journal homepage: www.elsevier.com/locate/jmatprotec Copper and graphite electrodes performance in electrical-discharge machining of XW42 tool steel C.H. Che Haron ∗, J.A. Ghani, Y. Burhanuddin, Y.K. Seong, C.Y. Swee Department of Mechanical and Materials Engineering, Faculty of Engineering, National University of Malaysia, 43600 Bangi, Selangor, Malaysia article info
Keywords: EDM Current Material removal rate Electrode wear rate
abstract
Electrical-discharge machining (EDM)
is
a process
of
utilizing the removal phenomenon of electrical-discharge in dielectric,
therefore,
the
electrode
plays an important role,
which affects
the material removal rate and the
tool wear rate. In this paper the machining char- acteristics were investigated when machining XW42
tool steel at two current
settings
(3
A
and 6 A),
three diameter sizes (10, 15 and 20 mm) and kerosene
as the
dielectric.
The results show that the material removal rate is
higher
and the relative electrode wear ratio is lower
with copper electrode than graphite electrode. The increase in the current and electrode diameter reduced tool wear rate as well as the material removal rate. © 2007 Elsevier B.V. All rights reserved. 1. Introduction An electrical-discharge machining (EDM) is based on the erod- ing effect of an electric spark on both the electrodes used. To obtain the maximum metal removal with minimum wear on the tool electrode, the work material and the tool must be set at positive and negative electrodes, respectively. EDM is commonly
used in tool, die and mould making industries for machining heat-treated tool
steel
materials.
The heat-treated tool steels material falls in the difficult-to-cut material group when using conventional machining process. The high rate of tool wear is one of the main problems in EDM. The
wear ratio, defined as the volume of metal lost from the tool divided by the volume of metal removed from the
work material, varies with the tool and work materials used (Boothroyd, 1981; McGeough, 1988). High tool wear rates result in inaccurate dimension. Due to these reasons, a suitable combination between workpiece and tool materials has been studied by previous researchers. Bhattacharyya et al. described influence of rise in the surface temperature on both the electrode and the workpiece (Bhattacharya et al., 1981). Patel et al. stated that the anode erosion rate increases as the current increases, but decreases as pulse time increases
(Patel et al., 1989).
Che Haron
et al.
studied
the
correlation between
current and
the machinability of the
AISI 1045 tool steel with
the
hardness
230
HB, using copper electrodes
(Che Haron et al., 2001). Singh et.al investigated the
material removal rate, tool wear, surface quality and
diametric over-cut
on
En-31 tool steel, in detail and concluded that copper and aluminium electrode resulted in the best machining rate (Singh et al., 2004). Ho and Newman stated that research areas in EDM could be arranged into three major headings: machining performance measures, the effects of process parameter and design and manufacture electrode. They concluded that machining per- formance depends on the wear and surface quality (Ho and Newman, 2003). This paper describes the wear behaviour of copper and graphite electrodes when machining XW42 tool steel.
The material removal rate and the electrode wear
rate
were
selected as
the
machinability factors to be investigated. The results will be utilised to select an optimum electrode and work material combination ∗ Corresponding author. Tel.: +603 8921 6516; fax: +603 8925 9659. E-mail address: chase@vlsi.eng.ukm.my (C.H. Che Haron).
0924-0136/$ – see front matter
© 2007
Elsevier B.V. All rights reserved. doi:10.1016/
j.jmatprotec.2007.11.285 571 Table 1 – Physical properties of graphite and copper electrode
Physical properties
Graphite
Copper Electrical resistivity (? /cm) Electrical conductivity compared with silver (%) Thermal conductivity (W/m K) Melting point
(◦C)
Specific heat (cal/g
◦C)
Specific gravity at
20 ◦C
(g/cm3) Coefficient of thermal expansion (×10−6
◦C
−1)
0.12 0.11 160 455 0.17–0.2 1.75 7.8 1.96 92 380.7 1083 0.092 8.9 6.6 2. Experimental details 2.1.
Work piece and
electrode
material The work material used in this study was
XW42
tool steel.
The chemical composition
of
the work material are as fol- lows: C = 1.42%, S = 0.3%, Mn = 0.4%, Cr = 11.2%, Mo = 0.8% and V = 0.2%. The dimension of the work material was 80 mm in diameter and 30 mm in length. The mean hardness value of the one end work material was 43 HRC. The EDM experiments were performed on NC EDM Mitsubishi M35J. For the purpose of this study, copper and graphite electrodes were selected. The
electrodes with diameter of
10, 15
and 20 mm were used
for the said material. The physical properties of copper and graphite electrodes are given in Table 1. 2.2. Machining tests The machining tests were performed for each type of electrode, graphite electrode followed by copper electrode subsequently. The tool and material were mounted in the machine and immersed about 5 cm in kerosene as dielectric. Machining test was carried out for 20 min at two different cur- rent settings, 3 A and 6 A. The mass lost was measured in every 5 min. The mass lost from the electrode and work material was weighed using a digital weighing scale and recorded. Machin- ing procedures were repeated for electrodes with diameters of 10, 15 and 20 mm.
3. Results and discussion 3.1.
Electrode wear
rate The
percentage
of mass
lost from both the copper and graphite electrodes with diameter of 10, 15
and 20 mm when machining at two current settings of 3
A
and 6 A are shown in Figs.
1–5. Generally, the tool wear rate decreases with the increasing size of the electrode diameter and machining time for both of the two current settings. The
percentage of mass
lost
at
a
higher current setting is greater than
that
at
a
lower current setting. The
ratio
of
mass lost percentage between the higher current setting and the lower setting is up to 10 times for the graphite electrode and 15 times for the copper electrode. Fig. 1 –
Mass loss
percentage
of
copper
electrode at a current setting of 3 A.
