International Journal of Electrical Machining, No. 1, January 1996

Removal Amount Difference between Anode and Cathode in EDM Process

Heng XIA*, Masanori KUNIEDA** and Nobuhiko NISHIW AKI** (Received on June i3, 1995)

*Tokyo University of Agriculture & Technology (now, Kuroda Precision Industries Ltd.) **Tokyo University of Agriculture & Technology, Tokyo 184, Japan

Abstract The pulse duration of discharge current greatly influences the anode and cathode removal amounts in the EDM

process. The purpose of this research is to determine the reason for such a phenomenon. First, the energies distributed into anode and cathode in the case of a single discharge process are obtained by measuring the temperature rise and removal amount of electrodes. Then the influence of carbon adhesion onto the anode surface on the anode removal amount is investigated. It is found that the energy distribution into the anode is more than that into the cathode, and the influence of pulse duration on the energy distribution is small. It is also concluded from the experimental results that the removal amount difference between the anode and cathode is mainly caused by carbon adhesion, not by energy distribution, when both anode and cathode are made of copper. Key words: EDM, removal amount, energy distribution, carbon adhesion

1 INTRODUCTION

The anode and cathode removal amounts in the EDM process are greatly dependent on the pulse duration of the discharge current, and they are quite different even when the materials of the two electrodes are the same. Motoki and Hashiguchi 1) and Kimoto et at. 2) explained that such a phenomenon is caused by the variation of the energies distributed into the anode and cathode with the pulse duration, on the basis of the cathode electron emission theory. However, it is also obvious that the removal amount difference cannot be explained well by only the energy distribution. Some papers3

)4)5)6) indicate that the carbon layer on the anode surface, which is formed because of the carbon adhesion phenomenon, influences the anode removal amount greatly, especially under the condition of longer pulse duration.

The finding that the energy distribution cannot explain well the removal amount difference between the anode and cathode is also reported in the papers of Hashiguchi7), DiBitonto et a1.8

) and Patel et aI.9). In

their studies, the energy distributions into the anode and cathode were obtained through the thermal conduction calculation on the basis of the removal amount, and it was reported that the energy distribution does not change with the pulse duration.

WelO) have also reported that the influence of pulse duration on the energy distribution is small when both anode and cathode are made of steel. It is, however, difficult to obtain highly reliable results

concerning the removal amount, and to determine the interrelation between the energy distribution and the removal amount, because the machining process is unstable due to the occurrence of short circuit when the steel anode and cathode are used in the continuous EDM process. In this research, the energies distributed into anode and cathode are investigated when both anode and cathode are made of copper, which gives high machining stability in both single discharge and continuous process. The energies are obtained by measuring the electrode temperatures and removal amounts. Also, in order to determine the main factors affecting the anode and cathode removal amount difference, the phenomenon of carbon adhesion onto the anode is also investigated under different experimental conditions.

2 MEASUREMENT OF ENERGY DISTRIBUTION

The total discharge energy supplied into the gap is first distributed among three parts as shown in Fig.l, that is the energy into the anode, the energy into the cathode and the energy into the dielectric. In this research, the energies into the anode and cathode were measured. 2.1 Principle

Let us consider the case of measuring the energy distribution ratio into the anode XA. The energy into the anode is distributed among the energy carried away by debris, the energy loss due to heat conduction

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in the electrode and the energy loss due to convection and radiation from the surface. The sum of the energy carried away by debris and the energy loss due to heat conduction represents the energy distributed into the anode in this research, because it is clear from our

........................ ... -:.:.:.:.:.:.:.:.:.:.:.:.:.:.:

:::::::: ::: :e~:~ r:~.y :.i~:~:~: ::~:~i~:~ .: : :· :: : :~:::~: ~ :: . : :. .... :)C<:::\:: anode .:::

energy loss into gap

total discharge energy

a) Distribution of discharge energy

previous research10) that the energy loss due to

convection and radiation can be neglected in comparison with the other two energies.

The flowchart for measuring the energy distribution ratio into the anode is shown in Fig.2. The

energy distributed into electrode

energy carried away by debris

energy loss due to convection and radiation

. . . . . . . . . . . ............ .. .............. .. , ... .. . .... . .. , ... ... .. .. . .... .......... . . ...... ...... ..... ......... . . . . . . . . . . . . . . .............. .... .. ..........

; .. ;:·;:·.:·.·:·ff~~~~·~~~~~~~~:!·!:!·:!: ~~~{~~~~~~~~~:~:) : ~ :~:~:~:~:~: ~: ~:~:~:~: ~:~ :~:~:~:~:~:~:::::: :: :::~:::::

b) Energy distribution in electrode

Fig.l Energy distribution in the EDM process

Assume the distribution ratio due to conduction as xAc

Calculate the anode temperature rise Tit) due to a single discharge

N

Measure the anode temperature rise Te(t) due to a single discharg

Measure the removal amount due to single discharge

Calculate the energy ratio carried away by debris XAd

Determine the distribution ratio as XA(=XAc+XAd)

Fig.2 Method for determination of energy distribution

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anode temperature rise at a given point P is measured first when a single discharge is supplied. Then the temperature rise at point P is calculated by a finite­difference method, with an assumed ratio of energy loss due to heat conduction within the anode, XAc. TIle calculation is repeated for different values ofXAc until the calculated temperature coincides with the measured one. By this tuning method, the ratio of energy loss due to heat conduction is obtained. At the same time, the removal amount due to a single discharge is detennined by measuring the anode removal amount as a result of several hundred thousand repetitions of an independent single discharge by setting a sufficiently long pulse interval between each discharge. The ratio of energy carried away by debris XAd is then calculated on the assumption that all the debris is removed from the anode surface when the temperature reaches the boiling point of copper. Although it is not yet clear at what temperature the debris is removed from the surface in the EDM process, our previous studies10)11) show that the energy carried away by debris is somewhat smaller than that due to heat conduction in the electrode even when the debris is assumed to be removed at the boiling point of copper. Therefore, the influence of the assumption concerning the temperature at which the debris is removed on the experimental results of the energy distribution ratio is negligible even if some debris is removed at the melting point. In this research the calculation is carried out on the assumption that the energy carried away by debris is maximum, that is the debris is removed at the boiling point.

copper needle electrode

,----Jl---4 D ----- shield box

amplifier

personal

computer

D GP-IB

Fig. 3 Experimental setup for anode temperature

measurement

In the above way, the energy distribution ratio into the anode XA(=XAc+XAd) is obtained. The same method is also used for the cathode. 2.2 Experimental setup for temperature measurement

The experimental setup for measuring the anode temperature is shown in Fig.3. The anode is a piece of copper foil of O.lmm thickness and 20mm diameter, while the cathode is a copper needle of Imm diameter. A constantan wire of 0.1 mm diameter is connected to the copper foil on the reverse of the discharge surface to form a copper-constantan thermocouple. The output signal of the thermocouple is sent to a storage oscilloscope through an amplifier (AD594, frequency characteristics 1kHz) and stored in a personal computer. The amplifier is placed in the shield box formed by the steel frame and the copper foil to reduce electromagnetic noise. The needle and foil electrodes are separated by an EDM oil when a single discharge occurs. 2.3 Temperature calculation model

Figure 4 shows the calculation model for the temperature rise of the foil electrode. The analysis in the foil electrode is three-dimensional, and in the constantan wire it is one-dimensional. The heat flux from the arc column enters the foil electrode through area A In this research, the arc column diameter is assumed to be constant and equals the diameter of the discharge crater formed by a single discharge. This

E E ,...... d

measuring point P

constantan wire (<p O.lmm)

FigA Temperature calculation model

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assumption little influences the computed results when the measuring point is far from the discharge point, which is proven by our computation.

