminate debris from the contact zone between diamond grinding wheel and ground surface. However, it is more difficult for ECD dressing of finer diamond grinding wheel to determine its process variables for realizing electric discharge and insulation recovery
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J. Xie, J. Tamaki / Journal of Materials Processing Technology 180 (2006) 83–90
Fig. 1. Research mode. (a) ECD dressing behavior; (b) Impulse discharge machining; (c) and (d) Discharge current trace; (e) Grit protrusion feature on wheel surface; (f) Discharge crater on metal bond surface (SEM).
alternatively. This is because finer diamond grit size produces less discharge gap between wheel and electrode. Therefore, this paper is concern with how to remove metal bond and protrude diamond grit from wheel surface in connection with process variability of electrical discharge machining. Because discharge current Ie and discharge pulse duration τ e as EDM process parameters have a main influence on surface topography of workpiece [10], in this paper the discharge parameters (Ie and τ e ) derived from discharge current trace are considered as index to evaluate grit protrusion feature of grinding wheel during ECD dressing. The goal is to establish its in-process evaluation method for valid ECD dressing by monitoring the discharge current trace concerning its discharge parameters (Ie and τ e ), which is used to determine process variables for actual dressing of fine diamond grinding wheel. The research procedure is illustrated in Fig. 1. First, removal mode of metal bond was established in impulse discharge machining experiment as shown in Fig. 1b to investigate action of discharge parameters (Ie and τ e ) defined in Fig. 1d on the removal of metal bond, namely discharge crater volume as shown in Fig. 1f. Then ECD dressing experiment was carried out to investigate the effect of discharge parameters (Ie and τ e ) derived from discharge current trace shown in Fig. 1c on grit
protrusion feature shown in Fig. 1e, concerning the interaction of mean discharge crater height and mean diamond grit size. Finally the requirement of discharge parameters (Ie and τ e ) for valid ECD dressing was experimentally introduced to realize in-process evaluation of the grit protrusion feature on wheel surface. 2. Impulse discharge machining experiment 2.1. Means and conditions Fig. 2 shows a schematic of impulse discharge machining setup. In this experiment, a 45 ◦ conical electrode (φ3 mm), composed of conductive resin bond used as dresser material, was controlled by a stepping motor with 0.2 m per step. It was moved down gradually to the rectangular workpiece composed of metal bond used for diamond grinding wheel until an electrical discharge occurred between them. The discharge current trace was measured by A/D-conversion of the output from a hall-effect current sensor and achieved by a digital memory. 3D shape of impulse discharge crater on workpiece surface was measured by the use of an auto-focusing laser beam microscope NH-3SP. The experimental conditions are shown in Table 1. In
J. Xie, J. Tamaki / Journal of Materials Processing Technology 180 (2006) 83–90
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Fig. 4. Geometric mode of impulse discharge crater.
Fig. 2. Impulse discharge machining setup. Fig. 5. Removal me of metal bond vs. Ei for impulse discharge machining.
Table 1 Impulse discharge machining conditions Workpiece Electrode Open-circuit voltage Coolant
Metal bond (Bronze) Conductive resin 5, 10, 15, 20, 30 and 40 V Dry
order to investigate the correlations between discharge crater size and discharge parameters (Ie and τ e ), total 60 experiments were carried out, in which the 10 same experiments were repeatedly taken for each Ei . The actual valid experiment number was 54 because it was ignored when a short circuit occurred between workpiece and electrode. 2.2. Impulse discharge crater and removal Fig. 3 shows 3D topography of an impulse discharge crater on metal bond surface. It may be approximately considered as a part of sphere whose 3D shape is evaluated by its diameter de and depth he shown in Fig. 4. In measurement, the de is an average of maximal and minimal diameters directly measured, and the he a maximal depth. Therefore, the bond removal me per
Fig. 3. 3D topography of impulse discharge crater.
electrical discharge can be given by 2 d e × he h3 + e . me = π × 8 6
(1)
Fig. 5 shows bond removal me per electrical discharge versus Ei . The me increases with increase of Ei , but it may be persisted in very small magnitude as Ei < 20 V. For example, the me for Ei = 15 V is about 40 times less than the one for Ei = 30 V. From the research result conducted before, the spark discharge regime (Ei < 20 V) is transferred to the arc discharge regime (Ei > 20 V) accompanied with an arc pillar at the critical open-circuit voltage Eg = 20 V [9]. This results in remarkable difference of removal behavior between spark and arc discharge regimes. 2.3. Discharge parameters Fig. 6 shows discharge current Ie and discharge pulse duration τ e versus Ei . The Ie and the τ e maintain stable in small magnitude as Ei < 20 V, but they rapidly increase with increase of the Ei as Ei > 20 V. This trend responds to the changes of material removal
Fig. 6. Discharge current Ie and discharge pulse duration τ e for impulse discharge machining.