Fig. 2 –
Mass loss
percentage
of
graphite
electrode at a current setting of 3 A.
Fig. 3 –
Mass loss
percentage
of
copper
electrode at a current setting of 6 A.
Fig. 4 –
Mass loss
percentage
of
graphite
electrode at a current setting of 6 A.
572 Fig. 5 – Electrode wear
rate of electrode material at two current
setting,
3
A
and 6 A:
(a) copper electrode, (b) graphite electrode. In this study, it was found that the wear rate of copper elec- trode was lower than that of graphite electrode. This is due to the higher melting point of copper electrode material, which is less eroding than that of the lower melting point graphite elec- trode material. As shown in Fig. 1, the mass lost percentage of copper electrode at a current setting of 3 A decreased for the electrode with the diameter of 15 and 20 mm, while the mass lost percentage increased for the electrode with the diameter of 10 mm. However, the mass lost percentage hereafter 15 min of machining is equal for all diameters. While the
percentage of mass
lost
of
graphite
electrode at current setting of 3 A
(Fig. 2), electrode wear with 20 mm diam- eter size is lower than those of graphite electrodes with 10 and 15 mm diameter, is still higher than those of graphite with 10 and 15 mm diameter size above 15 min. machining time. The non-linear curve of the
percentage of mass
lost
of
graphite
electrode at a current setting of 3 A
is caused by current insta- bility contrary to the curves of the percentage of mass lost of graphite electrode in Fig. 4. 3.2.
Material removal rate The percentages of mass
lost
of
work
material when
using copper electrodes
at two current
setting
(3
A
and 6 A) with diameter of
10, 15,
and 20 mm are shown in Figs.
6
and
7,
respectively.
The percentages of mass
lost
of
work
material
using graphite electrodes
at two current settings
with diame- ter
of
10, 15,
and
20 mm are shown in Fig. 8. Fig. 6 –
Percentage of mass loss of workpiece material
with copper electrode
at a current setting of 3 A.
From these figures, generally, the trend of the curves is sim- ilar. Percentages of mass lost at 6 A current setting is greater than that at 3 A current setting for both types of electrodes material. The percentage of mass lost of workpiece with larger diameter is greater than that with smaller diameter. The curve of the percentage
of mass
lost
of workpiece material
using copper electrode
at two current
settings
(3
A
and 6 A)
in Figs. 6 and 8(a) shows a similar pattern. Electrode with 15 mm diameter size removed the workpiece with the highest mass removal and the smallest mass removal was achieved using electrode with 20 mm diameter.
The percentages of mass
lost
of workpiece material
using graphite electrode
at
the
two current settings
also display the same pattern. At 3 A current setting, electrode with 15 mm diameter size removed the higher material mass than other two diameter size electrodes. At 6 A current setting, electrode with 20 mm diameter size removed the higher material mass than 10 and 15 mm diameter size electrode. Che Haron et al. (2001) found that the bigger electrode diameter will lose more heat when machining, because this heat will be transferred to electric fluid. The temperature of electric fluid will increase close to the melting point of the material. Based on above argument,
it can be concluded that the material removal rate not only
depends
on the diameter of electrode
and supply of current, but also the type of elec- trode material used. Copper electrode is found more suitable for roughing process, whilst graphite electrode is suitable for finishing process. Fig. 7 –
Percentage of mass loss of workpiece material
with graphite electrode
at a current setting of 3 A.
573 Fig. 8 –
Percentage of mass loss of workpiece material at
the 6
A current setting. (a)
Copper electrode and (b) graphite electrode. 4. Conclusions The EDM performance of copper and graphite tool electrodes was examined with XW42 tool steel. The
important results are summarized as follows: 1. The material removal rate of
XW42 tool steel with copper electrode is greater than that with graphite electrode. 2. Copper electrode is suitable for roughing process, whilst graphite electrode is suitable for finishing process. Com- bination of both electrodes will improve machining characteristics and surface finish. 3. The electrode wear rate of copper is lower than graphite electrode when machining XW42 tool steel. references Boothroyd, G., 1981. Fundamentals of Metal Machining and Machine Tools. McGraw-Hill, New York. Che Haron, C.H., Deros, B.Md., Ginting, A., Fauziah, M., 2001. Investigation on the influence of machining parameters when machining tool steel using EDM. J. Mater. Proc. Technol. 116, 84–87. Ho, K.H., Newman, S.T., 2003. State of the art electrical discharge machining (EDM). Int. J. Mach. Tools Manuf. 43, 1287–1300. McGeough, J.A., 1988. Advanced Methods of Machining. Chapman & Hall, London. Patel, M.R., Barrufet, M.A., Eubank, P.T., DiBitonto, D.D., 1989. Theoretical models of the EDM process. Part II. The anode erosion model. J. Appl. Phys. 66/9, 4104–4111. Singh, S., Maheswari, S., Pandey, P.C., 2004. Some investigations into the electrical discharge machining of hardened tool steel using different electrode materials. J. Mater. Proc. Technol. 149, 272–277. Bhattacharya, S.K., El-Menshawy, M.F., Garber, S., Walbank, J., 1981. A correlation between machining parameters and machinability in EDM. Int. J. Prod. Res. 19/2, 111–122. journal of materials processing technology 201 (2008) 570–573 journal of materials processing technology 201 (2008) 570–573 journal of materials processing technology 201 (2008) 570–573 journal of materials processing technology 201 (2008) 570–573
