 Hello and welcome back to this course on Microsystems Fabrication by Advanced Manufacturing Processes Lecture 18. So we will quickly recap what we tried to learn in last lecture. Here as you see we tried to plot the workpiece profile into the tool profile so that we can do it in a 2D and a 3D two-dimensional and three-dimensional basis. We saw that if we plot a curve on the with a certain fixed relationship parametric form between X and Y what would be the corresponding tool equation or vice versa provided you have a certain gap equilibrium gap and uniform field. We also said that if a surface is made up of many topologies of different regular surfaces connected together then it is a good idea to individually replicate the corresponding part of the tool piece surface based on whatever functional relationship exists into the part of the workpiece surface assuming the constant field criteria and then integrating the overall surface to have the whole complex profile where closed form expression of electric fields are available particularly for regular cases there is no problem as such the whole surface can be taken into picture. We also learned about electrolyte flow and the designing of these flows and in that context I would like to mention that the most important part in a ECM is to see if the tool is designed for the flow to cover all the zone of between the workpiece and the tool surface. There is some residual area which is left over from the flow field and there is a tendency of the machining to be uneven non-uniform and then the disinking process or basis of the ECM does not come into picture. We also looked at some properties which are desirable in electrolytes for example they should be as non-toxic as possible they should be able to allow charge transport in a smooth manner thus precipitating whatever emanates from the anode the workpiece side so that it can be carried away as a debris material then should be amenable to slightly higher temperatures and it should also be less viscous as less viscous as possible so that there are diffusion you know based flow of debris into the flowing fluid so on so forth and a variety of other properties like thermal stability, electrochemical stability etc should be analyzed when we talk about different properties. We also looked at some standard metals and the electrolytes which are used therein for the purpose of ECM and then we designed certain electrochemical machining plans looking into the very basic requirements of such ECM units the basic requirement is a rigid stage and a rigid workpiece tool holder which is important because the gaps which are maintained while in equilibrium of an ECM process is typically of the order of a few microns few tens of microns and then it necessitates the surface properties become more prominent it necessitates the viscous forces over the inertial forces so there is a dominance and therefore because of the viscous forces there are large pressures which are felt and the tool should be able to handle such pressures without getting warped or wobbled and therefore the stage needs to be very very rigid while feeding the tool. We also learnt about the various aspects of the electrochemical machining system as such for example you have to have a slurry tank you have to have a process of filtration where the primary debris which comes out can be removed selectively and then you know the slurry can be recirculated back into the system at a certain velocity there should also be a bleeding mechanism where whatever slurry is going in the corresponding amount of air which is closed in that enclosure tank like enclosure where ECM is carried out should be bleated out. We also should try to avoid as far as possible metals in contact with the electrolyte other than the workpiece zone so everything whatever is holding the electrolyte should be properly coated so that there is no electrochemical machining action on the same although the most amenable to such actions is the tool holder and so therefore proper coating of these tool holders are needed to prevent any stray machining to take place because of splashes etc as is the normal case with such electrolyte so wherever there is a field and wherever there is an electrolyte there is a possibility of machining which gets introduced. So today we will be looking into a slightly different aspect of ECM and the other corollary processes or associated processes like electro stream drilling, ESD or electrochemical grinding ECG and what are the basis what are what are the basic mechanisms associated with material removal in such processes of course the primary mechanism is electrochemical machining but there are certain other important aspects which are coupled to the ECM process in order to formulate these new processes which exist and for micro systems application these learning these nitty gritty of the associated processes of the electrochemical machining is very important from a standpoint of manufacture. So let's look at these processes and to begin with we start with just electrochemical drilling operation so the fundamentally this process is illustrated here so you have a tubular electrode which is used as a cathode in this case which is also the tool okay so this is the tool side the electrolyte is pumped from the center of the tool and exists throughout the side machining gaps so therefore the electrolyte is transported here in this axis of the tool and it emanates sidewise so it goes between the workpiece and the tool gap and the machining gap which is formulated here okay so it basically also gives the question of how much protection you should give in terms of shielding the electric field which is between this electrode here and the tool surface so you have to provide proper installation in the sides so that there is no stray machining effect which happens because of sides of the electrode now it's a drilling operation meaning thereby this hole that you want to drill is a through hole so definitely it's a high aspect ratio structure that we are trying to fabricate in a bulk micro machining sense across the thickness of the material the