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J. Xie, J. Tamaki / Journal of Materials Processing Technology 180 (2006) 83–90 Table 2 ECD dressing conditions
Fig. 7. ECD dressing setup.
me versus the Ei as shown in Fig. 5. Therefore, the Ei is chosen as 15 and 30 V in following ECD dressing experiment to stand for spark discharge regime and arc discharge regime, respectively.
Grinding wheel
SD600 N100M (Bronze bond) Diameter 200 mm, width 10 mm
Rotary electrode Power supply Dressing condition
GC600 L200B (conductive resin bond) Diameter 70 mm, Rim width 10 mm
Coolant
Dry
Open circuit voltage Ei = 15 and 30 V DC (switching regular, unlimited current 5 A) Wheel speed N = 3000 rpm Dresser speed n = 300 rpm Dresser feed Vf = 100 mm/min Depth of cut a = 1.0 m
3. ECD dressing experiment
image processing of the SEM photo. The discharge current trace was recorded by the same digital memory scope used in impulse discharge machining experiment as shown in Fig. 2. The ECD dressing conditions are shown in Table 2.
3.1. Means and conditions
3.2. Grit protrusion feature
Fig. 7 shows a schematic of ECD dressing equipment. ECD dressing was performed by a rotary dresser as an electrode, composed of conductive resin and #600 GC grits whose concentration is 200, ground by metal-bonded diamond wheel. The GC grits distributed inside dresser may disperse pulse electrical discharges during ECD dressing. The dressed wheel surface was investigated by SEM, which was utilized to analyses distribution of active grit number and size on wheel surface by the use of the
Figs. 8 and 9 shows, respectively, the discharge current trace and grit protrusion feature during ECD dressing of diamond grinding wheel surface in the case of Ei = 15 V (spark discharge regime) and Ei = 30 V (arc discharge regime). It illustrates that there exists a correlation between discharge current trace and grit protrusion feature. The Ei = 15 V may produce better grit protrusion on wheel surface than the Ei = 30 V. This also means that the discharge energy for Ei = 30 V could produce overfull
Fig. 8. Discharge current trace during ECD dressing. (a) Spark discharge (Ei = 15 V); (b) Arc discharge (Ei = 30 V).
Fig. 9. Wheel surface (SEM) during ECD dressing. (a) Spark discharge (Ei = 15 V); (b) Arc discharge (Ei = 30 V).
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removal of metal bond around grit so as to arouse diamond grit pullout.
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crater sizes may be predicted by the discharge parameters (Ie and τ e ) and controlled in micro order by monitoring the discharge current trace and regulating process variables simultaneously.
4. Data analysis results and discussion 4.2. Formation mode of grits protrusion feature with reference to discharge crater height
4.1. Correlations between impulse discharge crater size and discharge parameters The multiple regressive analyses on the correlation between discharge crater sizes (he and de ) and discharge parameters (Ie and τ e ) were carried out using the data of the 54 experiments for impulse discharge machining. Their regressive equations were respectively obtained as follows: he = 5.41 × Ie0.77 × τe0.19 , de = 29.00 × Ie0.78 × τe0.17 ,
R = 0.8582, R = 0.8585,
(2) (3)
where he is the crater depth in m, de the crater diameter in m, Ie the discharge current in A, τ e the discharge pulse duration in ms and R is the multiple regression coefficient. Their experimental data and regression curve are, respectively, shown in Figs. 10 and 11. It shows that there exit good correlations between discharge crater sizes (he and de ) and discharge parameters (Ie and τ e ). It also means that the discharge
The electrical discharges between wheel and electrode produce many discharge craters on metal bond surface so as to remove metal bond around grits and protrude them gradually. In contrast, oversize discharge crater could result in grit pullout, thus there exists appropriate discharge parameters (Ie and τ e ) dominating the size of discharge craters on metal bond surface for protruding grits without pullout. In order to obtain the discharge parameters (Ie and τ e ) of ECD dressing, the Ie and the τ e may be defined as mean discharge current and mean discharge pulse duration of np impulse discharges derived from discharge current trace for certain dressing duration, respectively: np Ien Ie = n=1 , (4) np np τen τe = n=1 , (5) np where Ien is the peak discharge current of nth pulse discharge, τ en the pulse discharge duration of nth impulse discharge and np is the number of pulse discharge peak. To investigate discharge crater shape and size on wheel surface, mean discharge crater height he and diameter de can be obtained from Eqs. (2) and (3), respectively, by using the Ie and the τ e derive discharge current traces shown in Fig. 8. The results for Ei = 15 and 30 V are shown in Table 3. Fig. 12 shows the quantitative illustration of grit protrusion modes with reference to discharge crater for Ei = 15 and 30 V by the use of the data of Table 3. It indicates that the Ei = 15 V may remove gradually so little metal bond material around diamond grit that fine
Fig. 10. Discharge crater he vs. discharge parameters (Ie and τ e ). (a) Experimental data; (b) Regression curve.
