Development of an intelligent electrolytic in process dressing (ELID) grinding system 4

Dressing optimization by controlling the voltage with cutting force feedback Chapter Dressing optimization by controlling the voltage with cutting force feedback 4.1 Introduction ELID (Electrolytic In-Process Dressing) by name itself is an in-process dressing technique, which is mostly applied on metal bonded ultra fine diamond wheel to ensure constant protrusion of the diamond grits. The concept was pioneered by Murata et al. [23] and later widely established by Ohmori et al. [24]. In ELID grinding the in-process dressing of the wheel is being carried out by electrochemical reaction. The mechanism of this is discussed comprehensively in the previous chapter (chapter 1). In order to carry out the electrochemical dressing of the grinding wheel a pulsed DC power source is mostly used. In ELID grinding an insulating oxide layer is formed during the dressing of the wheel because of the electrochemical reaction. This layer plays a polishing effect during grinding which enhances the quality of the finished product [81]. However the formation of the layer actually softens the wheel bond material which leads to higher tool wear. Murata et al. [23] in their investigation varied the pulse ON-OFF time by monitoring the grinding condition, although from further studies of ELID it is realized that the 75 Dressing optimization by controlling the voltage with cutting force feedback monitoring of the grinding condition in order to vary the dressing power was ignored. As a result the amplitude and the ON-OFF time of the pulsed DC power supply output are usually kept constant in conventional ELID grinding. In the case of BK7 glass previous researchers used 90v as the peak dressing voltage [30,55], but this technique of applying constant high peak voltage may overdress the grinding wheels, consequently the tool life shall be shortened. In order to address this problem one of the objectives of this research was set to optimize the dressing power by monitoring the grinding condition. In this chapter the author explains the implementation of an intelligent system that will monitor the sharpness of the grinding wheel and the dressing peak voltage shall be varied accordingly. Further a detail comparison between this new dressing method and conventional ELID grinding has been carried out to understand different aspects of grinding for the new technique such as grinding stability, ground surface quality, grinding wheel wear etc. The monitoring of the grinding wheel sharpness is one of the key factors for implementing such a system. There are several methods available to monitor the grinding wheel as explained elaborately in chapter 2. Malkin [63] studied the grinding force variation with wheel wear and found that grinding force ratio changes as wheel gets worn out. Marinescu [81] also proposed grinding force ratio as an index for monitoring the sharpness of the grits. D Kramer [16] and Murata et al. [23] used grinding force ratio as a feedback signal to control the wheel dressing power. It can be understood from the above discussion that grinding force ratio monitoring is more suitable for dressing control. In this current research, the author uses grinding force ratio [K value] as the indicator of the 76 Dressing optimization by controlling the voltage with cutting force feedback wheel sharpness and the dressing power for ELID grinding is varied according to the change of the wheel sharpness index. 4.2 Theoretical background and concept implementation In this section theoretical background will be given on how grinding force ratio can be used as the wheel sharpness monitoring index. Moreover detail explanation will be given on the implementation of the concept for development of ‘dressing on-demand’ technology in ELID grinding. 4.2.1 Theory It is a well accepted phenomenon that force ratio (Normal Force (Fn)/ Tangential Force (Ft)) in grinding increases as the grits of the grinding wheel becomes worn out and blunt [81]. A sharp cone grit model of figure 4.1 explains the force ratio [82] where infeed/indentation direction is shown in the figure and the feed direction is perpendicular to the page. The area ABC (Ap) swept by the acting grit (effective area for tangential force) from the work-piece is given by, Ap = ⋅ 2rb ⋅ rb tan α = rb .tan α ……………………………………………………………………… . (4.1) The effective area for the normal force is given by the following equation, An = Π. ( rb ) ……………… ……………………………………………………… (4.1a) Then the normal (Fn) and tangential force (Ft) can be expressed as follows, 77 Dressing optimization by controlling the voltage with cutting force feedback Ft = Ap . p = rb . tan α . p (4.2) Fn = An ⋅ p = Π. ( rb ) . p (4.3) where, p is the force per unit area. 2rb