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Precision Engineering 34 (2010) 124–132

Contents lists available at ScienceDirect

Precision Engineering

journa l homepage: www.e lsev ier .com/ locate /prec is ion

knowledge based feed-back control system for precision ELID grinding

. Fathima ∗, M. Schinhaerl, A. Geiss, R. Rascher, P. Sperberepartment of Mechanical and Optical Engineering, University of Applied Sciences Deggendorf, Edlmairstr. 6+8, 94469 Deggendorf, Germany

r t i c l e i n f o

rticle history:eceived 23 July 2008eceived in revised form 24 December 2008ccepted 11 May 2009vailable online 28 May 2009

eywords:LIDrinding

a b s t r a c t

Newer materials with excellent properties are of recent interest in the optical, electrical and electronicsindustries. Finishing of new materials for the required stringent specification stated by those industrialapplications emerges the innovation of new techniques and processes in the field of manufacturing. Elec-trolytic in-process dressing (ELID) is one of those new manufacturing techniques which may producemirror surface finish on various optical and non-optical materials. The easy implementation and the effi-ciency of the ELID technique have been drawing the attention of the optical manufacturing industries inthe recent years. However, further improvements are essential in order to minimize the difficulties expe-rienced during implementation and to extend its suitability for future applications. The authors propose

nowledge baseeed-back controlilicon waferuartz

a knowledge based feed-back control system for ELID grinding to eliminate the application difficultiesand to improve the effectiveness of the process. This study aims to experiment and to analyze the variousfeatures of the developed feed-back control system. The main objective is to examine the effectiveness ofthe system for precision finishing of optical and non-optical harder materials. The possibilities to reducethe geometrical inaccuracies of the workpiece have been examined in this study. The results show thatapplication of the feed-back control system minimizes the number of correction cycles necessary for

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precision finishing of har

. Introduction

The applications of hard and brittle materials have been increas-ng in the optical, electronic and mechanical industries due to thexcellent properties suitable for the recent requirements in thosendustries. Every year several new materials are invented and testedor different applications, and finishing of those materials for theesired stringent specifications becomes a challenge for today’sanufacturing. Therefore, modification of conventional processes

nd implementation of new processes is inevitable in order to facehe newer manufacturing challenges.

Grinding is one of the versatile manufacturing processesommonly used in the process chains of optical, electrical andechanical industries. Among those, grinding plays a vital role in

he optical industries. Grinding is used as a stock removal pro-ess as well as a fine finishing process in the optical industries.owever, grinding is used for producing components with certain

oughness, and the final finishing mainly depends on loose abra-

ive processes such as polishing. The conventional grinding andolishing cycle is more efficient for producing basic optical com-onents with simple geometrical shapes, such as, spherical andlano surfaces. Rapidly increased demands on aspherical and free-

∗ Corresponding author. Tel.: +49 991 3615 368; fax: +49 991 3615 399.E-mail address: fathima.kader-mohideen@fh-deggendorf.de (K. Fathima).

141-6359/$ – see front matter © 2009 Elsevier Inc. All rights reserved.oi:10.1016/j.precisioneng.2009.05.004

erials, such as, quartz.© 2009 Elsevier Inc. All rights reserved.

form optical components enlighten the urge of new manufacturingtechniques for today’s optical industries.

Grinding with fixed abrasives may produce the desired free-form surfaces with minimal shape error. Finishing of free-formsurfaces with fine fixed abrasive wheels is a simple way to main-tain the geometrical accuracy and to improve the surface finish.Therefore, a process of grinding with fine abrasive grits, commonlyknown as deterministic micro-grinding, has been gaining impor-tance in these days. However, grinding with fine fixed abrasivesis not a simple task as expected. Firstly, the conventional grindingwheels are not suitable for fine grinding of tough materials due tofaster diameter diminishing and wheel loading, and secondly, a dif-ferent bond matrix is required when the abrasive sizes decrease tosubmicron level.

