Development of an intelligent electrolytic in process dressing (ELID) grinding system 3

Development and performance evaluation of an intelligent ELID grinding machine Chapter Development and performance evaluation of an intelligent ELID grinding machine 3.1 Introduction Nano-scale surface finish with no subsurface defect is a principal requirement for the optical glasses. Conventional processes for achieving this are lapping and polishing, though they have their own disadvantages as mentioned earlier in chapter [2]. An alternative to replace polishing and lapping for the final surface treatment is ELID (Electrolytic in-process dressing) grinding. ELID technique uses a metal bonded diamond grinding wheel that is electrolytically dressed during grinding process such that the protrusion of abrasive particles remains consistent; detailed mechanism is explained in chapter 1. One of the main objectives of this current research is to develop a sensor assisted intelligent ELID grinding machine. In this chapter the main aim has been focused to present the development of a new machine primarily dedicated for ELID grinding and also the performance evaluation of the machine and its in-built measurement system. Curvilinear surfaces with good surface finish and form accuracy are also badly needed in the optical industries. Single point diamond turning with a fast tool servo is the more conventional way for generating free form surface, but the number of materials 48 Development and performance evaluation of an intelligent ELID grinding machine machinable with this method is limited. Wet grinding is thought to be another manufacturing option due to its ability of generating transparent surface on hard and brittle materials. Loading, dulling and shedding frequently occur while grinding hard and brittle materials like glass with super fine abrasive grinding wheel and results an opaque surface after machining. Electrolytic in process dressing (ELID) grinding, is a very efficient process for achieving mirror surface finish on hard and brittle materials even without any post-grinding polishing process because of its inherent ability to overcome loading, dulling of the wheels. So introduction of ELID grinding can be considered as a milestone for generating free-form optical quality surface with the elimination of polishing processes and enhancement of dimensional accuracy. However in ELID grinding the tool wear is significant because of its nature of in-process dressing which gives rise to chances of form error in spherical/aspheric surface grinding. In this chapter a novel method of tool wear compensation technique in ELID grinding for curvilinear surface machining has been described which was implemented and tested on the newly developed ELID grinding machine. 3.2 Development of a sensor integrated ELID grinding machine The proposed intelligent ELID grinding system comprises of a four-axis CNC machine which is sensor integrated and has a pulsed DC ELID power supply. In this section the detailed development of this machine and its different smart monitoring systems shall be described elaborately. 49 Development and performance evaluation of an intelligent ELID grinding machine 3.2.1 ELID grinding machine and the power supply A 4-axis CNC machine along with a DC power supply was developed to perform the ELID grinding process as shown in the photograph of figure.3.1 (a). Detail information about the newly developed grinding machinetool is given in Table 3.1. All the four axes are controlled by AC servo motors. The machine structure was re-engineered from an existing wire-cut EDM machine and configured as a gantry structure and is formed of two pillars, a crossbeam and a base. ELID grinding is meant for producing nano surface with high dimensional and form accuracy, for which machine structural rigidity is a big concern. That is why a gantry structure has been chosen for developing the machine which provides better rigidity. A special type of electrode was used, which had cavity inside the body as shown in the figure 3.1 (b) for better flushing of the electrolytes and a pump is used to inject electrolyte right at the gap between the wheel and electrode [51]. (a) (b) Fig 3.1 (a) Sensor integrated ELID grinding machine (b) specially designed injection electrode 50 Development and performance evaluation of an intelligent ELID grinding machine The electrode covers 1/4th peripheral area of the wheel; the conceptual design of the electrode is described in the figure 3.2. Previous literature [30] shows that the electrode covers 1/6th peripheral area of the wheel; here a bigger electrode has been chosen to increase the electrolysis area so that faster wheel dressing is possible. The electrode mounting mechanism was designed with the help of a wedge mechanism so that the gap between the electrode and the wheel could be varied. A rotary turn table was also designed and fabricated to give a rotary motion to the workpiece for machining curvilinear and axi-symmetric surfaces. Sealed-type ball bearings were used in the turntable to protect them from the corrosion. Table 3.1: Specification of the developed machine Parameters X Y Z Full stroke (mm) 250 250 150 Resolution (microns) 0.1 0.1 Natural frequency (Hz) 817 2000 1429 Damping ratio 0.0765 0.086 0.014 A square pulsed DC power supply was developed to generate power for the ELID cell. In most of the previous studies [74] researchers used maximum peak current 10 A, though any strong theory is yet to be developed for calculating how much current is actually needed to perform ELID grinding. In this research it was aimed to investigate the effect of low peak current on ELID. Several technical data of the developed power supply are given in the Table 3.2. Table 3.2 Technical data of the dc power supply Parameters Values Voltage (V) 20,40,60,80,100 51 Development and performance evaluation of an intelligent ELID grinding machine Maximum peak current (A) T-on (micro second) 8,10,14,30 T-off (micro second) 5,8,20 The first version of the power supply was an open loop type. However later it was converted into a closed loop power supply to implement the ELID truing control as described in the chapter 4. 3.2.2 Development of an in-process wheel monitoring system In order to study the wheel topography change and to develop the controlled wheel truing system (explained in chapter 5) it is necessary to monitor the wheel outer circumference and its metal bond profile in-process respectively. The developed grinding machine has the ability for in-process wheel monitoring. Two types of sensors were used in this purpose. The wheel metal bond profile measurement system uses an inductive sensor, whereas the wheel topography monitoring arrangement uses a laser sensor. (a) (b) Fig.3.2: (a) inductive displacement sensor (b) laser displacement sensor 52 Development and performance evaluation of an intelligent ELID grinding machine START Initial radius of the wheel is Rinitial Calculate the gap between wheel circumference and sensor head and assign it as AB After grinding measure the gap between wheel circumference and sensor head at different point and assign it as CD[i] At different point on the wheel profile the radius is EF[i] = Rinitial + (AB-CD[i]) n Final mean radius after grinding Rfinal= ∑ i =1 EF (i ) / n STOP (c) Flow chart to calculate wheel radius from in-process wheel monitoring system. The measured profile by the laser sensor can be used to calculate the tool wear, as well as this can be used for machining error compensation by measuring the change in diameter of the wheel due to its wear. The flow chart of the measurement method is described in the figure 3.2(c). Experiments showed that laser sensor is not ideal for wet grinding, it affects the measured profile, this adverse effect can be overcome by taking the data during the dry condition, i.e. first wheel profile is measured just before beginning of the grinding, and after few runs subsequent profile variation is measured with the grinding fluid flow turned off. 53 Development and performance evaluation of an intelligent ELID grinding machine An inductive sensor is only sensitive to the metal bond of the wheel and cannot measure the thickness of the oxide layer formed on the circumference of the wheel. Therefore the inductive sensor output was used as the feedback signal for the controlled truing of the grinding wheel which is explained in the chapter more comprehensively. 3.2.3 Development of on-machine wheel wear compensation In this subsection detail description is given on the development of the on-machine wheel wear compensation for spherical surface grinding. The section includes discussion on the software algorithm development to generate the tool path for spherical lens grinding, hardware design of the measurement unit and total system’s working principle. Tool-path generation Axis movement of this newly developed ELID grinding machine was controlled by the linear interpolation principle, which made it mandatory to provide the coordinates of the curved tool path. An image of the orientation of grinding wheel and workpiece during the experiment is shown in figure 3.3(a). Ideally the grinding wheel should be moved in a way such that there will always be single point contact between the wheel and workpiece. Grinding wheel radius (QN), radius of the workpiece (AN) and radius of the spherical surface to be machined (ON) were taken into consideration to accomplish the tool path for this single point contact. Taking more points to define a certain circular path makes the smaller interpolation segment, which in a way ensures better accuracy in achieving curved tool path by linear interpolation. So number of points was considered as a factor while determining the co-ordinates of this tool path. Firstly, the grinding wheel was in 54 Development and performance evaluation of an intelligent ELID grinding machine contact with the workpiece surface to set the origin of the working coordinate system as shown in figure 3.3(b). If C is the position of the spindle axis at the starting point then from simple geometry the location of the spindle can be calculated in a way such that the wheel will always be touching the workpiece at one point. Wheel Work piece (a) Electrode (b) Fig 3.3 (a) Wheel and workpiece orientation during spherical lens grinding (b) Cutting path for lens grinding In this way it is possible to calculate all the points of the tool path until the wheel reaches point N, which is also the end point of the circular arc with a single-point contact between workpiece and wheel. X coordinate of the point Q is given by, OP = OM .OQ …………………………………………………………………………(3.1) ON and the Y coordinate is CB = OC − OB ……………………………………………………………………… .