This paper deals with quality control of the electro-discharge texturing. Automobile manufacturers have strict requirements for micro-geometry parameters of rolled metal plates from contractors of body sheet metal. The micro-geometry of the roll surface has significant influence on the final quality of the rolled metal plate. In this paper electro-discharge texturing parameters are analyzed, for example, the texturing current, impulse time, technological pause, and their impact to roughness average and peak counts. Quality control is discussed based on mathematical model for estimating the texturing current, impulse time, and technological pause for roughness average or peak counts required value.Rad opisuje kontrolu kvalitete elektroerozijske teksture. Proizvođači automobila postavljaju prema dobavljačima lima za karoserije stroge zahtjeve za parametre mikrogeometrije valjanih limova. Mikrogeometrija valjane površine ima značajan utjecaj na završnu kvalitetu valjanih limova. U ovom su radu analizirani parametri elektrolizijske teksture, npr., jakost struje, trajanje impulsa, tehnološka pauza i utjecaj na prosječnu hrapavost i broj vrhova. Kontrola kvalitete se razmatra na osnovu matematičkog modela za procjenu jakosti struje, trajanja impulsa i tehnološke pauze za zahtjevanu hrapavost i broj vrhova

J. Revaj

J. Terpák

L\u27. Dorčák

English

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J. TERPÁK, L’. DOR^ÁK, J. REVAJQUALITY CONTROL OF THE ELECTRO-DISCHARGE TEXTURINGReceived – Prispjelo: 2008-10-24Accepted – Prihva}eno: 2009-01-24Original Scientific Paper – Izvorni znanstveni radINTRODUCTIONAutomobile manufacturers specify in their require-ments the characteristics of the microgeometry of metalplate quality. These quality criteria are a necessary con-dition for realization of supply of metal plates to themanufacturers. Each manufacturer has its own specifi-cations. For example the Ford company requires that theroughness average on both sides of the metal plate bewithin the range 1,1 to 1,7 m, while the peak count hasto be more than 50 cm-1 1.These requirements are important mainly for goodpressability and varnishability of the metal plates andalso to achieve their long lifetime in the automobile in-dustry.The roughness of the surface is a part of geometricunevenness of the surface with relatively small dis-tances of uneven parts. For the purpose of evaluation ofthe surface relief statistical parameters were chosenwhich we call microgeometric quantities. Themicrogeometry of the surface is given by the roughnessaverage and the peak count.Ra (roughness average) is a generally known andmost widely used international roughness parameter. Itis the mean arithmetic deviation of the roughness profilefrom the median line in the range of basic length 2.Geometrically, this parameter is interpreted by theheight of the rectangle constructed on the median linewith the same area as the profile unevenness, enclosedby the profile from the median line. The values obtainedshow considerable accuracy in repeated measurement.Pc (peak count) is the number of peaks that intersectthe defined upper and then lower level of the section,measured in the range of the evaluated length, trans-formed to the length of 1 cm 2.In addition to the above quantities the followingcharacteristics are used:– Rq – average quadratic deviation of the profile inthe range of basic length, so called medium effec-tive value– Rv – depth of the largest dip of the roughness pro-file within a single measured path– Rp – height of the tallest peak of the roughnessprofile within a single measured path– Rz – sum of Rv and Rp is the total depth of theroughness profile of a single measured path– Rmax – largest height of the profile (tallest peak tolargest dip) measured in the range of entire lengthbeing evaluatedMETALURGIJA 49 (2010) 1, 19-22 19This paper deals with quality control of the electro-discharge texturing. Automobile manufacturers have strictrequirements for micro-geometry parameters of rolled metal plates from contractors of body sheet metal. Themicro-geometry of the roll surface has significant influence on the final quality of the rolled metal plate. In thispaper electro-discharge texturing parameters are analyzed, for example, the texturing current, impulse time,technological pause, and their impact to roughness average and peak counts. Quality control is discussed ba-sed on mathematical model for estimating the texturing current, impulse