Machining neo magnets

[neilho]

A buddy and I are in the early design stages of an axial flux wind turbine and are trying to figure out the best way to attach the neos to the magnet rotors.

The most reliable attachment seems to me to be to fasten the magnets to the plate with screws. So...I'd like to drill and counterbore the neos.

Which brings up some obvious questions, like: How hard are the neos?, What kind of tooling is required- will carbide or CBN work? Coolant? How to hold the part? and of course- Has anyone here done it?

TIA for your comments.

Neil

[kurt]

best bet would be to get the magnets from that canadian supplyer that have the countersunk holes already in them. otherwise talk to zubbly he has drilled neos before......

[satmanuk]

i've only built a few mill's, so i'm not an expert, but i think the Consensus would be not to machine them,

a) you would weaken the structure of the magnet, b) you would damage the protective coating, c) create distortions in the flux of the magnet by removing material etc.

solutions, use an epoxy like everyone else does, or buy magnets that holes in them already.

although i might be completely wrong : -)

satman

[coldspot]

Drilling, cutting magnets causes heating and that will in turn causes losses of magnet strenght.

I've cut some HD, (Hard Drive) magnets in half

using a tile saw, (liquide cooling). Messy but seemed to work without hurting magnets.

Since have learned to score n snap them to cut in half. Of course then needed protetive coating must be added to opened ends, (used spray can clear coat).

If you haven't bought yet, Go with the factory

drilled ones.

$0.02

[RP]

If you're concerned about adhesives being strong enough, consider that the forces on the magnets only extend in one direction (radially outward).  Additional support would be a simple as a couple brass dowels hammered into the rotor on the outboard side of each magnet.

If you go this route, I'd suggest a slight countersink on the dowel pin holes to prevent any burrs under the magnet.

[neilho]

Thanks for the responses.

I'm more concerned with the longevity of the adhesive bond than the strength. Our plan so far is to put the magnets on the back of the 1/2" steel plate that also holds the blades on the front side. As the wind speed varies, thrust will vary and the plate will flex a bit, then return. The adhesive needs to be flexible enough to accomodate this movement, thermal expansion differentials and eventually, rust. (It's wet and humid in Vermont in the summer). It only takes one failed bond, out of 40 or so in our case, to trash a rather expensive alternator, so the magnets should be securely held. And it seems way easier to install magnets with screws. Being a machinist, I have more confidence in a couple of screws than adhesives. Maybe a chemical engineer would feel the opposite.

Kurt- do you have a link or reference to the Canadian supplier?  If they can do countersunk holes, they can probably counterbore them, too.

Neil

[vawtman]

Im more concerned about the initial bond once that goes the screws will be gone in a matter of time.Moisture will get in and really wreck havoc.

 An opinion.

[gotwind2]

Anyone considered a process called 'spark erosion' to form any shape in neo magnets.

The equipment is very expensive using a copper electrode, shaped to required hole size/countersink and the whole process is 'burnt' in a di electric solution.

http://en.wikipedia.org/wiki/Spark_erosion

A local toolmaking company could probably help you out.

Just a thought

Ben

[fungus]

Maybe it could work but I'd still be worried about local overheating of the magnets. A more in-depth explanation is here. I'd say to get predrilled mags if you're going down that road.

[dinges]

I have considerede EDM (Electrical discharge machining = spark erosion) too for a while.

There's only one problem: the magnetic fields of a strong NEO magnet would probably play havoc with the EDM process. At least, that's what I have been told by a friend who has (wire and sink) EDM capability. Apart from that, the process is slow and expensive. If you have to drill multiple magnets, it would take quite a bit of machining time.

I once tried TIG welding some pieces together, that were held in place by a few Neo magnets, about 20 cm away from the place that I was welding. It didn't work: the arc went everywhere, except where I wanted it to be.

One would do better to pay a few more $ for magnets with holes in them and proper surface coating than trying to save a few cents here by DIY-ing. Either by EDM or grinding or drilling. Just my humble opinion.

[alancorey]

If you could machine them, think about where the chips would go.  Wherever they wanted, but they'd cling in the wrong places.

