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Scot's FSG --Abbreviations but no acronyms
Hi all,
I have completed a very nice version of Scot D's fixed spark gap.
The FSG consists of nine 1.5 inch diam Cu sweat couplings sandwiched between
two
4 x 12 inch pcs of G-10. The G-10 was cut on a bandsaw using a new, metal-
cutting blade. The cutting was easy. I rigged the shop vacuum cleaner to
suck-up the
dust as the cuts were made. The blade is no longer new. :-((
The layout for the hole drilling is crucial if the copper cylinders are going
to lineup
properly to give an accurate and adjustable gap. Using zirconium nitride
drills, I drilled
the two slabs of G-10 exactly alike by wrapping the two slabs with duct tape,
carefully
centerpunching all nine holes, and drilling both slabs at the same time for
the nine
8-32 screws plus the six 1/4-20 threaded brass support rods. This means that
the bottom
piece of G-10 has five cylinders and four dummy holes, while the top slab
holds the four
cylinders with five dummy holes. I put screws in the blank holes. I drilled
the holes in two or three steps rather than ruining the drill by boring thru
in one hit.
I did the layout directly on the duct tape. The nine holes must be located
along a centerline
and spaced exactly 0.75 inches apart so that there is a spacing of 1.5
diameters for
each cylinder. The first of five cylinders mounts to the first hole, the
second of five cylinders
mounts to the third hole....and so on. On the other slab of G-10 the first of
the four cylinders mounts to the second hole, the second of the four
cylinders mounts to the fourth hole....and so on.
My major change in Scot's design is not milling the G-10 for parallel ditches
to hold
the cylinders parallel. The cylinders are held solidly in place with the 8-32
screw, a nut
and a lockwasher. In addition to eliminating the need to mill out the
ditches, I think this
method gives better accuracy and the parallelism can be easily set using a
gauge or
shims. Also, I think the engineering is better because you do not remove
almost
50% of the G-10 from right under the cylinder where you need the mass to
carry the
heat load. Another minor change in Scot's design is I added two additional
1/4 x 20 support rods in the center of the long sides of the slab. They help
in the final lockup.
The finishing touch is four pieces of 3/4-inch Lucite rod, drilled and tapped
to
fit the bottom of the corner rods to serve as insulated mounting feet. The
overall
appearance is impressive if you're impressed by spark gaps. :-))
Disclaimer: All the abbreviations in this posting are abbreviations.
They are not acronyms. :-))
Happy day,
Ralph Zekelman