(3.2) where OB = PQ = OQ − OP …………………………………………………………… .(3.3) 55 Development and performance evaluation of an intelligent ELID grinding machine Value of OM was reduced from the maximum value, which is equal to the radius of the workpiece to zero in as many steps possible so that it is possible to get more coordinates to define the curved tool path. As a result of grinding, wheel diameter changes due to wear which is significant in ELID grinding and this change was taken into consideration to calculate the tool path based on the new wheel diameter. So coordinates of the tool path were updated regularly in the NC program and in this way effort was made to compensate the wheel wear. Software was designed and developed based on the above mentioned principle to generate the tool path. Hardware design for on-machine form measurement An on-machine measurement (OMM) system based on co-ordinate measurement method (CMM) principle has been developed in this study to check the profile of a ground surface. A photograph of the developed OMM system is shown in figure 3.4. Fig 3.4: OMM system for surface form measurement 56 Development and performance evaluation of an intelligent ELID grinding machine CMM has become more and more prominent in measurements and authentication of dimensional excellence of machined parts. In most cases touch trigger probe is an integral part of CMM to evaluate the position of the axis indicated by the contact of the probe tip. The measurement uncertainty of CMM is mainly due to the very machine where the probe is attached, although some common errors result from the probe system. A touch trigger probe head along with a ruby ball tipped stylus and a female socket for holding the probe head was used in the newly developed on-machine profile measurement system. The stylus is attached to a tripod structure, whose three cylindrical arms are supported by three pairs of crossed cylinders. An electric current normally flows through the tripod arms and cylinders. When the stylus moves, one of the contacts breaks and a binary signal comes out from the probe head [75]. The female socket powered from a 24V supply contains an integral interface, which converts this binary signal into a voltage free solid state relay (SSR) output for transmission to the CNC machine controller. The internal structure of the probe is such that during undisturbed state signal continuously comes out from the output of the probe. Output signal of the probe was reversed by using a photo-coupler in a simple circuit to get the output signal only in the disturbed condition and thereby detect the moment of stylus deflection. With the built-in interface and compact size of φ25mm X L45.5 mm, this socket eliminates the need for a separate interface within the control cabinet and make the installation simpler. The detailed specification of the touch probe has been given in table 3.3. The probe is fixed in a block, which is attached with the piston rod of an air cylinder. A two-way solenoid valve controlled by a relay was used to control the flow of compressed air in and out of the cylinder. The piston rod is also linked with a block, which vertically moves the probe on 57 Development and performance evaluation of an intelligent ELID grinding machine system. After this the probe moves in the Y direction until it touches the machined surface. As soon as it touches the surface a signal is sent to the controller, which stops movements of all machine axes. Then another query is sent to the controller to know the real position of the machine, which is converted into coordinates of the contact point of the stylus tip with the aspheric ground surface. A graphical user interface (GUI) was developed for measuring the co-ordinates After setting all the parameters the system will start the measurement right after pushing the ‘Scan’ button in the GUI. the workpiece is first scanned at different points in a square matrix arrangement. All the scanned points’ coordinates are then fed into a custom made program to calculate the centre coordinate and the radius of curvature of the spherical surface. The program calculates all the circle radius for different z coordinates then assigns the largest radius as the radius of the spherical surface. The radius of the spherical surface thus measured is then fed to a tool wear compensation algorithm. This algorithm is particularly applied for spherical surface machining. Figure 3.6 above shows the flow chart of the developed algorithm. In this algorithm, firstly the tool path for spherical lens is generated by taking consideration of the tool (grinding wheel) radius as explained in the earlier section. The radius of the wheel can be measured by using the in-process wheel monitoring unit as described in the previous section. After completion of the machining the work-piece is scanned and the radius of curvature of the