I've only been working with neos for a couple of months, but already my vise, drill bits, even my dial calipers are magnetized, and I don't try to machine the magnets.  It's getting to be a real nuisance.  The only practical way to get them off a piece of steel is to slide them off, and as everyone learned in grade school, rubbing something like a nail against a magnet is the best way to magnetize it.

The magnets are quite brittle, almost like a ceramic.  Snapping hard drive magnets in half works most of the time, but they can also break into sharp shards like glass.  They can chip if you let them fly together.  Maybe something like a glass drill, a rod with grit paste under it and plenty of coolant might work.

  Alan

One state south of you, where the weather's about the same.

[Jon Miller]

Hi,

First thing, why are you going along the lines of a 1/2" steel plate on the front?  Trying to cut down cost, weight, manufacture time or maybe just the number of bits to assemble?  Would be far simplar to just keep to the wooden blades with metal plats.  

If your concerned about twisting and moving, why?  How big are your blades that they will twist a 1/2" plate?  

Have you considered using non foaming polyurethane it's been used in car and aviation industry for a notable time now.  Its flexible and strong, stands up to heat as well.  Its most common use is in wind bonded wind screens which flex.

If you are worried about the magnets twisting of this much then buy the ones with holes and screw them on, (if you don't seal them fully) give it time a little bit of water gets in and the screws expand, use stainless steel then? Different expanding rates then that of Neo's cracks may form?  

Not putting the idea down but just trying to understand why you need to bolt the blades to the 1/2" plate?

Hope the ideas of some help.

[neilho]

Peter,

 I hadn't even considered EDM. A friend has a diesinker, so I'll get him to try putting holes in a neo.

Jon,

 I'm not quite sure what you mean when you say "keeping to wooden blades with metal plates"....We're using the 1/2" steel plate with blades bolted to the front because it's a system that I have experience with and confidence in. It's from the 4kW Enertech that (to my knowledge) has not had a single failure in the plate, blade attachment system, hub or main shaft in 25 years. Not a great machine, all in all, but these parts of the machine are structurally sound. (And I just happen to have a few.:-) So mounting one set of magnets on the back side of this same plate saves making one magnet rotor. Fewer parts mean a simpler, more reliable, lighter and less expensive machine.

I'm concerned about the magnets twisting and moving because there'll be some relative movement between the magnet and the plate. We're planning a 15-20' blade diameter machine and each blade, via thrust, puts a considerable bending moment into the hub plate. My hunch is that yawing also adds considerable stress via gyroscopic loads, but don't yet have a good handle on the mechanism and just what loads are added where. Adhesives would have to be flexible enough to accomodate the bending. Obviously, it won't be much over the length/width of the magnet, but with 20 or so magnet/plate bonds  it's got to be very reliable over a 20 year period, and this is where screws excell. I'm sure there are adhesives out there that would work well (the urethane you mentioned sounds good- do you have a brand name or a source?), I just don't know what they are and would like to avoid having to buy and test a bunch of nasty chemicals and still not know if the bond will last.

Stainless steel screws will work fine. They're elastic enough to stretch a bit when the neos and the steel plate change temperature. And they'll make it really easy to change individual magnets if necessary.

Neil

[Flux]

Yes you can machine neo but I go along with the others, either buy magnets with holes or stick them on.

Your worry about corrosion is a real one and if you machine holes in your own magnets you will have a more serious corrosion problem than ever you will have from a broken glue bond.

I would regard any process that damages the manufacturers plating or epoxy coating of any magnets used in a damp environment as being a last resort.

I don't know how you intend to mount your blades, but if the discs are thick enough there should be no problem. The blade root support doesn't have to extend out to the outer part of the disc where you may cause flexing.

Use urethane or something that can stand flexing to keep moisture from finding its way between the magnets and discs. Even if you screw the magnets on you still have this issue to solve.

Flux

[Flux]

Having read your reply to Jon, you will indeed flex the plates with 20ft blades bolted directly to 1/2" plate without a front support plate.