machining occurs at high current densities in the frontal interior electrode gap between the workpiece and the tool face this gap right here and of course as you know here because of this presence of the flow dynamics across the emanating surface of the tool of the electrolyte there is a ridge which is automatically formulated okay so because this is a sort of place where the fields may not be that homogeneous or uniform the fields may exist between for example these two sides okay so the electric fields and the current densities are highest here and the field reaching here because of these electrodes here may be smaller in value and therefore this region is depreciated of the overall electric field that it faces okay so there is a tendency of this ridge to develop and that's only because of this coaxial portion done in the tool for the electrolyte flow side electrochemical dissolution acts laterally between the side walls of the tool and the component somewhere in these regions here so the side gaps on both sides which exist is where the side electro dissolution would happen and let's say if you want to produce a certain whole diameter CD this overall machining dia which comes out would be definitely more than CD okay so if this is the machining dia the machining dia which comes out is greater than CD in this particular case because of side machining so the CD is calculated by the following manner so if supposing DW is the workpiece dia which eventually gets formulated and DT is the tool dia so definitely this CD or the side machining gap happens because of the difference between DW and DT so DW minus DT is CD so electrochemical drilling produces diameters ranging from about 1 to 20 millimeter and using feed rates from 1 to 5 millimeter per minute typically that is what the range is and for high machining accuracy and smaller diametral oversize high feed rates are recommended so under such condition high removal rates and better surface quality are also ensured so the method in which you feed the electrolyte sometimes influences the overcut as such and the overcut is basically the amount of CD which is basically the difference between the workpiece dia which gets finally formulated and the tool dia that you are using okay so for example in this regard the reverse electrolyte flow mode under back pressure reduces the overcut so if instead of making the electrolyte flow in this direction you are trying to flow the electrolyte in the reverse direction entering from the side and going towards the axial center will definitely have a smaller CD or a smaller overcut of the hole that you want to produce the only thing you have to ensure that there has to be a back pressure of about 0.62 megapascal for this so what it does is that its procedure flushes away the gaseous products of electrolysis for machining gap without reaching the side machining zone and therefore whatever conductivity changes locally would happen because of the dissolved gases those changes will not happen and so the machining will be more precise because of the back pressure it ensures that the gas kind of oozes out of the whole electro electrolyte tank as such so that is one aspect of it electrochemical drilling now one thing which is important here to mention is that the increase in the gap pressure as you have seen before between the workpiece and the tool so this gap raises the electrolyte conductivity and it enhances the dissolution process due to increase in machining current naturally because the gases are no longer getting trapped here because there is a back pressure and the gas gets diffused away from the electrolytic system and you are assuming that you are flowing the electrolyte under the back pressure from the side to the center so generally what people have found is that this electrolyte back pressure would reduce the flow lines which otherwise happens on a machine surface for example you can have a look at the surfaces here which are products of the electrochemical drilling and you can see that there are varied level of surface roughnesses which are achieved because of different current values that you are using so if the value of current is 44 amps for example the roughness is 0.41 microns if it is 105 amps the roughness is about 0.29 microns and if it's 130 amps it is about 0.175 microns so on so forth so as you are seeing here that if the current density is more or the current per unit area is more that you are pumping in the roughness is overall reducing from 0.41 microns to about 0.175 microns so one aspect is of course electrolyte back pressure of reduction on the roughness as I have already told the other aspect is the amount of current that is being used for the purpose of the electrochemical machining and one again disadvantage a major disadvantage of such a system is that beside the tooling cost the overall system dynamics if the hydraulic forces are more would go up okay so the cost of the system also for designing a electrochemical system which can handle increased hydraulic forces increased pressures would be very high remember that rigidity aspect of the tool holder etc which would discuss while designing the electrochemical machining process of course you need to use or properly insulate the sides of the drill the electrochemical machining has the same parameters as any other ECM process so if side insulation is not happening then there is a huge amount of CD which is created because of additional electrochemical transport between the side and the tool face itself and therefore we can say that use of proper tool insulation would reduce side machining effect and this in turn would limit the widening of the side gap so you can have more accurate processes if insulation is proper so one important aspect which we have actually studied also while doing this theorizing of electrochemical machining and iron transport as we have seen before is that there are certain passivating agents which you would use from time to time in electro electrolytic solutions so that you can get a better surface finish for example it has been seen that if NaNO3 sodium nitrate solution is