Abstract: Reduction or loss of rotary hammermill grinding action for a tub grinder is prevented by effectively extending the surface of the floor of the tub grinder across the gravity feed opening for the hammermill chamber area. In one form, the floor extension includes a plurality of relatively narrow cross-bars which extend completely across the chamber opening in the floor so as to block relatively heavy or dense clumps of unground feed from resting on the hammermill discs or jamming between the hammermill and its chamber sidewalls. The pivoted hammers are permitted to rotate in an arc extending above the cross-bars. In another form, the tub floor extension is formed by a multiplicity of generally triangular shaped members which extend from the feed side of the opening across the opening with sufficient distance to prevent jamming against the hammermill discs while providing sufficient vertical support to allow initial grinding of such dense clumps by the pivoting hammermill hammers.