Magnet dimensions

[Thom]

I debated where to put this and other than another forum, this seemed safest.

Can the magnets in JBL compression drivers be replaced in the field. (by a shop counts)

Are exact dimensions given anywhere?

Does anyone Know for sure if you can use an electric discharge mill on neodymium without demag (I would suspect so but do not know for sure)

Can neodymium be field annealed / hardened or does any machining have to be done by grinding.

I'm not asking anyone to do any research. If you know an answer and could take the time to post that would be great . If not ok. If you know where an answer might be and want to post that that would be great also.

thank you

[Robh3606]

Why the question??

Rob:)

[Thom]

There are lots of neodymium magnets on EBAY. Every thing I have read about HF drivers says the stronger the field the better or the higher before roll off, and I'm crazy. Everything you see with neodymium is to save weight, but what if it wasn't? I'm pretty sure you could easily get enough field on a woofer to get terrible results but compression driver might be different. It's interesting when it says have anything magnetic out of your pockets and not loose within 5 feet. On the 2in by 6in disk it warns you that two of them may be attracted towards each other strong enough to break bones. A D130 might be a heck of a dome tweeter and not have the stability problems associated with domes. I don't think I was serious about one. As far as magnetic shielding, you might just have to say not for use in the vicinity of CRTs

[moldyoldy]

Out of sheer curiousity, I bought a few (1"x2"x1/4") of those eBay neo mags a couple years ago. The warnings they give may actually be understated, as even small ones like mine can be very dangerous to handle. Not to mention where to store them where they won't have undesired effects on everything else in the vicinty. I finally ended up sticking them under my truck bumper for lack of a better place to keep them. I wouldn't let one the size you describe into my house, even if my ex was carrying it (although that does bring to mind some interesting scenarios...).

As far as DIY driver magging, keep in mind that in production, the magnets remain uncharged until assembly is complete for the obvious reasons. Read up on the difficulties experienced by those of us that have reglued loose pole pieces and such with live mags, and if you have any sense, you'll abandon all thoughts of attempting what you describe.

[Thom]

You know I'm not sure if I'll take your advice or not but I might. It has sort of a reality to it that is missing from a warning label. I will definitely remember your advice. sounds like the computer might have to be move to another house

Thank you

[Paul C.]

Thom, I use a large neo magnet, appx 1.5" dia x 1.5" long, mounted in a special fixture with handle, for removing dents in brass musical instruments. Large diameter ball bearings (1" - 3" dia) are dropped into the bore, and rolled around with the magnet to roll out the dents. This works great as you don't have to unsolder anything.

And these large magnets are quite dangerous.

First, they will demag the strips on the backs of your credit cards in your wallet. They will ruin a mechanical watch. And anyone with a pacemaker stay away.

A repair tech I know said when theirs was delivered, the package was set on the desk with the computer... and ruined the monitor and hard drive.

All steel/iron tools and parts must be removed from the workbench. And I have lots of needle springs, razor blades, and other such objects. I have had other friends have hand injuries with tools, smashed fingers and such.

I can roll balls around on the floor with the magnet holding it in hand and not bending over.

If two of these magnets were to be on the workbench at the same time they would attract each other and accellerate geometrically, crashing together and exploding, sharp chips flying all over the place... this happened to another guy I know. There are still chips of magnets on beams in the metal workshop.

I have a small metal plate screwed on the wall opposite the work bench and store the magnet on there well away from everything else.

Neo magnets are nothing like the biggest most powerful ceramic or alnico magnet you have ever seen... they are far more powerful. I hope you don't think I am exaggerating or joking... I am not.

[Thom]

Does anyone know how easy/ difficult they are to degause and if somenne set up to remagnetise a particular driver would be able to remagnetise it. Thinking of bachining a piece of mild steal to fit where old gag was and accecpt new mag. Appreciate all feedback. Without feedback would probably own a magnet by now. Many mag materials an AC field strong enough long enough, probably tru here too but strong enough might be a kicker.

[moldyoldy]

The same equipment does both operations. Field strength isn't time related, so a pulse of the required intensity is all that's needed. Most production units are capacitive-discharge to the fixture coil, which is designed for a single application, so different target sizes and shapes need different fixtures.

Maggers with capacity to fully charge the ferrites and AlNiCos we're familiar with start around $10K less fixtures for a low duty cycle. I've no idea how much a high-energy unit for neos would run. The only facilities to have them would be the manufacturers that have to. Since neos don't weaken, and it's cheaper to replace than repair, there is no service application or market. Unless you develop a cooperative contact at a facility that has the equipment (and a satisfactory fixture) you're SOL.

[Robh3606]

Since neos don't weaken, and it's cheaper to replace than repair, there is no service application or market. Unless you develop a cooperative contact at a facility that has the equipment (and a satisfactory fixture) you're SOL.

So true. With a damaged 2435, shifted top plate JBL says replace the driver. They will not/cannot repair like an Alnico with a shifted top plate. After messing with that driver I wouldn't go near Neo's that are fully charged. Too damn strong, too easy to get hurt. Had that core grab a razor blade from 6" away. Scared the daylignts out of me, was lucky it didn't take part of my finger off.

Rob:)

[Thom]

Hey, if I never do anything, I know things now that I didn't a few days ago.

Is there a grain to the blank that needs to line up a certain way, or can you magnetize the "blank" (didn't know what else to call it) any direction?

[Ian Mackenzie]

So true. With a damaged 2435, shifted top plate JBL says replace the driver. They will not/cannot repair like an Alnico with a shifted top plate. After messing with that driver I wouldn't go near Neo's that are fully charged. Too damn strong, too easy to get hurt. Had that core grab a razor blade from 6" away. Scared the daylignts out of me, was lucky it didn't take part of my finger off.

Rob:)


Watch out JAWS!

[moldyoldy]

Hey, if I never do anything, I know things now that I didn't a few days ago.

Is there a grain to the blank that needs to line up a certain way, or can you magnetize the "blank" (didn't know what else to call it) any direction?


http://www.oersted.com/magnetizing.PDF

[moldyoldy]

High-energy magnetics are important in the medical field as well, where MRI systems commonly employ large permanent magnets in the 1-2Tesla range. (the big tube they slide you up inside of is one such magnet). There are more than half a dozen documented accident reports involving ferrous compressed gas cylinders (oxygen, nitrous, etc.) being snatched from up to 20 feet away and ending up in the MRI tube, sometimes with a patient, sometimes not. :( Kinda like terrorism in that it's impossible to foresee all they ways they can get ya....

[Hoerninger]

http://www.oersted.com/magnetizing.PDF
Thank you ... I've always wondered.
___________
Peter