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Contents lists available at ScienceDirectCIRP Annals - Manufacturing Technologyjournal homepage: http: / / ees.elsevier. com/cirp/ default. asp CIRP Annals - Manufacturing Technology 59 (2010) 235238Wire electro discharge trueing and dressing of ne grinding wheelsA. KlinkWZL, RWTH Aachen University, Steinbachstr. 19, 52074 Aachen, GermanySubmitted by F. Klocke (1) (RWTH Aachen, Germany).A R T I C L E I N F O Keywords:Electrical discharge machining (EDM) DressingTrueingA B S T R A C T In this paper fundamental investigations are presented regarding the capabilities of Wire-EDM for trueing and dressing of ne grained metal bonded diamond grinding wheels. These wheels are often used for precision grinding operations of hard and brittle mould materials like ceramics or cemented carbides. They are characterised by high prole constancies and wear resistances. Due to the electrical conductivity of the bond material, Wire-EDM offers an efcient and powerful alternative to conventional trueing and dressing. Achievable grit protrusion and possible thermal damage to smallest diamond grits are theoretically and experimentally examined. Additionally, machining strategies for high prole accuracy are presented.。 2010 CIRP.1. IntroductionPrecision grinding is a key technology for the mould and die making industry. Especially the optical industry needs highest form accuracies and surface nishes. Fine grained metal bonded diamond grinding wheels can be used efciently for the precision machining of cemented carbide or ceramic mould inserts which are subsequently used in glass moulding operations 1,2. Due to the combination of nest diamonds with a metal bond the grinding operation can be carried out with a high prole constancy and high wear resistance. Best surface nishes can be achieved with Ra values smaller than 10 nm (depending on the applied grinding kinematics). In combination with an efcient and powerful trueing and dressing technology these grinding wheels offer the advantages of longer tool lives, the possibilities for an individual adaptation of the metal bond composition and higher proling exibility compared to nest grained single-layered electroplated diamond grinding wheels recently presented in 3 or coarse grained single-layered electroplated wheels described in 4.Conventional trueing and dressing of the ne grained metal bonded diamond grinding wheels is difcult and very time- consuming due to the high wear resistance of both grain and bond material. Thus, effective technologies must be available especially when small and precise tool prole geometries are needed. Due to the electrical conductivity of the metal matrix the unconventional material removal processes of EDM and ECM are often suggested as alternatives. These technologies can be used as in-process dressing mainly focussing on grit protrusion like Electrolytic In-Process Dressing (ELID), Electrochemical In-Process Controlled Dressing (ECD) and Electrochemical Point Grinding (ECPG) 57. EDM can also be used as a pre-process trueing and dressing technology focussing both on grit protrusion and proling of the grinding wheel. This can be done on-machine with an EDM adjusted grinding machine avoiding clamping deviations 8,9 or as an ofine technology using conventional EDM machines. Combina-tions of EDM and ECM are also known. A comprehensive survey of all different technologies can be found in 10. Basic research in the area of EDM trueing and dressing was limited so far to biggerabrasive grain sizes (20 mm), compare to examples given in9,11.This paper presents the capabilities of Wire-EDM trueing and dressing for ne grained (average grain size between 5 mm and 15 mm) metal bonded (bronze) diamond grinding wheels.Achievable grit protrusion and possible thermal damage to smallest diamond grits are theoretically and experimentally examined. Finally, machining strategies for high prole accuracies are presented. The process is applied on a conventional Wire-EDM machine.2. Principle of Wire-EDM trueing and dressingWire-EDM trueing and dressingcompared to Sinking-EDM offers the advantages that no complex shaped electrode and no additional wear compensation are necessary. In a rst assumption, the discharges of the EDM process mainly take place between the tool electrode and the electrically conductive metal bond material which therefore can be removed, Fig. 1. The electrically not conductive diamond grits are not directly involved in the process and remain in the eroded zone. By sticking out of the surface a certain grit protrusion can be achieved after the process.Due to the thermal material removal principle heat is locally applied to the grinding wheel surface and depending on the amount of heat conduction of the involved materials the ne diamond grains can be thermally damaged. Therefore a possible surface graphitisation of the ne grits has to be estimated and evaluated. In order to keep a possible damage to a minimum amount low discharge energiesas they are commonly used for Wire-EDMand longer pulse interval times for better ushing and cooling are favourable. The use of smaller discharge energies also avoids too much wire deviation during the process due to the geometrically caused long free wire length, Fig. 1.0007-8506/$ see front matter 。 