De-Magnatizing digital callipers

[John HaineParticipant @johnhaine32865]

Jason, ouch!

The primary expects mains voltage but only if it has its complete magnetic circuit. With open-circuit secondary the current is limited by the primary inductance but it certainly doesn't saturate the core!! If it did then the transformer wouldn't work at all when you try to take current from the secondary because the core would saturate even more!

In a normal transformer the magnetic circuit has no air gap to maximise the inductance and minimise the no-load current ("Magnetising current&quot. If you create an air gap so as to put your tool in, the inductance greatly decreases (even a small air gap has a dramatic effect) and the current when connected to the mains will be MUCH bigger than it would be in the original transformer. The coil will probably get very hot and smoke, if the fuse doesn't blow.

If you use a transformer winding, ditch the laminations. Drive the coil either by (a) using a variac to limit the voltage (if you have one), measuring the current with an AC ammeter; or (b) connect a 100W incandescent light bulb in series (if you still have one, but they are still available for special purposes).

My demag coil was made by winding a pound of 28 gauge wire on a 2 inch plastic tube former about 2 inches long with a couple of cheeks made of thin ply araldited in place. I have driven it both from a variac and using the light bulb. Made short work of demagnetising the quill of my VMB.

Finally I doubt that welder cables kick in the earth's field. Any loop of wire (such as formed by the two cables from a welder when you touch the rod on the workpiece) will try to expand due to the interaction of the fields from the two wires – so-called Ampere force. This is the basis of the Ampere Balance used to define the amp in terms of other basic SI units.

[Ian PParticipant @ianp]

Posted by John Haine on 04/07/2013 16:00:36:

Finally I doubt that welder cables kick in the earth's field. Any loop of wire (such as formed by the two cables from a welder when you touch the rod on the workpiece) will try to expand due to the interaction of the fields from the two wires – so-called Ampere force. This is the basis of the Ampere Balance used to define the amp in terms of other basic SI units.

I be surprised if welding cable DID NOT jump or twitch when the arc is struck. The earths magnetic may only be weak but the current in the cable is very high.

I have not tried the following trick myself, it sounds good fun but BE WARNED it is hazardous.

Cut a long (3m?) strip of aluminium foil about 20mm wide and suspend it between two posts a couple of metres apart. connect the two ends of the foil to a car battery via thick wire and a decent switch.

Stand back! wearing goggles and protective clothing close the switch and watch the foil try to align itself in the earths magnetic field before it disintegrates, apparently spectacularly.

Ian P

[mechman48Participant @mechman48]

Wow! what a lot of post on demagnetizing a caliper; respect to all you electrickery wiz's; why not just use a demagnetizer bought from 'Boyes' DIY dept about 2" square with two slots, one to magnetize & one to demag', mine cost me about £4.99p IIRC. M/Mart sell a larger version approx 8" square with similar identified slots..not sure on price! (usual disclaimers ), I demagged a 6" rule with a couple of passes through one slot & magnetized a set of jewellers drivers through the other slot.. job done.. simples!

George

[Ian PParticipant @ianp]

Posted by mechman48 on 04/07/2013 16:42:37:

Wow! what a lot of post on demagnetizing a caliper; respect to all you electrickery wiz's; why not just use a demagnetizer bought from 'Boyes' DIY dept about 2" square with two slots, one to magnetize & one to demag', mine cost me about £4.99p IIRC. M/Mart sell a larger version approx 8" square with similar identified slots..not sure on price! (usual disclaimers ), I demagged a 6" rule with a couple of passes through one slot & magnetized a set of jewellers drivers through the other slot.. job done.. simples!

George

Who's 'Boyes DIY'? is that another name for B&Q, although on the occassions I go in, I have never seen a demagnetiser there.

Did you really mean to magnetise jewellers screwdrivers? if so why?