machined spherical surface is calculated. If the measured radius of curvature is different from that of the desired value due to some machining error the first step is repeated until the achieved radius of curvature is within tolerance. In this algorithm consideration of the 59 Development and performance evaluation of an intelligent ELID grinding machine START Generate the tool path for lens grinding by considering the grinding wheel radius using equations 3.1 to 3.3 Scan the machined sample to calculate the radius of the lens Measure diameter change by inprocess wheel monitoring system Is the radius within tolerance? No Yes Remove work-piece from the machine STOP Fig 3.6: Tool wear compensation algorithm wheel radius change for tool path generation is very crucial as it eliminates the wheel wear effect on the form accuracy. 3.3 Experimental evaluation of the performance In order to evaluate the performance of the developed ELID grinding machine tool two types of experiments were carried out namely spherical surface grinding and vertical 60 Development and performance evaluation of an intelligent ELID grinding machine groove grinding. The workpiece and wheel used for all the experiments included in this thesis were BK7 optical glass and cast iron bonded fine diamond wheel respectively. BK7 is a borosilicate crown optical glass with high homogeneity, low bubble and inclusion content. Its good physical and chemical properties make it widely used in visible and near IR range. Most of windows, lenses and prisms, which used in laser, optical system, optical communication, are made from BK7 glass. Some of the properties of BK7 glass are given below [80]: • Transmission range: 330 nm~2100 nm • Thermal Expansion Coefficient: 7.5X10-6 /K • Density: 2.51 g/cm3 Grinding wheels used in this research consists of very fine abrasive grains known as grits and the bonding metal that holds the grits together. Diamond grits were preferred in this study for their extreme hardness suitable for machining brittle material like glass. The bonding material was selected as cast iron because of its better electrolytic behavior. [28] In this section experimental procedure and setup as well as the results for both type of investigation shall be discussed in details. 3.3.1 Experimental setup and procedure The experimental setup for vertical groove grinding is schematically described in the figure 3.7. The feed motion and the in-feed direction are also shown in the figure. 61 Development and performance evaluation of an intelligent ELID grinding machine Fig 3.7: Experimental setup for vertical groove machining. The injection type electrode [51] used in this experiment is made of copper and it covers 1/4th of the perimeter of the wheel. The wheel is positive compared to the electrode. The gap between the electrode and the wheel was set to 0.2-0.4 mm. During groove grinding following parameters were varied in order to find out the optimum cutting condition to achieve minimum surface roughness. Table 3.4 shows the range of the parameter variation. Table 3.4 Parameters varied in vertical groove machining experiments Cutting and ELID parameters Value Feed rate (mm/min) 125-500 In-feed (micron/pass) 1-4 Wheel speed (RPM) 750 RPM Wheel mesh size #4000 Wheel diameter(mm) 75 Pulse On time ( micro second) 10 Pulse Off time (micro second) 62 Development and performance evaluation of an intelligent ELID grinding machine After machining the grooves the morphological study was carried out by Keyence optical microscope with high magnification. The surface roughness of the machined groove was measured by Taylor-Hobson surface profile-meter. A complete schematic of the experimental setup for spherical surface machining is shown in the figure 3.8. All values of the parameters of this experiment are mentioned in table 3.5. First of all the BK7 glass piece was fixed with the base plate in a way such that the center of the work-piece coincides with the rotational axis of the turntable. The turntable was fixed on the machine after checking the alignment properly. At second stage coordinates of the tool path were generated for grinding a spherical surface of profile radius 100mm and NC program was prepared for this tool path. Grinding was started at the third stage and economic grinding was aimed for generating the spherical surface by rough grinding followed by finishing with higher grade of grinding wheel to optimize the Fig 3.8 Experimental setup for spherical lens grinding 63 Development and performance evaluation of an intelligent ELID grinding machine wheel wear [76]. After a few hours at forth stage, machining was stopped for measuring wheel diameter and ground surface profile. So there are two parallel procedures after this stage. One is to measure the wheel diameter and then update the tool path in the NC program based on this new wheel diameter. The second of these two parallel procedures is measurement of ground surface profile using the touchprobe. Then the measured coordinates on the ground surface were used to calculate the radius of the spherical surface as explained in the earlier section. Comparing the achieved and desired values, error was calculated for the ground