19-04-2012, 05:11 PM
A study to achieve a fine surface finish in Wire-EDM
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Introduction
Accompanying the development of mechanical industry,
the demands for alloy materials having high hardness,
toughness and impact resistance are increasing. Nevertheless,
those materials are difficult to be machined by
traditional machining methods. Hence, non-traditional machining
methods including electrolytic grinding, supersonic
machining and electrical discharging machining (EDM) are
applied. Wire-EDM process with a thin wire as an electrode
transforms electrical energy to thermal energy for
cutting materials. With this nature, alloy steels, conductive
ceramics and aerospace parts can be machined irrespective
to their hardness and toughness. Furthermore, Wire-EDM
is capable of producing a fine, precise, corrosion-resistance
and wear-resistance surface.
Experimental equipments
The Wire-EDM machine used in this research was
developed by Industrial Technology Research Institute
(ITRI), Taiwan. The original ac pulse-generating circuit
for roughing operation is shown in Fig. 1. In this research,
the pulse-generating circuit for finishing operation was
designed by modifying the original circuit, by way of removing
the high-voltage discharging circuit. During the
whole machining process, a low-voltage pulse-generating
circuit was adopted for both ignition and machining as
shown in Fig. 2. In addition, all other elements in the
circuit such as resistance and capacitance are adjustable.
Based on the structure of the full-bridge circuit, machining
which uses ac alternating pulse-generating circuit,
dc pulse-generating circuit of positive polarity (taking
the wire as the anode) or dc pulse-generating circuit of
negative polarity can be achieved by two sets of power
switches.
Approach to achieving a fine surface
Based on the above analysis, the significant factors affecting
surface roughness in finishing are noted as machining
voltage, current-limiting resistance, capacitance and type of
pulse-generating circuit together with the use of a dielectric
of lower conductivity. Experiments of finishing operation
employing the dc pulse-generating circuit of positive
polarity having the voltage of 100V and no capacitance
are conducted under the dielectric having a conductivity of
15S/cm. By gradually increasing the values of resistance
the measured surface roughness is depicted in Fig. 8. A fine
surface of roughness Ra = 0.22 m can be obtained eventually
using dc pulse-generating circuit of positive polarity
and a low discharging energy. Although, a large resistance,
such as 125, can further decrease the discharging energy.
Conclusions
To obtain good surface roughness, the traditional circuit
using low power for ignition is modified for machining as
well.With the assistance of Taguchi quality design, ANOVA
and F-test, machining voltage, current-limiting resistance,
type of pulse-generating circuit and capacitance are identified
as the significant parameters affecting the surface roughness
in finishing process. A dc pulse-generating circuit of
positive polarity (wire electrode is set as anode) can achieve
a better surface roughness in finishing operation. In addition,
it is found that a low conductivity of dielectric should
be incorporated for the discharge spark to take place.
After analyzing the effect of each relevant factor on
surface roughness, appropriate values of all parameter are
chosen and a fine surface of roughness Ra = 0.22 m is
achieved. The improvement is limited because finishing
process becomes more difficult due to the occurrence of
short circuit attributed to wire deflection and vibration when
the energy is gradually lowered.