While using burs, 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 when operating them.
A tool commonly used in stonesetting to cut a seat for the gemstone or diamond is a bur.
Use light even pressure when you are using burs. Excessive pressure to vigorously cut metal will overheat the burs. This heat will remove the temper from the tool and it will no longer be able to keep a sharp cutting edge. Let the tool do the work. DO NOT USE EXCESSIVE SPEED OR PRESSURE TO FORCE A DULL BUR.