This can be used to grind chemicals to a very fine grain or to polish rocks.
Wiki says "A ball mill is a type of grinder used to grind materials into extremely fine powder for use in paints, pyrotechnics, and ceramics."
Many instructables refer to United Nuclear Ball Mills. Their small ball mill cost between $70 and $80 dollars.
For no more than $30 and in 5 minute you can build a ball mill of appreciable performance.
Check out my other Instructables:
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Step 1: What You Need
1. A rugged container (You can use PVC pipes or big plastic bottles)
2. An electric screwdriver (these are fairly cheap, I got mine for $10)
3. A bolt, a nut and maybe a washer.
4. Epoxy putty.
5. Steel or lead balls which in my case I substituted with screwdriver bits that I got for $3.
6. A vise clamp to hold down your ball mill.