Aircraft manufacturers spend millions of dollars reworking blown fastener holes, especially in portable tool drilling situations. Oval, tapered, rifled, and oversize holes are costly rework issues currently commonplace in the industry. The most common causes of imperfectly drilled holes include spindle runout, insufficient clamp and feed force, out-of-balance drill feed forces, spindle windup, and lack of adequate feed control.
This paper will focus on a next-generation drilling machine that utilizes a unique combination of hydraulics and pneumatics to solve the problems associated with legacy drilling units. Several design elements will be examined, such as the use of an on-board, 1000 PSI hydraulic pump, controlling both drill feed and clamp force. This greatly reduces the size and weight of the clamp and feed cylinders compared to legacy air systems, while increasing their force and rigidity. Additionally, by utilizing a hydraulically actuated clamping system, the drilling machine, work piece, and spindle guide elements are rigidly held together, in effect, in a one-piece system with no sliding or wearing joints. This maintains the feed force in-line with the drill bit, greatly enhancing drill straight-line motion.
Examination will be made of the use of a planetary gear train, made possible by designing the air motor in-line with the drill spindle. Planetary gears, with all gear forces internally balanced, have greater mechanical efficiency and stiffness than spur gear trains and eliminate the inaccuracy caused by side loading the end of the drill spindle. Less energy is wasted drilling the hole and spindle bearing wear is reduced.
Finally, the paper will present production data that focuses on the use of the tool in a production environment. Such issues as efficiency, reliability, and hole accuracy will be examined. A comparison will be made to legacy tooling in similar applications.