Rule 105.
SHAPE TYPES OF GRINDING WHEELS
SHAPE TYPES OF GRINDING WHEELS CONES AND PLUGS
On all cones and plugs grinding is performed by all surfaces except the flat (dimension "D") on the mounting side. These shapes normally have a blind hole threaded bushing for mounting.
RELIEVED AND/OR RECESSED WHEELS
A relieved side is a depression in the side of the wheel which is tapered from a radial flat at the periphery (A) to an inside flat (K) or recess diameter (P).
Mich. Admin. Code R. 408.10105