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De-Magnatizing digital callipers

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Ian P05/07/2013 10:33:38
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Ian

Certainly a hand held coil or even a bench mounted one is safer than a 240V version of the same thing, all I was indicating was that properly constructed a mains hand held device is just as safe. Think of all the power tool and houshold objects that run directly off the mains without any problems.

Having a separate transformer in a box just make the whole thing more complicated with no real benefit.

I don't see the point of connecting the secondary of a microwave transformer to a low voltage, all it will do is reduce it to a lower voltage. The primary (mains), to secondary (HV) ratio is high (maybe 10:1) so in reverse it will just divide by that ratio.

I thisnk its worth me repeating that voltages from microwave transformers are lethal 2000V upwards! All the articles I have seen on re-using old transformers is to REMOVE the secondary winding first.

Ian P

Russell Eberhardt05/07/2013 10:58:51
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Posted by Ian Phillips on 05/07/2013 10:33:38:

I don't see the point of connecting the secondary of a microwave transformer to a low voltage, all it will do is reduce it to a lower voltage. The primary (mains), to secondary (HV) ratio is high (maybe 10:1) so in reverse it will just divide by that ratio.

I thisnk its worth me repeating that voltages from microwave transformers are lethal 2000V upwards!

I haven't measured one but connecting the secondary to the mains will result in a lower current due to the higher inductance while producing a stronger magnetic field than the primary at the same current. Thus it will reduce the power dissipated in any current limiting resistor.

A microwave oven transformer only produces dangerous voltages if the primary is connected to the mains.

Russell.

Ian P05/07/2013 11:07:27
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Russell

My understanding of microwave oven transformers is that one end of the secondary winding is permanently connected to the transformer laminations which in turn are connected to mains earth.

As the secondary is then NOT isolated connecting that to the mains will mean the transformer is live, so dangerous!

Ian P

Ian S C05/07/2013 15:29:38
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7468 forum posts
230 photos

Ian P, the secondary winding of the micro wave tranny is completely harmless when used as a demagnetiser, it (or the primary)is removed from the laminations, and is just a coil of wire, it's only dangerous in it origional configuration as a HT transformer.  One of the main reasons for using the low voltage is that if the coil is used without the laminations it will overheat on 230V in a fairly short time, but on 12V it may run continuously quite safely.

I dischage the HT capacitor with a well insulated screw driver across the terminals, of the dozen or so ovens I'v stripped, none have actually had any charge in them, but better safe than sorry.

My 4" Record vise became magnetised the other day, just sat the demag coil on it for a few miniutes, then lifted it of slowly, and moved it away, then switched it off,   demagnetised.   Ian S C

Edited By Ian S C on 05/07/2013 15:40:19

Ian P05/07/2013 16:02:22
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Posted by Ian S C on 05/07/2013 15:29:38:

Ian P, the secondary winding of the micro wave tranny is completely harmless when used as a demagnetiser, it (or the primary)is removed from the laminations, and is just a coil of wire, it's only dangerous in it origional configuration as a HT transformer.

I dischage the HT capacitor with a well inulated screw driver across the terminals, of the dozen or so ovens I'v stripped, none have actually had any charge in them, but better safe than sorry. Ian S C

Ian

Totally agree with you about it being harmless when removed from the transformer, however anyone reading this thread will be by now totally confused by the whole demagnetiser topic.

I know there are many ways of skinning a cat, but to demagnetise tools in the workshop we dont need to go into complications of current measurement, iron saturation, load resistors and all the other paraphenalia that has been mentioned in the last couple of days. A coil of wire connected to an alternating supply does it all.

These days, fewer and fewer electrical (domestic and freely available) products contain a heavy lump of metal with an expensive coil of copper wire wound round it, motors, and microwave ovens though are two objects that can provide the donor parts for a demagnetiser. Neither of them generally have the windings encased in such a way that they convert easily for our purposes.

As I suggested earlier in this topic one ready made totally self contained (in a cast iron casing) item that is optimum/ideal/perfect/ for a demagnetiser is the type of central heating circulating pump with thin SS barrier between the electrical and the pumping part. The one I have has a three position switch which I have left in the 'low' position and has more than enough power for the job.

I have a tape recorder head demagnetiser (the original Ferrograph one) but as with TV tube degaussers it does not have enough oomph to have any useful effect.

IanP

Removed from the transformer it is no longer a secondary, come to think of it

Stub Mandrel05/07/2013 17:05:41
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4315 forum posts
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1 articles

My dad had a proper TV tube degausser - a big coil between two 8" paxolin octagons witha push button. He used to wave it around the front of a TV then walk about four-six feet away before releasing eth button. If left on it would start to overheat.

Neil

Muzzer05/07/2013 21:24:09
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2904 forum posts
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No, Stub. A real proper degausser would be something like this: **LINK** (scroll down to the description under the second row of pics)

Admittedly this wouldn't fit in most peoples sheds!

I have one of those plastic things with 2 holes and a magnet in it. It cost peanuts but might as well be made of toffee. My calipers remain well magnetised. Perhaps I need something in between these 2 extremes...

Muzzer

Edited By Muzzer on 05/07/2013 21:26:12

I.M. OUTAHERE05/07/2013 22:47:17
1468 forum posts
3 photos

Thanks Andy , it makes it much easier to understand how these work when you have a picture showing how they are made .

