Wood and plastic spark gap construction

[Terry Fritz <twftesla-at-uswest-dot-net>]

Hi All,

	I lost track of who originally ask this somewhere in the 50,000 e mails I
have :-))

I have used hardwood (oak and maple for spark gaps with great results.

http://users.better-dot-org/tfritz/RGAP.ZIP

The wood is very strong and very easy to work if you have a lot of wood
tools like I do.  The cautions I would give are:

1.	If you live in a humid place (i live in a dry desert) then arcing trough
the wood could be a real problem.  However, you can easily make most of the
structure out of wood and use plastic for the high voltage parts.

2.	Of course, be careful of using steel screws and such that can attract
the high voltage.  I use "really good" wood glue and clean and clamp the
wood.  So I only use glue to hold it together.  Wood dowels would help too
but I get great results just with the glue.

3.	Wood expands and contracts a little with the seasons so you have to
watch the grains and all so it does not crack.  Best to check gap
clearances too if it has been sitting awhile.

4.	I don't use any paint.  I just leave the wood naturally bare.
Polyurethane might be a good idea but I didn't know if it would trap water
in the wood in areas.

5.	Beautiful kiln dried clear 3/4" hardwood is not cheap!  Price it out
before you start. 

6.	Unprotected wood will soak up water like a sponge, so you have to keep
it dry and wait forever for it to dry if beer gets spilled on it ;-)

7.	If you go all out in finishing the wood and make it out of walnut or
something fancy.  The gap can try be beautiful.  The motor may need a
little body work and a nice paint job too.  The "barn blower" motors are
not known for aesthetics...

8.	Don't bother using pine, plywood, or particle board.  They really are
too cheap for this stuff.  Pine will shrink and do odd things like attract
bugs.  Plywood will just fall apart and be difficult to "machine" in tight
places,  Particle board does not even burn well...  The gap materials have
to be very strong and resilient.


I also have a gap made of lexan:

1.	Lexan is very flexible and it likes to twist and bend even when it is
thick.  This makes my gaps crash if I am not careful.  The way I have it
mounted now fixes this, but assume the plastic will not be "solid".

2.	Plastic will melt and soften with heat.  Watch the motor and the
electrode areas for melting.  It will start to loose strength at
surprisingly low temperatures.  I would not even think about using
polypropylene (again :-)).

3.	Although lexan is pretty easy to work with, It will scratch and show
mistakes.  You can't just belt sand out the "boo boos" like you can with
wood ;-)  It also shows dust, dirt, and finger prints.  But looks cool
otherwise...

4.	Static electricity on the plastic can cause long surface arcs so leave
good clearances.  Anti-static sprays and such would probably help this but
it is usually not big a concern.  Just be aware of it.

5.	Lexan is pretty good about not cracking and shattering but I prefer
flying parts to hit 3/4" hardwood...

6.	I used a LOT of nylon screws to attach the plastic parts.  That requires
tapping the holes and all which is a bit of a chore.  I assume lexan can be
glued but I have never tried it.  Too many horror stories from my previous
"attempts" at gluing plastics :-)

7.	If a gap is going to get a lot of rough use.  Hardwood will stand up
better than lexan (or not show hits as well).  I think my wood gap could
take a lot harder hit than the plastic (but the plastic may just bend and
flex off a hard hit...)


Today, I think I would combine hardwood with plastic taking advantage of
the best properties of both in the right places.  LE and G-10 Phenolic are
rock solid and could be used in many strategic areas too but the cost is
very high to use everywhere.  "Pros" like LE phenolic but they spend far
more money that most of us on such things.  However, if your really want a
rock solid gap that will last forever, phenolics are the best choice.  You
can easily bounce bullets off 1/2" G-10, solder pipefittinge on it, and it
flexes about as much as granite...

Cheers,

	Terry