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Characteristics of the surfaces obtained in electro-discharge machining
Characteristics of the surfaces obtained in electro-discharge machining P.V. Ramarao* and M.A. Faruqi**
According to the established thermal theories on electrodischarge machining (edm), the size of the craters produced in edm is proportional to the discharge energy, which is equal to the product of mean pulse current and pulse duration for a constant gap voltage. As a practical edm surface is a random superposition of such craters, the behaviour of the surfaces largely depends on the two important parameters, pulse current and pulse duration. With the help of multiple linear regression analysis, the effect of pulse duration and pulse current on the surface topography parameters Rq and [J* has been studied. sem photomicrographs have been taken in support of the qualitative evidence
Keywords:electro-dischargemachining, roughness (surface), mathematical analysis
A clear characterization of surface topography is essential to predict the quality and functional behaviour of contacting surfaces, such as static and dynamic stiffness, damping capacity, wear resistance, fretting corrosion etc 1 . The surfaces obtained during electro-discharge machining (edm) are the result of a combination of various contributing factors, which differ from conventional machining processes. A practical edm surface is a random superposition of craters formed by the discrete removal of metal by the thermal effects of successive discharges. As such, the most common method of representing the surface roughness by R a or maximum peak-to-valley distance (Rmax) fails to give a true picture of the surface and the need for a statistical approach to the problem has been established 2 . The random function approach for characterizing surface topography gives a comprehensive statistical description of the surface 3. Process correlation length ~* (at which the autocorrelation function (acf) becomes 0.1), as defined by Peklenik4 , is an important topographical index and can be used as a parameter for system identification and characterization of the pPofile. Since the surface obtained in edm is largely influenced by the discharge energy, it is more appropriate to have the energy terms split between pulse duration and current for precise understanding of their effect on/~* and the rms value of surface roughness, Rq. Further, since the pulse current and pulse duration can be varied and set independently, this kind of relationship will be useful in adaptive optimization and control of the process for material removal rate, tool wear and surface characteristicss. *Mechanical Engineering department, R.E. College, Durgapur713209, India **Mechanical Engineering Department, Indian Institute of Technology, Kharagpur-721302, India
PRECISION ENGINEER ING
Experimental procedure The experiments were conducted on Electra Puls Spark Erosion machine ModeI-M 100 having a solid state edm power supply (30 A). The open gap voltage across the electrodes is 90 V. As pulses with the same energy can have remarkably different combinations of average pulse current (if) and pulse duration (tp) and may cause different amounts of erosion, a 3 x 5 experiment was selected for the two variables of pulse current and pulse duration. Three values of pulse current, 10, 15 and 25 A, and five levels of pulse duration, 20, 50, 100, 200 and 500 #s, were chosen for the investigation. The dielectric employed was kerosene and two electrode materials, copper and graphite, were used. Tool steel (IS : T105 W2 Cr 60 V25) having an average hardness of Rc 56 was chosen as the workpiece material. A duty cycle of 50% was employed and the duration of each test was 1000 s. The surface profiles were obtained on a Rank Taylor Hobson Talysurf Model-4, which was provided with a diamond pyramid stylus of tip dimension 2.5/zm. Since the samples within the stylus tip dimension are unnaturally correlated6'7, a sampling interval of 5/Jm was selected for digitization of the profiles. The roughness of the surfaces produced during some of these tests was generally outside the normal range of the instrument. In fact the integrating meter was used only to compare the R a values with that of the estimated RQ values for a few cases. The scanning electron microscope (sem) photomicregraphs were obtained on an ISI-60 model sem with an accelerating potential of 30 kV. Some typical photomicrographs are given in Figs 1-5. The estimation of Rq and the plot of acf for each of the profiles was obtained using the standard procedure as outlined in Bendat and Piersol s. From the plots of acf the values of ~* were determined for all the profiles. The results are shown in Tables 1 and 2.