used along with salt water there is an action which creates a smoothening a self-smoothening effect of the surfaces that are produced and also it can remove process inaccuracies so you can have exact size of the CD the over cut very small amount of over cut and all those processes related to the accuracy of the tool and the reason why that is so is that the passivating electrolyte is creating generally an iron atmosphere which shields which is which is a which is maybe a non participating iron in the whole process okay and NaNO3 for example is a non participating iron when the electrolyte is NaCl and water normally salt water which is used for the machining of iron work pieces okay FE so if it is non participating the goal that that that particular solution passivating solution would have is it would produce a high density field as such where the iron of interest that is being machined would easily get emanated out and there may not be a problem about getting influenced by the the fields that are emanated from the work piece or the tool as such okay so you are creating a situation which is free for the iron of interest to move freely that means it comes out into the solution does the precipitation everything free of the fields that are imposed to this iron by either the tool or the work piece so that kind of a goal is being achieved by a passivating electrolyte and the purpose of it is also to create a large background field in the solution itself so that the effects of the tool and the work piece gets minimized that way so as we know that as we have seen that electrolyte flow definitely in this particular case in drilling particularly has a huge impact on the process over cut okay so if the flow rates are slightly higher the velocity is higher and the over cut may be more if the velocity is an optimum best the over cut may be lesser so on so forth so there are certain other aspects of this machining for example the machining current increases proportionately to the tool feed rate if the tool approaches the work piece at a higher speed and the machining current would be more because you can obviously think of it because the amount of field which is created is also inversely proportional to the tool electrode gap and as this gap is reducing in a faster manner thereby meaning the tool is approaching faster to the work piece the the amount of increase in the electric field because of that approach also is more and subsequently field and current density are connected by an equation j equals field times of k where k is the conductivity if the conductivity is assumed to be the same in that situation we can say that the current density or ion transport would be more if the field is more okay so the rate of approach with the reduction of distance between the work piece and the tool thereby increasing the electric field ensures greater current densities which would then machine at a higher machining rate or material removal rate so there are certain ill effects which take place particularly in electrochemical drilling process for example one such undesirable effect is sparking and sparking typically takes place when the tool advance rate towards the work pieces greater than the anodic resolution rate so you already know from the electrochemical machining theory that if the tool is approaching the work piece there is one aspect of the tool getting dissolved so that the surface of the work piece getting dissolved that the surface of the work piece recedes away from the tool and then there is other aspect of the tool getting fed in and the overall equilibrium happens when the amount of the rate of at which the work piece surface is dissolving is the linear rate at least is same as the feed rate that's how you calculate so if there is a difference as such because of a fastly approaching tool or a rapidly approaching tool if there is a difference in this rate of dissolution then there would be a tendency of the two surfaces to touch each other and even when it comes to just about touching there is this sparking phenomena which is essentially a discharge an ionic discharge which would happen because of the very small hills and valleys produced on both the surfaces that is the tool and the work piece surfaces and they getting in very very close proximity to each other so the field is so high that it boils off the electrons from these small hills and valleys and that results in a sparking so you have a tendency of sparking particularly at a higher advance rate tool advance rate where this rate is much much more than the dissolution rate of the work piece and we should not go towards a higher tool advance rate by the by because the process is disturbing and there is a huge amount of impact on the power source because of this sparking and we should by and large avoid that in case of electrochemical machine so it can cause damage particularly to the tool and the work piece so of whatever has been experimentally determined there exists in this empirical relationship between the diametral oversize CD which is actually equal to this work piece diameter minus tool diameter we have just seen it in the last example the last schematic and the gap voltage between the frontal end of the tool and the work piece surface as such this is V particularly for a particular feed rate let us say if A is the feed rate in that case empirically it has been determined by experimental observation that the CD what happens is proportional to the power of voltage to the power 0.74 minus 0.056 a so this a is basically in millimeters per minute so that's that's about what we have for electrochemical drilling process the next example that I would like to consider is another corollary or an extension of the electrochemical machining and it is called ECG or electrochemical grinding and let me just recall a little bit about what conventional grinding typically does ok so conventional grinding is a very fine machining process which in general produces a superior surface finish of the components and the conventional grinding is typically used as a finishing operation on most of the components that are sent