For the foil, the discharge surface and the reverse surface are assumed to be adiabatic, and the foil electrode is assumed to be at room temperature for radii larger than 6mm, according to the experimental setup. 2.4 Experimental results

The measured and calculated anode temperatures when the energy distribution ratio due to the heat conduction XAc is assumed to be 0.40 are shown in

>< .2 "§ s::

.2 -; ..0 ·c Vi ~ ;>.. on 1-0 ~ s:: ~

50~--Tl~--'-~--'-~--~

30

20

0 4

copper anode

- calculated

ie=25A

te=500 fJ. S

1=1.17mm XAc=OAO

8

time ms

12

Fig.5 One example of measured and calculated

transient anode temperatures

o

0.2 •

0.1 -0- copper anode (XA)

--- copper cathode (Xc)

ie=25A gap length 40 fJ. m

600 800

pulse duration te fJ. S

Fig.6 Relationship between energy distribution ratio and

pulse duration in a single discharge process

Fig.S. Time 0 in the graph indicates the moment of the initiation of the discharge. It is found that the calculated result coincides well with the measured one for the period after 2ms. The inconsistency between the two temperatures during the pulse duration (O.Sms) is caused by the voltage drop between the two terminals of the thermocouple due to the flow of discharge current through the shield box, while, the reason for the inconsistency during the period O.Sms to 2ms is thought to be that the amplifier does not work well immediately after an overrange load is removed.

Figure 6 shows the influence of the pulse duration on the energy distribution ratio into the anode XA and into the cathode Xc when the energy carried away by debris is taken into consideration. It is found that the distribution ratios into both the anode and cathode are nearly independent of the pulse duration, and the energy into the anode is slightly greater than that into the cathode. It will be shown in the next section that this result appears contradictory to the removal amount difference between the anode and cathode in the continuous EDM process.

3 INTERRELATION BETWEEN REMOVAL AMOUNT AND ENERGY DISTRIBUTION

3.1 Anode-to-cathode removal ratio in a single discharge process

It is difficult to conclude that there is a relation between the energy distribution and the removal amount, because it is not yet clear when and how the debris is removed from the electrode surfaces. However, it is considered that the anode-to-cathode

1

0 .~

;----- ie=25A ... C; te=100 fJ. S :> 0 copper-copper

~ ~ 0.5 oS

COd 0

J/

V single pulse

continuous process I

0 +t tU

"0 0

~

0

Fig.7 Anode-to-cathode removal ratio in single

discharge and continuous processes

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removal ratio increases linearly with the anode-to­cathode energy distribution ratio which is defined as the ratio of the energy into the anode to that into the cathode, if the removal mechanism and the arc column diameter at two electrode surfaces are the same. In order to investigate the anode-to-cathode removal ratio in a single discharge process, two copper cylinders of 15mm diameter were used as the electrodes. The differences in electrode weight before and after several hundred thousand successive single discharges with a sufficiently long interval between each discharge were measured, and the anode-to-cathode removal ratio was obtained by dividing the anode weight difference by the cathode one. It is found from the experimental result in Fig.7 that the anode-to-cathode removal ratio is near 1 in the case of the single discharge process, which is close to the measured energy distribution result in the above section. However, the energy distribution result shows that the energy into the anode is slightly greater than that into the cathode, while Fig.7 shows that the anode removal amount is slightly smaller than the cathode one. 3.2 Removal amount in continuous process

Figure 7 also shows the result of anode-to-cathode removal ratio for an ordinary continuous process. In this case, the anode-to-cathode removal ratio becomes quite small as shown in the graph, which appears contradictory to the energy distribution results. Figure 8 shows the relationship between the pulse duration and the removal amount per pulse in the continuous process. It is found that not only is the anode-to­cathode removal ratio very small except for the shorter pulse region, but it changes greatly with the pulse duration, which is quite different from the measured results of energy distribution shown in Fig.6.

From the above findings, it is obvious that there are other factors affecting the removal amounts, besides the energy distribution.