The electrolytic in-process dressing (ELID) technique has beengaining popularity in the early 90s [1,2]. This technique extends thepossibilities of using very fine abrasive sizes from 3 �m to 500 nm,and this technique reestablishes the conventional grinding processinto newer manufacturing. Special ELID grinding wheels have beenintroduced for establishing mirror finish on hard and brittle materi-als. The fine abrasives are bonded with usually metal or metal–resin

bonds in order to provide better bond strength during grinding. TheELID technique uses in-process electrolysis for oxidizing metal bondof the wheel to a certain amount for reducing the bond strength inorder to provide self-sharpening. Early reports and results showthat ELID is an efficient technique for establishing mirror finish on

K. Fathima et al. / Precision Engineering 34 (2010) 124–132 125

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the machine stiffness, tool spindle rotational errors and precision ofthe machine axes. A highly stable ultra-precision grinding machinemay minimize the machine inaccuracies and might improve thegeometrical accuracy of the workpiece. However, grinding with anultra-precision machine is not very much helpful in order to obtain

Fig. 1. (a) ELID grinding s

oth hard and brittle optical and non-optical materials [3–6]. Apartrom this, there are several advantages such as the technique doesot need major modifications in terms of machineries, which leadso low investment costs. An external power supply, a simple elec-rode attachment and an electrolyte are the essential elements formplementing this new technique.

Though the ELID technique is simple, successful and effective,everal practical difficulties have been experienced and reported inhe recent years [7]. The selection of good correlation between therinding and the ELID parameters and the undefined wheel wearre some of the important factors to be considered [8,9]. The aboveentioned factors significantly influence the geometrical accuracy

nd the surface finish of the workpiece, and hence it is very impor-ant to optimize the parameters for utilization and implementationf the ELID technique in an efficient way.

The advancements in the simulation and virtual technologies inhe recent years simplify several complicated manufacturing tasks.he authors developed a knowledge based feed-back control sys-em for optimizing and controlling the ELID process suitable forarious materials and applications. The knowledge database devel-ped which is used along with the machine may simplify variousifficult tasks in today’s manufacturing. The major objective of thisaper is to experiment the main features of the feed-back controlystem for the feasibility of future manufacturing.

. ELID grinding setup and mechanism

This section gives a brief introduction about the mechanismf the ELID [2,8]. The ELID technique enhances the possibilitiesf finishing extremely hard and brittle materials using fine abra-ive wheels by dressing the grinding wheels during the grindingrocess. The ELID grinding system basically consists of an elec-rolytic cell that contains a conductive metal bonded grindingheel, an electrode, a DC pulse power supply and an electrolyte

s illustrated in Fig. 1(a). The positive pole of the power supplys connected with grinding wheel, and the negative pole is con-ected with the electrode. A small gap of 0.1–0.3 mm is maintainedetween the wheel and the electrode in order to enable the elec-rolyte to pass. Electrolysis is stimulated when the power supplys switched ON. At the beginning of electrolysis the wheel sur-ace (a new or a trued wheel) has good conductivity, and hencehe current is as great as the current set at the power supply.ig. 1(b) shows the mechanism of the ELID. Due to electroly-is, the metal ions flow out from the wheel surface and form

etal oxide as shown in Fig. 1(b)-1. The prolonged electrolysis

eads to the oxidized layer deposition on the wheel active sur-ace (the wheel surface exposed to electrolysis). The depositionf the oxidized layer reduces the conductivity of the wheel andrevents the wheel from further oxidation (Fig. 1(b)-2), which is

nd (b) ELID mechanism.

noticed by a lower constant current. When a certain amount ofoxidized layer is scrubbed off during grinding due to the wheel-work interaction (Fig. 1(b)-3) the electrolysis is automaticallystimulated (Fig. 1(b)-4). This cycle (Fig. 1(b)-2–4) is called theELID cycle which repeatedly occurs throughout the grinding pro-cess.

3. A comparison between the conventional control and thefeed-back control system

This section gives an overview of the procedures adopted in theconventional ELID grinding and the newly developed knowledgebased feed-back controlled ELID grinding for precision finishing.The flow diagram shown in Fig. 2 depicts the procedure adoptedfor precision grinding using conventional ELID grinding. A grindingmachine is equipped with an external ELID power supply whichprovides the electrical power necessary for stimulating the in-process electrolysis in order to dress hard metal-bonded grindingwheels. Both the grinding parameters and the ELID parameterswere set by the operator before performing grinding. The param-eter selection mainly depends on the experience of the operator.There could be two main factors which influence the geometricalaccuracy of the work. The first factor is the change of wheel geom-etry due to the wheel wear and the second factor is the inaccuracycaused by the grinding machine. The machine accuracy depends on

Fig. 2. Flow diagram of conventional ELID grinding.

