Literature review Chapter Literature review In-process dressing of grinding wheel has been under research interest for several years A lot of studies were carried out by many researchers on this subject area ELID is one of the methods of in-process dressing In order to understand its behavior and the field of applications a lot of research works were done previously In this chapter comprehensive discussion will be given on different in-process dressing as well as on ELID grinding In the first section of this chapter various methods of in-process dressing are discussed extensively Previous reports and publications related to ELID grinding and its application are included in the second section of this chapter Furthermore, publications related to grinding wheel monitoring, wheel wear detection and dressing power control are also presented Finally some concluding remarks on the literature study are discussed briefly 2.1 Different dressing/truing techniques Several methods of in-process dressing of grinding wheels are available as mentioned in the previous chapter Researchers are trying to explore those technologies to investigate their feasibility In the following subsections literatures on all the major in-process dressing techniques are discussed in details 12 Literature review 2.1.1 Study on mechanical contact dressing Albert J Shih [3] studied rotary diamond truing and dressing technique for vitreous bond wheels for ceramic grinding The author investigated the effects of truing parameters like speed ratio (truing disk surface speed over wheel disk surface speed), feed, overlap ratio (width of contact over truing disk lead), etc on tangential truing force, roundness error of the wheel, surface roughness of the work-piece It was found that a wide range of surface finish, roundness can be generated by varying the truing parameters, which is advantageous because grinding result can be tailored by adjusting truing parameters Nakagawa et al [4] used stationary dressing stick for in-process dressing of the wheel in creep feed grinding It was found that normal grinding force is much smaller when inprocess dressing was used The stock removal rate was also found to be higher when used this method of in-process dressing I Inasakai [5] examined the dressing force for resinoid bonded grinding wheels by means of dynamometer His experimental study suggests that monitoring of the dressing force helps to decide on the time needed for dressing, which will reduce the chances of excessive dressing of the grinding wheel A.A Torrance et al [6] developed a model for dressing of grinding wheel by diamond blade or a single point diamond tool The model successfully predicts the topography of the grinding wheel Grinding forces in surface grinding and the surface roughness of the 13 Literature review ground workpiece can also be predicted with good accuracy by this model in combination with abrasive wear model 2.1.2 Electro-thermal method of dressing/truing K Suzuki et al [7] and T Nakagawa et al [8] developed and studied EDM and WEDM methods for in-process dressing of the metal bonded grinding wheel It was found that WEDM method produces less electrode wear thus to provide more precise truing and dressing of the grinding wheel whereas die sinking EDM causes more electrode wear Due to the high wear of the block electrode in die sinking case, the electrode has to be machined periodically to maintain the form accuracy Later this group developed a grinding center equipped with their on-machine wire electro-discharge truing/dressing unit for metal bonded diamond grinding wheel They have found it to be very much efficient for different ceramic parts grinding Parametric study on EDM dressing was carried out by Xiankui Wang et al [9] They found that truing efficiency depends not only on the electrical parameters of the EDM power but also on wheel speed, wheel eccentricity etc The truing efficiency reduces drastically with the wheel speed after certain threshold level The eccentricity of the wheel plays a positive role on the truing efficiency; the efficiency increases as the eccentricity of the wheel increases Yan Wang et al [10] proposed dry EDM method of dressing and truing of the grinding wheel, which helps to reduce the chances of electrolytic corrosion caused by electrolytic 14 Literature review current flowing through water encountered in conventional EDM dressing/truing They compared experimentally the dressing efficiency between the proposed method and the conventional single point mechanical dressing It was found that dry EDM method of wheel dressing is advantageous over the conventional dressing/truing method for metal bonded grinding wheels Another experimental study for optimization of EDM dressing/truing of metal bonded CBN grinding wheel was carried out by N Ortega et al [11] It was found that the larger electrode size gives better stability for truing and dressing process The study also shows an asymptotic trend in grit protrusion as the operating