is the grit contact width Thus, the force ratio can be estimated from the above two equations, as shown below, K = Fn/Ft = Π …………………………………………………………………… (4.4) tan α In-feed Fig 4.1: Sharp cone grit model However the above force ratio model yields a constant value for K and it does not describe the actual situation. Sin, Saka and Suh [83] introduced further improvement by assuming the grits to be a combination of cone and sphere as shown in the figure 4.2(a) where r is the radius of the arc, rb is the grain contact width and α is the inclination angle. At small depths of penetration, when 2.rb ≤ 2.r.sin α then the swept area by the grit during sliding ABC (Ap) is as follows, 78 Dressing optimization by controlling the voltage with cutting force feedback 1/2   rb   −1 rb Ap = r sin − rb r 1 −    …………………………………………………….(4.5)  r  r   Sin, Saka and Suhs’ [83] grit model assumes area An as follows, An = Π. ( rb ) / ……………… (4.5a) Expression for Fn and Ft can be obtained by the same method as applied for equation 4.2 and 4.3. Therefore the K value for cone-sphere model of the grit with small depth of indentation can be redefined as follows; −1 1/2 −2 −2     r r ∏  rb        −1 b b  …………………………………………(4.6) K= .sin − −          r  r r        In the case of higher depth of indentation of the grit , where 2.rb ≥ 2.r.sin α the active area A′BC ′ (Ap) can be expressed as,  r .α  Ap = tan α .  rb − r +  ………………………………………………………….(4.7) tan α   and subsequently K can be defined for higher indentation as follows, −1 −2 Π   rb   tan α − α   K= 1 −    ……………….………………………………(4.8) ⋅ tan α   r   tan α   In ELID grinding the in-feed of the wheel is very small therefore equation 4.6 is more appropriate to express K. The value of K has been calculated for different range of the parameter ⋅ rb / r and figure 4.2(b) shows the variation of K with ⋅ rb / r . If it is assumed that α = 45o then the correct range of ⋅ rb / r [for equation 4.6 to work] is to 1.4. As machining progresses the cutting grit tip becomes flat and value of r increases. Thus lower value of ⋅ rb / r indicates the worn out and dull grits. It is clearly 79 Dressing optimization by controlling the voltage with cutting force feedback understandable from the figure that K increases drastically as the wheel becomes worn out and blunt. (a) Fig 4.2: (a) Grit model (b) Variation of K with 4.2.2 (b) rb r Concept implementation The grinding wheel sharpness monitoring approach mentioned above has been implemented in the ELID power supply to adjust the peak dressing voltage. The overall algorithm is described in the block diagram shown in the figure 4.3(a). The acceptable value of K for grinding depends on the cutting grits’ material. In the current study thr author used diamond grinding wheel for which the standard range of K for sharp wheel is 11 to 15 [16]. Therefore the maximum permissible limit for K has been set to be 15. 80 Dressing optimization by controlling the voltage with cutting force feedback (a) (b) Fig 4.3: (a) Proposed dressing on demand model (b) Variation of K and Dressing peak voltage in one machining cycle Figure 4.3(b) shows the variation of the peak dressing voltage and grinding force ratio in one machining cycle. The two peaks as shown in the figure describe the high value of grinding force ratio during the initial engagement between wheel and workpiece because of the sudden impact during the start of the machining cycle. Software was written in order to automate the process and the complete flow chart of the system is shown in figure 4.4. At first the pre-dressing is done at a constant voltage. During grinding cutting 81 Dressing optimization by controlling the voltage with cutting force feedback force is measured with the help of the analog force dynamometer. An analog to digital converter converts the analog force signal to its digital form and data are fed back to the computer. After that K value is measured during each sampling time and the control signal is sent to the serially interfaced ELID power supply to vary the dressing voltage accordingly. Fig 4.4: Flow chart for ‘Dressing on demand’ for ELID grinding 4.3 Experimental setup and machining conditions An intelligent machine tool equipped with several sensory systems was developed to implement the proposed controlled dressing concept for ELID grinding. The detail 82 Dressing optimization by controlling the voltage with cutting force feedback description of the machine is given in chapter 3. In order to carry out the experimentation BK7 glass work-piece and cast iron bonded ultra fine grinding wheel were used as mentioned earlier. Detailed experimental setup is illustrated in the figure 4.5. The copper injection type electrode [explained in chapter 3] was used to ensure radial flow of the electrolyte. A commercially available