An assembly for mounting an abrasive sheet on a rotary abrading tool having a circular mounting member fastenable to the tool drive shaft and having an opposite surface bearing a typically square raised backup pad to which a square sheet of abrasive material can be fastened. Other straight edged backup pad configurations can be used if desired. Entrance holes are formed through the mounting member adjacent to the backup pad. A circular shroud is provided on the mounting member surface opposite the backup pad covering the exit holes. Air and dust are evacuated from said shroud and holes with a vacuum or built-in impeller evacuation system. The shroud has a diameter less than that of the mounting member and may have an edge engaging a circular groove in the mounting member to minimize air leakage between shroud and mounting member. The back up pad may be a reversible, double sided pad snapped into a mounting member recess. Additional dust evacuation holes and grooves may be formed in the backup pad itself. This assembly provides improved abrasion efficiency in sanding, grinding and polishing operations and substantially eliminates abrader edge grooving or damaging of workpiece surfaces.