time, and technological pause for ro-ughness average or peak counts required value.Key words: quality control, electro-discharge texturing, mathematical modelKontrola kvalitete elektroerozijske teksture. Rad opisuje kontrolu kvalitete elektroerozijske teksture.Proizvo|a~i automobila postavljaju prema dobavlja~ima lima za karoserije stroge zahtjeve za parametre mikro-geometrije valjanih limova. Mikrogeometrija valjane povr{ine ima zna~ajan utjecaj na zavr{nu kvalitetu valjanihlimova. U ovom su radu analizirani parametri elektrolizijske teksture, npr., jakost struje, trajanje impulsa, teh-nolo{ka pauza i utjecaj na prosje~nu hrapavost i broj vrhova.Kontrola kvalitete se razmatra na osnovu matemati~kog modela za procjenu jakosti struje, trajanja impulsa itehnolo{ke pauze za zahtjevanu hrapavost i broj vrhova.Klju~ne rije~i: kontrola kvalitete, elektroerozijska tekstura, matemati~ki modelISSN 0543-5846METABK 49(1) 19-22 (2010)UDC – UDK 621.9.015:667.661.3:519.673=111J. Terpák, L’. Dor~ák, J. Revaj, Berg Faculty, Technical University ofKo{ice, Ko{ice, SlovakiaIndividual parameters of the metal plate are achievedby rolling the plate with textured (roughened) metalplate rolls. The quality of the roll surface can be influ-enced by several technological parameters which aregiven by individual technologies.To date there are five known methods of texturingthe surface of work rolls 3, 4 for rolling steel belts:blasting with steel grit, electro-discharge texturing, lasertexturing, texturing with electron beam, and topochrommethod. The roll surface can be achieved with thesemethods that are stochastic or fully deterministic.ELECTRO-DISCHARGE TEXTURINGElectro-discharge texturing (EDT) is a technologicalprocess characterized by the fact that the melting downof the roll’s material is caused by electric discharges thatare temporally and spatially separated. The dischargesoccur in the work slot between the electrode and the roll.The electrodes are immersed in a dielectric fluid duringthe process of texturing 5, 6.The result of the process is textured, stochastic sur-face, characterized by non-oriented, randomly locatedcraters after individual discharges. The resulting valuesof the microgeometry after electro-discharge texturingare affected by several parameters having differentweight of influence on the required characteristics.Among the most important are the following 7, 8 ef-fect of the value of texturing current, of the impulselength, of the value of technological pause, and of thevoltage. Among further important parameters of theEDT are: revolutions of the roll, velocity of the shift,shape and size of the area of the electrodes used, mate-rial of the electrodes being used, etc.The evaluation of the microgeometry of the workrolls was performed according to the DIN methodologywhich represents a generally used standard 9. Two ba-sic characteristic of surface microgeometry were evalu-ated, namely Ra and Pc, measured in nine locationsin four quadrants along the roll.The results of individual measurements of the char-acteristics were recorded in tables. Subsequently, meanvalues were determined of Ra and Pc from each set ofmeasurements and also the value of mean deviation. Themeasurements were taken by using the Hommel TesterT1000 device.ANALYSIS OFTHE EFFECT OF PARAMETERSThe data for analysis were provided by the companyNEO Slovak Inc. realizing electro-discharge texturingof chrome rolls, which is more than 4500 measured dataitems. Processed rolls are grouped into individual pro-grams characterized by required value Ra and Pc (Fig-ure 1). Reaching the desired values is then dependent onthe setting of main parameters of the process, i.e. by tex-turing current, impulse length, and the length of thetechnological pause.Classical histograms and the Shewart regulation dia-grams were used for the evaluation. For those sets ofvalues with more than 20 measurements regulation dia-grams for arithmetic mean value and standard deviationwere used. For the sets with less than 20 measurementsregulation diagrams for arithmetic mean value andvariational range were used. After excluding the valuesoutside of regulation boundaries, the parameters of de-pendence and correlation coefficients between the mon-itored parameters and Ra and Pc were calculated withthe method of least squares.Effect of texturing currentThe value of the texturing current has fundamentaleffect on the roughness achieved. It is obvious from themeasured values