surface profile. This whole cycle of profile and wheel radius measurement, error calculation and NC program generation Table 3.5 Parameters for lens grinding Work-piece Desired BK glass of 40mm dia profile 200mm diameter Grinding wheel CIB-D; Diameter75mm thickness 3mm #1200 (roughing); #4000 (finishing) Electrode Injection type copper electrode Electrolyte CG-7 (diluted in water with a ratio of 50:1) Spindle speed 1500 rpm Workpiece speed 275 rpm Feed rate 300 mm/min (roughing); 50 mm/min (finishing) Depth of cut µm (roughing); µm (finishing) 64 Development and performance evaluation of an intelligent ELID grinding machine continues until error value reaches below a tolerable limit. Finally the finished part was removed form the machine to measure the profile radius in the CMM machine. Surface roughness was measured in the Taylor–Hobson form Talysurf-120 machine and information required for calculating form accuracy was gathered from the Mitutoyo CS500 form tracer. Surface of the finished workpiece was observed under SEM and Keyence optical microscope to check the ground surface integrity. In order to observe the influence of software compensation over the dimensional accuracy of the machined part, another BK7 glass workpiece was machined in the same setup but without software compensation to achieve a profile of 100mm radius. In this case ground surface profile or wheel wear was not measured during the grinding process to update the tool path in the NC program. 3.3.2 Results and discussions Vertical groove grinding Vertical grooves with optical surface quality have been machined using the developed ELID grinding machine. The best surface quality was achieved at 125 mm/min feed speed with µm depth of cut (DOC). Figure 3.9 shows the photograph of the machined surface. Fig 3.9: Photograph of the machined surface 65 Development and performance evaluation of an intelligent ELID grinding machine Figure 3.10 shows the effect of DOC /in-feed and feed speed on the surface roughness. In order to ensure the repeatability of the results each experiment was carried out three times and the mean surface roughness values are plotted in figure 3.10. The error bar showing the maximum and minimum values for all experiments is also included in figure 3.10. Form the figure it can be observed that the best surface finish can be achieved at a certain depth of cut whereas lower feed rate gives better surface finish. Fig 3.10 Effect of in-feed and feed speed on the surface roughness Ra Hahn [77] identified that there are three types of material deformation as a grain interacts with the work-piece: rubbing, ploughing, and cutting. These are illustrated in figure 3.11. In rubbing mode of deformation the material removal is negligible though friction is apparent which may even cause work-piece burn. Ploughing occurs as the depth of penetration is increased; however the chips are yet to form. With further penetration (the cutting stage) of the grits the actual chip formation occurs, which is desirable for grinding. 66 Development and performance evaluation of an intelligent ELID grinding machine Fig 3.11 Three modes of cutting in abrasive machining Again too low in-feed/DOC produces bad quality surface which is due to the reason that rubbing takes place rather than cutting at a very low indentation of the grits as explained earlier. However at a too high in-feed/DOC the cutting occurs in brittle mode rather than in ductile mode which can be seen from the figures 3.12 (a) and (b). Brittle fracture Fig. 3.12. (a) Ductile surface generated at 500 mm/min feed speed and 3µm DOC using #4000 wheel (magnification 450 times) (b): Brittle fracture on the surface at 500 mm/min feed speed and 4µm DOC using #4000 wheel (magnification 450 times). This explains the trend of the variation of Ra with different DOC. Also lower feed rate produces better surface at a constant depth of cut/in-feed, as seen from figure 3.10, 67 Development and performance evaluation of an intelligent ELID grinding machine because higher the feed rate higher the probability to occur brittle mode cutting, which can be better understood from the figures. 3.13 (a) and (b) shown below. Fig. 3.13. (a) Smoother surface at 250 mm/min feed speed and 4µm DOC using #4000 wheel (magnification 450 times) (b): bad surface at 500 mm/min feed speed and µm DOC using #4000 wheel (magnification 450 times). Spherical lens fabrication The decision of when to stop the machining was taken after checking the profile radius with the OMM system. In order to check the repeatability of the OMM system a convex lens surface profile was measured for a grid size of 10mm X 1mm at a feed speed of 250 mm/min and it was repeated five times for the same setup. Different values of the radius were then plotted and found to be very much repeatable as shown in figure 3.14. The average of the five radii values measured in the OMM system was 75mm. 68 Development and performance evaluation of an intelligent ELID grinding machine Radius of the measured surface 100 Radius of the surface 95 90 85 80 75 70 65 60 55 50 1.5 2.5 3.5 4.5 No of observation Fig 3.14: Repeatability test for the on-machine measurement system Profile radii of both the workpieces