Int J Mater Form (2010) Vol. 3 Suppl 1:1107–1110 DOI 10.1007/s12289-010-0965-z © Springer-Verlag France 2010
STUDY ON THE DRY ELECTRICAL DISCHARGE MACHINING Irina Beşliu1*, Hans-Peter Schulze2, Margareta Coteaţă3, Dumitru Amarandei1 1
University Ştefan cel Mare of Suceava, Romania Otto-vonGuericke University Magdeburg, Germany 3 “Gheorghe Asachi” Technical University of Iaşi, Romania 2
ABSTRACT: The dry electrical discharge machining is considered by the researchers as an environment friendly machining process and different specific aspects of this machining method are presented in the specialty literature. To obtain holes by electrical discharge machining, tubular rotating electrodes tools could be used. The paper presents a relatively simple device proposed to be used for applying the dry electrical discharge drilling. The rotation of the electrode tool could contribute to the diminishing of the errors of the circular shape in a cross section through the hole. Some preliminary experiments proved the possibilities to use this device to achieve holes in workpieces of low thickness. Test pieces made of stainless steel were machined by dry EDM and by common EDM. For the applied operating conditions, the dry electrical discharge drilling presents some advantages regarding the machined surface quality and the electrode tool wear, in comparison with the machining that uses liquid dielectric. KEYWORDS: Dry electrical discharge machining, Tubular electrode tool, Device, Electrode tool wear, Working fluids
1 INTRODUCTION Nowadays green production is the only rational strategy for companies to obtain competitive advantages over the business competitors. Environment friendly manufacturing is the new trend even in the field of metal parts production. Researchers focus in developing nopolluting ways to cut materials that also ensures the technical requirements like high material removal rates, surface accuracy, low tool wear, minimum roughness values etc. If the electrical discharge machining (EDM) process is analyzed, the main cause of environmental concerns is related to the use of mineral oil based dielectric liquids. High temperature, chemical breakdown of these dielectric liquids associated to the machining process generate toxic fumes and sometimes even fire hazards. Another problem related to the use of dielectric liquids is the low recyclables. All these facts were the reasons for developing the dry electrical discharge machining processes which use different gases as dielectric fluids instead of the working liquids. The first approach in this field was mentioned in a report elaborated by NASA and dating from 1985; this report was stipulating that some electrical discharge drilling processes were conducted in dielectric fluid mediums such as argon and helium [1, 2]. Some years later, a group of researchers [3] referred to the immersing of the oxygen in the discharge gap. Thus, they highlighted a higher removal rate in comparison with the use of distilled water as dielectric liquid. Another research was made by Kunieda et.al. [4] in
1997. They showed that the use of compressed air in the electrical discharge machining with tubular copper electrodes can ensure a minimum wear of the tool electrode [4]. In the United States of America, an important research centre in the field of dry EDM field was developed at the Michigan University [4, 2, 5]. In recent years, the studies made by the researchers from this university stoked important information referring to the main aspects of the dry electrical machining processes. They made experiments on dry EDM and near-dry EDM for wire electrode and also for drilling method [2, 5]. The specialty literature [6, 7, 8] regarding the dry electrical discharge drilling process shows that the flow of high velocity gas through the tool electrode into the dielectric gap can efficiently substitute the liquid dielectric. In association with a tool electrode rotation movement, these gases prevent the extreme heating of the tool and workpiece electrodes, and facilitate the removal of the debris. The common performance variables used to characterize the dry EDM processes are the material removal rate, the tool wear rate, the machined surface roughness, shape accuracy of the machined surfaces etc. Some advantages noticed by the researchers in this field are the lower discharge gap, the low tool wear, the thinner heat affected zone, the higher material removal rate and the lower residual stresses [ 5, 6]. Even if there are numerous papers referring to this topic, the current track recorded in the field is not yet ready for an industrial implementation of the process.