time of the dressing process increases It was also found in the study that the electro discharged wheel produces much reduced grinding force as compared to the fresh new grinding wheel Brian K Rhoney et al [12] studied WEDM (wire electro discharge machining) truing/ dressing of metal bonded diamond wheel for silicon nitride machining The study shows WEDM trued/dressed grinding wheel produces significantly (20% to 40%) less grinding force than the single point diamond trued wheel This study also concludes that tool life for the WEDM trued wheel is longer J A Sanchez [13] developed a new method of EDM based wheel truing method where the electrode is single point and the movement of the electrode is programmed in such a way that the thickness of the material removed in each dressing pass are constant This 15 Literature review new method removes the manufacturing cost of complex shaped electrode as proposed by the other researchers and it can produce virtually any shape of wheel profile 2.1.3 Laser technology for dressing/truing X-Z Xie et al [14] used Nd: YAG laser to dress the resin bonded super abrasive wheel The research focused on studying the variation of ablation crater depth by varying pulse power density It was observed that pulse power density directly influences ablation crater depth They also suggested method of selecting feasible laser parameter for which a favorable surface topography can be obtained It was concluded in the paper that acousto-optic Q-switched Nd:YAG pulse laser is more suitable for dressing resin-bonded superabrasive grinding wheels than corundum block and CW laser dressing on surface topography M J Jackson et al [15] studied the effect of high power laser dressing for vitrified grinding wheels High power laser was also used to remove the clogged chips from the surface of the grinding wheel The change in the properties of the bonding between grinding grains after laser dressing allows the grains to adhere to the surface just long enough so that, after the grits become dull, they may get detached from the surface of the grinding wheel and expose new sharp particles below them thus introducing self sharpening effect in the grinding wheel It was observed in the study that laser dressed grinding wheel performance is equivalent to that of conventionally cleaned and dressed grinding wheel 16 Literature review 2.1.4 Electrochemical method of dressing One of the ways of doing electrochemical dressing of the grinding wheel is the ELID grinding This method of dressing has several important advantages over other dressing methods as described in the table 1.1 of the chapter and this technology is the leading the research area of in-process dressing of super abrasive grinding wheels Therefore its application and other research works on this area will be discussed in the next section separately In this section another method of Electrochemical dressing will be discussed, which is known as Electrochemical in-process controlled dressing D.Kramer et al [16] introduced this method of in-process dressing The principle of operation of this technique is quite similar to that of ELID grinding The main difference is that, this method does not produce any insulating layer during the electrolytic dressing of the wheel In this process the metal bond is just dissolved and thus the new sharp grains protrude out Another interesting feature of this technology is that it uses force feed back to control the dressing current The authors also discussed the performance evaluation of the process It was found that the average protrusion of the grit was around 100-120% of the grit diameter The control of the current flow causes the wheel to be worn out less and also the technique eliminates the non-productive dressing time 2.1.5 Miscellaneous methods Some studies propose other methods of grinding wheel dressing/truing which cannot be categorized directly under any of the above mentioned methods C.Zhang et al [17] 17 Literature review introduced a combined method of vitrified wheel dressing where laser is used as an assisting medium and actual dressing is carried out by the diamond dresser The laser power softens or even sometime melts the vitrified bonding material thus to facilitate the removal of the bond material by the diamond dresser It was found that truing efficiency is much higher and wear of the dresser is much less in this case compare to the conventional mechanical contact dressing method The truing/dressing force is also much reduced in the case of laser assisted dressing In their next study [18] experimental investigation on the wear rate of the dresser was studied, which was found to be much smaller compared to the mechanical dressing The wear nature in this case is progressive (no trace of fracture wear was found on the worn out dresser) in nature They also evaluated the depending factors of the wear ratio (namely laser power, depth of cut and feed rate) of the dresser for laser assisted diamond dressing Mingxia et al [19] developed a vibration assisted abrasive belt truing/dressing method for super abrasive wheel The vibration medium helps to improve the truing and dressing efficiency Super abrasive wheel dressed by this method produces better ground surface than if dressed by other technology (such as Electro discharge method, mechanical contact method.) ECDM (Electro Chemical Discharge Machining) proposed by M Schopf et al [20] is another method of dressing super abrasive wheels It is a hybrid process combining both electrochemical dressing [16] and EDM truing for the grinding wheels The truing efficiency of the grinding wheel is higher in this case when compared to conventional 18 Literature review truing method The higher protrusion of the sharp grits result in reduced grinding force Furthermore this truing and dressing method produces better surface roughness on the ground cermets workpieces than conventional mechanically dressed/trued grinding wheels W.K Chen et al [21] proposed the loose abrasive dressing/truing method for resin bond ultrafine diamond cup wheels for grinding spherical end faces of fibre connectors The truing using a cast iron profiled plate with diamond pastes as lapping media produces a good wheel profile with little wheel damage the loose abrasive dressing was found to be able to produce a much better grit protrusion This method is more suitable for the wheels with finer grit sizes This method of wheel dressing also improves the grinding performance significantly Ohmori et al [22] also proposed a hybrid method of dressing and truing of ultra fine grinding wheels The process combines electrolytic in-process dressing (ELID) and electro-discharge truing (ED truing) in one setup It was found that after using this hybrid technology the surface quality, precision of the workpiece and the process stability improved a lot when compared to the conventional grinding without ED truing and ELID 2.2 ELID Grinding The best known method for super abrasive grinding wheel dressing is electrolytic inprocess dressing (ELID) grinding which was first proposed by Murata et al [23] and later established by Ohmori et al [24] back in 1990 The important feature of this ELID 19 Literature review grinding is that it can be adapted to any existing CNC machine with a very little significant modification The basic principle of ELID grinding has already been discussed in the first chapter of this report In this section study on different types of ELID grinding, research on fundamental investigations on ELID, and different experimental and development works on ELID grinding have been discussed in details 2.2.1 Preliminary studies on ELID grinding & its classifications Murata et al [23] are the pioneers of ELID grinding as mentioned earlier They proposed a current controllable power supply for electrolytic in-process dressing of the metal bonded grinding wheel in order to achieve efficient grinding condition, back in 1985 In their study, the grinding condition was constantly monitored to adjust dressing power by introducing a simple ON-OFF controller Murata et al observed that the grinding stability improved significantly when controlled electrolytic in-process dressing is applied In 1990, Ohmori et al [24] popularized ELID grinding by applying it for grinding of silicon wafers, however this time monitoring of the grinding condition to adjust the ON-OFF cycle of the ELID power supply was ignored unlike Murata et al [23] work In their study Ohmori et al investigated the pre dressing current profile and explained the phenomenon of decreasing trend of the current with dressing time During grinding the grinding force was found to be very stable and continuous because of the continuous protrusion of the sharp grits They also investigated the effect of feed speed on the surface roughness of the ground sample The effect is less significant in the case of ELID grinding, and mirror surface can be achieved at higher feed speed as well 20 Literature review Researchers proposed three major classifications of ELID grinding, based on the materials to be ground and application method However the basic principle of dressing of the wheel remains the same The various methods of ELID grinding are as follows: Electrolytic In-Process Dressing (ELID 1) Electrolytic Interval Dressing (ELID 2) Electrode-less ELID (ELID 3) Electrolytic In-Process Dressing (ELID 1) This method of ELID grinding is mostly used In this case the wheel is dressed continuously in-process [24] Figure 2.1 shows the basic mechanism of ELID grinding method A cathode electrode covers usually 1/4th to 1/6th of the cutting edges of the wheel whereas the other exposed portion of the wheel takes part in the cutting action The gap between the electrode and the wheel is maintained to be 0.1mm to 0.3mm Fig 2.1: ELID grinding principle [24] Electrolytic Interval Dressing (ELID 2) In order to carry out precision grinding on the small holes on ceramic materials Zhang et al [25] introduces ELID method In this method