De-Magnatizing digital callipers

Ian( Phillips)... a full disscussion of magnetic properties is beyond this thread( and me)..what I was trieng to relate was some limes it is the debries not the tool that is magnetic ( yeah both have to be magnetic but only one might be permantly magnetic).. this is what we found in our hairy components...

putting a hairy component through the deguasser leaves swarf behind and a hairless part ...checking hairless part with a compass yields attraction of both ends of part to north end of compass and egually the same for the south end...the part itself is not a MAGNET even though it is made of a MAGNETIC material .. applying the same test to a "hairy " part shows attraction one end and repulsion the other...hariy part is a MAGNET..wiping off hair and clean with air line...part attracts notrth both ends and also south...part is magnetic material but not magnetisied...these un technical definitions served to resolve the issue for me..

BTW a simple permanent magnet demagnetiser is offered just wipe the tool through the hole for example ( just the first that google offered today)http://www.rapidonline.com/Tools-Equipment/Toolcraft-Demagnetiser-and-Magnetiser-524455

Edited By jason udall on 04/07/2013 13:58:14

Ian

I presume its an error but the secondary of a microwave cooker transformer is,

LETHAL LETHAL LETHAL!

sorry for shouting but if someone who is not that knowleable fries themselves or their family its not funny.

Using transformers to drive transformers (or a demagetising coil) is just adding complication.

Ian P

BTW...whats all this about throttling the primary to reduce the current ?

the primary of the trans former is already expecting mains ( assume mains transformer)...the current will depend (mostly) on the secondary load.. and if that is unconnected /used then the primary will idled with just enough current to saturate the iron in the magnetic circuit

now if you make your tool part of that magnetic circuit..you might well increase the primary current.. but it is most likely that the new arrangement will have a much lower requirement not higher.. ( talk of magnetising force susectability and permability follows......).. so

if you could

1 select "beefy" trans former...think pounds of iron...

2 saw slot in laminations to " cut" the magnetic circuit.. so that

3 the tool can be introduced in to air gap created in 2 above

4 a) idealy slowly ramp up hold and ramp down the current to demagnetise the tool..

4 b) alternatively, ( to 4 a) move the tool into and out of the air gap from infinity in each case...( for practical purposes 5 -6 times the diameter of tranformer coil)

me..? I wound a 1000 turn 2 foot long solenoid on 4" tube( thats only 300 m of wire) and fed it with 10 amps.(80 V).. before thinking about why my components were hairy...

why didn't I use the transformer trick from above?..ever seen a cheap redily available trand former that can have a 4" air gap added?

further thought.. readily avalable..CONTACTOR coils/solenoids ...if big enough no fafing around disassebling transformers....

btw welder wires "hop" in just the earths field when you strike up...

Edited By jason udall on 04/07/2013 14:32:20

Right, as promised I've checked my home-made one out. The whole caboodle draws 80mA at 230VAC. I can't remember the specs of the transformer, but its windings are on a former which is roughly a 1" cube, so it was no biggie. I suspect its secondary produced 9 or 12V.

Circuit:

L -------- 2350ohms---------X------------- transformer primary ----------N

L to point X 190VAC, X to N 40VAC. The resistor is two wirewound 10W 4K7 ceramic cased jobs in parallel. These seemed to get a bit hot. They are square in cross-section, so werte clamped to the ally box with a heatsink on top, which keeps their temperature within reason. There is a salvaged neon indicator between L and N, though to stop the thing being left on, the push switch mentioned on the other thread is a good idea.

Andy

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". 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.

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

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

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

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

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 .

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

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

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

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

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

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