02-10-2012, 04:32 PM
Electrical discharge machining
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Generalities
Electrical discharge machining is a machining method primarily used for hard metals or those that would be very difficult to machine with
traditional techniques. EDM typically works with materials that are electrically conductive, although methods for machining
insulating ceramics [5 ] [6 ] with EDM have also been proposed. EDM can cut intricate contours or cavities in pre-hardened steel without the
need for heat treatment to soften and re-harden them. This method can be used with any other metal or metal alloy such
as titanium, hastelloy, kovar, andinconel. Also, applications of this process to shape polycrystalline diamond tools have been reported.[7]
EDM is often included in the ‘non-traditional’ or ‘non-conventional’ group of machining methods together with processes such
aselectrochemical machining (ECM), water jet cutting (WJ, AWJ), laser cutting and opposite to the ‘conventional’ group
(turning, milling,grinding, drilling and any other process whose material removal mechanism is essentially based on mechanical forces).[8]
To obtain a specific geometry, the EDM tool is guided along the desired path very close to the work, ideally it should not touch the
workpiece, although in reality this may happen due to the performance of the specific motion control in use. In this way a large number of
current discharges (colloquially also called sparks) happen, each contributing to the removal of material from both tool and workpiece,
where small craters are formed. The size of the craters is a function of the technological parameters set for the specific job at hand. They
can be with typical dimensions ranging from the nanoscale (in micro-EDM operations) to some hundreds of micrometers in roughing
conditions.
Material removal mechanism :-
The first serious attempt of providing a physical explanation of the material removal during
electric discharge machining is perhaps that of Van Dijk.[15] Van Dijk presented a thermal model together with a computational simulation
to explain the phenomena between the electrodes during electric discharge machining. However, as Van Dijk himself admitted in his
study, the number of assumptions made to overcome the lack of experimental data at that time was quite significant.
Further models of what occurs during electric discharge machining in terms of heat transfer were developed in the late eighties and early
nineties, including an investigation at Texas A&M University with the support of AGIE, now Agiecharmilles. It resulted in three scholarly
papers: the first presenting a thermal model of material removal on the cathode,[16] the second presenting a thermal model for the erosion
occurring on the anode[17] and the third introducing a model describing the plasma channel formed during the passage of the discharge
current through the dielectric liquid.[18] Validation of these models is supported by experimental data provided by AGIE.
These models give the most authoritative support for the claim that EDM is a thermal process, removing material from the two electrodes
because of melting and/or vaporization, along with pressure dynamics established in the spark-gap by the collapsing of the plasma channel.
However, for small discharge energies the models are inadequate to explain the experimental data. All these models hinge on a number of
assumptions from such disparate research areas as submarine explosions, discharges in gases, and failure of transformers, so it is not
surprising that alternative models have been proposed more recently in the literature trying to explain the EDM process.
CNC Wire-cut EDM machine
In wire electrical discharge machining (WEDM), also known as wire-cut EDM and wire cutting,[22] a thin single-strand metal wire, usually
brass, is fed through the workpiece, submerged in a tank of dielectric fluid, typically deionized water.[20] Wire-cut EDM is typically used to
cut plates as thick as 300mm and to make punches, tools, and dies from hard metals that are difficult to machine with other methods.
The wire, which is constantly fed from a spool, is held between upper and lower diamond guides. The guides, usually CNC-controlled,
move in the x–y plane. On most machines, the upper guide can also move independently in the z–u–v axis, giving rise to the ability to cut
tapered and transitioning shapes (circle on the bottom square at the top for example). The upper guide can control axis movements in x–y–
u–v–i–j–k–l–. This allows the wire-cut EDM to be programmed to cut very intricate and delicate shapes.
Applications
Prototype production:-
The EDM process is most widely used by the mould-making tool and die industries, but is becoming a
common method of making prototype and production parts,[23] especially in the aerospace, automobile and electronics industries in which
production quantities are relatively low. In Sinker EDM, a graphite, copper tungsten or pure copper electrode is machined into the desired
(negative) shape and fed into the workpiece on the end of a vertical ram.
Coinage die making:-
For the creation of dies for producing jewelry and badges by the coinage (stamping) process, the positive
master may be made from sterling silver, since (with appropriate machine settings) the master is significantly eroded and is used only once.
The resultant negative die is then hardened and used in a drop hammer to produce stamped flats from cutout sheet blanks of bronze, silver,
or low proof gold alloy. For badges these flats may be further shaped to a curved surface by another die. This type of EDM is usually
performed submerged in an oil-based dielectric. The finished object may be further refined by hard (glass) or soft (paint) enameling and/or
electroplated with pure gold or nickel. Softer materials such as silver may be hand engraved as a refinement.
Small hole drilling
Small hole drilling EDM is used in a variety of applications.
On wire-cut EDM machines, small hole drilling EDM is used to make a through hole in a workpiece in through which to thread the wire
for the wire-cut EDM operation. A separate EDM head specifically for small hole drilling is mounted on a wire-cut machine and allows
large hardened plates to have finished parts eroded from them as needed and without pre-drilling.
Small hole EDM is used to drill rows of holes into the leading and trailing edges of turbine blades used in jet engines. Gas flow through
these small holes allows the engines to use higher temperatures than otherwise possible. The high-temperature, very hard, single crystal
alloys employed in these blades makes conventional machining of these holes with high aspect ratio extremely difficult, if not impossible.
Small hole EDM is also used to create microscopic orifices for fuel system components, spinnerets for synthetic fibers such as rayon, and
other applications.