Lost tripod mounting screw

"JG" wrote in message
news:313030303331393045D0BE0B06@crescentcomputing. co.uk...

from "Roger Hamlett"
"JG" wrote
from "Roger Hamlett"

"Nats" wrote
It ok thanks I got a couple of 3/8 Whitworth screws of various
lengths from a manufacturer and they work perfectly just need
to cut one down and geta washer and I'll be great to go. Thanks
for your reply though.

The point is that 3/8 Whit, will fit a 3/8 UNC nut, but will reduce
the
overall strength. The sides of the teeth are steeper on the Whit
bolt,
so the thread doesn't engage right across the surface of the tooth.
I
would be _very_ suprised if it was 3/8 Whit. Even in the UK, Whit
as dropped by most of industry, in the second World War, so that
stuff would work with kit from the US.

Fortunately, the base thread, doesn't need to be done up terribly
tightly, so the loss of strength probably won't matter...

I can't just let that mis-information go uncorrected!

The thread will most certainly be BSW. Whilst it is true that BSW in
general engineering was depricated in 1948 (well after WWII) in
favour
of UNC, the British Standard 84 which describes BSW was last updated
in
1956 and is still current and Camera/Tripod fixings have always been
BSW, wherever the camera originates - Germany/Japan/UK/USA.

Use of a UNC screw/bolt in a BSW nut should never be tolerated even
though the OD-Pitch is generally the same (the notable excption being
1/2" (BSW-12 UNC-13))
Whoa.
Look at my original post, where I warned about exactly this problem for
the 1/4 Whit thread on cameras, and the damage this can cause. However
_this is not a 1/4" camera thread_. This is the 3/8 thread used on a
tripod to hold a Gem. Now most Gem mounts in this area, are all
actually
manufactured by just two Chinese companies, who till very recently,
_both
used UNC_. The very recent examples, have in some cases switched to
M10*1.5.
I really do feel this will not be Whit.

Appologies Roger - I did read your post out of context and if, as now
seems the case, the 'nut' actually is a mounting thread for a telescope
then 'all bets are off' as far as what thread it might be - I have no
knowledge of the industry to draw on.

The point of my argument was purely based on the fact that the standard
'small' camera thread is still 1/4 BSW and the larger one used on medium
and large format cameras is /8 BSW - why it hasn't changed to metric I
can't really fathom - there has been ample time.

Another small point is that the only reason a BSW screw might fit a UNC
nut (of the same pitch) is down to the tolerance in manufacture. If both
were made to 'size' then there would be interferance between the top
radius of the screw and the 'root' flat of the nut - essentially because
the round 55° BSW is too fat to fit the UNC form but a 3% (ish)
reduction in diameter will avoid that.
Yes. The tolerances on this sort of kit are actually quite loose. The only
reason I suspect that most of these tripods use UNC, is that I have stuck
a tooth shape gauge on some of the bolts. So far, only a couple of very
old units used Whit.
In fact it gets worse on the camera threads. Though 1/4Whit,is the
standard, some kit ships using 1/4UNC. For instance, the 'tripod bush', on
SBIG cameras, is listed in their engineering drawings, as "1/4-20 UNC".
Ouch... :-(

Best Wishes