Starrett magnetic base

This is because it uses Alnico type metal magnets which must NEVER be removed from their associated pole pieces (not even for milliseconds!) unless you have the facilities to re-magnetise in situ after final assembly.

The characteristic that controls this is the product of the magnet length and the material Coercive Force

- roughly speaking this controls the ability of the material to retain much of it's magnetic strength during or after exposure to an airgap in the magnetic circuit.

A magnet is open circuit if it's on the bench with no pole pieces attached - the magnetic circuit is closed if a continuous path through soft iron connects North pole to South pole.

All the older types of magnet steels and even the latest Alnicos behave this way and 50 to 90% loss of field strength from incautious disassembly is possible.

Ceramic (ferrite) and rare earth (samarium etc) magnets behave differently. Their coercive force is so high that they can withstand being magnetically open circuited without serious loss of field strength.

Jim

On Tue, 1 Feb 2005 03:13:52 -0800, the inscrutable "Harold and Susan Vordos" spake:

SWAG: He didn't quite get the mag release assembled properly and the magnet wasn't seating to the bottom when down.

As there are no wires, releasing of the Magic Smoke is -not- a possibility here. My SWAG is interpreted from ownership of a fine HF (Chiwanese) mag base (which has never been disassembled.)

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Because the type of magnet used in it lost its magnetism very quickly on dissassembly (low coercive force) when the pole pieces were removed. This site explains about magnet properties and coercive force.

Or, he put one of the magnet assemblies in backwards? That or the whole flux-capacitor theory the other guy posted, that's good too. (???)

Larry answered cryptically: "SWAG: He didn't quite get the mag release assembled properly and the magnet wasn't seating to the bottom when down." And further, picquing our curiosity, he goes, "As there are no wires, releasing of the Magic Smoke is -not- a possibility here. My SWAG is interpreted from ownership of a fine HF (Chiwanese) mag base (which has never been disassembled.)"

Now I, suspecting Larry was trying to interleague himself with co-responding English friends "Jim" and Dave, looked up "SWAG". Sure 'nuf, there it was in Newton's Telecom Dictionary: "Scientific Wild-Ass Guess".

Bob ( how to you make the grin symbol?) Swinney

On Tue, 01 Feb 2005 11:53:54 +0000, the inscrutable snipped-for-privacy@yahoo.com spake:

Alnicos are semi-permanent magnets? Interesting. I'll have to read more about them. I'd never heard of them.

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They're permanent, just not forgiving. It was _the_ magnetic material until the hard-ferrite ceramics came about.

AlNiCo can be cast, and magnetized in place (this is the exact flip side of being demagnetized in place), it requires a lot of length to develop the coercivity necessary to cross an airgap.

Ferrite can be mass-produced cheaply, it's much less likely to demagnetize than AlNiCo, but it _can_ be demagnetized if you run too much current in a DC motor with ferrite magnets.

Neodymium-Iron-Boron is tough stuff, "supermagnet" material. Its strong and nearly (magnetically) indestructible. It tends to temporarily loose magnetization with elevated temperatures; AlNiCo beats it out there. Physically it corrodes readily and it's very brittle, and if you let two magnets whang into each other the impact can shatter one or both magnets and let little sharp bits of shrapnel fly around.

Does this mean that turning the base to the "on" position when it is not in contact with a surface to complete the magnetic circuit, will ruin it?

Jim

That's dibble and fluff. Alnico magnets do decay, albeit slowly (over years). But no magnetic base uses a "keeper" while it's in use (none I know of have a keeper, at all), or it wouldn't stick to anything. The keeper is designed to _complete_ the flux circuit, in order to lengthen the life of the magnet. If the circuit were completed with a keeper (shunting the field between the poles), the magnet couldn't stick to your work surface.

He's confused.

More likely, anyway (if the base is relatively new - say twenty years or less), it has a ceramic/ferrite magnet. They hardly decay in strength at all, with or without a keeper.

LLoyd

Jim sez: "Does this mean that turning the base to the "on" position when it is not in contact with a surface to complete the magnetic circuit, will ruin it?"

Naw! "Leakage" magnetic effect would still practically emulate a "keeper".

Bob Swinney

Thanks, Jim. You likely hit the answer right on the head. This incident occured back in ealy '67----when mag bases were, indeed, Alnico.

Harold

The "associated pole pieces" are the soft iron parts of the magnetic base.

The geometry and physical arrangement of these pole pieces is carefully chosen so that there is just sufficient alternate path (i e leakage flux) to enable the magnet to operate close to it's maximum energy point when NOT in contact with an iron surface. This is the BH max point - it corresponds to the most efficient use of the magnet material Because of this, the magnet is not degraded when left 'ON' and not in contact with an iron surface. This deliberate leakage flux path acts as carefully chosen partial 'keeper' to protect the magnet from the demagntising effect of a wholly open magnetic circuit.

When in close contact with an iron surface the overall magnetic circuit reluctance is very much lower and almost all the available flux is then diverted away from the designed in leakage path and now flows through the pole pieces into the work surface.

Jim

The decay rate of Alnico magnets in a properly designed assembly is negligible. A familiar example is the use of Alnico magnets in moving coil meters which directly rely on the long term stability of the magnetic field strength.

The leakage function of the pole pieces in partially closing the magnetic circuit is covered in my reply to the valid query raised by Jim Rozen.

Pentagrid

Alnico magnets need about 8,000 ampere turns per inch length of magnet to fully re-magnetise. This assumes that you've arranged suitable lumps of soft iron or mild steel to form a closed magnetic circuit with one leg of the coil passing through the middle.

The current needs only to be present for a few milliseconds.

Jim

As a corollary to all the comments, Anyone have an idea how many thousand ampere turns I might need to re-magnetise an old mag base after building a good core around it?

Mark Rand RTFM

Nope, he's completely off-base. Totally confused. LLoyd

The magnet needs a "keeper" to keep the magnetic flux high. The body works as one normally. When you pull the magnet out, you have to slide it into a mild steel cylinder as it exits the one in the base.

It should be possible to re "pole" the magnet with a serious magnetic pulse applied to the pole pieces of the base once it is reassembled and switched to the "on" position. But it is probably cheaper to just buy a new one, if you don't have the materials on hand to build a device to "pole" the magnet.

Now to see what others have said. :-)

Enjoy, DoN.

Nope! The base acts as an inefficient keeper when the magnet is switched to the "on" position, and a very good one when in the "off" position. (There is a non-magnetic metal separating the two pole pieces, probably brass fused to the steel of the base.)

Because the keeper action in the "on" position is not particularly strong, it is easy to divert the field into whatever it is set upon as long as it is a magnetically attractable material.

That may be -- but was the age of the base listed? That part has been trimmed out of the quoting, and I can't go back while in the middle of a reply.

Enjoy, DoN.

The copper involved would probably cost more than a new mag base.