This Grindability Test or Bond Ball Mill Work Index Procedure is used to determine the Bond Work Index of minus six mesh or finer feed ore samples These equation application methods are used to process d 0 05 mm W is the work index measured in a laboratory ball mill kilowatt hours per metric or short ton P 80 is the mill circuit product size in micrometers F 80 is the mill circuit feed size in micrometers Rod mill A rotating drum causes friction and attrition between steel rods and ore particles citation needed But note that the term rod mill is also used }