Magnetic Bench

I have a problem . for some unknown reason the metal top on my bench work bench has suddenly become Magnetic, the top is approx 24" x 60" and there aren't any electrical connections on the bench or any batteries. can you help me to de - magnetise it ? . Ken.

This does seem to be an odd thing that occurs now and then. I have heard that hammering the metal will stop magnetism?

Some time back all the tools for my lathe suddenly became magnetic, picking up ferrous swarf which sometimes stuck in my hand but eventually it seemed to disappear.

Clive

I read somewhere that one could magnetise metal rods by hammering them with ends N-S...? I think you can also get induced currents/magnetism from power lines and electrified railways?

Perhaps turning the top around before hitting it or turning it at 90deg to any power line might help?

WW2 ships were demagnetised using a large elctromagneti coil surrounding the ship, see

**LINK**

It would be a lot of work, but if you want to go down that route you could start with the transformer out of a microwave, a length of welding cable and a variac to control the current. Noneed to move the bench through the coil, wrap it round, then ramp the current up and down. Having set this running hopefully someone with relevant experience will tell us where I've gone wrong.

You need to use an alternating current if this is to work, and reduce the current steadily down to nothing. Anything else is likely to magnetise the bench, and anything else within a few inches. And take your watch off before you start.

The process is called de-gaussing, and ships were done to avoid magnetic mines.

Cheers, Tim

I would have thought a microwave transformer even when fed with a variac would have been a bit dangerous, if anything went wrong you could 2000V on the loose.

I've seen a homemade TV tube de-gaussing coil which was a few turns about the size of the tube and fed from the mains in series with an electric fire, the coil is then moved around in front of the tube and gradually moved away. No idea if that would work on a steel table though.

My understanding of Microwave Oven Transformers derives from an interest in building Tesla Coils. Tesla coils require a source of high-voltage high-current power, ideally more than 100mA at 6000V. The voltage can be obtained by connecting the high-voltage outputs of two or more MOTs in series. There is a high risk of insulation failure. For this and other reasons you have to be very careful using MOTs in a Tesla Coil. Perhaps the best advice is don't.

Individual transformers are also very dangerous. In normal operation in an oven their output will be at least 0.5A at 2000V. Although the shock alone could easily be fatal, being killed outright might be the least painful option. This is because the 3 or 4 kW available at the output will have no problem striking an arc into flesh and turning you into an electric fire. Burning will continue until someone switches you off.

An MOT could be used for degaussing only by rewinding the secondary with a few turns of very thick wire. 3000 amps at a volt or less would be good. It will still get very hot though!

Ships become magnetised when the steel they are being made of is struck repeatedly in alignment with the earth's magnetic field. The same mechanism probably magnetised Ken's worktop. Rotating the top through 180 degrees and beating it again might undo the effect. Alternatively after using a compass to establish which way round the top is magnetised, stroking it with a strong permanent magnet in the opposite sense might counteract the unwanted magnetism.

Heat also destroys magnetism but you would need to make the whole top very hot. Not sure that's practical!

Cheers,

Dave

I think Duncan Webster's intention was that the high voltage secondary was removed and replaced with a few turns of very thick wire (Hence the reference to welding cable.) to make it into a low voltage very high current transformer.

Les.

Maybe the best idea is to replace it with a sheet of Aluminium? Much safer to work on and it will not damage files etc.

Clive

Exactly what I meant to say, as described in recent MEW for making a spot welder, but it would still be a lot of effort.

Sure thing Les. Good point.

Apologies if what I said came over in any way as being critical of Duncan. Not my intention at all - I respect him.

I was trying to pass on some info about a subject where I have some experience. No offence intended.

Regards,

Dave

None taken I didn't express it very well

Stainless steels are not all non-magnetic. Of the three common types two are fairly magnetic and one in almost non-magnetic.

I once tried a micro wave transformer back to front, hooked the secondary to 230v, I think the out put was about 10v, open voltage/no load. One of these transformers with the secondary removed and replaced with heavy cable gives about 3v. I'v got 4 or 5 of these things waiting for me to think up some way of welding a seam in thin stainless steel.

Ian S C

Why not just replace it with plywood top, or as mentioned aluminium sheet, unless you use it for welding things on in which case any residual magnetism is/may be taken back to earth through the earthing cable... I think ... I'm not that far electrikery compos mentis.

George.

Frances, Yes about modern power supplies, smaller lighter, and a whole lot more RF interference, as in the charger for my camera, and the power supply in my PC, and the 10W LED spot light that in the workshop. Copper is a semi precious metal, perhaps it's getting more valuable.

Ian S C