coolant, namely CG7 [30], was mixed with water to a ratio of 1:50 and used as electrolyte and coolant for the experiment. The in- feed and feed direction are same as the experiments explained in the section 3.3.1 of chapter and also shown in this figure. A axis Kistler dynamometer has been sandwiched between the workpiece and the base plate to measure the normal and tangential cutting force during machining as shown in the figure. The power supply used for the wheel dressing is voltage controllable. The maximum and minimum peak voltages that can be maintained are 100V and 30 V respectively. Fig 4.5: Experimental setup for dressing optimization 83 Dressing optimization by controlling the voltage with cutting force feedback A Keyance laser displacement sensor shown in figure 4.6 was used for the experiment to measure the wheel profile so that the wheel wear can be assessed. The sensor uses a triangulation measurement system. This laser sensor employs CCD (Charged coupled device) as the light receiving element which enables a stable, highly accurate gap measurement. Extensive experiments have been carried out to compare conventional ELID grinding and the proposed dressing on demand ELID grinding. The experimental conditions for both the cases are described in the table 4.1. The in-feed was kept to be constant at micron because it was found to be the optimum in-feed for BK-7 glass machining as explained in chapter (section 3.3.2). Table 4.1: Machining Condition Conventional ELID Dressing on Demand ELID grinding grinding Wheel Diameter 75mm 75mm Wheel grit size #4000 #4000 Wheel Speed 900 RPM 900 RPM In-feed/machining 3microns 3microns Feed Speed 150mm/min 150mm/min Dressing peak voltage 90 V (const) Variable (According to K value) Duty ratio 30% 30% cycle 84 Dressing optimization by controlling the voltage with cutting force feedback Laser Sensor Grinding Wheel Gap adjusting mechanism for electrode Fig 4.6: Laser sensor arrangements to measure the tool wear 4.4 Results and discussions The phenomenon of grinding force ratio variation with wheel wear described in the previous section was verified by the experimental study. Several vertical slots with same dimensions were machined without any wheel dressing. Figure 4.7 shows that progressive grinding without any intermediate dressing shall eventually lead to high force ratio value and bad surface roughness as worn out grits come into the grinding action. Fig 4.7: Variation of the force ratio and Ra with progressive grinding. Each slot depth is 0.1mm. 85 Dressing optimization by controlling the voltage with cutting force feedback 4.4.1 Study of grinding stability and ground surface quality The methodology of ELID power control proposed in this current study has been implemented and compared with the conventional ELID grinding. The new system dresses the grinding wheel less than conventional ELID grinding because the dressing power is varied according to the change in the force ratio of grinding as explained earlier. Hence the normal grinding force (Fn) in grinding are slightly higher in this method as the comparison between the both systems is shown in the figure 4.8. However the K value is still well within the acceptable range for the new method and the (a) (b) Fig 4.8: Variation of Grinding force and Force ratio (a) Conventional ELID (b) Dressing On Demand ELID 86 Dressing optimization by controlling the voltage with cutting force feedback surface characteristic produced by both the systems are equally comparable. Figure 4.9 shows the results of comparison of surface quality between constant peak voltage dressing and controlled peak voltage dressing ELID grinding. It can be clearly observed from the figure that surface roughness (Ra) and surface uniformity (Rt) of the machined workpiece are comparable and equivalent for both Fig 4.9: Comparison of surface properties (Ra and Rt) and force ratio between constant peak voltage dressing and Controlled peak voltage dressing. the cases. Figures 4.10(a) and 4.10(b) show the ground surface profile measured by the Taylorsurf using conventional ELID grinding and dressing on demand ELID grinding. The surface roughness and uniformity (Ra and Rt) are very good and equivalent for both methods. Figure 4.11(a) and 4.11(b) show the microscopic view of conventional ELID ground surface and dressing on demand ELID ground surface. These images prove that equally clean surface can be produced using the two dressing technologies. 