as indicated as s referring now more particularly to fig2 and the hammermill 3 the mill has a drive shaft 23 which is coupled to a driving motor not shown the shaft 23 inwardly of the mill is provided with an enlarged axial portion 24 extending over the primary length of the mill shaft portion 24 carries a plurality of parallel arranged circular plates 25 fig2 and 12 the plates 25 each have a plurality of hammers 26 secured thereto by retainer bolts 27 the hammers 26 fig2 and 12 in the operation of the mill rotate with their peripheries moving closely adjacent wall section 28 of the hammermill and so as to abut against and break into fiber pieces the fiber materials fed through feed rolls 2 the internal diameter of ring 5 is somewhat greater than that of ring 6 air under the influence of fan 11 is drawn through opening 4 and assumes a helical path p fig4 in which the fibrous material being broken up under the action of the hammers tends to move the internal ring diameter of each ring 5 6 is less than the diameter of the working zone 32 of the hammermill 3 shown to the right of the classifying rings 5 6 in.