[HBR] Eddystone Three Section Variable Capacitor Question

[Walt Hutchens]

Whitebear1122 said:

> The 6-40's 40 turns/inch doesn't
> quite work out but the Whitworth's 1/8" (0.125") seems a bit small.

It does, but I just mic'ed the screws I have: The diameter over the
threads is .137 and as near as I can count on a 1/4" length, they
would be 40 TPI.

I'm betting that the Whitworth rounded peak and valley is simply
omitted -- that some makers use V threads of appropriate nominal
diameter and pitch and call them 'Whitworth.' They would be fine as
nut-bolt pairs, though not when mixed with the real thing. And that
would increase the overall diameter somewhat over the nominal .125.

It's far easier to make the tooling for a V thread than for a real
Whitworth. You can freehand a workable 60 (or 55) degree cutting tool
to make a V thread on a lathe but good luck making the tool for
Whitworth which needs both a rounded tip and a shoulder at the proper
place. That would require a precision set-up, particularly in such
small sizes.

Unfortunately I have only the 1/4" screws so I can't help you with the
1" issue. I'd suggest going on line to find what you need. All kinds
of stuff is out there.

Here's another idea: Check the dimensions of an 8-40NS (national
special) screw. It's going to be VERY close, and those screws
are extensively used in mounting scopes and similar gunsmithing jobs;
you should be able to buy them in lengths of 1". Or failing all else
you might get a particularly well-equipped gunsmith to make them for
you.

Walt Hutchens