the dressing of the grinding wheel is 21 Literature review ELID on Coated film K Katahira et al [43] also investigated ELID grinding on TiAlN films In their study authors used ELID grinding and other finishing process for comparison ELID grinding can achieve an average surface roughness of 2.4nm whereas other finishing process can generate a surface finish of 56.2 nm Tribological study of the ground surface suggests that the friction coefficient of the ELID-ground TiAlN film obtained in this study is approximately two-thirds that of the non-ELID-ground TiAlN film; the wear rate of the ELID-ground TiAlN film obtained in this study is approximately one-half of that of the non-ELID-ground TiAlN film; and the wear rate against an Al2O3 ball for ELID-ground TiAlN film is approximately one-tenth of that for non-ELID-ground TiAlN film According to the authors the rationale behind these improvements in the tribological behavior of ELID ground samples is the formation of a diffused oxide layer on the TiAlN film However the detail mechanism of the diffusion of oxygen and formation of oxide layer on the work-piece is not presented in the study Zhang et al [44, 45] compared the grinding of CVD-SiC (chemical vapor deposited silicon carbide) film by two methods; one is ELID grinding and the other one is ordinary grinding with saw like abrasive stone dressed cast iron bond diamond wheel The authors observed in their study that during ELID study an oxide layer of few microns thickness was formed which holds the new sharp grits The layer is soft in nature and it changes the contact status between the wheel and workpiece which is a unique feature of ELID grinding However in the case of conventional dressing of the wheel, no such layer is formed and only the cutting edge of the diamond grain protruded at a certain amount The 33 Literature review surface roughnesses of the ground samples were found to be better in quality in the case of ELID ground samples The variation of the surface roughness with grinding time was found to be more stable in the case of ELID grinding Moreover the morphological study of the samples suggests that in ELID grinding ductile regime machining is the predominant mode of machining whereas in ordinary grinding both ductile regime and brittle facture machining occurs equally Therefore the authors suggested in their study that ELID grinding is a better choice in ultra-precision grinding ELID on Metals Qian et al [46] studied the feasibility of ELID technology on finishing of bearing steel materials by cylindrical grinding Both traverse and plunge mode grinding were used and compared Further ELID grinding was compared with typical finishing operation of bearing steel, such as honing and electro-finishing methods Experimental findings showed ELID grinding produces better surface finish on the work-piece while comparing with other finishing processes In ELID plunge mode grinding generates poorer surface finish than traverse grinding, especially with coarser grits Moreover better and stable waviness can be achieved in traverse mode grinding with ELID In this study the effect of spark out process on waviness and roundness of the work-piece were investigated Both waviness and roundness were improved with 3-4 spark out passes Metrological results of the samples showed that ELID grinding produces better waviness result on the samples than honing and electro-finishing processes Residual stress on the ELID ground sample was also found to be very low, however the cycle time for ELID ground samples was found to be high when compared to existing honing and electro-finishing processes 34 Literature review Therefore authors suggested that ELID technology should be used for small batch production situations Saito et al [47] studied the effect of bond material on the performance of ELID grinding In their study ELID grinding was used to produce cemented carbide alloys lens mold Two types of metal bonded diamond wheels were used for the study, namely Cr bond and Cu bond The finding shows Cr bonded wheels produced best surface roughness on the work-piece Authors further investigated the electrolytic behavior of the Cr bonded grinding wheel In ELID grinding most of the metal bonded wheels produce an insulating layer after the initial electrolytic dressing, however for Cr bonded wheel the phenomenon was found to be non-matching In Cr bonded wheel no oxide layer is formed after and during the electrolytic dressing moreover oxygen is diffused to the workpiece material and enhances the surface quality Recently Ohmori et al [48] investigated mirror finishing of stainless steel by ELID grinding Three different types of abrasive particles (diamond, alumina, silicon dioxide) were used for comparison All the three abrasives produced similar surface roughness of to 10 nm The observation also suggests that ELID mirror surface grinding process produced a stable oxide layer on the workpiece surface, and also that the abrasive elements of the grinding wheel