Ian P

[jason udallParticipant @jasonudall57142]

OK. Re welding wires…1 swell due to ampere force…individual conductor try to part..normally the insulation holds them together. A possible cause foe jumping cable considered was magnetic interaction cable to cable and/or iron work locally. .laying the cables out in other shapes..remove the effect..loosely tacking cables to board and removing as much iron as possible then testing..NS one effect WE no or different effect..done the science know what I believe..

[jason udallParticipant @jasonudall57142]

As to alifoil and ampere definition..one of the first experiment done when I first did science at secondary school…

As to effect of messing with transformers magnetic cct. well never done it so can’t comment futher..
Back to op as some one said simple blue box mag demag available. .I think someone posted a link..oh yeah me…

[Sub MandrelParticipant @submandrel]

I've noticed that a blunt endmill tends to produce magnetic swarf. This could be the reason why toolbits sometimes appear magnetic and sometimes don't. As mentioned above I've always put this down to the heavier vibration.

Neil

[john fletcher 1Participant @johnfletcher1]

Evening all. I have a home made "Growler" which is used primarily to carry out a rudimentary test on car/motor bike armature, but also to de-magentise tools cutters etc. I obtained a large transformer 240 volts in 55 volts out, about 4" X 4" X 5", much smaller will do. I remove the 55 volt secondary winding, taped up the primary as best I could and milled a VEE completely cutting through the lamination so that an amature would sit in the VEE. Most garages around in the 1920/1960 had a Growler made by Cryton or some other maker. The tape was to prevent swarf and coolant getting into the primary winding, once the machining was completed the tape was removed. To de magentise any tools etc, I switch on the mains to the primary, pass the object through the air gap, switch off job done. As some one else suggested use an instant heat soldering iron. Don't get involved with overheating and a lot of theory you are using the thing for a minute or so. Ted

[AndyfParticipant @andyf]

Posted by jason udall on 04/07/2013 17:18:31:
Back to op as some one said simple blue box mag demag available. .I think someone posted a link..oh yeah me…

I'm sure the blue ones work fine for screwdriver blades and lathe tools, Jason, and there's no possibility of electric shock, but calipers might not fit through the holes. Andy. (why, all of a sudden, can I not put my name on a line of its own? Because the software on this forum is *^%! came the chorused reply&nbsp.

[I.M. OUTAHEREParticipant @i-m-outahere]

Hi all,
Thanks Andy , if you are finding 80ma sufficient with no serious heating issues i will aim for that and experiment a little .
I only want something that will do small tools and my calipers, although the transformers from microwave ovens are plentiful and maybe a little larger in size so i may look into this .

Not being an electrical Engineer or the like although i do have a basic knowledge of how transformers work and what inductance is, i know that changing the structure of the laminated core will change the inductance and the amount of current that will flow in the circiut .

One thing with microwave ovens that has not been mentioned is the high voltage capacitor !
These must be approached with caution as they are probably ( read treat as they are ) still charged so need to be discharged and have thier terminals tied together with some wire so they cant build up another charge .

I think that the unit described by Ian sc has some merit in the fact that he is using a transformer to step down the voltage to a safe level but still achieving the same result – much safer in my opinion .
By doing this i would wonder if using the secondary winding of the microwave transformer would work better as it is a step up transformer and must have more turns on it than the primary so may provide a stronger magnetic field ?

I mentioned the use of an old welder because they are not much more than a big transformer with a sliding choke so maybe could be modified to make a big degaussing unit – if you had an old one laying around to strip that is .
I remember back when i used to weld for a living that the welding leads would arrange the grinding dust on the floor to follow the lead and it would stay there when you moved the leads .

I tried one of those square demagnetising tools and they do work -but there is a fascinating side effect !
If i stick the external jaws in this it works but the internal jaws stay magnetic , so i ran them thru and they worked but now the external ones are now magnetic again!
I feel the problem may be trying to degausse two pieces at once as the magnetic field seems to jump from one to the other !