126 K. Fathima et al. / Precision Engineering 34 (2010) 124–132

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Fig. 3. Flow diagram of feed-b

he stringent specification of free-form workpieces without con-rol and compensation of inaccuracies caused by the wheel wear.he geometrical accuracy of the ground workpiece was measuredsing an interferometer. The deviations from the desired ideal pro-le were used for the new tool path generation. The tool path is

he trajectory of the tool during grinding for obtaining the desired

eometry on the workpiece. The tool path has been generated byhe grinding machine based on the workpiece geometry, grind-ng wheel dimensions and the desired geometry required on the

orkpiece. The correction procedure is repeated until the desiredolerance is achieved. However, the complex workpiece geometry of

Fig. 4. Experime

ocess control system for ELID.

aspherical and free-form surfaces may make the profile correctioncycles cumbersome.

Fig. 3 shows the flow diagram of the feed-back process controlsystem developed for ELID grinding. The feed-back system consistsof a computer controlled pulsed power source (current and voltagein the form of pulse), the software necessary for the user interface

and the knowledge database. The main task of this work was to buildthe knowledge database for commonly used and specific materialsto be ground with ELID. In this system, it is possible for the userto describe the grinding job in terms of work material, wheel bondmaterial and the machining parameters required for the job. The

ntal setup.

K. Fathima et al. / Precision Engineering 34 (2010) 124–132 127

Table 1Grinding wheel specifications.

Mesh size of the wheel Average grit size (�m) Bond material Grit concentration Wheel type

#325 45 FCI-X 100 Straight#1200 12 FCI-X 100 Straight#4000 3.8 FCI-X 100 Straight

Table 2Machining parameters.

Mesh size of the wheel Cutting speed (m/s) Spiral speed (mm/min) Spiral distance (mm) Depth-of-cut (�m)

#325 30 4,000–10,000 0.1 50#1200 30 4,000–5,000 0.1 10#4000 21 2,000–5,000 0.1 5

Table 3Materials and properties.

Material Hardness (kg/mm2) Fractal toughness (MPa m1/2) Passion ratio Young’s modulus (GPa)

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used Quartz 615 0.79-BK7 559 0.82ilicon wafer 〈1 0 0〉 1150 0.91

nformation inputs are used to select the optimized ELID parame-ers for the process, and to set the values for the process control fromhe knowledge database. The optimized parameter settings reducehe excessive wear rate of the wheel and improve the geometricalccuracy of the work. The feed-back control system controls andaintains the optimized set values for ELID during grinding. How-

ver, it is difficult to eliminate the geometrical inaccuracies of theorkpiece in a single cycle. The main aim of the knowledge based

eed-back control system is to minimize the profile errors occurreduring ELID grinding and to minimize the number of correctionycles necessary for obtaining the desired geometrical accuracy. It isxpected that the number of correction cycles required to finish theorkpiece with the desired specifications using the feed-back con-

rol system (Nf) is expected to be less than the number of correctionycles required for finishing the workpiece with the conventionalontrolled ELID process (Nc).

. Experimental setup and procedure

This section gives a general view on the experimental setup, pro-edures and methodologies used in this study. The first subdivisionxplains the machineries, measuring equipments and the experi-ental setup used in this study. The second subdivision explains theheel wear measurement method used for conventional grinding

nd for the ELID grinding.

.1. Experimental setup and procedure

Fig. 4 shows the schematic illustration of the experimentaletup used in the Laboratory of Optical Engineering. The exper-

ments were carried out using a five axes twin spindle LOH G-IIptical grinding machine (Satisloh, Germany). The workpiece wasounted on the workpiece spindle which rotates up to 900 rpm

uring grinding. The pre-grinding wheels were used with the roughrinding spindle of the machine, and the fine grinding process

able 4aterials, sample dimensions and the surface quality.

aterial Diameter (mm) Thickness (mm) Initial sh

used Quartz 40 30 Plano-BK7 40 30 Planoilicon wafer 〈1 0 0〉 100 0.200 Plano

0.165 71.70.210 81.00.170 150.0

was performed with the fine grinding spindle. Plano surfaces wereground using the aspherical spiral grinding mode as shown inFig. 4. The experiments were performed using the constant materialremoval mode, in which the rotational speed of the workpiece spin-dle and the feed rate were adjusted to obtain a constant materialremoval rate (MRR). The novel computer controlled pulse powersource developed was used to provide the power required forthe ELID process. A knowledge database was developed with theoptimized grinding and ELID parameters for various optical andnon-optical materials. The user interface allows the user to spec-ify the grinding job, for example, to select the work material, toset the grinding wheel specifications and the desired shape ofthe lens, etc. From the user specification, the search engine willfind the closest optimized grinding and ELID parameters and theexpected grinding ratio from the database [10]. The optimizedgrinding parameters and the expected grinding ratio are sent tothe machine controller. The optimized ELID parameters (peak cur-rent (Ip), voltage (Vp) and current duty ratio (Rc)) are sent tothe pulse power source developed. The output current (I′p) andthe voltage (V ′

p) are used to monitor and control the ELID pro-cess using the feed-back control system developed as shown inFig. 4.