4 INTERRELATION BETWEEN REMOVAL AMOUNT AND CARBON ADHESION PHENOMENON

4.1 Influence of pulse duration on removal amount and carbon adhesion amount

A carbon layer is usually observed on the copper anode surface when pulse duration is long. In some papers3

)4)5) it was reported that this carbon layer prevents the anode from being eroded. It is also well known that the carbon adhesion amount is small and the anode-to-cathode removal ratio is high in the short pulse duration region. In our experiments, carbon is

not observed on the anode surface and the anode removal amount is greater than the cathode one for the pulse duration of 20~(see Fig.8), which is in agreement with the experimental result concerning the energy distribution. This fmding convinces us that there is some interrelation between the anode removal amount and the carbon adhesion phenomenon.

In order to investigate the relationship between the pulse duratiop and the carbon adhesion amount, elements present on the anode crater surface were quantitatively analyzed with an X-ray microanalyzer after a single pulse was supplied between a copper anode and a steel cathode. It is found from the result shown in Fig.9 that the carbon adhesion amount onto

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bl) c-

I 0 M

X 0 VJ :; 0-~ 0 0-

~ > 0 E ~

~

0 u

-t ::l CIl ~ 0 ~ ~ u 0

-5 t:: 0

600~~--~~--~~--~~--~

-0--.-

400

200

anode cathode

400

ie=25A copper-copper

600

pulse duration te /1 S

800

Fig.8 Relationship between removal amount

per pulse and pulse duration

100

crater ~ ....

~p analyzed area

carbon 50

e-'Vi t:: 0

"'0 CIl CIl

'" E

ie=16A copper anode

80 100 pulse duration te /1 S

Fig.9 Mass percentage on the anode crater surface

the anode becomes greater with increasing pulse duration, which was also reported by Mukoyama et al. l2) for the continuous EDM process. It is also found that there is a marked change in the anode-to-cathode removal ratio as shown in Fig.8, in the area where the mass percentages of copper and carbon reverse as shown in Fig.9. This means that the carbon adhesion phenomenon influences the anode-to-cathode removal ratio greatly. 4.2 Influence of polarity on removal amount

It is well-known that carbon usually adheres on the anode surface but not on the cathode. Carbon will, therefore, adhere on both electrode surfaces, if the alternative pulse shown in Fig. 1 0 with which the polarities of electrodes change for every discharge is used. The sum of two electrode removal amounts in the cases of the conventional pulse and alternative

r-

gap voltage

OV----~-------r-.~------_I~ I I ::J"

discharge current

OA -------r-------r---L---t------t-

Fig.IO Waveform of alternative pulse

-0- conventional pulse --+- alternative pulse

ie=16A copper-copper

pulse duration te f.1 s

Fig.II Sum of removal amount with the conventional and alternative pulses

pulse was investigated in order to determine the influence of carbon adhesion. The result in Fig.ll shows that the sum of removal amounts with the alternative pulse is smaller than that with the conventional one. The reason for this is considered to be that protective carbon layers adhere on both electrodes when the alternative pulse is used. This protective effect is prominent in the longer pulse duration region where carbon adhesion occurs easily. 4.3 Variation of removal amount along dielectric flushing flow

It was reported that the electrode wear near the entrance and exit of the dielectric flushing is quite different13) 1 4). This phenomenon is thought to be related to carbon adhesion. The copper electrodes shown in Fig. 12 were used in order to investigate the electrode removal amount along the dielectric flow. The dielectric was flushed through the center hole of 5mm diameter in the anode. Because the energy distribution would be influenced by the different gap widthlO) at the entrance and exit if the flushing flow rate were highl5), the flushing conditions shown in table 1 were used to maintain uniform gap width along the flushing flow (see Fig. 13).

Distributions of the removal length of anode and cathode along the radius direction were measured with

anode datum plane

datum plane \.

cathode

Fig.12 Electrodes used in dielectric flushing experiment

Table 1 Flushing conditions

flushing 1 110 ml/min

flushing 2 220 ml/min

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a contour shape measuring instrument after machining, and the local wear ratio was defined as the ratio of anode removal length to cathode removal length.

0.08 r-r----.----.---,----,----,----..-.