87 Dressing optimization by controlling the voltage with cutting force feedback Ra = 11.7 nm Rt = 0.24 microns (a) Ra = 10.8 nm Rt = 0.26 microns (b) Fig 4.10: Surface profile (a) conventional ELID ground surface (b) and Dressing On Demand ELID ground surface (a) (b) Fig 4.11: Surface topography for conventional ELID ground surface (a) and Dressing On Demand ELID ground surface (b). (450X optical zoom) 88 Dressing optimization by controlling the voltage with cutting force feedback 4.4.2 Study of dressing current and wheel wear In the case of conventional ELID grinding researchers recommend peak dressing voltage as 90V for BK glass [30]. However in the case of dressing on demand ELID grinding average peak dressing voltage during one machining cycle is about 58V (as shown in the fig 4.3(b)), which is only 65% of the conventional dressing voltage. This result is very significant because this indicates that the proposed method of wheel dressing can save the wheel from unnecessary dressing without compromising the ground surface quality. Figure 4.12 shows the comparison of the dressing current for two dressing methods. Fig 4.12: Comparison between controlled voltage dressing and constant voltage dressing for average dressing current Dressing on demand ELID grinding generates much less and uniform dressing current than conventional ELID grinding. It is realized from the previous studies of ELID grinding that during the dressing of the grinding wheel a soft insulating oxide layer is formed and it may break as the grinding force increases which occurs throughout the 89 Dressing optimization by controlling the voltage with cutting force feedback grinding process. During the whole grinding operation formation and break down of the layer continues. The breakage of the insulating layer eventually causes increase in the dressing current flow. As the average dressing voltage in the new system is much lower than the conventional ELID this increase in the dressing current due to the insulating layer breakage is not much significant which results much uniform dressing current profile as shown in figure 4.12. In order to compare the tool wear between the two techniques of electrolytic dressing of the wheel, grinding experiments were carried out. A 0.1 mm deep, 10mm long and 2mm wide slot were machined for both grinding experiments. The duty ratio was kept constant to 30% for both cases to maintain same experimental condition. The tool wear was calculated by comparing the grinding wheel profile. (Before grinding and after grinding) Figure 4.13 (a) shows profile of the grinding wheel before the grinding and after grinding for conventional ELID. Figure 4.13 (b) shows the same for the dressing on demand ELID grinding. The tool wear can be calculated from the figures and the results are summarized in the table 4.2. Table 4.2: Results of the wheel wear experiment Dressing Ground slot dimension Dressing Voltage Wheel (L*W*D) after grinding On 10mm*2mm*0.1mm Varied according to 15 microns the Demand ELID wear change in K value Conventional ELID 10mm *0.1mm Constant voltage of 31 microns 90V 90 Dressing optimization by controlling the voltage with cutting force feedback (a) (b) Fig 4.13. Grinding Wheel profile before and after grinding (a) Conventional ELID (b) Dressing on demand ELID. It is clear from the above comparison that dressing on demand ELID grinding dresses the wheel less compared to conventional ELID grinding; thus tool wear is significantly reduced (48% of the conventional ELID grinding). However the surface quality is not compromised during the proposed method of controlled dressing of the grinding wheel. 91 Dressing optimization by controlling the voltage with cutting force feedback 4.5 Concluding remarks This chapter explains one of the two major improvements applied to the ELID power supply. A newly developed variable voltage dressing methodology has been introduced in contrast with the conventional ELID grinding where constant peak voltage is applied to the grinding wheel throughout the grinding cycle. The proposed mechanism uses grinding force ratio as the feed back signal to adjust the peak dressing voltage and hence the dressing power. The new system has been proven to be more efficient and several improvements can be achieved over conventional ELID grinding. • The average dressing power applied to the new system is much less compared to typical ELID. However the grinding system is equivalently stable and the ground surface quality are comparable for both the cases. • The dressing current profile over the whole grinding cycle is much more uniform for the proposed methodology of wheel dressing. The tool wear is found to be only 48% of conventional ELID grinding for equal volume of material removal. However the ground surface quality was not sacrificed as mentioned earlier. Hence it can be confidently concluded that dressing parameters in ELID grinding needs to be adjusted according to the wear of the wheel to save it from excessive dressing. In the next chapter a comprehensive discussion is given explaining how ELID technology can be used for the truing of the grinding wheel by implementing a feedback control mechanism on the dressing power supply. 