Metal bonded grinding wheels were generally used for ELIDgrinding. Among the metal-bonded ELID grinding wheels, cast-ironcobalt bonded grinding wheels (FCX-I, Fuji Die, Japan) were cho-sen for the experiments due to the excellent non-linear electrolyticbehavior of the grinding wheels. Both optical and non-optical workmaterials were used in this study. The grinding wheel specifica-tions and the grinding parameters used in this study are listed inTables 1 and 2. The dimension and the initial surface quality of the

materials are listed in Table 4. The ground samples from the experi-ments were measured for both flatness and surface roughness. Theflatness of the ground samples was examined using an interferome-ter (Fisba � shape), and the roughness measurements were carriedout with a white light interferometer (Zygo NewView).

ape Final shape Av. surface roughness (ground using #325) (�m)

Plano 0.578Plano 0.508Plano 0.586

128 K. Fathima et al. / Precision Engineering 34 (2010) 124–132

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Fig. 5. Methodology used for con

.2. Wear measurements

Fig. 5 describes the methodology used for conventional on-achine wear measurement. The grinding machine is equippedith a measuring device, which is used to measure the center thick-

ess of the workpiece with an accuracy of 1 �m. The measuringevice is used to measure the actual center thickness of the blanknd the ground lens. The difference between the center thicknessess used to calculate the actual volume of material removed. Con-entional wheels of known grinding ratio (the ratio between theolume of material removal from the workpiece and the volumef material removal from the wheel) are used for grinding. Fromhe center thickness measurements and the known grinding ratio,he diameter reduction is calculated and then automatically com-ensated by the machine for the next cycle. However, firstly, thisethod is limited for commercially available wheels and commonly

sed optical materials because the grinding ratio of new harderaterials is not the same as of conventional materials. Secondly,

he conventional wear measurement method described above isot useful for in-process control of grinding.

The expected wheel wear calculation method for ELID grind-ng is described in Fig. 6. The expected grinding ratio has beenbtained from the knowledge database based on several inputarameters, such as, lens material, wheel bond material and abra-ive size of the wheel, and grinding and ELID parameters. Thectual volume of material removed from the workpiece was calcu-ated from the measurements obtained from the measuring device.he theoretical expected diameter diminishing may be calculatedrom the expected grinding ratio from the knowledge database andhe actual volume of material removed from the workpiece. Theredicted theoretical expected diameter diminishing may thus beompensated automatically for the next cycle.

. Results and discussion

Three important results will be presented and discussed in thistudy. The first shows the “response” of the ELID process while

Fig. 6. Methodology used for the knowled

onal wheel wear compensation.

grinding different materials. The second subdivision explains theadvantages of the knowledge based feed-back control system devel-oped, and the third subdivision discusses the advantages of usingthe system for larger workpiece diameters.

5.1. The response of the ELID processes

The term “wheel dressing” generally means to produce a newwheel-active surface (where the active grits are bonded) free fromloaded chips and worn out grits. In electrolytic dressing, pulsedpower is supplied in order to oxidize a certain amount of bond mate-rial to establish in-process dressing. However, the efficiency of theelectrolytic dressing depends on the selection of electrical param-eters such as voltage (V), peak current (Ip) and the pulse ON (Ton)and OFF (Toff) times [7–9]. Among those parameters, the dressingcurrent (current supplied during ELID grinding) is a useful measure-ment value in order to determine the condition of the oxidized layerpresent on the grinding wheel surface. The thickness of the oxidizedlayer directly influences the dressing current due to the non-linearelectrolytic behavior. The variations of the dressing current werenoticed during in-process dressing due to the thickness changes ofthe oxidized layer on the wheel surface [9]. For better grinding per-formance, a uniform thickness of the layer has to be maintained onthe wheel surface. However, the thickness of the layer present onthe wheel surface during grinding depends on the rate of formationand the wear rate. The rate of formation of the layer depends on thechosen ELID parameters such as current, voltage and duty ratio [7].However, the wear rate of the oxidized layer during ELID grinding isa cumulative effect of the grinding parameters, work material andthe properties of the oxidized layer [12]. The measured ELID dress-ing current indicates this cumulative effect in a certain way, whichis described as “response” of ELID in this paper.

