Oil sucks, wind blows.
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How bad would it be to go ahead and cast the magnets on the metal discs and hope that the casting and mold will be strong enough to hold the metal flat?
p.s. This is my first attempt making an axial flux alternator.
Some always choose to do things the hard way, if you must struggle then use at least 4 discs and bolt them together or something but it is a hard way to make something that at best is not very satisfactory. If you can't do something then it is better in the end to get someone else to do it unless you value your time and effort at zero. By the time you have cut 8 discs and paid for the steel and spent hours on it you could probably have bought some discs but I suppose it does depend on the part of the world you are in and the cost of shipping.
If you are in a mild climate then you may be able to get away with 2 discs each side if you use plywood or something to give the mechanical strength but I don't regard it suitable for much other than experimenting .
Flux
Also consider the jacking arrangements if you haven't already done so.
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I wanted to keep the mass moment of inertia low and with respects to the added weight of thicker mag rotor metals as in some of the brake rotor versions I've seen. I guess I was thinking that most of the strength in the mag rotors would come from the fiber glass casting, but you guys have helped show me that the bending force on the mag rotor is quit substantial and trumps other design considerations. Capt Slog, if you see some dishing on 10" then the bending force would be worse for 12" for the same magnet configuration. I'm using N52 1x2x1/2's 12 per rotor.
Were you suggesting that I make a lot of cuts like line a. tangent to the circle then spin the roughed circle and grind it circular? Or can you somehow just cut along circle b -- can the cut off wheel conform to the circular shape or would that just destroy the cut off wheels? Let me know what you had in mind!
Do not worry about the weight.
Worry, and worry a lot, about "you can't believe the amount of attraction these discs will have for each other (or to any metal surface) when the mags are installed".
My fridge is dented (anti-dented?) from trying to remove a smaller magnet with no flux return path from a surface with less "pounds of attraction".G-
http://www.magnetnerd.com/Neodymium%20Magnets/Dirks%20Accident.htm
It's like playing with a loaded gun.Think each step through before you do it!!!
Ralph