I claim the ornamental design for grating mill, as shown and described.
FIG. 1 is a front elevational view of a grating mill showing my new design,
the rear view being a mirror image and, therefore, not shown;
FIG. 2 is a left side elevational view thereof, the right side view being a
mirror image except for the opposite side of the handle being shown and,
therefore, not shown;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof; and,
FIG. 5 is an exploded front elevational view thereof.