Page 31 - Fall 2023 EDM Today

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As the blistering heat of the summer was fast approaching, the plant had only two valid options: One, they could
leave everything alone and derate the plant – producing a reduced amount of energy. Two, they could try fi nding an
alternate solution to create a new load path to apply torque to the valve disc and open the valve.

While derating the plant was an option, it was an extremely expensive option. At a typical full capacity revenue gen-
eration rate of $55,000 per hour, derating to 60% capacity would cost the company more than 47 million dollars in
lost revenue during the summer season. Moreover, the energy would have to be purchased from another source.
The plant contacted MPR Associates Inc, an engineering company that specializes in fi nding solutions to unique
challenges. Founded in 1964 by a group of engineers that came out of Admiral Rickover’s Naval Reactors
group, MPR has since tackled some of the most demanding engineering tasks. MPR had worked in the past
with Hydratight as an engineering partner on similar projects.
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Based on past collaborative successes, MPR contacted Hydratight and discussed a solution where EDM would be
used to cut a female spline shaped cavity into the non-driven valve stem on the other side of the valve disc from
the driven stem that failed. On a 78” CCW valve, the stem is 5” in diameter and really more like a ball screw. Once
the female spline was cut, a male spline shaft can be inserted and a hydraulic torque wrench can be used to rotate
the valve. Based on MPR and Hydratight’s reputation, and previous successes with similar applications of splined
cavities, the plant quickly approved the proposed fi x.
With the project a go, Hydratight contacted EDM Zap for assistance. The Zap team reviewed the application
and considered what needed to be done. For many similar situations, the part needed to be cut is removed and
brought to the EDM machine. But in this case, the EDM needed to come to the workpiece. This work needed to be
done at the site, with the reactor in operation. This created many specifi c challenges, the least of which the work
needed to be done in a radiologically active area. The bigger challenge was that a specialty CNC EDM cutting ma-
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chine needed to be designed and built within a few weeks.
The Zap team rolled up their sleeves and got to work.
The Zap engineering team worked concurrently on the
tasks at hand. One engineer worked on the design of a
3-axis portable machine, while another engineer worked
on the design of the electrodes.
Figure 1 shows the machine concept. It is based on high-
rigidity linear actuators mounted to an extrusion frame.
The work tank is a commercially available polystyrene
plastic storage tank. This design concept allowed for pro-
curement of materials and assembly within a few days.
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The valve stem was in a horizontal orientation, embedded
within the valve.

As strange as it seems, the EDM cutting was done outside
of the work tank. A major challenge was to create a seal
Figure 1 Machine Concept
between the EDM plastic tank wall and the valve body.
MPR designed an interface that would bolt up to the valve stem access port on one side, and allow connection
to the Zap work tank fl ange on the other side. O-Ring seals were used on both sides to assure that there was no
leakage of river water into the tank and prevent successful operation of the EDM equipment.
The dielectric fl uid used was water; it is temperature and conductivity
controlled, much like a wire EDM machine. River water entering
the work tank would impact conductivity and slow down the cut.
But yes….you can use a graphite electrode for cutting in water.
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In operation, the spline electrode would move through the fl ange into
the valve body and cut into the valve stem. Figure 2 shows the spline
electrode mounted horizontally, and the fl ange that the electrode will
move through to contact the valve stem.
While the machine was being designed, another engineer collaborated
with Hydratight and worked on the design of the spline electrode.
The spline needed to transmit tremendous torque from the hydraulic
wrench with plenty of safety margin. Once the electrode was designed,
it was a huge challenge to fi nd an electrode manufacturer with 5-axis
CNC electrode milling capabilities that had the bandwidth to turn the
project around in just a few days. Fortunately, Matt Gawlik and the
team at 3D Graphite was able to provide a fast turnaround on the elec-
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trodes required. Roughers were produced at -.012 per side undersize.
Figure 2 Spline Electrode and Sealing Flange
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EDM Today, Fall 2023 Issue