What material works best for the table that the bearing rolls on?
Thanks
Probably depends on the size of the table. But in any case the closer the better and it has to be reasonably rigid to avoid sagging under gravity or magnet force.
For my mid-size table I got a 21x21" sheet of simple 1/8" glass cut to size by a local glass shop. It allowed the magnet to be less than 0.2"/5mm distance with a little margin. I had considered aluminum but the glass is as rigid and much cheaper. To help avoid seeing through I had it cut from opaque etched glass.
Works perfectly with baking soda. I tried simple quartz sand first but the sand grains make a horrible crunching noise between the steel ball and the glass.
To be on the safe side I installed 2 very small transfer rollers on the magnet slider to prevent the magnet from rubbing on the glass underside when not well adjusted. But they should normally not be engaged.
I used 1/4" MDF on mine. As long as you don’t use something ferric and you keep the total distance low you should be good to go. Be aware that too much ‘sand’ (I use baking soda) will allow you to lose the bearing on occasion. I keep mine down to about 1/2 the bearing radius.