I did build a new stirplate using a good quality potentiometer and a motor that is mounted on a metal plate that is mounted on 4 screws that are setting through the bottom of the box (see the picture). I mounted two earth magnets on a plastic disk that is mounted on the rotating axle. The distance between the two earth magnets is equal to the length of the stir bar that Im using, 1. Al spins fine with slight wobbling, but in all it spins fast and slow and it does fine without water. However, when I put water in the flask, the bar will spin fine at slow speeds but when I increase speed (slowly or fast) the bar spins out to the side of the flask. The best I could get is a start of a vortex. I even have put two earth magnets at each end which did help little bet but not good enough. Any thoughts?