Page 32 - Fall 2023 EDM Today

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Finishers were produced at -.005 per side undersize. While the project was done with CNC orbiting, the reason
for the two different size electrodes was due to a need to maintain the dimensional accuracy for maximum
power transmission.

In short order, the machine was built and electrodes were
fabricated. As this was essentially a zero-failure project
where the team had just one shot at the cut, the decision was
made to mock-up the cut and verify the accuracy, surface
fi nish, and fi t of the male spline.

The mock-up was conducted at Hydratight’s Gaffney, SC
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facility. In attendance were engineers from the plant, from
MPR, and from EDM Zap. Figure 3 shows the mock-up. The
assembly attached to the left side of the tank represents that
attachment to the valve body and the 5” diameter valve stem.
The mock-up cut worked as expected and the male
spline was a perfect fi t! Once plant personnel and all the
engineers involved were happy, the equipment was mobilized
to the site.

Up to this point everything was smooth. However, this is
where the challenges really began. The valve is located
in a lower level of the plant, below the turbine, under the
condensers. The machine needed to be taken down stairs
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and through a doorway that was too small to accommodate
the machine.
Fortunately, the rapid assembly characteristics of the
machine frame also meant that it could be taken apart.
The team disassembled the machine, carried the parts down
the stairs, and reassembled it in the correct location.
Figure 3 Mock-Up Cut
The CNC power supply used was a Zap model E-100 portable
unit – see Figure 4. This extremely compact unit was light enough
and small enough to get down the stairs and into position.
However, the E-100 supports arc cables that can be several
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hundred feet long. It could have remained outside of the room
and operated remotely. This is often the case when Hydratight
operates in high radiation environments.

The next step was to remove the valve stem cover and attach the
MPR designed interface. The team had a surprise when pulling the
cover. There was a very signifi cant water leak. The team saw 10-30
GPM of water fl ow out of the valve with the cover off. While this was
unexpected, it was something the team was prepared to handle.
They were able to stop the leak with simple EDM putty. The setup
was done in a single shift.
It was extremely important that the spline cavity be placed exactly in
the center of the valve stem. With the valve stem being buried inside
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of the valve body, it was impossible to do this visually. The team
needed to use the built-in CMM functions of their EDM control and
perform an automated centering routine. A graphite rod was used as
a probe – see Figure 5.
After centering, the cut was started. The team that was brought in
was eight people, which included two supervisors and six techni-
cians. Once setup began, the teams worked back-to-back 12-hour
shifts until the project was complete and the valve was moving
again. The EDM cutting itself took approximately 34-hours, with
the orbital fi nishing cuts taking only a few minutes. After the cavity
was cut, the male spline was inserted and checked for fi t. With the
mock-up work being done to assure a good fi t, this was a somewhat
Figure 4 E-100 CNC Power Supply
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superfl uous check, but it assured that the fi t was correct.
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