See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/239406230 Investigation of ElectroDischarge Mechanical Dressing (EDMD) of Diamond Abrasive Wheels with Conductive Bonds Using Brush Electrodes Article in Proceedings of the Institution of Mechanical Engineers Part B Journal of Engineering Manufacture · June 2006 DOI: 10.1243/095440505X32922 CITATIONS READS 12 89 3 authors, including: B. Nowicki Sławomir Spadło Warsaw University of Technology Politechnika Świętokrzyska 19 PUBLICATIONS 136 CITATIONS 60 PUBLICATIONS 182 CITATIONS SEE PROFILE All content following this page was uploaded by Sławomir Spadło on 14 January 2015. The user has requested enhancement of the downloaded file. SEE PROFILE SPECIAL ISSUE PAPER 421 Investigation of electro-discharge mechanical dressing (EDMD) of diamond abrasive wheels with conductive bonds using brush electrodes B Nowicki1, R Pierzynowski1*, and S Spadło2 1 Faculty of Production Engineering, Warsaw University of Technology, Warsaw, Poland 2 Faculty of Mechanical Engineering, University of Technology in Kielce, Poland The manuscript was received on 2 February 2005 and was accepted after revision for publication on 22 September 2005. DOI: 10.1243/095440505X32922 Abstract: The authors have presented a new method of electro-discharge mechanical dressing of ultra-hard abrasive tools with metal bonds in which brush electrodes were used. This kind of machining is denoted as EDMD (electro-discharge mechanical dressing) and combines features of electro-discharge machining with mechanical treatment so that thin layers of the bond are removed. Moreover, the brush electrode offers the additional possibility of introducing the advantageous (having the anchorage of the abrasive grains in mind) state of compressive stress and strain hardening of the outer layer of the bond. Keywords: electro-discharge machining, surface layer, dressing, diamond abrasive wheels 1 INTRODUCTION The progress observed in the field of materials engineering and the improved quality of the mechanical treatment (hardness, material strength, wear resistance) of the construction materials is accompanied by an increase in the abrasive machining share in manufacturing processes and by an increase in demand for modern abrasive tools. The abovementioned considerations necessitate notable progress in the development of modern abrasive tools including metal-bonded grinding wheels and it has recently taken place. The diamond tools, taking the forms of wheels, honing stones, abrasive strips, etc., with metal bonds, are used more and more widely in industries such as mechanical engineering, glasswork, optical, stonework, geological drilling, dental services, and many others and their share in abrasive machining approaches 20 per cent. The principal drawback of dressing the ultra-hard materials results from the abrasive hardness and high strength of the metal bond. A few methods are used in profiling and sharpening diamond *Corresponding author: Faculty of Production Engineering, Warsaw University of Technology, Al. Niepodleglosci 222, r. 162, Warsaw, 00-663, Poland. email: rpierzyn@meil.pw.edu.pl JEM294 IMechE 2006 wheels with metal bond apart from dressing: electro-discharge machining (EDM), electrochemical machining (ECM), abrasive machining of honing sticks, abrasive jet, and so on [1, 2]. The operation of such grinding wheels includes wear of the abrasive grains, which results in lowering the height of protrusions over the bond surface, dulling the corners and cutting edges, gumming the wheel active surface with the erosion products, and changes in the wheel macrogeometry. In the wake of these processes, grinding wheels become deprived of their operational features (mainly their cutting properties) [3, 4]. Sharpening of the diamond wheels should imply minimum wheel loss because of their high manufacturing costs [5, 6]. The standard methods of sharpening used so far are connected with the technical problems and are in numerous cases impractical or inefficient. This requirement is met in the case of a new method for electro-erosion wheel sharpening, as suggested by the authors, where rotary brush electrodes have been employed. This method has been denoted as EDMD (electro-discharge mechanical dressing). It combines features of electro-erosion and mechanical machining without disadvantageous thermal interactions. EDMD enables thin bond layers to be removed and contributes to the Proc. IMechE Vol. 220 Part B: J. Engineering Manufacture 422 B Nowicki, R Pierzynowski, and S Spadło introduction of a favourable state of stress, advantageous for locking the abrasive grains in the wheel bond and for strain