Surface characteristics R o o t mean square Multiple linear regression analysis was carried out for the logarithms of the variables to quantify the variation of Rq with pulse current (if) and pulse duration (tp). From the results of the statistical tests, a good correlation exists between Rq and the two independent variables, pulse current and pulse duration taken together, whereas their interaction has no significance. This is evident from the high value obtained for the multiple correlation coefficient. Further, the estimated values of 't' from 't'-tests for the coefficients are greater than the tabled values of 't' corresponding to 12 degrees of freedom at the 1% significance level9 . With Rq in micrometres, if in amps and tp in microseconds, the relations are as follows:
0141--6359/82/020111--03 $03.000 1982 Butterworth & Co (Publishers) Ltd
111
Ramarao and Faruqi - Surfaces obtained in electro-discharge machining Copper electrode Rq = 0.596 if °'399tp °'22s
(1)
Graphite electrode Rq = 0.340 if °'s63 tp °'281
(2)
Process c o r r e l a t i o n length
The above expressions show that Rq increases with pulse duration and pulse current, which is to be expected since the increase in pulse energy leads to greater metal removal z°. However the impact on Rq of pulse current is more than that of pulse duration.
The results of the statistical tests of process correlation length (/3*) indicate that effects of pulse current and pulse duration are significant at the 1% level of significance and the tests also show a very high correlation which implies that their interaction has no significance. Thus the linear regression models are justified. The following results are obtained relating/3* to if and tp. Copper electrode 3* = 13.53 if °'2s9 tp °'124
(3)
Graphite electrode 3* = 9.56 if°'328tp°A88
(4)
It is observed that/3* increases with both pulse current and pulse duration. However the values of the exponents for current are higher than for pulse duration, indicating that 3* is more influenced by the pulse current than the pulse duration. The theoretical predictions of crater radii by Van Dijck and Snoeys 11 using thermal models also indicated an increase in a similar manner with pulse current and pulse duration. It is therefore proposed that ~* may be taken as a measure representative of the radius of the crater obtained by theoretical models.
Scanningelectron microscopy Fig 1 sem photomicrograph (xlO0) (Electrode: Cu, tp = 50 Ps, i f = 10 A)
In order to verify the results obtained in the preceding sections, qualitatively, sem photomicrographs of the eroded surfaces were obtained. Typical photomicrographs to assess the affect of pulse duration and pulse current are given in
Fig 2 sem photomicrograph (x 100) (Electrode: Cu, tp = 200 ps, if = IO A)
Fig 3 sem photomicrograph (x 100) (Electrode: Cu, tp = 100 ps, if = 15 A)
Fig 4 sem photomicrograph (x 100) (Electrode: Cu, tp = lOOps, if = 25 A)
Fig 5 sem photomicrograph (x1100). Enlarged view of crater. (Electrode: Cu, tp = I00 ps, if = 15 A)
112
A P R I L 1982 V O L 4 NO 2
Ramarao and Faruqi - Surfaces obtained in electro-discharge machining
Table 2 Values of ~*,/.tm
Table 1 Values of R q , / l m Copper Pu Ise duration, /~s
10
15
20 50 100 200 500
3.00 3.75 4.35 5.12 5.62
3.37 4.12 5.00 6.12 6.75
Copper
Graphite Pulse current, A 25 10
4.15 4.75 6.51 7.63 8.84
3.05 3.85 4.45 5.75 6.90
15
25
Pulse duration, /~s
10
15
3.49 4.57 6.08 7.34 8.21
4.79 5.56 8.27 10.18 12.06
20 50 100 200 500
38 42 44 46 48
40 43 45 54 62
Figs 1-5. The effect of pulse duration is evident from Figs 1 and 2, for respectively 50 and 200/~s, at the same pulse current of 10 A. Although it will be impossible to predict the exact crater size from the photomicrographs, because of their random positioning, it is apparent that the crater size is increasing with pulse duration, suggesting higher values of Rq and/~*. It can be seen that the surface obtained from a 50/~s discharge duration was smoother compared to the surface obtained at 200/zs pulse duration. The effect of pulse current can be observed from Figs 3 and 4, for respectively 15 A and 25 A, at the same pulse duration of 100/~s. Increase in pulse current results in larger crater sizes indicating more eroded material and high values of Rq and ~*, thereby supporting the quantitative predictions by regression expressions. Some other metallographic features such as overlapping layers and pock marks can be seen from photomicrographs at very high magnification. More prominent pock marks near the walls and spheroidised particles at the bottom of the crater can be observed from Fig 5. References 1. BeardsC.F. and NeroutsopoulosA.A. The Control of Structural Vibration by Frictional Damping in ElectroDischarge Machined Joints. Trans. ASME, J. Mechanical Design, 1980, 102, 5 4 - 5 7
PRECISION ENGINEERING
2.
