Well, I'm no expert but I have a few theories:
1) If the shaft was maybe something like cold rolled with forming skin tension or whatever you call it, then cutting the key slot down the whole length might allow stress relief & curl over time? Maybe 'good' shafts are post stabilized or start with different material? One would think it should be a natural bow, but in my case where it starts rubbing, its being constrained by the carriage worm bracket itself.
2) A bad worm/bracket design where the thrust force starts to erode the CI bracket by power traversing over time. If things get out of alignment, maybe puts wonky force to the shaft & gets distorted. You can see my fix which has yet to be trialed, milled out the bracket slightly & inserted a bronze wear ring. I have a better plan, but first need to get this resolved & lathe back together. (This is exactly what Keith Fenner remedied on his lathe too, but he silver soldered the ring on).
3) Some lathe mover slipped a strap under my bar when I wasn't looking, haha. (That explains the first problem I had with poor surface finish when lathe was new, I actually cant recall if shaft was straightened at the time or bent back. But that doesn't explain many years of happy turning so I suspect 1. or 2.
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