from what I understand this mounting magnet idea is really good because it will reduce the magnetic field perpendicular to the magnetisation direction. However I see that the mounting magnet sizes I need are smaller then whats on offer. The diameter of the mounting magnet must be no more then 10mm. Also it could be that the mounting magnet leads to forces that are too big actually.
I may look into it further if the current magnets and board I am ordering does not do a good job. Thanks for the info.

 

Hi tmer1,
We use Neodymium magnets in our products - we also make use of a custom designed cup to direct the flux. Our application is different to yours, as we require the magnet to actuate a plastic bodied solenoid (with windings around the bobbin of the solenoid body).
We custom made our first cups by having them wire cut - think of a four pronged star that gets formed into a cup with four small slits around the periphery. We now get them stamped and formed on a manufacturing basis, but a couple of practical points for you to keep in mind, based on our learning curve - you will need to stabilize the magnet in the centre of the cup, irrespective of your application. Our preferred adhesive for this is Locktite 480 (expensive, but does the job). A micro applicator is used to apply a tiny, specific amount to each cup.
Also, commercially produced stamped parts will have a light sheen of oil on them - dipping in Acetone prior to applying the glue is required to make the adhesive effective.
Neodymium magnets are extremely efficient and exert good anisotropic properties (a resistance to being demagnetised, or re-magnetised in a different axis), but they're not ideal for this type of resistance unless alloyed with Gadolynium or Dysprosium. This may become problematic for your application, and as an economic consideration, whilst Neodymium is relatively wide spread and economically mineable, the other two elements are really only available from China, and every now and then China threatens to restrict supply.
The magnets we use are 3mm in diameter and 3mm in height. We are based in South Africa and I'm fairly sure that we import them, so let me know if you're looking for a supplier and I'll check with our production manager.
Also, if you're designing a game, you should keep in mind that there have been reported cases of children ingesting one or more of these small magnets, which has resulted in them losing parts of their digestive tracts, as the magnets have been attracted to each other within the body. Some sort of resin encapsulation would probably be a good idea.
Have you considered 3CR12 as an alternative to the 400 series stainless steels? Good magnetic properties (for your application), a lot cheaper and reasonably corrosion resistant - far more so than hot or cold rolled mild steels.

 

Thanks for the detailed information K.I.S.S.

I will think about these points more if I get past the next testing phase.

Cheers,

Tmer

 

Tape them.

I don't know why in particular neo magnets are a good idea. If they fly together they break. You could use ferrite ceramic magnets, alnico, and so on. Neo is there for difficult high force applications. They seem misplaced here.

If I were you I would work out the distance and attractions issues with a roll of masking tape and a handful of different size magnets. Wrap enough tape around them so they don't get close enough to move each other, but they still stick to the board (needs to be thin steel). Probably figure it out very well in less than an hour of messing around.

I say this having designed permanent magnetic circuits for Hall Effect sensors where there were air gaps involved. One problem is the literature on magnetic circuit design is sort of old school. Simulation software is VERY expensive. There is a free and open one that does 2D called FEMM. It will take you a long time to do this with math and theory. I have never seen this addressed in textbooks.

Small magnets don't do much across more than a quarter of an inch. Keep that in mind. You can use smaller magnets if you put a ring of steel around them to concentrate the flux, that is a good way to get holding power, but then maybe you can't get them free of the board once they are latched. Or even just a small bar along side a cylindrical magnet, less power, but a smaller magnet still works.

This is not to scare you off from the theory, it is very interesting, but it is non-trivial. And practical advise hard to come by. I was blessed to have real production as a goal so I could call magnet manufacturers and resellers for design advice. Arnold Magnetics was excellent, but the old timers are probably gone now.

MR