92 [...]... 4. 12 shows the comparison of the dressing current for two dressing methods Fig 4. 12: Comparison between controlled voltage dressing and constant voltage dressing for average dressing current Dressing on demand ELID grinding generates much less and uniform dressing current than conventional ELID grinding It is realized from the previous studies of ELID grinding that during the dressing of the grinding. .. 10mm long and 2mm wide slot were machined for both grinding experiments The duty ratio was kept constant to 30% for both cases to maintain same experimental condition The tool wear was calculated by comparing the grinding wheel profile (Before grinding and after grinding) Figure 4. 13 (a) shows profile of the grinding wheel before the grinding and after grinding for conventional ELID Figure 4. 13 (b)... force feedback 4. 4.1 Study of grinding stability and ground surface quality The methodology of ELID power control proposed in this current study has been implemented and compared with the conventional ELID grinding The new system dresses the grinding wheel less than conventional ELID grinding because the dressing power is varied according to the change in the force ratio of grinding as explained earlier... grinding wheel a soft insulating oxide layer is formed and it may break as the grinding force increases which occurs throughout the 89 Dressing optimization by controlling the voltage with cutting force feedback grinding process During the whole grinding operation formation and break down of the layer continues The breakage of the insulating layer eventually causes increase in the dressing current flow... properties (Ra and Rt) and force ratio between constant peak voltage dressing and Controlled peak voltage dressing the cases Figures 4. 10(a) and 4. 10(b) show the ground surface profile measured by the Taylorsurf using conventional ELID grinding and dressing on demand ELID grinding The surface roughness and uniformity (Ra and Rt) are very good and equivalent for both methods Figure 4. 11(a) and 4. 11(b) show the... microns 90V 90 Dressing optimization by controlling the voltage with cutting force feedback (a) (b) Fig 4. 13 Grinding Wheel profile before and after grinding (a) Conventional ELID (b) Dressing on demand ELID It is clear from the above comparison that dressing on demand ELID grinding dresses the wheel less compared to conventional ELID grinding; thus tool wear is significantly reduced (48 % of the conventional... were machined without any wheel dressing Figure 4. 7 shows that progressive grinding without any intermediate dressing shall eventually lead to high force ratio value and bad surface roughness as worn out grits come into the grinding action Fig 4. 7: Variation of the force ratio and Ra with progressive grinding Each slot depth is 0.1mm 85 Dressing optimization by controlling the voltage with cutting force... glass [30] However in the case of dressing on demand ELID grinding average peak dressing voltage during one machining cycle is about 58V (as shown in the fig 4. 3(b)), which is only 65% of the conventional dressing voltage This result is very significant because this indicates that the proposed method of wheel dressing can save the wheel from unnecessary dressing without compromising the ground surface... normal grinding force (Fn) in grinding are slightly higher in this method as the comparison between the both systems is shown in the figure 4. 8 However the K value is still well within the acceptable range for the new method and the (a) (b) Fig 4. 8: Variation of Grinding force and Force ratio (a) Conventional ELID (b) Dressing On Demand ELID 86 Dressing optimization by controlling the voltage with cutting... conventional ELID grinding where constant peak voltage is applied to the grinding wheel throughout the grinding cycle The proposed mechanism uses grinding force ratio as the feed back signal to adjust the peak dressing voltage and hence the dressing power The new system has been proven to be more efficient and several improvements can be achieved over conventional ELID grinding • The average dressing power . calculated by comparing the grinding wheel profile. (Before grinding and after grinding) Figure 4. 13 (a) shows profile of the grinding wheel before the grinding and after grinding for conventional. to vary the dressing voltage accordingly. Fig 4. 4: Flow chart for Dressing on demand’ for ELID grinding 4. 3 Experimental setup and machining conditions An intelligent machine tool equipped. Fig 4. 3: (a) Proposed dressing on demand model (b) Variation of K and Dressing peak voltage in one machining cycle Figure 4. 3(b) shows the variation of the peak dressing voltage and grinding