This section shows the response of the ELID process for mate-

rials with different mechanical properties and applications. Fig. 7shows the schematic illustration of the methodology used for theexperiments. Identical ELID parameters (Ip, V, Ton and Toff), grind-ing parameters (cutting speed and MRR), PID controller parameters(P, I and D) and the set value for the dressing current were cho-

ge based wheel wear compensation.

K. Fathima et al. / Precision Engineering 34 (2010) 124–132 129

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Fig. 7. Setup for measu

en to grind different materials. An optical material (fused quartz)nd a non-optical material (silicon wafer) with different mechan-cal properties were chosen for the experiments. Quartz blanksf 40 mm diameter and a silicon wafer of orientation 〈1 0 0〉 withiameter 100 mm were used for this experiment. The parameterettings used for ELID in both experiments were Ip: 20 A, V: 90 V,on: 5 �s and Toff: 5 �s. The workpieces were ground using the con-tant material removal mode (explained in Section 4.1) with cuttingpeed of 21 m/s. The increase of the dressing current during grind-ng is proportional to the wear rate of the oxidized layer from the

heel surface, and hence the dressing current was used as the mea-urement value in order to characterize the influence of the workaterial properties on the ELID process.

Fig. 8 shows the controlled dressing current while grindinguartz and silicon wafer. When the grinding processes were tried toe controlled in the same manner with identical ELID and grindingnd PID parameters, significant deviations in the dressing current

ere noticed. The material removal from the quartz sample may beifferent from the silicon wafer sample for the grinding and ELIDarameters. However, it is difficult to know which of the propertiesf the materials (Table 3) influences the material removal duringLID grinding. The variation of the dressing current acts as an indi-

Fig. 8. The controlled dressing current durin

e response of the ELID.

cation of the controlled ELID process. The larger variations showthat the control method is not suitable for the quartz sample. Theresults of the surface measurements show that the parameters cho-sen for control were more suitable for grinding silicon wafer thanfor grinding quartz. As a result, different materials require a differ-ent set of parameters in order to optimize the ELID process. Thus,the optimization of the ELID grinding process can be expressed as

ELIDoptimized = f (Ep, Gp, Pw, Pm) for conventional (1)

ELIDoptimized = f (Ep, Gp, Pw, Pm, PIDp) for feed-back control (2)

where Ep: ELID parameters; Gp: grinding parameters; Pw: proper-ties of the bond/layer; Pm: properties of the work material; PIDp:PID controller parameters.

5.2. The advantages of the feed-back control system

The parameters and factors to be considered for optimization ofELID grinding have been discussed in the previous section. Optimiz-ing the ELID process with a feed-back control system also requirestuning of the PID controller for various materials as shown in Eq. (2),which may further complicate the optimization procedure. There-

g grinding of silicon wafer and quartz.

130 K. Fathima et al. / Precision Engineering 34 (2010) 124–132

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Fig. 9. The average current r

ore, it is necessary to know the effectiveness and the efficiency ofhe feed-back control system before implementation. A compara-ive study has been performed without and with application of theeed-back control system in order to evaluate the effectiveness ofhe system.

Experiments were conducted using conventional ELID grind-ng with optimized parameters (Ip, V, Ton and Toff) and usinghe knowledge based feed-back control system developed. The

ethodologies used for both processes are described in Section. Optical materials with different properties were chosen for thexperiments. The first test material was N-BK7 and the second

aterial was quartz. Both workpieces had a diameter of 40 mm and,thickness of 30 mm. Plano surfaces were ground on the samplessing the aspherical spiral mode as described in Section 4.1. Theesired flatness requirement on the samples was less than 1 �m

Fig. 10. The average current record

d during grinding of NBK-7.

and the surface finish was expected to not exceed a few nanome-ters. The glass blanks (NBK-7 and quartz) were examined after twopasses and the wheel wear compensations in between the passeswere calculated using the method described in Section 4.2.