____________________ * Corresponding author: Blvd D. Mangeron 59 A, 700050 Iasi, Romania, phone: + 40 232 278680 2313, fax: +40 232 217290, email:
[email protected]
1108 Work fluid
vf
nET
Figure 2: Geometrical conditions in the case of the rotating electrode tool for achieving closed holes Figure 1: Machining schema valid in the case of the dry electrical discharge drilling of closed holes
dh e
2 BACKGROUND A machining schema corresponding to the dry electrical discharge machining is presented in figure 1. To obtain penetrated holes, a tubular electrode tool could be used. To avoid the welding of the electrode tool to workpiece, usually this electrode tool achieves a rotation motion; generally, the rotation motion of the electrode tool ensures the diminishing of the errors from the circular shape of the machined hole in a cross section. Of course, the work fluid could be introduced by the tubular electrode. The solution presented in figure 1 could be used only for open end holes. As known, the main functions of the work fluid are the following: a) to remove the material detached from the workpiece and electrode tool from the work zone and, thus, to diminish the proportion of the wasted side electrical discharges developed between the electrodes and the metallic debris existing in the fluid; b) in a certain proportion, to contribute to the localization of the electrical discharge and to the distribution of the energy on the surfaces affected by the electrical discharges etc. Such functions were adequately fulfilled by the liquids dedicated to the electrical discharge machining. In the case of the dry electrical discharge machining, if the pressure of the gas sent through the tubular electrode is not high enough, there is an increased probability to not remove the small debris detached from the electrodes as consequence of the electrical discharges; this means that a higher pressure is necessary to efficiently remove the metallic chips found initially in the space between the electrodes. If the holes are blind, a solution able however to allow their machining is based on the use of a rotating electrode tool which has an eccentric longitudinal hole (fig. 2). If the diameter of the electrode tool is DET, the machined hole has a diameter dh and the eccentricity between the electrode tool axis and the hole axis is e, the conditions to obtain a plan surface at the end of the holes are the following:
DET , 2
(1)
dh . 2
(2)
Of course, closed holes could be also obtained by using an electrode tool whose axial hole has a diameter dh lower than the double of the work gap s:
dh
2s
(3)
The last solution could not be convenient due to the fact that the low diameter of the axial hole does not allow an adequate circulation of the compressed air in the work zone. It is known that the initiation of the electrical discharge is possible either by the breakdown of the electrical resistance specific to the fluid existing in the work gap or by the breakage of the contact between the electrodes. The condition of the electrical discharge existence is the following:
s
U , E
(4)
where U is the voltage applied to the electrodes and E is the dielectric rigidity of the fluid existing between the electrodes.
3
EXPERIMENTAL RESEARCHES
Some preliminary experimental researches were developed to prove the possibility to perform holes by dry electrical discharge machining and using relatively simple equipment. With this aim in view, equipment initially used to materialize an electrochemical discharge machining [11] was modified so that it could be used for dry electrical discharge drilling. Essentially, the tubular electrode tool was clamped in a spring collet attached to the end of a rotating shaft (fig. 3); this shaft received the motion by means of a belt drive, from a direct current motor. The rotation speed of the electrode tool was established at 30 rev/min. The shaft is placed on two shielded bearings placed into a sleeve; a system brush - collector ring is used to
1109 connect the rotating electrode tool to a relaxation generator. The compressed air enters the body sleeve by a connecting sleeve; afterwards, it arrives in the axial hole of the shaft by a cross hole. If there is not a specialized ram electrical discharge machine, as generator of electrical discharges a simple relaxation generator could be used (fig. 4). To test the possibilities to use the device for electrical discharge drilling, it was mounted firstly on the slotter ram of a milling machine; the slotter ram was used to avoid the continuous contact of the electrode tool with the workpiece surface. A rod lever mechanism included in the slotter ram allows the periodical removal of the electrode tool from the workpiece. The test piece was fixed on a table which allows a certain vertical displacement under the pressure exerted by the electrode tool; when the electrode tool is removed from the workpiece, the table and the test piece come in the initial position due to the presence of a spring. A compressor was used as gas source; the pressure of