Press fitting neodymium magnets`

[PAULatREDWORKS]

Hello all,

I got a couple of projects that would call for press fitting some neodymium magnets into anodized aluminium. I've talked to K & J magnetics and they obviously don't recommend it. But I see press fit magnets everywhere, specially anodized aluminium. They're almost a given on any aluminium pocket flashlight. AND I found this supplier (Industrial Magnet inc.) that has magnet assems with brass sleeves.

Am I missing some magical technique that allows this? I know that the tolerances on the nickle plated magnets are yet to be desirable, let alone the brittle nature of the magnets. The hardening of the anodized aluminium can't help either.

Can anyone help me out here?

 

[JNieman]

Magnets with either a threaded hole or threaded stud integrated are very common.

Then of course there is the option to use adhesives.

 

[PAULatREDWORKS]

The application is consumer level. As novelty as those cheap flashlights with the magnets in them. One of the fittings involves pushing .250in magnet through a 2mm/.080in anodized aluminium plate. So adhesives aren't the best option there.

I've already read though this thread:

http://www.eng-tips.com/viewthread.cfm?qid=371826

And the adhesives suggested in that post would be great for my other application but I need to think of cost on the manufacturing side when this goes to production, not to mention the quality control that goes into goopy adhesives.

 

[SwinnyGG]

I think you're not seeing any press fit magnets, but you are probably seeing a lot of magnets put in place with epoxy.

 

[btrueblood]

Rather than a true press fit, machine a slight lip at the edge of the hole. The hole should be an easy slip fit for the magnet, then the lip is peened/pressed over to snug up against the magnet. A small chip of plastic or similar behind the magnet will help the assembly have enough compliance for the magnet to stay put thru thermal cycling, if it's a concern.

You can also find "canned" Nd magnets, like these:

http://in.misumi-ec.com/asia/ItemDetail/10100147350.html

 

[JNieman]

I was just about to type a suggestion about pressing metal-clad neodymium magnets instead. Since you're truly married to the pressing-in of them, it seems.

 

[PAULatREDWORKS]

I already looked into those.

https://www.magnetics.com/product.asp?ProductID=42

They are impressive. But maybe a little over engineering the problem. It would definitely be the easiest fix. I think I'm just going to go for it in my second phase of prototyping. I'll get pictures of what I have thus far.

 

[JNieman]

Seems like you're really hellbent on pressing neodymium itself directly into anodized aluminum. Have you thought about making the hole oversized and sleeving it with some kind of nylon or thermoplastic?

The only other way I can see it working too well, without chipping the magnets, is to possibly use a heat shrink fit rather than pressing it in. Personally I haven't use heat shrink fits with anodized aluminum, only bare aluminum, so I haven't really looked into how it might go differently with the surface changes.

 

[PAULatREDWORKS]

The thing is I'm not hell bent on it. I'm under pressure from the high echelon to get this solved by way of press fit without any bells or whistles. I'm just going to do it.

I just ordered a ranged index of drill bits sizes (D-F), a ton of magnets, and get to it. Ill have to spec each magnet, unfortunately american magnet tolerances seem to be tighter than over seas, go figure.

 

[hendersdc]

Have you thought about using plastic coated magnets? They may be more likely to survive a press fit than regular nickel plated options.

 

[PAULatREDWORKS]

umm now that's actually a good idea. I'll look into that. thanks. Here's some samples that I've tried out.

The cylinder was just epoxy-ed in and the other was press fit.

 

[btrueblood]

Not sure I'd leave it protruding (and thus susceptible to impact damage), presumably that is a "feature" wanted by the upper echelon as well.

 

[3DDave]

Neodymium magnets are sintered powder protected with plating. There's a reason you never see unplated magnets, because the magnetic material will flake off easily. They don't handle the loads required to press fit. Try staking if you want to retain the magnets mechanically. You may need to use adjustable reamers to follow the variation in sizes of your supply.

 

[PAULatREDWORKS]

correct. It is a .080 in plate with a .2 in L magnet centered on each other. I think I have this figured out. I'll get back to this post when I have more made in a different way.

 

[benta]

Heat the aluminium and you'll be able to drop the magnets right in. The thermal expansion coefficient of Al is so high, you don't even need exorbitant temperatures. 150...200 C should be enough.

You'll need to experiment a bit with the tolerances for the right clamping. But the magnet will sit better than with a cold press fit.

Benta.

 

[PAULatREDWORKS]

The max temp on Nd magnets is 80C. I know it's not constant heat but I would assume that kind of immediate heat cant be good.

 

[benta]

80 C?
Wow, didn't know that. Poses a problem, then.

 

[3DDave]

Just in case management relents, I think a UV cured adhesive would be ideal. Very fast processing and adequate retention without the need for finicky fits. The guys from Masterbond or Loctite-Henkle would be able to advise on a suitable material.

https://youtu.be/CbY7zfp1g-A?t=220

example Youtube video of bonding to aluminum. Since the joint is accessible from both sides the metal-metal joint should cure just fine.

 

[KENAT]

Only issue with UV might be getting the light to the effective bond.

Having broken a few neodymium magnets I'm on the sceptical side of getting a good press fit.

You can get them with adhesive backing, though the size range may not be impressive and I've had at least one case where the adhesive wasn't really enough.

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[MikeHalloran]

Aren't those the kind of magnets that fracture explosively when they are abused?


Mike Halloran
Pembroke Pines, FL, USA