Here's how I make my truss rods. I cut a curved slot in the neck with this jig on my table saw. It consists of a raised insert with a curved top surface, and a pair of slats with the same curve which attach to the neck with double-sided tape. I mount three blades on the saw to cut a 1/4" wide slot. The slats and insert make the resulting slot curved, with the deepest point at roughly the center of the neck.
The rods themselves are just a length of pre-threaded 3/16" rod, which go for about 50 cents for a 3' length (which makes two truss rods). I cut a 1/2" piece of brass rod and pin it in place with a 1/16" drill bit. I cut a little shelf in the brass rod to expose the end of the drill bit. That way I can theoretically pull the pin and pull the truss rod if it ever stripped (although I've never had to do that to date). I make my truss rod nuts out of steel PC board standoffs and socket head cap screws, fusing the latter into the former with high strength thread lock compound.
Note that in this photo, the rod is not threaded. That was back when I was threading plain rod myself. The pre-threaded rod is much less work and the threads are of better quality than I ever managed to produce. Moreover, it's actually stronger than a partially threaded rod due to the principle of stress concentration.
I cut a spline to the curve of the rod slot and glue it in over the rod. This presses the rod into its curve. The fretboard is then glued over the whole thing.
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