that roughness Ra increases with in-creasing texturing current (Figure 2). In the case of con-stant value of the impulse this increase is negligible afterreaching specified value Ra. Based on this finding it canbe stated that, if higher values of Ra are needed, from theviewpoint of efficiency of electro-discharge texturingand concurrently with increasing the value of texturingcurrent, it is necessary to increase the value of the im-pulse length. The dependence of Ra on current deter-mined with the least squares method is given by the for-mula20 METALURGIJA 49 (2010) 1, 19-22J. TERPÁK et al.: QUALITY CONTROL OF THE ELECTRO-DISCHARGE TEXTURINGFigure 1. Considered programs of chromed rollsFigure 2. Dependence of Ra and Pc on the currentRa I I  0 0015 01493 0 63622, , , / m (1)where the correlation coefficient has value 0,8661 forcurrent within 5 to 30 A (Figure 2).The value of the texturing current for the value Pchas reverse character as with the value Ra. That meansthat with increasing value of the texturing current thepeak count decreases because of increasing area of themelted-out crater. From certain value of the texturingcurrent, with unchanged value of the impulse, this de-crease stopsPc I I  01317 75978 179822, , , / cm-1 (2)while the correlation coefficient has value 0,9313 forcurrent within 5 to 30 A (Figure 2).Effect of impulse lengthThe length of impulse affects the roughness in a fun-damental way. With increasing value of the impulselength (On) increases the value of Ra. However, at cer-tain value of the impulse length at constant value of thetexturing current a considerable slowdown in increasingthe value of Ra occurs and subsequently it stops. Thisvalue is directly proportional to the value of texturingcurrent IRa On On  0 003 02442 0 38022, , , / m (3)while the correlation coefficient has value 0,9969 for Onwithin 5 to 20 s (Figure 3).Based on the evaluated results there is an obvious de-crease in the peak count Pc at increasing value of the im-pulse length (On). After certain value of Pc has beenreached at unchanged value of texturing current, theprocess is stoppedPc On On  0 4084 1500 205262, , , / cm-1 (4)while the correlation coefficient has the value 0,9521 forOn between 5 and 20 s (Figure 3).Effect of technological pauseThe obtained data did not confirm substantial effectof the value of technological pause (Off) on the rough-ness 1. There is a dependence of roughness Ra on tech-nological pause, this, however, does not equal the de-pendencies Ra on the strength of current and impulse.However, it was shown that in the case of excessiveshortening of the technological pause regeneration ofthe channel in the discharge gap does not occur anda phenomenon appears that is called roll burning. Rollburning is an unacceptable fault from the viewpoint ofthe quality of the surface. The optimum value of the du-ration of technological pause is related to the value ofthe texturing current and the duration of the impulse. Ingeneral, it can be stated that with increasing value of thetexturing current it is necessary to increase the value oftechnological pause.The measured data did not confirm direct influenceof the duration of technological pause on the peak count.Effect of other parametersOther parameters, i.e. electrical voltage, method ofelectrode motion, revolutions of the roll, speed of elec-trode shift, shape and size of the area of the electrodesused, material of the electrodes do not have significanteffect on the values of the characteristics Ra and Pc.PREDICTION MODELIn view of the requirements Ra and Pc in texturingnew rolls, that have not yet been used, it is necessary toset the most accurate estimates possible for the currentand length of impulse. Therefore a prediction model wasdesigned for determination of the value of current andimpulse for required value of Ra.Prediction model for the currentThe prediction model for determination of the cur-rent was created on the basis of measured data by usinga nonlinear regression model. The obtained dependenceis given by the formulaI Ra Ra  2 7441 26 545 33312, , , / A (5)where the index of correlation has the value 0,8885 forRa within 1,5 to 4,0 m Figure 4.Prediction model for impulseSimilarly, the prediction model for determination ofthe length of the impulse has been set (Figure 4)On Ra Ra  0 6698 21215 0 39222, , , / A (6)where the correlation index is 0,9967 for Ra in the range1,5 to 4,0 m.In the same way we could determine the