What Is the Definition of Depressed Center Wheels?

Grinding wheels with a depressed center, which helps keep the mounting hardware out of the grinding process. ANSI type 27, 28 and 29 wheels fall under this category. ANSI type 27 wheels have a flat configuration with a depressed center.  ANSI type 29 wheels fold back away from the workpiece and wheel center while ANSI type 28 wheels project out in front of the wheel center toward the workpiece. Depressed center wheels are applied in rough offhand grinding or snagging of castings, weld beads, flash or parting lines or defects in metal parts.