penetrated and diffused into the workpiece ELID ground samples exhibited superior surface properties including hardness, tribological and fatigue properties, corrosion and hightemperature oxidation resistances, and adhesive strength with coating films over conventionally polished samples In addition, authors proposed 35 Literature review that by varying the processing conditions and grinding wheel one can possibly control the electrical potential characteristics and even hydrophilicity of the surfaces ELID on Silicons Chao et al [49] studied the effect grinding with metal bonded grinding wheel and resin bonded grinding wheel on silicon wafers Amorphous layer, nanocrystals, polycrystals, dislocations and micro-cracks were observed in both cases depending on the wheel and grinding parameters It was found that the ELID ground surfaces were very sensitive to the ELID parameters and the optimized conditions were difficult to obtain and be maintained However in the case of grinding with resin bond wheel, the surface integrity was relatively less dependent to the machining conditions in comparison to ELID grinding In the case of ELID grinding, thick oxide layer and high feed rate favors wheel to produce sharp new protruding grains so that a stable cutting condition can be reached but there is still chances for some protruding grains getting excess attritious wear before it is pulled out in ELID grinding The worn grains will dull the wheel and generate much friction heat and subsurface damage This chance is very low for resin bond wheel because of its self sharpening effect Moreover In the case of resin bond wheel, except when feed rate is very low (say 2µm/min) or wheel is loaded, the amorphous layer generated were normally thinner than those generated by ELID grinding under the same grinding conditions Itoh et al [50] performed an experimental investigation on ELID lap grinding of silicons The results of the experiments showed that stable grinding can be achieved with #8000 to #30000 series wheel and ultra fine surface with average surface roughness of 0.38 nm can be achieved using #30000 wheel Moreover morphological study of the ground samples 36 Literature review with SEM and AFM also showed that produced surfaces are very smooth in the order of several nano meters M.M Islam et al [51] studied the injection electrode(IE) assisted ELID grinding for thinning of silicon wafer With the help of IE assisted ELID grinding process authors succeeded to thin down the silicon wafer from 750 micro meters to approximately 70 micro meter with sub-ten nano surface finish and without any visible cracking, pilling and chipping Moreover it was found that IE assisted ELID grinding exhibits less force than conventional ELID grinding because more uniform wheel dressing M.M Islam et al [52] further characterized the ELID lapping process for machining silicon wafers and following salient conclusions were drawn in this study The rate of formation of oxide layer was found to be dependent on the current duty ratio, however total oxide layer growth is independent of current duty ratio Compared to other conditions, only intermediate range of current duty ratio and grinding parameter (feed rate) showed better wheel surface condition However the detail reasoning behind these optimized conditions is not presented here Conventional grinding process exhibited unstable grinding forces with very high magnitude, on the contrary ELID grinding especially under intermediate range of value, showed stable grinding force with very low magnitude The ELID grinding process under intermediate range of experimental conditions (current duty ratio and feed rate) showed better grinding performance (smaller percentage of wear flat and smaller value of specific grinding energy) with superior ground wafer surface quality (Ra =4nm) Under all experimental conditions, the ELID 37 Literature review grinding process performs better compared to the conventional grinding process Moreover the subsurface damages exhibited on the samples are significantly lower for ELID grinding process than conventional grinding ELID on Optical Glasses Ohmori et al [53] used ELID grinding to fabricate spherical glass lens and compared the characteristics of the machined lens with ordinary ground samples With the help of ELID grinding the average surface roughness (Ra) achieved was 0.018 microns whereas by ordinary grinding method it was found to be 0.138 microns Moreover the morphological study of the samples suggests that ELID grinding can produce much smoother surface than conventional grinding method The shape accuracy of the lens ground by ELID was also found to be less than micron range Kumar et al [54] studied the effect of ELID grinding on BK7 glasses It was found that current duty ratio plays an important role on the grinding performance and surface finish of the product Both surface finish and grinding force improves (cutting force reduces) with the increase of current duty ratio This is because