Fig. 11. Discharge crater de vs. discharge parameters (Ie and τ e ). (a) Experimental data; (b) Regression curve.
Fig. 12. Grit protrusion mode relative to discharge crater. (a) Spark discharge (Ei = 15 V); (b) Arc discharge (Ei = 30 V).
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Table 3 Discharge parameters and discharge crater size during ECD dressing process Ei (V)
15 30
Discharge parameters
Discharge crater size
Dressing time (ms)
Pulse discharge number, np
Ie (A)
τ e (ms)
he (m)
de (m)
30 30
56 68
1.43 6.89
0.13 0.31
4.84 19.12
27.16 106.97
Fig. 13. Image processing result of wheel surface. (a) Spark discharge (Ei = 15 V); (b) Arc discharge (Ei = 30 V).
grit is able to be protruded from wheel surface without pullout because the mean discharge crater height he is less than half of mean diamond grit size dgm , resulting in the good grit protrusion shown in Fig. 9a. In contrast, the Ei = 30 V easily arouses grit pullout because the he is larger than half of the dgm , resulting in poor grit protrusion shown in Fig. 9b. 4.3. Requirement for discharge parameters for ECD dressing The image processing of SEM photos on wheel surface shown in Fig. 9 was carried out to distinguish active grit protrusion area and investigate numerically distribution of grit sizes on wheel surface. The result is shown in Fig. 13. In order to analyze grit protrusion diameter statistically, the grit protrusion diameter dg may be defined as the diameter of a circle whose area Sg is equal to grit protrusion area S as shown in Fig. 14. Fig. 15a and b show histograms of girt protrusion diameter distribution on wheel surface obtained from the image process results of wheel surface for Ei = 15 V and Ei = 30 V, respectively. It is shown that the active grit protrusion number ng of Ei = 15 V is about twice larger than that of Ei = 30 V. The reason is that the mean discharge crater height he for Ei = 15 V is much less than dgm /2 and the one for Ei = 30 V is much larger than dgm /2. Therefore, the requirement of discharge parameters (Ie and τ e )
Fig. 14. Mode of grit protrusion diameter dg . (a) Actual shape; (b) Grit diameter.
for ECD dressing may be given by dgm . (6) 2 Fig. 16 shows the critical curves for three kinds of diamond grit mesh such as #270 (dgm = 56.0 m), #600 (dgm = 24.0 m) and #1500 (dgm = 10.5 m) according to Eq. (6). It indicates that ECD dressing may be best done when a limit range of discharge parameters (Ie and τ e ) is chosen on the left side of the critical curve for certain grit size. In addition, the discharge current trace need to be more strictly controlled when the finer diamond grinding wheel is dressed. It is concluded that its discharge parameters (Ie and τ e ) should conform to Eq. (6) so as to realize valid ECD dressing.
he = 5.41 × Ie0.77 × τe0.19 <
Fig. 15. Distribution of grit protrusion diameter. (a) Ei = 15 V; (b) Ei = 30 V.
J. Xie, J. Tamaki / Journal of Materials Processing Technology 180 (2006) 83–90
Fig. 16. Requirement of discharge parameters for different grit mesh of diamond grinding wheel.
4.4. Dressing of #1500 diamond grinding wheel ECD dressing of #1500 diamond grinding wheel was carried out by in-process monitoring of the discharge current trace between wheel and electrode. The GC grit inside electrode, namely dresser, was chosen as #1500, matching #1500 diamond grinding wheel. The Ei was chosen as 15 V. The other conditions were same as Table 2. At the beginning of this experiment, a short circuit often occurred during ECD dressing as the depth of cut a = 1.0 m. This is because little gap distance between metal bond and electrode dominated by fine grit size cannot accommodate overfull electrode swarf. Therefore, the depth of cut a was regulated and decreased gradually until the discharge parameters (Ie and τ e ) derived from discharge current trace satisfied the Eq. (6). The suitable discharge current trace shown in Fig. 17 for valid ECD dressing was produced by the a of 0.2 m. Its mean discharge crater height he of 4.36 m obtained from Eqs. (2) and (3) by using the Ie and τ e derived from the discharge current trace shown in Fig. 17 is less than the half of the dgm of #1500 diamond grinding wheel. The #1500 diamond grinding wheel dressed with the above conditions was used to grind hard-brittle materials such as Al2 O3 and optical glass (BK10) as compared with mechanical dressing, namely GC cup dressing without electrical erosion. The ground result is shown in Fig. 18. It indicates that ECD dressing may obtain better ground surface than GC cup dressing. It is concluded that the in-process monitoring of discharge current trace during ECD dressing may be utilized to determine dressing process variables for fine diamond grinding wheel.
