But surely bronze would be okay?
You would need to be well sure the cast nut could be removed from the spindle once it had solidified. It will be intimate contact with the shaft when poured and as it shrinks when it cools, it might become even more tight.
With a slight modification, you could do a lower risk version of this: make a cast of the threads on the spindle in wax or foam, remove that and then use in a traditional casting mould to make the bronze casting.
The threads in the cast nut would almost certainly need some hand finishing and that would be very tricky.
Maybe at the wax stage, you could scrape out a little material from the soft wax to give clearance.
It would be great if there were something you could apply to the existing threads that made them uniformly bigger in all directions so that the wax casting would have clearance without any need for adjustment.
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A traditional solution to repairing the shafts of motors (often when the keyway has been wallowed out) is to turn them down a little, weld them back up oversize and then machine a new keyway.
A similar approach could be adopted here: turn off the old threads, weld back up, turn down to 16mm diameter and then cut an M16 x 2 LH thread on the shaft. The secret in the initial diameter reduction is to go below the root diameter of the new M16 thread, so that the new thread is cut into fresh weld material, not some unknown alloy of old and new material in a heat affected zone.
- Could a nut not be cast around the threaded spindle? Maybe some form of reinforced epoxy or even bronze?