Just as an aside i was playing with my calipers and noticed there was a little play in the moving jaw so i nipped up the adjustment srews and found that with the 3 sets i own that i could get no difference in readings but before there was a .005 discrepency .
This was between the old and now magnetic set and a relatively un- used set so i guess they wear also even though the newer set had a litle play also so it was nipped up as well .
By the way these are not brand name units as i would bet they are all made in CHINA and if i want total accuracy i use a Micrometer/snap gauge / inside micrometer etc .

Once agian thanks to those who have replied to this post .

Ian

[Ian PParticipant @ianp]

CAUTION

Ian (slotdriller) and others.

• Do not even think about using a microwave secondary winding! its very hazardous and the only way to make the transformer safe is to totally remove the secondary windin before using it.
• Capacitors cannot charge up on their own (although they can recover some charge if not discharged for long enough.)
• Using low voltage for demagnetising does not make the equipment any 'safer.' I would assume that everyone uses mains electricity to power their equipment and the demagnetiser is connected to the mains. If there is just one, mains connected winding and its magnetic components as long as it is properly insulated and safely constructed there is no need to involve any other transformers. The total amount of mains connected and live components is the same with one or even ten subsequent transformers.
• As others have said, why not buy one ready made.

Ian P

Edited By Ian Phillips on 05/07/2013 08:23:25

[jason udallParticipant @jasonudall57142]

In all seriousness.
Any thing producing significant levels of magnetic field needs a caution..like microwaves .. people with pacemakers and the like should avoid exposure. I aways wonder if patients are warned and that the signs etc. are redundant..but anyway a caution is needed.

[Clive HartlandParticipant @clivehartland94829]

I wonder if I could set up a business de-magnetising tools. Now theres a thought.

Perhaps a, 'Walk in' set up. Pay a fee and de-magnetise all you like. Perhaps a conveyor style, bung it in one end and run around and catch it at the other.

Clive

[mechman48Participant @mechman48]

Ian

Boyes are a local general general dealers in my area (Teesside) which cater to all manner of requirements from socks & footwear, toiletries, stationary, toys, fishing tackle & DIY tools etc,etc. I don't believe they are related to B & Q. As for the jewellers drivers I have on more than one occasion lost tiny screws from grandaughters toys or the back of radios so I magnetised a set to hold on to small screws & the like for future use.

George

[I.M. OUTAHEREParticipant @i-m-outahere]

Ian ,
I would much rather have a coil at 12v than at 230v as it is isolated from the mains supply by using a step down transformer .
True there is mains supply to the unit and the unit would be earthed accordingly but the degaussing coil could be something mounted seperately to this power supply .

I know capacitors cannot charge by themselves but one does not know what will happen down the line that is why the electronics magazine that i read suggested to tie the leads together and this was from a few articles they did for using a microwave housing to make an exposure cabinet for pcb's -short it out- make it dead-peg it in the bin !
The transformer cannot hold a charge and once isolated from any power supply is inert but a capacitor wil hold a charge that you cannot see and these can kill !

I'm not suggesting to use the secondary winding of a microwave transformer to be fed with more than 12v so one would remove or negate the primary winding and feed 12v to the secondary instead so it is just a coil with more windings .

Ian

[AndyfParticipant @andyf]

In case you're interested, Ian, I've created a temporary page on my website and put a couple of pics on it here. I'll leave the page up for a couple of days before deleting it. The page isn't in the navigation menu, so if you move to another page, you will have to use the above link to get back to it.

Andy

[Ian PParticipant @ianp]

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 EberhardtParticipant @russelleberhardt48058]

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 PParticipant @ianp]

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 CParticipant @iansc]

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 PParticipant @ianp]

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

[Sub MandrelParticipant @submandrel]

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

[MuzzerParticipant @muzzer]

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. OUTAHEREParticipant @i-m-outahere]

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. OUTAHEREParticipant @i-m-outahere]

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

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