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The science of magnets?
http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10102&t=14355		 Page 1 of 1
Author:  Brock Poling [ Sat Nov 03, 2007 6:33 am ]
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I am looking for some basic info about magnets. Particularly rare earth magnets. I can find info at both ends of the spectrum, either way way to high tech, or way too basic.

I am looking for answers to some specific questions. If anyone knows the answers or can point me toward a resouce it would be greatly appreciated.

I am using these in a series of jigs, and I need to figure out what the best size & strength is for the application.


1. Generally speaking, (for rare earth magnets) does strength increase with wider surface area on N/S poles or in the length of the magnet? (BTW, I know about the ratings, n40, 48, etc.)

2. If I want double the pull, use 2 magnets side by side? (Parallel).    Does the field double if you stick them end to end in series?

I know if you cut a magnet in half you end up with two N/S combos but I am unsure about what happens to the magnetic field. Is it only half as strong once you cut the magnets? If so, wouldn't it stand to reason then that if you stacked several smaller ones in series you would end up with a stronger magnet?

3. Say a given magnet has a 50lb pull. I presume this means 50lbs against some type of ferrous metal. If instead it was attracted to another rare earth magnet of equal size and strength woudl the pull now be 100lbs, or is it the same 50lb pull?

4. Part a) And here is the question I am really after. When magnets are separated by a non-ferrous material (MDF, Plywood, air, paper, whatever...) is there a (usable) forumula to predict the change in strength?

Part b) Does the type of material separating the magnets (again assuming non ferrous) have any impact on the changes in strength? (For instance, would separating the magnets with air be any different the separating them with MDF.


I have a whole series of clamps and jig ideas using these magnets.. I will show them when I get them figured out, but now I am just trying to understand these magnets well enough to know what to go buy.

I have done a little trial and error, but that gets expensive.

Thanks in advance.


Author:  John How [ Sat Nov 03, 2007 6:46 am ]
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Hi Brock
Last year I bought a bunch of these magnets with the intent of using them to do much the same. My first attempt was to install them around my building form and us another magnet to clamp the sides in. I discarded the idea after the first try because these little guys are somewhat hard to control and the potential for damage around thin pieces of expensive wood was too great for me and I just went back to my old ways. I can send you some for testing of you'd like. I got tired of rebuilding my jigs and just decided to carry on. The ones I have are about 1" in diameter and 1/4" thick. I may get ambitious and try again but not right away.
Author:  Brock Poling [ Sat Nov 03, 2007 6:54 am ]
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Certainly putting them inside the spreaders is one of my thoughts. I have played with this some and I have a solution that seems to work ok. As long as you deliberately put them in and take them out all is fine.

I have other ideas for them too, particularly places where it is hard and slow to put clamps.

Author:  Rick Turner [ Sat Nov 03, 2007 6:54 am ]
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The science is too dense for a lot of practical knowledge. You'll get from here to there a lot quicker by just making prototypes and learning from practical experience.
Author:  David Collins [ Sat Nov 03, 2007 6:59 am ]
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I have plenty of reference books, but not enough formulas memorized to
recite off hand. I can tell you this much though. I think it would be easier
for a few folks here to simply give a specific size or recommendation
based on some specific criteria you aim to meet, than to try to teach how
it is figured out. There are certainly formulas to answer your questions,
but the ones I'm looking at have an awful lot of greek characters that
aren't available in the fonts here.

I'm sure there are plenty of online calculators available for the strength/
distance question. Here's one.

http://www.arnoldmagnetics.com/mtc/calc_gauss_cyl.htm

If you want a reference book, a good high school or early college physics
textbook will have a number of chapters on magnetics to make for
relatively simple and understandable reference. I keep a number of these
around.
Author:  jhowell [ Sat Nov 03, 2007 7:14 am ]
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Brock--


In general, the strength of the magnetic field vs the distance obeys the inverse square law -- twice as far away = 25% of the force.  Solving one of Maxwell's differential equations will give it to you more exactly, but I had a h%%l of a time doing that in college as a physics major!   If the material between the magnets in non-ferrous, I don't think it matters in any practical way. 


If you know anyone who does computer repair or has access to crashed hard drives, the magnets in the hard drives work really well for crack repair and remote sanding and I suspect in jigs.  I think that Rick is right, you really won't know for sure without just messing about with some.