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Fig. 18. Ground surface roughness of hard-brittle materials.
5. Conclusion In this paper, the quantitative action of discharge parameters (Ie and τ e ) on grit protrusion feature of fine diamond grinding wheel was experimentally investigated and its in-process evaluation method for valid ECD dressing was established. From the experiments and analysis, the following conclusions can be drawn: (1) There exist good correlations between discharge removal and discharge parameters (Ie and τ e ) for impulse discharge machining of metal bond. The grit protrusion feature of diamond grinding wheel is sensitive to the discharge parameters (Ie and τ e ) derived from discharge current trace during ECD dressing, which depends on mean discharge crater height and mean diamond grit size dgm . (2) The requirement of discharge parameters (Ie and τ e ) for valid ECD dressing of diamond grinding wheel should conform to the expression of 5.41 × Ie0.77 × τe0.19 = (dgm )/(2), which may be used to monitoring discharge current trace and evaluate grit protrusion feature during ECD dressing process. (3) The in-process evaluation method for grit protrusion feature on wheel surface may be applied to the ECD dressing of fine diamond grinding wheel to realize valid grinding of hardbrittle materials. Acknowledgements The authors express their thanks to Nitolex Co., Ltd., Read Co., Ltd., and Noritake Diamond Industries Co., Ltd. for providing the electrode and diamond grinding wheel. The Project was sponsored by Research Collaboration Committee of the Japan Society for Precision Engineering and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. References
Fig. 17. Suitable discharge current trace for valid ECD dressing.
[1] P. Koshy, A. Iwasaki, M.A. Elbestawi, Surface generation with engineering diamond grinding wheels: insights from simulation, Ann. CIRP 52 (1) (2003) 271–274. [2] P. Koshy, L.K. Ives, S. Jahanmir, Simulation of diamond-ground surfaces, Int. J. Mach. Tools Manuf. 39 (9) (1999) 1451–1470.
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[3] X. Chena, W.B. Rowe, Analysis and simulation of the grinding process. Part I: Generation of the grinding wheel surface, Int. J. Mach. Tools Manuf. 36 (8) (1996) 71–882. [4] H. Ohmori, T. Nakagawa, Analysis of mirror surface generation of hard and brittle materials by ELID (electrolytic in-process dressing) grinding with superfine grain metallic bond wheels, Ann. CIRP 44 (1) (1995) 287– 290. [5] K. Suzuki, T. Uematsu, T. Nakagawa, On-machine truing/dressing of metal bond grinding wheel with electro-discharge machining, Ann. CIRP 40 (1) (1991) 363–366. [6] M. Schopf, I. Beltrami, M. Boccadoro, D. Kramer, ECDM (electro chemical discharge machining), a new method for trueing and dressing of
[7] [8]
[9]
[10]
metal-boned diamond grinding tools, Ann. CIRP 50 (1) (2001) 125– 128. H.K. Tonshoff, T. Friemuth, In-process dressing of fine diamond wheels for tool grinding, Precision Eng. 24 (1) (2000) 58–61. J. Tamaki, K. Kondoh, T. Iyama, Electrocontact discharge dressing of metal-bonded diamond grinding wheel utilizing a hybrid electrode, J. Jpn. Soc. Precision Eng. 65 (11) (1999) 1628–1632. J. Xie, J. Tamaki, A. Kubo, T. Iyama, Application of electro-contact discharge dressing to a fine-gained diamond grinding wheel, J. Jpn. Soc. Precision Eng. 67 (11) (2001) 1844–1849. H. Ramasawmy, L. Blunt, Effect of EDM process parameters on 3D surface topography, J. Mater. Process. Technol. 148 (2004) 155–164.
