On the description of the bearing capacity of electro-discharge machined surfaces G.P Petropoulos a, C. Pantazaras a, N.M. Vaxevanidis b and A. Antoniadis c a Department of Mechanical and Industrial Engineering, University of Thessaly, Pedion Areos, 383 34 Volos, Greece b Hellenic Air-Force Academy, Dekelia Air-Force Base, Attica, Greece c Department of Natural Resources Engineering, Technological Institute of Crete, 3 Romanou Str., 73133 Chania, Crete, Greece Abstract: The present study concerns with the investigation of a set of non-common surface topography parameters of EDMed surfaces that describe their bearing capacity in tribological applications. The parameters considered are the Abbott (bearing) curve parameter at 10 per cent of the raw unfiltered and the roughness profile height P tp10% and R tp10%, respectively and the R k family of parameters (DIN 4776, ISO 13565-2). Keywords: EDM, surface topography, Abbot curve parameters, R k family of parameters. 1. INTRODUCTION Among the various non-conventional processes electro-discharge machining (EDM) is the most widely and successfully applied for the machining of various workpiece materials. The material is removed by means of repetitive spark discharges that cause local melting and/or evaporation of the workpiece material and the resulted surface is characterized by overlapping craters and features indicative of the intense thermal impact involved [1]. Besides the study of surface integrity changes (including surface topography) induced by EDMachining [2] of major interest is the correlation of the surface topography parameters with the machining conditions towards the advanced control and optimisation of the EDM. In a previous work [3] emphasis was directed towards the multi-parameter analysis of the EDMed surface texture interrelationship to process parameters and statistical regression models were developed to correlate the machining conditions with the imparted surface finish characteristics. The present study concerns with the investigation of a set of non-common surface topography parameters of EDMed surfaces that describe their bearing capacity in tribological applications; to the authors knowledge this topic is somehow overlooked in technical literature. Surface analysis followed deals with three aspects, namely: (a) correlation of bearing curve parameters with machining conditions and other surface texture parameters, (b) representation
of bearing curves through polynomial fitting models and (c) description of bearing curves by the set of R k parameters. 2. EXPERIMENTAL EDMachining was performed on a HOSTEK SH-38GP (ZNC-P type) electro-discharge machine-tool with working voltage of 30V and open circuit voltage of 100V. Experiments were conducted in a typical oil dielectric (BP250) with electrolytic copper being used as the tool electrode (anode). The pulse current, i e and the pulse-on time, t p considered to be the main operational parameters varied over a range from roughing to finishing, namely: i e : 5, 10, 20, 30 A and t p : 100, 300, 500 µsec, thus resulting in 12 discrete pulse energies. Specimens of plain carbon steel Ck60 in the form of square plates of dimensions 70mm x 70mm x 10 mm were used as workpieces (cathode). The surface texture parameters under study are the Abbott (bearing) curve parameter at 10 per cent of the raw unfiltered and the roughness profile height P tp10% and R tp10%, respectively and the R k family of parameters (DIN 4776, ISO 13565-2), namely: R k : core roughness depth, M r1 and M r2 : material portion, R pk : reduce depth height and R vk : reduced valley height. The multi-parameter surface texture analysis was performed using a Rank Taylor-Hobson Surtronic 3+ profilometer equipped with the Talyprof software. The cut-off length was selected at 0.8 mm whilst 40 measurements were conducted on every specimen at random directions. 3. RESULTS AND DISCUSSION The Abbott (material ratio or bearing) curve is nowadays established as a mean for providing a working representation of the portions of the surface at different depths combining texture aspects related to contact area and contact mechanics, wear, lubricant retention and others [4]. Other views of the surface bearing capacity can be provided by the skewness of the profile height distribution and the fractal dimension. These topographic parameters apart from connecting the EDM process to functional behaviour of the machined surfaces can also describe useful surface features and generally to give useful insights into the nature of roughness regarding the EDM process control. 3.1 Correlation of bearing curve parameters with machining conditions The bearing curve parameters corresponding to the raw and the roughness profile were selected at 10% depth in order to be functionally significant. The variation of P tp10% and R tp10% parameters with pulse energy is presented in Figs 1 and 2 respectively. Both parameteres increase when the pulse energy increases, over a relatively wide range. This behaviour implies that the bearing capacity of the surface becomes pronounced at the same level, when intensifying the machining condition. A similar trend is followed if the Abbot parameters are considered with respect to R a and R sk. Note, that R sk (kurtosis) can be considered as a rough measure of the surface bearing capacity and as it is shown generally correlates with the R tp (Abbot curve) parameter.
Figure 1. Variation of P tp10% parameter of unfiltered profile with pulse energy, W e. Figure 2. Variation of Rtp10 % parameter of roughness profile with pulse energy, W e. 3.2 Representation of bearing curves through polynomial fitting models Preliminary analysis and previous researches indicated that 3 rd order polynomial fitting model is the best representation of bearing curves [5]. Due to limitation of space only the raw profile bearing curves fitting according to equation, y = k o + k 1 x + k 2 x 2 + k 3 x 3 is presented. Calculated fitting coefficients are shown in Figs 3 and 4. An intense dropping trend of the cubic coefficient appears at high pulse energies. All these changes in the shape of the bearing curves are closely related to contact mechanics and wear behaviour [6]. Figure 3. Polynomial fitting coefficient against pulse energy, W e (k o : constant, k 1 : linear). Figure 4. Polynomial fitting coefficient against pulse energy, W e (k 2 : square, k 3 : cubic). 3.3 Description of bearing curves by the set of R k parameters
The variation of these parameters with pulse energy for all EDMed specimens is plotted in Figs 5-9. Judging from these plots it is obvious that ISO 13565-2 standard suits EDM, as the bearing curves are of a general s- shape and the parameters vary in a monotonous way. Note however, that this standard was originally formulated for stratified textures and therefore present results need further clarification. Figure 5. Variation of R k with pulse energy, W e. Figure 6. Variation of R pk with pulse energy,w e. Figure 7. Variation of R vk with pulse energy, W e. Figure 8. Variation of M r1 parameter with pulse energy, W e. Figure 9. Variation of M r2 parameter with pulse energy, W e. 4. CONCLUSIONS Close correlation exists between the tp bearing curve parameters of the raw and the roughness profile and the pulse energy regarded as the main machining independent variable. as well as the roughness parameters considered. The bearing curves are represented by a 3 rd degree polynomial model which fits them satisfactorily. In general, the coefficients of the model change monotonously. Changes in the shape of the bearing curves are closely related to contact mechanics and wear behaviour of machined surfaces. The R k set of parameters (ISO 13565-2) standard can provide description of crucial portions of the profile and, are also related to the machining condition. This is expectable owing to the near s- shaped bearing curves of the EDMed surfaces. REFERENCES
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