hardening of the outer layer of the bond. EDMD is an effective method of sharpening the abrasive tools of the following kinds: abrasive wheels, hones, multilayer abrasive tapes containing metal bonds, etc. 2 REVIEW OF THE EXISTING METHODS FOR SHARPENING THE METAL-BONDED ABRASIVE TOOLS Analysis of the methods used so far for the ultra-hard abrasive wheels indicates their diversified usefulness. Conventional methods of wheel dressing, which used to be simple and popular, cannot be used for the effective shaping of the ultra-hard active wheel surfaces. Their disadvantages consist in superfluous wear of the expensive wheels and many problems with automation of the sharpening process [5]. Methods of abrasive microdressing that can be used for removing the abrasive layers of several mm in thickness are difficult for practical realization and can be employed in a limited range for sharpening wheels intended for particularly fine grinding [7]. Methods of abrasive jet sharpening do not ensure uniformity of the operating conditions within the jet range and are difficult to employ on the grinders because they are hazardous for the environment and for the machine tool durability. Electro-discharge sharpening employing typical conditions of the process run can introduce thermal-induced damage of the abrasive grains and it results in an unfavourable state of stress in the surface layer. The practical implementation of this method necessitates the use of expensive instruments such as generators, control systems for the working electrode feeding movement, systems for the dielectric supply etc. Because of the above, such methods are rarely used [8]. Electrochemical dressing employing direct current is actually considered as the most prospective method for shaping the ultra-hard active wheel surfaces [1]. Trials concerning the ECM including typical conditions are much limited. The basic disadvantages consist of difficulties in shaping the wheel active surface and restoring its cutting properties which have been lost during grinding. A lot of research has been devoted to this issue [1, 7, 8]. Disadvantages of the above-mentioned methods justify the purposefulness of continuing research in the field of the unconventional methods of sharpening while paying special attention to the combined erosion and mechanical methods being analysed by the authors. Proc. IMechE Vol. 220 Part B: J. Engineering Manufacture 3 PRINCIPLES OF THE EDMD METHOD Analysis of the erosion–mechanical machining with a rotary brush electrode application [9, 10] and preliminary research show that it is possible to sharpen the abrasive tools with the EDMD method. The method is featured by discharges at a low energy level and by mechanical interaction of the elastic elements where the rotary brush is an electrode, resulting in material removal from the active wheel surface. Then, the abrasive grains are exposed because of the bond material removal due to electric erosion (which is responsible for the sharpening effect in the first place); however, mechanical hardening of the bond superficial layers also takes place due to the dynamic interaction of the brush elements with the metal bond. Thus, in the method presented, the controllable compressing stress can be introduced into the surface layer of the metal bond (it can be controlled by the appropriate selection of the process parameters). The authors’ own experimental research proved that machining with the pulse generator in the ‘off ’ position results in inducing compressive stress in the surface layer of the grinding wheel subjected to the machining process [9]. The advantage of the presented method is that the spark voltage employed in this EDM variety is considerably smaller than in the case of standard EDM (the power supply voltage in the beginning is considered to amount to 3–5 V and is just 1/50 of the voltage that is necessary to initiate discharges in a typical EDM case). The low-voltage discharges are initiated by the sudden rupture of contact between the bond and the brush filaments (due to the brush rotary motion or vibrations of the brush wires). Generation of the electric discharges is possible only in the area of contact between the brush and the conductive bond. Selection of the proper machining parameters (voltage, wire diameter, wire material, etc.) makes it possible to remove thin layers of bond in a controllable way, without any thermal damage to the diamond grains. The diagram in Fig. 1 shows the erosion– mechanical dressing process used in the case of abrasive tools. In the EDMD process, the work electrode is in the form of a rotary brush (2). The machining processes are