Magnets

[Klaynos]

I was speaking about a magnet which was already manufactured and magnetized, as is the present situation. How would the domains because "stuck" in the 'wrong' position after manufacturing and magnetization? How are they aligned when melted? I assumed that domains are aligned when the magnet is magnetized. Is that not true?

 

To magnetise a magnet you need to have all (or the majority and the more you have the stronger it is) of the domains aligned in the same direction. Over time due to external effects (like other magnetic fields near by) the domains become misaligned (become "stuck" in the "wrong" direction), so to remagnetise a magnet you need to re-align the domains, to do this you apply a magnetic field, the warmer the magnet the lower field is required to re-align the domains as the temperature makes them more 'free'...

[Pete]

i believe there are physical limits to the strength of permanent magnets and it is really difficult to machine already magnetised parts as the shavings stick to everything.

I'd still like to read more about what you're saying. Do you have a textbook reference for me?

 

Have you ever worked with a metal lathe? I did in High School (one of those things I actually remember from those days. ) and from that experience I can't see any reason to assume that machining a magnetized magnet is any more difficult than maching an unmagnetized one.

..so to remagnetise a magnet you need to re-align the domains' date=' to do this you apply a magnetic field, the warmer the magnet the lower field is required to re-align the domains as the temperature makes them more 'free'...

[/quote']

That is clear to me. But I don't see the need for melting it, just heating it.

 

Pete

[insane_alien]

I'd still like to read more about what you're saying. Do you have a textbook reference for me?

 

unfortunately no, it only came up in conversation with my physics teacher some years ago.

 

Have you ever worked with a metal lathe? I did in High School (one of those things I actually remember from those days. ) and from that experience I can't see any reason to assume that machining a magnetized magnet is any more difficult than maching an unmagnetized one.

 

yep, i've lathed my share of wonky things that were supposed to have turnedout nice and smooth.

 

basically, the shavings can jam up and/or put wear and tear on the parts. normally this isn't a problem as the shavings are not attracted to the metal of the lathe so just fall away. magnetised shavings will stick quite firmly and be difficult to remove while lathing.

[swansont]

I was speaking about a magnet which was already manufactured and magnetized, as is the present situation. How would the domains because "stuck" in the 'wrong' position after manufacturing and magnetization? How are they aligned when melted? I assumed that domains are aligned when the magnet is magnetized. Is that not true?

 

Pete

 

The present situation is a magnet that has lost much of its magnetization.

 

 

basically, the shavings can jam up and/or put wear and tear on the parts. normally this isn't a problem as the shavings are not attracted to the metal of the lathe so just fall away. magnetised shavings will stick quite firmly and be difficult to remove while lathing.

 

I would think the local heating during machining would affect the magnet, which is another reason to machine it first.

[Pete]

The present situation is a magnet that has lost much of its magnetization.

That's what I was talking about. All such magnets have already been manufactured and thus any aligning of the crystaline structure has already been established. Melting and resolidifying is merely repeating the process its already gone through. While magnetic domains may realign due to various reasons I don't see how the crystaline structure could have changed. Domains can be realigned even when the crystaline structure is out of line.

 

Pete

[booker]

Url]

Edited August 17, 2008 by booker
multiple post merged

[Pete]

Neodymium (rather than neodium) magnets--the ones I know of having an N45 rating--consist of a powdered material in an expoxy, or other, matrix. No remelting allowed. The lower temperature magnets are lower temperature due to the lower melting point of the matrix. I think these magnets at this rating run fairly low in max temperature, as low as 150C, which won't get you much thermal advantage for domain realignment.

 

The powdered neodymium alloy could be come in domain sized chunks, for all I know.

 

Edit I found some specs.

 

http://www.kjmagnetics.com/specs.asp

Very helpful. Thanks!

 

Pete

[booker]

Very helpful. Thanks!

 

Pete

 

Sorry, I got my facts embarassingly scrambled. I couldn't delete fast enough

 

http://www.kjmagnetics.com/faq.asp#stack

 

"The powdered mixture [Nd and Fe, and B] is pressed under great pressure into molds. The material is then sintered (heated under a vacuum), cooled, and then ground or sliced into the desired shape. Coatings are then applied if required. Finally, the blank magnets are magnetized by exposing them to a very powerful magnetic field in excess of 30 KOe.

 

OK, so I haven't completely lost my mind.

 

http://www.stanfordmagnets.com/magnet.html

 

"Both Nd-Fe-B and Sm-Co magnets can be made either in sintered or polymer-bonded magnets. The polymer (such as epoxy)-bonded magnets can be produced with close tolerances off tool, with little or no finishing required. "

 

Reading a little further, notice that N45 is sintered NIB. The injection molded (therefore thermoplastic I suppose) magnets supplied by this company are far weaker.

Edited August 17, 2008 by booker
multiple post merged

[Jacky.Wang]

First of all,I need to explain the Neodymium magnets to you,this kind of magnet should be the strongest magnets now.And there are many grades too ,of this kind of magnets.Its character is that they can not work over 200℃,and do they will lose the magnetism as the time over,for the outer magnetic field.And another bad news of this magnet,they can not be magnetized again easily when they lose their magnetism.As I know Neodymium magnets were manufactured by Cast or Sintered.This kind alloy can be magnetized in the special environment.Of course,I just know little of Neodymium,but I know little more about another magnets Alnico.