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 egory. 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 ...