In a stack of flexible sheets on a substrate having an outer edge, some of the sheets project laterally past one of the edges. The stack is tamped together by a stabilizing element shiftable horizontally toward and away from the one edge of the substrate and a drive that shifts the stabilizing element horizontally toward the stack and substrate for engaging the projecting sheets and pushing same inward on substrate to a position lying on or inward of the outer edge without vertically bending or deflecting the sheets.