According to its developers, the Robofil 240cc and 440cc wire EDM models are
capable of cutting tool steel at 42 square inches per hour under realistic shop
conditions. Currently, these are the fastest cutting wire machines from Charmilles Technologies
Corp. (Lincolnshire, Illinois). Other builders are also offering wire
machines that cut at astonishing speeds, making it a highly competitive
market.
One of the challenges created by high speed wire EDM is developing filtration
systems that can keep up with the high rate at which debris enters the
dielectric fluid. The quantity of this debris is not the only issue. Removing it
from the dielectric water quickly, effectively and thoroughly is also critical.
Without effective filtration, a wire machine quickly loses its ability to
maintain high cutting speed, achieve the desired surface finish and obtain
expected dimensional accuracy.
Charmilles’ approach to solving this problem has been to develop a filtration
system that doubles the number of filter cartridges, from two to four. The
system arranges the filter cartridges in two circuits that run independently or
simultaneously. All four filters are used during peak metal removal (very tall
cuts, for example). At other times, one circuit can be cut out for replacing
spent paper filtering elements without interrupting the EDM process. Two of the
cartridges are located directly in the worktank. These filters draw in water in
such a way that constant pressure is maintained on the filter element, forcing
particles to be trapped more evenly and densely across the filter surfaces. All
of the cartridges use paper filtering elements with a new design that increases
surface area about 54 percent, boosting the capacity to trap particles.
Charmilles reports that doubling the number of filters more than doubles the
life of each filter. Filter elements tend to last 2.6 times longer under similar
operating conditions, allowing the machine to run unattended 3.8 times longer.
This reduces filter costs by 40 percent despite having twice the number of
filters in the filtration system.
Effective filtration means that only water that is virtually free of
particles flows into the spark gap, the minute space between the electrode wire
and the workpiece surface. The absence of particles allows sparks to more
consistently follow the precise pattern of rising and falling energy levels set
by the generator. The Robofil 240cc and 440cc use Charmilles’ new CC-Generator,
which is capable of producing an increased ignition voltage and higher maximum
amplitude. In part, this spark “shape” enables these machines to remove material
very efficiently.
Dielectric fluid with a minimum number of particles cools the wire more
effectively, too, reducing heat stresses that can cause wire breakage. This is
especially important on tall cuts and steep tapers (the 440cc can cut a
workpiece as tall as 24 inches and taper at 30 degrees maximum at full
height).
Finally, the spark gap is not the only place where unwanted particles have a
negative effect. When particles reach the power contacts that allow the wire to
be energized, sparks can occur, causing erosion that shortens the life of the
contacts. Excessive particles can cause premature wear of the seals that allow
the lower guide arm to move without leakage of the worktank. This makes the
fixed table design of the machines practical, which in turn enables them to load
very heavy parts without affecting accuracy, the company says.
MMS Online is a trademark of Gardner
Publications, Inc, copyright 1997-2008.
MMS Online and all contents are
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Inc.
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