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how does magnetic chuck turn on and off?

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  • how does magnetic chuck turn on and off?

    aboard_epsilon was talking about his magnetic chuck in his latest post, and it brought to mind a question i had. i scored an old Brown & Sharpe surface grinder for $25 last year and it came with the 18"x6" magnetic chuck. before anyone gets too excited, the magnetic chuck was probably the best part of the deal. anyway, the chuck has a lever on the front that i have been afraid to force too hard to one side or the other. just how does it turn the magnetism of the chuck off? i mean this thing is no electromagnet. and how hard to you need to shove the lever to turn it off (and to which side)?

    just wondering (sort of like the "how do balls get in ball bearings" thread).

    andy b.
    The danger is not that computers will come to think like men - but that men will come to think like computers. - some guy on another forum not dedicated to machining

  • #2
    light finger pressure on mine.
    should be an oiling hole underneath it
    if its an eclipse
    try giving it a drop or two
    all the best...mark


    • #3
      In a non=electric magnetic plate - that is a Permanent Magnet plate the lever operates a cam system that gently closes a magnetic circuit between the iron poles of the plate. This is much like the "keeper bar" that used to be on the old horseshoe magnets - you are in effect moving the magnet into contact with the iron poles of the plate. When you have a ferrous opject on the plate, this closes the magentic circuit and the workpiece is held firmly to the plate. The finer the plates poles, the higher its grip.


      • #4
        The handles will require some force to operate, and usually turn through a 180* arc. Age, coolant and neglect often result in gummed up operation. There should be lubrication points arount the operator, open these and work WD 40 or Kroil into the innards and work the handle back & forth to free it up.
        Most manufacturers caution not to dismantle as it may cause loss of magnetism.
        Jim H.


        • #5
          I too have heard the warning to NOT take apart a magnetic chuck. This also applies to many other items like stepper motors also.

          So, why os the magnetism lost when you take the device apart? On the stepper motor I took apart, I didn't see any magic smoke escape but the magnetism was gone just the same.



          • #6
            Magnets are made up of a lot of small "domains" that can be magnetized in various directions. The more that align, the stronger the magnet. If they point in different directions, they don't work together, and may "short circuit" locally.

            It takes a strong external field to align most of them.

            If you maintain a piece of iron across the poles after removing the external field, the remaining magnetic field strength is enough to hold most of the previously aligned domains aligned. Most of the field goes thru the iron, concentrating the field in the magnet and iron. The low "reluctance" of the "magnetic circuit" allows the magnet to maintain a strong field through itself Thermal energy will still flip some domains, and strength is lost from that existing just at the moment the external field was removed.

            If you remove the "keeper" iron, the field must pass through air, not iron, which has a higher "reluctance" and the field strength within the magnet is reduced. The reduced field cannot hold the domains aligned as well, and so more flip out of alinement from thermal energy, reducing field strength more.

            Since field strength has been lost, even putting the keeper back cannot bring back those domains into alinement. You would have to both put the keeper on, AND apply an external field which is much stronger than the magnet itself in its reduced strength condition.


            Keep eye on ball.
            Hashim Khan