the compressed air was of about 6 atm (0.607 MPa). The tubular electrode tool was made of copper and his external diameter was of 2 mm. The test piece material was a stainless steel containing 0.055 % C, 0.076 % Mo, 18.824 % Cr, 0.035 % W, 9.79 % Ni, 1.20 % Mn and 0.225 % Cu. The rectilinear alternative motion of the electrode tool was of 80 strokes per minute. Due to the periodical removal of the electrode tool from the test piece, one can consider that the proper machining time was of about 1.5 minutes (for these preliminary experiments, the duration of the process being of 5 minutes). The relaxation generator used within these preliminary experiments included a source of variable voltage and capacitors which can be connected in different ways, offering a capacity variable between 33 and 840 μF. A voltage of 45 V was applied to the electrodes; the capacity of the capacitors included in the discharge circuit of the relaxation generator was of 840 μF. After a machining duration of 5 minutes, a ring-shaped hole was obtained on the workpiece surface. To make a comparison between dry electrical discharge drilling and electrical discharge machining, a second
Compressed air
nET
vf
Cv
S
Figure 4: Connecting the electrode tool in the circuit of the relaxation generator
machining test was made by using the electrical discharge machining with the immersion of the electrode tool in liquid dielectric. The aspects of the two ring shaped holes obtained in the above mentioned operating conditions could be seen in figure 5. For the duration of the process of 5 minutes, the depth of the machined cavity was of about 0.19 mm in the case of the dry electrical discharge process and of about 0.49 mm in the case of developing the machining process in liquid dielectric. The images presented in figure 6 highlights a better quality of the machined surface in the case of the dry electrical discharge machining process, when the gas flow it does not allow the adhesion of the melted debris to the test piece surfaces. For these operating conditions, the wear of the electrode tool was lower in the case of the dry electrical discharge machining process, as one can see in figure 6; the shape of the active zone of the electrode tool was less affected in the case of the dry electrical discharge machining.
a
b
Figure 5: Ring-shaped holes obtained by electrical discharge machining: a – using the liquid dielectric as working fluid; b – dry electrical discharge drilling (images achieved by means of an optical microscope; magnification 60 x)
Figure 3: Equipment for ensuring the electrode tool rotation
In order to analyse evolution in time of the material removal ratio (MRR) and tool wear ratio of the dry electrical discharge machining process another set of experiments were conducted in dry conditions on a AD3L Sodick electrical discharge machining. The tubular electrode tools chosen had an external diameter of 2 mm and were made of copper. The same device for rotating motion was used. The workpiece material was a stainless steel with the following composition: 0.063 % C, 0.251 % Mo, 18.307 % Cr, 0.417 % W, 9.93 % Ni, 1.18 % Mn and 0.328 % Cu. The machining times were
1110 machine in order to establish the optimal electrical working parameters for dry electrical discharge drilling.
ACKNOWLEDGEMENT The research was carried out as part of project no. ID_625, financed by the National Council of Scientific Research for Higher Education (Romania). a
b
Figure 6: External electrode tool wear in the case of the obtaining of ring-shaped holes: a – by using liquid dielectric as working liquid; b – by using dry electrical discharge machining (electrode tool diameter: 2 mm; images achieved by means of an optical microscope; magnification 60 x)
10, 30 and 40 minutes. The results obtained in the experiments are presented in the graphic from figure 7. As seen in the graphic, the material removal ratio is higher than the tool electrode wear ratio, both evaluated by mass loss.
Figure 7: The evolution in time of the MRR and tool wear
4 CONCLUSIONS One of the evolution tendencies of the researches and application in the field of the electrical discharge machining concerns the use of the dry or near – dry fluids as working medium. Generally, to obtain holes by dry electrical discharge machining, tubular electrodes tools are used; the researchers found that good results are also obtained by the rotation of the electrode tool. Relatively simple equipment was used to test the possibilities to materialize a dry electrical discharge drilling process; some preliminary experiments were developed by using both the liquid dielectric and the compressed air as work fluids. These preliminary experiments showed that for the considered operating conditions, the surface quality and the electrode tool wear are more advantageous in the case of the dry electrical discharge process. In the future, the researches will be directed to the use of the above mentioned device on a proper ram electrical discharge
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