Starrett magnetic base

He's right on. Trolling?

Reply to
Don Foreman
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Depends mostly on the internal airgaps. Figure about 30 amp-turns per .001" of total airgap to saturate the steel parts and the alnico. (That's 15,000 oersted) Non-magnetic material (brass or aluminum) in the flux path counts as airgap. You can get several hundred amps out of an auto battery for the short time it would take to blow the wire. #16 or #14 wire might be about right; it'd fuse in a second or two but the job would be long done by then. 50 turns of #16 should get its attention. More turns of finer wire might work too, but take more winding.

I won't strongly assert that this will work, but it's easy to try.

Reply to
Don Foreman

I tried re-magnetizing a base a while ago. I screwed around with it a lot and never did get very good results. I dumped charged capacitors to get the large currents. It is definitely not worth the effort, but I knew that and was just doing it for kicks, er ... science, yeah "science".

I used the wisdom of this group and posted about it. Google-group the RCM archives on "magnetic base", author "engelhardt".

Bob

Reply to
Bob Engelhardt

On Tue, 01 Feb 2005 11:13:13 -0800, the inscrutable Tim Wescott spake:

To clarify, I was aware of alnicos (most speakers had them--for most of my pre-teen and teen years, anyway) but wasn't aware of the possibility of demagnetizing any "permanent" magnets.

I have some tiny NIB mags which are strong enough to give my fingers blood blisters when I'm careless with 'em. Amazing. I wish I'd had them during experiments when I was a small child.

Reply to
Larry Jaques

Excellent explaination, thank you, Jim.

Jim

Reply to
jim rozen

I'm in my late fifties. I have a fairly large collection of odd-n-ends alnico magnets from speaker pull-outs I've kept since my teens.

They're all weaker than they were, but they're all still magnets, and still useful for holding drawings, etc.

To those insisting that alnico magnets lose their strength almost instantly in the absence of a keepe, consider: Millions of kitchen pot holders were made in the fifties, sixties, and seventies with a small 3/16^2 x 3/4" alnico bar magnet within the hanging loop (for hanging the thing on your fridge or oven door).

They were made, shipped, stored, and usually used in the home without any keeper of any sort -- usually when thrown in a drawer, they didn't even have the benefit of being near a ferrous substance. But they kept their strength - enough to hang the things up - for _years_.

LLoyd

Reply to
Lloyd E. Sponenburgh

Just because they shipped magnets without keepers doesn't mean that they are at their fullest possible strength. And if alnicos they weren't. I've seen the mag base effect where taking one apart weakens the magnet. Curious, I asked an engineer I know about it and he told me what had happened. A similar effect can be seen in stepper motors. They are assembled and then magnetised. If the motor is taken apart it will lose much of it's torque. I've been told that some of the newer motors are not as sensitive to this. Even if this is true, the motors I modify on a regular still come with instructions saying not to remove the rotor. The rotors are not alnico though. ERS

Reply to
Eric R Snow

The flux density in those alnico household magnets is considerably less than that of saturated alnico. It became so almost instantly when removed from the magnetizer at the factory, before it ever arrived at the store.

Reply to
Don Foreman

Anyone who works on magnetos knows that the rotors should always be stored with 'keepers' on the pole pieces.

I always wondered what would happen if I took one of those old mags, and set it up with some of the new supermagnets.

Jim

Reply to
jim rozen

The keeper discussion still seems to be simmering gently so a few extra comments might be helpful.

For the moment consider only the iron alloy based magnets. This includes Alnico, Alni and the earlier alloys using cobalt,tungsten or carbon additives.

These alloys, when magnetised to saturation in a fully closed magnetic circuit magnetising jig, all retain about the same flux density after the magnetising field is removed. This is quoted in the manufacturers literature as the B remanence figure. It is typically about 12 Kilogauss.

If, now, a small air gap is introduced there is an immediate drop in flux density which is only partially recovered when the air gap is returned to zero.

Provided that any later experimentally introduced airgap is smaller than this first gap there is no further degradation.

If however a subsequent air gap is greater than any preceding gap it ratchets the closed magnetic circuit flux density down further and establishes a new range of airgaps that the magnet can accept without further degradation.

This is the behaviour that we observe when we disassemble a magnetic base or remove the armature from a servomotor that uses this type of magnet.

It doesn't explain the role of keepers commonly used to protect magnets when not in use. Once a magnet has been exposed to the demagnetising influence of a large airgap, putting the keeper back on will NOT restore it to it's previous state.

The keeper is there for a different reason. Permanent magnets exhibit their magnetism as a result of the saturating magnetising field forcing their randomly oriented internal magnetic domains into a nice orderly additive arrangement. As noted above this is fairly easily disturbed by the demagnetising effect of an airgap.

It is also disturbed by mechanical shock and it's sensitivity to shock or stray external magnetic fields is increased if it's also fighting with the demagnetising influence of an airgap. The older permanent magnet materials which have low intrinsic coercive force are particularly sensitive to this problem and this is why you see the old schoolboy bar magnets and ancient magneto horshoe magnets religiously stored with keepers on when not in use.

A keeper is also used occasionally to provide a temporary alternate flux path when it is necessary to remove part of the normal main flux path. This avoids exposing the magnet to the demagnetising effect of a large disasembled airgap. The keeper must, of course, be installed BEFORE the main flux path is removed.

The mechanical shock effect is easily demonstrated in the workshop. Magnetise a screwdriver or a piece of hardened carbon steel by stroking it against a decent permanent magnet. Left undisturbed on the bench it will retain it's newfound magnetism indefinitely. Bash it hard against any sustantial lump of metal and much of its magnetism is immediately destroyed.

Similar effects, but to a much smaller degree occur with ferrite and the rare earth supermagnets. With most designs, additional airgap induced degradation is not enough to matter.

Designs using the old magnet materials can be uprated by changing to rare earth magnets but the vastly different optimum length to diameter ratio of the new material means that a pretty major mechanical redesign is necessary.

Jim

Reply to
pentagrid

Very nice explanation, thank you. I've learned so much in this NG! Bob

Reply to
Bob Engelhardt

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