Pre-connected Process Optimization Of Aircraft Stacks For Automatic Drilling System

Thousands of connecting holes on aircraft structures are processed in the stacked condition, when the stacks do not fit tightly in the automatic drilling, interlayer burrs will be formed, which not only cause stress concentration, but also reduce the friction between the stacks. The processing quality of the connecting holes directly impacts on the fatigue resistance and reliability of the aircraft. Thus, it is particularly important to ensure the processing quality of the connecting holes and inhibit the formation of the interlayer burrs between the stacks in order to meet the high quality and long life requirements of the aircraft.During aircraft parts’assembly processes, an initial gap often exists between the stacks due to manufacturing errors, positioning errors and other factors. At present, burr control technology is mainly applying to overcome the separation of stacks and reduce drilling gap. However, the unidirectional pressing force usually only applies on the stacked structure with strong rigidity constituted by skins and frames, which achieve the effect of "bidirectional pressing". When facing the stacked structure with weak rigidity constituted by skins and beams, the unidirectional pressing force cannot effectively eliminate the initial gap and drilling gap between them, which provide physical space for the formation of burrs in the automatic drilling. In this case, we need to use pre-connectors to tighten skins and beams, the pre-connectors can optimize the contact stiffness and reduce drilling gap of the stacks, also play an important role to ensure the quality and efficiency of bolt connection afterwards.Therefore, a simplified pre-connect ion finite element model without considering of the pressing force is established in this paper, and the residual gap and drilling gap under the conditions of different pre-connector’s quantity, installation location and pre-load are studied, then the optimal pre-connection scheme is obtained. Finite element results show that residual gap and drilling gap of the model is significantly reduced with the increasing of the preload and then slightly increased, because excessive elastic deformation will be occurred by too large preload. Smaller residual gap is obtained after pre-connection, which does not mean smaller drilling gap will be gained, this is because better pre-connector’s installation location can optimize the contact stiffness of the stacks so that the drilling gap can be effectively reduced.Finally, each scheme is compared and verified by automatic drilling experiment of stacks. Experimental results show that the interlayer burr size is significantly reduced with the increasing of the preload and then slightly increased, pre-connector’s quantity and installation location has significant effect on the interlayer burr size. The average interlayer burr height and root thickness of the optimal pre-connection scheme is 0.051 mm and 0.015 mm.