In-process evaluation of grit protrusion feature for fine diamond grinding wheel by means of electro-contact discharge dressing J. Xie a,∗ , J. Tamaki b a
College of Mechanical Engineering, South China University of Technology, Guangzhou 510640, China b Department of Mechanical Engineering, Kitami Institute of Technology, Kitami 090, Japan Received 2 November 2004; received in revised form 4 October 2005; accepted 9 May 2006
Abstract This paper introduces a new in-process evaluation method for grit protrusion feature on wheel surface by monitoring discharge current trace during electro-contact discharge (ECD) dressing of metal-bonded fine diamond grinding wheel. First an impulse discharge machining experiment was carried out to investigate the correlation between metal bond removal and discharge parameters, namely discharge current Ie and discharge pulse duration τ e . Then ECD dressing experiment for #600 diamond grinding wheel was conduced to analyses the quantitative effect of the discharge parameters (Ie and τ e ), derived from discharge current trace between wheel and dresser (electrode), on grit protrusion feature of wheel surface. The result shows that the grit protrusion feature is sensitive to the discharge parameters (Ie and τ e ) with reference to mean diamond grit size dgm . Further, the discharge parameters (Ie and τ e ) in ECD dressing should conform to the discharge variables’ requirement of 5.41 × Ie0.77 × τe0.19 < (dgm )/(2), by which the grit protrusion feature may be evaluated and the dressing process variables may be determined. Finally, the in-process evaluation method was successfully applied to ECD dressing of #1500 diamond grinding wheel for valid grinding of hard-brittle materials. © 2006 Elsevier B.V. All rights reserved. Keywords: Diamond grinding wheel; Electro-contact discharge dressing; Grit protrusion; Discharge current; Discharge pulse duration
1. Instruction Grit protrusion feature of diamond grinding wheel strongly affects integrity of ground surface [1]. Dressing is a prerequisite process to protrude fine grits from grinding wheel surface. Because grit protrusion feature of grinding wheel can not be precisely specified due to random nature of grit distribution in grinding wheel, the simulation of grinding wheel surface has often be carried out to explain the importance of dressing and grinding parameters [2,3]. However, it is very difficult to evaluate in-process grit protrusion feature of grinding wheel surface for determining process variables of dressing for actual grinding. In addition, the metal bond of high strength can hold firmly diamond grits in grinding wheel to realize valid grinding as compared with resin bond, whereas it obstructs fine grit protrusion due to its strength. In order to remove metal bond and protrude fine diamond grits from wheel surface, ELID (electrolytic
∗
Corresponding author. Tel.: +86 20 3333 0124; fax: +86 20 8711 4634. E-mail address: [email protected] (J. Xie).
0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.05.006
in-process dressing) [4], EDM (electro-discharge machining) dressing [5] and ECDM (electro-chemical discharge machining) dressing [6] have been introduced and successfully applied to grinding of hard-brittle materials. However, they all need a pollutive coolant to realize electrical erosion of metal bond and a complex system to control the discharge gap distance between wheel and electrode. Therefore, electro-contact discharge (ECD) dressing [7–9] has been considered to be a means for dressing metal-bonded diamond grinding wheel by plunge grinding of an electrode, dresser, without a special control system whose discharge gap may be filled with air instead of coolant. ECD dressing of metal-bonded diamond grinding wheel attributes to the action of thermal erosion derived from electric discharges between metal bond (anode) and electrode swarf (cathode) to which an open circuit voltage Ei is applied as shown in Fig. 1a. It may generate superior protrusion topography without damage to diamond grits [8,9], which is useful to eliminate debris from the contact zone between diamond grinding wheel and ground surface. However, it is more difficult for ECD dressing of finer diamond grinding wheel to determine its process variables for realizing electric discharge and insulation recovery
84
J. Xie, J. Tamaki / Journal of Materials Processing Technology 180 (2006) 83–90
Fig. 1. Research mode. (a) ECD dressing behavior; (b) Impulse discharge machining; (c) and (d) Discharge current trace; (e) Grit protrusion feature on wheel surface; (f) Discharge crater on metal bond surface (SEM).
alternatively. This is because finer diamond grit size produces less discharge gap between wheel and electrode. Therefore, this paper is concern with how to remove metal bond and protrude diamond grit from wheel surface in connection with process variability of electrical discharge machining. Because discharge current Ie and discharge pulse duration τ e as EDM process parameters have a main influence on surface topography of workpiece [10], in this paper the discharge parameters (Ie and τ e ) derived from discharge current trace are considered as index to evaluate grit protrusion feature of grinding wheel during ECD dressing. The goal is to establish its in-process evaluation method for valid ECD dressing by monitoring the discharge current trace concerning its discharge parameters (Ie and τ e ), which is used to determine process variables for actual dressing of fine diamond grinding wheel. The research procedure is illustrated in Fig. 1. First, removal mode of metal bond was established in impulse discharge machining experiment as shown in Fig. 1b to investigate action of discharge parameters (Ie and τ e ) defined in Fig. 1d on the removal of metal bond, namely discharge crater volume as shown in Fig. 1f. Then ECD dressing experiment was carried out to investigate the effect of discharge parameters (Ie and τ e ) derived from discharge current trace shown in Fig. 1c on grit
protrusion feature shown in Fig. 1e, concerning the interaction of mean discharge crater height and mean diamond grit size. Finally the requirement of discharge parameters (Ie and τ e ) for valid ECD dressing was experimentally introduced to realize in-process evaluation of the grit protrusion feature on wheel surface. 2. Impulse discharge machining experiment 2.1. Means and conditions Fig. 2 shows a schematic of impulse discharge machining setup. In this experiment, a 45 ◦ conical electrode (φ3 mm), composed of conductive resin bond used as dresser material, was controlled by a stepping motor with 0.2 m per step. It was moved down gradually to the rectangular workpiece composed of metal bond used for diamond grinding wheel until an electrical discharge occurred between them. The discharge current trace was measured by A/D-conversion of the output from a hall-effect current sensor and achieved by a digital memory. 3D shape of impulse discharge crater on workpiece surface was measured by the use of an auto-focusing laser beam microscope NH-3SP. The experimental conditions are shown in Table 1. In
J. Xie, J. Tamaki / Journal of Materials Processing Technology 180 (2006) 83–90
85
Fig. 4. Geometric mode of impulse discharge crater.
