Would it be the same for magnetic chucks?

Mark P.

I know it's not a DTI base, but I presume that the mechanism is similar:

Magnetic chuck .

See bottom of page 4 ; note the page order is a bit odd, perhaps the booklet was disassembled before scanning.

Bill

Unfortunately, permanent magnets aren't permanent, although, modern magnets, made from high-coercivity materials are pretty good at retaining their magnetic properties. Older, low-coercivity materials, such as Alnico, really needed 'keepers' to preserve the magnet's strength.

'Permanent' magnets can be de-magnetized by varying magnetic fields, heating to above their Curie temperature, vibration and shocks, and probably other mechanisms I don't know about. Again, 'modern' materials are more resilient (although not physically – ferrites and neodymium 'super magnets' are very brittle). I think that it's correct to say that repeated 'sticking' and 'un-sticking' an old-style magnet from whatever it was 'sticking to' would slowly weaken the magnet.

It's best for a magnet to be kept with the magnetic circuit closed – hence keepers between poles of horse-shoe magnets. Whether the circuit is effectively 'more closed' when 'off', or when 'on' + a keeper presumably depends on the design of the mag. base or chuck. It's an interesting question – perhaps an authority can enlighten us…

The strongest magnet in the world.

steve.

We have digressed, but this forum is more like a chat in the clubroom than Wikipedia! I understand why wandering off subject can be annoying, but I think it adds interest. Anyway, magnetic fields as such aren't destroyed by heat – artificial fields are used to contain plasma, and the Sun and Earth (molten Iron Core) are both magnetic. Magnets made of steel are destroyed by heat because the field depends on alignments inside the metal, billions of tiny North-South elements all pointing the same way. Mechanical shock and heat tend to disrupt the alignment which reduces the strength of the field, or removes it entirely. The atomic magnets aren't destroyed, they point in different directions and cancel out.

For the same reason, magnets can be created by hammering steel, because if steel or iron is frequently beaten in the same position, the molecules tend to align with the earth's magnetic field. It doesn't make a strong magnet, but the effect is significant. For that reason iron ships found their compasses influenced as much by the ship as the earth, making them difficult to navigate. A Binnacle is a remarkable bit of engineering that compensates for the ship's magnetism. That ships are also magnets is used in by Magnetic Mines. These detect the approach of a ship and can be set to explode just as the ship passes closest. They are much more effective than a contact mine because they explode under the keel making a giant bubble that leaves the damaged keel unsupported, causing it to break like an overloaded bridge. In WW2 ships were demagnetised electrically to reduce the risk.

So hammering a dynamo isn't completely insane if it restores enough residual magnetism to start the process. However, it's an example where 'common sense' experience is likely to mislead. If your mate's garage happens to be aligned in the right direction hammering will improve the magnet. But his recipe fails when applied elsewhere. He swears blind it works, some agree, whilst others fail.

Personally I'm against hammering anything delicate as part of a repair. While it might lift a dynamo's residual magnetism, hammering sends shock waves through the structure, possibly bending the frame, damaging bearings, and loosening wires. Don't buy a refurbished dynamo from that man!

I was middle-aged before twigging that magnets needn't have a North and South pole! A toroid magnet is broken to make a classic horseshoe magnet, and there are no physically separate North and South poles in a 3-phase motor. Unfortunately older materials gradually lose magnetic alignment if the field is broken, hence the need for a keeper. As modern magnet materials are much less likely lose alignment, they don't need keepers. (Though no harm in having one.)

Dave

In a similar vein, magnetised steel can be demagnetised by subject it to a varying magnetic field.

After magnetic crack detection, crankshafts, in particular, are demagnetised by passing through a coil carrying an AC voltage. Each cycle magnetises / demagnetises the steel, and the effect diminishes (inverse square law ) as the item moves away through the reducing field.

Howard

Edited By Howard Lewis on 29/02/2020 13:35:18

Were "Keepers" as provided by magnet manufacturers in the past to "Keep" the field localised or to "Keep" the magnet's strength? 😀

In the early 50's I bought a pair of Eclipse bar magnets from a shop close to Canterbury Cathedral, I still have them hardlyn used in the original box base with the keepers, sadly there is hardly enough strength to hold the keepers on