CIRP Annals - Manufacturing Technology 59 (2010) 231–234

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CIRP Annals - Manufacturing Technology

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Evaluations of spark distribution and wire vibration in wire EDM by high-speedobservation

A. Okada (2)a,*, Y. Uno a, M. Nakazawa a, T. Yamauchi b

a Graduate School of Natural Science & Technology, Okayama University, Okayama 7008530, Japanb R&D Department, Tokusen Kogyo Co., Ltd., Ono 6751361, Japan

A R T I C L E I N F O

Keywords:

Wire EDM

Analysis

Spark location

A B S T R A C T

In fine wire EDM using thin wire electrode, uniform distribution of spark location is necessary to achieve

stable machining performance. However, it is difficult to precisely evaluate the distribution of spark

location by the conventional branched electric current method when the workpiece is thin. A new

evaluation method by using a high-speed video camera is proposed. The locations of spark are measured

by analyzing the recorded images. Then the effects of machining parameters, such as servo voltage, pulse

interval time, wire running speed and others on the distribution of spark location are investigated. The

possibility of evaluating the wire vibration is also discussed.

� 2010 CIRP.

1. Introduction

Discharge concentration is easy to occur in wire EDM using a finewire electrode because the workpiece is relatively thin and thedischarge gap is narrow. Frequent discharge concentration causesunstable machining performance and wire breakage [1–3]. There-fore, uniform distribution of spark location is necessary to achievethe high-performance machining. Branched electric current method[4,5] has been conventionally used for evaluating the distribution ofspark location. In the branched electric current method, thedischarge current is supplied from two points separated with asufficient distance, for example upper and lower current feedingcontacts in a wire EDM machine. Each current is measured bycurrent sensor, and the spark location can be obtained from the ratioof the currents from the upper and the lower contacts. However,when the workpiece is thin, it is difficult to measure the sparklocation, and precise evaluation of the distribution of spark locationis impossible by the conventional method.

In this study, a new evaluation method of spark location by ahigh-speed observation is proposed. Sparks generated in theworking gap during the process are recorded by using a high-speedvideo camera and the position of spark in recorded images iscalculated by image analysis. Thus, variations of spark locationwith time are investigated to evaluate spark location distributionin wire EDM. Relative spark location is also evaluated, in whichvariation of the distance between the present spark and theprevious spark is calculated, since just one spark is recorded in oneframe because of its extremely high recording speed. The effects ofmachining conditions, such as servo voltage, pulse interval timeand wire running speed on the distribution of spark location arediscussed using a thin tungsten wire of 50 mm in diameter. Also thewire movement during wire EDM can be recorded by the high-

* Corresponding author.

0007-8506/$ – see front matter � 2010 CIRP.

doi:10.1016/j.cirp.2010.03.073

speed video camera. Then the possibility of evaluating the wirevibration is also discussed.

2. High-speed observation and image analysis

Fig. 1 schematically illustrates a high-speed observation systemof wire EDM. The system consists of a running tungsten wire of50 mm in diameter, a workpiece of 1 mm in thickness and a smalltank made of acrylic resin. The workpiece material is SKD11 in JISspecifications used widely as a metal mold. Using kerosene typeworking fluid in the model, the state of discharges was observedfrom the rear side of wire by a digital high-speed video camera(KEYENCE VW-6000). The discharge voltage and current wave-forms were also monitored with an oscilloscope.

The recording conditions are shown in Table 1. The view size is1.3 mm � 0.6 mm, which covers the workpiece thickness. Just onespark can be recorded in one frame because the recording speed is8000 frames per second (fps), and the discharge pulse frequency isabout 6000 Hz. The acrylic tank is set on the working table ofcommercial EDM machine (Sodick AP-200L). The machining in theacrylic tank is carried out using the discharge circuit and the tablecontrol system of the wire EDM machine.

The wire EDM conditions are shown in Table 2. Basically, EDMmaker recommended 1st cut conditions for steel of 1.0 mm inthickness with tungsten wire of 50 mm is applied. Servo voltage,pulse interval time and wire running speed are varied in order toinvestigate their effects on the distribution of spark location.

3. Image analysis

A commercial image analysis software (DITECT DIPP-Macro)was used for the analysis of recorded images with high-speedvideo camera. In order to clearly catch the sparks in the imageanalysis process, the high-speed observation was carried outwithout any illuminations. The recording was started after the kerf

Fig. 1. High-speed observation system of wire EDM.

Table 1High-speed observation conditions.

Recording speed 8000 fps

Shutter speed 1/8000 s

Max. recording time 1.0, 5.0 s

View size 1.3 mm�0.6 mm

Table 2Wire EDM conditions.

