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Grinding Ferrite

Discussion in 'Electronic Design' started by Syd Rumpo, Apr 17, 2013.

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  1. Syd Rumpo

    Syd Rumpo Guest

    I need to gap a core, specifically, taking 3mm off the centre pole of an
    RM12 core (or 1.5mm of each half).

    I could use spacers to gap the whole thing - which would be easy - but
    if grinding isn't too hard, I'd prefer it. Total quantity < 50 cores.

    Anyone done this?


  2. Use transformer tape at 1.5 mils and gap the entire core. You end up
    with the same gap as twice the number on the center pole. and no, it
    does NOT affect coupling to not have one mating face. The whole reason
    you are gapping it IS to "affect coupling".

    You tape off one core face. It is easy to trim with a nice scalpel.

    You can get 1 mil and 1.5 mil transformer tapes in short qtys no

    Trimming out 50 core faces doesn't take that long, and the number is
    exact because the tape is consistent.

    You then have to remember to pair an untaped core and a taped core
    together in the build.

    Grinding works, but you can call Farnell or the like and get them
    already gapped too. So you could find and buy 100 pre-gapped cores and
    have 100 sets to use. As to the grinding, abrasive cloth on a rubber
    stab works, but precision is a grind-test-grind-test matching and culling
    hand operation. Some transformer houses might sell you a gapped core.
  3. Syd Rumpo

    Syd Rumpo Guest

    Thanks, but that's 1.5 millimetres (or 59 mils (which we call 'thous')).
    That means there's some flux leakage around the outside which might
    conceivably couple to adjacent wiring. Probably not an issue if I'm
    careful with wiring (although there's not much room), but gapping the
    centre only would be more certain.

  4. Bill Sloman

    Bill Sloman Guest


    DigiKey sells the EPCOS parts too.

    Only a 1.3mm gap but they are off-the-shelf. Using transformer tape to gap the outer faces works, but there is more external leakage inductance (though not a lot).
  5. Syd Rumpo

    Syd Rumpo Guest

  6. Wrap it with a copper strip. Still worried? Use Mu metal strip.

    Switchers use gapped cores all the time and the gap is of this nature.

    Some go through the trouble of moving to a hard gapped core set, but
    tape gaps have been the std for a long time. Your gap is huge though.
    So not knowing the application I can't comment about radiance yet.
  7. Robert Macy

    Robert Macy Guest

    yes, had a machine shop do it, but single quantity.

    the guy had to learn how to work with the ceramic material, but in the
    end, nice grind job. He learned to grind it off just like grinding off
    any ceramic lamp base, or such. found out it was SLOW, and he broke a
  8. 3mm is a pretty huge gap.

    A few weeks ago I tried a diamond tool in a fairly rigid benchtop mill
    (450lbs) and got results that were a bit underwhelming.. possibly due
    in part to the limited spindle RPM (only 1800 RPM). Surface finish was
    similar to the raw parts of the core, not the mirror finish on the
    ground spots where they touch.

    At >= 1mm, I'm sure it would be fine. If you could make a fixture, a
    Dremel would probably work quite well, go for a fine grit and maybe
    some mist coolant.

    Downside is that it leaves bits of abrasive ferrite and probably a few
    diamond bits on the mill ways, so you will want to cover them and
    clean up well afterward. A magnet will help suck up the ferrite bits,
    but not the diamond obviously.

    Suggest clamping with some urethane sheet to help spread out the
    forces and reduce the number of broken cores.
  9. Syd Rumpo

    Syd Rumpo Guest

    My mistake, it's 1.5mm total or 2 x 0.75mm. Thanks for that, I'll have
    a go and see just how hard it is.

  10. Fred Abse

    Fred Abse Guest

    RM 12 cores have enough open side to get a half inch wheel onto the
    center limb. Find someone with a proper surface grinder and a magnetic
    chuck. It's a toolroom job, expect to pay
  11. Shielding for the air gap?
  12. Syd Rumpo

    Syd Rumpo Guest

    (My mistake, total gap is 1.5mm or 2 x 0.75mm) Yeah, powder toroids are
    an option, but an RM12 makes good use of the space I have. Easier to
    wind, too, and I can easily get them with the required high temperature
    rating. And I need decent isolation which is a bit easier to achieve.

  13. Bill Sloman

    Bill Sloman Guest

    Must have been a high-frequency modem. Mullard used to sell Nickel-Zinc ferrite RM and pot cores for use between roughly 1MHz and 10MHz which came with a hole down the middle into which you could plug an adjustable bridging slug, which you could indeed move up and down with an adjusting screw to fine-tune the inductance. Used one once to fine-tune a nominally 7.5MHz VCO sothat the control voltage sat bang in the middle of it's range. It was varactor tuned, so if the control voltage got too far away from nominal, the feed-back loop dynamics stopped being dead-beat.
  14. Robert Macy

    Robert Macy Guest

    Gotta watch those machine shop people and be SPECIFIC!

    I had 1 inch square about 6 inch long I core that I had turned to be
    round. When the guy got done with turning, he cut the ends off to
    remove the 'unsightly' notch he'd made in the ends to hold it in the
    turning spindle! arrrgggg start over guy! ignore notch, needed the
  15. legg

    legg Guest

    Thr Gryphon offers adjustible diameter grinding heads for center leg
    work, and water to cut down on dust and heat.

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