Graphite
Pulse current, A 25 10
44 46 56 64 70
40 43 45 54 62
15
25
41 47 60 65 71
46 53 64 75 100
Crookall J.R. and Khor B.C. Electro-DischargeMachined Surfaces. Proc. 15th Int. Conf. Machine Tool Des. Res., Macmillan, 1974, I - 3
3.
PeklenikJ. New Developments in Surface Characterization and Measurementby Meansof Random ProcessAnalysis. Proc. I. Mech. E., 1968, 182(3), 108-112
4.
PeklenikJ. Investigation of Surface Typology. C.I.R.P. Ann., 1967, 15, 381-385
5.
SnoeysR., Dauw D. and Kruth J.P. Selection of Optimal Working Conditions in EDM. Proc. 20th Int. Conf. Mach.
6.
Chetwynd D.G. The Digitization of Surface Profiles.
7.
Whitehouse D.J. and Phillips M.J. Discrete Properties of Random Surfaces.Phil. Trans. R. Soc. Lond., 1978,
Tool Des. Res., Birmingham, 1979, 583--590 Wear, 1979, 57, 137-145
290(1369), 267-298 8.
BendatJ~. and Piersol A.G. Random Data: Analysis and Measurement Procedures.Wiley-lnterscience Pub/., 1971, 311--324
9.
Volk W. Applied Statistics for Engineers.2nd Ed., McGraw-Hill Book Co., 1969, 109-148
10. KahngC.H. and Rajurkar K.P. Fundamental Theories of the Parameters of EDM Process.SME Technical Paper, 1977, MR77-285
11. Van Dijck F. and Snoeys R. A Theoretical and Experimental Study of the Main Parameters Governingthe EDM Process. Mecanique, 1975, (301-302), 9 - 1 6
113
According to the established thermal theories on electrodischarge machining (edm), the size of the craters produced in edm is proportional to the discharge energy, which is equal to the product of mean pulse current and pulse duration for a constant gap voltage. As a practical edm surface is a random superposition of such craters, the behaviour of the surfaces largely depends on the two important parameters, pulse current and pulse duration. With the help of multiple linear regression analysis, the effect of pulse duration and pulse current on the surface topography parameters Rq and [J* has been studied. sem photomicrographs have been taken in support of the qualitative evidence
Keywords:electro-dischargemachining, roughness (surface), mathematical analysis
A clear characterization of surface topography is essential to predict the quality and functional behaviour of contacting surfaces, such as static and dynamic stiffness, damping capacity, wear resistance, fretting corrosion etc 1 . The surfaces obtained during electro-discharge machining (edm) are the result of a combination of various contributing factors, which differ from conventional machining processes. A practical edm surface is a random superposition of craters formed by the discrete removal of metal by the thermal effects of successive discharges. As such, the most common method of representing the surface roughness by R a or maximum peak-to-valley distance (Rmax) fails to give a true picture of the surface and the need for a statistical approach to the problem has been established 2 . The random function approach for characterizing surface topography gives a comprehensive statistical description of the surface 3. Process correlation length ~* (at which the autocorrelation function (acf) becomes 0.1), as defined by Peklenik4 , is an important topographical index and can be used as a parameter for system identification and characterization of the pPofile. Since the surface obtained in edm is largely influenced by the discharge energy, it is more appropriate to have the energy terms split between pulse duration and current for precise understanding of their effect on/~* and the rms value of surface roughness, Rq. Further, since the pulse current and pulse duration can be varied and set independently, this kind of relationship will be useful in adaptive optimization and control of the process for material removal rate, tool wear and surface characteristicss. *Mechanical Engineering department, R.E. College, Durgapur713209, India **Mechanical Engineering Department, Indian Institute of Technology, Kharagpur-721302, India
PRECISION ENGINEER ING
