外文原文.pdf

Journal of Materials Processing Technology 160 2005 348 353 Controlling of metal removal thickness in ECM process M S Hewidy Department of Production Engineering and Machine Design Faculty of Engineering Menoufi a University Shebin El Kom Egypt Received 18 April 2001 received in revised form 6 March 2002 accepted 6 August 2003 Abstract Electrochemicalmachining ECM offerstheuniqueadvantageofbetteraccuracyandhighsurfaceintegrityofhard machinedcomponents A new technique has been developed to utilize a simultaneously moving and rotating electrode to remove a specifi c amount of material from pre machined holes and rods of hardened steel specimens One of the electrodes was provided with two simultaneous movements traverse speed and rotational speed The electrolyte was pumped into the gap between the tool and the workpiece through a matrix of fi ne holes distributed along one of the electrode surface A mathematical model has been proposed for accurately estimating the thickness of the workpiece layer under different working conditions Experimental results revealed that this technique could lead to the removal of a surface layer thickness up to 200 which consequently classifi ed this method as a super fi nishing process Finally the results of the experiments and the simulation are compared with each other The obtained results are an endeavour to enhance the controllability of the ECM process 2004 Elsevier B V All rights reserved Keywords Electrochemical machining ECM Hole fi nishing Electrolyte Shape prediction Surface integrity 1 Introduction Electrochemical machining ECM is being increasingly used in aircraft and aerospace industries due to its ability of producing complex shapes of very hard materials 1 Sur face fi nishing of the advanced hard materials poses many problems particularly those related to accuracy and surface integrity of the components Recently the ECM process has been recommended as the unique solution to get better ac curacy and high surface integrity of the machined surfaces after the electrodischarge machining process 2 4 On the other hand it is felt that the capabilities of the ECM pro cess of fi nishing of the pre machined shapes have not been fully mature due to the diffi culties encountered in the sup ply and the distribution of the electrolyte in the interelec trode gap 5 6 Electrochemical fi nishing of holes has been investigated by different researches 7 11 Modifi cation of tool shape 7 rotating of the tool 8 9 and electrochemi cal honing 10 were the main techniques which have been Tel 20 123506972 fax 20 403310505 E mail addresses mhewidy m hewidy3 M S Hewidy adopted to improve the electrolyte distribution In the ECM process ithasbeenreportedthattheelectrolytepressuredrop is usually associated during these methods Also the chance of changing the electrolyte conductivity was associated with the electrolyte circulation in the machining gap 12 This re sulted in decreasing the control of the dimensional accuracy and sometimes leads to poor surface quality It has also been observed that there is a deviation of the fl atness of the ma chined workpieces at the place where the electrolyte enters and leaves 13 In the present work it has been suggested to apply the ECMprocessusingsimultaneouslymovingandrotatingelec trode to remove amounts of material from hole or from rod surface The electrolyte was pumped into the gap between tool and workpiece through a matrix of fi ne holes distributed along one of the electrodes as shown in Fig 1 2 Theoretical model For accurate modeling of the ECM process the succes sive removal thickness of the workpiece surface can be cal culated considering the effect of each tool element length 0924 0136 see front matter 2004 Elsevier B V All rights reserved doi 10 1016 j jmatprotec 2003 08 007 M S Hewidy Journal of Materials Processing Technology 160 2005 348 353349 Fig 1 Hole and rod fi nishing by ECM process Fig 1 shows that the tool and workpiece geometry at side gap is equal to Ysi During the ECM process the initial side gap value will increase according to the forward motion of the electrode tool Each element of the tool electrode b causes an increase in the workpiece radius value equal to Ysi Depending on the tool length B and the other work ing conditions of the ECM process the workpiece diameter Dwi will be determined according to the relationship Dwi Dt 2Ysi 1 where Dtis the tool diameter The electrolytic area A can be calculated as follows A 4 D 2 wi D2 t 2 The electrolyte velocity Vsi can be computed from the following equation Vsi Q Ai 3 where Q is the electrolyte fl ow rate mm3 min The electrolyte velocity varies according to the change in the cross sectional area of the fl ow The metal removal thicknessalongthesidegapcanbecomputedasfollows 13 Ysi ZJsi t 4 where Zistheeffectivemetalremovalrate Amm3 min and can be computed as follows Z w F 5 