I like the ideal of a stainless shaft - or brass if it's strong enough. But you might have corrosion issues near the join in an outdoor environment.
Other possibilities:
- As long as you're machining the "flowers", machine out the center of (at least) one for a significant clearance (like larger than the gap between the "petals") and press in a brass insert. Then you might be able to get away with the original shaft and still have most of your mag field unshorted by the shaft. (You'd retain the splines in one "flower" and if the other tends to slip you can always cast it into place by filling the space between the fingers.)
- Ditto but for brass substitute something non-metallic to avoid corrosion - like more casting material. (Or just leave it open and count on the "petals" to center you on the magnets until it's cast.)
- Iffy: Machine down the shaft and press on a brass pipe.
Remember that the structure was open mainly to cool the coil. With permanent magnets this is no longer an issue.
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