Fig. 2. Impulse discharge machining setup. Fig. 5. Removal me of metal bond vs. Ei for impulse discharge machining.
Table 1 Impulse discharge machining conditions Workpiece Electrode Open-circuit voltage Coolant
Metal bond (Bronze) Conductive resin 5, 10, 15, 20, 30 and 40 V Dry
order to investigate the correlations between discharge crater size and discharge parameters (Ie and τ e ), total 60 experiments were carried out, in which the 10 same experiments were repeatedly taken for each Ei . The actual valid experiment number was 54 because it was ignored when a short circuit occurred between workpiece and electrode. 2.2. Impulse discharge crater and removal Fig. 3 shows 3D topography of an impulse discharge crater on metal bond surface. It may be approximately considered as a part of sphere whose 3D shape is evaluated by its diameter de and depth he shown in Fig. 4. In measurement, the de is an average of maximal and minimal diameters directly measured, and the he a maximal depth. Therefore, the bond removal me per
Fig. 3. 3D topography of impulse discharge crater.
electrical discharge can be given by 2 d e × he h3 + e . me = π × 8 6
(1)
Fig. 5 shows bond removal me per electrical discharge versus Ei . The me increases with increase of Ei , but it may be persisted in very small magnitude as Ei < 20 V. For example, the me for Ei = 15 V is about 40 times less than the one for Ei = 30 V. From the research result conducted before, the spark discharge regime (Ei < 20 V) is transferred to the arc discharge regime (Ei > 20 V) accompanied with an arc pillar at the critical open-circuit voltage Eg = 20 V [9]. This results in remarkable difference of removal behavior between spark and arc discharge regimes. 2.3. Discharge parameters Fig. 6 shows discharge current Ie and discharge pulse duration τ e versus Ei . The Ie and the τ e maintain stable in small magnitude as Ei < 20 V, but they rapidly increase with increase of the Ei as Ei > 20 V. This trend responds to the changes of material removal
Fig. 6. Discharge current Ie and discharge pulse duration τ e for impulse discharge machining.
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J. Xie, J. Tamaki / Journal of Materials Processing Technology 180 (2006) 83–90 Table 2 ECD dressing conditions
Fig. 7. ECD dressing setup.
me versus the Ei as shown in Fig. 5. Therefore, the Ei is chosen as 15 and 30 V in following ECD dressing experiment to stand for spark discharge regime and arc discharge regime, respectively.
Grinding wheel
SD600 N100M (Bronze bond) Diameter 200 mm, width 10 mm
Rotary electrode Power supply Dressing condition
GC600 L200B (conductive resin bond) Diameter 70 mm, Rim width 10 mm
Coolant
Dry
Open circuit voltage Ei = 15 and 30 V DC (switching regular, unlimited current 5 A) Wheel speed N = 3000 rpm Dresser speed n = 300 rpm Dresser feed Vf = 100 mm/min Depth of cut a = 1.0 m
3. ECD dressing experiment
image processing of the SEM photo. The discharge current trace was recorded by the same digital memory scope used in impulse discharge machining experiment as shown in Fig. 2. The ECD dressing conditions are shown in Table 2.
