a rotary grinding disc in an apparatus for refining fibrous pulp material , comprising a plurality of arcuate grinding plate segments disposed annularly about the disc and supported thereon by means of wedge - shaped tongues projecting from the segment plates and dovetailed into grooves of corresponding profile in the disc , the wedge - shaped tongues and dovetail grooves decreasing in width from an outer radial portion toward an inner radial portion , or vice versa , for wedging the plate segment into the disc . the plate segments may be additionally secured against disc placement by the effect of the centrifugal force during rotation of the disc by means of a peripheral ring surrounding the disc and engaging the wedged tongues .