~ 0.06 .~

~ f 0.04 fIl

:.a

/. without flushing

~ / flushing 2 ~

~~ ie=16A tc=100 f.1 S

0.02'-'----:!5~-"''--~1."...0 ---'--"""'1"=5---LJ

distance from the electrode center mm

Fig.13 Measured discharge gap width

Figure 14 shows the influences of pulse duration and flushing flow rate on the distribution of local wear ratio. It is found that a uniform local wear ratio along the flushing flow is obtained for pulse duration of 20f.1S, while the local wear ratio becomes uneven when pulse duration of 100 f.1S is used. The greater the flushing flow rate is, the more uneven the local wear ratio becomes. Occurrence of the uneven distribution of the local wear ratio at the longer pulse duration 100 f.1S may be attributed to the different carbon adhesion amounts at the flushing entrance and exit, which is caused by the different rates of carbon production due to the thermal dissociation of dielectric and the carbon adhesion onto the anode surface, because the dielectric temperature, the bubble volume fraction and the debris concentration along the dielectric flow are different.

5 CONCLUSIONS

In this research, the energies distributed into the copper anode and cathode were measured under intermediate machining conditions, by means of measuring the electrode temperature and removal amount caused by a single discharge. The influence of carbon adhesion to the anode on the removal amount was also investigated under different machining conditions, such as pulse duration, polarity, and dielectric flushing flow rate. The following

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3

.9 2 '§ ..... ~ ~

~ ~ u ..s

0

3

.9 2 '§ ..... ~ ~

~ C;; u ..s

without flushing I

flushing 1 flushing 2

ic=16A te=20 f.1 s copper-copper

5 10 15

distance from the electrode center mm

a) Pulse duration of 20 J1 s

ie=16A tc=100 f.1 S

copper-copper

distance from the electrode center mm

b) Pulse duration of 100 J1 s

Fig.14 Influence of dielectric flushing flow on local

wear ratio

conclusions were drawn from the results.

1 )The energy distribution ratios into anode and cathode are nearly independent ofthe pulse duration.

2)The energy distributed into the anode is slightly greater than that into the cathode.

3)The above results 1) and 2) concerning the energy distribution are contradictory to the finding that the anode-to-cathode removal ratio is influenced greatly by the pulse duration and it becomes quite small in the longer pulse duration region.

4)The carbon adhesion amount onto the anode surface increases and the anode-to-cathode removal ratio decreases with increasing pulse duration.

5)The sum of the anode and cathode removal amounts with the alternative pulse is smaller than that with the conventional pulse because carbon adhesion occurs on both electrode surfaces.

6)The uneven local wear ratio under radial flushing flow conditions is caused by the uneven carbon adhesion amount along the dielectric flow.

It can be concluded from the above results that the removal amount difference between anode and cathode is mainly caused by the phenomenon of carbon adhesion onto the anode surface, not by the energy distribution, when both anode and cathode are made of copper.

ACKNOWLEDGMENTS

This research was funded by the Ministry of Education, Science and Culture of Japan (Grant-in­Aid for Scientific Research on Priority Areas (2), Project No.05239203).

REFERENCES

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2)Y.Kimoto, K.Tamiya and K.Hirata, Study of electrode erosion due to transient arc discharge in liquid, J.IEEJ, 89-964(1969), 133. (in Japanese)

3)M.Motoki, C.Lee and T.Tanimura, Research on electrode erosion caused by transient arc discharge in dielectric liquid, J.IEEJ, 87-943(1967), 793. (in Japanese)

4)H.Xia, K.Kondo, M.Kunieda and N.Nishiwaki, Influence of carbon adhesion onto electrode on electrode wear, Proc. 3rd Annual Meeting of JSEME, (1993),9. (in Japanese)

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9)M.R.Patel, et aI, Theoretical models of the electrical discharge machining process 11. The anode erosion model, IAppl.Phys., 66-9(1989), 4104.

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13)H.E.De.Bruyn, Some aspects of the influence of gap flushing on the accuracy in fmishing by spark erosion, Annals of the Cl RP, 18(1970), 147.

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