Experimental procedure The experiments were conducted on Electra Puls Spark Erosion machine ModeI-M 100 having a solid state edm power supply (30 A). The open gap voltage across the electrodes is 90 V. As pulses with the same energy can have remarkably different combinations of average pulse current (if) and pulse duration (tp) and may cause different amounts of erosion, a 3 x 5 experiment was selected for the two variables of pulse current and pulse duration. Three values of pulse current, 10, 15 and 25 A, and five levels of pulse duration, 20, 50, 100, 200 and 500 #s, were chosen for the investigation. The dielectric employed was kerosene and two electrode materials, copper and graphite, were used. Tool steel (IS : T105 W2 Cr 60 V25) having an average hardness of Rc 56 was chosen as the workpiece material. A duty cycle of 50% was employed and the duration of each test was 1000 s. The surface profiles were obtained on a Rank Taylor Hobson Talysurf Model-4, which was provided with a diamond pyramid stylus of tip dimension 2.5/zm. Since the samples within the stylus tip dimension are unnaturally correlated6'7, a sampling interval of 5/Jm was selected for digitization of the profiles. The roughness of the surfaces produced during some of these tests was generally outside the normal range of the instrument. In fact the integrating meter was used only to compare the R a values with that of the estimated RQ values for a few cases. The scanning electron microscope (sem) photomicregraphs were obtained on an ISI-60 model sem with an accelerating potential of 30 kV. Some typical photomicrographs are given in Figs 1-5. The estimation of Rq and the plot of acf for each of the profiles was obtained using the standard procedure as outlined in Bendat and Piersol s. From the plots of acf the values of ~* were determined for all the profiles. The results are shown in Tables 1 and 2.
Surface characteristics R o o t mean square Multiple linear regression analysis was carried out for the logarithms of the variables to quantify the variation of Rq with pulse current (if) and pulse duration (tp). From the results of the statistical tests, a good correlation exists between Rq and the two independent variables, pulse current and pulse duration taken together, whereas their interaction has no significance. This is evident from the high value obtained for the multiple correlation coefficient. Further, the estimated values of 't' from 't'-tests for the coefficients are greater than the tabled values of 't' corresponding to 12 degrees of freedom at the 1% significance level9 . With Rq in micrometres, if in amps and tp in microseconds, the relations are as follows:
0141--6359/82/020111--03 $03.000 1982 Butterworth & Co (Publishers) Ltd
111
Ramarao and Faruqi - Surfaces obtained in electro-discharge machining Copper electrode Rq = 0.596 if °'399tp °'22s
(1)
Graphite electrode Rq = 0.340 if °'s63 tp °'281
(2)
Process c o r r e l a t i o n length
The above expressions show that Rq increases with pulse duration and pulse current, which is to be expected since the increase in pulse energy leads to greater metal removal z°. However the impact on Rq of pulse current is more than that of pulse duration.
The results of the statistical tests of process correlation length (/3*) indicate that effects of pulse current and pulse duration are significant at the 1% level of significance and the tests also show a very high correlation which implies that their interaction has no significance. Thus the linear regression models are justified. The following results are obtained relating/3* to if and tp. Copper electrode 3* = 13.53 if °'2s9 tp °'124
(3)
Graphite electrode 3* = 9.56 if°'328tp°A88
(4)
It is observed that/3* increases with both pulse current and pulse duration. However the values of the exponents for current are higher than for pulse duration, indicating that 3* is more influenced by the pulse current than the pulse duration. The theoretical predictions of crater radii by Van Dijck and Snoeys 11 using thermal models also indicated an increase in a similar manner with pulse current and pulse duration. It is therefore proposed that ~* may be taken as a measure representative of the radius of the crater obtained by theoretical models.