2010 CIRP. doi:10.1016/j.cirp.2010.03.076236A. Klink / CIRP Annals - Manufacturing Technology 59 (2010) 235238Fig. 1. Principle of Wire-EDM trueing and dressing and example of machine set-up.Table 1Grit sizes and concentrations of used diamond super abrasives.D0 C0D1 C50D7 C50D15 C50D15 C100D46 C100Grit size (mm)013510122012203845Conc. (vol.%)012.512.512.52525D: Diamond C: Concentration.Three different bronze bondsa copper bronze (CuBz), an iron-based bronze with 70% of Fe (FeBz) and a cobalt-based bronze bond with 70% of Co (CoBz)were investigated. The material variation results in an increase of bond hardness for an individual adaptation to the grinding task. The additionally varied grit sizes and concentrations during trials are listed in Table 1.The machining of the composed grinding wheel layers by Wire- EDM is comparable to the machining of common steel or cemented carbide workpieces. Similaror at least in the same order of magnitudecutting rates and working gap widths can be achieved. Therefore, the already existing machining technologies can be adapted for the according trueing and dressing tasks. Fig. 2 shows a comparison of cutting rates of the different bonds for the machining in a CH-based dielectric under constant main cut conditions. No major negative inuence due to the increasing amount of electrically not conductive diamond gritslike in the case of PCDcan be identied. Even bigger cutting rates can generally be achieved for the initial addition of diamond particles (D0, D1 and D7) in all materials and for CuBz due to the changed thermo-physical properties.3. Analysis of grit protrusionMost important for the obtainment of a sharp grinding wheel is the achievement of a certain diamond grit protrusion by the trueing and dressing process. Best grit protrusion is dened as half of the grit diameter. If grits are excavated more by the process the bond clamping will be insufcient. Basic research on the different bond materials and diamond grit sizes revealed that during the primary material removal of the bond material by the Wire-EDM process a dened diamond grit protrusion can be achieved. This is valid for average grit sizes down to D7. For smaller sizes (D1) only aFig. 3. Diamond grit protrusion of cobalt-based bronze grinding wheel bond (CoBz D15) after application of Wire-EDM trim cuts without further offset changes to smooth roughness tips in water- and CH-based ling effect can be identied due to the micro topography of the EDM process. Within the crater-landscape no grit protrusion of the smallest grits can be identied.The grit protrusion can generally be achieved for both main and trim cuts, see example in Fig. 3a. Additional trim cuts offer the advantage of higher prole accuracy and a more at bond surface between the grits. But further basic research revealed that for CH- based dielectrics too many trim cuts (only for the removing of roughness tips and without any additional offset change) will result in at surfaces without grit protrusion, see Fig. 3b.This effect can be explained by the different formation of electric elds for water-based and CH-based dielectrics, Fig. 4. For CH-based dielectrics with a smaller relative permittivity compared to the permittivity of diamond the highest eld strength can be identied on the already protruding tips of the diamond grits. For water-based dielectrics with a much higher permittivity the highest eld strength appear on the metal bond and especially at the areas besides the grits. At the areas with the highest electric eld and the lowest dielectric strength the next discharge is likely to happen. Therefore, diamond grits in CH-based dielectrics have a higher risk for a complete removal especially when a thermal surface damage due to graphitisation of a former trim cut already exists. A more detailed description of these effects can be found in 12. Thus, too many trim cuts have to be avoided during Wire- EDM trueing and dressing.For the quantitative measurement of the grit protrusion a Stereo-SEM can be used ideally, Fig. 5. It allows the identication of diamonds in the surface and the according height measurement.Fig. 2. Wire-EDM cutting rates of bronze bonds (Bz) in comparison to steel, cemented carbide and PCD for machining in CH-based dielectrics.Fig. 4. Formation of the electric eld in CH-based and water-based dielectrics after the supply of voltage and before dielectric breakdown.Fig. 5. Quantitative analysis of diamond grit protrusion with a Stereo-SEM for CH- based dielectric under main cut conditions.A. Klink / CIRP Annals - Manufacturing Technology 59 (2010) 235238237Fig. 6. Edge and corner rounding effects of the diamond grains after the Wire-EDM trueing and dressing.General Rz or Ra topography values turned out not to be suitable for this.4. Analysis of thermal grit surface damageDiamond is known to metamorphose at higher temperatures from diamond to graphite modication. Under inert conditions this graphitisation takes place at 1500 8C. For catalytic environmentsthis temperature is much lower 1,13. Due to the thermal material removal principle of EDM, the diamond grits could thermally be damaged. This effect can be amplied by the changed surface/ volume-ratio of the smallest grit sizes. A detailed view on the grits reveals that a surface change takes place during process, Fig. 6. It can be seen that a rounding of edges and corners appears on the grits even for smallest discharge energies.The amount of this graphitisation was investigated using crystallographic analysis. Therefore, a small TEM-lamella was prepared out of one diamond grit in the eroded surface (CoBz D15) by FIB. First EDX-analysis of the grit revealed three carbon layers under two preparation layers, Fig. 7a. A HRTEM-analysis showed a crystalline structure for the inner layer and amorphous structures for the outer layers indicating graphitisation, Fig. 7b. The two outer layers have a thickness of about 500 nm.An EELS-analysis along a line scan of the three layers explicitly approved the existence of graphite in the two outer carbon layers and of diamond in the inner layer due to the appearance and non-appearance of the p*-peak in the according spectra, Fig. 8. It can beconcluded that a thermal damage of the grits takes place under theFig. 