[ARC5] (Screw threads) was Part 6743 - tuning cranks etc

[Leslie Smith]

Hello list-members,
Following the informative replies to my questions here about the tuning
crank and the 27 tpi thread in the locking nut I'll digress a little
from the list topic.  I have an acquaintance, Peter Johnson, who is
knowledgeable in workshop practice.  (I'd love to know as much as Peter,
but life is short and knowledge hard to win, so "them's the breaks.") 
He noted that before WWII a good many organizations cut screw threads to
a "private" standard.  His comment was directed to British practice, not
US practice, and the point of his comment was that WWII brought a need
for standardization that ended the use of "private" threads.  I found
Peter's comment most interesting, since Whitworth threads have been
around since the year "dot".  Still, I'd be very surprised if Peter's
comment wasn't based on sound knowledge and probably something he
learned early in his apprenticeship.

As for the 27tpi, John, yes your explanation is wonderfully simple, and
I just love it when a puzzle turns out to have a disarmingly simple
explanation.  OTOH, looking at American Thread tables, I see only 2 odd
numbers from 39 threads in the table, and one (1/2 13tpi) has an even
"double" (1/2 20tpi), and so I wonder why threads are more commonly cut
to an even number rather than an odd number.  Then there is the
wonderfully obscure BA thread, where 2 BA = 31.3 threads to the inch.  

But I digress, and return to re-wiring the filament circuits in my
BC-453, (ser nr) 3154, where all the valves think they can fool my power
supply into delivering 12 volts.  Ha, I'll teach them that the WWII
standard calls for 24V.  Standardization is one of the GREAT lessons
that can be learned from the "command" sets.  

Best wishes from lovely Australia, where we have seen more rain in the
last 6 months than in the previous 10 years.


Les
(end) 


 
-- 
  Leslie Smith
  vk2bcu at operamail.com


On Fri, 08 Apr 2011 09:24 -0400, "Mike Hanz" <aaf-radio-1 at aafradio.org>
wrote:
> On 4/8/2011 6:13 AM, Leslie Smith wrote:
> > While it's not possible to fabricate such a "nice" part as the original
> > knob and crank (part 6743) it should be possible to make a functional
> > item that isn't a dog's breakfast.  That's the good news.
> 
> You can actually make an identical knob if you have a lathe, the 1/2-27 
> tap, and some steel stamping letters (the 6743 is actually engraved on 
> the front, but a careful stamping will come close.)  I'll have the 
> sketches and photos up by this afternoon.  You will, however, have to 
> come up with a female spline or a reasonable facsimile for part 6788.  A 
> piece of vinyl tubing pinned to the adapter might work fine.
> 
> > (27tpi is a REALLY
> > strange thing.  Bendix or ARC or who-ever must have had an unusual
> > combination of thread cutting gears in the workshop.)
> 
> Yeh...as I discovered (and noted in the article), that thread form was a 
> standard only used for gas lighting in the 1800s, dying back until it 
> was reapplied for these radio control cables in the late 1920s.  Perhaps 
> there was a touch of nostalgia there by one of the ARC or Bendix or 
> Signal Corps engineers - the rationale is lost to the ages now.  
> Interestingly enough, most lathes still have a 27 TPI setting, at least 
> the ones I'm familiar with.
> 
> Best of luck,
>    - Mike  KC4TOS
> 

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