Use magnets with holes if you can get them , but you still need to prevent moisture penetration between magnets and discs. This solution is not likely to be any better than the suggestion using dowels to stop the magnets flying out, but it will give you more confidence.

Twenty years is a long time and I doubt that there is much data on the survival of neo in harsh environments under these sort of conditions. Electrolytic corrosion and fretting may occur even with magnets screwed on unless you find an effective potting medium.

Flux

[dinges]

Ok Neil,

You seem to have made up your mind. I hope EDM works, but have my doubts.

http://www.fieldlines.com/comments/2006/6/16/202945/707/9#9

Personally, I can't economically drill or EDM holes in magnets for the slight premium that Zubbly's vendor seems to charge. Let alone the corrosion issue that arrises.

Anyway, please report back in this forum whether EDM-ing the magnets worked out or not. AFAIK, no one in this board has actually yet tried it.

[Seaspray0]

I'm a newbie, but I have a hard time picturing a powerful magnet popping off a 1/2" steel plate... they stick to metal pretty darn good without any help and take quite a bit of effort to break the magnetic bond.  It seems to me that if there was going to be any movement, it would be accross the surface and not seperating from it (sheer load and not a tensile load).  Filling in the gaps with epoxy would prevent them from sliding around on the plate and also secure them as well.  If you still want screws, how about filling in between and around the magnets with a urethane epoxy and then drilling and screwing the epoxy to the plate (use non-magnetic stainless screws)?  It's a compromise.  The epoxy arrests the sheer load and the screws through the epoxy provide your "fastened" link.

[Flux]

you are quite right, the only tendency to move is radially due to cf. Those that use polyester to hold magnets on are relying almost entirely on the fact that the whole block prevents radial movement. Keying the resin block to the metal plates as you suggest is an extra precaution worth taking.

Magnets usually come off when the stator warps and rubs the magnet rotors. With enough heat the magnets demagnetise and the resin block is damaged and then magnets fly off.

If properly glued on or even if potted in a keyed resin block they will never come off unless there is a major rub on to the stator. The issue of moisture and corrosion is a different one and may occur in cases where there is no danger of magnets coming off.

I have run machines on a test rig at normal operating speeds with magnets just held on by their own attraction with no trouble at all. I wouldn't like to take them up to the overspeed that could arise under wind conditions with no load, but even simple polyester potting should take care of this.

Flux

[neilho]

Hmmm. Some food for thought here. Thanks.

Zubbly's ebay supplier (tropical_son for others who are interested) is supplying skewed magnets for induction motor conversions. Not quite what we need, but maybe he'll be willing to have the magnets we need machined for us. I'm still curious as to how the machining is done, but it looks as if it might not be necessary for us to do it, though it still seems to me that counterbored holes would be better than countersunk, since flathead screws exert some radial force on what seems to be a very brittle neo.

 It also seems that the epoxy coating that's supplied on the magnets isn't very durable, suggesting a few coats of epoxy paint which could also be used to coat bare magnet surfaces. Epoxy automotive paint has a good track record for longevity and we'll probably be coating the rest of the machine with it (speaking of nasty chemicals!) so painting the hub plate and magnets before assembly and depending on the paint system to resist corrosion (both fretting and moisture) seems like an effective strategy.

Just FYI, the blades will be mounted on the windward side of the 1/2" steel plate and each blade will be "topped" with a 1/4"x8"x8" plate that covers the root end. Four 1/2" grade 8 bolts and nylocs hold each blade plate and blade to the steel hub plate. It could be more structurally elegant, but it's simple, proven to last and allows easy individual component replacement on the tower.

"If you still want screws, how about filling in between and around the magnets with a urethane epoxy and then drilling and screwing the epoxy to the plate (use non-magnetic stainless screws)?"

Maybe I'm misunderstanding, but I don't quite see the point of doing both adhesive and screws. Either method works by itself, why make it more complicated than necessary?

News flash- just found a hardness value of Vickers 500 for neos, which converts to Rockwell 49C, easily machinable with carbide tooling. (Don't think EDM is necessary.) Now how to get rid of those pesky chips?.

 Will keep you posted.