Development of an intelligent electrolytic in process dressing (ELID) grinding system 5

Design and development of in-process truing for ELID grinding Chapter Design and development of in-process truing for ELID grinding 5.1 Introduction Ultra-precision grinding is a finishing process which is usually used for producing nanoscale surface finish on both hard and brittle materials like ceramics, glasses, hardened steels etc. In-order to achieve accuracy and process quality in precision grinding, maintaining wheel profile uniformity is one of the major factors that should be taken into consideration. The profile inaccuracy in the grinding wheel resulting from clamping error, spindle run out and/or non-uniformity of the wheel may cause process instability and/or surface damage on the finished products. Hence truing of the wheel is needed to eliminate this error. There are several wheel truing methods available, which includes mechanical contact methods (diamond dresser), electrothermal methods (electrical discharge machining), laser technology, loose abrasive machining and electrochemical techniques [3-15]. The working principle and the pros and cons of all these techniques were discussed elaborately earlier in chapter and chapter 2. Electrochemical truing of the grinding wheel has more benefits over other methods because of the system simplicity, low noise level, and less damage to the grinding wheel 93 Design and development of in-process truing for ELID grinding as described in chapter 1. Electrolytic in-process dressing (ELID) is one of the electrochemical methods for in-process dressing and truing of grinding wheels. However, conventional ELID grinding does not use controlled dressing of the grinding wheel to ensure acceptable grinding wheel profile [39-57]. Note that truing is the dressing of the grinding wheel to maintain the profile uniformity. The work performed by Ashizuka et al. [38] contributes greatly to the truing control of the grinding wheel by ELID, but it ignores the concept of convolution, which is discussed in the later part of this chapter. To address these drawbacks, a methodology has been proposed to control ELID power by measuring the circumferential profile of the grinding wheel while considering the convolution between the wheel and the electrode to ensure concentric wheel rotation. In this setup, an inductive displacement sensor is used to measure the metal-bonded grinding wheel profile. However, other methods of profile measurement such as hydrodynamic pressure sensor, laser scanning micrometer are also available [70, 72]. This wheel profile data after proper conditioning is fed back to the controller to adjust the pulse duty cycle of the ELID power supply. In this chapter, the theory and the simulation of this novel concept as well as the experimental implementation are discussed thoroughly. In ELID grinding the metal bonded diamond wheel is continuously dressed and an insulating oxide layer is formed along the circumference of the wheel. This layer is very soft and brittle in nature and easily breaks off as it comes in contact with the workpiece. The wheel truing method proposed in the current study makes use of this phenomenon. 94 Design and development of in-process truing for ELID grinding The example of a non uniform wheel in figure 5.1(a) is used as a brief introduction to this unique concept. However the idea is equally applicable in the case of wheel clamping error or spindle run out problem. Figure 5.1(a) shows a high zone in the metal bond of the grinding wheel. In order to achieve a uniform metal bond controlled dressing current is applied during the pre-dressing of the wheel. This will cause higher metal dissolution from the peak region of the wheel. Eventually after few cycles the grinding wheel will become like the diagram in figure 5.1(b) which shows that a thicker ELID layer is formed at the high portion of the wheel because of the controlled pre-dressing. However, this excess layer will quickly wear off as grinding starts and finally the wheel-workpiece contact shall be maintained uniform throughout the grinding cycle as shown in figure 5.1(c). In this hypothetical case, it is assumed that metal bond shall become completely uniform during the pre-dressing, though this is not the case in actual machining. During fifteen to twenty minutes of pre-dressing of the wheel the metal bond profile improves a lot but it does not become completely uniform; therefore the controlled dressing of the wheel has to be maintained throughout the grinding cycle to ensure consistent grinding. Fig 5.1: (a) A typical example of non-uniform grinding wheel for ELID grinding (b) Grinding wheel profile after applying proposed in-process truing (c) Wheel profile after the grinding 95 Design and development of in-process truing for ELID grinding 5.2. Experimental procedure and signal processing 5.2.1 Experimental setup A detailed schematic illustration of the experimental setup is shown in figure 5.2. In order to implement the new wheel truing idea, the sensor integrated ELID grinding machine (mentioned in the earlier chapters) was used. The experimental conditions are almost same as the earlier experiments; explained in chapter and chapter except the pulse duty ratio. However for readers’ better understanding these are described briefly in the table 5.1. For the current study a Keyance inductive displacement sensor was used to measure the metal bond profile of the grinding wheel. The optical sensor shown in the figure is used to get the reference signal from the spindle mark so that the wheel profile could be measured from the same starting point for every rotation. Figure 5.3 shows the photograph of the inductive and optical sensor attached to the machine. Raw signal thus obtained needs to undergo signal processing stage to use it for the control unit of the current system. Fig 5.2: Experimental setup for in-process truing 96 Design and development of in-process truing for ELID grinding Optical sensor Wheel Inductive sensor Fig 5.3: Sensor arrangement on the machine Table 5.1: Experimental Conditions Wheel Grit Size #4000 Wheel Diameter 75mm 5.2.2 Bond Material Cast Iron Spindle Speed 750 RPM In-Feed 3micron/cycle Feed Rate 150mm/min ELID voltage 90 v constant Duity ratio Controlled by the wheel profile feed back Signal processing Inductive sensor applied in this study measures the distance between the sensor head and the wheel metal bond. It is mounted on the setup in such a way that the distance measured is equal the distance between cathode electrode and wheel metal bond. Usually in ELID grinding, the cathode electrode covers the ¼th of the circumferential