I have an old Montgomery Wards cement mixer that I fixed up just for something to do. I was out feeding the chickens the other day and noticed that the oyster shell that I have is a little too big for them. I also have some granite grit that is a little large too so I was thinking that if I used the cement mixer with 5 or 6 large steel ball bearing's I could crush the rock and shell even finer. I'm a genius. I load half a feed scoop of granite with the ball bearings and turn the mixer on medium. Mix for about 20 minutes and dump in a full scoop of oyster shell. Mix another 10 minutes and add two scoops of chicken scratch and mix for a minute more. Dump it in the bucket.