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  • Magnet strength

    I recently purchased two magnetic base lights for my milling machine. Both are identical. One, however, has great holding power while the other will simply not hold to the side of the mill. Switching the lights around culminates in the same result ..... the same weak light lacks holding power. It is the light that's unable to stay in place rather than the lights's position on the mill.

    I have managed to cannibalize a similar magnet (same diameter and thickness) from a 6 meter magnetic base antenna. My question is, will magnetic strength (holding power) be increased by adding a second magnet to the original magnet? If so, is there any reason why the two magnets should not be joined using crazy glue?

    Harold
    For those having fought for it, Freedom has a flavor the protected will never know.
    Freedom is only one generation away from extinction.

  • #2
    The less air that comes between the magnet poles, the magnetic flux path, and the thing it's holding to, the stronger the force up to a maximum for the particular magnet. Saying the same thing another way, the larger the air gap, the less will be the holding force. Increasing the size of the magnet will increase the force, as will increasing the strength of the magnet. In very rough terms, a 'super magnet' will hold about ten times as well as a similar mass of ferrite magnet (the typical magnet material, used in speakers, etc). Either one of your bases has a weakened magnet, or for some reason it doesn't allow a fully closed magnetic circuit. Could be the pole pieces don't fully contact the faces of the magnet, or the pole pieces can't both sit flush against the steel side of the machine-

    Note that if you add another magnet, you will have to determine the proper orientation for it, otherwise it could actually decrease the holding power. Note also that a 1 inch super magnet disc 1/8 inch thick, inset into a steel disc and sitting flush with the edge will have about 40 lbs of holding power to any steel surface with at least 1/8 inch thickness. You can get those recessed discs at some woodworking suppliers, already drilled for a mounting hole. Pretty easy to make your own fixture, and very compact for the holding power they deliver.
    Last edited by darryl; 09-05-2010, 03:30 AM.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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    • #3
      Get some magnets out of junked hard drives.
      Any products mentioned in my posts have been endorsed by their manufacturer.

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      • #4
        Originally posted by darryl
        ..........Note that if you add another magnet, you will have to determine the proper orientation for it, otherwise it could actually decrease the holding power.........
        First light has come to Alaska and the single comment shown above has reduced me to "scratching my head" before full sunrise. Admittedly, by any stretch of the imagination, a comprehensive knowledge of magnetic flux is "not my strong suit". Additionally, I have a tendency of "reading too much" into what others have stated so bear with me with the following statement/question.

        I realize that two magnets can repel each other in one orientation while attracting each other in the opposite orientation. Are you taking this a step further by implying that magnetic attraction, between two magnets, is only the beginning of getting the most out of joined magnets and that once magnetically joined, further orientation (tweaking) is necessary for greatest holding power?

        To further demonstrate my ignorance, I simply took for granted that when two magnets "found themselves", maximum attraction was achieved without any "outside help". Now I have a strange vision, e.g., that when joining two magnets of same shape, size, and thickness, further orientation (rotation??) is necessary to achieve maximum attraction. Am I totally "off to see the wizard" by reading too much into your statement or were you telling me that when the magnets are joined using crazy glue, care must be taken because it's possible to inadvertently impart a slight rotation thus effectively decreasing attraction (strength).

        Harold
        For those having fought for it, Freedom has a flavor the protected will never know.
        Freedom is only one generation away from extinction.

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        • #5
          Not really an answer to the question, but I have a segmented cow magnet. If you take it apart, the magnets (each separated by a steel disc) are repelling each other. If it is reassembled so the magnets attract, it is nowhere near as strong.

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          • #6
            Harold:

            Try flipping the weak magnet over before you do anything else.
            It's possible the weak base was assembled incorrectly.
            Mike

            My Dad always said, "If you want people to do things for you on the farm, you have to buy a machine they can sit on that does most of the work."

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            • #7
              Originally posted by Harvey Melvin Richards
              Not really an answer to the question, but I have a segmented cow magnet. If you take it apart, the magnets (each separated by a steel disc) are repelling each other. If it is reassembled so the magnets attract, it is nowhere near as strong.
              Originally posted by Motorrad Mike
              Harold:

              Try flipping the weak magnet over before you do anything else.
              It's possible the weak base was assembled incorrectly.
              Harvey: Glad you said that. This bears further investigation regarding orientation.

              Mike: I will, in fact, flip the magnet over and give that a go. Though I have not carefully scrutinized the base on the weak magnet (light) and compared it to the stronger light, there may be an "air gap" between the magnet housing and the mill causing less strength. I will look at that also.

              Harold
              For those having fought for it, Freedom has a flavor the protected will never know.
              Freedom is only one generation away from extinction.

              Comment


              • #8
                Assuming that this is a simple magnet with two pole pieces type of thing, here's a mental exercise- cut the magnet in half lengthwise. The two magnets if placed together again in the same way will repel. Before the magnet was cut in half, the pole pieces would have gathered all of the S magnetism from one side and all of the N from the other. When this is placed against metal, the flux will complete a circuit through the magnet, pole pieces, and the metal it's stuck to.

                Now if one section of the magnet is stuck to the other in the opposite way, in other words the way they are attracted to each other, the flux path will be from one magnet segment through the length of the pole piece and into the other, with the same happening on the other side of the magnets. The magnetic flux makes a circuit through both magnets and the pole pieces. Little flux remains to make a circuit through the metal you try to stick it to.

                It gets more complicated if you have a series of magnets and pole pieces. You could for instance have three pole pieces and two magnets in series. That way you have a center pole and two outer poles. It could be that the center pole is one polarity and the outer poles are the same other polarity. Or it could be that one outer pole is S for instance, and the center pole N for the magnet between them, but the second magnet gives S to the center pole and N to the last outer pole. You would still have a magnetic flux path through the metal you stick it to, but here I don't know which configuration gives the best holding power. It might even be the same- I'd have to test this to find out.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                • #9
                  Any suggestions as to how to measure how strong a magnet is?