carried out in the presence of a working fluid which is supplied by a nozzle (3). The mutual location of the tool and the workpiece is such that the individual wires of the brush are deflected by the D value during the brush motion along the machining surface and the pressure value is high enough to initiate a spark discharge. After the discharge has been initiated and the plasma channel has been formed, a sudden local temperature increase of the microroughness peaks is noticed and melting, evaporating, and bond material JEM294 IMechE 2006 Investigation of EDMD of diamond abrasive wheels 423 Fig. 1 Schematic diagram of the erosion–mechanical dressing process (EDMD) principle removal take place. The main factors contributing to the EDMD process are: electro-erosion, mechanical, and thermomechanical interactions. 4 RESEARCH AND RESULTS Initial experiments [9, 10] made it possible to determine the most important parameters that influence the machining process. In order to determine the ranges of machining parameters, a series of singlefactor experiments have been performed where the following variables have been taken into account: machining voltage (U), brush electrode deflection (D), wire diameter (d), peripheral speed of the electrode (v0), and feed rate (vf). The wires made of constructional steel of standard quality St5, tungsten, and stainless steel 1H18N9 were used in the experiment. In the preliminary investigations a water solution of sodium silicate of low concentration, air, water, and a typical coolant for the grinding process were used as working media. Research has shown that using air as the natural environment initiates the graphitization effect for much lower machining voltages than technological water and water–oil emulsion. The results of investigations of the efficiency of diamond wheel sharpening in the above-mentioned media were shown in Fig. 2. Water–oil emulsion has been selected for further experiments as the natural medium for the grinding process. It ensures good results of diamond wheel sharpening and because it is a regular grinding medium there is no need to use extra hydraulic systems for the liquids that are employed in wheel sharpening. JEM294 IMechE 2006 Machining with wire electrodes at low voltages is sensitive to selection of proper force applied by the brush to the machined surface. Optimum forces Fn range between 1 and 3 N. For the force values above 3 N short-circuits are prevalent and for forces lower than 1 N the interactions are too weak to initiate sparks. The diagrams of voltages for various loads were presented in Fig. 3. Analysis of the experimental results made it possible to determine the variance ranges of the following machining parameters: deflection D ¼ 0.1–1 mm, wire diameter d ¼ 0.1–0.7 mm, peripheral speed of the wheel v0 ¼ 3.76 m/s, feed rate vf ¼ 4.7 mm/min, and voltage U ¼ 3–12 V. The advantageous localization of the discharges and their low energy do not have an adverse effect on the diamond grains (graphitization, cracking, and crush dressing) and they do not cause tensile stresses or microcracks in the bond surface layers, or even high roughness of the active wheel surface that diminishes the cutting properties of the wheels. It has been confirmed in the hitherto performed investigations that the spark discharges of low-energy level do not contribute to the diamond grain destruction and that they effectively remove the small chips gummed up to the active wheel surface. The usage of brush electrodes in the process of sharpening metal-bonded grinding wheels is connected with the additional abrasive grain support. It is because of overshadowing of the areas placed directly behind the abrasive grains that are attacked simultaneously by the unilateral interaction of the brush wires and erosion (Fig. 4). As a result of the favourably shaped stereo metric active wheel surface features, the force of locking the grains in the bond is Proc. IMechE Vol. 220 Part B: J. Engineering Manufacture 424 B Nowicki, R Pierzynowski, and S Spadło Medium mass decrement ∆m [mg] 12,5 10 7,5 5 2,5 0 air water deionised water water-oil emulsion Fig. 2 Relationship between the efficiency of diamond, metal-bonded hone sharpening, and the employed medium type (a) (b) (c) Fig. 3 Typical voltage diagrams for various forces applied by the brush: (a) Fn ¼ 4 N – excessive, (b) Fn ¼ 1 N – insufficient, (c) Fn ¼ 2 N – proper larger than for any other methods. The strain hardening phenomenon of the surface bond layers is an important factor, contributing to the increase in the locking force mentioned above. It results from the dynamic interaction between the working