7. TEM overview and HRTEM detailed view of a diamond lamella after Wire- EDM trueing and dressing of a grinding wheel CoBz D15 C50.Fig. 8. Examination of the surface layer graphitisation of diamond grain lamella with STEM/EELS in the nanometre range.Fig. 9. Quality of prole and radial runout of the Wire-EDM trueing and dressing with controlled or constant wire feed in the processing direction.given conditions but is limited to a surface graphitisation in a magnitude of 0.5 mm.5. Machining strategies for high prole accuracyThe achievement of high prole accuracies is additionally very important for the Wire-EDM trueing and dressing. Due to the high exibility of the Wire-EDM process virtually any user-dened prole can be implemented on the grinding wheel only by adapting the programmed NC-code. The only restrictions for minimum inner radius is dened by the given wire diameter. For a reduction of wire deviations due to the high free wire length generally large wire diameters and high wire pre-loads are favourably.During the Wire-EDM of rotating parts the machining with a constant wire feed instead of a machine-controlled wire move- ment in processing direction is used, Fig. 9. Unfavourable frequency ratios of rotational grinding wheel speeds and machine control loops often result in only partial machining on the grinding wheel circumference. By the use of the constant wire kinematics a full proling of the whole circumferential area of the grinding wheel can be achieved with high process stability. Independent of the initial clamping deviation a high concentricity smaller than1 mm can be reached by the Wire-EDM trueing.In order to determine the best constant feed speed it has to be distinguished between the axial and radial wire kinematics, Fig. 10a. All proling geometries can be composed out of these two basic movements. The axial movement offers a relative open cut with good ushing conditions but the pure radial cut in direction of the centre is characterised by narrow gaps with worse conditions resulting in the need for a technology adaptation.Fig. 10. Process kinematics for the Wire-EDM trueing and maximum constant feed rate against proling depth for the axial and radial cut.238A. Klink / CIRP Annals - Manufacturing Technology 59 (2010) 235238Fig. 11. Surface roughnesses depending on the rotational speed of disks.Fig. 12. Prole examples of Wire-EDM trueing and dressing of ne grained diamond grinding wheels.The maximum constant wire feed rate nfmax can be calculated from the theoretically achievable material removal rate VW under the given process conditions (used materials, generator para-meters, etc.). This material removal rate has to be disposed for the material allocation at the rotating disk. For the axial movement Eq. (1) and for the radial movement Eq. (2)which can easily be derived from the drawings in Fig. 10acalculate the maximum possible feed rate. An exemplary calculation is done in Fig. 10b. For a given disk and material removal rate the maximum wire feed rate is shown against the designated prole height. Depending on a given minimum feed speed of the machine tool and a minimal processing depth (in order to equalise initial eccentricities) a resulting process window for a stable Wire-EDM trueing can be identied. V WAcknowledgementsThis work is funded by the German Research Association DFG within the Transregional Collaborative Research Center SFB/TR4 Process Chains for the Replication of Complex Optical Elements.References1 Klocke F, Klink A, Kamenzky S (2007) Electro Discharge Dressing of Fine Grained Metal Bonded Grinding Wheels. Proceedings of the 15th International Symposium on Electromachining 153158.2 Klocke F, Klink A, Schneider U, Gru ntzig A (2007) EDM- and ECM-Dressing of Fine Grained Bronze-Bonded Diamond Grinding Layers. Proceedings of 7th Euspen International Conference 2:253256.3 Aurich JC, Engmann J, Schueler GM, Haberland R (2009) Micro Grinding Tool For Manufacture Of Complex Structures In Brittle Materials. Annals of CIRPy f max p r22 a ra hP y V Wf max 2pra hPb(1)(2)58(1):311314.4 Heinzel C, Rickens K (2009) Engineered Wheels For Grinding Of Optical Glass.Annals of CIRP 58(1):315318.5 Ohmori H, Nakagawa T (1990) Mirror Surface Grinding Of Silicon Wafers With Electrolytic In-Process Dressing. Annals of CIRP 39(1):329332.6 Kramer D, Rehsteiner F, Schumacher B (1999) ECD (Electrochemical In-Process Controlled Dressing), a New Method for Grinding of Modern High-Perfor-In order to minimise geometrical deviations during the Wire-EDM trueing a small rotational speed of the grinding wheel is preferable, Fig. 11. With smaller speeds a better surface quality can be achieved on the circumferential surfaces due to less vibrations and more stable process conditions. Finally, in Fig. 12 and based on the considerations bef