 
Author:  BruceH [ Sat Nov 03, 2007 8:27 am ]
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Ditto to what John said. I bought some last year with the intent of using them for some quick clamping fixtures. They are extremely hard to control and I decided I didn't want them anywhere around my guitars.

Try this: get a couple of .5" x.5" x 1" and put one in each front pocket.
Author:  PaulB [ Sat Nov 03, 2007 11:50 am ]
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This site has calculators and info on magnets.
Author:  erikbojerik [ Sat Nov 03, 2007 1:29 pm ]
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Don't bother with trying to cut them; they are quite hard, and the heat
generated will cause the magnet to lose a fair bit of its field.
Author:  JohnAbercrombie [ Sun Nov 04, 2007 6:22 am ]
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Brock-
I've bought and used some magnets from LeeValley, and you have probably looked at their selection. You can get a pretty extensive collection for experiments for $25 or so. They have some useful 'accessories' like the steel cups and also plastic facing discs which can help in controlling the action of the magnets in use. Steel backing plates increase the apparent attractive force of these magnets a lot.

Off-topic:
One neat trick is to put a small magnet on your steel stationary power tools- no more searching for the chuck key on the drill press or the allen wrench to adjust the bandsaw fence- the tools live on the machines now.

Cheers
John
Author:  Brock Poling [ Mon Nov 05, 2007 1:33 am ]
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Actually I have found tons of magnet stores on the internet. Lots of very interesting offerings and (for magnets) very reasonable prices too.

Great idea about the wrenches and chucks.

Author:  Rick Turner [ Mon Nov 05, 2007 1:53 am ]
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Using steel backing plates or "U shaped pole pieces brings the opposite polarity around and does greatly increase the magnetic attraction to ferrous objects.

If you want to see what's going on with magnetic fields, crumble up some 0000 steel wool and sprinkle the bits onto your magnetic structure. The bits will line up with the flux field.
Author:  JohnAbercrombie [ Mon Nov 05, 2007 3:57 am ]
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[QUOTE=Brock Poling]

Great idea about the wrenches and chucks.

[/QUOTE]
Brock-
I can't take credit for the idea- it's another one I spotted at Sergei deJonge's. I gave myself an 'imaginary dope slap' when I saw it there. You can hang screwdrivers, wrenches, etc on the bandsaw with a suprisingly small magnet. Keeps 'em out of the sawdust pile!

Cheers
John
Author:  Dennis Leahy [ Mon Nov 05, 2007 7:47 am ]
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When a repair guy showed a repair using magnets on one of the forums, it looked pretty slick, and I started think about using them in jigs too. Interesting to note that those who have tried them now see their potential for damaging the guitar.

...which made me think about using electromagnets instead. To bother with the initial setup, it would have to be a jig that gets a lot of use (like a female form for a production guitar), but you would be able to easily control when the magnetic force was present.

Dennis
Author:  JohnAbercrombie [ Mon Nov 05, 2007 8:24 am ]
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[QUOTE=DennisLeahy] When a repair guy showed a repair using magnets on one of the forums, it looked pretty slick, and I started think about using them in jigs too. Interesting to note that those who have tried them now see their potential for damaging the guitar.
[/QUOTE]

Dennis-
Some builders use magnets with no problems. Pretty well any time you get something harder than cedar near a guitar, you have potential for damage. It's just a matter of figuring out what's appropriate, and how to work under control, the same as with any tool.
With the steel cups, and CA or epoxy, you can attach handles to magnets- this can help keep them under control.

Cheers
John
Author:  Brock Poling [ Mon Nov 05, 2007 8:38 am ]
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I am not using the magnets directly. I am installing them INTO the jigs and they are much more managable like that. But what I am struggling with is controlling the strength. It has pretty much been trial and error up to this point. That is why I was hoping there was some simple forumula to predict the strength variance as you put add distance between them.

but I am having some good success with them.

As I refine these ideas I will post some of the stuff I am working on.

Author:  Bob Garrish [ Mon Nov 05, 2007 8:48 am ]
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I've got rare earth magnets all over the shop for holding rulers, chuck keys, etc. The Fadal is covered with rulers, vise grips, little wrenches for tools, etc etc etc all stuck to magnets.

If you want twice the grip, put them side by side. Stacking them is relatively ineffectual. They're also really handy for finding things that may have fallen into the sawdust pile.
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