electrode wires and the wheel bond material. A further improvement in the grain locking will be possible if the following two processes are consecutively applied: erosion–mechanical machining and mechanical interaction of the brush without the electric power supply. The latter treatment is used for diminishing the tensile stress level or generating the compression stress in the surface layer, and consecutively for the improved locking of the abrasive grains. The desired level of the stress and strain hardening can be obtained by appropriate selection of the mechanical process. The applied experimental investigations have been performed as a two-stage procedure: (a) investigation of how the EDMD conditions influence the results, i.e. efficiency of Proc. IMechE Vol. 220 Part B: J. Engineering Manufacture sharpening and state of the cutting grains and wheel active surface; (b) investigation of the changes of the wheel active surface state and results of grinding using wheels that were sharpened by the presented method, compared with the results obtained using the same wheels, which were sharpened by a conventional method. Examination of the influence of sharpening a diamond wheel with a metal bond on the wheel active surface state using the EDMD method has been performed with the metal-bonded hones of size L · B · H ¼ 10 · 5 · 8 mm and grain size a ¼ 400 mm. The layout of the experimental set-up for investigating the wheel sharpening process is presented in Fig. 1. The hones were fixed in a vice and were connected to the positive pole of a d.c. generator, with a rotary brush as the working electrode connected to the negative pole of the generator and slightly pressed against the hone surface. The brushes of diameter D ¼ 80 mm and wires of conventional carbon steel and diameter d ¼ 0.2 mm were used. Preliminary investigations of the EDMD process, which were carried out for the following media, industrial water, water–oil emulsion, and the air, have shown that water–oil emulsion should be further investigated as a natural media for the EDMD processes using grinders. The most important investigations were carried out according to an experimental design principle, with the following independent variables: (a) brush rotational speed; (b) force exerted by brush wires on the surface; (c) interelectrode voltage. Photographs of typical hone active surfaces after being sharpened by the EDMD method are shown in Figs 5(a) and (b). The hones which were sharpened at U ¼ 9 V are featured by distinctly exposed grains with sharp corners and edges which are present well above the bond level. JEM294 IMechE 2006 Investigation of EDMD of diamond abrasive wheels (a) 425 (b) Fig. 4 SEM photograph of an individual grain on the hone surface: (a) sharpened by the EDMD (with visible effect of the grain support), (b) worn grain (·100) (a) (b) Fig. 5 Image of surfaces of hones sharpened at working voltages: (a) U ¼ 9 V, (b) U ¼ 12 V (visible traces of graphitization have been marked) (a) abrasive grains after machining at voltages U lower than 9 V (a ¼ 400 mm) do not display any signs of graphitization and are featured with sharp and not damaged corners and edges; JEM294 IMechE 2006 80 Mass decrement [mg] 70 60 50 40 30 ` 20 1300 10 700 3 4 6 5 8 7 9 0 10 The surface of the hone sharpened at U ¼ 12 V is presented in Fig. 5(b). The dark diamond grains with rounded corners and edges are visible on the surface and result from graphitization induced by the interaction of spark channels transporting high energy. The relationship between machining conditions and hone mass decrement has been shown in Fig. 6. This relationship is useful when calculating the time necessary for grinding wheel sharpening with the predetermined thickness of the removed wheel layer. The analysis of the investigation results has shown that: 100 Rotational speed n [rpm.] Voltage U [V] Fig. 6 Efficiency of diamond hone sharpening versus machining parameters (water–oil emulsion as medium, d.c. generator) Proc. IMechE Vol. 220 Part B: J. Engineering Manufacture B Nowicki, R Pierzynowski, and S Spadło (b) behind the grains there is the so-called shaded zone, a strip (elevation) of the bond material that can operate as an additional grain support during the grinding process; (c) after a prolonged machining process without the transverse movement of the hone relative to the brush electrode, microgrooves may appear on its surface as a result of multiple interaction from the wires in the same place of the hone; (d) application of voltages U greater than 10 V (a ¼ 400 mm) may result in partial or total graphitization of the diamond grains; The performed investigations