While using burs you should use a steady, slow to medium speed. Flex-shafts can operate at speeds up to 14,000 rpm, some even faster. Burs and drills should not approach this speed while cutting metal (carbide and diamond tipped burs and drills are the exception to this). You should operate the flex-shaft at no more than medium speed while using steel burs and drills, and the larger the bur the slower the speed. High speed steel burs are a description of the type of steel used and not indicative of the speed used to operate.