where is the chemical equivalent wis the workpiece den sity g cm3 and F is the Faraday s constant The current density at the workpiece surface Jsiat the initial side gap value Ysiis commonly expected as follows 13 Jsi V V Ki Ysi 6 where Kiis the electrolyte conductivity and Ysiis the side gap V is the applied voltage and V is the over potential value 10 The successive machining time interval t can be esti mated from the tool land interval b and the feed rate f according to this equation t b f 7 Thewidthofthesidegapattheendof btoolelementlength can be expressed as follows Ysi 1 Ysi Ysi 8 where Ysiis the metal removal thickness for tool element length b at side gap Ysi The conductivity of the two phase medium Kican be de termined as follows 10 Ki K0 1 1 n 9 where K0is the conductivity of the electrolyte in the tank is the temperature coeffi cient of the electrolyte conductivity atT0 istemperatureriseforeachelectrodeintervaland is the void fraction of hydrogen Both and can be calculated as mentioned in Eqs 10 and 11 The temperature rise for tool element length can be rep resented by the following Eq 10 i Ki V V 2 b VsiY2 si ece 10 The void fraction of hydrogen can be calculated as follows 10 i AiX i 1 Ai X i 11 where X i b aJsi eVsiYsi 12 gi P Rg i 13 Ai e g gi a 14 Vg Vel 15 where giis the hydrogen density for each interval g mm3 Rgis the gas constant iis the temperature for each interval 350M S Hewidy Journal of Materials Processing Technology 160 2005 348 353 C eis the electrolyte density g mm3 gis the electro chemical equivalent of gas ais the electrochemical equiv alent of electrolyte b is the distance along the electrolyte fl ow direction mm and P is the electrolyte pressure Mpa The major factor that contributes to the temperature rise is the Joule heating 10 However in this work the temper ature rise was low since the process is a fi nishing and not a forming process The pre elementary tests of the present work revealed that the change of electrolyte temperature did notexceed1 C Thiswasduetotherotationoftheelectrolyte and the fast feed rate of the tool Therefore the assumption of constant conductivity is reasonable 7 Ki K0 16 Substituting Eqs 5 7 in Eq 4 the equation can be rewritten as follows Ysi wF V V K0 f b Ysi 17 Substituting Eq 17 in Eq 10 leads to Ysi 1 Ysi wF V V K0 f b Ysi 18 Fig 2 Calculation of metal removal thickness The fi nal side gap can be estimated as follows Yf Ysi wF V V K0 f I j i 1 b Ysi 19 According to the previous equation each tool element b enters the interelectrode gap at different side gap lengths Ysi and each element causes different and specifi c metal removal thickness dg of workpiece surface The fi nal workpiece diameter will be the summation of all differ ent metal removal thickness plus the initial diameter of the hole For steel material at K0 0 02 1mm 1 ap plied voltage V 20V V 2 3V 14 feed rate f 20mm minandinitialsidegap Ysi 0 5mm Eq 19 equals to Yf 0 5 0 04 I j i 1 b Ysi 20 Thesecondtermofthepreviousequationrepresentsthesum mation of the fi nal value of the metal removal thickness For rood fi nishing ECM process the same techniques are valid withlittlechangetopredicttheaccuratevalueofthefi nalrod anode diameter Dtf Dtf Dtj 2dg 21 M S Hewidy Journal of Materials Processing Technology 160 2005 348 353351 Fig 3 Effect of applied voltage on dg at different feed rates where Dtf is the initial rod diameter and dg is the fi nal metal removal thickness of the rod and it can be expressed as fol lows dg wF V V K0 f L Ysi 22 where L is the cathode interval value Thepreviousequationsaresimplifi edfromthemainradial gapconfi guration Theequationsarebasedonaplaneparallel gapconfi guration whichproveditsadequacyinsimilarcases 7 Forholefi nishingbyECM acomputermodelisproposed for accurate calculation of the removed workpiece surface layer Fig 2 Fig 3 shows the effect of the applied voltage on the re sultant workpiece diameter at different feed rates The sig nifi cant effect of feed rate on the resultant hole diameter at different conditions of the initial side gap values can be seen in Fig 4 It has been revealed that as the initial gap value increases metal removal thickness decreases This is due to Fig 4 Effect of initial side gap on dg at different feed rates Fig 5 Effect of machining strokes on dg at different feed rates the decrease of the current density value at the higher values of the side gap Fig 5 reveals that as the number of tool strokes in creases accumulatedmetalremovalthicknessincreases This result was expected However it has been submitted to emphasize the possibility that this technique can achieve the required metal removal thickness through the increase of tool strokes instead of the increase of the tool length which sometimes leads to poor controllability of the ECM process 3 Experimental work A special test rig has been designed to integrate with the featuresoftheECMprocess