3.1. Means and conditions
3.2. Grit protrusion feature
Fig. 7 shows a schematic of ECD dressing equipment. ECD dressing was performed by a rotary dresser as an electrode, composed of conductive resin and #600 GC grits whose concentration is 200, ground by metal-bonded diamond wheel. The GC grits distributed inside dresser may disperse pulse electrical discharges during ECD dressing. The dressed wheel surface was investigated by SEM, which was utilized to analyses distribution of active grit number and size on wheel surface by the use of the
Figs. 8 and 9 shows, respectively, the discharge current trace and grit protrusion feature during ECD dressing of diamond grinding wheel surface in the case of Ei = 15 V (spark discharge regime) and Ei = 30 V (arc discharge regime). It illustrates that there exists a correlation between discharge current trace and grit protrusion feature. The Ei = 15 V may produce better grit protrusion on wheel surface than the Ei = 30 V. This also means that the discharge energy for Ei = 30 V could produce overfull
Fig. 8. Discharge current trace during ECD dressing. (a) Spark discharge (Ei = 15 V); (b) Arc discharge (Ei = 30 V).
Fig. 9. Wheel surface (SEM) during ECD dressing. (a) Spark discharge (Ei = 15 V); (b) Arc discharge (Ei = 30 V).
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removal of metal bond around grit so as to arouse diamond grit pullout.
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crater sizes may be predicted by the discharge parameters (Ie and τ e ) and controlled in micro order by monitoring the discharge current trace and regulating process variables simultaneously.
4. Data analysis results and discussion 4.2. Formation mode of grits protrusion feature with reference to discharge crater height
4.1. Correlations between impulse discharge crater size and discharge parameters The multiple regressive analyses on the correlation between discharge crater sizes (he and de ) and discharge parameters (Ie and τ e ) were carried out using the data of the 54 experiments for impulse discharge machining. Their regressive equations were respectively obtained as follows: he = 5.41 × Ie0.77 × τe0.19 , de = 29.00 × Ie0.78 × τe0.17 ,
R = 0.8582, R = 0.8585,
(2) (3)
where he is the crater depth in m, de the crater diameter in m, Ie the discharge current in A, τ e the discharge pulse duration in ms and R is the multiple regression coefficient. Their experimental data and regression curve are, respectively, shown in Figs. 10 and 11. It shows that there exit good correlations between discharge crater sizes (he and de ) and discharge parameters (Ie and τ e ). It also means that the discharge
The electrical discharges between wheel and electrode produce many discharge craters on metal bond surface so as to remove metal bond around grits and protrude them gradually. In contrast, oversize discharge crater could result in grit pullout, thus there exists appropriate discharge parameters (Ie and τ e ) dominating the size of discharge craters on metal bond surface for protruding grits without pullout. In order to obtain the discharge parameters (Ie and τ e ) of ECD dressing, the Ie and the τ e may be defined as mean discharge current and mean discharge pulse duration of np impulse discharges derived from discharge current trace for certain dressing duration, respectively: np Ien Ie = n=1 , (4) np np τen τe = n=1 , (5) np where Ien is the peak discharge current of nth pulse discharge, τ en the pulse discharge duration of nth impulse discharge and np is the number of pulse discharge peak. To investigate discharge crater shape and size on wheel surface, mean discharge crater height he and diameter de can be obtained from Eqs. (2) and (3), respectively, by using the Ie and the τ e derive discharge current traces shown in Fig. 8. The results for Ei = 15 and 30 V are shown in Table 3. Fig. 12 shows the quantitative illustration of grit protrusion modes with reference to discharge crater for Ei = 15 and 30 V by the use of the data of Table 3. It indicates that the Ei = 15 V may remove gradually so little metal bond material around diamond grit that fine
Fig. 10. Discharge crater he vs. discharge parameters (Ie and τ e ). (a) Experimental data; (b) Regression curve.
Fig. 11. Discharge crater de vs. discharge parameters (Ie and τ e ). (a) Experimental data; (b) Regression curve.
Fig. 12. Grit protrusion mode relative to discharge crater. (a) Spark discharge (Ei = 15 V); (b) Arc discharge (Ei = 30 V).
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Table 3 Discharge parameters and discharge crater size during ECD dressing process Ei (V)
15 30
Discharge parameters
Discharge crater size
Dressing time (ms)
Pulse discharge number, np
Ie (A)
τ e (ms)
he (m)
de (m)
30 30
56 68
1.43 6.89
0.13 0.31
4.84 19.12
27.16 106.97
Fig. 13. Image processing result of wheel surface. (a) Spark discharge (Ei = 15 V); (b) Arc discharge (Ei = 30 V).