for grinding because their dimensional tolerance as such is low but the dimensional tolerance gets significantly affected with associated problems like burrs or comparatively large heat affected zone or thermal residual stresses which would create a sort of breakdown of the surface structure right because grinding is a multiple cutting tool with a lot of abrasive particles which are embedded on to a matrix which rotates with respect to a workpiece and there is a heavy amount of deformation on the surface plastic deformation on the surface which happens because of the small abrasive grains continually hitting the surface at a very small depth of cut ok so in fact the grinding is an operation where you get mostly splinters coming out which means that the metal which is emanating is so small that it gets oxidized and burnt away as it goes out of the surface so therefore in a normal mechanical grinding as such you can say that although it is a high tolerance high you know super finish so basically lower tolerances can be achieved using that grinding but then this dimension dimensional tolerances are significantly associated with these different problems burrs heat affected zone of the workpiece or residual stresses which is there so the surface gets modified because of these problems now in order to take care of such problems in a conventional grinding people have switched on to electrochemical grinding or ECG which is actually a process where at least a part of the material removal which takes place is by electrochemical dissolution and as such the surface finish would be much more in case of ECG than in case of a conventional grinding system mechanical grinding system so we can say that electrochemical grinding process has mostly these defects taken care of and you can get anodic work pieces which are mostly free of burst because of the ECG process so during electrochemical grinding the material is removed by mostly as I told you electrochemical dissolution which is about 90 percent of the overall material and then a little bit by mechanical abrasive action which is about only 10 percent of the the material removal so one has to design the tool system in a manner so that the dissolution is at such a rate that it removes 90 percent of the material work material at a very high rate and by the time some abrasive action mechanical abrasion action starts taking place already a lot of material has been dissolved away so that's how you have to design the ECG or the electrochemical grinding process so one very important and significant aspect of ECG that is that the work piece and the grinding tool both have to be conductive in nature because it's a it's an electron flow process okay so you cannot have one of the components non conductive okay or insulating in nature both of them have to be essentially metallic as far as the surface is going therefore the ECG tools so created are having this component ensured that they have overall a high conductivity on the surface not all grinding tools are conductive in nature but at least the electrochemical grinding tool is designed in a manner so that it has high conductivity on the surface so the electrolyte is circulated in a ECG process and there should have there have to have so it has to have an effective recirculation system which includes a flow supply of the electrolyte and also a flow filtration system where whatever residue of the process comes out as dissolved electrolyte and the precipitate which is coming out for precipitate of machining that has to be somehow taken away from the electrolyte for recirculating it back into the process and we have seen that this sodium chloride and a passivating agent sodium nitrate NaNO3 is typically used most of the time for ECM processes so we can say that in this ECG process also a similar composition can be given as I already illustrated earlier the role of a passivating electrolyte is to create a high field so that as such the ions which come out don't get influenced by the work piece in the tool surface so if we look at the basic process mechanics in electrochemical grinding operation it is represented in the schematic here so you have a grinding wheel which is almost similar to a conventional grinding system and here as I already illustrated the bonding material is not any epoxy or any non-conducting system it has to be essentially a electrically conductive binding material to make the ECG wheel so the electrolyte is supplied through the IEG or the inter electrode gap here in this particular case this gap right here is the IEG or the inter electrode gap and that happens between the wheel and the work piece so the work the gap here is basically between this wheel ECG wheel and the work piece so one more aspect is the height of these abrasive particles which are bonded onto the metallic wheel the height sticks out to just about an extent where it is equal to the inter electrode gap okay so for one instance of operation there is a cell which is formulated between let's say the two abrasive grains which are there on the surface the work piece and the matrix the conducting matrix binding the grains on the wheel side so there is a small electrochemical cell a local electrochemical cell which is created in this manner between the ECG wheel and the work piece so you have to ensure that the height of the abrasive particles protruding outside the bonding material would be kind of all similar in shape or constant in size and therefore it has to help in maintaining this inter electrode gap because of the abrasive particles which act as spacers in turn and then it tries to create a compartment holding an electrolyte for high rate of dissolution on a local area between the wheel and the surface in question so some facts and figures about this ECG process a conventional you know grinding wheel if you look at the lifetime and then you compare it with the ECG wheel it's about 10 times more if you consider electrochemical grinding wheel so the surface the overall effective lifetime of such a ECG wheel is much more than a conventional grinding wheel here because you're not doing any mechanical