The ornamental design for a rotating grind stone, as shown and described.
FIG. 1 is a front view of a rotating grind stone showing our new design, a rear view being a mirror image thereof;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a right side view thereof, a left side view being a mirror image thereof;
FIG. 5 is a perspective view thereof;
FIG. 6 is a partial enlarged perspective view thereof; and,
FIG. 7 is a partial enlarged perspective thereof.
The broken lines shown in the drawing are for the purpose of illustrating the portions of the rotating grind stone and form no part of the claimed design.