predictionmodel also for Pc, however, based on analysis a strongdependence between Ra and Pc follows which is givenby the following relationPc Ra Ra  12 717 10126 269 662, , , / cm-1 (7)METALURGIJA 49 (2010) 1, 19-22 21J. TERPÁK et al.: QUALITY CONTROL OF THE ELECTRO-DISCHARGE TEXTURINGFigure 3. Dependence of Ra and Pc on the length of impul-sewhile the correlation index has the value 0,9644 for Rain the range 1,5 to 4,0 m (Figure 5).Verification of prediction modelsFor verification of prediction models (5) and (6) 12measurements were used for texturing new rolls with re-quired roughness Ra=3,0 m and minimum number ofpeaks Pc=75 that did not appear before in the analyzedmeasurements. The recommended value of the currentaccording to (5) is 21,63 A and the value of the length ofthe impulse according to (6) is 12,78 s.Operators decided on the value of the current 20 A,while the possibility of setting the current in the textur-ing device is with minimum step size 1 A. The value ofthe impulse and technological pause was set at the samevalue 13 s. From the measured values resulted the av-erage value Ra=3,04 m and Pc=81,9. If we substitutedthe value Ra=3,04 m as the desired value into the for-mula (5) then the predicted value of the current would be22,03 A which in the case of possible settings of the de-vice corresponds to the value 22 A. The difference be-tween the predicted and real value is 2 A which corre-sponds to relative error 10 %.Multiple linear modelBecause of quite large relative error, a multiple lin-ear model was subsequently designed and realized:I a a On a Off a Ra   0 1 2 3 / A (8)where a0 = –21,287; a1 = –2,5878; a2 = 0,5595; a3 =22,6574.The value of the predicted current according to theabove model for the values of the impulse 13 µs, techno-logical pause 13 µs, and required roughness 3 µm is20,317 A. In view of the possibilities of the device thiscorresponds to setting the value of the current to 20 A.CONCLUSIONBy analyzing the effects of individual parameters ofthe EDT process on the microgeometry of the surface ofwork rolls used for cold-rolling metal sheets it was foundthat the most important parameters are the intensity of thetexturing current and the length of its impulse. These pa-rameters have the largest effect on the values of Ra andPc, while the parameters are mutually independent. Fromthe above results followed the need of creating a predic-tion model for determination of the texturing current.Based on verification of predicted and measured valueswe can conclude that the designed models can be used inthe system of quality control of the process with the pa-rameters set for new, not yet used, texturing of rolls.AknowledgementsThis work was supported by grant VEGA 1/0365/08,1/0404/08 and 1/4194/07 from the Slovak Agency forScience.REFERENCES1. J. Revaj: Vplyv parametrov procesu elektroiskrovéhotextúrovania na mikrogeometriu povrchu valcov, diplomo-vá, Technická univerzita Ko{ice, 2007, 73.2. M. Gombar: Statistical Model of the Surface Roughness inMATLAB, Manufacturing engineering, 5 (2006) 1, 14-17.3. A. Dzierwa: Chosen Properties of Chromium Coating afterPneumatic Ball Peening, Manufacturing engineering, 5(2006) 2, 62-65.4. K. Steinhoff, W. Rasp, O. Pawelski: Formation of Paint Sur-face on Different Surface Structure of Steel Sheet, Iron andSteel Engineer, 74 (1997) 3, 43-49.5. F. El-Menshawy, B. Snaith: Advances in Electro-DischargeTexturing for Cold Mill Work Rolls, Iron and Steel Engine-er, 68 (1991) 8, 57-59.6. J. A. McGeough, H. Rasmussen: A model for the SurfaceTexturing of Steel Rolls by Electrodischarge Machining,Mathematical and Physical Sciences, 436 (1992) 1896,155-164.7. M. Simao et al.: Elektrical Discharge Texturing of ColdMill Work Rolls Using Different Tool Electrode Materials,Iron and Steel Engineer, 73 (1997) 3, 42-47.8. D. Desoete, D. Pans, K. Steinhoff: EDT Technology and itsApplications, Iron and Steel Engineer, 74 (1997) 9, 36-40.9. DIN 4768, Determination of Surface Roughness Values ofthe Parameters Ra, Rz, Rmax by Means of Electrical Con-tact (Stylus) Instruments; Terminology, Measuring Condi-tions, Beuth Verlag, Berlin, 1990.Note: The responsible translator for English language is Ladislav Pivka,Ko{ice, Slovakia.22 METALURGIJA 49 (2010) 1, 19-22J. TERPÁK et al.: QUALITY CONTROL OF THE ELECTRO-DISCHARGE TEXTURINGFigure 4. Prediction model for the current and the impulseFigure 5. Dependence of Pc on Ra

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