Fig 6showsthemachiningcell thathasbeenusedforholeandrodfi nishing Thecellhasbeen adaptedtoworkonaradialdrillingmachinetogettheadvan tages of the controlled feed rate and the rotational motion of the tool The electrolyte was sodium chloride 200g l and hasbeenfedintothecellthroughacentrifugalpumpdrawing from a plastic tank 1m3 352M S Hewidy Journal of Materials Processing Technology 160 2005 348 353 Fig 6 Electrochemical machining cell The test specimens were made of hardened steel with a circular cross section as shown in Fig 1 The pre machined holes of the workpiece were machined through a fi ne turn ing process to get different initial side gap values between tool and workpiece surface A matrix of fi ne holes 2 5mm diameter were arranged on the tool surface to supply the electrolyte into the interelectrode gap The outer frame of the machining cell was made from a tube of transparent Perspex Table1summarizestheworkingconditionsusedinthiswork process Somemodifi cationshavebeenintroducedontheconstruc tion of the cell to adapt it to rod outer surface fi nishing to confi rm with the features of this process especially for tool fi xture These results concern only hole fi nishing Table 1 Summary of working conditions ParameterValue Workpiece materialSt 45 Workpiece outer diameter20mm Workpiece height20mm Tool materialBrass Tool diameter9 12mm Tool rotational speed10rpm Electrolyte typeNaCI Electrolyte conductivity0 02 1mm 1 Electrolyte pressure1kg cm2 Applied voltage12 24V Feed rate10 30mm min Machining time40s to 1min Fig 7 Effect of applied voltage on metal removal thickness 4 Results and discussions The accuracy and the productivity of the ECM process with a rotating electrode are infl uenced not only by the aver age electrolyte fl ow rate but also by the entire cross section of the interelectrode gap For hole fi nishing investigations have been carried out using fi ne perforated electrodes with the objective of achieving optimum machining performance in terms of high surface quality and minimal metal removal thickness Experimental results revealed that in order to keep the machining process without sparking it was necessary to provide an electrolyte fl ow rate more than 3l min It was ob served that using electrodes with closely packed perforations caused tendency to sparking Fig 7 shows the effect of the applied voltage on the metal removal thickness It has been observed that as the applied voltage increases the fi nal metal removal thickness also in creases This has been attributed to the increase of current densityastheappliedvoltageincreases Fig 8showsthesig nifi cant effect of the feed rate on the metal removal thickness at different working conditions of the initial side gap values It has been observed that as the initial gap value increases metal removal thickness decreases This is due to the drop of Fig 8 Effect of initial side gap on metal removal thickness M S Hewidy Journal of Materials Processing Technology 160 2005 348 353353 Fig 9 Effect of number of machining strokes on metal removal thickness current density value at higher values of the initial side gap Fig 9 shows the effect of the number of strokes on the metal removal thickness Surface roughness has been measured for some speci mens at different working conditions Current density Eq 3 could be considered as a reasonable measure to ex press surface quality For applied voltage equal to 24V K0 0 02 1mm 1and Ysi 0 5mm estimated current density reached 86A cm2 For applied voltage equal to 12V K0 0 02 1mm 1 and Ysi 1mm current density was about 24A cm2 The measured surface quality ranged from 0 62 m to 2 m It hasalsobeenfoundthatasmetalremovalthicknessincreases surface roughness decreases 5 Conclusion The present technique has proved its adequacy to fi nish outer and inner surfaces providing high surface quality by a low cost operation In the present work the possibility of increasing feed rates has been achieved which could lead to the removal of wall thickness up to 0 02mm This enhances the controllability of the ECM process Theoretical model and computer calculations facilitate the prediction of metal removal thickness through different combinations of ECM parameters The proposed technique of electrolyte supply can be ex tended to deal with complex shapes of moulds and dies Acknowledgments The author would like to express his thanks to Prof S J Ebied of An Shams University and Prof A M Abdel Maaboud of Menoufi a University for their helpful sugges tions He also thanks Dr M F Safti of Menoufi a University for his kind support of this research References 1 M S Amalnik J A McGeough Intelligent concurrent manufactura bility evaluation of design for electrochemical machining J Mater Proc Tech 61 1996 130 2 T Masuzauwa S Sakai Quick fi nishing of WEDM products by using a mate electrode Annals of CIRP 35 1987 123 3 J A McGeough EDM ECM fi