Most air-driven, high-speed dental handpieces provide a means of accessing the turbine assembly for maintenance purposes. The turbine assembly is located in a chamber formed in the head of the drill and includes chuck, bearings and impeller. The turbine assembly has a much shorter service life than the handpiece body and must be replaced periodically. Routine cleaning and lubrication may also necessitate accessing the turbine assembly periodically. Typically, a removable threaded insert is provided for the purpose of removing or servicing the turbine assembly. This threaded insert frequently incorporates a push button mechanism for disengaging the chuck and removing the cutting tool or burr. Removal of the threaded insert is generally facilitated by the use of a specially designed key or wrench which may be hexagonal or octagonal in shape and matches the exterior profile of the threaded inset. Other designs incorporate slots or flats in the threaded insert for receiving a corresponding key or wrench. Such prior art designs are exemplified, for example by reference to U.S. Pat. No. 5,334,013 issued on Aug. 12, 1994.
Air-driven, high-speed dental handpieces incorporate one or more air passages for supplying air to the turbine assembly and one or more exhaust passages for evacuating the return air from the turbine chamber. The effective area of the air exhaust passage is of great importance since a larger opening reduces the back pressure in the turbine chamber and increases the efficiency of the energy transfer by the air flow through the turbine and out through an exhaust passage produced in the chamber side wall. Air exhaust passages produced by conventional manufacturing processes such as drilling, produce holes with a circular cross-section which do not take full advantage of the available area within the handpiece head. Furthermore, the total available area for the air exhaust passage leading from the turbine chamber is limited by the fact that the handpiece head is generally designed to be as small as possible in order to improve accessibility and maneuverability within the confines of the patient's mouth. Certain handpiece designs incorporate multiple, circular air exhaust passages in order to increase the effective exhaust area.