abrasive action in fact in conventional grinding you have to do an operation of dressing where after every grinding process you have to redress the tool in order to start up an issue and one of the reasons is that most mechanical abrasions result in clogging of the gap between the abrasive particles on the surface of the wheel in this particular case that's not the case because you're also carrying away the precipitate which is coming out of the material in the electrochemical machining aspect of the ECG and the tendency or the chances of the the pockets to get redeposited with the debris is miniscule because you have designed the electrolyte in a manner in that manner so that whatever comes out goes as a precipitate and not as a deposit so therefore ECG wheels automatically are about 10 times more in terms of their life than the conventional grinding system so the effective area in which machining is taking place can be divided into three zones and you can see them marked here as zone one two and three respectively okay let's see individually what happens in all these three different zones in the ECG process so as I already illustrated before the electrochemical grinding is a combination of electrochemical dissolution as well as mechanical abrasion or removal of material mechanically there is a 90-10 combination so 90 percent of the material gets removed by ECM process and 10 percent gets removed by mechanical abrasion process so there has to be some parity between these zones so called zone one two three and these different aspects of the ECM and we will just see that zone by zone so in zone one the material removal is purely due to electrochemical dissolution because you can see here that the abrasives are hardly in touch or hardly in contact with the workpiece okay so there is no mechanical abrasion as such which happens in this particular zone okay zone one so it is only whatever iron transport is taking place between the workpiece surface as such this surface okay and the tool surface which is a conducting matrix with particles abrasive particles so rotation of this ECG will of course helps in drawing the electrolyte into the IEG so it is a self-priming process so even if electrolyte is in the near vicinity and you are moving the grinding heel in a certain direction it is obvious to assume that whatever small fragments are coming out or sticking out as abrasive particles on the wheel they would pedal the electrolyte so so it's a self-paddling action which would happen of the electrolyte which is around here and it gets pushed into the IEG or the inter electrode gap so as a result of this whatever machining you are having in zone one and whatever reactions electrochemical reactions in zone one the products which are coming out of this zone as such start contaminating the electrolyte okay so in zone 2 and 3 whatever is the available conductivity of the electrolyte which is available here this conductivity is decreased because of whatever by products like debris and hydrogen gas is packed in zone one because of electrochemical dissolution so in fact there is a counter effect so if the sludge is present or the debris is present the conductivity goes up and if gas products are present or gaseous products of the reactions are present the conductivity goes down so one is exactly counter to the other so if sludge is more than the conductivity is higher if gases are more the conductivity is lower so the net result is that there is a overall decrease in the value of conductivity of the electrolyte because gases are emanated and dissolved into the electrolyte system much faster in comparison to the dissolution of the debris which is solid and which is limited by diffusion kinetics and therefore it yields a lower value of equilibrium gap equilibrium gap as such if you may decipher from your previous analysis on ECM is really determined by this lambda by f okay so that is what the equilibrium gap is and lambda is dependent on the conductivity so if conductivity is low then the equilibrium gap also automatically falls down okay so the i gene decreases because of a decrease in conductivity so now although the conductivity is lower and there is a certain smaller dissolution rate in zone 2 but as you can see here that between 1 and 2 there is a automatic transition which happens right and the particles here start to be in direct contact with the workpiece so the transition between 1 and 2 zones brings in the additional contact of the abrasive particles with the workpiece so there is certain 1 to 1 contact between the particles on the wheel and the workpiece surface which happens on the transition between zone 1 and 2 and therefore a small part of the material that is removed is in forms of chips as well the moment it enters into zone 2 as you can very well illus see here by this blown up schematic or portion of this 2 here because of the direct contact of the the abrasive particles with respect to the workpiece there is a formation of the small localized electrochemical cells and these cells are formulated throughout the width of the grinding wheel because there are many such particles which are there and in fact we are talking about a very thin film of electrolyte and this thin film is more amenable to surface effects because the gap is very small and it prefers to stick to the surface because forces of adhesion are more to the surface than the forces of cohesion it is a very very small thin film ok so it basically tries to contain itself in that small gap along with the tool and the workpiece and formulates a small electrochemical cell so there are several such cells across the width of the grinding wheel between the workpiece and the grinding surfaces which are formulated in this machining zone 2 so therefore in zone 2 the electrolyte is being forced into the equilibrium gap by the rotational motion of the wheel and therefore because of these formations of electrochemical cell etc the local electrolyte pressure increases in this part of the IEG so the pressure is very high and of course it suppresses the formation of gas bubbles and that results in yielding