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Thread: Rare Earth Magnet strength

  1. #1

    Rare Earth Magnet strength

    Hello,

    I'm working on a cribbage board, and plan to put a a little compartment in the bottom to hold the pegs. I was planning on using rare earth magnets to hold the 1/4" (or so) thick door in place. I bought some online based on size alone (I beleive they were 1/4" dia, and 1/8" thick maybe), and man those suckers are strong. I'm now worried that for my purpose, two pairs of them might be too much to pry apart easily to access the compartment. Obviously I can glue some magnets to a couple peices of wood and see, but, I thought maybe someone here might have a suggestion re/ pull strength.

    Also, would the pull strength (if that's what it's even called) be 1/2 as strong if it was a magnet and a piece of metal, rather than two magnets? If so, maybe I could use a washer or something on one side.

  2. #2
    Calculating the coupling forces (relative "grip") of a magnet requires you to consider the surface area, the Gauss field, the Energy, distance, and magnetic properties of the item being held in the gaussian field.

    Magnetic coupling (grip or magnetic pull) tends to drop on a logarythim of the distance.

    Neodymium Iron Boron (the most common rare earth magnet) is a 30 million energy product when made correctly.

    I am guessing the calculations necessary would stump a fair number of engineers not intimate with gaussian heuristics.
    Why not just get your magnets and try it ??
    Last edited by Cliff Rohrabacher; 09-15-2006 at 9:44 AM.

  3. #3
    I bought the magnets from this place, which lists a pull force for the magents they sell. I wasn't planning on recalcuating the figure, especially after your description of how it's done! But I thought maybe based on experience for a similar application, someone might be able to say, "oh yeah, you'd be better off with such and such" or "those should be perfect for that." No big deal, though. I can glue up a few though, and experiment if needed.

  4. #4
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    While I can't give you a scientific explanation, I do play a lot of cribbage. And I play with these magnets quite a bit as well. Four magnes is a bit much to hold a piece of metal in place when it won't have much mass to it. Even if the metal went the full length of the board, you'd most likely only need two - one at each end.

    Without knowing your precise design, it's tough to say. If it's just a small cover to keep the pegs then one would probably do. I'm envisioning an oval compartment with the magnet positioned about 1/4 of the way along its length. You could then push on the piece of metal above the smaller section and have the magnet act as a hinge. Sorry if that's not very clear.

    Another thought would be to resaw the board and create a cavity in one of the halves to contain the cards and pegs. Join the two pieces back together with a swivel hinge in the middle and use the magnets to keep the board from inadvertently opening. This would let you use both pairs and decoupling these magnets from each other with a shearing motion is the easiest.

    Oh, but then you didn't ask for a cribbage board design. Sorry about that.

    Matt
    Matt

  5. #5
    Actually I wasn't very clear Im envisioning a router out compartment, maybe 1' by 3" at one end of the board. It would have a wood (not metal) lid, maybe 1/4" thick that sits in the compartment and rests on a ledge. There would be magnets recessed into each peice at that ledge to hold the door shut. However, it probably would be easier/better to just use a wide headed screw, or a washer or metal slug of some sort in the board soi that it would be magnet on metal, rather than magnet on magnet. I probably should have put this in the design forum....

  6. #6
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    I use the 3/8" RE magnets to hold cabinet doors closed...I suspect they'd work just fine for what you're doing.

    KC

  7. #7
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    It sounds like you're planning on having the mag exposed so you should get the full effect. I've done some experiments wth 3/8" and 1/2" mags hidden behind a plug (probably about 1/8" thick) and was disappointed in the results. It's really surprising how much that thin layer of wood does to decrease the pull strength. I'd like to use them in the future for small hidden door holds but think I need to experiment more with the longer rod type. The standard thickness ones just haven't given me as much pull as I'd like through wood. Once you get this project done, pics of how you did the magnetic catch setup would be nice.
    Use the fence Luke

  8. #8
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    Andy,

    I just had a strange experience with rare earth magnets. I don't know if my expeerience will help you, but you might want to do some preliminary experimenting with your magnets before you fix them permanantly to your project.

