In article , tomcas says...

If you cannot change the design to put a larger diameter where the
part mates to the vertical surface, then get rid of the turned
down section. Thread the part 3/8-24.

Now that's a good idea. Extending the thread way past what is needed
will move the stress concentration of the last tread out to a lower
stressed area and loctite will hold it in.

there will still be a stress concentration at the point where the
wall of the vertical support coincides with the one thread root
of the rod that occurs at that surface. If it fails in shear, that
is the location where it will fail.

If you can, keep the threads completely inside the vertical support
and relieve them to the root diameter where it passes out of that
material. From what you say though it sounds as if you don't have
enough hole depth to do that.

In that case you will have to rely on the larger root diameter
strength alone. Remember cross section goes like the square of
the diameter. This might be enough.

If it fails by snapping at the point where the threads run out into
the exterior portion of the rod, then you can improve matters somewhat
by relieving that section down to the root diameter for a length
of about 5 or 10 threads - but you MUST put gentle radii on the
reliefs. I like do do this with a ball-nose end mill in the toolpost.

Jim


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