[GreenKeys] ASR-33 mounting screws

[WA5CAB at cs.com]

In the Unified thread form system set up between the US and the British 
very early or just before WW-II (ast least our part of it), there were two 
common thread pitches established for US machine screws, cap screws and bolt 
diameters up to 1", plus a number of less common finer threads.  The two common 
US ones are known as Unified National Fine and Unified National Coarse.  
The full designators for the #6 machine screw threads are #6-40 UNF and #6-32 
UNC.  Teletype Corporation had nothing to do with it.  For some reason, 
their engineers seemed to prefer UNF, just as did the British automobile 
industry when they shifted briefly from BSF and BW to UN.  As time went on, the UNC 
pitches, being marginally cheaper to make, tended to dominate, #10-32 being 
an exception.  Plus UNC is the pitch used on stove bolts and carriage and 
plow bolts.  Otherwise, UNC was technically preferred for softer materials 
like aluminum and brass.

I also checked McMaster again this morning and found that they no longer 
seem to list anything for #6-40 or #8-40 but list both UNC and UNF for #10 
(this is a fairly recent change as they still carried both within the past two 
or three years).  They also list quite a few lengths in #5-40, which 
surprised me as #5 basically disappeared from the electronics industry after WW-II. 
 And wasn't very common during the War.  Maybe as someone mentioned 
earlier, the firearms industry continued to use it.

In a message dated 5/16/2010 1:12:06 PM Central Daylight Time, 
chrise at pobox.com writes: 
> Yes... I performed an exhaustive online search including McMaster-Carr
> and many, many other bolt and nut suppliers and the closest I came was
> one outfit offering #6-40 allen head cap screws at 3/4" long, which I
> would have had to cut to length.  They were only available in 100 pc
> boxes at .19 / screw so I continued the search.
> 
> Here in Minneapolis, we have a pretty robust market for this kind of
> stuff but every place I went or called had never heard of #6-40 and that
> includes quite a number of machine shops, hobby shops, sewing machine
> repair shops and gun shops.
> 
> So, I suspect this particular screw was one of those made by Teletype
> themselves or #6-32 clearly won whatever war took place over threads
> in the 1960's :-)
> 

Robert & Susan Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480