    I built my daughter a studio easel to use in graduate school. The easel has a small compartment (about 21" X 8" X 5") of 3/4" oak to store paint tubes and such. The door of the paint box is made of 1/2" Baltic Birch plywood, veneered with QSWO on the outside and white formica on the inside. Before I glued on the formica, I inlaid five 3/8" rare earth magnets along the top edge of the door that would butt up against the edge of the box. I epoxied them into holes drilled into the door and glued the formica over them. I did NOT use the little steel cups that are supposed to multiply the strength of the magnets. I figured 5 magnets would be plenty strong - maybe too strong. I then inlaid the little steel washers in the corresponding edge of the box, one opposite each magnet. To my surprise, the magnetic attraction beween the magnets in the door and the steel washers was very weak. It was probably too weak to insure the door would stay closed. What to do?? Rebuild the door?? The solution I came up with was to replace the washers with five magnets set in the little steel cups in the edge of the box, making sure that the polarity between the door magnets and the box magnets was reversed so they would attract each other rather than repelling. This worked fine and gave me enough attraction to keep the door closed. If' you're interested, pictures of the box with the magnets are here:
    http://www.sawmillcreek.org/showthread.php?t=40421

    I would have thought that the door magnets by themselves would have been plenty strong. If you had asked me before I started what would happen if I used 5 sets of opposing rare earth magnets, my response would have been that I wouldn't be able to open the door. As it turned out, I needed all ten magnets to adequately secure the door. I don't know if the steel cups made the difference, I.E., whether, if I had used steel cups with the magnets I imbedded in the door, the steel washers on the box edge would have been adequate. It's a mystery. In any case, I ended up with the result I wanted.

    My advice is to build a mock-up or find some way to test your application before you nail it down.

    My $.02

  9. #9
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    I have glued a piece of color matching veneer to a magnet face to reduce the grip between it and the metal disc it "grips". Reeeeeeel scientific.
    "A hen is only an egg's way of making another egg".


    – Samuel Butler

  10. #10
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    On the jewelry box I posted a while back the two main doors were held closed with two 1/4 rare earth magnets. LOML hasn't had any problems getting to her necklaces.

    Also, magnetic strength drops off according to the square of the distance if I recall correctly. Therefore even a thin piece of wood would reduce the pull strength significantly. Try thin layers of veneer over the magnet until the pull is what you want.

    As an alternate solution, if you can make the door a sliding door, teh force to slide the magnets apart is also significantly less than the full strength.

    Lee
    Lee Schierer
    USNA '71
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    My advice, comments and suggestions are free, but it costs money to run the site. If you found something of value here please give a little something back by becoming a contributor! Please Contribute

  11. #11
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    Calculating the coupling forces (relative "grip") of a magnet requires you to consider the surface area, the Gauss field, the Energy, distance, and magnetic properties of the item being held in the gaussian field.

    Magnetic coupling (grip or magnetic pull) tends to drop on a logarythim of the distance.

    Neodymium Iron Boron (the most common rare earth magnet) is a 30 million energy product when made correctly.

    I am guessing the calculations necessary would stump a fair number of engineers not intimate with gaussian heuristics.
    Why not just get your magnets and try it ??


    Huh????

  12. #12
    My buddy & I buy from kjmagnetics.com they tell you the pull force of each one....... (third paragraph down, Pull Force Tables)
    Tony

  13. #13
    If you put two round magnets together so they attract each other they are very strong but if you turn one the poles change and they repel.

    If you were to put one fixed to the box and one fixed to the top attached to a knob.... if you turn the knob so they attract it keeps the door closed, if you turn the knob the magnets repel and the door opens.
    Silence is golden but duct tape is silver.

  14. #14
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    Quote Originally Posted by glenn bradley
    I have glued a piece of color matching veneer to a magnet face to reduce the grip between it and the metal disc it "grips". Reeeeeeel scientific.
    Ooooohhhhh.....excellent solution to a "sticking" problem.
    Thanks & Happy Wood Chips,
    Dennis -
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