Abstract: A tool for filing or abrading surfaces or for sharpening instruments such as knives, chisels and the like includes two spaced thin ferromagnetic rails having co-planar upper edges. A magnet is located between the rails with a ferromagnetic plate magnetically held to the rail. The plate has an exposed abrasive coated surface. A first magnetic pole of the magnet concentrates and conducts magnetic flux to the plate through the rails with sufficient force to hold the plate in position against the upper edges of the rails without the need for a retaining cavity to further restrain the plate from moving during use.