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

In order to achieve a uniform metal bond controlled dressing current is applied during the pre-dressing of the wheel.. During fifteen to twenty minutes of pre-dressing of the wheel the m

[3-15] The working principle and the pros and cons of all these techniques were

discussed elaborately earlier in chapter 1 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

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

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

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 3 and chapter 4 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

Fig 5.3: Sensor arrangement on the machine

Table 5.1: Experimental Conditions

Wheel Grit Size #4000 Wheel Diameter 75mm 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

Optical sensor

Wheel Inductive sensor

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 0 < t < T/4

Si is the average distance from the wheel to the electrode for ith segment, as shown in figure 5.4 (a) ( i = 1 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 0< <t T / 4) the convolution integral y(t) between function A1 and A0 can be written as follows,

Figure 5.5(a) shows the raw wheel profile and figure 5.5(b) shows the corrected average profile derived from equations (5.1) to (5.4) After these calculations the data is fed back

to the truing controller to output the desired pulse duty ratio at different segment of the wheel In the next section the design and development of such controller has been discussed in detail

Fig 5.5: (a) Raw profile for one wheel revolution (b) Corrected average profile for one wheel revolution

5.3 Theory of a feedback controller for in-process truing in ELID grinding

5.3.1 Controller design

The objective of the proposed system is to vary the dressing power in a way that it ensures consistent wheel-workpiece contact This can be achieved if the metal bond of the wheel is dressed to become uniform This will lead to non-uniform ELID layer formation on the wheel circumference However, the excess layer will be broken off as it interacts with the workpiece as shown in figure 5.1 previously

In order to design such a controller a system model has been derived from Faraday’s basic law of electrochemistry Faraday’s law states the following,

where,

dm is the mass dissolved from the anode

I is the average current flowing thru the ELID cell

dt is the time of electrolysis

Fis the Faraday’s constant

M is the molar mass of anode

zis the valence number of the anode

In the current study the power supply is designed to produce pulsed DC current for the ELID cell Hence equation 5.5 can be re written as follows,

I is the peak current

ris the pulse duty ratio

Further from equation 5.6 the following can be deduced,

A is the peripheral area of the wheel covered by the electrode

As I peakis kept constant in the power supply, the only parameter that can be controlled to

achieve the uniformity of the wheel is r The following control unit illustrated in the

figure 5.6 is designed for this purpose

min

r

Fig 5.6: Control unit for ELID truing

In the figure 5.6, x(n) is the array of distance value between the wheel and electrode for different segment on the wheel after proper signal processing as explained in section 5.2.2 R is the reference target which is the maximum of x(n) K1 is the proportional gain and K2/s [K2 is electrolytic constant] is the system model in s domain as proposed in equation 5.7 The zero order hold block is necessary because the control signal needs to

be held constant for T/n time In this particular study n is 40 The control signal r(n) for nth segment is defined by the following equation,

where,

max

r is the maximum duty ratio allowed by the controller which is 0.99

min

r is the minimum duty ratio allowed by the controller which is 0.01

K1 can be deduced from equation 5.8 which is equal to (max min)

5.3.2 Mathematical explanation for progressive error reduction in the wheel

profile

D2

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 bond which can be achieved by controlled ELID truing The metal bond profile of the wheel shall become uniform as the truing control is applied, which is explained in the earlier section The insulating layer shall also become uniform as the wheel interacts with the workpiece which is discussed in this section Figure 5.7 shows three different angular positions (1,2 and 3) on a non-uniform wheel D1, D2 and Dn represents distances from the wheel center to the edge for the respective positions Now the initial profile error of the wheel at these three different angles can be described as follows

e10 = D1-Dn ……… (5.10)

e30 = D1-D2……….(5.12)

where,

e1, e2 and e3 are the wheel profile error at different angular position of the wheel

Let us further denote α1,α2 and αn as the increase in layer thickness in one machining cycle at different angular position of the wheel as shown in figure 5.7 Due to the controlled electrochemical reaction αn is zero and α1 is the maximum (αn<α2<α1) Let

δ be the in-feed (for simplicity δ< α1) in each machining cycle; after one machining cycle equations 5.10 to 5.12 become as described below,

From the above three equations it is clear that all the error except e2 are decreasing Although e2 is increasing it will cause D2 to become equal to D1 as predicted by equation 5.14 Then overall error shall be decreasing at a rate governed by the equation 5.13 Eventually all the segments of the grinding wheel will become uniform which will lead to a 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 8051 microcontroller The PC side program does all the necessary calculations for generating the desired pulse duty ratio for one wheel revolution whereas the controller side program generates the pulses and captures the wheel profile data for one wheel revolution The communication between the controller and the PC is being done by RS232 serial communication

At first a signal is sent from the PC to the microcontroller On receiving this signal, the microprocessor starts to wait for the interrupt input from the optical sensor as explained

in the section 5.2.1 On receiving the interrupt; it measures a wheel profile and sends it to the PC through RS232 communication The PC program takes care about all the necessary signal processing steps; calculates the desired pulse duty ratio for one wheel revolution and sends the data to the microcontroller After receiving the pulse duty ratio data from the PC the microcontroller program again waits for the interrupt signal Then it

generates the desired pulse duty ratio across the ELID cell upon receiving the interrupt signal from the optical sensor, and the process repeats throughout the grinding cycle

the index signal?

Processor received the index signal?

Processor generates the pulsed power for the ELID wheel truing

YES

Wait

NO Send command to processor to generate desired pulse for

the ELID Cell

Figures 5.9 (a) shows the ELID current profile for one wheel revolution without process truing where the pulse width is uniform throughout the revolution and figure 5.9 (b) shows the same after the above mentioned system is applied where the pulse width is varied according to the wheel profile during one wheel revolution

Fig 5.9: Current profile for one wheel revolution (a) Without in-process truing (b) With in-process truing

5.5 Results and discussions

The concept of wheel truing method described in the previous sections has been practically implemented and thoroughly experimented to assess its performance In this section simulation results and experimental findings shall be presented and compared extensively

Index Signal

Current pulses

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 be observed from the figure that the metal bond of the wheel becomes more uniform after several iterations

100 150 200 250 300 350

Profile initial Profile after 10min equivalent iteration

Profile after 20min equivalent iteration Profile after 30min equivalent iteration

Fig 5.10: 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 previous section whereas figure 5.11(b) is the actual metal bond profile Significant improvement in the wheel profile can be observed in both cases