Fig. 9 shows the average dressing current recorded during grind-ing of NBK-7 samples without and with feed-back control system.The average current recorded during grinding of quartz sampleswithout and with feed-back control system is shown in Fig. 10. Theground samples were measured for flatness and roughness afterthe process. The measured flatness and surface roughness resultsof the NBK-7 and quartz samples for uncontrolled and feed-back

controlled ELID are shown in Fig. 11. The results obtained fromthe NBK-7 samples were found to be in an acceptable tolerancefor both methods. However, application of the feed-back controlimproves both the geometrical accuracy and the surface finish. The

ed during grinding of quartz.

K. Fathima et al. / Precision Engineering 34 (2010) 124–132 131

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Fig. 11. Flatness and roughness results

atness and roughness results obtained from the quartz sampleshow that the application of the feed-back control system signif-cantly improves both the geometrical accuracy and the surfacenish. The surface profile obtained using the feed-back control sys-

em after the second pass was found to be in the acceptable rangeor profile correction, for example with magnetorheological finish-ng (MRF) [13], whereas the sample from the conventional processeeds further grinding. The results clearly evident that the appli-ation of the feed-back control system is inevitable while grindingaterials of high hardness as well as common optical materials.

.3. The feed-back control of ELID and the size of the workpiece

Apart from the hardness of the work material, the size of theorkpiece also plays an important role in achieving geometrical

ccuracy. The dimensional and the topographical changes of therinding wheel are expected to be greater while grinding largerized workpieces due to larger material removal per grinding pass.he wear compensation methods discussed in Section 4.2 are not

n-process control methods. They are used to correct the wheelear after every grinding pass. It is very important to control the

heel wear to a significant amount in order to minimize overall

eometrical inaccuracies of large workpieces. Though the wheelimensional and topographical changes during ELID grinding areependent also on the grinding time, an effective control of theast iron-bonded grinding wheels may produce better results. This

Fig. 12. The average current recorded

ut control and with feed-back control.

section examines the feasibility of reducing overall geometricalinaccuracies by the implementation of the feed-back control sys-tem.

Experiments were carried out using previously sawed andlapped silicon wafers of diameter 100 mm with orientation of 〈1 0 0〉and thickness 200 �m. Fine grinding of silicon wafers was carriedout using the uncontrolled and the feed-back controlled method. Acast-iron bonded grinding wheel of diameter 100 mm, 5 mm thick-ness and mesh size #4000 was used for grinding. The samples wereground using the aspherical spiral grinding mode as explained inSection 3. The flatness of the ground samples was examined aftersingle passes in order to evaluate the effectiveness of the feed-backcontrol system. The average dressing current recorded during thegrinding process is shown in Fig. 12. The flatness of the groundsamples was measured interferometrically. Fig. 13(a) and (b) showsthe flatness of the samples obtained using the uncontrolled andthe feed-back controlled method, respectively. The flatness of thewafer after a single pass of fine grinding was found to be 4.156 �mfor the uncontrolled method, whereas the flatness obtained usingthe feed-back control after a single pass was found to be 2.123 �m.The results show a significant difference in terms of flatness while

grinding with the feed-back controlled system.

This experimental study shows that the dimensional and topo-graphical deviations of the wheel may be minimized when thenon-linear electrolytical behavior of the wheels can be controlledin an efficient way. The number of corrective grinding cycles nec-

during grinding of silicon wafer.

132 K. Fathima et al. / Precision Engineering 34 (2010) 124–132

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[12] Fathima K, Rahman M, Senthil Kumar A, Lim HS. Modeling of ultra-precision

Fig. 13. The flatness of the ground wafers after a single grinding pass. (a) G

ssary to achieve the desired stringent specifications of a largeorkpiece may be minimized because the feed-back control sig-ificantly reduces the overall geometrical inaccuracies after eachass.

. Conclusions

From the feasibility studies conducted with the feed-back con-rol system developed for precision grinding of various optical andon-optical materials with different sizes, the following conclu-ions may be drawn:

1. The response of the ELID process is found to be different for dif-ferent work materials and hence the process has to be optimizedbased on the work material to be ground.

. The application of the feed-back control system reduces the geo-metrical inaccuracies and improves the surface finish especiallyon harder materials.

. The application of the feed-back control system developed isfound to be very efficient for reducing the overall geometricalinaccuracies while grinding large workpieces.

. The application of the knowledge based feed-back control sys-tem reduces the ambiguities experienced during ELID grinding,and increases the robustness of the process in a significant

amount.

. The feasibility studies conducted on the knowledge based feed-back control system developed clearly evident that the systemcan be successfully implemented in the field of precision opticalmanufacturing.

[

g with uncontrolled mode and (b) grinding with feed-back control mode.

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