Remagnetizing a weak magnetic v-block
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  1. #1
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    Default Remagnetizing a weak magnetic v-block

    I came across a cheap pos mag v-block that was very weak,that's probably why I ended up with it.
    So I pulled it apart to see if there was anything I could do to make work.It was a very simple deal,had 3 Alnico dead magnets,27.5mm od x 18mm ht with axial poles.A little searching on E-Bay turned up a lot of cheap Neyodium rare earth magnets.Previously when I had bought them they were pretty expensive,however the Chinese have dumped a lot of them on the market cheaply.
    I couldn't find any the exact size and although I could have installed some larger od ones in the holder. I settled on some 25mm od x 15mm ht and some 25mm od x 3mm ht grade n50 and stacked them to get the correct height.I still had to add some iron shim to get the fit.Cost about $35.00.
    The stack gave me approximately 60 lb.s pull ea.(If I could have found some 30mm N52 grade I would have approx.100 lb.s pull).Didn't want to machine any to fit or spend to much in case it didn't work.
    It wouldn't hold its own weight on a vertical surface,now it takes about 35-40 lbs pull to make it slide on a metal surface and holds a round pretty good in the v.
    20170310_133910-vee-block.jpg20170310_133829.jpg

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    Thanks rb427, found this from your reference in the other thread about V- blocks. Very useful, and now I’m wondering about re-magneting other types of permanent magnetic tooling, I’ve got a Brown and Sharpe tilting sine chuck that does indeed suck, very weak. Different kettle of fish there, for sure, but the rest of that that unit has considerably more value than a cheap V-block, so it might be worth investigating. Anyone looked into that?

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    I remagnetize things frequently. The technique is simple, just connect it to a magnet that is strong enough to pull the magnetic domains in it into alignment. In this case, I would first determine the magnetic polarity of the existing weak magnetism, no point in fighting more than necessary. Then lay a piece of iron across the poles with a heavy wire going through the V and discharge a large capacitor through it. You can do the same with a large DC current but the capacitor discharge produces a very large pulse without burning up the wire.

    Historically, magnets have progressed from nothing more than hardened steel, which has very poor retention to recent rare earths that are very strong and hold their strength. Alnico and ceramics are somewhere in between.

    In the case of this V block, you would have the magnet turned on with the bar across the poles, zap it, then switch the magnet off. Always switch it off before removing it from the material it is attached to because because that will move the magnets to an internal "keeper".

    The old steel magnets on early magnetos will lose their strength if the magnetic path is broken for milliseconds. You have to keep a keeper on them until they are in position where the magneto armature supplies the path.

    BTW, some of the new magnets are plated and grinding it off exposes the magnetic material to oxygen and they deteriorate. I have not opened one up, but I think they may fall apart if the supporting plate is removed.

    Bill

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    Quote Originally Posted by 9100 View Post
    I remagnetize things frequently. The technique is simple, just connect it to a magnet that is strong enough to pull the magnetic domains in it into alignment. In this case, I would first determine the magnetic polarity of the existing weak magnetism, no point in fighting more than necessary. Then lay a piece of iron across the poles with a heavy wire going through the V and discharge a large capacitor through it. You can do the same with a large DC current but the capacitor discharge produces a very large pulse without burning up the wire.
    Bill
    Yes, I was using a similar approach using a large capacitive discharge spot welder. The wire (or rather a 1/8" copper bar in my case)should be secured in the gap, I used wood wedges.

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    I was going to add in the previous thread about cheap mag v-blocks; The difference in accuracy between the cheap China v-blocks and quality ones is something that could be matched easily on a surface grinder.I don't know for sure but I think that most all the China magnet blocks are probably all rare earth.

    The cheap mag base indicator holders seem to be stronger than the older domestic I've had.

    So as far as the v-blocks go I would buy the cheapest dead ones and install rare earth mags and regrind if necessary to what ever accuracy I needed.Check on BangGood seller on E_Bay.

    As far as a mag chuck the price of them has really come down but if you want to resurrect one with dead magnets I wouldn't be surprised if you couldn't find some rare earth mags to fit.There are all kinds of different sizes available.


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