                  And to compare the strength of two different magnets?

                  TMT

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                  • #10
                    You can get gauss meters, but the strength of a magnet is important only when it's in use. Because it's usually used to stick something to metal, the measurement that's important is how much force does it take to pull it away. You can use a fish scale to figure that out. For comparison purposes, you wouldn't compare two magnets that are the same, but with one having pole pieces and the other without. If you are going to use a scale of some type to get a reading of 'pull force', especially when comparing magnets, the test surface should be considerably larger than the magnets, and should be thick enough to handle the density of the magnetic flux. Normally, a piece of 1/4 inch steel plate would be thick enough. You would find, for instance, that a super magnet will hold more strongly to that plate than it will to tin can material, but a weak magnet might not show much difference in attractive force from the thin material to the thicker one.

                    One way to get an impression of how powerful a magnet is is to trap a finger between it and the steel surface. The amount of pain you feel is directly proportional to the strength of the magnet- Kidding of course- have you read some of the disclaimers that come with super magnets? Some of those can actually crush fingers, and they don't have to be that large either. Some are large enough to be quite dangerous, as a lot of that literature suggests.
                    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                    • #11
                      Magnetic fields are volumetric. Because of this the field strength (density per unit volume) may be increased by reducing the volume it occupies. There is no configuration of a pair of magnets the reduces or increases the total strength of either one. What does change is the shape of the magnetic field and the amount of space it occupies.

                      If you place a pair of disc magnets South to South with the North poles on either end you don't cancel the south poles in any way. What does happen is the both S fields will be strongly compressed and will extend further from the magnet as it forms a loop that connects the flux to each north pole.

                      Different configurations produce different flux densities. An extreme example of greatly increasing the flux density by combining magnets is the Halbach Array.



                      Not shown is the flux distribution in the array. Nearly all the flux is forced to one side of the magnet array so that one side is almost twice as strong as it would be if the magnets were stacked NSNSNS. The other side has almost no field extending at all and acts as a very weak magnet.

                      These effects are especially evident with magnets because of the volumetric nature of the magnetic field. Since it is volumetric and not areal the field strength falls off as the inverse 3rd power of distance. Compress the field and the strength curve with distance steepens by the third power.
                      Last edited by Evan; 09-05-2010, 07:15 PM.
                      Free software for calculating bolt circles and similar: Click Here

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                      • #12
                        I have a foot long stack of hard drive magnets. Cannot get them within 4-5 inches of each other or they collide with enough force to shatter. Wonder what would happen if I put them through the US postal system?
                        mark costello-Low speed steel

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                        • #13
                          I agree with what you've said, Evan, about the configuration not affecting the total strength of the magnets, except where the magnet or magnet assembly has to hold to something. No good if much of the magnetic field is confined within the magnets and not available to loop through the metal you're trying to stick it to. This is like putting a keeper on a horseshoe magnet and expecting it to hold to a surface as well as it would without the keeper.

                          However much flux passes through the keeper is that much less that contributes to the holding power, even while the total flux remains unchanged. Same goes for magnets oriented in such a way that a flux loop passes largely through the magnets and pole pieces without having to go through the surface you're trying to stick it to. Just making a clarification here.
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                          • #14
                            Originally posted by Evan
                            Magnetic fields are volumetric. Because of this the field strength (density per unit volume) may be increased by reducing the volume it occupies. There is no configuration of a pair of magnets the reduces or increases the total strength of either one. What does change is the shape of the magnetic field and the amount of space it occupies.

                            If you place a pair of disc magnets South to South with the North poles on either end you don't cancel the south poles in any way. What does happen is the both S fields will be strongly compressed and will extend further from the magnet as it forms a loop that connects the flux to each north pole.

                            Different configurations produce different flux densities. An extreme example of greatly increasing the flux density by combining magnets is the Halbach Array.



                            Not shown is the flux distribution in the array. Nearly all the flux is forced to one side of the magnet array so that one side is almost twice as strong as it would be if the magnets were stacked NSNSNS. The other side has almost no field extending at all and acts as a very weak magnet.

                            These effects are especially evident with magnets because of the volumetric nature of the magnetic field. Since it is volumetric and not areal the field strength falls off as the inverse 3rd power of distance. Compress the field and the strength curve with distance steepens by the third power.
                            Such as that shown above spawns yet another question. Am I to assume that pole labeling of North & South is arbitrarily assigned to the first magnet in an array OR is there truly a North and South (even on any simple magnet be it in an array or singular), and if so how does one know which is North and which is South?

                            In passing, the magnet on my lights are "flat & circular" rather than square or rectangular. I failed to give out that information when starting this thread.

                            Harold
                            For those having fought for it, Freedom has a flavor the protected will never know.
                            Freedom is only one generation away from extinction.

                            Comment


                            • #15
                              The north pole of a magnet is the one that is attracted to the south pole of the Earth's magnetic field. The Earth's field is the standard so it isn't arbitrary. One does need standards for this sort of thing.

                              To discover the actual polarity of the field attach the magnet to a thread and see which way it aligns when suspended. In the case of a coin shaped magnet the poles are on the faces so it should be suspended from the rim via a bit of tape or whatever. The face that points north is the south pole of the magnet.

                              BTW, this is what the field of a Halbach Array looks like. A Halbach Array may be made arbitrarily long.

                              Last edited by Evan; 09-06-2010, 01:36 AM.
                              Free software for calculating bolt circles and similar: Click Here

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