grit is able to be protruded from wheel surface without pullout because the mean discharge crater height he is less than half of mean diamond grit size dgm , resulting in the good grit protrusion shown in Fig. 9a. In contrast, the Ei = 30 V easily arouses grit pullout because the he is larger than half of the dgm , resulting in poor grit protrusion shown in Fig. 9b. 4.3. Requirement for discharge parameters for ECD dressing The image processing of SEM photos on wheel surface shown in Fig. 9 was carried out to distinguish active grit protrusion area and investigate numerically distribution of grit sizes on wheel surface. The result is shown in Fig. 13. In order to analyze grit protrusion diameter statistically, the grit protrusion diameter dg may be defined as the diameter of a circle whose area Sg is equal to grit protrusion area S as shown in Fig. 14. Fig. 15a and b show histograms of girt protrusion diameter distribution on wheel surface obtained from the image process results of wheel surface for Ei = 15 V and Ei = 30 V, respectively. It is shown that the active grit protrusion number ng of Ei = 15 V is about twice larger than that of Ei = 30 V. The reason is that the mean discharge crater height he for Ei = 15 V is much less than dgm /2 and the one for Ei = 30 V is much larger than dgm /2. Therefore, the requirement of discharge parameters (Ie and τ e )
Fig. 14. Mode of grit protrusion diameter dg . (a) Actual shape; (b) Grit diameter.
for ECD dressing may be given by dgm . (6) 2 Fig. 16 shows the critical curves for three kinds of diamond grit mesh such as #270 (dgm = 56.0 m), #600 (dgm = 24.0 m) and #1500 (dgm = 10.5 m) according to Eq. (6). It indicates that ECD dressing may be best done when a limit range of discharge parameters (Ie and τ e ) is chosen on the left side of the critical curve for certain grit size. In addition, the discharge current trace need to be more strictly controlled when the finer diamond grinding wheel is dressed. It is concluded that its discharge parameters (Ie and τ e ) should conform to Eq. (6) so as to realize valid ECD dressing.
he = 5.41 × Ie0.77 × τe0.19 <
Fig. 15. Distribution of grit protrusion diameter. (a) Ei = 15 V; (b) Ei = 30 V.
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Fig. 16. Requirement of discharge parameters for different grit mesh of diamond grinding wheel.
4.4. Dressing of #1500 diamond grinding wheel ECD dressing of #1500 diamond grinding wheel was carried out by in-process monitoring of the discharge current trace between wheel and electrode. The GC grit inside electrode, namely dresser, was chosen as #1500, matching #1500 diamond grinding wheel. The Ei was chosen as 15 V. The other conditions were same as Table 2. At the beginning of this experiment, a short circuit often occurred during ECD dressing as the depth of cut a = 1.0 m. This is because little gap distance between metal bond and electrode dominated by fine grit size cannot accommodate overfull electrode swarf. Therefore, the depth of cut a was regulated and decreased gradually until the discharge parameters (Ie and τ e ) derived from discharge current trace satisfied the Eq. (6). The suitable discharge current trace shown in Fig. 17 for valid ECD dressing was produced by the a of 0.2 m. Its mean discharge crater height he of 4.36 m obtained from Eqs. (2) and (3) by using the Ie and τ e derived from the discharge current trace shown in Fig. 17 is less than the half of the dgm of #1500 diamond grinding wheel. The #1500 diamond grinding wheel dressed with the above conditions was used to grind hard-brittle materials such as Al2 O3 and optical glass (BK10) as compared with mechanical dressing, namely GC cup dressing without electrical erosion. The ground result is shown in Fig. 18. It indicates that ECD dressing may obtain better ground surface than GC cup dressing. It is concluded that the in-process monitoring of discharge current trace during ECD dressing may be utilized to determine dressing process variables for fine diamond grinding wheel.
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Fig. 18. Ground surface roughness of hard-brittle materials.
5. Conclusion In this paper, the quantitative action of discharge parameters (Ie and τ e ) on grit protrusion feature of fine diamond grinding wheel was experimentally investigated and its in-process evaluation method for valid ECD dressing was established. From the experiments and analysis, the following conclusions can be drawn: (1) There exist good correlations between discharge removal and discharge parameters (Ie and τ e ) for impulse discharge machining of metal bond. The grit protrusion feature of diamond grinding wheel is sensitive to the discharge parameters (Ie and τ e ) derived from discharge current trace during ECD dressing, which depends on mean discharge crater height and mean diamond grit size dgm . (2) The requirement of discharge parameters (Ie and τ e ) for valid ECD dressing of diamond grinding wheel should conform to the expression of 5.41 × Ie0.77 × τe0.19 = (dgm )/(2), which may be used to monitoring discharge current trace and evaluate grit protrusion feature during ECD dressing process. (3) The in-process evaluation method for grit protrusion feature on wheel surface may be applied to the ECD dressing of fine diamond grinding wheel to realize valid grinding of hardbrittle materials. Acknowledgements The authors express their thanks to Nitolex Co., Ltd., Read Co., Ltd., and Noritake Diamond Industries Co., Ltd. for providing the electrode and diamond grinding wheel. The Project was sponsored by Research Collaboration Committee of the Japan Society for Precision Engineering and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. References
Fig. 17. Suitable discharge current trace for valid ECD dressing.
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