higher current duty ratio dresses the grinding wheel more to ensure more protrusion of the sharp cutting grits However the grinding ratio also decreases with high current duty ratio Authors also found that #4000 series wheel with proper grinding condition can generate a flawless ductile surface on BK7 glass 38 Literature review Stephenson et al [55] investigated AE detection method to identify wheel loading and to assess the grinding state of a wheel for machining of BK7 and zerodur glass This investigation suggests that the acoustic emission energy has a potential to be used as grinding condition indicator because of its strong dependence on wheel loading It was found in the study that ELID grinding with a fine grit size cup wheel is less likely to encounter wheel loading compared to a resin bonded wheel when wheel work contact zone is big High level of AE signal during grinding with resin bonded wheel indicates occurrence of excessive friction and rubbing between wheel and work-piece, which implies that a resin bond wheel cannot perform effective self dressing when the machining arcs of the abrasive grits were long However, resin bond wheels tend to generate lower AE amplitude when the wheel/workpiece contact area is small Authors also found that if the dressing parameter is low in ELID grinding with finer wheels the AE signal level was also considerably high Therefore they recommend in-process dressing with more aggressive dressing parameters for finer grit size grinding wheels to minimize wheel loading However this selection of dressing parameters depends on many factors like wheel configuration, grinding parameters and material properties of the workpiece etc Thus the authors suggested the use of AE sensing technique for monitoring the complex ELID grinding process to ensure optimum grinding/dressing conditions are maintained Yin et al [56] studied the characteristics of ELID grinding on zerodur glass where they achieved an average surface roughness of 5-10 nm The flatness and perpendicularity were also found to be very good AFM study of the ground surfaces also indicated that 39 Literature review the material removal mode was in ductile region The influences of the grinding parameters on the grinding performances were also studied It was observed that higher feed rate causes high grinding force and more thermal damages to the surface Depth of cut was another important factor for ELID grinding of zerodur glass Ideal depth of cut for better surface finish and stable grinding condition was found to be 0.4 microns Yin et al [57] further developed a new grinding process and system for the fabrication of large non axisymmetric aspheric surface of fused silica In their study they employed precision truing method for the grinding wheel as well as the form error compensation of the machined workpiece By implementing these authors achieved better form precision and surface finish on the work-piece simultaneously 2.2.5 Miscellaneous research on ELID grinding Zhu et al [58] analyzed the fluid flow system between the solid electrode and the rotating wheel Their simulation results showed a good agreement with the experimental findings The analysis revealed that at high wheel speed the inlet velocity of the electrolyte into the gap should also have to be very high to maintain good electrolytic condition However for low inlet velocity of the electrolyte in high speed ELID grinding the authors suggested the use of elastic foil electrode It was shown that the foil ELID system is more effective for high-speed grinding with low electrolyte supply rates Uehara et al [59] developed a desktop axes machine equipped with ELID grinding system All the axes are stepper motor driven and semi closed loop controlled A concave 40 Literature review mirror with 37.3 mm radius was ELID ground to understand the feasibility of the machine The average surface roughness of the ground mirror was 20nm and the form accuracy was 20microns which are reasonably good Itoh et al [60] investigated the flattening and smoothing of micro composite parts by ELID lap grinding The parts were fabricated from copper and resin by film lamination method Special grinding technique known as simultaneous grinding was used in this experimental study After 15minutes of grinding the surface irregularity was reduced from 15 micorn to 0.4 microns No abnormal dressing current, etc were observed indicating that stable grinding was successfully achieved through out the experiment Uehara et al [61] further developed a similar [59] compact cylindrical grinding machine equipped with ELID system to develop micro tools Using the machine they fabricated angular micro shafts with good dimensional accuracy Further they used these micro shafts for micro engraving of characters 2.3 Study on grinding wheel wear and topography monitoring There are several measurement methods available for monitoring the grinding wheel wear and topography monitoring, namely force measurement, vibration measurement, acoustic