An electrode wear compensation model regarding the EDM process.
Tirla, Andrei ; Popa, Marcel Sabin ; Pop, Grigore Marian 等
1. INTRODUCTION
For a long time now in other words from the beginning of industrial
usage of EDM in 1950, it was admited that this procedure, due to
it's capacities of eroding through electric sparks in any spacial directions is kinematically very flexible (Popa et al., 2009). The
manufacturing of complex surfaces by electro erosion is one of the most
used methods, especially because it can generate practically surfaces as
complicated as it needs, and not depending on the hardness of the
material. The process is used mainly in the work tools departments and
also for the large series of machining. The main advantage is that the
work tool is performed from materials that are easy to process and the
conditions of working don't depend on the hardness of the material.
Concerning the fact that the finishing works of the surfaces occur after
the process thermal treatment, the process is difficult and expensive.
Electric discharge machining becomes very efficient. Without a contact
between tool and work piece, it is avoided the appearance of the
distortion in the work piece and of the internal stress in the
superficial layer of the manufactured surface (Nichici & Achimescu,
1983). The major advantage of EDM in comparison with other manufacturing
processes is represented by the fact that the hardness of material is
not important, the only necessary condition is that the processed
material must be electro conductive (Popa M, et al, 2008).
[FIGURE 1 OMITTED]
During the process, a voltage is applied between two
electrodes--the tool and the workpiece--closely placed inside a liquid
dielectric medium. When electrodes are very close to each other (gap
distance) an electric spark discharge occurs between them forming a
plasma channel between the cathode and the anode (Figure 1 shows a
close-up of the machining region).
The tool wear at the electro discharge machining is a matter of
interest at present and its reduction is discussed by many researchers.
The tool wear process (TWP) is quite similar to the MRM (material
removal mechanism) as the tool and workpiece are considered as a set of
electrodes in EDM. Mohri claimed that tool wear is affected by the
precipitation of carbon from the hydrocarbon dielectric onto the
electrode surface during sparking (Mohri et al., 1995). He also stated
that the rapid wear on the electrode edge was due to the failure of
carbon to precipitate at difficult-to-reach regions of the electrode.
From this simple understanding of TWP, some useful applications
exploiting both the advantages and disadvantages of electrode wear have
been developed. Marafona and Wykes introduced a wear inhibitor carbon
layer on the electrode surface by adjusting the settings of the process
parameters prior to normal EDM conditions (Marafona & Wykes, 2000).
Although the thickness of the carbon inhibitor layer made a significant
improvement on the TWR (tool wear rate), it has little effect on the MRR (material removal rate). On the other hand, for applications requiring
material accretion, a large pulse current is encouraged to increase
electrode wear implanting electrode material onto the workpiece.
2. ELECTRODE WEAR COMPENSATION MODEL
Research on electrode wear compensation focuses on geometric
modeling of electrodes. The methodology consists of determining the wear
of electrodes and its compensation by adding material on the active
surface. To determine the minimum number of experiments required the
ANOVA method will be used. The experimental tests will determine the
electrode wear over time, depending on processing parameters of the
machine. To determine the evolution of wear, there will be made 3D
electrodes scanning after a At processing time. A database consisting of
electrode wear based on the values of used parameters will be prepared.
For electrodes compensation there will be produced two mathematical
algorithms that describe:
* Dimensional compensation of the electrodes
* Geometrical compensation of the electrode.
With Visual C programming language will be redesigned the electrode
shape and the form will be rendered using CAD software. For redesigned
electrodes the CAM program will be automatically generated and their
execution will be carried through the processing center. Testing will be
resumed in order to validate the generated mathematical algorithms. The
project will be completed through a program that will automatically
generate the electrode depending on the form that is intended to be
processed.
[FIGURE 2 OMITTED]
3. EXPERIMENTAL RESEARCH
This paper presents the results for the first set of experiments
where it is desired to observe the impact of tool wear on the
electrodes. For the experiments it was used stainless steel 1.4112 and
graphite as raw materials. There were made two types of electrodes, one
with a flat active surface and the other one with a conic active
surface. The purpose was to notice the surface's modifications and
to observe if the conical surface flattens out. This flatten will appear
due to the wear effect. For a better understanding the conical dimension
was established to 2 mm. Because the machine is capable of achieving a
roughness between 0,1 Lim and 18 Lim it was decide to make three
machining sets with the next values 0,3 [micro]m, 1,1 [micro]m si 6,3
[micro]m. The machining depht was set to 3 mm.
After the machining process there were measurements made and the
active surface of the electrodes was scanned in order to observe the
radius modifications that apears to the conical ones. For this operation
it was used a Carl Zeiss CONTURA measuring center, equipment that also
brings the 3D model of the electrodes. After that with a CAD (computer
aided design) software the electrodes profiles were overlaid and we
obtained the needed information.
[FIGURE 4 OMITTED]
After the measurements it was observed that the radius increases at
the same time with the processing blank, the wear being more pronounced
at the extremities of the electrode. A strange phaenomenon take place in
the center of electrode were it was obtained a constant wear, regardless
of the roughness required and, implicit, regardless of the machining
parameters.
4. CONCLUSIONS
The tool wear at the electro discharge machining is a matter of
interest at present and its reduction is discussed by many researchers.
The proposed concept involves designing a model of dimensional and
geometrical compensation of the electrodes used in electrical erosion
processing, performance to meet current world level. Regarding the
interpretation of experimental data obtained can be concluded that the
direction of research is good.
5. REFERENCES
Popa, M.S.; Kunz, A. & Kennel, T. (2009). Innovative
Technologien und kreative produktionprozesse, U.T. Press, ISBN 978-973-662-421-6, Cluj-Napoca, Romania
Nichici, Al. & Achimescu, N. (1983). Prelucrarea prin eroziune
electrica in constructia de masini, Editura Facla, Timisoara, Romania
Popa M. S., et. al., New Trends in Non--Conventional technologies
and Electric Discharge Machining, The 2nd European DAAAM International
Young Researchers' and Scientists' Conference, ISBN
978-3-901509-66-7, Trnava, 22-25th October 2008, Slovakia
N. Mohri; M. Suzuki; M. Furuya & N. Saito (1995). Electrode
wear process in electrical discharge machining, Ann. CIRP 44 (1),
165-168, ISSN 0007-8506
J. Marafona & C. Wykes (2000). A new method of optimising
material removal rate using EDM with copper-tungsten electrodes, Int. J.
Mach. Tools Manuf. 40 (2), 153-164, ISSN 0890-6955
Tab. 1. Results for the electrode with conical active surface
Exp. Initial Initial
in. Electrode type lenght /final [mm] diameter
4 [OMEGA]30 Conical 22,8 22,426 30,46
5 [OMEGA]30 Conical 22,426 22,207 29,98
6 [OMEGA]30 Conical 22,207 22,09 29,92
Exp. Wished /obtained
in. Electrode type /final [mini roughness [[micro]m]
4 [OMEGA]30 Conical 29.98 0,3 0,442
5 [OMEGA]30 Conical 29,92 1,1 1,99
6 [OMEGA]30 Conical 29,81 6,3 6,375
Wished /obtained Machining Edge
depht [mm] program difference
3 2,626 P6 [right arrow] P2-P4 0.374
3 2,781 P8 [right arrow] P6 0,15
3 2,883 P8 [right arrow] PG 0,093
Conical
Wished Central surface
depht difference radius
3 0,06 47,7S
3 0,06 54,15
3 0,62 56,23
58,03