Some tests were made, on the basis of the fundamental concept of comminution formerly proposed by the author, to study the relation between the performance and the essential grinding mechanism of the tumbling mill, and further to make clear why the industrial radial mill shows a lower grindability.As the result, it has been proved that the new concept is satisfactorily applicable to the industrial equipments as follows:where K and S∞ can be expressed by the following equations when other factors remain constant:K=(1-1.1)×104(g/kWH), almost independent of any of mill types, speeds, or diameters.S∞=C·f(Ncr)·Dm, where m=φ(Ncr)for ball mill, m=0.52-0.53 (Ncr=46-74%)for radial mill, m=0.13-0.20 (_??_)The above results are significant in that they help to theoretically explain the correlation concerning the cylindrical mill as:which equations, however, do not apply to the case of the radial type mill, where, According to .an approximate analysis in which earlier results2) were employed, the milling action in a ball mill seems to be greatly influenced by the impact of falling balls, while in a radial mill, attrition-grinding between balls and the material might be a controlling factor.The special features of the industrial radial mill are that although the mill requires only half the energy needed by a cylindrical one of the same size, its capacity is so much smaller than that of the latter that it is not recommendable to connect this type of mill to a continuous long tube mill as a fine grinding compartment.