In article ,
Ed Huntress wrote:

It isn't the sharp threads, Ned. It's the fact that the walls are weak at
each end of the taper, because one side or the other is thin at either end.
Only a couple of threads in the middle of the joint can produce the full
strength you would get with straight threads, and that isn't enough to
exploit the strength of the material. There aren't enough threads where the
material on *both* elements is equally thick.

Uh, in that respect the taper is actually an *advantage*, not a
disadvantage. The thin walls at the end of the taper, where there is
less strength, are also more stretchy, so they don't attempt to bear as
much load. The normal rule of thumb that only about three threads bear
the stress (and that it is actually concentrated on the first or last
thread) can be improved by using a taper.

(Consider, as a thought experiment, two tubes of the same OD and ID
screwed together using an extremely fine tapered thread, with the taper
extending all the way from the OD to the ID, so that looking at the joint
as a whole, there are no stress risers on the inside or outside. The
whole thing stretches like a solid mass; there is no stress concentration
on any first three threads or last three threads. Of course this
situation -- both OD and ID matching, and ultrafine threads -- is not the
situation we're considering here, and any other situation will be worse,
but it does serve as the extreme example of the advantages a taper can
bring.)


--
Norman Yarvin http://yarchive.net