have shown that for a wide range of voltages, wire diameters of the brush, and kinematics parameters of the EDMA process, a satisfactory condition of the wheel active surface can result. The investigations of the results of machining with the grinding wheels have been carried out on a type SPC-20 plane grinder. The experiments included the following measurements: (a) cutting forces (Fy and Fc) using a strain gauge dynamometer; (b) volume of material (tungsten carbide) removed in the grinding process; (c) linear removal rate (for standard times of 5 and 10 s) for a special sample made of tungsten carbide, pressed against the wheel surface with constant force; (d) roughness of the ground element and a special sample of tungsten carbide; (e) wheel active surface roughness as measured on the replication surface; (f) visual inspection of the wheel surface condition as based on the microscopic images. The wheel active surface has been examined by special replication surfaces which mirrored the active surface of the grinding wheel. Then, the active wheel surface was measured with a Talysurf-10 profilometer equipped with a computer system for analysis of the surface geometric structure. The wheel decrement (wear) has been determined using an artificial basic method. The wheel surface was marked with scratches of special geometry before the machining and after each experiment a replication of this scratch was made and then its depth was measured using a profilometer. The experiments included wheels sharpened by EDMD with appropriate parameter levels applied. Moreover, for comparison purposes these wheels were sharpened using a block of the abrasive material; this method is the most widely used in industrial practice. The voltage applied when sharpening by the EDMA method has been selected within the Proc. IMechE Vol. 220 Part B: J. Engineering Manufacture range where graphitization does not occur, namely U 6 9 V, and sharpening time was selected based on the results of the previous hone sharpening so that the cutting grain edges are exposed up to a given level above the bond and it is usually equal to 30–50 per cent of the average grain size a. All measurements have been carried out for strictly defined periods of the cutting wheel operation time t ¼ 1, 2, 4, 8, 16, 32, 48, and 64 min. Figure 7 shows diagrams of the tangential force for grinding of tungsten carbide with the diamond grinding wheel, grain size a ¼ 100 mm, metalbonded, sharpened conventionally, and by EDMD applying various parameters. Analysis of the diagrams shows that only for very short machining times there are differences between the tangent component of the cutting force measured for conventional and EDMD sharpening. The Fc force in the latter case is up to 40 per cent larger when the grinding time t ¼ 1–2 min. There are no significant differences between the relevant forces for t > 4 min. The relationship between productivity and machining time is similar (Fig. 8). Differences that count appeared only in the case of conventionally sharpened wheels when machining times were lower than t ¼ 4 min (right after the wheel sharpening). For a longer machining time the differences are insignificant and the highest efficiency was obtained for the wheel sharpened by EDMD and for the highest parameters of sharpening. Figure 9 shows typical roughness profilograms for the tungsten carbide ground in the experiments. Analysis of the profilograms shows that the roughness height for surfaces sharpened by the EDMD method is a little lower than in the case of machining with wheels sharpened by the conventional method. 6 5 Fc force [N] 426 ceramic 4 E 3V 17 min 3 E 3V 30 min 2 E 4V 5 min 1 0 0 1 2 4 8 16 32 46 64 machine time [min] Fig. 7 Tangential grinding force versus time for a grinding wheel sharpened by EDMD (with various machining parameters applied:E3V17min denotes the EDMD method, U ¼ 3 V, t ¼ 17 min; E3V30min denotes the EDMD method, U ¼ 3 V, t ¼ 30 min; E4V5min denotes the EDMD method, U ¼ 4 V, t ¼ 5 min) and by the abrasive method which denotes ceramic JEM294 IMechE 2006 Height decrement [mm] Investigation of EDMD of diamond abrasive wheels 427 ceramic E 3V 17 min E 3V 30 min E 4V 5 min 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0 0 1 2 4 8 16 32 46 64 machine time [min] Fig. 8 Grinding efficiency for sintered carbide sample ground versus time for a grinding wheel sharpened by EDMD (various machining parameters were used:E3V17min denotes the EDMD method, U ¼ 3 V, t ¼ 17 min) and by the abrasive method, which denotes ceramic (a) (b) (c) (d) Fig. 9 Surface profilograms for sintered carbide sample ground by the wheel sharpened by EDMD (various machining parameters were used) and by the abrasive method. Conventional method: (a) t ¼ 2 min (Ra ¼ 0.54 mm, Rt ¼ 4.7 mm); (b) t ¼ 48 min (Ra ¼ 0.51 mm, Rt ¼ 4.0 mm); EDMD method: (c) t ¼ 2 min (Ra ¼ 0.43 mm, Rt ¼ 4.2 mm); (d) t ¼ 46 min (Ra ¼ 0.47 mm, Rt ¼ 3.8 mm) For both methods, the roughness of ground surfaces decreases slowly with passing time. 5 CONCLUSIONS Experimental investigations of diamond metalbonded grinding wheels sharpened by the EDMD method have shown the following. 