emission measurement, direct wheel geometry measurement Many researches have been carried out on these techniques, some of which are discussed below 41 Literature review Kwak et al [62] used force signal to detect the correct time for wheel dressing In this study the wavelet decomposition technique was introduced and applied to the grinding process with force feed back The de-noising method of the wavelet technique and FFT filtering technique were compared and analyzed It was observed that the wavelet denoising was nearer to the original signal During the grinding operation the time at which it was necessary to redress a grinding wheel was determined with the wavelet technique When a higher level of the wavelet deposition was applied, the easiest detection of the redressing time was made in a processed signal From the results of this study, it was concluded that the wavelet transform technique has high potential to detect the wheel redressing time in grinding S Malkin [63] studied the relationship between the grinding force and the wear flat of the cutting grits It was observed that grinding force has a strong dependence over the wear flat area of the wheel He proposed that the grinding force consists of two components, cutting component and sliding component Sliding component due to the rubbing between the grits and workpiece increases with wear flat area of the wheel Thus by monitoring the grinding force the wear of the wheel can be identified Inasaki [64] is one of the pioneer researchers to introduce acoustic emission signals to monitor grinding operation and grinding wheel dressing operation It was observed in his study that grinding wheel wear may be non uniform along the width of the wheel However to maintain the dimensional accuracy the wheel needs to be flat and uniform along the width When the grinding wheel is flat the AE amplitude during the wheel dressing remains constant, therefore the AE signal output can be used as the monitoring 42 Literature review signal during dressing of the wheel so that a flat wheel profile can be obtained In this study author also studied the applicability of the AE signal to detect the wheel work-piece contact It was observed that AE signal monitoring is more reliable and repeatable to identify wheel work-piece contact than grinding power monitoring method Further, author investigated the feasibility of AE signal to monitor the grinding process The results suggest that AE signal has a strong relationship with Ra (average surface roughness) and grinding power Therefore author concluded that AE signal can be used to monitor the grinding process hence to identify the wheel life Hundt et al [65] also investigated the application of AE signal to identify grinding wheel wear, however they analyzed the signal in frequency domain In their study the authors assumed that in grinding every grain interacts with the work material to emit its acoustic signal that can be identified Experimental results also confirmed this assumption It was also assumed that the abrasive grain material fracture and the bond fracture shall generate different AE signal in frequency domain, which was found to be completely matching in actual grinding experiments Thus the operator can identify the different type of wear mechanism in grinding wheel by observing the AE signal in frequency domain and can take action accordingly Mokbel et al [66] studied the spectral amplitude variation of AE signal during the grinding process to understand wheel condition The wheel condition was varied from blunt to sharp by varying the truing/grinding ratio Specimens were ground with different wheel conditions and the surface roughness was measured It was observed that both 43 Literature review surface roughness and AE spectral amplitude correspond to surface condition of the wheel Small surface roughness values were found to be associated with high AE spectral amplitude and sharp wheel condition whereas rough surfaces were associated with low AE spectral amplitude and blunt wheel condition With these findings authors concluded that AE spectral amplitude has a good potential to use as a wheel monitoring tool A Hassui et al [67] studied monitoring wheel wear by vibration and acoustic emission It was found that both vibration and A.E signal increase as the wheel becomes worn out and so as the work-piece surface roughness, which gives a hint of ending of the wheel life The RMS AE signal obtained using a high pass filter with frequencies higher than 10 kHz and the vibration signal obtained using a high pass filter with frequencies higher than 100 Hz showed good dependence on wheel wear and can be used to indirectly establish the end of wheel life, with proper signal conditioning Fan et al [68] developed a CCD camera based non contact wheel wear measurement system An image processing technique was introduced to successfully detect the grinding wheel edges The images before and after grinding were taken and analyzed to detect the position