Wire ED machine Sodick AP-200L

Wire Ø50 mm tungsten

Workpiece SKD11 (JIS)

Workpiece thickness, t 1.0 mm

Dielectric fluid Kerosene

Pulse duration, te 1.0 ms

Discharge current, ie 20 A

Servo voltage, Sv 50–90 V

Pulse interval time, to 7.0–9.0 ms

Wire running speed, Ws 3.0–15.0 m/min

Fig. 2. Recorded and analyzed images.

A. Okada et al. / CIRP Annals - Manufacturing Technology 59 (2010) 231–234232

of 0.1 mm in length was machined. The recording speed was set to8000 fps, and the recording time was 5.0 s.

Fig. 2 shows the schematic arrangement of wire and workpiece(a), the high-speed observation image of spark (b), and the imageanalyzed result (c). As can be seen from (b), the original image ofspark recorded by high-speed video camera is not clear, but theimage was sharpened and binarized into white and black by theimage analysis software (c). Then, y position of center of the sparkwas calculated. The image analysis was done in this manner for allimages in the high-speed observation.

Fig. 3. Absolute sp

4. Analysis of spark location distribution

The distribution of spark location is evaluated by two methods,absolute spark location and relative one. Fig. 3 shows one example ofresults evaluated by absolute spark location. The left side graph isspark locations whenthe evaluationtime is1.0 s,and theverticalaxisindicates the distance of spark from the lower edge of workpiece. It isunderstood that the spark locations can be precisely detected byusing the high-speed observation system, even if the workpiecethickness is as thin as 1 mm. In this case, sparks uniformly occuralong the thickness direction of workpiece without dischargeconcentration. Furthermore, the workpiece thickness is divided into

ark location.

Fig. 6. Variation of (Cd)abs with servo voltage Sv.

Fig. 4. Relative spark location.

A. Okada et al. / CIRP Annals - Manufacturing Technology 59 (2010) 231–234 233

5 regions, and thenumber ofsparksoccurringineachregion isshownin the right side histogram. The maximum diameter of arc columnobserved with high-speed video camera under this machiningcondition was about 200 mm, which was about 5 times larger thanthe crater diameter [6]. Then the thickness of one region in thehistogram was 200 mm. As a matter of course, if the spark locationdistribution is perfectly uniform, all frequencies become 20%.

On the other hand, Fig. 4 shows relative spark location as anotherevaluation method. Observation speed by high-speed video camerawas as high as 8000 fps and it exceeds discharge frequency.Therefore, just one spark is recorded in one frame image, thus thedisplacement between the present spark location and the previousspark location can be calculated. The displacements are plotted inthe graph. Small displacement indicates discharge concentration.There is the possibility to discuss the phenomenon which is difficultto evaluate with absolute spark location. A dashed line in the graphshows perfectly uniform distribution of spark location. This wascalculated by using random number.

5. Effects of machining conditions on spark distribution

Servo voltage is one of the important parameters affectingmachining characteristics and controls the average gap distance.The gap distance becomes narrower as the servo voltage is smaller.Fig. 5 shows absolute spark locations and the histogram with fiveregions when the servo voltage is 50 V. As shown in the figure,sparks more concentrate at lower region than those at upperregion, especially in the first half. The histogram obviouslyindicates the discharge concentration around lower region. Inthe same way, absolute spark locations and the histograms wereobtained under other servo voltage conditions. Then, the differencebetween the maximum frequency and the minimum one wascalculated from the histogram in order to quantitatively discussthe discharge concentration. In this index, high difference meansrelatively frequent occurrence of discharge concentration. There-fore, this index calculated from absolute spark location is definedas discharge concentration index (Cd)abs.

Fig. 6 shows the variation of discharge concentration index(Cd)abs with servo voltage. As can be seen from the figure, (Cd)abssignificantly decreases with an increase of servo voltage. In thewire EDM experiments, wire breakage occurred and the machiningwas impossible when the servo voltage was set to less than 50 V. Itis guessed that the wire breakage was brought by frequent

Fig. 5. Absolute spark locations when Sv = 50 V.

discharge concentrations. As the servo voltage is smaller, the gapdistance becomes narrower, and the exclusion of debris and bubblegenerated in the gap becomes more difficult. Consequently,discharge concentration, short circuit and secondary dischargesfrequently occur. From the results, the discharge concentrationindex (Cd)abs defined in this study can quantitatively representsthe state of actual discharge concentration in the gap.

Next, the effect of pulse interval time on spark distribution wasdiscussed with discharge concentration index (Cd)abs. As a result,the (Cd)abs decreased slightly with an increase of pulse intervaltime. Pulse interval time is needed to extinguish the plasmagenerated by the previous spark and to recover the dielectric of thegap. If the pulse interval time is insufficient, discharge is easy toconcentrate. The actual discharge concentration phenomenon withpulse interval time was well represented by the variation of(Cd)abs. When the effect of pulse interval time was compared withthat of servo voltage discussed above, the change in (Cd)abs forpulse interval time was small. Therefore, the effect of servo voltageon discharge concentration is significant.

