PSI - Issue 10

E.L. Papazoglou et al. / Procedia Structural Integrity 10 (2018) 235–242

238

E.L. Papazoglou et al. / Structural Integrity Procedia 00 (2018) 000 – 000

4

The equation which has been used for the calculation of the performance parameter is:

W W

- st fin

MRR

(1)

exp

t  

m

with MRR exp being the material removal ratio in g/min, W st , W fin the workpiece weight in g before and after machining, respectively, ρ the workpiece material density in g/mm 3 and t m the machining time in min. As the specific EDM machine has no option to directly define the T off time, either the duty factor, it must be calcu lated indirectly, for using it in the mean machining power calculation. Researches on the current and voltage wave forms during EDM provide useful data about a typical current waveform, coming from a transistor resistor power unit (Fan et al. (2016); Wang et al. (2017)). Based on the previous mentioned references, the assumption that the current waveform has a trapezoid form has been made, reaching the peak current value in the first 10% of the pulse-on time, and zeroes in the last 5% (Fig. 2). Taking in mind that the indication of ammeter is the mean value of pulse current Ī P during the T on and T off time and knowing the nominal peak current value (I P ) the duty factor η can be calculated as:

T

T

0.85  





on

on

T T 

T

I

1.85



2



1

1.85

on off

P

2

  I t dt

0 

I

I

I    

















on

(2)

P





P

P

P

T T 

T T 

I

2

1.85



T

T

2





on off

on off

P

on off

Fig. 2. Theoretical current waveform The mean machining power and the nominal discharged energy per pulse are given by Eqs.(3) and (4), respectively:

P V I 

f    

(3)

av

P P

eff

(4)

3 10

nom P P on E V I T    

with P av in W, V P in V, I p in A, E nom in mJ and T on in μ s. 3. Results and discussion

The results of the above mentioned experimental procedure and the calculations results are shown in Table 2 and in graphical form in Fig. 3.

Table 2. Experimental and calculations results # 1 2 3

4

5

6

7

8

9

10

11

12

P av [W]

454.05 415.14 363.24 311.35 525.41 363.24 285.41 165.41 376.22 324.32 207.57 389.19

E nom [mJ]

81

72

63

54

243

162

135 57.8

108

315

225

180

54

MRR [mm 3 /min]

87.18

76.92

76.2

48.78

101.21

78.94 11.68

29.24

78.43

56.61

29.63

81.45

Ra [ μ m] Rt [ μ m]

8.64

9.04 68.8

9.4

9.24

11.16

10.04

7.88 56.4

9.84 67.2

8.48

7.52 48.6

8.76 61.8

59

66

62

69.8

77.4

68

64