1910.243(c)
Portable abrasive wheels -
1910.243(c)(1)
General requirements. Abrasive wheels shall be used only on machine provided with safety guards as defined in paragraph (c) (1) through (4) of this section.
1910.243(c)(1)(i)
Exceptions. The requirements of this paragraph (c)(1) shall not apply to the following classes of wheels and conditions.
1910.243(c)(1)(i)(a)
Wheels used for internal work while within the work being ground;
1910.243(c)(1)(i)(b)
Mounted wheels used in portable operations 2 inches and smaller in diameter; (see definition 1910.241(b) (1)); and
1910.243(c)(1)(i)(c)
Types 16, 17, 18, 18R, and 19 cones, and plugs, and threaded hole pot balls where the work offers protection.
http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9850