1. Localization of discharges and application of the brush electrodes and d.c. generator which enables the generation of low-voltage electric discharges of low-energy level do not cause graphitization of the diamond grains. 2. Application of the mechanical interaction of the brush elements in the final stage of dressing JEM294 IMechE 2006 results in reinforcing the surface layer of the bond and contributes to generation of compression stresses that are advantageous with regard to locking the grains in the bond material. 3. The open-circuit nature of the discharges causes the electric erosion processes to be localized in the areas of contact between the working electrode elements and the conductive bond material. 4. Application of the flexible electrodes in the form of rotary brushes enables both preparation of the active abrasive tool surface and restoration of its cutting ability. 5. The cutting capabilities of the diamond wheels sharpened by EDMD are close to the capabilities of conventionally sharpened wheels. Proc. IMechE Vol. 220 Part B: J. Engineering Manufacture 428 B Nowicki, R Pierzynowski, and S Spadło 6. The EDMD process enables sharpening time reduction by a few times when compared with the conventional method. 7. Essential differences of the above features appear only for short machining times, t ¼ 1–4 min, and for these time intervals conventionally sharpened wheels display more favourable properties than their EDMD counterparts. These differences do not mean any significant influence on the grinding wheel properties when considering its whole life range after dressing (60–90 min). 8. The EDMD process can be used for removing very thin bond layers which economizes usage of expensive diamond and cubic boron nitride materials. 9. Sharpening of diamond metal-bonded grinding wheels by the EDMD can be used effectively directly on the grinders with the presence of water–oil emulsion, which is widely used as a grinding coolant. 10. The equipment needed for the EDMD process is simple and can be installed on any grinder. 6 7 8 9 10 Szkoła Obróbki Ściernej on Podstawy i Technika Obróbki Ściernej, Koszalin, 1993, pp. 129–144. Schopf, M. Dressing of metal-bonded grinding wheels by ECDM. Ind. Diamond Rev., 2002, 64(2), 82–85. Schöpf, M. Truing and dressing of high precision grinding tools with unconventional machining. In Proceedings of 2-dn Euspen Information Conference, Turin, Italy, 2001, pp. 816–819. Bachtiarov, S. A. Rabotaspasobnost almaznych krugov posle kontaktno-erozionnoj prawvki. Stanki i Instrument, 1989, 18–20. Nowicki, B. and Spadło, S. Smoothing the surface by brush electro-discharge mechanical machining – BEDMM. Central European Exchange Program for University Studies Project PL-1 CEEPUS, Project Report, 1998, pp. 129–137. Nowicki, B., Pierzynowski, R., and Spadło, S. The forming of surface roughness by anodic-mechanical machining with a discrete electrode. In CIM’99, Opatija, Croatia, 1999, pp. 33–40. APPENDIX Notation REFERENCES 1 Goła˛bczak, A. Elektrochemiczne ostrzenie ściernic z zastosowaniem pra˛du przemiennego (Electrochemical wheel sharpening using alternate current), Zeszyty Naukowe Politechniki Łódzkiej. Nr 762, Łódź, 1996. 2 Tönschoff, H. K., Karpuschewski, B., Andrae, P., and Türich, A. Grinding performance of superhard abrasive wheel. Ann. CIRP, 1998, 47(2), 723–732. 3 Oczoś, K. and Porzycki, J. Szlifowanie (A grinding process), 1986 (WNT, Warszawa). 4 Koziarski, A. Czynna powierzchnia ściernicy (An active wheel surface), 1996, (Wyd. P.Ł.). 5 Koziarski, A. Metody obcia̧gania ściernic ze ścierniw supertwardych (Methods for dressing of grinding wheels made of ultra-hard materials). In XVI Naukowa Proc. IMechE Vol. 220 Part B: J. Engineering Manufacture View publication stats a d D E ECM EDM EDMD Ra, Rt t U vf v0 grain size diameter of the wire diameter of the brush electrode impulse energy electrochemical machining electro-discharge machining electro-discharge mechanical dressing roughness parameters machining time voltage feed rate rotational speed of the brush electrode D value of the deflection of the tool components impulse time duration t JEM294 IMechE 2006