Originally Posted by Anthony_Lopez
For those of you that are saying that your bar is spinning off center, I'd suggest fine tuning the centering of your magnet until it spins in the center of your enclosure or the desired spot.
the problem is not so much the position of the magnet its the physics behind it. A nice stir bar is magnitized through the length giving it a N and S poll on each end. Hardrive magnets are polarized through the thickness giving each face a N and S poll. If you are using a small stir bar this is not a problem cause the hardrive magnet is large enough to keep it centered. However, If you are using a larger stir bar the bar spins off axis and gets tossed at very low speeds. The best was to make one is either to get a bar magnet polarized through the length similar to the stir bar or to buy small magnets that you can put on either side (one with N facing up and the other with S facing up) with a space in the middle. I have both harddrive magnets and small cirlce magnets that I have tested with 1", 1.5", 2", 3". The small circle magnets work well with all 4. The harddrive magents works perfect with the 1" so/so with the 1.5", okay at slow speed with 2" and not at all with the 3".
as a quick reference to people building these from now on follow this chart depending on what you already have or how much you are will to spend:
Magent type ............... stir bar size
1 hardrive magent ....... 1"
circle magnets ........... 1-2"
bar magnets* ............ 1-3"
*Bar magnets needs to be polarized through length and matching the length to the stir bar is ideal
hope this helps