machined with and without software compensation were measured with the MAHR OMS 400 CMM machine and values obtained are shown in table 3.6. Taking the values measured in the CMM machine as the reference profile radius, error values were calculated and are shown in this table. These error values are the evidence of improvement of profile accuracy in the finished work-piece ground with software compensation. After setting the starting point, an area in the middle region of the machined surface was measured using the newly developed system to read the coordinates at many locations inside this area with a grid size of 5mm by 1mm and a surface plotted by these points is shown in figure 3.15. Profile accuracy of the finished workpiece was also checked in the Mitutoyo form tracer machine, which is shown in figure 3.16 and value obtained from this measurement is 1.234 µm. Table 3.6 Comparison of profile accuracy Experimental condition Desired profile radius Actual profile radius Deviation (mm) (mm) (%) With compensation 100 99.96 0.04 Without compensation 100 98.17 2.83 69 Development and performance evaluation of an intelligent ELID grinding machine Fig 3.15: Surface generated by the points measured in the OMM system. Fig 3.16: Profile accuracy of the machined BK7 Workpiece. In order to verify form accuracy of the ground surface it was set in the Mitutoyo CS-500 form tracer. Then coordinates along a line right at the middle of the surface, were measured for a length of 30mm and with a pitch size of 0.01 mm. Radius was calculated at each point and finally all those radii were plotted with the desired radius as shown in figure 3.17. This figure showed a symmetric form inaccuracy valued at 15.35 mm P–V 70 Development and performance evaluation of an intelligent ELID grinding machine for the finished surface. From the symmetric nature of this graph and repetitiveness for other workpieces machined in the same setup, it can be said that positional inaccuracy of the machine axes is one reason for this form inaccuracy. Shape of the grinding wheel cutting edge is another factor determining form accuracy. For making a precise spherical form, it should be single-point contact between workpiece and wheel. Toshiyuki Enomoto and Yutaka Shimazaki [78] have found in a study that, due to higher rotational speed grinding force is also larger on the outward region of the workpiece surface than on the inner one. This variation of grinding force causes differential rate of material removal and initiate form inaccuracy. Fig 3.17: The form accuracy of the machined lens Roughness of the final ground surface was measured using Taylor–Hobson form Talysurf-120 machine with a stylus speed of 0.05 mm/s. Different values of roughness achieved along the line of measurement are plotted in figure 3.18. The Ra value obtained from this measurement was 0.015 mm. Grinding wheel abrasive grain size has a profound 71 Development and performance evaluation of an intelligent ELID grinding machine effect on surface roughness. In order to obtain better surface finish by grinding, ultra-fine abrasives wheel is necessary [79]. In this study the grinding wheel used was CIB-D with mesh number of 1200 and 4000 for roughing and finishing, respectively. Surface roughness would be improved a lot if finer grit grinding wheels are used. Fig 3.18: Surface profile of the machined surface In order to investigate the presence of surface integrity the finished surface was observed under SEM and Keyence microscope and the images obtained are shown in figures 3.19 and 3.20, respectively. It is evident from these figures that the machining was carried out in ductile regime without any brittle fracture. A photograph of the finished BK7 glass piece is also shown in figure 3.21. From these figures it can be stated a very high-quality optical surface has been generated where with minimum grinding marks. Ren et al. [14] have explained that passivating film is the major reason behind generation of submicron level surface by ELID grinding. In this process materials are removed with a manner combining the micro-scale grinding of ultra fine abrasives and the lapping and polishing actions of the abrasives wrapped in the film. So it is possible to generate high-quality 72 Development and performance evaluation of an intelligent ELID grinding machine surface even without lapping and polishing, which reduces the amount of time and money usually needed for these post processes. Fig3.19: Surface quality of the machined lens under Keyence optical microscope (X 3000) Fig 3.20 SEM observation of the surface (a) X1500 (b) X2500 Fig 3.21: Photograph of the machined lens 73 Development and performance evaluation of an intelligent ELID grinding machine 3.4 Concluding remarks In this chapter efforts have been made to describe the development of an intelligent ELID grinding machine tool for nano-surface generation. Detailed discussions are also carried out on the performance of the developed system and following conclusions can be briefly summarized from this chapter: • A machine tool has been re-engineered with integrated sensory system for ELID grinding. The machine is capable of producing optical surface with less than 20 nm of surface finish. • Some crucial algorithm has been introduced to the newly developed machine such as tool wear compensation for spherical surface machining. A spherical lens was ground and tested to evaluate the performance. It was found that the proposed algorithm enhanced the profile accuracy of the machined surface significantly. A very important element of the ELID processing technology is its dressing power supply. Two major improvements are proposed and implemented on ELID power supply which is explained in the next two chapters. 