Ball mills. ball mills have been the primary piece of machinery in traditional hard rock grinding circuits for 100+ years. they are proven workhorses, with discharge mesh sizes from ~40m to 200m. use of a ball mill is the best choice when long term, stationary milling is justified by an.Be sure it is perfectly straight. the sheave on the drive shaft is a 6 inch o.d. by 5/8 inch (grainger #3x919). this cost me $3.72. you will also need two self-centering ball-bearing pillow-block bearings to mount the drive shaft (grainger #2x898). i had to sand the drive shaft a little bit to allow the bearings to slide onto the.