Scanningelectron microscopy Fig 1 sem photomicrograph (xlO0) (Electrode: Cu, tp = 50 Ps, i f = 10 A)
In order to verify the results obtained in the preceding sections, qualitatively, sem photomicrographs of the eroded surfaces were obtained. Typical photomicrographs to assess the affect of pulse duration and pulse current are given in
Fig 2 sem photomicrograph (x 100) (Electrode: Cu, tp = 200 ps, if = IO A)
Fig 3 sem photomicrograph (x 100) (Electrode: Cu, tp = 100 ps, if = 15 A)
Fig 4 sem photomicrograph (x 100) (Electrode: Cu, tp = lOOps, if = 25 A)
Fig 5 sem photomicrograph (x1100). Enlarged view of crater. (Electrode: Cu, tp = I00 ps, if = 15 A)
112
A P R I L 1982 V O L 4 NO 2
Ramarao and Faruqi - Surfaces obtained in electro-discharge machining
Table 2 Values of ~*,/.tm
Table 1 Values of R q , / l m Copper Pu Ise duration, /~s
10
15
20 50 100 200 500
3.00 3.75 4.35 5.12 5.62
3.37 4.12 5.00 6.12 6.75
Copper
Graphite Pulse current, A 25 10
4.15 4.75 6.51 7.63 8.84
3.05 3.85 4.45 5.75 6.90
15
25
Pulse duration, /~s
10
15
3.49 4.57 6.08 7.34 8.21
4.79 5.56 8.27 10.18 12.06
20 50 100 200 500
38 42 44 46 48
40 43 45 54 62
Figs 1-5. The effect of pulse duration is evident from Figs 1 and 2, for respectively 50 and 200/~s, at the same pulse current of 10 A. Although it will be impossible to predict the exact crater size from the photomicrographs, because of their random positioning, it is apparent that the crater size is increasing with pulse duration, suggesting higher values of Rq and/~*. It can be seen that the surface obtained from a 50/~s discharge duration was smoother compared to the surface obtained at 200/zs pulse duration. The effect of pulse current can be observed from Figs 3 and 4, for respectively 15 A and 25 A, at the same pulse duration of 100/~s. Increase in pulse current results in larger crater sizes indicating more eroded material and high values of Rq and ~*, thereby supporting the quantitative predictions by regression expressions. Some other metallographic features such as overlapping layers and pock marks can be seen from photomicrographs at very high magnification. More prominent pock marks near the walls and spheroidised particles at the bottom of the crater can be observed from Fig 5. References 1. BeardsC.F. and NeroutsopoulosA.A. The Control of Structural Vibration by Frictional Damping in ElectroDischarge Machined Joints. Trans. ASME, J. Mechanical Design, 1980, 102, 5 4 - 5 7
PRECISION ENGINEERING
2.
Graphite
Pulse current, A 25 10
44 46 56 64 70
40 43 45 54 62
15
25
41 47 60 65 71
46 53 64 75 100
Crookall J.R. and Khor B.C. Electro-DischargeMachined Surfaces. Proc. 15th Int. Conf. Machine Tool Des. Res., Macmillan, 1974, I - 3
3.
PeklenikJ. New Developments in Surface Characterization and Measurementby Meansof Random ProcessAnalysis. Proc. I. Mech. E., 1968, 182(3), 108-112
4.
PeklenikJ. Investigation of Surface Typology. C.I.R.P. Ann., 1967, 15, 381-385
5.
SnoeysR., Dauw D. and Kruth J.P. Selection of Optimal Working Conditions in EDM. Proc. 20th Int. Conf. Mach.
6.
Chetwynd D.G. The Digitization of Surface Profiles.
7.
Whitehouse D.J. and Phillips M.J. Discrete Properties of Random Surfaces.Phil. Trans. R. Soc. Lond., 1978,
Tool Des. Res., Birmingham, 1979, 583--590 Wear, 1979, 57, 137-145
290(1369), 267-298 8.
BendatJ~. and Piersol A.G. Random Data: Analysis and Measurement Procedures.Wiley-lnterscience Pub/., 1971, 311--324
9.
Volk W. Applied Statistics for Engineers.2nd Ed., McGraw-Hill Book Co., 1969, 109-148
10. KahngC.H. and Rajurkar K.P. Fundamental Theories of the Parameters of EDM Process.SME Technical Paper, 1977, MR77-285
11. Van Dijck F. and Snoeys R. A Theoretical and Experimental Study of the Main Parameters Governingthe EDM Process. Mecanique, 1975, (301-302), 9 - 1 6
113