a higher material removal rate. So whatever gas bubbles are formulated here it gets suppressed and it was already the pressure ambient pressure is very high and therefore slightly higher MRR is desirable in this particular zone because you are suddenly squeezing the electrolyte into a very small area region thus increasing the pressure likelihood of increasing of pressure so whatever gas has been dissolved from the previous zone sort of bubbles out of the system so whatever chemical or electrochemical reactions which is the principal removal mechanism in zone 2 here takes place between the work and the tool it will result in the formation of a passive layer particularly on the workpiece surface ok. So here in this particular surface there may be a passive layer which is a deposit because it is already very small and then there are diffusion limitations and the debris which has been generated here goes into the cell and comes back once you know the wheel rotates and the next cell comes in place so there is always a formation of a passive layer because of debris re-deposition in this particular zone 2 of course because the abrasive grains are also in contact so when this grain moves with the motion of the wheel in a direction opposite to that of the feed of the workpiece there is a tendency of scratching the workpiece particularly the surface layer which has been formulated. So whatever reactive oxides sorry non-reactive oxides are formulated here because of maybe deposit of debris or deposit of some electrochemical products from this flow cell they are being scratched. So there is a removal of that layer ok the passive layer formulated here or the passive layer because of any other electrochemical deposition as such which is taking place in this region. So at the end of the day when this zone 2 is crossed the surface is kind of brought back to fresh because there is already a scratched mechanical action initiated by these abrasive grains towards the end of zone 2 and there is a general removal of the non-reactive oxide layer. Finally the zone 3 comes into picture where there is mostly electrochemical removal because there is a field which exists there is no direct contact of course between the grains sticking out of the wheel and the surface but then because of the ambient field and the presence of the electrolyte which itself has been thrown by the grinding wheel forward there is always a tendency of the material to remove in a electrochemical dissoluting mode. So it starts at the point where the wheel lifts from the work surface point here maybe right where the there is a separation between the grain grinding grain and the surface and suddenly there is a pressure release also which happens in this particular zone because earlier the electrolyte was being trapped as a flow cell between the work piece and these two grains and the wheel on the other side and suddenly the electrolyte has been thrown into the open. So therefore this zone sort of contributes the removal of those scratches or bars which have been formulated because of the mechanical abrasion in zone 2. So those bars are eliminated in zone 3. So that is about the basic process of electrochemical grinding there are certain other aspects associated with the machine tool as such it has to be a rotor driven tool as any other normal grinding process because there has to be relative rotation between the electrochemical wheel and the work piece surface. So this slide here illustrates briefly how the electrochemical grinding machine tool would look like. So you have a work piece here for example and then you have a grinding spindle and this is the grinding wheel and there is a flow of electrolyte past the grinding wheel. So you are flowing the electrolyte very close to this wheel here and the wheel starts working on the work piece surface by positioning relatively with respect to a high precise work table and whatever material comes out as debris or gases in the electrolyte is stored here in the tank which is again reused by a set of filtration techniques and pumping techniques and there is of course a power supplied between the work piece and the grinding tool to ensure the electrochemical transport. So by and large the following things need to be taken care of for designing such a system. The first thing is that material the machining area should be made as non-corrosive as possible because it is in direct touch with the electrolyte. So things like let us say the work piece holder or even the tool holder here right here has to be as non-corrosive as possible. Power must be supplied through the spindle either with the help of brushes or mercury coupling. So there has to be a continuous power. So this right here probably is the arrangement for the brush. So there is always tendency of the brushes to go past the rotating spindle so that there is continuous supply ensured on the wheel. From the power supply side there should not be any break and if you have mercury in between which sort of is a metal which is of you know it has a tendency to cohes more than adhere. So it basically formulates a thick layer between the outer which is the stator and the rotor which is the inner spindle. So there is a film of mercury which is continuously able to give a pathway to the flow of electrons and mercury coupling is always better than the brush contacts because brushes may get damaged or depreciated with time but mercury going to its cohesiveness can get reformulated as a continuous thin film between the stator and the rotor. So people prefer normally mercury coupling in comparison to the brushes although it is expensive and of course one thing you have to ensure is that the probability of short circuiting during ECG should not occur in the remaining part of the circuit because the ECG as such is a process which by virtue of the way it has been defined is very you know very less probable to the short circuiting. Because the it is a self material removal process as such. So after the dissolution has taken place after the debris has get gotten deposited after there is a formation of a non reactive oxide layer there