1 he rotary cutting instruments used in dentistry remove tooth structure by cutting or abrading. Prior to the early 1940’s, the abrasion of tooth structure was accomplished with Carborundum disks and mounted stones. The introduction of diamond instruments, together with increased rotational speeds (20,000 to 30,000 r. p. m.) resulted in increased cutting efficiency at lighter cutting pressures. Tungsten carbide steel burs became available in 1947 and have proved to be a significant improvement over the carbon steel burs previously used. The basic design of the tungsten carbide bur evolved from the 6 bladed carbon steel bur. This design utilized engineerin, u theories for cutting tools. However, considering the material being cut (enamel and dentin) and the variations in speed and pressure used by different dentists, it is not possible to predict optimum design and performance accurately. As a result, design changes have been based upon empirical findings rather than scientific research. Because of this, the United States Bureau of Standards, the United States Armed Forces, and the Australian Bureau of Dental Standards have established specifications which influence the quality of dental burs. In an attempt to cope with the forementioned variables, the manufacturer considers tooth profile, helix angle, sharpness and number of cutting edges, concentricity, and quality of carbide steel in a series of compromises to produce a satisfactory product. The profile of the bur tooth is determined by the rake and relief angle. A small rake and large relief angle result in a small bur tooth profile and faster cutting. However, the resultant small cross section of the bur tooth produces a weaker edge more subject to dulling. Also, problems of longevity of the grinding tools used in the manufacturing process have a role in the profile design of the bur tooth. The helix angle influences the smoothness of the cut, the larger angle producing a smoother cut. Quality control in the grinding of the cutting edges of the