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How do I demagnetise tools and parts of machines

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David Williams 607/05/2010 13:46:15
2 forum posts
A number of my engineering tools and parts of my machines have become magnetic. I bought the small demagneising box with the two holes but it is ineffective and anyway it cannot be used on the motor housing of my small Chester Mill etc. Would someone be kind enough to show me where I am going wrong with the small demagnetiser and if possible suggest a solution to bigger items. I am aware of the degaussing of ships to protect against magnetic mines, are there machines for this(?) as I cannot simple bash the motor housing with a large hammer or for that matter my engineering tools...... Any help gratefully received.
dcosta07/05/2010 14:52:15
496 forum posts
207 photos
Hello David.
 
I'm Writing from Portugal.
A while ago I bought (I think from RDGTools) a demagnetizing table made by Vertex and with it I could demagnetize all the tools which were magnetized and even my precision
 toolmaker's vice was demagnetized. It was enough to slide the table on the vice or, as was the case with my twist drills, to slide them on the surface of the demagnetize. Before
writing this message I looked for the demagnetiser in the RDGTools site.
 
 
 


I can only say I'm happy with it.
 
 
Best regards
Dias Costa
 
 
Ian S C07/05/2010 15:00:54
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7468 forum posts
230 photos
As far as your mill goes I imagine that it has a DC motor and electronic speed control, it's supposed to be magnetised. For small tools I built a demagnetizer. The coil is the primary coil from the HT transformer of a microwave oven, it is fed with AC current from a transfomer at about 12V (a little more or less won't hurt just as long as it can be left on for a few minutes without getting too hot), I built a box around it leaving a hole about 2"x 2 1/2" through which the object being demagnetized is passed, you have to move the object away from the magnetic field before switching off. In my early days in aircraft engineering we had a bigger version of this (ex RNZAF), we used it for magntizing steel parts for crack testing, then demagnetizing after the test was completed, the system seemed crude but it worked, and the powers that be thought it was OK. Someone else will have something more practical I'm sure. Ian S C
Pierre EHLY07/05/2010 18:25:13
5 forum posts
Hi David,
 
For tools demagnetisation use an magnetic field generated by an AC current.
Use the stator (fixed part) of  an old AC (240Vac) motor. Connect a light bulb
(40 to 75w) in serie with the  field winding an power it up with 240v.
Pass slowly the tool in the stator, thats all.
 
IMPORTANT: 
Isolate all connections (240v may be lethal) & connect all metallics part to ground
Watch also the T° rise of the stator
 
For machine part , built a fixture to move the stator over the affected zone
 2 or 3 time (distance varie with the power )
 
Select a bulb that give you the half of the normal motor current
 
pm 
 
 
 
Stub Mandrel07/05/2010 20:54:50
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4318 forum posts
291 photos
1 articles
My Dad used to have a degaussing coil for TV tubes - It was about 10" diameter on (IIRC) a tufnol frame and you demagnetised a tube by pressing a button on the (mains) coil and waving it around the tube then walking slowly away to, I think, about 10' distance.  I don't know how many turns the coil had, but I can imagine it would work well for fair sized tools. Note it didn't have a steel core - the steel core on a transformer traps the magnetic field. I'm sure demagnetising tables must have steel cores, but with open poles at the demagnetising areas.
 
When I had a blue spot on an old CUB computer monitor some ten years ago, I managed to demagnatise it by waving a ceramic speaker magnet at it - but it took a few goes! Not sure THAT would work for tools
 
Neil
Steve Garnett07/05/2010 22:38:50
837 forum posts
27 photos
Following on from what Neil said, years ago I made a cheap, flat demagnetiser like the Vertex one out of an old mains transformer, and I can tell you what you need to do to get over the trapped field problem. What you have to do is to knock out all of the E and I laminations, which are normally interleaved, and put all the E ones back facing in the same direction. This will give you two pole faces available, and that's essentially what you need to pass whatever you need to demagnetise over.
 
As with Pierre's motor arrangement, a light bulb in series with the primary, which is still mains-connected is a good idea, otherwise the whole thing will overheat rather quickly. But for not too much effort, you can get a workable result and it's a damn sight cheaper than the Vertex one is. If you make one, it's generally a good idea to use a spring-loaded switch biased to the off position, so that you have to hold it on to use it. That way, it doesn't get left on accidentally.
john fletcher 108/05/2010 08:51:25
893 forum posts
De-magetising. 
I have a home made growler, which I use for simple testing of armatures,and for de-magnetising, growlers  were  found in most auto electrical shops in the days of dynamos. To make the growler I used  a mains transformer 240/55 about 10amp rating with the secondary (55volts) coil removed,the core is best described as square 8 shape. I bound up the remaining  coil to ensure no steel particles could get in whilst I milled away a vee in the top portion of the 8 shape, deep enough to lay an armature in. To de-magentise a piece of metal all I do is switch on and pass the tool,metal or what ever through or across the vee. My growler doesn't over heat and it can be used to remagnetise small DC motors, like those found in childrens train set. Observing polarity place the complete motor in the vee connect the coil to a source of DC, switch on for a moment and the motor is like new. Alternativelydischarge a large capacitor across the coil and that has the same effect.Ted
Geoff Theasby08/05/2010 14:05:35
615 forum posts
21 photos
I used to have a tape head demagnetiser for my reel-to-reel tape recorder.   They may still be available.
It was a coil wound on a steel core and plugged in to the mains.   The core was extended into a probe covered in plastic which was touched onto the tape head and then withdrawn to a few feet before being switched off.
I used it to demagnetise screwdrivers and other tools.
 
You can demagnetise screwdrivers and other small tools by stroking them with a small permanent magnet.
 
Regards
Geoff

Edited By Geoff Theasby on 08/05/2010 14:07:43

V8Eng08/05/2010 15:50:44
1826 forum posts
1 photos
I keep a couple of screwdrivers magnetised.  They will hold small steel screws in line when working in difficult locations.
Certainly beats dropping the things.
Ian S C09/05/2010 01:41:09
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7468 forum posts
230 photos
Hi V8Eng,magnetised screw drivers are also handy for getting swarf out of difficult places on the mill and lathe.Ian S C
V8Eng09/05/2010 16:10:51
1826 forum posts
1 photos
 
 

 

 

Edited By V8Eng on 09/05/2010 16:16:14

Edited By V8Eng on 09/05/2010 16:17:07

David Williams 611/05/2010 20:39:58
2 forum posts
Dear All, May I say a really BIG THANKS to everyone who has been so believably helpful and I am now considering what to do next. I think that with all the options and help that has been posted, then unless anyone has anything really special/useful to add, I will call it a day and suggest we informally close the thread line. As I am sure you all realised, I really meant the motor fixings or  vices or tables etc. rather than the actual motor itself. Not the best of examples to suggest - anyway it probably gave you a laugh. Many thanks again, David 

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