Will said:
"My guess is because 4-40 screws have more holding power than a 4-36 screw
does. This is due to more threads in the thickness of whatever it's
screwed into, either tapped material or a nut. Fine threads are always
used in places where more holding power is needed in a thin section.
Coarse threads are for thicker sections."
This is true up to a point. It's generally reckoned that if you have three
threads engaged, that will give sufficient strength in most materials. However,
softer materials benefit from using a coarse thread, because although the core
and overall diameters are the same, there's more meat in each thread. All of
which is why I tend to use Unified coarse threads (or even BSW!)in places where
I'm using soft materials like aluminium, teflon, tufnol, polythene and even
fibreglass. The last is very rough on tools, blunting them very rapidly, and
carbide tipped tools are desirable. However, I haven't seen any carbide taps
yet!!!
One point worth remembering is that plain carbon steel is harder than HSS (High
Speed Steel) at low temperatures. HSS is used extensively where speed of metal
removal is important, as in industrial machining, but for a lot of the stuff we
do in home brewing, it's not worthwhile - certainly for taps and dies which are
only used in hand tools.
73
Peter W6/G3RZP (where it's warm and sunny, for you guys in the mid west and on
the east coast!)
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