area of the wheel. Therefore the data obtained by the sensor for one wheel revolution is divided into four 97 Design and development of in-process truing for ELID grinding segments and the average values are calculated. As the wheel is rotating at a constant speed, it can be seen from figure 5.4(a) and 5.4(b) that during one revolution of the wheel moments will occur when two successive segments come together in the dressing zone. Thus, the effective distance between the wheel and electrode shall be the weighted average of the distances of two successive segments. The weighing factors can be calculated by taking into account the convolution effect between wheel segments and electrode area. Convolution is nothing but denotes the overlapped area between a moving function and a stationary function in time domain. In-order to understand the data processing let us denote the followings, Fig 5.4: (a) The grinding wheel divided in four segments. (b)Two segments together into the dressing zone (c) Relative position between A1 and A0 at time t=0 (d) Relative position between A1 and A0 at < t < T/4 98 Design and development of in-process truing for ELID grinding Si is the average distance from the wheel to the electrode for ith segment, as shown in figure 5.4 (a) ( i = to 4) Ai is the projected area function of ith segment which is assumed to be a rectangle function of unit height in time domain as shown in figure 3(c) and figure 3(d) A0 is projected area function of the electrode which is also assumed to be the rectangular function of unit height in time domain as shown in figure 3(c) and figure 3(d) T is the time for one revolution t is time s(t) is the actual average distance between wheel and electrode at time t. y(t) is the convolution integral At any time t (for < t < T / ) the convolution integral y(t) between function A1 and A0 can be written as follows, t y (t ) = ∫ A0(τ ) A1(t − τ )dτ = t for (0[...]... time of grinding without in- process truing, whereas figure 5. 12(b) shows the same with in- process truing 0 340 5 3 20 After 40 secs of grinding 40 After 360 secs of grinding 2 60 4 320 300 1 280 80 0 -1 260 100 240 120 220 140 200 160 180 (a) 112 Design and development of in- process truing for ELID grinding 0 340 25 20 After 40 seconds of grinding 40 After 360 seconds of grinding After 680 seconds 60 of. .. figure 5. 11(b) is the actual metal bond profile Significant improvement in the wheel profile can be observed in both cases 110 Design and development of in- process truing for ELID grinding 0. 35 0.3 s(t) in mm 0. 25 0.2 0. 15 Profile after 10 min 0.1 Profile after 20 min 0. 05 Profile after 30 min Initial profile 0 Distance from the sensor head in mm 0. 35 0.3 0. 25 0.2 0. 15 0.1 Initial Profile Profile after... metal bond of the wheel becomes more uniform after several iterations Profile after 10min equivalent iteration Distance s(t) (microns) Distance s(t) (microns) Profile initial 350 300 250 200 150 100 5 10 15 20 25 30 35 40 350 300 250 200 150 100 5 10 nth segment Distance s(t) (microns) Distance s(t) (microns) 300 250 200 150 5 10 15 20 25 25 30 35 40 30 35 40 35 40 350 300 250 200 150 100 5 10 nth segment... controlled dressing is applied during grinding as well which is explained in the subsequent section 104 Design and development of in- process truing for ELID grinding 5. 3.2 Mathematical explanation for progressive error reduction in the wheel profile D 2 Fig 5. 7: A non-uniform wheel touching the workpiece at one point The non uniformity in the wheel should be eliminated both form insulating layer and metal... segment Fig 5. 10: 20 Profile after 30min equivalent iteration Profile after 20min equivalent iteration 350 100 15 nth segment 15 20 25 30 nth segment Simulation result of the proposed controller Figure 5. 11 shows the experimental proof of improvement in the metal bond profile of the grinding wheel after the pre -dressing Figure 5. 11(a) is the wheel profile after the signal processing as explained in the... steady grinding 5. 4 Development of the system algorithm to implement in- process truing concept A system was developed to implement the truing concept in ELID grinding The flow chart of the developed system is shown in figure 5. 8 There are two programs one is in the PC and the other one is programmed in the Intel 8 051 microcontroller The PC side program does all the necessary calculations for generating... Send command to processor to generate desired pulse for the ELID Cell Processor received the index signal? NO Wait YES Processor generates the pulsed power for the ELID wheel truing Fig 5. 8: Flowchart of the truing control for ELID grinding 108 Design and development of in- process truing for ELID grinding Figures 5. 9 (a) shows the ELID current profile for one wheel revolution without inprocess truing where... 60 of grinding 20 320 15 300 10 5 280 80 0 -5 260 100 240 120 220 140 200 160 180 (b) Fig 5. 12: Normal cutting force signal for one wheel revolution at different time of grinding (a) without in- process truing (b) with in- process truing It is very clear from these two figures that the wheel workpiece contact becomes more uniform with progressive grinding when in- process truing is used 5. 6 Concluding remarks... implemented and thoroughly experimented to assess its performance In this section simulation results and experimental findings shall be presented and compared extensively 109 Design and development of in- process truing for ELID grinding 5. 5.1 Study on the wheel metal bond profile Figure 5. 10 shows the simulation results of the designed controller using wheel profile data measured by the inductive sensor It can... previous system [38] 113 Design and development of in- process truing for ELID grinding • A pulse width controllable power supply was developed for the in- process truing of the grinding wheel The wheel roundness and the wheel workpiece contact have been improved significantly after implementing this power supply Hence it can be concluded that ELID is not only meant for wheel dressing alone, but in addition . and development of in- process truing for ELID grinding 93 Chapter 5 Design and development of in- process truing for ELID grinding 5. 1 Introduction Ultra-precision grinding is a finishing. grinding (b) Grinding wheel profile after applying proposed in- process truing (c) Wheel profile after the grinding Design and development of in- process truing for ELID grinding 96 5. 2. Experimental. Electrolytic in- process dressing (ELID) is one of the electrochemical methods for in- process dressing and truing of grinding wheels. However, conventional ELID grinding does not use controlled dressing