deviation of the grinding wheel edge Thus the wheel wear was measured and the accuracy was found to be micron E.Brinksmeier et al [69] used the laser sensor to monitor the wheel topography change The measurement technique works based on the principle of triangulation By measuring the wheel topography the bearing ratio curve can be generated for a particular wheel 44 Literature review surface profile The authors showed that bearing ratio curve profile depends on the wheel condition; the profile differs from the condition when wheel has microscopic grain wear to the case when the wheel is loaded with chips By monitoring the B.R.C profile online one can identify the current wheel condition Ju et al [70] introduced laser scanning micrometer to measure the wheel wear It was observed that the grinding wheel profile changed significantly during grinding indicating the occurrence of wheel wear Authors suggested quantifying this wear with LSM method so that machining error due to incorrect depth of cut can be eliminated Moreover the profile trend showed that wear increased abruptly at the end of grinding This is because at the beginning the wear is predominated by grain wear whereas at the end of grinding grits fall-out’s from the wheel is the dominant factor The authors proposed to use this phenomenon to detect the appropriate dressing time of the wheel SunHo Kim et al [71] used eddy current sensor to detect the wheel loading As wheel loading causes change of metal chip density on the wheel circumference, it can be correlated to the output of the eddy current sensor To decide on the dressing depth the authors used laser sensor to generate the wheel topography Authors suggested deciding the optimum dressing depth through the analysis of the variance of the dressed wheel surface topography, which is almost inversely proportional to the accumulated dressing depth However for grinding of no metallic substances this method may not be suitable as eddy current sensor is not applicable for non metallic object 45 Literature review Furutani et al [72] also developed a grinding wheel topography measurement using hydrodynamic pressure sensor Authors studied different frequency components of the sensor output and observed that when the loading and dulling of the wheel occurs some of the frequency components of the signal increased Thus this method can be used as an indicator of wheel loading/ dulling An automated system of measuring the wear flat by camera was proposed by LaChance et al [73] The objective of this development was to develop models to explain the life cycle of the wheel and to correlate wear flat with grinding force, heat generation and thermal damage The wear flat measurement done by this method was compared with scanning electron microscope measurement and found to be in good agreement 2.4 Concluding remarks From the above discussion of the literature study following conclusions can be drawn, There are various techniques available in order to achieve in-process dressing of the grinding wheel [3-22], though ELID grinding is drawing more attention by the researchers This is not only because of its efficient dressing technique, but also the insulating layer formed during the dressing of the wheel The soft oxide layer holding the diamond grit is like the lapping pad and the bonding material acts like a supporting pad Thus it is a combination of grinding and polishing process, which is indeed a very unique feature 46 Literature review Most of the works done in the field of ELID grinding are primarily related to its application on different engineering materials [39-57] Some studies were carried out on the fundamental study of the behavior of ELID grinding [28- 34] Very few works [35-38] were done on the process control of ELID grinding to optimize the technology and plenty of opportunities are available to contribute in this area In order to meet two main objectives of this research work as mentioned in the chapter (2 & 3) it is necessary to estimate wheel wear and the topography There are different methods like vibration detection, force monitoring, acoustic emission monitoring etc available to monitor the wear or loading of the grinding wheel as well as its topography [62-73] A suitable technique can be chosen from the above mentioned methods in order to use it as a feed back signal to develop an intelligent power supply for ELID grinding 47 ... schematic illustration of the interval ELID dressing and grinding Fig2 .2: ELID grinding principle [25 ] Qian et al [26 ] also investigated ELID method to grind the internal surface of cylindrical objects... finer grit sizes This method of wheel dressing also improves the grinding performance significantly Ohmori et al [22 ] also proposed a hybrid method of dressing and truing of ultra fine grinding. .. [23 ] work In their study Ohmori et al investigated the pre dressing current profile and explained the phenomenon of decreasing trend of the current with dressing time During grinding the grinding