EF6 – High or Low Ratio of Reduction Rod Milling – the equation to be used, unless Rr is between Rro = - 2 and Rro = +2, is:

This factor always applies to low ratios of reduction but its application to high ratios of reduction does not always apply and should be used only if the Wioc ÷ Wi grinding efficiency factor indicates that it should be used.
   
EF7 – Low Ratio of Reduction Ball Mill – the need to use this factor foes not occur very often as it only applies to ball milling when the Ratio of Reduction is less than 3. This shows up particularly in regrinding concentrates and tailings. The equation for this is:

While this factor is used in selecting rod mills, the inability to measure and define it accurately its value and significance in calculating Wioc and, therefore, should probably not be used in determining the efficiency of rod mill performance. However, knowledge of its existence can be helpful in analyzing rod mill performance.
   
EXAMPLES: Two examples are given to show how to calculate Wio and Wioc for single stage ball mills. The first example is a comparison of two parallel mills from a daily operating report. Mill size 16-1/2’ x 20’.

 Ball mill grindability test at 65 mesh gave a Wi = 14.5. Using this to calculate oversized feed factor: