RE: SRSG disk material

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "David Dean by way of Terry Fritz <twftesla-at-uswest-dot-net>" <deano-at-corridor-dot-net>

Hi Bill,

G-10 is a high pressure laminate of fiber glass cloth and epoxy resin. It is
very good stuff. Most G-10 you will find nowadays is really FR-4, which is
the flame retardant version. FR-4/G-10 is the stuff that high quality PC
boards are made of. It is usually green in color. The advantages are high
tensile strength, high impact resistance, high temperature resistance, and
dimensional stability. The disadvantages are high cost, and difficulty
machining. FR-4/G-10 is very hard, and is also abrasive. It has a tendency
to dull cutting tools rapidly. This is the material of choice, and most, if
not all, professionally made rotary spark gaps are made with it. It is what
I use and my experience is that it is not so difficult to machine as its
reputation would suggest.

Another material that some people have used is GPO-3. The cost is
approximately 1/4 that of FR-4/G-10. It is a fiber glass reinforced material
which uses polyester resin as the bonder. The fiberglass is not in cloth
form, it is random strands, and the tensile strength, impact resistance,
etc. are not as good as G-10. If you go to www.mcmaster-carr-dot-com they have
specifications on GPO-3. G-10/FR-4 is listed under Garolite. I would take
the temperature rating listed on their site for GPO-3 with a grain of salt,
however. They state the temperature resistance of GPO-3 to be higher than
that of G-10. I have a piece of GPO-3 on the desk in front of me that has
been discolored and appears to be somewhat "charred" by being too close to a
light bulb. In my personal opinion, I would not use GPO-3 as a rotor disk in
a SRSG. Certainly not at 3600 RPM.

Some folks use lexan and report that it works very well. I have no
experience with lexan in this application, however, In my opinion, I would
not use it, and again, not at 3600 RPM. Keep in mind that the "centrifugal"
force increases as the square of the angular velocity, so a disk spinning
3600 RPM will be subjected to forces four times that of a disk spinning 1800
RPM.

As to the cutting board, they are made of HDPE (high density polyethylene)
or polypropylene, and these materials do not have mechanical properties
suitable for a rotary disk. Even if heat were not a problem, these materials
tend to deform under pressure, and may allow the electrodes to become loose.
These materials are not recommended at all. There are enough potential
hazards in a Tesla coil as it is without adding a mechanical explosion
hazard to the mix.

later
deano

PS lexan can be had at any good hardware store or home center. It can be
cut, drilled, etc. with ordinary tools made for use with wood.


>
> I've read the disk for a SRSG should be made of lexan or C-10 (don't
> even know what C-10 is).  Where can I get lexan, and how do I cut it?
> What about the heavy white plastic cutting boards are made of?
> (polyethylene?)  Is there anything that will work that is readily
> available?
>
> 	Thanks,
> 	Bill Vanyo
>
>
>