Section 13. qxd

wire EDM
(Fig. 13.4.21). A slowly
moving wire travels a prescribed path along the workpiece and cuts the
metal with the sparks acting like saw teeth. The wire, usually about 0.01
in (0.25 mm) in diameter, is made of brass, copper, or tungsten and is
generally used only once. The process is also used in making tools and
dies from hard materials, provided that they are electrically conducting.
Electric-discharge grinding (EDG)
is similar to the electric-discharge
machining process with the exception that the electrode is in the form of
Fig. 13.4.20
Schematic diagram of the electric-discharge machining process.
Fig. 13.4.21
Schematic diagram of the wire EDM process.
Wire
Wire 
diameter
Spark gap
Slot (kerf)
Reel
Wire 
guides
Workpiece
Dielectric 
supply
Section_13.qxd 10/05/06 10:32 Page 13-70

---

a grinding wheel. Removal rates are up to 1.5 in
3
/h (25 cm
3
/h) with prac-
tical tolerances on the order of 0.001 in (0.025 mm). A graphite or brass
electrode wheel is operated around 100 to 600 surface ft/min 
(30 to 180 m/min) to minimize splashing of the dielectric fluid. Typical
applications of this process are in grinding of carbide tools and dies, thin
slots in hard materials, and production grinding of intricate forms.
The 
electrochemical machining (ECM)
process (Fig. 13.4.22) uses
electrolytes which dissolve the reaction products formed on the work-
piece by electrochemical action; it is similar to a reverse electroplating
process. The electrolyte is pumped at high velocities through the tool.
A gap of 0.005 to 0.020 in (0.13 to 0.5 mm) is maintained. A dc power
supply maintains very high current densities between the tool and the
workpiece. In most applications, a current density of 1,000 to 5,000 A
is required per in
2
of active cutting area. The rate of metal removal is
proportional to the amount of current passing between the tool and the
workpiece. Removal rates up to 1 in
3
/min (16 cm
3
/min) can be obtained
with a 10,000-A power supply. The penetration rate is proportional to
the current density for a given workpiece material.
The process leaves a burr-free surface. It is also a cold machining
process and does no thermal damage to the surface of the workpiece.
Electrodes are normally made of brass or copper; stainless steel, titanium,
sintered copper-tungsten, aluminum, and graphite have also been used.
The electrolyte is usually a sodium chloride solution up to 2.5 lb/gal
(300 g/L); other solutions and proprietary mixtures are also available.
The amount of overcut, defined as the difference between hole diame-
ter and tool diameter, depends upon cutting conditions. For production
applications, the average overcut is around 0.015 in (0.4 mm). The rate
of penetration is up to 0.750 in/min (20 mm/min).
Very good surface finishes may be obtained with this process.
However, sharp square corners or sharp corners and flat bottoms cannot
be machined to high accuracies. The process is applied mainly to round
or odd-shaped holes with straight parallel sides. It is also applied to
cases where conventional methods produce burrs which are costly to
remove. The process is particularly economical for materials with a
hardness above 400 HB.
The