Sheet Metal for use in Mag. Rotors?
By Wes, Section Newbies Posted on Tue May 05, 2009 at 12:51:17 AM MST
One of the discs is slightly bowed such that when it is laying face down on a table I can push the center down a bit.

I cut 12" diameter discs out of 1/8th" sheet metal (mild steel I'm guessing) I got from a local hardware store. One of the discs is slightly bowed such that when it is laying face down on a table I can push the center down a bit. 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.

Sheet Metal for use in Mag. Rotors? | 16 comments (16 topical)

Re: Sheet Metal for use in Mag. Rotors? (3.00 / 0) (#1) by hiker on Mon May 04, 2009 at 08:44:39 PM MST

kinda thin for a 12" rotor--can you double up? ive used doubled up saw blades in the past--with fender whashers for added stringht...i only use one hole in the center for  mounting... WILD IN ALASKA

Re: Sheet Metal for use in Mag. Rotors? (3.00 / 0) (#3) by Wes on Mon May 04, 2009 at 10:15:19 PM MST

I can double up...I am guessing you just bolted the two together if you were using washers to hold the two together? How did you cast two discs with bolts through them? Thanks! [ Parent ]

Re: Sheet Metal for use in Mag. Rotors? (3.00 / 0) (#5) by hiker on Tue May 05, 2009 at 12:08:15 AM MST

no cast... just superglued the mags--with epoxy around the ouside of them.. its just a mill i built for my old rv.. so its not out in the elements all the time.......... WILD IN ALASKA [ Parent ]

Re: Sheet Metal for use in Mag. Rotors? (3.00 / 0) (#2) by Electron Pump on Mon May 04, 2009 at 08:49:12 PM MST

Hardware stores charge a premium for steel. Call a lockal steel destributer. I bet you can get the thicker stuff way, cheaper. Steel is down right now. Electron Pump

Re: Sheet Metal for use in Mag. Rotors? (3.00 / 0) (#4) by Wes on Mon May 04, 2009 at 10:27:30 PM MST

Good point. The only metal cutting tool I have is a jig saw, it'll probably cut 1/4" thick, but I've never tried cutting anything that thick. I could just take is slow, and buy a few metal cutting blades in case the 1/4" stuff eats them up.   [ Parent ]

Re: Sheet Metal for use in Mag. Rotors? (3.00 / 0) (#6) by Flux on Tue May 05, 2009 at 01:33:18 AM MST

1/8" steel at 12" diameter is not mechanically strong enough, it is not really adequate magnetically but the main issue is mechanical. 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

Re: Sheet Metal for use in Mag. Rotors? (3.00 / 0) (#7) by Electron Pump on Tue May 05, 2009 at 07:57:37 AM MST

Harbor Freight has a 4.5" angle grinder for $16.95. Get ya one of those. And get a few packs of the thin cut-off wheels for it. I have cut some thich stuff with mine. I'm talking in the 1 5/8' ball park. Then you can spin it from the middle hole and gring the edge round. Electron Pump

Re: Sheet Metal for use in Mag. Rotors? (3.00 / 0) (#8) by Electron Pump on Tue May 05, 2009 at 07:59:56 AM MST

1 5/8 (inch) not foot. Electron Pump [ Parent ]

Re: Sheet Metal for use in Mag. Rotors? (3.00 / 0) (#11) by Wes on Wed May 06, 2009 at 11:00:10 AM MST

I haven't used an angle grinder for cutting like this before, so I have a question or two about what you had in mind. I made a diagram to help clear things up: 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! [ Parent ]

Re: Sheet Metal for use in Mag. Rotors? (3.00 / 0) (#12) by Wes on Wed May 06, 2009 at 11:02:38 AM MST

I got my a's and b's mixed up. I should have said line b. and circle a. That's my short term memory, woops! [ Parent ]

Re: Sheet Metal for use in Mag. Rotors? (3.00 / 0) (#9) by Capt Slog on Wed May 06, 2009 at 09:51:17 AM MST

Wes, 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.  They are really dangerous to handle, and if they should stick to each other you'll struggle to ever get them apart. You won't be able to stop them bowing if the metal is thin, and the casting will probably give way under the strain.  I can find some dishing on my 6mm steel discs (1/4") and these are around 10" diameter. Also consider the jacking arrangements if you haven't already done so. . "Slowly changing the world, one watt at a time!"

Re: Sheet Metal for use in Mag. Rotors? (3.00 / 0) (#10) by Wes on Wed May 06, 2009 at 10:34:31 AM MST

Capt Slog, this info's been very helpful. I am planning on 4 point jacking as of now. I'll defiantly use at least 1/4" basted on everyones' valued recomendations and comment. Maybe I should go with 5/8th" to be safe! 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.   [ Parent ]

Re: Sheet Metal for use in Mag. Rotors? (3.00 / 0) (#13) by ghurd on Wed May 06, 2009 at 03:05:29 PM MST

I take it you do not have the magnets yet. 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- Ghurd.info [ Parent ]

Re: Sheet Metal for use in Mag. Rotors? (3.00 / 0) (#14) by Wes on Wed May 06, 2009 at 11:17:09 PM MST

Actually I do have the magnets. I know you're not supposed to bang them around and play with them, but I did take one out and muck around for a bit. They are remarkably strong when in direct contact. I found out that cast iron pipes really have an affinity for the magnet. I know they're strong enough not to toy around with...at least not too much. Now that I realize that most of the mechanical strength comes from the steel rotors themselves I am going to use a more reasonable thickness. [ Parent ]

Re: Sheet Metal for use in Mag. Rotors? (3.00 / 0) (#15) by Wes on Wed May 06, 2009 at 11:21:15 PM MST

p.s. point taken though. Definitely not going to "play around" with the two 12 magnet rotors. [ Parent ]

Re: Sheet Metal for use in Mag. Rotors? (3.00 / 0) (#16) by luv2weld on Fri May 08, 2009 at 07:56:05 AM MST

Wes, In case you missed the posting on how dangerous these magnets are---- Here is a link 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 "The best way to kill time is to work it to death!!" [ Parent ]

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