Automating the diesinking process Essay

Until recently, it was considered impractical, if not downright
foolhardy, to operate an EDM unit untended around the clock. Three
factors prompted this feeling:

* High wear necessitated frequent electrode changes.

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* Less than reliable technology made predicting results almost

* Unsophisticated generator controls resulted in DC arcing, which
frequently caused fire or damaged workpieces.

Happily, recent advancements in die-sinking technology have
overcome these problems and untended operation is becoming quite common.
In some cases, continuous machining for up to 300 hours without operator
assistance is both possible and practical with virtually no risk to the
machine or the workpiece.

Many of the difficulties associated with untended operation were
first overcome in 1977 with the introduction of EDM units with orbiting
quills and integrated adaptive control. The finished cavity in
traditional diesinking is slightly tapered with the deeper portions
having a rougher surface finish than the part nearer the surface. This
is caused by residual particles that continue the machining process as
they are flushed along the sides of the electrode (Figure 1).

This phenomenon is accentuated during finishing operations because
of high-wear characteristics in low-amperage finishing modes. Since the
frontal gap (sparking distance) is smaller than the lateral gap, most
machining is done across the leading electrode surface. The subsequent
wear creates a distortion in cavity shape (Figure 2).

Advantages provided by an orbiting system include improved
flushing, reduced electrode wear, and shorter finishing times. It also
minimizes the effect of residual particles reducing the tapering action.
Another advantage is that the sides and bottom of the cavity will have
identical surface finishes and electrode wear will be evenly distributed
(Figure 3).

Prior to the introduction of a fully automatic adaptive control,
EDM required continuous monitoring and fine tuning. Because ideal
machining conditions seldom exist, conditions that result in
short-circuits, abnormal discharges, and other operating difficulties
can develop quickly. These and other dangers associated with DC arcing
ruled out untended machining. The solution seemed to lie in optimizing
effective machining current. This quest led to developing fully
automatic adaptive controls, which today are capable of performing the
following functions:

* The control breaks down the succession of machining pulses into
short trains of pulses with an interval of “off” time. This
is done while maintaining the total pulse interval time. Through
control of these pulses effective machining current is optimized.

* The electrode is given a high-speed automatic pulsation movement
until any machining abnormalities are eliminated.

* Machining is stopped before the workpiece is damaged if the above
operations are unsuccessful.

Adaptive controls also adjust secondary parameters resulting in
less electrode wear and faster machining rates. Setting errors are
reduced and repeatability is enhanced. Abnormal discharges, which can
degenerate into destructive arcs, are totally eliminated.

An example

Figure 4 shows a cavity being machined for a universal joint
forging die using both orbiting and fully automatic adaptive control.
In the past, these dies were produced using traditional techniques.
Holes in the electrode were necessary to facilitate flushing during the
EDM process. These holes left small posts in the finished cavities that
had to be removed in a secondary operation.

The ability to orbit the electrode under fully automatic adaptive
control eliminated the need for secondary operations. One electrode was
used for both roughing and finishing each cavity to a 4.5 micron RA
(arithmetic mean) finish. Machining was performed completely untended.

Shortly after the introduction of orbiting and automatic adaptive
control came the integration of CNC for table movement, orbit, and
generator settings.

The EDM time for single-cavity molds can be greatly reduced by CNC.
All electrodes necessary to rough and finish a cavity can be shank mounted for use with automatic electrode changers. This reduces setup
time and increases the efficiency of both the operator and the machine.

Another example

An excellent example of the capabilities of CNC applied to EDM is
the production of a keyboard mold, Figure 5. The mold consists of 96
cavities and required 300 hours to machine. Estimates are that
production costs were cut in half by using CNC to automate the EDM
operation. Apart from initial programming, every stage of production
was automatic with no need for supervision.

A wirecut EDM unit produced the electrolytic copper electrodes for
the diesinking operation. The 96 cavities on the cavity half of the
mold are square and measure 0.160″ x 0.160″ x 0.240″
deep. On the core half, they are U, L, or square shaped and contained
in a pocket 0.520″ x 0.520″ x 0.480″ deep with a 2-degree
taper on all sides. Accuracy was specified within 0.0004″, both in
size and distance between cavities. Surface finish was 1.12 micron RA.

This precision mold was finished two months ahead of the expected
date. The total machining time of 300 hours–a record in its own
right–is even more impressive because there was no operator
intervention. Electrodes were changed automatically.

A third example

Another application for CNC diesinking EDM is multiple-workpiece
production. Figure 6 shows how knob mold workpieces are fixtured in a
holder block that was designed for multiple-cavity work. The electrodes
are mounted on shanks. Flushing is through the electrode. A graphite
electrode is used to rough cut the cavities. The graphite roughing
electrode is exchanged for a copper finishing electrode automatically.
A 0.40 micron RA surface finish is attained.

All generator settings, table movements, tool changes, and
translations are preprogrammed. The total machining time is 1.5 hours
per cavity. This includes EDM polishing of the cavities that eliminates
the need for hand polishing. Another benefit of polishing with EDM is
that it preserves the dimensional integrity of the machined cavity.

Recent developments in controls and software have shattered many of
the old concepts of diesinking EDM. At the forefront is contour EDMing,
which permits simultaneous machining of the X, Y, and Z axes in either a
positive or negative direction. This is a significant contrast to the
traditional “sinking” application.

Programmed cycles such as vectoral, conical, orbital, directional,
and others have simplified EDM diesinking. Further, advances in
tooling, fixturing programming, and integration of machining strategies
will continue to expand use of CNC diesinking EDM.

In the machine-tool industry, there has often been an inherent fear
of anything that breaks away from tradition. This is especially true
for EDM. Today, however, the impact of CNC on EDM is reshaping the
framework of practical applications.

Everyday we see new applications, and new approaches to old
applications, that eliminate many former restraints. It’s a time
of change and overwhelming growth in an industry whose potential is just
being realized.

For more information on EDM equipment, circle E67.


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