Ian

I.M. OUTAHERE06/07/2013 10:42:11
1468 forum posts
3 photos

You could use the transformer from an old battery charger as well and they are a reasonable physical size .
Ian p ,
Thank you for for your input into this thread and i hope that anyone who reads this thread in the future will heed your warnings in regards to using the transformer from a microwave ovens and they will use the information wisely if they choose to use microwave internals for what ever purpose they choose.

Ian

Russell Eberhardt07/07/2013 11:00:48
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2736 forum posts
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Posted by Ian Phillips on 05/07/2013 16:02:22:A coil of wire connected to an alternating supply does it all.

Agreed, and the more turns the better such as the secondary of a microwave oven transformer. I don't think anyone here has suggested using one with the primary still connected to the mains. I would hope that anyone on this forum would have more sense.

Russell.

BTW., can anyone explain the theory behind these little permanent magnet with holes demagnetisers?  I don't see how they can work.

Edited By Russell Eberhardt on 07/07/2013 11:04:26

swarfmaker07/07/2013 14:45:53
2 forum posts

Have a look on tinternet for a magnetic bulk eraser, it is intended for the erasing of cassette tapes, floppy discs etc. mine is about a 4in cube and it has demagnetised everything. I paid about 6 quid for it at sandown park about 3 years ago.

Ian P07/07/2013 21:11:32
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2590 forum posts
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Russell

I dont see how the permanent magnet erasers work either, it sort of defies common sense.

However I still maintain that microwave secondaries should be left out of demagnetiser considerations. A coil of wire with more turns is not automatically better, in fact too many will have higher resistance so less current and a weaker magnetic field (unless you put a higher voltage in).

The main reason to not connect the secondary to the mains is that generally only one wire of the secondary is accessible, the second wire being grounded within the transformer construction leading to a potentially dangerous situation.

Ian P

Russell Eberhardt08/07/2013 11:26:58
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it sort of defies common sense.

... and physics!

Russell.

joegib08/07/2013 14:12:24
154 forum posts
18 photos

Posted by Ian Phillips on 07/07/2013 21:11:32:

I dont see how the permanent magnet erasers work either, it sort of defies common sense.

I think the permanent magnet demagnetisers are made up of 3 bar magnets enclosed in a plastic case like so:

magnet.jpg

The magnets forming the left-hand aperture 'M' have opposite poles at each end so a magnetising field develops between them. The magnets forming the right-hand aperture 'DM' have like poles at each end so a repulsion field develops between them. So, an article passed through one or other aperture will be magnetised/de-magnetised.

They work well enough for small items like screwdrivers. Indeed, mine is more commonly used to magnetise a screwdriver — it's useful to be able to hang a screw on the end of the screwdriver when working in the depths of a computer.

Joe

Andyf08/07/2013 14:34:29
392 forum posts

I must say that, had a defunct central heating pump been available, I would have followed Ian Phillip's suggestion of discarding the "wet side" and rotor and using the stator windings in the dry side, under the diaphragm which separates wet from dry. I've read that tip before.

But not having an old pump, I took the modified transformer route. At least the result is lightweight and reasonably compact .

Andy

Russell Eberhardt08/07/2013 16:31:31
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2736 forum posts
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Posted by joegib on 08/07/2013 14:12:24:The magnets forming the right-hand aperture 'DM' have like poles at each end so a repulsion field develops between them. So, an article passed through one or other aperture will be magnetised/de-magnetised.

I don't see how that follows. Your diagram is wrong in that between the two magnets to the right there will be a strong magnetic field running vertically between the two magnets.

Perhaps if the screwdriver gets magnetised from side to side instead of end to end it appears to be less magnetised??

I would still like to see a good explanation.

Russell.

Russell Eberhardt08/07/2013 17:09:07
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2736 forum posts
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Here's the demagnetiser I made (lashed up) some time ago to demagnetise chuck jaws. Cost; nil. I already had the solder gun for cutting and sealing ropes and had some copper wire in the scrap box.

No high voltages and totally safe. Works fine on chuck jaws, screwdrivers and verniers.

Russell.

joegib08/07/2013 18:00:52
154 forum posts
18 photos

Yes, I made the mistake of theorising based on recollection of the device rather than rooting it out and examining it. I've since done that and a photo of the unit is here:

magnet2.jpg

— and its innards here:

magnet3.jpg

I'll not theorise further but let the assembled company pick the bones!

Joe

Michael Gilligan08/07/2013 21:55:19
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20182 forum posts
1053 photos

Joe,

Thanks for posting the pic.

From this; it looks like the "demagnetize" magnet is actually an array of magnets bonded into a block.

If the poles are suitably aligned, then the applied magnetic field would alternate as you withdraw the screwdriver.

Any chance you could check the fields ? [iron filings on a sheet of paper, or maybe use a compass] ... If I'm right, the two magnet blocks will be very different.

MichaelG.

joegib09/07/2013 10:17:27
154 forum posts
18 photos

Hi Michael,

The striations visible in the above photo are perhaps misleading — they're slightly curved so are either grinding marks (Blanchard ground?) or sawing marks (diamond sawn?). The short sides of the blocks are smooth with no machining marks — diecast or sintered maybe? Testing with a compass indicates that the poles are located across the broad sides of each block.

Here's a diagram of the construction:

magnet4.jpg

Joe

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