74 [...]... behind generation of submicron level surface by ELID grinding In this process materials are removed with a manner combining the micro-scale grinding of ultra fine abrasives and the lapping and polishing actions of the abrasives wrapped in the film So it is possible to generate high-quality 72 Development and performance evaluation of an intelligent ELID grinding machine surface even without lapping and... performance In order to evaluate the performance of the developed ELID grinding machine tool two types of experiments were carried out namely spherical surface grinding and vertical 60 Development and performance evaluation of an intelligent ELID grinding machine groove grinding The workpiece and wheel used for all the experiments included in this thesis were BK7 optical glass and cast iron bonded fine... values of roughness achieved along the line of measurement are plotted in figure 3. 18 The Ra value obtained from this measurement was 0.015 mm Grinding wheel abrasive grain size has a profound 71 Development and performance evaluation of an intelligent ELID grinding machine effect on surface roughness In order to obtain better surface finish by grinding, ultra-fine abrasives wheel is necessary [79] In. .. 275 rpm Feed rate 30 0 mm/min (roughing); 50 mm/min (finishing) Depth of cut 2 µm (roughing); 1 µm (finishing) 64 Development and performance evaluation of an intelligent ELID grinding machine continues until error value reaches below a tolerable limit Finally the finished part was removed form the machine to measure the profile radius in the CMM machine Surface roughness was measured in the Taylor–Hobson... stage) of the grits the actual chip formation occurs, which is desirable for grinding 66 Development and performance evaluation of an intelligent ELID grinding machine Fig 3. 11 Three modes of cutting in abrasive machining Again too low in- feed/DOC produces bad quality surface which is due to the reason that rubbing takes place rather than cutting at a very low indentation of the grits as explained earlier... radius of curvature is different from that of the desired value due to some machining error the first step is repeated until the achieved radius of curvature is within tolerance In this algorithm consideration of the 59 Development and performance evaluation of an intelligent ELID grinding machine START Generate the tool path for lens grinding by considering the grinding wheel radius using equations 3. 1... type of investigation shall be discussed in details 3. 3.1 Experimental setup and procedure The experimental setup for vertical groove grinding is schematically described in the figure 3. 7 The feed motion and the in- feed direction are also shown in the figure 61 Development and performance evaluation of an intelligent ELID grinding machine Fig 3. 7: Experimental setup for vertical groove machining The injection.. .Development and performance evaluation of an intelligent ELID grinding machine an LM guide So it was possible to move the probe up during grinding and save it from hazardous grinding conditions Table 3. 3 Specification of the touch probe Length (mm) 40.8 Diameter (mm) 2.5 Sense directions ± X, +Y, ± Z Unidirectional repeatability 1µm Working principle of on-machine form measurement and tool... and polishing, which reduces the amount of time and money usually needed for these post processes Fig3.19: Surface quality of the machined lens under Keyence optical microscope (X 30 00) Fig 3. 20 SEM observation of the surface (a) X1500 (b) X2500 Fig 3. 21: Photograph of the machined lens 73 Development and performance evaluation of an intelligent ELID grinding machine 3. 4 Concluding remarks In this chapter... the development of an intelligent ELID grinding machine tool for nano-surface generation Detailed discussions are also carried out on the performance of the developed system and following conclusions can be briefly summarized from this chapter: • A machine tool has been re-engineered with integrated sensory system for ELID grinding The machine is capable of producing optical surface with less than 20 . Development and performance evaluation of an intelligent ELID grinding machine 48 Chapter 3 Development and performance evaluation of an intelligent ELID grinding machine 3. 1 Introduction. desirable for grinding. Development and performance evaluation of an intelligent ELID grinding machine 67 Fig 3. 11 Three modes of cutting in abrasive machining Again too low in- feed/DOC. machine and its different smart monitoring systems shall be described elaborately. Development and performance evaluation of an intelligent ELID grinding machine 50 3. 2.1 ELID grinding