[Boatanchors] SI units and the rest

[WA5CAB at cs.com]

The other basic design flaw in metric threaded fasteners (to be reusable 
after a few years in the normal dirty world) is that screw and bolt heads and 
nuts are smaller relative to diameter.  As are screw slots.  So that the 
amount of torque that you can put on a partially rusted fastener before the 
wrench rounds off the corners and turns without turning the fastener is much 
lower.  And actually, in this regard, the Whitworth (the oldest) thread 
standard is better than either USS or SAE.as the hex sizes are slightly larger on 
both BW and BSF.  Whoever designed it (I assume someone named Whitworth) 
obviously put some thought into the overall design of the system.  Which 
obviously didn't happen with any of the several metric systems that you run into 
today.

I will point out (although you seldom find (or can find) actual screws, 
bolts or nuts in these sizes) that many standard diameters in the UN thread 
tables have three or more standard thread pitches listed.  The additional ones 
beyond two mostly being finer than UNF.  These are all generally identified 
as UNEF.  And most decent machine shop suppliers stock most of them.  But 
you won't find them in any "Home Improvement" store.

Boatanchor content - a few years ago I had to overhaul an MT-299/GRC.  I 
found that the cable glands in the junction box had UNEF threads (and the nuts 
were interference fit so to make the things easier to reassemble after I 
stripped and painted the mount I wanted to chase the threads and use 
Locktite)..  The antenna cap on the BC-611 is tapped 3/4-18 UNEF except for some 
French made ones that do not fit original US made top covers and have been 
recently sold on eBay.  The tuning slugs in the IF transformers in the BC-611-F 
are threaded #12-28.  Not UNEF but uncommon today.  In all three cases, I 
walked into one or another of the nearest suppliers and bought a tap and/or 
die.  There are also several thread pitches where the pitch is the same across 
multiple diameters.  -16 being perhaps most common in the smaller sizes.  I 
must have run into one of these in a boatancher sometime in the past as 
well, as I have a 3/4-16 UN tap in my tap cabinet.  Anyway, if you need a thread 
pitch finer than UNF in a given diameter it may be available.  Or taps and 
dies may at least.

If anyone wonders, the "U" in UNF and UNC stands for "Unified".  Dates to 
early WW-II when US and British thread forms (the actual "vee" shape of the 
thread and the radii at peak and trough - has nothing to do with pitch or 
diameter) were standardized or "Unified" so that machine shops on both sides of 
the pond could make interchangable parts, regardless of whether the threads 
were UNF, UNC, BW, BSF or BA. 

In a message dated 11/16/2010 10:26:36 PM Central Standard Time, 
wh7hg.hi at gmail.com writes: 
> Robert, your earlier comments on metric threads are quite accurate.  It's
> kind of like using NF where any real engineer would call out NC instead.  
> I
> have found one place metric's good and that's where I need fine adjustment
> capability like my watchmaker's lathes.  Metric threads are pretty useful
> then although I'm planning to convert at least one cross slide to a fine
> decimal pitch.

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