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Mag Base

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Vic28/02/2020 17:54:03
2917 forum posts
8 photos

What is best practice for parking a mag base such as those used for DTI’s? Should it be left with the magnet turned off or is it ok to leave it switched on?

Steviegtr28/02/2020 17:58:40
2244 forum posts
311 photos

I was always taught at school. Oh a couple of years ago to always have the keep on a magnet or it looses it. I would think the same applied to a mag base. I have mine stuck to a sheet of steel.


Mark P.28/02/2020 18:25:48
621 forum posts
8 photos

Would it be the same for magnetic chucks?

Mark P.

Bill Davies 228/02/2020 18:53:49
246 forum posts
11 photos

I was taught that permanent magnetic chucks circulate their magnetic field through the iron segments when turned off. When on, the flux flows out through one segment, through the work, and returns to the magnet via the adjacent segment. I've seen a diagram somewhere, but can't find it!


Bill Davies 228/02/2020 19:06:52
246 forum posts
11 photos

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.


Martin Thomson28/02/2020 22:43:58
11 forum posts

Leave it switched on, or off, or halfway in between, or whatever you want, it makes no difference.

Mag bases and mag chucks (with the exception of electronic mag chucks) consist of several permanent magnets and a mechanism that moves them around such that in one position they reinforce each other's magnetic field and in the other position they (almost) cancel each other out. Or, sometimes they are a permanent magnets combined with a movable conductive element that short circuits the magnetic flux. Either way, the key thing is that they're permanent magnets..

You can't do anything to a permanent magnet (short of melting it) to remove its magnetism - the clue is in the name. Magnetism can't flow out of a magnet and somehow get lost, that's just not how magnets work.

If you were taught to leave a mag base switched on because it would lose its magnetism, then your teacher was not paying attention in physics class...

Kiwi Bloke29/02/2020 00:04:14
609 forum posts
1 photos

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...

Steviegtr29/02/2020 00:20:54
2244 forum posts
311 photos

I will reply again. As a young un I worked on many small motorcycle engines. My favourite was always the Villiers. From 9E to 2T 3T & 4T. One of the most prolific problems with these engines was the generator. The flywheels would loose there magnetism & you ended up with little charge output & a weak spark. I never overcome these problems & usually had to bump start the engines. A friend of mine builds Classic motorcycles. Namely the BSA Goldstar & Rocket Goldstar.

He says that if you have a weak generator magnetic field, the way to fix it is to hit it with a hammer. He reckons that this re-energises the magnetic field. Now I was always under the impression that to destroy a magnetic field you either heated it up or impacted it with force. I know I have totally digressed from the subject but isn't that what we do. Steve.

Steviegtr29/02/2020 00:45:55
2244 forum posts
311 photos

The strongest magnet in the world.


John Olsen29/02/2020 02:38:04
1198 forum posts
92 photos
1 articles

The older types of permanent magnet do indeed tend to lose their suck over time. There are people who will remagnetise them which will make them as good as new. The basic idea is to apply a very strong magnetic field, for example by placing a few turns of heavy wire around the magnet and then applying a pulse of current from a large battery or a bank of capacitors. The field does not need to be applied for long.

The generator is a different kettle of fish to the magneto or the permanent magnet alternator. The generator does not have a permanent magnet in it, it relies on residual magnetism to get it started when it begins turning. If it has been standing around long enough to lose the residual magnetism it may need a pulse of current from a battery through the field to get it going. That can also be done in the reverse direction if you are changing the polarity of the system.

If the magnetic base is the type with a "switch" to turn it on and off, leaving it in the switched off position will effectively be putting a keeper across the poles. The same applies to magnetic chucks. If there is no control, clamping it onto a piece of steel when not in use will help.


SillyOldDuffer29/02/2020 10:00:21
7562 forum posts
1680 photos
Posted by Steviegtr on 29/02/2020 00:20:54:


A friend of mine builds Classic motorcycles. Namely the BSA Goldstar & Rocket Goldstar.

He says that if you have a weak generator magnetic field, the way to fix it is to hit it with a hammer. He reckons that this re-energises the magnetic field. Now I was always under the impression that to destroy a magnetic field you either heated it up or impacted it with force. I know I have totally digressed from the subject but isn't that what we do. 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.)


duncan webster29/02/2020 13:21:14
3522 forum posts
63 photos

There was a design for a remagnetiser in ME many moons ago, I'd guess around 2005,m but not sure why

Howard Lewis29/02/2020 13:32:23
5320 forum posts
13 photos

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.


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

old mart01/03/2020 19:28:52
3347 forum posts
208 photos

I always leave the mag bases clamped on to a steel girder behind the machines. The exception is the magnetic vee block which lives turned off in its wooden box, its not the thing to leave gathering dirt in the open.

Nick Hulme05/03/2020 19:09:51
750 forum posts
37 photos

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

Nicholas Farr05/03/2020 19:30:26
2999 forum posts
1369 photos

Hi Nick, as far as I'm aware and was told by my farther many moons ago, "keepers" are for stopping the magnets from losing their strength. They are normally on horse shoe magnets and the pot magnets where the north and south poles are in close proximity with each other.

Regards Nick.

Adam Mara05/03/2020 19:32:51
156 forum posts
3 photos

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

Steviegtr06/03/2020 01:04:12
2244 forum posts
311 photos

Oh the problems of lodestone.

Probably spelling mistake in there.


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