Fig. 7 shows the effect of wire running speed. As shown in thefigure, (Cd)abs increases with wire running speed. In general,higher wire running speed tends to higher flow rate of workingfluid in the gap and smooth exclusions of debris and bubblegenerated with discharges [7]. Therefore, it was guessed that thedischarge locations were uniformly distributed as wire runningspeed increased. However, opposite result was obtained in thiscase. Actually, the removal rate significantly increased with wirerunning speed. Large removal rate indicates large amount of debrisgeneration in the working gap. Therefore, it is guessed that thedebris accumulates in the gap and the discharge concentrationeasily occurs with wire running speed.

As another evaluation method, relative spark location explainedabove was applied to this effect of wire running speed. In theevaluation, the frequency of spark occurrence within 200 mm isevaluated and it is defined as discharge concentration ratio inrelative spark location (Cd)rel, because the diameter of dischargeplasma is about 200 mm in maximum. Fig. 7 also shows the variationof (Cd)rel under various wire speed conditions. From the relative

Fig. 7. Variations of (Cd)abs and (Cd)rel with wire running speed Ws.

Fig. 8. Recorded wire image and movement analysis.

Fig. 10. Variation of removal rate with wire tension.

A. Okada et al. / CIRP Annals - Manufacturing Technology 59 (2010) 231–234234

spark locations in 5 s as shown in Fig. 4, the values of (Cd)rel werecalculated every 1 s under each wire running speed condition, andthe average of five values was plotted in the graph with red circle.Dashed line shows (Cd)rel value of uniform distribution of sparklocation, which is calculated by using random number. The variationof (Cd)rel with wire running speed is very similar to that of (Cd)abs.Therefore, (Cd)rel is also effective index for quantitatively evaluatingthe distribution of spark location. Besides, the scattering of (Cd)rel,that is the difference in the minimum and maximum (Cd)rel valuesshown as an error bar in the graph, becomes a little larger with wirerunning speed. Large scattering of (Cd)rel indicates that dischargessometimes distribute uniformly but sometimes concentrate, and thestate of discharge distribution is unsteady.

6. Evaluation of wire movement

The movement analysis of wire during the machining was triedusing high-speed observation images. Fig. 8 shows the schematicarrangement of wire and workpiece, the high-speed observationimage of wire during machining, and the wire movement analysisresults. The observed area is above the upper edge of workpiece. Thewire is observed from the rear side for machining direction. Theedges of wire at point A and point B in the recorded images areautomatically detected and the x positions are calculated by theimage analysis. Their distances from the upper edge of workpiece are1.0 mm and 0.2 mm respectively, In this case, the upper wire guide isas far as 5.0 mm from the upper edge of workpiece, so themovements of wire are almost the same as each other. Bythe variations of x position, the amplitude of wire vibration, thefrequency and others can be investigated as well as the conventionaloptical method using light emitting diode and detecting diode [8,9].

Fig. 9 shows the relationships between the amplitude of wirevibration and wire tension. The variation of machined kerf width isalso shown. As shown in the figure, the amplitude decreases with thewire tension, and takes a minimum at around 3 N. When the wiretension is 4 N, the amplitude slightly increases. The elastic limit of

Fig. 9. Variation of wire vibration amplitude and kerf width with wire tension.

tungsten is 3.2 N when the diameter of wire is 50 mm. Therefore, it isguessed that the wire locally extends at spark location during themachining when the wire tension exceeds the elastic limit of wire,which cannot keep its small amplitude of wire vibration. Moreover,It is obvious that the variation of kerf width well corresponds withthe variation of amplitude. Therefore, the kerf width significantlydepends on the amplitude of wire vibration during machining.

Fig. 10 shows the variations of machining rate with wiretension. With an increase of wire tension, the machining rateincreases because the kerf width becomes narrower. However, themachining rate becomes small when the wire tension is too high,because of larger amplitude of wire vibration shown in Fig. 9 andwire extension due to loading of excessive wire tension.

The details of wire movement during machining will beclarified by further discussions of high-speed observation imagesand the image analysis.

7. Conclusions

In this study, high-speed observation system for fine wire EDMprocess was built and a new evaluation method of spark locationdistribution was proposed. The spark locations were analyzedusing the recorded images, and the effects of servo voltage, pulseinterval time and wire running speed on the distribution of sparklocation were investigated. Experimental results clarified thatspark distribution becomes uniform when servo voltage is high,pulse interval time is long, and wire running speed is low. Then,high applicability of the method to evaluate spark locationdistribution was proved.

Acknowledgement

This research was financially supported by Research forPromoting Technological Seeds 2007 (No. 12-046), Japan Scienceand Technology Agency.

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