is always a tendency of the material to get freshly exposed. So therefore there is a tendency of this ECG to not really have any short circuit because short circuits typically formulate when there is a chance of the metal of the metal binding of the grinding wheel coming in close operation to the surface here you are removing the surface there is mechanical abrasion which is following always the electrochemical deposition. So there is very less tendency of the ECG process as such to deposit a residue which may somehow interconnect the workpiece to the the matrix of the abrasive wheel and short circuit is by enlarge self avoided in the ECG process. So therefore you have to be extra careful about designing circuitry. So there should not be any issues about short circuiting of the circuitry as such particularly when you are talking about the grinding wheel getting in close proximity to the workpiece holder you should avoid any such systems which would provide any shorting between the two. So four kinds of different ECG performed operations can be performed they can be cylindrical grinding can be form grinding or can be surface grinding and internal grinding and variety of component shapes complex shapes can result from these four processes out of which the cylindrical grinding probably is the slowest process and that is because there is always a limited area of contact between the tool surface and the machine surface as such. So there is always almost a line of contact cylindrical grinding is done when there is a grinding roller which is moving with a straight line contact with another workpiece. So the zone of machining is very small and therefore there is a tendency of the dissolution rate to also fall down because of a line contact or probably at the most a very small surface area and contact during the contact between the tool and the workpiece. So therefore the ECG is slowest when you talk about cylindrical grinding and among all the processes the surface grinding happens to be the fastest process because the whole surface is in contact as such. So that is some information about the ECG machine. Of course you know you can further add some additional actions to improve the efficiency of the ECG process and one of them is that the wear can be controlled of the grinding wheel by oscillating motion provided on to the workpiece. So if you have the workpiece moving from one side to another and you are having the grinding action by rotation motion may be the clockwise manner or even the anticlockwise manner it is always a high efficiency system. So you can oscillate so that there is always a tendency of the work zone to come back and forth between this grinding wheel and that way by the frequency of the oscillation as such the material or the material surface can be smoother based on and also the wheel wear rate can also be reduced because of that because in one motion you are typically going in the direction of the feed and in the other motion you are going away from the direction of the feed. So the abrasive action of one cycle is kind of a dressing action for the other cycle. So it is a self dressing mechanism which happens. So the EC surface grinding typically necessitates the workpiece to reciprocate with respect to the tool. For example this is a tool which is shown where you can see this stage here which is containing the workpiece and it has a rectilinear motion in both the x and minus x direction as the tool moves in clock or anticlockwise direction. So in the EC internal grinding and form grinding the basic concept is same as the conventional internal and form grinding operations. The only thing is that you are using a conducting ECG wheel and electrolyte because the material removal mechanism has connotations which have been already discussed. There are some other associated benefits of the ECG process one of them is that as I told you the ECG wheel is made up of metals. So instead of a resinoid you know combination with particles you are actually having a metal wheel combining the metal wheel or making the metal of the wheel as the resinoid and the abrasive grains sticking out of that metal wheel. So therefore there is a tendency of those wheels to have long lasting effects because resinoid otherwise can needs a frequent amount of dressing which this process will not. So the ECG wheel again commonly uses materials that are copper, brass, nickel or copper impregnated resins for the purpose of you know making the grinding wheel and such metal bonded wheels are effectively dressed using the electrochemical process. So there does exist a dressing option here but it has to be very less frequently used and the only thing which is needed for dressing is that because in the ECG process you have electrochemical dissolution. So you have to just reverse the dissolution so that the dissolution happens from the tool side rather than the workpiece side. So if it is a copper impregnated resin then you make sure that there is the electrolyte which goes in dissolves the copper with respect to a workpiece which displaces copper from its metal state into the solution. So it is the reverse of the ECM process as such. So it is the machining on the tool that you are talking about but the dressing has to be in electrochemical sense which is done in this particular operation. So the commonly used abrasive that is used is alumina grid size about 60 to 80 and as we have already indicated the ECG does not need frequent wheel dressing and because already there is some mechanical action followed by electrochemical dissolution action which takes away the debris or carries away the debris. So I think we are probably at the end of this lecture and we have completed two processes which are associated processes the electrochemical drilling and electrochemical grinding. We would also like to explore a little bit of electro stream drilling which is important for high aspect ratio, hole formation, etc. and members of structures following which we will give some practical applications from the industry where ECM is used for micro manufacturing or micro systems fabrication. Thank you.