Koz wrote:
....
Here's the situation: We need to take a long 1/2" dia CRS rod, drill
the ends 1-1/2" deep x 5/16 and insert a 3" long hardened and ground
dowel pin in the end. The pin doesn't get much tension but gets some
torque in the form of tens of thousands of cycles over it's life.
....

Maybe try longer pins, eg 4" long pins in 2.5" deep hole.

We've tried using a hydraulic compression fit (heavy press in) which
held ok but didn't like the repetative stress cycling for long enough.
We've also tried perimeter welding the joint which
....

Are brazing temps too high? Could try grooved or necked pins wrapped
with brazing wire or sheet, pressed in, then locally heated.

Any thoughts on a better procedure to use off the shelf dowel pins (or
whatever) and get the cycle strength we need?
....

Perhaps modify pins with "hose barb" pattern or a light knurl,
and swage rod end after pressing pin in. I think swaging without
a pattern on the pins won't help.