The mill includes a milling chamber (1) containing a rotor (2) provided with hammers (4), and at least one impact screen mounted pivotably with the aid of an adjusting jack in order to move the bottom end of the screen toward or away from the rotor so as to define, between the bottom end of the screen and the free end of the hammers, a spacing which determines a given particle size. The mill includes a device for positioning the rotor that includes indexing elements for positioning the end of one of the hammers as close as possible to the screen for adjusting the spacing. It includes a geared motor driving a pinion capable of engaging with a toothed wheel fixed in rotation to the rotor so as to rotate it from a rest state until one of the hammers is as close as possible to the screen.