OCTOBER 19-20, 2012 - YMCA University of Science & Technology

Proceedings of the National Conference on
Trends and Advances in Mechanical Engineering,
YMCA University of Science & Technology, Faridabad, Haryana, Oct 19-20, 2012
spark to occur simultaneously in all such locations. These sparks happen in huge numbers at
seemingly random locations between the electrode and the workpiece. As the base metal is
eroded, and the spark gap subsequently increased, the electrode is lowered automatically by the
machine so that the process can continue uninterrupted. Several hundred thousand sparks occur
per second, with the actual duty cycle carefully controlled by the setup parameters. These
controlling cycles are sometimes known as "on time" and "off time", which are more formally
defined in the literature.
The on time setting determines the length or duration of the spark. Hence, a longer on time
produces a deeper cavity for that spark and all subsequent sparks for that cycle, creating a
rougher finish on the workpiece. The reverse is true for a shorter on time. Off time is the period
of time that one spark is replaced by another. A longer off time, for example, allows the flushing
of dielectric fluid through a nozzle to clean out the eroded debris, thereby avoiding a short
circuit. These settings can be maintained in micro seconds. The typical part geometry is a
complex 3D shape, often with small or odd shaped angles. Vertical, orbital, vectorial,
directional, helical, conical, rotational, spin and indexing machining cycles are also used.
7.2 Wire EDM
In wire electrical discharge machining (WEDM) a thin single-strand metal wire, usually brass,
is fed through the workpiece, submerged in a tank of dielectric fluid, typically deionized water.
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.
The upper and lower diamond guides are usually accurate to 0.004 mm, and can have a cutting
path or kerf as small as 0.021 mm using Ø 0.02 mm wire, though the average cutting kerf that
achieves the best economic cost and machining time is 0.335 mm using Ø 0.25 brass wire. The
reason that the cutting width is greater than the width of the wire is because sparking occurs
from the sides of the wire to the work piece, causing erosion. This "overcut" is necessary, for
many applications it is adequately predictable and therefore can be compensated for (for instance
in micro-EDM this is not often the case). Spools of wire are long—an 8 kg spool of 0.25 mm
wire is just over 19 kilometers in length. Wire diameter can be as small as 20 micrometres and
the geometry precision is not far from +/- 1 micrometre.
The wire-cut process uses water as its dielectric fluid, controlling its resistivity and other
electrical properties with filters and de-ionizer units. The water flushes the cut debris away from
the cutting zone. Flushing is an important factor in determining the maximum feed rate for a
given material thickness.
7.3 EDM With Water
F.N. Leao and I.R. Pashby [15] published a review paper on dielectric fluid which is safe for the
environment. Deionized/distilled water is having the ability to be used as dielectric and can
achieve higher MRR in several condition even though hydrocarbon oil is proven to be more
competent. Norliana Mohd Abbas et al. [11] revealed the advantages of machining with water.
Information is presented in chronological order according to the year of published manuscript.
Some of the benefits are 1) promote high MRR and low TWR, 2) harmless machining
operations, 3) cost effective, 4) less microcracks and 5) no crack breeding.
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